Change search
Refine search result
1234 1 - 50 of 160
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the 'Create feeds' function.
  • 1.
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Spin-diode effect and thermally controlled switching in magnetic spin-valves2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis demonstrates two new device concepts that are based on the tunneling and giant magnetoresistance effects. The first is a semiconductor-free asymmetric magnetic double tunnel junction that is shown to work as a diode, while at the same time exhibiting a record high magnetoresistance. It is experimentally verified that a diode effect, with a rectification ratio of at least 100, can be obtained in this type of system, and that a negative magnetoresistance of nearly 4000% can be measured at low temperature. The large magnetoresistance is attributed to spin resonant tunneling, where the parallel and antiparallel orientation of the magnetic moments shifts the energy levels in the middle electrode, thereby changing their alignment with the conduction band in the outer electrodes. This resonant tunneling can be useful when scaling down magnetic random access memory; eliminating the need to use external diodes or transistors in series with each bit.

    The second device concept is a thermally controlled spin-switch; a novel way to control the free-layer switching and magnetoresistance in spin-valves. By exchange coupling two ferromagnetic films through a weakly ferromagnetic Ni-Cu alloy, the coupling is controlled by changes in temperature. At room temperature, the alloy is weakly ferromagnetic and the two films are exchange coupled through the alloy. At a temperature higher than the Curie point, the alloy is paramagnetic and the two strongly ferromagnetic films decouple. Using this technique, the read out signal from a giant magnetoresistance element is controlled using both external heating and internal Joule heating. No degradation of device performance upon thermal cycling is observed. The change in temperature for a full free-layer reversal is shown to be 35 degrees Celsius for the present Ni-Cu alloy. It is predicted that this type of switching theoretically can lead to high frequency oscillations in current, voltage, and temperature, where the frequency is controlled by an external inductor or capacitor. This can prove to be useful for applications such as voltage controlled oscillators in, for example, frequency synthesizers and function generators. Several ways to optimize the thermally controlled spin switch are discussed and conceptually demonstrated with experiments.

  • 2.
    Andersson, Sebastian
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Exchange coupling and magnetoresistance in CoFe/NiCu/CoFe spin valves near the Curie point of the spacer2010In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 107, no 9, p. 09D711-Article in journal (Refereed)
    Abstract [en]

    Thermal control of exchange coupling between two strongly ferromagnetic layers through a weakly ferromagnetic Ni-Cu spacer and the associated magnetoresistance is investigated. The spacer, having a Curie point slightly above room temperature, can be cycled between its paramagnetic and ferromagnetic states by varying the temperature externally or using joule heating. It is shown that the giant magnetoresistance vanishes due to a strong reduction in the mean free path in the spacer at above similar to 30% Ni concentration-before the onset of ferromagnetism. Finally, a device is proposed which combines thermally controlled exchange coupling and large magnetoresistance by separating the switching and the readout elements.

  • 3.
    Andersson, Sebastian
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Thermoelectrically Controlled Spin-Switch2010In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 46, no 6, p. 2140-2143Article in journal (Refereed)
    Abstract [en]

    The search for novel spintronic devices brings about new ways to control switching in magnetic thin-films. In this work we experimentally demonstrate a device based on thermoelectrically controlled exchange coupling. The read out signal from a giant magnetoresistance element is controlled by exchange coupling through a weakly ferromagnetic Ni-Cu alloy. This exchange coupling is shown to vary strongly with changes in temperature, and both internal Joule heating and external heating is used to demonstrate magnetic switching. The device shows no degradation upon thermal cycling. Ways to further optimize the device performance are discussed. Our experimental results show a new way to thermoelectrically control magnetic switching in multilayers.

  • 4. Aurino, P. P.
    et al.
    Kalabukhov, A.
    Borgani, Riccardo
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Bauch, T.
    Lombardi, F.
    Claeson, T.
    Winkler, D.
    Retention of Electronic Conductivity in LaAlO3/SrTiO3 Nanostructures Using a SrCuO2 Capping Layer2016In: PHYSICAL REVIEW APPLIED, ISSN 2331-7019, Vol. 6, no 2, article id 024011Article in journal (Refereed)
    Abstract [en]

    The interface between two wide band-gap insulators, LaAlO3 and SrTiO3 (LAO/STO) offers a unique playground to study the interplay and competitions between different ordering phenomena in a strongly correlated two- dimensional electron gas. Recent studies of the LAO/STO interface reveal the inhomogeneous nature of the 2DEG that strongly influences electrical-transport properties. Nanowires needed in future applications may be adversely affected, and our aim is, thus, to produce a more homogeneous electron gas. In this work, we demonstrate that nanostructures fabricated in the quasi-2DEG at the LaAlO3/SrTiO3 interface, capped with a SrCuO2 layer, retain their electrical resistivity and mobility independent of the structure size, ranging from 100 nm to 30 mu m. This is in contrast to noncapped LAO/STO structures, where the room-temperature electrical resistivity significantly increases when the structure size becomes smaller than 1 mu m. High-resolution intermodulation electrostatic force microscopy reveals an inhomogeneous surface potential with "puddles" of a characteristic size of 130 nm in the noncapped samples and a more uniform surface potential with a larger characteristic size of the puddles in the capped samples. In addition, capped structures show superconductivity below 200 mK and nonlinear currentvoltage characteristics with a clear critical current observed up to 700 mK. Our findings shed light on the complicated nature of the 2DEG at the LAO/STO interface and may also be used for the design of electronic devices.

  • 5.
    Balatsky, Alexander V.
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Institute for Materials Science, Los Alamos National Laboratory, Los Alamos, NM, United States .
    Balatsky, Galina I.
    Borysov, Stanislav S.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Resource Demand Growth and Sustainability Due to Increased World Consumption2015In: Sustainability, ISSN 2071-1050, E-ISSN 2071-1050, Vol. 7, no 3, p. 3430-3440Article in journal (Refereed)
    Abstract [en]

    The paper aims at continuing the discussion on sustainability and attempts to forecast the impossibility of the expanding consumption worldwide due to the planet's limited resources. As the population of China, India and other developing countries continue to increase, they would also require more natural and financial resources to sustain their growth. We coarsely estimate the volumes of these resources (energy, food, freshwater) and the gross domestic product (GDP) that would need to be achieved to bring the population of India and China to the current levels of consumption in the United States. We also provide estimations for potentially needed immediate growth of the world resource consumption to meet this equality requirement. Given the tight historical correlation between GDP and energy consumption, the needed increase of GDP per capita in the developing world to the levels of the U.S. would deplete explored fossil fuel reserves in less than two decades. These estimates predict that the world economy would need to find a development model where growth would be achieved without heavy dependence on fossil fuels.

