Change search
Refine search result
123 1 - 50 of 121
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.
    Abergel, David S. L.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Center for Quantum Materials, Sweden.
    Robustness of topologically protected transport in graphene-boron nitride lateral heterostructures2017In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 29, no 7, article id 075303Article in journal (Refereed)
    Abstract [en]

    Previously, graphene nanoribbons set in lateral heterostructures with hexagonal boron nitride were predicted to support topologically protected states at low energy. We investigate how robust the transport properties of these states are against lattice disorder. We find that forms of disorder that do not couple the two valleys of the zigzag graphene nanoribbon do not impact the transport properties at low bias, indicating that these lateral heterostructures are very promising candidates for chip-scale conducting interconnects. Forms of disorder that do couple the two valleys, such as vacancies in the graphene ribbon, or substantial inclusions of armchair edges at the graphene-hexagonal boron nitride interface will negatively affect the transport. However, these forms of disorder are not commonly seen in current experiments.

  • 2.
    Ahuja, Rajeev
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Osorio-Guillen, J. M.
    de Almeida, J. S.
    Holm, B.
    Ching, W. Y.
    Johansson, Börje
    KTH, Superseded Departments, Materials Science and Engineering.
    Electronic and optical properties of gamma-Al2O3 from ab initio theory2004In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 16, no 16, p. 2891-2900Article in journal (Refereed)
    Abstract [en]

    We report on a density functional theory calculation of the electronic structure and optical properties of gamma-Al2O3. We have made a comparison between the optical and electronic properties of the alpha and gamma phases of alumina. The calculated bulk modulus of the gamma phase is slightly lower than that of the a phase. The calculated static dielectric function and the optical constant of the gamma phase are very close to those of the alpha phase.

  • 3. Araujo, C. M.
    et al.
    Blomqvist, A.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ti-induced destabilization of NaBH4 from first-principles theory2008In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 20, no 12Article in journal (Refereed)
    Abstract [en]

    In this work, we have employed first-principles total energy calculations and ab initio molecular dynamics simulations to investigate the Ti doping of NaBH4. We show that Ti destabilizes the BH4 cages, which in turn increases the mobility of hydrogen atoms. Such an effect is shown to be due to the formation of B-Ti bonds, rather than the lowering of the BH4 charge state as expected. These results indicate that Ti may catalyse the dehydrogenation process in NaBH4 as it does for NaAlH4.

  • 4. Belhadji, Brahim
    et al.
    Bergqvist, Lars
    Institut fuer Festkoerperforschung, Forschungszentrum Juelich.
    Zeller, Rudi
    Dederichs, Peter H.
    Sato, Kazunori
    Katayama-Yoshida, H
    Trends of exchange interactions in dilute magnetic semiconductors2007In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 19, no 43, p. 436227-Article in journal (Refereed)
    Abstract [en]

    We discuss the importance of different exchange mechanisms like double exchange, p–d exchange and anti-ferromagnetic as well as ferromagnetic superexchange in dilute magnetic semiconductors (DMSs). Based on the coherent potential approximation for the electronic structure of the DMSs we show that the different mechanisms exhibit different dependences on the concentration of the magnetic impurities, on the hybridization with the wavefunctions of neighbouring impurities and on the position of the Fermi level in the band gap. However, common to all mechanisms is that, as long as half-metallicity is preserved, they are determined by the hybridization with the orbitals of neighbouring impurities and of the resulting energy gain due to the formation of bonding and anti-bonding hybrids. By calculating the exchange coupling constants Jij(EF) as a function of the position of the Fermi level we obtain a universal trend for the exchange interactions with band filling.

  • 5.
    Belonoshko, Anatoly
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Arapan, S.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    An ab initio molecular dynamics study of iron phases at high pressure and temperature2011In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 23, no 48Article in journal (Refereed)
    Abstract [en]

    The crystal structure of iron, the major component of the Earth's inner core (IC), is unknown for the IC high pressure (P; 3.3-3.6 Mbar) and temperature (T; 5000-7000 K). There is mounting evidence that the hexagonal close-packed (hcp) phase of iron, stable at the high P of the IC and a low T, might be unstable under the IC conditions due to the impact of high T and impurities. Experiments at the IC P and T are difficult and do not provide a conclusive answer as regards the iron stability at the pressure of the IC and temperatures close to the iron melting curve. Recent theory provides contradictory results regarding the nature of the stable Fe phase. We investigated the possibility of body-centered cubic (bcc) phase stabilization at the P and T in the vicinity of the Fe melting curve by using ab initio molecular dynamics. Thermodynamic calculations, relying on the model of uncorrelated harmonic oscillators, provide nearly identical free energies within the error bars of our calculations. However, direct simulation of iron crystallization demonstrates that liquid iron freezes in the bcc structure at the P of the IC and T = 6000 K. All attempts to grow the hcp phase from the liquid failed. The mechanism of bcc stabilization is explained. This resolves most of the earlier confusion.

  • 6.
    Bergqvist, Lars
    et al.
    Institut für Festkörperforschung, Forschungszentrum Jülich.
    Dederichs, Peter H.
    A theoretical study of half-metallic antiferromagnetic diluted magnetic semiconductors2007In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 19, no 21, p. 216220-Article in journal (Refereed)
    Abstract [en]

    Based on electronic structure calculations and statistical methods, we investigate a new class of materials for spintronic applications: half-metallic antiferromagnetic diluted magnetic semiconductors (HMAF-DMSs). As shown recently by Akai and Ogura, these DMS systems contain equal amounts of low-valent and high-valent transition metal impurities, such that their local moments exactly compensate each other. We present ab initio calculations using the KKR-CPA and the PAW-supercell methods, and show that quite a few half-metallic antiferromagnets should exist. Our calculations demonstrate that the exchange coupling parameters in these systems are dominated by a strong antiferromagnetic interaction between the two impurities. The Néel temperatures are calculated by Monte Carlo simulations and in mean-field approximation. It is shown that the latter method strongly overestimates the critical temperatures and that the more realistic values obtained by Monte Carlo techniques are rather low.

  • 7.
    Bergqvist, Lars
    et al.
    Uppsala University.
    Eriksson, Olle
    Theory of weakly coupled two-dimensional magnets2006In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 18, no 20, p. 4853-4860Article in journal (Refereed)
    Abstract [en]

    We demonstrate that the experimental findings of the magnetic properties of the weakly coupled trilayer system Ni4/CuN/Co2are reproduced by a theory that combines first principles calculations of the exchange interactions in a classical Heisenberg model with Monte Carlo simulations. Through an analysis of the spin–spin correlation function we show that two distinct temperatures can be identified; a higher temperature where long range magnetic order disappears and a lower temperature where the spin–spin correlation of the Ni atoms undergoes a drastic change. We argue that our findings hold in general for 'weak exchange link' systems.

  • 8. Brinkley, M. K.
    et al.
    Abergel, David S.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Clader, B. D.
    Two-photon absorption in gapped bilayer graphene with a tunable chemical potential2016In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 28, no 36, article id 365001Article in journal (Refereed)
    Abstract [en]

    Despite the now vast body of two-dimensional materials under study, bilayer graphene remains unique in two ways: it hosts a simultaneously tunable band gap and electron density; and stems from simple fabrication methods. These two advantages underscore why bilayer graphene is critical as a material for optoelectronic applications. In the work that follows, we calculate the one-and two-photon absorption coefficients for degenerate interband absorption in a graphene bilayer hosting an asymmetry gap and adjustable chemical potential-all at finite temperature. Our analysis is comprehensive, characterizing one-and two-photon absorptive behavior over wide ranges of photon energy, gap, chemical potential, and thermal broadening. The two-photon absorption coefficient for bilayer graphene displays a rich structure as a function of photon energy and band gap due to the existence of multiple absorption pathways and the nontrivial dispersion of the low energy bands. This systematic work will prove integral to the design of bilayer-graphene-based nonlinear optical devices.

  • 9.
    Cao, Hui
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Ma, Jing
    Yi, Luo
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Conductance Oscillation in Dithiolated Oligoacene JunctionsIn: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648XArticle in journal (Refereed)
  • 10. Cao, Peiyu
    et al.
    Ni, Xiaodong
    Tian, Fuyang
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Univ Sci & Technol Beijing, Dept Phys, Beijing 100083, Peoples R China.
    Varga, Lajos K.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Inst Solid State Phys & Opt, Wigner Res Ctr Phys, Hungary.
    Ab initio study of AlxMoNbTiV high-entropy alloys2015In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 27, no 7, article id 075401Article in journal (Refereed)
    Abstract [en]

    The AlxMoNbTiV (x = 0-1.5) high-entropy alloys (HEAs) adopt a single solid-solution phase, having the body centered cubic (bcc) crystal structure. Here we employ the ab initio exact muffin-tin orbitals method in combination with the coherent potential approximation to investigate the equilibrium volume, elastic constants, and polycrystalline elastic moduli of AlxMoNbTiV HEAs. A comparison between the ab initio and experimental equilibrium volumes demonstrates the validity and accuracy of the present approach. Our results indicate that Al addition decreases the thermodynamic stability of the bcc structure with respect to face-centered cubic and hexagonal close packed lattices. For the elastically isotropic Al0.4MoNbTiV HEAs, the valence electron concentration (VEC) is about 4.82, which is slightly different from VEC similar to 4.72 obtained for the isotropic Gum metals and refractory-HEAs.

  • 11. da Silva, A. F.
    et al.
    Araujo, C. M.
    Sernelius, B. E.
    Persson, Clas
    Ahuja, R.
    Johansson, Börje
    Influence of Si doping on optical properties of wurtzite GaN2001In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 13, no 40, p. 8891-8899Article in journal (Refereed)
    Abstract [en]

    The band gap shift (BGS) of Si-doped wurtzite GaN for impurity concentrations spanning the insulating to the metallic regimes has been investigated at low temperature. The critical impurity concentration for the metal-non-metal transition is estimated from the generalized Drude approach for the resistivity to be about 1.0 x 10(18) cm(-3). The calculations for the BGS were carried out within a framework of the random phase approximation, taking into account the electron-electron, electron-optical phonon, and electron-ion interactions. In the wake of very recent photoluminescence measurements, we have shown and discussed the possible transitions involved in the experimental results.

  • 12.
    Delczeg, Lorand
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delczeg, Erna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Density functional study of vacancies and surfaces in metals2011In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 23, no 4, p. 045006-Article in journal (Refereed)
    Abstract [en]

    We compare the performances of three common gradient-level exchange-correlation functionals for metallic bulk, surface and vacancy systems. We find that approximations which, by construction, give similar results for the jellium surface, show large deviations for realistic systems. The particular charge density and density gradient dependence of the exchange-correlation energy densities are shown to be the reason behind the obtained differences. Our findings confirm that both the global (total energy) and the local (energy density) behavior of the exchange-correlation functional should be monitored for a consistent functional design.

  • 13.
    Delin, Anna
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Tosatti, E.
    The electronic structure of 4d transition-metal monatomic wires2004In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 16, no 45, p. 8061-8074Article in journal (Refereed)
    Abstract [en]

    Monatomic nanowires of the nonmagnetic transition metals Ru, Rh, and Pd have been studied theoretically, using first-principles computational techniques, in order to investigate the possible onset of magnetism in these nanosystems. Our fully relativistic spin-polarized all-electron density functional calculations reveal the onset of Hund's rule magnetism in nanowires of all three metals, with mean-field moments of 1.1, 0.3, and 0.7 mu(B), respectively, at the equilibrium bond length. An analysis of the band structures indicates that the nanocontact superparamagnetic state suggested by our calculations should affect the ballistic conductance between tips made of Ru, Rh or Pd, leading to possible temperature and magnetic field dependent conductance.