  • 6. Balkashin, O. P.
    et al.
    Fisun, V. V.
    Korovkin, I. A.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Ferromagnetic resonance in copper-permalloy point contacts2014In: Fizika Nizkih Temperatur, ISSN 0132-6414, E-ISSN 1816-0328, Vol. 40, no 10, p. 1187-1197Article in journal (Refereed)
    Abstract [en]

    The systematic investigations of the response signal of Copper-Permalloy (Ni80Fe20) point contacts to microwave irradiation (8-12 GHz) are performed. The influence of external magnetic field strength, transport current flowing through the contact, the intensity and frequency of the RF exposure on the measured signal is studied. The contributions to the contacts resistance from the effects of giant and anisotropic magnetoresistance are analyzed. It is established experimentally that the position of resonance features is in good agreement with the Kittel formula calculation for FMR in the film at a parallel H field. The resonance signal was observed on the background of an additional contribution due to the rectification of RF current on the nonlinear contact IV-curve. It is found that there are two mechanisms for the resonant response are revealed: a synchronous self-detection by means of mixing of RF current with temporal variations of resistance and a bolometric mechanism associated with a steady-state change of contact resistance under excitation of magnetization precession (a bolometric response). It is found that the resonance signal amplitude is linearly proportional to transport current through the contact and microwave irradiation intensity.

  • 7. Balkashin, O. P.
    et al.
    Fisun, V. V.
    Korovkin, I. A.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Ferromagnetic resonance in copper-permalloy point contacts2014In: Low temperature physics (Woodbury, N.Y., Print), ISSN 1063-777X, E-ISSN 1090-6517, Vol. 40, no 10, p. 929-936Article in journal (Refereed)
    Abstract [en]

    The response of copper-permalloy (Ni80Fe20) point microcontacts to microwave irradiation (8-12 GHz) is investigated systematically. The effects of the external magnetic field strength, transport current flowing through the contact, and microwave intensity and frequency on the measured signal are studied. The contributions to the contact electrical resistance owing to giant and anisotropic magnetoresistance are analyzed. The experimentally determined positions of the resonance features are in good agreement with Kittel's formula for FMR in films in parallel magnetic fields. The resonance signal is observed against the background of an additional contribution owing to rectification of the rf field on the nonlinear current-voltage characteristic of the contact. Two mechanisms for the resonance response are discovered: synchronous self-detection from mixing of the rf current with temporal variations in the resistance and a bolometric response associated with the steady-state variation in the contact resistance when magnetization precession is excited. The amplitude of the resonance signal is linearly proportional to the transport current through the contact and to the intensity of the microwave irradiation.

  • 8. Balkashin, O. P.
    et al.
    Fisun, V. V.
    Triputen, L. Yu
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Naidyuk, Yu G.
    Spin-valve effects in point contacts to exchange biased Co40Fe40B20 films2014In: Low temperature physics (Woodbury, N.Y., Print), ISSN 1063-777X, E-ISSN 1090-6517, Vol. 40, no 10, p. 915-918Article in journal (Refereed)
    Abstract [en]

    Nonlinear current-voltage characteristics and magnetoresistance of point contacts between a normal metal (N) and films of amorphous ferromagnet (F) Co40Fe40B20 of different thickness, exchange-biased by antiferromagnetic Mn80Ir20 are studied. A surface spin valve effect in the conductance of such F-N contacts is observed. The effect of exchange bias is found to be inversely proportional to the Co40Fe40B20 film thickness. This behavior as well as other magneto-transport effects we observe on single exchange-pinned ferromagnetic films are similar in nature to those found in conventional three-layer spin-valves.

  • 9. Balkashin, O. P.
    et al.
    Fisun, V. V.
    Yanson, I. K.
    Triputen, L. Yu
    Konovalenko, Alexander
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Nonstationary magnetization dynamics of point contacts with a single ferromagnetic film2009In: Low temperature physics (Woodbury, N.Y., Print), ISSN 1063-777X, E-ISSN 1090-6517, Vol. 35, no 8-9, p. 693-701Article in journal (Refereed)
    Abstract [en]

    The electric conductivity of point nanocontacts between 5, 10, and 100 nm thick ferromagnetic (F) cobalt films and a nonmagnetic (N) metal (copper or silver) needle is investigated. Two fundamentally different mechanisms for the formation of the response, signal to microwave irradiation have been observed for the first time. One mechanism is due to the effect of the rectification of high frequency ac current on the nonlinearity of the current-voltage characteristics due to the precession of the magnetization under the action of a constant transport current flowing through the contact. The second one is associated with the resonance excitation of the precession of the magnetization vector at the fundamental frequency and its harmonic by an external high-frequency field. The experimental results support a previous contention that a "surface spin valve" whose static and dynamic properties are similar to conventional F-1-N-F-2 spin valve [Nano Letters 7, 927 (2007)] is formed in the experimental F-N nanocontacts.

  • 10.
    Balkashin, O. P.
    et al.
    B. Verkin Institute for Low Temperature Physics and Engineering, National Academy of Sciences of Ukraine.
    Fisun, V. V.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Yanson, I. K.
    B. Verkin Institute for Low Temperature Physics and Engineering, National Academy of Sciences of Ukraine.
    Triputen, L. Yu.
    B. Verkin Institute for Low Temperature Physics and Engineering, National Academy of Sciences of Ukraine.
    Konovalenko, Alexander
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Spin dynamics in point contacts to single ferromagnetic films2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 9, p. 092419-1-092419-4Article in journal (Refereed)
    Abstract [en]

    Excitation of magnons or spin waves driven by nominally unpolarized transport currents in point contacts of normal and ferromagnetic metals is probed by irradiating the contacts with microwaves. Two characteristic dynamic effects are observed: a suppression of spin-wave nonlinearities in the point contact conductance by off-resonance microwave irradiation and a resonant stimulation of spin-wave peaks in the differential resistance of the nanocontacts by the microwave field. These observations provide direct evidence that the magnetoresistance peaks observed are due to gigahertz spin dynamics at the ferromagnetic interface driven by the spin transfer torque effect of the transport current.

  • 11. Ban, S.
    et al.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Pattern storage and recognition using ferrofluids2006In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 99, no 8Article in journal (Refereed)
    Abstract [en]

    An implementation of an associative memory based on a ferromagnetic nanocolloid is proposed. The design contains inductive input and output units for training the ferrofluid as well as sensors incorporated into the output units for performing recall. Using Monte Carlo simulations of the system we demonstrate the possibility of creating nanoparticle configurations that can serve to associate input/output pattern pairs.