  • 14. Dubrovinsky, L. S.
    et al.
    Dubrovinskaia, N. A.
    McCammon, C.
    Rozenberg, G. K.
    Ahuja, R.
    Osorio-Guillen, J. M.
    Dmitriev, V.
    Weber, H. P.
    Le Bihan, T.
    Johansson, Börje
    KTH, Superseded Departments, Materials Science and Engineering.
    The structure of the metallic high-pressure Fe3O4 polymorph: experimental and theoretical study2003In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 15, no 45, p. 7697-7706Article in journal (Refereed)
  • 15.
    Elenius, Måns
    et al.
    KTH, School of Computer Science and Communication (CSC), Numerical Analysis and Computer Science, NADA.
    Dzugutov, Michail
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Evidence for compact cooperatively rearranging regions in a supercooled liquid2009In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 21, no 24, p. 245101-Article in journal (Refereed)
    Abstract [en]

    We examine structural relaxation in a supercooled glass-forming liquid simulated by constant-energy constant-volume (NVE) molecular dynamics. Time correlations of the total kinetic energy fluctuations are used as a comprehensive measure of the system's approach to the ergodic equilibrium. We find that, under cooling, the total structural relaxation becomes delayed as compared with the decay of the component of the intermediate scattering function corresponding to the main peak of the structure factor. This observation can be explained by collective movements of particles preserving many-body structural correlations within compact three-dimensional (3D) cooperatively rearranging regions.

  • 16. Eriksson, O
    et al.
    Bergqvist, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Sanyal, B
    Kudrnovsky, J
    Drchal, V
    Korzhavyi, Pavel A.
    KTH, Superseded Departments, Materials Science and Engineering.
    Turek, I
    Electronic structure and magnetism of diluted magnetic semiconductors2004In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 16, no 48, p. S5481-S5489Article in journal (Refereed)
    Abstract [en]

    The electronic structure and magnetism of selected diluted magnetic semiconductors (DMS) is reviewed. It is argued that the effect of antisite defects plays an important role in the magnetism of DMS materials and that these defects lower the saturation moment and ordering temperature. We also show that the interatomic exchange of these materials is short ranged. By combining first principles calculations of interatomic exchange interactions with a classical Heisenberg model and Monte Carlo simulations, we show that-the observed critical temperatures of a broad range of diluted magnetic semiconductors, involving Mn-doped GaAs and GaN as well as Cr-doped ZnTe, are reproduced with good accuracy. We show that agreement between theory and experiment is obtained only when the magnetic atoms are randomly positioned on the Ga (or Zn) sites. This suggests that the ordering of DMS materials is heavily influenced by magnetic percolation and that the measured critical temperatures should be very sensitive to details in the sample preparation, in agreement with observations.

  • 17. Eriksson, Olle
    et al.
    Bergqvist, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Holmström, Erik
    Bergman, Anders
    LeBacq, Olivier
    Frota-Pessoa, S.
    Hjörvarsson, Björgvin
    Nordström, Lars
    Magnetism of Fe/V and Fe/Co multilayers2003In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 15, p. 599-Article in journal (Refereed)
    Abstract [en]

    We discuss in this paper the magnetic and structural parameters of Fe/V and Fe/Co multilayers. The electronic structure, magnetic moments (spin and orbital) and Curie temperatures as well as the magneto-crystalline anisotropy are calculated using first principles theory. Although theory is fairly successful in reproducing the experimental data we argue that the observed difference between theory and experiment most likely is due to lattice imperfections and that the interface between e.g. Fe and V is not perfectly sharp. We also present a model, based on the theory of elasticity, for analysing the structural properties of multilayers.

  • 18. Etz, Corina
    et al.
    Bergqvist, Lars
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Bergman, Anders
    Taroni, Andrea
    Eriksson, Olle
    Atomistic spin dynamics and surface magnons2015In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 27, no 24, article id 243202Article, review/survey (Refereed)
    Abstract [en]

    Atomistic spin dynamics simulations have evolved to become a powerful and versatile tool for simulating dynamic properties of magnetic materials. It has a wide range of applications, for instance switching of magnetic states in bulk and nano-magnets, dynamics of topological magnets, such as skyrmions and vortices and domain wall motion. In this review, after a brief summary of the existing investigation tools for the study of magnons, we focus on calculations of spin-wave excitations in low-dimensional magnets and the effect of relativistic and temperature effects in such structures. In general, we find a good agreement between our results and the experimental values. For material specific studies, the atomistic spin dynamics is combined with electronic structure calculations within the density functional theory from which the required parameters are calculated, such as magnetic exchange interactions, magnetocrystalline anisotropy, and Dzyaloshinskii-Moriya vectors.

  • 19. Finazzi, M.
    et al.
    Ghiringhelli, G.
    Tjernberg, Oscar
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Brookes, N. B.
    Photon energy dependence of the perpendicular geometry magnetic circular dichroism in the 2p3p3p resonant photoemission from Ni2000In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 12, no 9, p. 2123-2133Article in journal (Refereed)
    Abstract [en]

    The magnetic circular dichroism in the perpendicular geometry of the resonant 2p3p3p photoemission (PE) spectroscopy has been investigated in metallic Ni as a function of the photon energy across the Ni Lj absorption edge. Within the experimental error bars, the photon energy dependence of the PE dichroism signal is the same as the one shown by the magnetic circular dichroism of the corresponding x-ray absorption (XMCD), obtained in the collinear geometry. This is attributed to the fact that, in metal Ni, the orbital [L-z] and dipolar [T-z] moments are smaller than the spin angular moment [S-z]. The latter is the dominating term in both the expressions that give the integrated values of the PE dichroism or XMCD intensities, Although the respective photon energy dependence is very similar, the normalized PE dichroism intensity is a factor similar to 5.6 smaller than the normalized XMCD signal, while only a factor similar to 1.6 is expected from theoretical considerations. This factor is observed even below the L-3 threshold, thus we exclude that the small intensity of the perpendicular geometry dichroism in the Ni 2p3p3p resonant photoemission is due to fast relaxation processes in the intermediate state.

  • 20.
    Fransson, Jonas
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Eriksson, O
    Asymmetric negative differential conductance in double quantum dots2004In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 16, no 8, p. L85-L91Article in journal (Refereed)
    Abstract [en]

    An explanation for the recently observed asymmetric negative differential conductance (NDC) in a double-quantum-dot system attached to metallic external contacts is proposed. The NDC was observed only for one half of the bias voltage range (-V, V). The theory, which is based on a diagrammatic technique for non-equilibrium many-body operator Green functions, suggests that scattering between the states in the double quantum dot suppresses the total current dynamically as the bias voltage is increased. The effect is present in systems where the double-quantum-dot states are asymmetrically coupled to the left- and right-hand contacts, and we predict that a symmetric coupling will suppress the negative differential conductance completely.

  • 21.
    Fransson, Jonas
    et al.
    KTH, Superseded Departments, Physics.
    Sandalov, I
    Eriksson, O
    A perfect spin-filter quantum dot system2004In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 16, no 16, p. L249-L254Article in journal (Refereed)
    Abstract [en]

    The discovery of a novel effect in the transport through a QD spin-dependently coupled to magnetic contacts is reported. For a finite range of source-drain voltages the spin projections of the current cancel exactly, resulting in a completely suppressed output current. The spin down current behaves as one normally expects whereas the spin up current becomes negative. As the source-drain voltage is increased the spin up current eventually becomes positive. Thus, tuning the source-drain voltage such that the spin up current vanishes will result in a perfect spin filter.

  • 22.
    Fu, Ying
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Han, Tiantian
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Dynamic analysis of multiple-photon optical processes in semiconductor quantum dots2006In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 18, no 39, p. 9071-9082Article in journal (Refereed)
    Abstract [en]

    Semiconductor quantum dots (QDs) have been gaining much attention because of their outstanding properties for multiple-photon microscopy applications. By solving nonperturbatively the time-dependent Schrodinger equation, it has been shown that the large number of energy states densely compacted in both the conduction and valence bands of the QD greatly enhance the inter-band and intra-band optical couplings between two energy states induced by multiple photons from ultra-fast and ultra-intense lasers. The multiphoton absorption processes are further enhanced by many energy relaxation processes in commonly used semiconductors, which are generally represented by the relaxation energy in the order of tens of meV. Numerical calculation of multiphoton processes in QDs agrees with experimental demonstration. After proper designing, QDs can be activated by infrared radiation to emit radiation in the visible optical regime (up-conversion) for bioimaging applications.

  • 23. Gebhardt, T.
    et al.
    Music, D.
    Ekholm, M.
    Abrikosov, I. A.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Dick, A.
    Hickel, T.
    Neugebauer, J.
    Schneider, J. M.
    The influence of additions of Al and Si on the lattice stability of fcc and hcp Fe-Mn random alloys2011In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 23, no 24, p. 246003-Article in journal (Refereed)
    Abstract [en]

    We have studied the influence of additions of Al and Si on the lattice stability of face-centred-cubic (fcc) versus hexagonal-closed-packed (hcp) Fe-Mn random alloys, considering the influence of magnetism below and above the fcc Neel temperature. Employing two different ab initio approaches with respect to basis sets and treatment of magnetic and chemical disorder, we are able to quantify the predictive power of the ab initio methods. We find that the addition of Al strongly stabilizes the fcc lattice independent of the regarded magnetic states. For Si a much stronger dependence on magnetism is observed. Compared to Al, almost no volume change is observed as Si is added to Fe-Mn, indicating that the electronic contributions are responsible for stabilization/destabilization of the fcc phase.

  • 24. Gebhardt, T.
    et al.
    Music, D.
    Hallstedt, B.
    Ekholm, M.
    Abrikosov, I. A.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Schneider, J. M.
    Ab initio lattice stability of fcc and hcp Fe-Mn random alloys2010In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 22, no 29, p. 295402-Article in journal (Refereed)
    Abstract [en]

    We have studied the lattice stability of face centred cubic (fcc) versus hexagonal close packed (hcp) Fe-Mn random alloys using ab initio calculations. In the calculations we considered the antiferromagnetic order of local moments, which for fcc alloys models the magnetic configuration of this phase at room temperature (below its Neel temperature) as well as their complete disorder, corresponding to paramagnetic fcc and hcp alloys. For both cases, the results are consistent with our thermodynamic calculations, obtained within the Calphad approach. For the room temperature magnetic configuration, the cross-over of the total energies of the hcp phase and the fcc phase of Fe-Mn alloys is at the expected Mn content, whereas for the magnetic configuration above the fcc Neel temperature, the hcp lattice is more stable within the whole composition range studied. The increase of the total energy difference between hcp and antiferromagnetic fcc due to additions of Mn as well as the stabilizing effect of antiferromagnetic ordering on the fcc phase are well displayed. These results are of relevance for understanding the deformation mechanisms of these random alloys.

  • 25. Guo, J. H.
    et al.
    Gupta, A.
    Sharma, P.
    Rao, K. V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Marcus, M. A.
    Dong, C. L.
    Guillen, J. M. O.
    Butorin, S. M.
    Mattesini, M.
    Glans, P. A.
    Smith, K. E.
    Chang, C. L.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    X-ray spectroscopic study of the charge state and local ordering of room-temperature ferromagnetic Mn-doped ZnO2007In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 19, no 17Article in journal (Refereed)
    Abstract [en]

    The charge state and local ordering of Mn doped into a pulsed laser deposited single-phase thin film of ZnO are investigated by using x-ray absorption spectroscopy at the O K-edge, Mn K-edge and L-edge, and x-ray emission spectroscopy at the O K-edge and Mn L-edge. This film is ferromagnetic at room temperature. EXAFS measurement shows that Mn2+ replaces the Zn site in tetrahedral symmetry, and there is no evidence for either metallic Mn or MnO in the film. Upon Mn doping, the top of O 2p valence band extends into the bandgap, indicating additional charge carriers being created.

  • 26. Guo, J. H.
    et al.
    Vayssieres, L.
    Persson, Clas
    Ahuja, R.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Nordgren, J.
    Polarization-dependent soft-x-ray absorption of a highly oriented ZnO microrod-array2005In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 17, no 1, p. 235-240Article in journal (Refereed)
    Abstract [en]

    Polarization-dependent x-ray absorption measurements were performed on a crystalline ZnO three-dimensional array consisting of highly oriented microrods as well as on particulate thin film consisting of monodisperse spherical nanoparticles. Strong anisotropic effects have been observed for the highly oriented ZnO rods, unlike for the isotropic spherical ones. Full-potential calculations of orbital-resolved x-ray absorption of a ZnO wurtzite periodic crystal, including the Zn 3d as part of the valence states, shows a very good agreement with the experimental findings. Comprehensive fundamental knowledge of the electronic structure of ZnO is obtained by probing and demonstrating the orbital symmetry of oxygen and its contribution to the conduction band of this important II-VI semiconductor.