  • 12. Bergqvist, J.
    et al.
    Tress, W.
    Forchheimer, Daniel
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Melianas, A.
    Tang, Z.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Inganäs, O.
    New method for lateral mapping of bimolecular recombination in thin-film organic solar cells2016In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 24, no 8, p. 1096-1108Article in journal (Refereed)
    Abstract [en]

    The best organic solar cells are limited by bimolecular recombination. Tools to study these losses are available; however, they are only developed for small area (laboratory-scale) devices and are not yet available for large area (production-scale) devices. Here we introduce the Intermodulation Light Beam-Induced Current (IMLBIC) technique, which allows simultaneous spatial mapping of both the amount of extracted photocurrent and the bimolecular recombination over the active area of a solar cell. We utilize the second-order non-linear dependence on the illumination intensity as a signature for bimolecular recombination. Using two lasers modulated with different frequencies, we record the photocurrent response at each modulation frequency and the bimolecular recombination in the second-order intermodulation response at the sum and difference of the two frequencies. Drift-diffusion simulations predict a unique response for different recombination mechanisms. We successfully verify our approach by studying solar cells known to have mainly bimolecular recombination and thus propose this method as a viable tool for lateral detection and characterization of the dominant recombination mechanisms in organic solar cells. We expect that IMLBIC will be an important future tool for characterization and detection of recombination losses in large area organic solar cells.

  • 13.
    Bondarenko, Artem V.
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Holmgren, Erik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Koop, Björn C.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Descamps, Thomas
    KTH.
    Ivanov, B. A.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Stochastic dynamics of strongly-bound magnetic vortex pairs2017In: AIP Advances, ISSN 2158-3226, E-ISSN 2158-3226, Vol. 7, no 5, article id 056007Article in journal (Refereed)
    Abstract [en]

    We demonstrate that strongly-bound spin-vortex pairs exhibit pronounced stochastic behaviour. Such dynamics is due to collective magnetization states originating from purely dipolar interactions between the vortices. The resulting thermal noise exhibits telegraph-like behaviour, with random switching between different oscillation regimes observable at room temperature. The noise in the system is further studied by varying the external field and observing the related changes in the frequency of switching and the probability for different magnetic states and regimes. Monte Carlo simulations are used to replicate and explain the experimental observations.

  • 14.
    Borgani, Riccardo
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Sensing photo-induced surface charges on OrganicPhoto-Voltaic materials using Atomic Force Microscopy2014Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 15.
    Borgani, Riccardo
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Forchheimer, Daniel
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Bergqvist, Jonas
    Thorén, Per-Anders
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Inganas, Olle
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Intermodulation electrostatic force microscopy for imaging surface photo-voltage2014In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 105, no 14, p. 143113-Article in journal (Refereed)
    Abstract [en]

    We demonstrate an alternative to Kelvin Probe Force Microscopy for imaging surface potential. The open-loop, single-pass technique applies a low-frequency AC voltage to the atomic force microscopy tip while driving the cantilever near its resonance frequency. Frequency mixing due to the nonlinear capacitance gives intermodulation products of the two drive frequencies near the cantilever resonance, where they are measured with high signal to noise ratio. Analysis of this intermodulation response allows for quantitative reconstruction of the contact potential difference. We derive the theory of the method, validate it with numerical simulation and a control experiment, and we demonstrate its utility for fast imaging of the surface photo-voltage on an organic photovoltaic material.

  • 16.
    Borgani, Riccardo
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Pallon, Love
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hedenqvist, Mikael S.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gedde, Ulf W.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Haviland, David
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Local Charge Injection and Extraction on Surface-Modified Al2O3Nanoparticles in LDPE2016In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 16, no 9, p. 5934-5937, article id 10.1021/acs.nanolett.6b02920Article in journal (Refereed)
    Abstract [en]

    We use a recently developed scanning probe technique to image with high spatial resolution the injection and extraction of charge around individual surface-modified aluminum oxide nanoparticles embedded in a low-density polyethylene (LDPE) matrix. We find that the experimental results are consistent with a simple band structure model where localized electronic states are available in the band gap (trap states) in the vicinity of the nanoparticles. This work offers experimental support to a previously proposed mechanism for enhanced insulating properties of nanocomposite LDPE and provides a powerful experimental tool to further investigate such properties.

  • 17.
    Borgani, Riccardo
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Thorén, Per-Anders
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Forchheimer, Daniel
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Dobryden, Illia
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Sah, Si Mohamed
    KTH.
    Claesson, Per Martin
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Background-Force Compensation in Dynamic Atomic Force Microscopy2017In: Phronimon, ISSN 1561-4018, E-ISSN 2331-7019, Vol. 7, no 6, article id 064018Article in journal (Refereed)
    Abstract [en]

    Background forces are linear long-range interactions of the cantilever body with its surroundings that must be compensated for in order to reveal tip-surface force, the quantity of interest for determining material properties in atomic force microscopy. We provide a mathematical derivation of a method to compensate for background forces, apply it to experimental data, and discuss how to include background forces in simulation. Our method, based on linear-response theory in the frequency domain, provides a general way of measuring and compensating for any background force and it can be readily applied to different force reconstruction methods in dynamic AFM.

  • 18.
    Borysov, Stanislav
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Roudi, Yasser
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. The Kavli Institute for Systems Neuroscience, NTNU, Trondheim, Norway.
    Balatsky, Alexander V.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Institute for Materials Science, Los Alamos National Laboratory, Los Alamos, NM, United States.
    U.S. stock market interaction network as learned by the Boltzmann machine2015In: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 88, no 12, p. 1-14Article in journal (Refereed)
    Abstract [en]

    We study historical dynamics of joint equilibrium distribution of stock returns in the U.S. stock market using the Boltzmann distribution model being parametrized by external fields and pairwise couplings. Within Boltzmann learning framework for statistical inference, we analyze historical behavior of the parameters inferred using exact and approximate learning algorithms. Since the model and inference methods require use of binary variables, effect of this mapping of continuous returns to the discrete domain is studied. The presented results show that binarization preserves the correlation structure of the market. Properties of distributions of external fields and couplings as well as the market interaction network and industry sector clustering structure are studied for different historical dates and moving window sizes. We demonstrate that the observed positive heavy tail in distribution of couplings is related to the sparse clustering structure of the market. We also show that discrepancies between the model’s parameters might be used as a precursor of financial instabilities.

  • 19.
    Borysov, Stanislav S.
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Balatsky, Alexander V.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Cross-Correlation Asymmetries and Causal Relationships between Stock and Market Risk2014In: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 9, no 8, p. e105874-Article in journal (Refereed)
    Abstract [en]

    We study historical correlations and lead-lag relationships between individual stock risk (volatility of daily stock returns) and market risk (volatility of daily returns of a market-representative portfolio) in the US stock market. We consider the cross-correlation functions averaged over all stocks, using 71 stock prices from the Standard & Poor's 500 index for 1994-2013. We focus on the behavior of the cross-correlations at the times of financial crises with significant jumps of market volatility. The observed historical dynamics showed that the dependence between the risks was almost linear during the US stock market downturn of 2002 and after the US housing bubble in 2007, remaining at that level until 2013. Moreover, the averaged cross-correlation function often had an asymmetric shape with respect to zero lag in the periods of high correlation. We develop the analysis by the application of the linear response formalism to study underlying causal relations. The calculated response functions suggest the presence of characteristic regimes near financial crashes, when the volatility of an individual stock follows the market volatility and vice versa.

  • 20.
    Borysov, Stanislav S.
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Forchheimer, Daniel
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Dynamic calibration of higher eigenmode parameters of a cantilever in atomic force microscopy by using tip-surface interactions2014In: Beilstein Journal of Nanotechnology, ISSN 2190-4286, Vol. 5, p. 1899-1904Article in journal (Refereed)
    Abstract [en]

    We present a theoretical framework for the dynamic calibration of the higher eigenmode parameters (stiffness and optical lever inverse responsivity) of a cantilever. The method is based on the tip-surface force reconstruction technique and does not require any prior knowledge of the eigenmode shape or the particular form of the tip-surface interaction. The calibration method proposed requires a single-point force measurement by using a multimodal drive and its accuracy is independent of the unknown physical amplitude of a higher eigenmode.

  • 21.
    Borysov, Stanislav S.
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Platz, Daniel
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    de Wijn, Astrid S.
    Forchheimer, Daniel
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Tolén, Eric A.
    Balatsky, Alexander V.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Reconstruction of tip-surface interactions with multimodal intermodulation atomic force microscopy2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 11, p. 115405-Article in journal (Refereed)
    Abstract [en]

    We propose a theoretical framework for reconstructing tip-surface interactions using the intermodulation technique when more than one eigenmode is required to describe the cantilever motion. Two particular cases of bimodal motion are studied numerically: one bending and one torsional mode, and two bending modes. We demonstrate the possibility of accurate reconstruction of a two-dimensional conservative force field for the former case, while dissipative forces are studied for the latter.

  • 22.
    Cherepov, Sergiy
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Resonant switching and vortex dynamics in spin-flop bi-layers2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis is a study of the static and dynamic behavior of the magne-tization in spin-flop bi-layers, which consist of two soft ferromagnetic layerscoupled by dipolar forces through a thin nonmagnetic spacer. The focus ofthe work is three fold: collective spin dynamics in the anti-parallel groundstate; resonant switching in the presence of thermal agitation; and static anddynamic behavior of the system in the vortex-pair state, with a particularemphasis on the interlayer core-core interaction.

    Two collective spin-flop resonance modes are observed and interpreted asacoustical and optical spin precessions, in which the moments of the two lay-ers oscillate in phase and out of phase, respectively. An analytical macrospinmodel is developed to analyze the experimental results and is found to ac-curately predict the resonance frequencies and their field dependence in thelow-field anti-parallel state and the high-field near saturated state. A micro-magnetic model is developed and successfully explains the static and dynamicbehavior of the system in the entire field range, including the C- and S-typespin-perturbed scissor state of the bi-layer at intermediate fields.

    The optical spin-flop resonance at 3-4 GHz is used to demonstrate resonantswitching in the system, in the range of the applied field where quasi-staticswitching is forbidden. An off-axis field of relatively small amplitude canexcite large-angle scissor-like oscillations at the optical resonance frequency,which can result in a full 180-degree reversal, with the two moments switchingpast each other into the mirror anti-parallel state. It is found that the switch-ing probability increases with increasing the duration of the microwave fieldpulse, which shows that the resonant switching process is affected by thermalagitation. Micromagnetic modeling incorporating the effect of temperature isperformed and is in good agreement with the experimental results.

    Vortex pair states in spin-flop bi-layers are produced using high amplitudefield pulses near the optical spin resonance in the system. The stable vortex-pair states, 16 in total, of which 4 sub-classes are non-degenerate in energy, areidentified and investigated using static and dynamic applied fields. For AP-chirality vortex-pair states, the system can be studied while the two vortexcores are coupled and decoupled in a single field sweep. It is found thatthe dynamics of the AP-chirality vortex pairs is critically determined by thepolarizations of the two vortex cores and the resulting attractive or repulsivecore-core interaction. The measured spin resonance modes in the system areinterpreted as gyrational, rotational, and vibrational resonances with the helpof the analytical and micromagnetic models developed herein.

    A significant effort during this project was made to build two instrumentsfor surface and transport characterization of magnetic nanostructures: a high-current Scanning Tunneling Microscope for studying transport in magneticpoint contacts, and a Current In Plane Tunneling instrument for characteriz-ing unpatterned magnetic tunnel junctions. The design and implementationof the instruments as well as the test data are presented.

  • 23.
    Cherepov, Sergiy
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Dzhezherya, Yu. I.
    Worledge, D. C.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Resonant Activation of a Synthetic Antiferromagnet2011In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, no 7, p. 077202-Article in journal (Refereed)
    Abstract [en]

    The magnetic decay time of a synthetic antiferromagnet comprised of two closely spaced magnetic dipoles is measured in the presence of microwave excitation. The system is known to be highly stable with respect to switching between its two antiparallel ground states under quasistatic magnetic fields. We show that an order of magnitude lower field can switch the pair, provided the field is applied in resonance with the optical eigenmode of the collective spin dynamics in the system. We furthermore show that thermal agitation can play an essential role in spin-flop switching for resonant excitations of near-or subcritical amplitude.