  • 27. Guo, J. H.
    et al.
    Vayssieres, L.
    Persson, Clas
    Ahuja, R.
    Johansson, Börje
    Nordgren, J.
    Polarization-dependent soft-x-ray absorption of highly oriented ZnO microrod arrays2002In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 14, no 28, p. 6969-6974Article in journal (Refereed)
    Abstract [en]

    Polarization-dependent x-ray absorption measurements were performed on crystalline ZnO three-dimensional arrays consisting of highly oriented microrods as well as on particulate thin films consisting of monodisperse spherical nanoparticles. Strong anisotropic effects have been observed for the highly oriented ZnO rods, but not for the isotropic spherical nanoparticles. Full-potential calculations of the orbital-resolved x-ray absorption of a ZnO wurtzite periodic crystal, including Zn 3d among the valence states, show very good agreement with the experimental findings. Comprehensive fundamental knowledge of the electronic structure of ZnO is obtained by probing and demonstrating the orbital symmetry of oxygen and its contribution to the conduction band of this important II-VI semiconductor.

  • 28.
    Göthelid, Mats
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Janin, E.
    Surface structures on sputtered/annealed WC(0001)2000In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 12, no 6, p. 773-782Article in journal (Refereed)
    Abstract [en]

    We use scanning tunnelling microscopy, Auger electron spectroscopy and low energy electron diffraction to study different tungsten and carbon terminated surface reconstructions on the sputtered/annealed WC(0001) surface. The tungsten terminated surface encompasses a (root 7 x root 7)R19 degrees W-trimer structure, a (root 3 x root 7) reconstruction representing a transition structure from the (root 7 x root 7)R19 degrees reconstruction to a (6 x 1) phase which consists of a quadratic W overlayer on the first close-packed carbidic carbon layer. The carbon terminated WC(0001) surface consists of a single graphitic carbon layer on top of the (6 x 1) structure.

  • 29.
    He, Sailing
    et al.
    KTH, Superseded Departments, Electromagnetic Theory.
    Qiu, M.
    Simovski, C. R.
    An averaged-field approach for obtaining the band structure of a dielectric photonic crystal2000In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 12, no 2, p. 99-112Article in journal (Refereed)
  • 30.
    Huang, Dan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Stability of the bandgap in Cu-poor CuInSe22012In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 24, no 45, p. 455503-Article in journal (Refereed)
    Abstract [en]

    Recent photoluminescence studies report that the bandgap energy E-g approximate to 1.0 eV of CuInSe2 is stable for Cu-poor compounds [Cu]/[In] < 1, despite the fact that Cu vacancies and (In-Cu + 2V(Cu)) complexes increase the energy gap. In this work, the impact on E-g due to the presence of native defects is analyzed using a screened hybrid density functional approach. We demonstrate that the formation energy of neutral (Cu-In + In-Cu) anti-site dimers decreases for CuInSe2 compounds when [Cu]/[In] decreases. This is explained in terms of the octet rule for the Se atoms next to the (In-Cu + 2V(Cu)) defects. As a consequence, Cu-poor CuInSe2 involves the large [(In-Cu + 2V(Cu)) + (Cu-In + In-Cu)] complexes where the anti-site defects stabilize E-g, in agreement with experimental findings.

  • 31. Iqbal, Asad M.
    et al.
    Jaffari, Ghulam Hassnain
    Saleemi, Mohsin
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Ceylan, Abdullah
    Relaxation dynamics and polydispersivity associated with defects and ferroelectric correlations in Ba-doped EuTiO32017In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 29, no 46, article id 465402Article in journal (Refereed)
    Abstract [en]

    We present the frequency- and temperature-dependent dielectric response of Eu1-xBaxTiO3 (0 <= x <= 0.5) in detail. Excluding grain boundary effects, four relaxation mechanisms were observed. Relaxation dynamics were observed to arise due to hopping conduction associated with defects, namely oxygen vacancies as well as Eu3+ and Ti3+ ions. Dielectric relaxation analysis led to the identification of Ti ions in two different environments with different relaxation rates in the overall EuTiO3 perovskite structure. The emergence of another relaxation mechanism associated with ferroelectric order as a consequence of the formation of polar regions was also observed for higher Ba concentrations. The addition of Ba led to the identification of relaxation dynamics associated with hopping conduction between Eu ions, Ti ions (in the regions with and without oxygen vacancies) and with the formation of ferroelectric polar regions. Furthermore, the polydispersivity and relaxation times were extracted within the framework of the modified Debye model. Relaxation times have been observed to increase with a decrease in temperature while larger values of polydispersivity reveal a wide distribution of relaxation times due to the presence of lattice parameter and energy barrier distributions.

  • 32. Jayasinghe, R. C.
    et al.
    Lao, Y. F.
    Perera, A. G. U.
    Hammar, Mattias
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Cao, C. F.
    Wu, H. Z.
    Plasma frequency and dielectric function dependence on doping and temperature for p-type indium phosphide epitaxial films2012In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 24, no 43, p. 435803-Article in journal (Refereed)
    Abstract [en]

    The optical properties of p-type InP epitaxial films with different doping concentrations are investigated by infrared absorption measurements accompanied by reflection and transmission spectra taken from 25 to 300 K. A complete dielectric function (DF) model, including intervalence band (IVB) transitions, free-carrier and lattice absorption, is used to determine the optical constants with improved accuracy in the spectral range from 2 to 35 mu m. The IVB transitions by free holes among the split-off, light-hole, and heavy-hole bands are studied using the DF model under the parabolic-band approximation. A good understanding of IVB transitions and the absorption coefficient is useful for designing high operating temperature and high detectivity infrared detectors and other optoelectronic devices. In addition, refractive index values reported here are useful for optoelectronic device designing, such as implementing p-InP waveguides in semiconductor quantum cascade lasers. The temperature dependence of hole effective mass and plasma frequency is also reported.

  • 33. Jenkins, A. C.
    et al.
    Temmerman, W. M.
    Ahuja, R.
    Eriksson, O.
    Johansson, Börje
    Wills, J.
    The relationship between interlayer spacing and magnetic ordering in gadolinium2000In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 12, no 50, p. 10441-10456Article in journal (Refereed)
  • 34.
    Jiang, Jun
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Kula, Mathias
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).
    Molecular modeling of inelastic electron transport in molecular junctions2008In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 20, no 37Article in journal (Refereed)
    Abstract [en]

    A quantum chemical approach for the modeling of inelastic electron tunneling spectroscopy of molecular junctions based on scattering theory is presented. Within a harmonic approximation, the proposed method allows us to calculate the electron-vibration coupling strength analytically, which makes it applicable to many different systems. The calculated inelastic electron transport spectra are often in very good agreement with their experimental counterparts, allowing the revelation of detailed information about molecular conformations inside the junction, molecule-metal contact structures, and intermolecular interaction that is largely inaccessible experimentally.