  • 24.
    Cherepov, Sergiy
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Konovalenko, Alexander
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Worledge, D. C.
    IBM T.J. Watson Researh Center.
    Micromagnetics of Spin-Flop Bilayers: S, C, and Vortex Spin States2010In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 46, no 6, p. 2124-2127Article in journal (Refereed)
    Abstract [en]

    Spin-flop tunnel junctions subjected to strong gigahertz excitations are found to exhibit highly stable resistance states, intermediate between the two spin-uniform ground states of high and low resistance. The associated spin distributions are necessarily nonuniform and differ significantly from the ground-state anti-parallel spin configuration in their static and dynamic properties. Detailed micromagnetic modeling reveals that inplane spin vortices in dipole-coupled thin-film bilayers are stable spin configurations, where the orientation of the vortex cores and the vortex chirality play an important role in the response of the system to external magnetic fields.

  • 25.
    Cherepov, Sergiy
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Koop, Björn
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Galkin, A. Yu.
    Khymyn, R.S.
    Institute of Magnetism, Ukrainian Academy of Science.
    Ivanov, B. A.
    Institute of Magnetism, Ukrainian Academy of Science.
    Worledge, D. C.
    IBM T.J. Watson Researh Center.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Core-Core Dynamics in Spin Vortex Pairs2012In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 9, p. 097204-Article in journal (Refereed)
    Abstract [en]

    We investigate nanopillars in which two thin ferromagnetic particles are separated by a nanometer thin nonmagnetic spacer and can be set into stable spin vortex-pair configurations. We find that the previously unexplored limit of strong vortex core-core coupling can dominate the spin dynamics in the system. We observe experimentally and explain analytically and numerically how the 0.2 GHz gyrational resonance modes of the individual vortices are transformed into a 2 GHz collective rotational resonance mode in the configurations where the two cores form a bound pair.

  • 26.
    Cherepov, Sergiy
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    Worledge, D. C.
    IBM T.J. Watson Researh Center.
    Resonant activation of asynthetic antiferromagnetManuscript (preprint) (Other academic)
  • 27.
    Cherepov, Sergiy
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Worledge, D. C.
    IBM T.J. Watson Researh Center.
    Resonant Switching of Two Dipole-Coupled Nanomagnets2010In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 46, no 6, p. 2112-2115Article in journal (Refereed)
    Abstract [en]

    The storage layer of recently developed spin-flop magnetic random-access memory consists of two closely spaced dipole-coupled nanomagnets and is highly stable in the ground state as well as in quasistatic fields applied off the easy axis. We show experimentally and confirm by using micromagnetic simulations that these spin-flop bilayers can be switched relatively easily by dynamic fields, applied at the frequency of the optical spin resonance of the bilayer. The field amplitude sufficient for this resonant switching can be an order of magnitude lower than the fields necessary for quasistatic reversal. Our data and micromagnetic analysis suggest that thermal agitation can play a role in the observed resonant switching behavior.

  • 28.
    Claesson, Per M.
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. SP Technical Research Institute of Sweden.
    Dobryden, Illia
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Li, Gen
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    He, Yunjuan
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Huang, Hui
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Thorén, Per-Anders
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    From force curves to surface nanomechanical properties2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, no 35, p. 23642-23657Article in journal (Refereed)
    Abstract [en]

    Surface science, which spans the fields of chemistry, physics, biology and materials science, requires information to be obtained on the local properties and property variations across a surface. This has resulted in the development of different scanning probe methods that allow the measurement of local chemical composition and local electrical and mechanical properties. These techniques have led to rapid advancement in fundamental science with applications in areas such as composite materials, corrosion protection and wear resistance. In this perspective article, we focussed on the branch of scanning probe methods that allows the determination of surface nanomechanical properties. We discussed some different AFM-based modes that were used for these measurements and provided illustrative examples of the type of information that could be obtained. We also discussed some of the difficulties encountered during such studies.

  • 29.
    Corlevi, Silvia
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Guichard, Wiebke
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Cooper pair transistor in a tunable environment2006In: Quantum Computing in Solid State Systems, Springer-Verlag New York, 2006, p. 63-69Chapter in book (Other academic)
    Abstract [en]

    We report an experimental study on the effect of high impedance environment on a Cooper pair transistor (CPT). The CPT consists of two small capacitance Josephson tunnel junctions in series with a gate electrode coupled through a capacitance Cg to a central island. In small-capacitance CPT the charging energy EC = e2/2C, where C ∼ 2CJ+Cg is the total capacitance of the island, becomes relevant at low temperature, and charging effects influence the transport properties.

  • 30. Dzhezherya, Yu. I.
    et al.
    Demishev, K. O.
    Korenivskii, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Kapitza problem for the magnetic moments of synthetic antiferromagnetic systems2012In: Journal of Experimental and Theoretical Physics, ISSN 1063-7761, E-ISSN 1090-6509, Vol. 115, no 2, p. 284-288Article in journal (Refereed)
    Abstract [en]

    The dynamics of magnetization in synthetic antiferromagnetic systems with the magnetic dipole coupling in a rapidly oscillating field has been examined. It has been revealed that the system can behave similar to the Kapitza pendulum. It has been shown that an alternating magnetic field can be efficiently used to control the magnetic state of a cell of a synthetic antiferromagnet. Analytical relations have been obtained between the parameters of such an antiferromagnet and an external magnetic field at which certain quasistationary states are implemented.

  • 31. Dzhezherya, Yu I.
    et al.
    Yurchuk, V. P.
    Demishev, K. O.
    Korenivskii, Vladislav N.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Remagnetization of synthetic antiferromagnetic cells by a magnetic field pulse2013In: Journal of Experimental and Theoretical Physics, ISSN 1063-7761, E-ISSN 1090-6509, Vol. 117, no 6, p. 1059-1065Article in journal (Refereed)
    Abstract [en]

    The theory of the dynamic remagnetization of a synthetic antiferromagnetic system and magnetic points located on a magnetic substrate in an external magnetic field has been considered. The energies of the equilibrium states of the system have been calculated. The conditions of switching between equilibrium states have been described. The conditions of applicability of this theory have been formulated. It has been shown that the process of remagnetization can be implemented in an inertialess regime, escaping the long-term relaxation of the system to a new equilibrium position with the use of a special shape of the field signal. The possibility of the reduction of the switching field amplitude by varying the pulse duration has been demonstrated.