  • 35.
    Jiang, Xue
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Zhao, Jijun
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    A novel superhard BN allotrope under cold compression of h-BN2013In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 12, p. 122204-Article in journal (Refereed)
    Abstract [en]

    Using first-principles calculations, we identify a new orthorhombic boron nitride (BN) phase (namely, P-BN; space group: Pmn2(1)), which has similar topological structure to Bct-BN and Z-BN, but without the six-membered ring. This P-BN allotrope is energetically more favorable than previously reported Pnma-BN, Bct-BN and Z-BN phases. With only 0.06 eV/atom less stable than h-BN at ambient pressure, it can be formed from h-BN under cold compression at a low pressure of 4 GPa. The theoretical hardness and bulk modulus of the P-BN crystal are 403 GPa and 60.5 GPa, respectively, comparable to those of c-BN. Moreover, the P-BN phase along with Bct-BN and Z-BN are suggested as possible intermediate phases between h-BN and w-BN, which can be qualitatively explained by two empirical rules of Ostwald and Ostwald-Volmer.

  • 36. Johansson, Malin B.
    et al.
    Baldissera, Gustavo
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Valyukh, Iryna
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. University of Oslo, Norway.
    Arwin, Hans
    Niklasson, Gunnar A.
    Österlund, Lars
    Electronic and optical properties of nanocrystalline WO3 thin films studied by optical spectroscopy and density functional calculations2013In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 20, p. 205502-Article in journal (Refereed)
    Abstract [en]

    The optical and electronic properties of nanocrystalline WO3 thin films prepared by reactive dc magnetron sputtering at different total pressures (P-tot) were studied by optical spectroscopy and density functional theory (DFT) calculations. Monoclinic films prepared at low P-tot show absorption in the near infrared due to polarons, which is attributed to a strained film structure. Analysis of the optical data yields band-gap energies E-g approximate to 3.1 eV, which increase with increasing P-tot by 0.1 eV, and correlate with the structural modifications of the films. The electronic structures of triclinic delta-WO3, and monoclinic gamma- and epsilon-WO3 were calculated using the Green function with screened Coulomb interaction (GW approach), and the local density approximation. The delta-WO3 and gamma-WO3 phases are found to have very similar electronic properties, with weak dispersion of the valence and conduction bands, consistent with a direct band-gap. Analysis of the joint density of states shows that the optical absorption around the band edge is composed of contributions from forbidden transitions (>3 eV) and allowed transitions (>3.8 eV). The calculations show that E-g in epsilon-WO3 is higher than in the delta-WO3 and gamma-WO3 phases, which provides an explanation for the P-tot dependence of the optical data.

  • 37. Kadas, K.
    et al.
    Iusan, D.
    Hellsvik, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Cedervall, J.
    Berastegui, P.
    Sahlberg, M.
    Jansson, U.
    Eriksson, O.
    AlM2B2 (M = Cr, Mn, Fe, Co, Ni): a group of nanolaminated materials2017In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 29, no 15, article id 155402Article in journal (Refereed)
    Abstract [en]

    Combining theory with experiments, we study the phase stability, elastic properties, electronic structure and hardness of layered ternary borides AlCr2B2, AlMn2B2, AlFe2B2, AlCo2B2, and AlNi2B2. We find that the first three borides of this series are stable phases, while AlCo2B2 and AlNi2B2 are metastable. We show that the elasticity increases in the boride series, and predict that AlCr2B2, AlMn2B2, and AlFe2B2 are more brittle, while AlCo2B2 and AlNi2B2 are more ductile. We propose that the elasticity of AlFe2B2 can be improved by alloying it with cobalt or nickel, or a combination of them. We present evidence that these ternary borides represent nanolaminated systems. Based on SEM measurements, we demonstrate that they exhibit the delamination phenomena, which leads to a reduced hardness compared to transition metal mono-and diborides. We discuss the background of delamination by analyzing chemical bonding and theoretical work of separation in these borides.

  • 38. Kamali, S.
    et al.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Yoda, Y.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Oxidation states and quality of upper interfaces in magnetic tunnel junctions: oxygen effect on crystallization of interfaces2014In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 26, no 2, p. 026004-Article in journal (Refereed)
    Abstract [en]

    In this study, we have deposited an Fe-57 sensor layer at the upper interface, i.e. the interface between the oxide barrier and the upper electrode in selected magnetic tunnel junctions (MTJs), in order to perform nuclear resonant scattering with the aim of obtaining direct information on the magnetic properties and quality of this interface. This is a unique approach as it makes use of this powerful technique to give information at the atomic level, and specifically from the interface where the sensor layer is deposited. By varying sample tunnel barrier thicknesses and oxidation times in the preparation of this barrier, we have observed that longer oxidation time results in not only an increase of the magnetic hyperfine fields, but also causes an interesting crystallization and smoothing of the interface. We also observed that boron atoms diffuse away from the lower part of the upper FeCoB electrode toward the capping layer. An important observation, which has a crucial effect in tunnel magnetoresistance values, is the absence of any magnetically dead FeO layer at the interface. Another finding is that the deposition of Fe on MgO is much smoother than the deposition of MgO on Fe.

  • 39. Kamali, S.
    et al.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Yoda, Y.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Oxidation states and the quality of lower interfaces in magnetic tunnel junctions: oxygen effect on crystallization of interfaces2013In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 13, p. 135302-Article in journal (Refereed)
    Abstract [en]

    Lower interfaces in magnetic tunnel junctions (MTJs), which are the basic components in many spintronic devices such as magnetoresistive random access memories, have crucial effects on the performance of these devices. To obtain more insight into such interfaces, we have introduced an ultrathin sensor layer of Fe-57 at the interface between the lower electrode and the oxide barrier in selected MTJs. This allowed us to perform nuclear resonant scattering measurements, which provide direct information on the magnetic properties and quality of the interfaces. The application of nuclear resonant scattering to study interfaces in MTJs is a unique approach in the sense that it gives information at the atomic level, and specifically from the interface where the sensor layer is deposited. Samples with different tunnel barrier thicknesses and varied oxidation times in the preparation of this barrier have been studied. These show that oxidation can not only increase the magnetic hyperfine fields but also cause an interesting smoothing and crystallizing of the interface. Another interesting finding is the observation of boron diffusion into the lower part of the FeCoB lower electrode towards the Ta seed layer.