  • 32.
    Ergül, Adem
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Nonlinear dynamics of Josephson Junction Chains and Superconducting Resonators2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis presents the results of the experimental studies on two kindof Superconducting circuits: one-dimensional Josephson junction chains andsuperconducting coplanar waveguide (CPW) resonators. One-dimensionalJosephson junction chains are constructed by connecting many Superconducting quantum interference devices (SQUIDs) in series. We have studied DC transport properties of the SQUID chains and model their nonlineardynamics with Thermally Activated Phase-Slips (TAPS). Experimental andsimulated results showed qualitative agreement revealing the existence of auniform phase-slipping and phase-sticking process which results in a voltage-independent current on the dissipative branch of the current-voltage char-acteristics (IVC). By modulating the effective Josephson coupling energy ofthe SQUIDs (EJ ) with an external magnetic field, we found that the ratio EJ /EC is a decisive factor in determining the qualitative shape of theIVC. A quantum phase transition between incoherent Quantum Phase Slip, QPS (supercurrent branch with a finite slope) to coherent QPS (IVC withwell-developed Coulomb blockade) via an intermediate state (supercurrentbranch with a remnant of Coulomb blockade) is observed as the EJ /EC ratio is tuned. This transition from incoherent QPS to the intermediate-statehappens around R0 ∼ RQ (RQ = h/4e^2 = 6.45kΩ). We also fabricated structured chains where a SQUID at the middle of the chain (central SQUID) has different junction size and loop area compared to other SQUIDs in the chain. Results showed that with these structured chains it is possible to localize andtune the amplitude of both TAPS and QPS at the central SQUID.

    The second part of the thesis describes the fabrication process and themeasurement results of superconducting CPW resonators. Resonators withdifferent design parameters were fabricated and measured. The transmissionspectra showed quality factors up to, Q ∼ 5 × 10^5 . We have observed bendingof the resonance curves to the lower frequencies due to existence of a nonlinear kinetic inductance. The origin of the nonlinear kinetic inductance isthe nonlinear relation between supercurrent density, Js, and superfluid veloc-ity, vs , of the charge carriers on the center line of the resonators. A simplemodel based on the Ginzburg-Landau theory is used in order to explain ob-served nonlinear kinetic inductance and estimates using this model showedgood agreement with the experimental results.

  • 33.
    Ergül, Adem
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Johansson, Jan
    Azizoglu, Yagiz
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Schaeffer, David
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Localizing quantum phase slips in one-dimensional Josephson junction chains2013In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 15, p. 095014-Article in journal (Refereed)
    Abstract [en]

    We studied quantum phase-slip (QPS) phenomena in long one-dimensional Josephson junction series arrays with tunable Josephson coupling. These chains were fabricated with as many as 2888 junctions, where one sample had a separately tunable link in the middle of the chain. Measurements were made of the zero-bias resistance, R-0, as well as current-voltage characteristics (IVC). The finite R-0 is explained by QPS and shows an exponential dependence on root E-J/E-C with a distinct change in the exponent at R-0 = R-Q = h/4e(2). When R-0 > R-Q, the IVC clearly shows a remnant of the Coulomb blockade, which evolves to a zero-current state with a sharp critical voltage as E-J is tuned to a smaller value. The zero-current state below the critical voltage is due to coherent QPSs and we show that these are enhanced when the central link is weaker than all other links. Above the critical voltage, a negative, differential resistance is observed, which nearly restores the zero-current state.

  • 34.
    Ergül, Adem
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Schaeffer, David
    KTH.
    Lindblom, Magnus
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Johansson, Jan
    Phase sticking in one-dimensional Josephson junction chains2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 10, p. 104501-Article in journal (Refereed)
    Abstract [en]

    We studied current-voltage characteristics of long one-dimensional Josephson junction chains with Josephson energy much larger than charging energy, E-J >> E-C. In this regime, typical I-V curves of the samples consist of a supercurrent-like branch at low-bias voltages followed by a voltage-independent chain current branch, I-chain at high bias. Our experiments showed that I-chain is not only voltage-independent but it is also practically temperature-independent up to T = 0.7T(C). We have successfully model the transport properties in these chains using a capacitively shunted junction model with nonlinear damping.

  • 35.
    Ergül, Adem
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. Stockholm University, Sweden.
    Weissl, Thomas
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Johansson, Jan
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Spatial and temporal distribution of phase slips in Josephson junction chains2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 11447Article in journal (Refereed)
    Abstract [en]

    The Josephson effect, tunnelling of a supercurrent through a thin insulator layer between two superconducting islands, is a phenomena characterized by a spatially distributed phase of the superconducting condensate. In recent years, there has been a growing focus on Josephson junction devices particularly for the applications of quantum metrology and superconducting qubits. In this study, we report the development of Josephson junction circuit formed by serially connecting many Superconducting Quantum Interference Devices, SQUIDs. We present experimental measurements as well as numerical simulations of a phase-slip center, a SQUID with weaker junctions, embedded in a Josephson junction chain. The DC transport properties of the chain are the result of phase slips which we simulate using a classical model that includes linear external damping, terminating impedance, as well as internal nonlinear quasiparticle damping. We find good agreement between the simulated and the experimental current voltage characteristics. The simulations allow us to examine the spatial and temporal distribution of phase-slip events occurring across the chains and also the existence of travelling voltage pulses which reflect at the chain edges.

  • 36.
    Forchheimer, Daniel
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Imaging materials with intermodulation: Studies in multifrequency atomic force microscopy2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The Atomic Force Microscope (AFM) is a tool for imaging surfaces at the microand nano meter scale. The microscope senses the force acting between a surfaceand a tip positioned at the end of a micro-cantilever, forming an image of the surface topography. Image contrast however, arises not only from surface topography, but also from variation in material composition. Improved material contrast, and improved interpretation of that contrast are two issues central to the further development of AFM.

    This thesis studies dynamic AFM where the cantilever is driven at multiple frequencies simultaneously. Due to the nonlinear dependence of the tip-surface force on the tip’s position, the cantilever will oscillate not only at the driven frequencies, but also at harmonics and at mixing frequencies of the drives, so-called intermodulation products. A mode of AFM called Intermodulation AFM (ImAFM) is primarily studied, which aims to make use of intermodulation products centered around the resonance frequency of the cantilever. With proper excitation many intermodulation products are generated near resonance where they can be measured with large signal-to-noise ratio.

    ImAFM is performed on samples containing two distinct domains of different material composition and a contrast metric is introduced to quantitatively evaluate images obtained at each response frequency. Although force sensitivity is highest on resonance, we found that weak intermodulation response off resonance can show larger material contrast. This result shows that the intermodulation images can be used to improve discrimination of materials.

    We develop a method to obtain material parameters from multifrequency AFM spectra by fitting a tip-surface force model. Together with ImAFM, this method allows high resolution imaging of material parameters. The method is very generalas it is not limited to a specific force model or particular mode of multifrequency AFM. Several models are discussed and applied to different samples. The parameter images have a direct physical interpretation and, if the model is appropriate, they can be used to relate the measurement to material properties such as the Young’s modulus. Force reconstruction is tested with simulations and on measured data. We use the reconstructed force to define the location of the surface so that we can address the issue of separating topographic contrast and material contrast.

  • 37.
    Forchheimer, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Borysov, Stanislav S.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Platz, Daniel
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. Max-Planck-Institute for the Physics of Complex Systems, Germany.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Determining surface properties with bimodal and multimodal AFM2014In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 25, no 48, p. 485708-Article in journal (Refereed)
    Abstract [en]

    Conventional dynamic atomic force microscopy (AFM) can be extended to bimodal and multimodal AFM in which the cantilever is simultaneously excited at two or more resonance frequencies. Such excitation schemes result in one additional amplitude and phase images for each driven resonance, and potentially convey more information about the surface under investigation. Here we present a theoretical basis for using this information to approximate the parameters of a tip-surface interaction model. The theory is verified by simulations with added noise corresponding to room-temperature measurements.

  • 38.
    Forchheimer, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Forchheimer, Robert
    Linköping University.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Improving image contrast and material discrimination with nonlinear response in bimodal atomic force microscopy2015In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 6, article id 6270Article in journal (Refereed)
    Abstract [en]

    Atomic force microscopy has recently been extented to bimodal operation, where increased image contrast is achieved through excitation and measurement of two cantilever eigenmodes. This enhanced material contrast is advantageous in analysis of complex heterogeneous materials with phase separation on the micro or nanometre scale. Here we show that much greater image contrast results from analysis of nonlinear response to the bimodal drive, at harmonics and mixing frequencies. The amplitude and phase of up to 17 frequencies are simultaneously measured in a single scan. Using a machine-learning algorithm we demonstrate almost threefold improvement in the ability to separate material components of a polymer blend when including this nonlinear response. Beyond the statistical analysis performed here, analysis of nonlinear response could be used to obtain quantitative material properties at high speeds and with enhanced resolution.

  • 39.
    Forchheimer, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. Division of Solid State Physics and the Nanometer Structure Consortium, Lund University, Sweden .
    Luo, G.
    Ye, L.
    Montelius, L.
    Molecularly selective nanopatterns using nanoimprint lithography: A label-free sensor architecture2011In: Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics, ISSN 2166-2746, E-ISSN 2166-2754, Vol. 29, no 1, p. 0110212-0110215Article in journal (Refereed)
    Abstract [en]

    Nanoimprint lithography (NIL) can generate well defined nanostructures with high efficiency and at very low cost. Molecular imprinting (MIP) is a "bottom-up" technique creating a polymer layer exhibiting structures with a molecular selectivity. Such polymer structures may be employed as molecular recognition sites for sensing applications. In this work, the authors combine NIL with MIP and they are able to obtain micro- and nanopatterns of polymer with features down to 100 nm that show high molecular selectivity.

  • 40.
    Forchheimer, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Platz, Daniel
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Tholén, Erik A.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Model-based extraction of material properties in multifrequency atomic force microscopy2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 19, p. 195449-Article in journal (Refereed)
    Abstract [en]

    We present a method to reconstruct the nonlinear tip-surface force and extract material properties from a multifrequency atomic force microscopy (AFM) measurement with a high-quality-factor cantilever resonance. In a measurement time of similar to 2 ms, we are able to accurately reconstruct the tip-surface force-displacement curve, allowing simultaneous high-resolution imaging of both topography and material properties at typical AFM scan rates. We verify the method using numerical simulations, apply it to experimental data, and use it to image mechanical properties of a polymer blend. We further discuss the limitations of the method and identify suitable operating conditions for AFM experiments.

  • 41.
    Forchheimer, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Platz, Daniel
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Tholén, Erik A.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Simultaneous imaging of surface and magnetic forces2013In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 103, no 1, p. 013114-Article in journal (Refereed)
    Abstract [en]

    We demonstrate quantitative force imaging of long-range magnetic forces simultaneously with near-surface van-der-Waals and contact-mechanics forces using intermodulation atomic force microscopy. Magnetic forces at the 200 pN level are separated from near-surface forces at the 30 nN level. Imaging of these forces is performed in both the contact and non-contact regimes of near-surface interactions.

  • 42.
    Forsman, Alexander
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Development of radio-frequencyscanning tunneling microscope formagnetic point contact measurements2013Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 43.
    Gromov, Andrey
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Gigahertz Sandwich Strip Inductors Based on Fe-N Films: The Effect of Flux Closure at the Flange2010In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 46, no 6, p. 2097-2100Article in journal (Refereed)
    Abstract [en]

    Planar strip inductors consisting of soft Fe-N films enclosing a conductor made of Cu are fabricated on Si substrates and exhibit 70 to 100% inductance gain at 1 Ghz with quality factors of 3 to 5. The magnetic part of the inductance is less than predicted theoretically, which is attributed to hardening of the magnetic material at the edges of the strip, where the deposition is close to 60 degrees incidence. Test films were fabricated on tilted substrates and found to develop very high anisotropy for deposition angles larger than 30 degrees. Optimizing the flux closure at the strip edges is essential for further improving the performance of sandwich strip inductors.

  • 44.
    Haviland, David
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. KTH, School of Biotechnology (BIO), Centres, Albanova VinnExcellence Center for Protein Technology, ProNova.
    SUPERCONDUCTING CIRCUITS Quantum phase slips2010In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 6, no 8, p. 565-566Article in journal (Refereed)
  • 45.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. Albanova.
    Quantitative force microscopy from a dynamic point of view2017In: Current Opinion in Colloid & Interface Science, ISSN 1359-0294, E-ISSN 1879-0399, Vol. 27, p. 74-81Article in journal (Refereed)
    Abstract [en]

    We discuss the physical origin and measurement of force between an atomic force microscope tip and a soft material surface. Quasi-static and dynamic measurements are contrasted and similarities are revealed by analyzing the dynamics in the frequency domain. Various dynamic methods using single and multiple excitation frequencies are described. Tuned multifrequency lockin detection with one reference oscillation gives a great deal of information from which one can reconstruct the tip–surface interaction. Intermodulation in a weakly perturbed high Q resonance enables the measurement of a new type of dynamic force curve, offering a physically intuitive way to visualize both elastic and viscous forces.