  • 40. Kanchana, V.
    et al.
    Vaitheeswaran, G.
    Svane, A.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    First-principles study of elastic properties of CeO2, ThO2 and PoO22006In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 18, no 42, p. 9615-9624Article in journal (Refereed)
    Abstract [en]

    Using first-principles density functional calculations, the structural and elastic properties of fluorite type oxides CeO2, ThO2 and PoO2 were studied by means of the full-potential linear muffin-tin orbital method. Calculations were performed within the local density approximation (LDA) as well as generalized gradient approximation (GGA) to the exchange correlation potential. The calculated equilibrium lattice constants and bulk moduli are in good agreement with the experimental results, as are the computed elastic constants for CeO2 and ThO2. For PoO2 this is the first quantitative theoretical prediction of the ground state properties, and it still awaits experimental confirmation. The calculations find PoO2 to be a semiconductor with an indirect band gap and elastic constants similar in magnitude to those of CeO2 and ThO2.

  • 41.
    Kanchana, Venkatakrishnan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Vaitheeswaran, Ganapathy
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Souvatzis, P.
    Eriksson, O.
    Lebegue, S.
    Density functional study of the electronic structure and lattice dynamics of SrCl22010In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 22, no 44, p. 445402-Article in journal (Refereed)
    Abstract [en]

    A theoretical study of the structural, electronic, optical and lattice dynamical properties of SrCl2 in the cubic fluorite structure has been performed using first-principles calculations. The calculated ground state and elastic properties are in good agreement with the experiments. The calculated band gap is underestimated within the generalized gradient approximation for the exchange and correlation functional. GW calculations have been performed in order to improve the band gap and good agreement with the experiment is obtained. The phonon dispersion relations are discussed in detail in addition to the ground state and elastic properties. Also, the optical properties are computed with DFT corrected by the GW approximation.

  • 42. Kapilashrami, Mukes
    et al.
    Wang, Yung Jui
    Li, Xin
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Glans, Per-Anders
    Fang, Mei
    Riazanova, Anastasia V.
    Belova, Lyubov M.
    Rao, K. Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Luo, Yi
    Barbiellini, Bernardo
    Lin, Hsin
    Markiewicz, Robert
    Bansil, Arun
    Hussain, Zahid
    Guo, Jinghua
    Understanding the magnetic interaction between intrinsic defects and impurity ions in room-temperature ferromagnetic Mg1-xFexO thin films2016In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 28, no 15, article id 156002Article in journal (Refereed)
    Abstract [en]

    Understanding the nature and characteristics of the intrinsic defects and impurities in the dielectric barrier separating the ferromagnetic electrodes in a magnetic tunneling junction is of great importance for understanding the often observed 'barrier-breakdown' therein. In this connection, we present herein systematic experimental (SQUID and synchrotron-radiation-based x-ray absorption spectroscopy) and computational studies on the electronic and magnetic properties of Mg1-xFexO thin films. Our studies reveal: (i) defect aggregates comprised of basic and trimer units (Fe impurity coupled to 1 or 2 Mg vacancies) and (ii) existence of two competing magnetic orders, defect- and dopant-induced, with spin densities aligning anti-parallel if the trimer is present in the oxide matrix. These findings open up new avenues for designing tunneling barriers with high endurance and tunneling effect upon tuning the concentration/distribution of the two magnetic orders.

  • 43.
    Kapilashrami, Mukes
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Xu, Jun
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Rao, K Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Belova, Lyubov
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Carlegrim, Elin
    Fahlman, Mats
    Experimental evidence for ferromagnetism at room-temperature in MgO thin films2010In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 22, no 34, p. 345004-Article in journal (Refereed)
    Abstract [en]

    Ferromagnetic ordering at room temperature (RTFM) in MgO thin films deposited by RF magnetron sputtering under various atmospheric conditions and temperatures is reported. A saturation magnetization (MS) value as high as 1.58 emu g(-1) is (0.046 mu B/unit cell) observed for a 170 nm film deposited at RT under an oxygen pressure of 1.3 x 10(-4) mbar. In contrast, films deposited at elevated temperature (under an identical oxygen pressure), or at higher oxygen pressures, as well as under a nitrogen atmosphere at RT show significantly suppressed magnetization. The ferromagnetic order in the MgO matrix is believed to be defect induced

  • 44. Khmelevska, T.
    et al.
    Khmelevskyi, S.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Mohn, P.
    Magnetism and origin of non-monotonous concentration dependence of the bulk modulus in Fe-rich alloys with Si, Ge and Sn: a first-principles study2006In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 18, no 29, p. 6677-6689Article in journal (Refereed)
    Abstract [en]

    Using the first-principles calculations based on the coherent potential approximation, we study the electronic structure, magnetic moments and the bulk modulus of FeX alloys with IVB group elements ( X = Si, Ge, Sn) in the Fe-rich concentration range (x = 0.0 - 0.25), which form a stability region of bcc-related phases. In agreement with experiment, our calculations reproduce well a peculiar non-monotonous behaviour of the bulk modulus in Fe - Si alloys with increasing Si concentration. Such a dependence is found for all bcc-related disordered and partially ordered Fe - Si phases A2, B2 and D0(3), which is in contrast with an earlier suggestion that the non-monotonous bulk modulus behaviour is related to partial ordering in Fe - Si. In addition, our results predict a similar behaviour in Fe - Ge and Fe - Sn alloys. It is shown that the observed behaviour of the bulk modulus is entirely related to the changes of the magnetic properties with chemical composition.

  • 45. Khmelevskyi, S.
    et al.
    Khmelevska, T.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Mohn, P.
    Magnetic exchange interactions in the paramagnetic state of hcp Gd2007In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 19, no 32Article in journal (Refereed)
    Abstract [en]

    We use the first-principles magnetic force theorem embodied in the Korringa-Kohn-Rostoker method to calculate pair magnetic exchange interactions in pure hexagonal close packed ( hcp) Gd metal in the ferromagnetic as well as in the paramagnetic state with disordered local 4f-moments. It is found that the exchange interactions between the localized 4f-moments, in particular also distant ones, depend on the state of magnetic disorder. Such a dependence is a consequence of the electronic structure changes of the conduction band that mediates the interaction between the local moments. The magnetic ordering temperature has been calculated using a Monte Carlo simulation technique and the results are compared with mean-field based studies.

  • 46. Khmelevskyi, S.
    et al.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Mohn, P.
    Electronic structure analysis of the pressure induced metamagnetic transition and magnetovolume anomaly in Fe3C - Cementite2005In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 17, no 46, p. 7345-7352Article in journal (Refereed)
    Abstract [en]

    Within the framework of ab initio electronic structure calculations we study a mechanism for the recently discovered pressure induced metamagnetic transition in Fe3C-cementite. It is shown that the transition occurs between a high moment ferromagnetic state and a non-magnetic (non-spin-polarized) state without stabilization of any intermediate antiferromagnetic-like state, which may explain the experimentally observed difference in character of the transitions in Fe3C and fcc Fe-Pt. Although both materials are Invar-type systems we find considerable differences between their electronic structures and the degrees of localization, or itinerancy, of the Fe magnetic moments. The possibilities for describing the thermal expansion anomaly in Fe3C within existing approaches to the Invar problem are discussed in detail.

  • 47. Koci, L.
    et al.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Belonoshko, Anatoly B.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Study of the high-pressure helium phase diagram using molecular dynamics2007In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 19, no 1Article in journal (Refereed)
    Abstract [en]

    The rich occurrence of helium and hydrogen in space makes their properties highly interesting. By means of molecular dynamics ( MD), we have examined two interatomic potentials for He-4. Both potentials are demonstrated to reproduce high-pressure solid and liquid equation of state (EOS) data. The EOS, solid - solid transitions and melting at high pressures ( P) were studied using a two-phase method. The Buckingham potential shows a good agreement with theoretical and experimental EOS, but does not reproduce experimental melting data. The Aziz potential shows a perfect match with theoretical melting data. We conclude that there is a stable body-centred-cubic (bcc) phase for He-4 at temperatures ( T) above 340 K and pressures above 22 GPa for the Buckingham potential, whereas no bcc phase is found for the Aziz potential in the applied PT range.

  • 48. Koci, L.
    et al.
    Kim, D. Y.
    de Almeida, J. S.
    Mattesini, M.
    Isaev, E.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Mechanical stability of TiO2 polymorphs under pressure: ab initio calculations2008In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 20, no 34Article in journal (Refereed)
    Abstract [en]

    First-principles calculations using plane-wave basis sets and ultrasoft pseudopotentials have been performed to study the mechanical stabilities of the rutile, pyrite, fluorite and cotunnite phases of titanium dioxide (TiO2). For these polymorphs, we have calculated the equilibrium volumes, equations of state, bulk moduli and selected elastic constants. Compared to the three phases rutile, pyrite and fluorite, the recently discovered cotunnite phase shows the highest c(44) for all pressures considered. Cotunnite also shows the highest bulk modulus amongst the four studied phases at an ambient pressure of B-0 = 272 GPa.

  • 49.
    Kravets, Anatolii
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. National Academy of Sciences of Ukraine, Ukraine.
    Tovstolytkin, A. I.
    Dzhezherya, Yu I.
    Polishchuk, Dmytro
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. National Academy of Sciences of Ukraine, Ukraine.
    Kozak, I. M.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Spin dynamics in a Curie-switch2015In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 27, no 44, article id 446003Article in journal (Refereed)
    Abstract [en]

    Ferromagnetic resonance properties of F-1/f/F-2/AF multilayers, where weakly ferromagnetic spacer f is sandwiched between strongly ferromagnetic layers F-1 and F-2, with F-1 being magnetically soft and F-2-magnetically hard due to exchange pinning to antiferromagnetic layer AF, are investigated. Spacer-mediated exchange coupling is shown to strongly affect the resonance fields of both F-1 and F-2 layers. Our theoretical calculations as well as measurements show that the key magnetic parameters of the spacer, which govern the ferromagnetic resonance in F-1/f/F-2/AF, are the magnetic exchange length (Lambda), effective saturation magnetization at T = 0 (m(0)) and effective Curie temperature (T-C(eff)). The values of these key parameters are deduced from the experimental data for multilayers with f = NixCu100-x, for the key ranges in the Ni-concentration (x = 54 divided by 70 at. %) and spacer thickness (d = 3 divided by 6 nm). The results obtained provide a deeper insight into thermally-controlled spin precession and switching in magnetic nanostructures, with potential applications in spin-based oscillators and memory devices.

  • 50. Kudrnovsky, Josef
    et al.
    Drchal, Vaclav
    Turek, Ilja
    Bergqvist, Lars
    Department of Physics, University of Uppsala.
    Eriksson, Olle
    Bouzerar, Georges
    Sandratskii, Leonid
    Bruno, Patrick
    Exchange interactions and critical temperatures in diluted magnetic semiconductors2004In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 16, no 48, p. S5571-S5578Article in journal (Refereed)
    Abstract [en]

    A first-principles approach to magnetic properties of diluted magnetic semiconductors (DMS) is presented that is based on the local spin-density approximation (LSDA) as implemented in the framework of the tight-binding linear muffin-tin orbital method, while the effect of randomness is described by the coherent potential approximation. Application of a real-space Green-function formalism yields the exchange pair interactions between distant magnetic atoms that are needed for quantitative studies of magnetic excitations including the Curie temperatures. We have found that the pair exchange interactions exhibit a strong directional dependence and are exponentially damped with increasing distance between magnetic atoms due to disorder and the half-metallic character of the DMS. As a case study we consider (Ga, Mn) As, (Ga, Mn)N, and (Zn, Cr)Te alloys. The calculations demonstrate that inclusion of disorder and, in particular, realistic distances among magnetic impurities, are needed to obtain critical temperatures which are in good agreement with available experiments.

123 1 - 50 of 121
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