  • 46.
    Haviland, David B.
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    van Eysden, Cornelius Anthony
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Forchheimer, Daniel
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Platz, Daniel
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Kassa, Hailu G.
    Leclere, Philippe
    Probing viscoelastic response of soft material surfaces at the nanoscale2016In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 12, no 2, p. 619-624Article in journal (Refereed)
    Abstract [en]

    We study the interaction between an AFM tip and a soft viscoelastic surface. Using a multifrequency method we measure the amplitude-dependence of the cantilever dynamic force quadratures, which clearly show the effect of finite relaxation time of the viscoelastic surface. A model is introduced which treats the tip and surface as a two-body dynamic problem with a nonlinear interaction depending on their separation. We find good agreement between simulations of this model and experimental data on polymer blend samples for a variety of materials and measurement conditions.

  • 47.
    Haviland, David B.
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    van Eysden, Cornelius Anthony
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Forchheimer, Daniel
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Platz, Daniel
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Kassa, Hailu G.
    Leclere, Philippe
    Probing viscoelastic response of soft material surfaces at the nanoscale (vol 12, pg 619, 2016)2016In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 12, no 2, p. 625-625Article in journal (Refereed)
  • 48.
    Heydari, Golrokh
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Sedighi Moghaddam, Maziar
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. SP Technical Research Institute of Sweden..
    Tuominen, Mikko
    Fielden, Matthew
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Haapanen, Janne
    Makela, Jyrki M.
    Claesson, Per M.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Wetting hysteresis induced by temperature changes: Supercooled water on hydrophobic surfaces2016In: Journal of Colloid and Interface Science, ISSN 0021-9797, E-ISSN 1095-7103, Vol. 468, p. 21-33Article in journal (Refereed)
    Abstract [en]

    The state and stability of supercooled water on (super)hydrophobic surfaces is crucial for low temperature applications and it will affect anti-icing and de-icing properties. Surface characteristics such as topography and chemistry are expected to affect wetting hysteresis during temperature cycling experiments, and also the freezing delay of supercooled water. We utilized stochastically rough wood surfaces that were further modified to render them hydrophobic or superhydrophobic. Liquid flame spraying (LFS) was utilized to create a multi-scale roughness by depositing titanium dioxide nanoparticles. The coating was subsequently made non-polar by applying a thin plasma polymer layer. As flat reference samples modified silica surfaces with similar chemistries were utilized. With these substrates we test the hypothesis that superhydrophobic surfaces also should retard ice formation. Wetting hysteresis was evaluated using contact angle measurements during a freeze-thaw cycle from room temperature to freezing occurrence at -7 degrees C, and then back to room temperature. Further, the delay in freezing of supercooled water droplets was studied at temperatures of -4 degrees C and -7 degrees C. The hysteresis in contact angle observed during a cooling-heating cycle is found to be small on flat hydrophobic surfaces. However, significant changes in contact angles during a cooling-heating cycle are observed on the rough surfaces, with a higher contact angle observed on cooling compared to during the subsequent heating. Condensation and subsequent frost formation at sub-zero temperatures induce the hysteresis. The freezing delay data show that the flat surface is more efficient in enhancing the freezing delay than the rougher surfaces, which can be rationalized considering heterogeneous nucleation theory. Thus, our data suggests that molecular flat surfaces, rather than rough superhydrophobic surfaces, are beneficial for retarding ice formation under conditions that allow condensation and frost formation to occur. 

  • 49.
    Heydari, Golrokh
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Sedighi Moghaddam, Maziar
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials. SP Technical Research Institute of Sweden.
    Tuominen, Mikko
    Fielden, Matthew
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Haapanen, Janne
    Mäkelä, Jyrki M.
    Claesson, Per Martin
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Wetting hysteresis induced by temperature changes: supercooled water onhydrophobic surfacesManuscript (preprint) (Other academic)
    Abstract [en]

    The state and stability of supercooled water on (super)hydrophobic surfaces is crucial for low temperature applications and for obtaining anti-icing and de-icing properties. Surface characteristics such as topography and chemistry are expected to affect wetting hysteresis during temperature cycling experiments, and also the freezing delay of supercooled water. We utilized stochastically rough wood surfaces that were further modified to render them hydrophobic or superhydrophobic. Liquid flame spraying (LFS) was utilized to create a multi-scale roughness by depositing titaniumdioxide nanoparticles. The coating was subsequently made non-polar by applying a thin plasma polymer layer. As flat reference samples modified silica surfaces with similar chemistries were utilized. With these sets of surfaces we test the hypothesis that superhydrophobic surfaces also should retard ice formation. Wetting hysteresis was evaluated using contact angle measurements during a freeze-thaw cycle from room temperature to freezing occurrence at -7 °C, and then back to room temperature. Further, the delay in freezing of supercooled water droplets was studied at temperatures of -4 °C and -7 °C. The hysteresis in contact angle observed during a cooling-heating cycle is found to be small on flat hydrophobic surfaces. However, significant changes in contact angles during a cooling-heating cycle are observed on the rough surfaces, with a higher contact angle observed on cooling compared to during the subsequent heating. This hysteresis is lower for hydrophobic wood samples with multi-scale roughness compared to those with predominantly micro-scale features. Condensation and subsequent frost formation at sub-zero temperatures induce the hysteresis. The freezing delay data suggests that the multi-scale roughness reduces the penetration of supercooled water into surface depressions, and enhances the freezing delay at low degrees of supercooling. However, the flat surface is even more efficient in enhancing the freezing delay than the rougher surfaces, which can be rationalized considering heterogeneous nucleation theory. Thus, our data suggests that molecular flat surfaces, rather than rough superhydrophobic surfaces, are beneficial for retarding ice formation under conditions that allow condensation and frost formation to occur.

  • 50.
    Holmgren, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Bodarenko, Artem
    KTH. Institute of Magnetism, National Academy of Science, 03142 Kiev, Ukraine.
    Koop, Björn
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Ivanov, Boris
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Non-Degeneracy and Effects of Pinning in Strongly Coupled Vortex Pairs2017In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 53, no 11, article id 4400505Article in journal (Refereed)
    Abstract [en]

    We study the effects of pinning on the quasi-static behavior of stacked, strongly coupled spin-vortex pairs in magnetic multilayered nanopillars, with vertical vortex separation small compared with the vortex-core size. The small separation causes the core-core interaction to be the dominant energy contribution for small applied fields and excitations, which results in highly non-linear dynamics. The properties of such a vortex pair are expected to only be dependent on the relative vortex core polarizations and relative chiralities, so that the individual configurations should be degenerated. We show how pinning can lift this degeneracy, which can be used to distinguish the individual chirality configurations.

1234 1 - 50 of 160
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf