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  • 51.
    Odell, Anders
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delin, Anna
    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.
    Bock, N.
    Challacombe, M.
    Niklasson, Anders M. N.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Higher-order symplectic integration in Born-Oppenheimer molecular dynamics2009In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 131, no 24Article in journal (Refereed)
    Abstract [en]

    The extended Lagrangian formulation of time-reversible Born-Oppenheimer molecular dynamics [A. M. N. Niklasson, C. J. Tymczak, and M. Challacombe, Phys. Rev. Lett. 100, 123004 (2008); Phys. Rev. Lett. 97, 123001 (2006)] enables the use of geometric integrators in the propagation of both the nuclear and the electronic degrees of freedom on the Born-Oppenheimer potential energy surface. Different symplectic integrators up to the sixth order have been adapted and optimized in the framework of ab initio self-consistent-field theory. It is shown how the accuracy can be significantly improved compared to a conventional Verlet integration at the same level of computational cost, in particular, for the case of very high accuracy requirements.

  • 52.
    Odell, Anders
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delin, Anna
    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.
    Cawkwell, Marc J
    LANL, Los Alamos, USA.
    Niklasson, Anders M N
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Geometric integration in Born-Oppenheimer molecular dynamics2011In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 135, no 22, p. 224105-Article in journal (Refereed)
    Abstract [en]

    Geometric integration schemes for extended Lagrangian self-consistent Born-Oppenheimer moleculardynamics, including a weak dissipation to remove numerical noise, are developed and analyzed.The extended Lagrangian framework enables the geometric integration of both the nuclear and electronicdegrees of freedom. This provides highly efficient simulations that are stable and energy conservingeven under incomplete and approximate self-consistent field (SCF) convergence. We investigatethree different geometric integration schemes: (1) regular time reversible Verlet, (2) secondorder optimal symplectic, and (3) third order optimal symplectic. We look at energy conservation,accuracy, and stability as a function of dissipation, integration time step, and SCF convergence. Wefind that the inclusion of dissipation in the symplectic integration methods gives an efficient dampingof numerical noise or perturbations that otherwise may accumulate from finite arithmetics in aperfect reversible dynamics.

  • 53.
    Odell, Anders
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delin, Anna
    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.
    Cawkwell, Marc
    Theoretical Division, Los Alamos National Laboratory.
    Niklasson, Anders M. N.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Geometric integration in extended lagrangian self consistent tight-binding molecular dynamicsArticle in journal (Other academic)
    Abstract [en]

    Geometric integration schemes for extended Lagrangian self-consistent tight-binding molecular dynamics, including a weak dissipation to remove numerical noise, are developed and analyzed. The extended Lagrangian framework enables the geometric integration of both the nuclear andelectronic degrees of freedom. This provides highly effcient simulations that are stable and energy conserving even under incomplete and approximate self-consistent field (SCF) convergence. We investigate three different geometric integration schemes: i) regular time reversible Verlet, ii) secondorder optimal symplectic, and iii) third order optimal symplectic. We look at energy conservation, accuracy and stabilitty as a function of dissipation, integration time step, and SCF convergence. We find that the inclusion of dissipation in the symplectic integration methods gives an efficient damping of numerical noise or perturbations that otherwise may accumulate from finite arithmetics in a perfect reversible dynamics. The modification of the integration breakes symplecticity and introduces a global energy drift. The systematic drift in energy and the broken symplecticity can be kept arbitrarily small without significant perturbations of the molecular trajectories. However, we have yet to find a formalism for the inclusion of the dissipation in higher-order symplectic integration methods with a more optimal balance between efficient damping and minimal global energy drift.

  • 54.
    Odell, Anders
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delin, Anna
    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.
    Rungger, Ivan
    School of Physics and CRANN, Trinity College, Dublin.
    Sanvito, Stefano
    School of Physics and CRANN, Trinity College, Dublin.
    Investigation of the Conducting Properties of a Photoswitching Dithienylethene Molecule2010In: ACS NANO, ISSN 1936-0851, Vol. 4, no 5, p. 2635-2642Article in journal (Refereed)
    Abstract [en]

    Photoswitching molecules are attractive candidates as organic materials for optoelectronics applications because light impulses can switch them between states with different conducting characteristics. Here, we report a fully self-consistent density functional theory calculation of the electron transport properties of photoswitching dithienylethene attached to Au leads in both the open and closed conformations. The molecule is found to be a good conductor in both conformations, with the low-bias current for the closed one being about 20 times larger than that of the open. Importantly, the current voltage characteristics away from the linear response are largely determined by molecular orbital rehybridization in an electric field, in close analogy to what happens for Mn-12 molecules. However, in the case of dithienylethene attached to Au, such a mechanism is effective also in conditions of strong electronic coupling to the electrodes. This makes the dithienylethene family an intriguing materials platform for constructing highly conducting organic optoelectronics switches.

  • 55.
    Odell, Anders
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delin, Anna
    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.
    Ulman, Kanchan
    Jawaharlal Nehru Centre for Advanced Scientic Research Jakkur.
    Narasimhan, Shobana
    Jawaharlal Nehru Centre for Advanced Scientic Research Jakkur.
    Rungger, Ivan
    School of Physics and CRANN, Trinity College.
    Sanvito, Stefano
    School of Physics and CRANN, Trinity College.
    Comparison between s- and d-electron mediated transport in a photoswitching dithienylethene molecule using ab initio transport methods2011In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 84, no 16, p. 165402-Article in journal (Refereed)
    Abstract [en]

    The influence of the electrode's Fermi surface on the transport properties of a photoswitching molecule is investigated with state-of-the-art ab initio transport methods. We report results for the conducting properties of the two forms of dithienylethene attached either to Ag or to nonmagnetic Ni leads. The I-V curves of the Ag/dithienylethene/Ag device are found to be very similar to those reported previously for Au. In contrast, when Ni is used as the electrode material the zero-bias transmission coefficient is profoundly different as a result of the role played by the Ni d bands in the bonding between the molecule and the electrodes. Intriguingly, despite these differences the overall conducting properties depend little on the electrode material. We thus conclude that electron transport in dithienylethene is, for the cases studied, mainly governed by the intrinsic electronic structure of the molecule.

  • 56.
    Odell, Anders
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Rungger, Ivan
    Sanvito, Stefano
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Investigation of conducting properties of a photochromic dithienylethene molecule using different lead atomic basis setsManuscript (Other academic)
  • 57.
    Pan, Fan
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Applied Physics.
    Chico, Jonathan
    Delin, Anna
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Applied Physics. Uppsala Univ, Sweden.
    Bergman, Anders
    Bergqvist, Lars
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Applied Physics.
    Extended spin model in atomistic simulations of alloys2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 18, article id 184432Article in journal (Refereed)
    Abstract [en]

    An extended atomistic spin model allowing for studies of the finite-temperature magnetic properties of alloys is proposed. The model is obtained by extending the Heisenberg Hamiltonian via a parametrization from a first-principles basis, interpolating from both the low-temperature ferromagnetic and the high-temperature paramagnetic reference states. This allows us to treat magnetic systems with varying degree of itinerant character within the model. Satisfactory agreement with both previous theoretical studies and experiments are obtained in terms of Curie temperatures and paramagnetic properties. The proposed model is not restricted to elements but is also applied to binary alloys, such as the technologically important material permalloy, where significant differences in the finite magnetic properties of Fe and Ni magnetic moments are found. The proposed model strives to find the right compromise between accuracy and computational feasibility for accurate modeling, even for complex magnetic alloys and compounds.

  • 58.
    Pan, Fan
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Chico, Jonathan
    Hellsvik, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Delin, Anna
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. KTH, Centres, SeRC - Swedish e-Science Research Centre. Uppsala University, Sweden.
    Bergman, Anders
    Bergqvist, Lars
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Systematic study of magnetodynamic properties at finite temperatures in doped permalloy from first-principles calculations2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 21, article id 214410Article in journal (Refereed)
    Abstract [en]

    By means of first-principles calculations, we have systematically investigated how the magnetodynamic properties Gilbert damping, magnetization, and exchange stiffness are affected when permalloy (Py) (Fe0.19Ni0.81) is doped with 4d or 5d transition metal impurities. We find that the trends in the Gilbert damping can be understood from relatively few basic parameters such as the density of states at the Fermi level, the spin-orbit coupling, and the impurity concentration. The temperature dependence of the Gilbert damping is found to be very weak which we relate to the lack of intraband transitions in alloys. Doping with 4d elements has no major impact on the studied Gilbert damping, apart from diluting the host. However, the 5d elements have a profound effect on the damping and allow it to be tuned over a large interval while maintaining the magnetization and exchange stiffness. As regards the spin stiffness, doping with early transition metals results in considerable softening, whereas late transition metals have a minor impact. Our result agree well with earlier calculations where available. In comparison to experiments, the computed Gilbert damping appears slightly underestimated, whereas the spin stiffness shows a general good agreement.

  • 59.
    Pan, Fan
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Delin, Anna
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Applied Physics. Department of Physics and Astronomy,Uppsala University.
    Bergman, Anders
    Bergqvist, Lars
    KTH, School of Engineering Sciences (SCI), Applied Physics. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Magnon properties of random alloysManuscript (preprint) (Other academic)
  • 60. Panda, S. K.
    et al.
    Bhowal, S.
    Delin, Anna
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Eriksson, O.
    Dasgupta, I.
    Effect of spin orbit coupling and Hubbard U on the electronic structure of IrO22014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 15, p. 155102-Article in journal (Refereed)
    Abstract [en]

    We have studied in detail the electronic structure of IrO2 including spin orbit coupling (SOC) and electronelectron interaction, both within the generalized gradient approximation plus Hubbard U (GGA+ U) and GGA plus dynamical mean field theory (GGA+ DMFT) approximations. Our calculations reveal that the Ir t(2g) states at the Fermi level largely retain the J(eff) = 1/2 character, suggesting that this complex spin orbit entangled state may be robust even in metallic IrO2. We have calculated the phase diagram for the ground state of IrO2 as a function of U and find a metal insulator transition that coincides with a magnetic phase change, where the effect of SOC is only to reduce the critical values of U necessary for the transition. We also find that dynamic correlations, as given by the GGA+ DMFT calculations, tend to suppress the spin- splitting, yielding a Pauli paramagnetic metal for moderate values of the Hubbard U. Our calculated optical spectra and photoemission spectra including SOC are in good agreement with experiment, demonstrating the importance of SOC in IrO2.

  • 61. Panda, S. K.
    et al.
    Di Marco, I.
    Delin, Anna
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Eriksson, O.
    Correlated electronic structure of CeN2016In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 208, p. 111-115Article in journal (Refereed)
    Abstract [en]

    We have studied in detail the electronic structure of CeN including spin orbit coupling (SOC) and electron electron interaction, within the dynamical mean-field theory combined with density-functional theory in generalized gradient approximation (GGA+DMFT). The effective impurity problem has been solved through the spin-polarized T-matrix fluctuation-exchange (SPTF) solver and the Hubbard I approximation (HIA). The calculated l-projected atomic partial densities of states and the converged potential were used to obtain the X-ray-photoemission-spectra (XPS) and Bremstrahlung Isochromat spectra (BIS). Following the spirit of Gunnarsson-Schonhammer model, we have coupled the SPTF and HIA 4f spectral functions to explain the various spectroscopic manifestations of CeN. Our computed spectra in such a coupled scheme explain the experimental data remarkably well, establishing the validity of our theoretical model in analyzing the electronic structure of CeN. The contribution of the various l-states in the total spectra and the importance of cross sections are also analyzed in detail.

  • 62. Panda, S. K.
    et al.
    Pal, Banabir
    Mandal, Suman
    Gorgoi, Mihaela
    Das, Shyamashis
    Sarkar, Indranil
    Drube, Wolfgang
    Sun, Weiwei
    Di Marco, I.
    Lindblad, Andreas
    Thunstroem, P.
    Delin, Anna
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, Centres, SeRC - Swedish e-Science Research Centre. Uppsala Univ, Sweden.
    Karis, Olof
    Kvashnin, Y. O.
    van Schilfgaarde, M.
    Eriksson, O.
    Sarma, D. D.
    High photon energy spectroscopy of NiO: Experiment and theory2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 23, article id 235138Article in journal (Refereed)
    Abstract [en]

    We have revisited the valence band electronic structure of NiO by means of hard x-ray photoemission spectroscopy (HAXPES) together with theoretical calculations using both the GW method and the local density approximation + dynamical mean-field theory (LDA+DMFT) approaches. The effective impurity problem in DMFT is solved through the exact diagonalization (ED) method. We show that the LDA+DMFT method in conjunction with the standard fully localized limit (FLL) and around mean field (AMF) double-counting alone cannot explain all the observed structures in the HAXPES spectra. GW corrections are required for the O bands and Ni-s and p derived states to properly position their binding energies. Our results establish that a combination of the GW and DMFT methods is necessary for correctly describing the electronic structure of NiO in a proper ab initio framework. We also demonstrate that the inclusion of photoionization cross section is crucial to interpret the HAXPES spectra of NiO. We argue that our conclusions are general and that the here suggested approach is appropriate for any complex transition metal oxide.

  • 63. Panda, S. K.
    et al.
    Thunstrom, P.
    Di Marco, I.
    Schott, J.
    Delin, Anna
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Dasgupta, I.
    Eriksson, O.
    Sarma, D. D.
    A charge self-consistent LDA plus DMFT study of the spectral properties of hexagonal NiS2014In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 16, p. 093049-Article in journal (Refereed)
    Abstract [en]

    The electronic structure and spectral properties of hexagonal NiS have been studied in the high temperature paramagnetic phase and low temperature anti-ferromagnetic phase. The calculations have been performed using charge self-consistent density-functional theory in local density approximation combined with dynamical mean-field theory (LDA+DMFT). The photoemission spectra (PES) and optical properties have been computed and compared with the experimental data. Our results show that the dynamical correlation effects are important to understand the spectral and optical properties of NiS. These effects have been analyzed in detail by means of the computed real and imaginary part of the self-energy.

  • 64. Paul, S.
    et al.
    Iuşan, D.
    Thunström, P.
    Kvashnin, Y. O.
    Hellsvik, Johan
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Pereiro, M.
    Delin, Anna
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Knut, R.
    Phuyal, D.
    Lindblad, A.
    Karis, O.
    Sanyal, B.
    Eriksson, O.
    Investigation of the spectral properties and magnetism of BiFeO3 by dynamical mean-field theory2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 12, article id 125120Article in journal (Refereed)
    Abstract [en]

    Using the local density approximation plus dynamical mean-field theory (LDA+DMFT), we have computed the valence-band photoelectron spectra and magnetic excitation spectra of BiFeO3, one of the most studied multiferroics. Within the DMFT approach, the local impurity problem is tackled by the exact diagonalization solver. The solution of the impurity problem within the LDA+DMFT method for the paramagnetic and magnetically ordered phases produces result in agreement with the experimental data on electronic and magnetic structures. For comparison, we also present results obtained by the LDA+U approach which is commonly used to compute the physical properties of this compound. Our LDA+DMFT derived electronic spectra match adequately with the experimental hard x-ray photoelectron spectroscopy and resonant photoelectron spectroscopy for Fe 3d states, whereas the LDA+U method fails to capture the general features of the measured spectra. This indicates the importance of accurately incorporating the dynamical aspect of electronic correlation among Fe 3d orbitals to reproduce the experimental excitation spectra. Specifically, the LDA+DMFT derived density of states exhibits a significant amount of Fe 3d states at the position of Bi lone pairs, implying that the latter are not alone in the spectral scenario. This fact might modify our interpretation about the origin of ferroelectric polarization in this material. Our study demonstrates that the combination of orbital cross sections for the constituent elements and broadening schemes for the spectral functions are crucial to explain the detailed structures of the experimental electronic spectra. Our magnetic excitation spectra computed from the LDA+DMFT result conform well with the inelastic neutron scattering data.

  • 65. Peters, Lars
    et al.
    Ghosh, Saurabh
    Sanyal, Biplab
    van Dijk, Chris
    Bowlan, John
    de Heer, Walt
    Delin, Anna
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Uppsala Universitet, Sweden.
    Di Marco, Igor
    Eriksson, Olle
    Katsnelson, Mikhail I.
    Johansson, Borje
    Kirilyuk, Andrei
    Magnetism and exchange interaction of small rare-earth clusters; Tb as a representative2016In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, article id 19676Article in journal (Refereed)
    Abstract [en]

    Here we follow, both experimentally and theoretically, the development of magnetism in Tb clusters from the atomic limit, adding one atom at a time. The exchange interaction is, surprisingly, observed to drastically increase compared to that of bulk, and to exhibit irregular oscillations as a function of the interatomic distance. From electronic structure theory we find that the theoretical magnetic moments oscillate with cluster size in exact agreement with experimental data. Unlike the bulk, the oscillation is not caused by the RKKY mechanism. Instead, the inter-atomic exchange is shown to be driven by a competition between wave-function overlap of the 5d shell and the on-site exchange interaction, which leads to a competition between ferromagnetic double-exchange and antiferromagnetic super-exchange. This understanding opens up new ways to tune the magnetic properties of rare-earth based magnets with nano-sized building blocks.

  • 66.
    Quellmalz, Arne
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Micro and Nanosystems.
    Smith, Anderson David
    KTH, School of Electrical Engineering and Computer Science (EECS), Micro and Nanosystems.
    Elgammal, Karim
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Fan, Xuge
    KTH, School of Electrical Engineering and Computer Science (EECS), Micro and Nanosystems.
    Delin, Anna
    KTH, School of Electrical Engineering and Computer Science (EECS), Micro and Nanosystems.
    Östling, Mikael
    KTH, School of Electrical Engineering and Computer Science (EECS), Electronics, Integrated devices and circuits.
    Lemme, Max C.
    Chair of Electronic Devices, RWTH Aachen University.
    Gylfason, Kristinn
    KTH, School of Electrical Engineering and Computer Science (EECS), Micro and Nanosystems.
    Niklaus, Frank
    KTH, School of Electrical Engineering and Computer Science (EECS), Micro and Nanosystems.
    Influence of Humidity on Contact Resistance in Graphene Devices2018In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 10, no 48, p. 41738-41746Article in journal (Refereed)
    Abstract [en]

    The electrical contact resistance at metal–graphene interfaces can significantly degrade the properties of graphene devices and is currently hindering the full exploitation of graphene’s potential. Therefore, the influence of environmental factors, such as humidity, on the metal–graphene contact resistance is of interest for all graphene devices that operate without hermetic packaging. We experimentally studied the influence of humidity on bottom-contacted chemical-vapor-deposited (CVD) graphene–gold contacts, by extracting the contact resistance from transmission line model (TLM) test structures. Our results indicate that the contact resistance is not significantly affected by changes in relative humidity (RH). This behavior is in contrast to the measured humidity sensitivity  of graphene’s sheet resistance. In addition, we employ density functional theory (DFT) simulations to support our experimental observations. Our DFT simulation results demonstrate that the electronic structure of the graphene sheet on top of silica is much more sensitive to adsorbed water molecules than the charge density at the interface between gold and graphene. Thus, we predict no degradation of device performance by alterations in contact resistance when such contacts are exposed to humidity. This knowledge underlines that bottom-contacting of graphene is a viable approach for a variety of graphene devices and the back end of the line integration on top of conventional integrated circuits.

  • 67.
    Råsander, Mikael
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Bergqvist, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Density functional theory study of the electronic structure of fluorite Cu2Se2013In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 12, p. 125503-Article in journal (Refereed)
    Abstract [en]

    We have investigated the electronic structure of fluorite Cu2Se using density functional theory calculations within the LDA, PBE and AM05 approximations as well as the non-local hybrid PBE0 and HSE approximations. We find that Cu2Se is a zero gap semiconductor when using either a local or semi-local density functional approximation while the PBE0 functional opens up a gap. For the HSE approximation, we find that the presence of a gap depends on the range separation for the non-local exchange. For the occupied part in the density of states we find that LDA, PBE, AM05, PBE0 and HSE agree with regard to the overall electronic structure. However, the hybrid functionals result in peaks shifted towards lower energy compared to LDA, PBE and AM05. The valence bands obtained using the hybrid functionals are in good agreement with experimental valence band spectra. We also find that the PBE, PBE0 and HSE approximations give similar results regarding bulk properties, such as lattice constants and bulk modulus. In addition, we have investigated the localization of the Cu d-states and its effect on the band gap in the material using the LDA + U approach. We find that a sufficiently high U indeed opens up a gap; however, this U leads to valence bands that disagree with experimental observations.

  • 68.
    Råsander, Mikael
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Bergqvist, Lars
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Delin, Anna
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, Centres, SeRC - Swedish e-Science Research Centre. Uppsala University, Sweden.
    Electronic structure and lattice dynamics in the FeSb3 skutterudite from density functional theory2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 1, article id 014303Article in journal (Refereed)
    Abstract [en]

    We have performed density functional calculations of the electronic structure and lattice dynamics of the binary skutterudite FeSb3. We find that the ground state of FeSb3 is a near half-metallic ferromagnet with T-c = 175 K. Furthermore, we find that FeSb3 is softer than CoSb3 based on an analysis of the relation of the elastic constants and the shape of the phonon density of states in the two systems, which is in agreement with experimental observation. Based on these observations we find it plausible that FeSb3 will have a lower thermal conductivity than CoSb3. Additionally, our calculations indicate that FeSb3 may be stable towards decomposition into FeSb2 and Sb. Furthermore, for ferromagnetic FeSb3 we obtain real-valued phonon frequencies and also a c44 greater than zero, indicating that the system is mechanically as well as dynamically stable.

  • 69.
    Råsander, Mikael
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics. Imperial College London, United Kingdom.
    Hugosson, Håkan W.
    Delin, Anna
    KTH, School of Engineering Sciences (SCI), Applied Physics. KTH, Centres, SeRC - Swedish e-Science Research Centre. Uppsala University, Sweden.
    Density functional study of carbon vacancies in titanium carbide2018In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 30, no 1, article id 015702Article in journal (Refereed)
    Abstract [en]

    It is well established that TiC contains carbon vacancies not only in carbon-deficient environments but also in carbon-rich environments. We have performed density functional calculations of the vacancy formation energy in TiC for C-as well as Ti-rich conditions using several different approximations to the exchange-correlation functional, and also carefully considering the nature and thermodynamics of the carbon reference state, as well as the effect of varying growth conditions. We find that the formation of carbon vacancies is clearly favorable under Ti-rich conditions, whereas it is slightly energetically unfavorable under C-rich conditions. Furthermore, we find that the relaxations of the atoms close to the vacancy site are rather long-ranged, and that these relaxations contribute significantly to the stabilization of the vacancy. Since carbon vacancies in TiC are also experimentally observed in carbon-rich environments, we conclude that kinetics may play an important role. This conclusion is consistent with the experimentally observed high activation energies and sluggish diffusion of vacancies in TiC, effectively causing a freezing in of the vacancies.

  • 70.
    Smith, Anderson D.
    et al.
    KTH, School of Information and Communication Technology (ICT), Electronics, Integrated devices and circuits.
    Elgammal, Karim
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Fan, Xuge
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Lemme, Max C.
    RWTH Aachen, Otto-Blumenthal-Str., 52074 Aachen, Germany .
    Delin, Anna
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Råsander, Mikael
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Bergqvist, Lars
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Schröder, Stephan
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems. SenseAir AB, Sweden..
    Fischer, Andreas C.
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems. Karlsruhe Institute of Technology (KIT), Germany..
    Niklaus, Frank
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Östling, Mikael
    KTH, School of Information and Communication Technology (ICT), Electronics, Integrated devices and circuits.
    Graphene-based CO2 sensing and its cross-sensitivity with humidity2017In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 7, no 36, p. 22329-22339Article in journal (Refereed)
    Abstract [en]

    We present graphene-based CO2 sensing and analyze its cross-sensitivity with humidity. In order to assess the selectivity of graphene-based gas sensing to various gases, measurements are performed in argon (Ar), nitrogen (N2), oxygen (O2), carbon dioxide (CO2), and air by selectively venting the desired gas from compressed gas bottles into an evacuated vacuum chamber. The sensors provide a direct electrical readout in response to changes in high concentrations, from these bottles, of CO2, O2, nitrogen and argon, as well as changes in humidity from venting atmospheric air. From the signal response to each gas species, the relative graphene sensitivity to each gas is extracted as a relationship between the percentage-change in graphene's resistance response to changes in vacuum chamber pressure. Although there is virtually no response from O2, N2 and Ar, there is a sizeable cross-sensitivity between CO2 and humidity occurring at high CO2 concentrations. However, under atmospheric concentrations of CO2, this cross-sensitivity effect is negligible – allowing for the use of graphene-based humidity sensing in atmospheric environments. Finally, charge density difference calculations, computed using density functional theory (DFT) are presented in order to illustrate the bonding of CO2 and water molecules on graphene and the alterations of the graphene electronic structure due to the interactions with the substrate and the molecules.

  • 71.
    Smith, Anderson D.
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Elgammal, Karim
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Fan, Xuge
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Lemme, Max
    Delin, Anna
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, Centres, SeRC - Swedish e-Science Research Centre. Uppsala Univ, Sweden.
    Niklaus, Frank
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Östling, Mikael
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Toward Effective Passivation of Graphene to Humidity Sensing Effects2016In: 2016 46TH EUROPEAN SOLID-STATE DEVICE RESEARCH CONFERENCE (ESSDERC), IEEE, 2016, p. 299-302Conference paper (Refereed)
    Abstract [en]

    Graphene has a number of remarkable properties which make it well suited for both transistor devices as well as for sensor devices such as humidity sensors. Previously, the humidity sensing properties of monolayer graphene on SiO2 substrates were examined - showing rapid response and recovery over a large humidity range. Further, the devices were fabricated in a CMOS compatible process which can be incorporated back end of the line (BEOL). We now present a way to selectively passivate graphene to suppress this humidity sensing effect. In this work, we experimentally and theoretically demonstrate effective passivation of graphene to humidity sensing - allowing for future integration with other passivated graphene devices on the same chip.

  • 72.
    Smith, Anderson D.
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Elgammal, Karim
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Niklaus, Frank
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Delin, Anna
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, School of Electrical Engineering (EES), Micro and Nanosystems. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Fischer, Andreas C.
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Vaziri, Sam
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Forsberg, Fredrik
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Råsander, Mikael
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Univ London Imperial Coll Sci Technol & Med, Dept Mat, England.
    Hugosson, Håkan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Bergqvist, Lars
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schröder, Stephan
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Kataria, Satender
    Östling, Mikael
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Lemme, Max C.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits. Univ Siegen, D-57076 Siegen, Germany.
    Resistive graphene humidity sensors with rapid and direct electrical readout2015In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 7, no 45, p. 19099-19109Article in journal (Refereed)
    Abstract [en]

    We demonstrate humidity sensing using a change of the electrical resistance of single-layer chemical vapor deposited (CVD) graphene that is placed on top of a SiO2 layer on a Si wafer. To investigate the selectivity of the sensor towards the most common constituents in air, its signal response was characterized individually for water vapor (H2O), nitrogen (N-2), oxygen (O-2), and argon (Ar). In order to assess the humidity sensing effect for a range from 1% relative humidity (RH) to 96% RH, the devices were characterized both in a vacuum chamber and in a humidity chamber at atmospheric pressure. The measured response and recovery times of the graphene humidity sensors are on the order of several hundred milliseconds. Density functional theory simulations are employed to further investigate the sensitivity of the graphene devices towards water vapor. The interaction between the electrostatic dipole moment of the water and the impurity bands in the SiO(2)d substrate leads to electrostatic doping of the graphene layer. The proposed graphene sensor provides rapid response direct electrical readout and is compatible with back end of the line (BEOL) integration on top of CMOS-based integrated circuits.

  • 73.
    Smith, Anderson D.
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Vaziri, Sam
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Östling, Mikael
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Lemme, Max C.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Strain engineering in suspended graphene devices for pressure sensor applications2012In: 2012 13th International Conference on Ultimate Integration on Silicon, ULIS 2012, IEEE , 2012, p. 21-24Conference paper (Refereed)
    Abstract [en]

    The present paper describes a device structure for controlling and measuring strain in graphene membranes. We propose to induce strain by creating a pressure difference between the inside and the outside of a cavity covered with a graphene membrane. The combination of tight-binding calculations and a COMSOL model predicts strain induced band gaps in graphene for certain conditions and provides a guideline for potential device layouts. Raman spectroscopy on fabricated devices indicates the feasibility of this approach. Ultimately, pressure-induced band structure changes could be detected electrically, suggesting an application as ultra-sensitive pressure sensors.

  • 74.
    Smith, Anderson D.
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Vaziri, Sam
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Fischer, Andreas C.
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Sterner, Mikael
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Östling, Mikael
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Lemme, Max
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Pressure sensors based on suspended graphene membranes2013In: Solid-State Electronics, ISSN 0038-1101, E-ISSN 1879-2405, Vol. 88, p. 89-94Article in journal (Refereed)
    Abstract [en]

    A novel pressure sensor based on a suspended graphene membrane is proposed. The sensing mechanism is explained based on tight binding calculations of strain-induced changes in the band structure. A CMOS compatible fabrication process is proposed and used to fabricate prototypes. Electrical measurement data demonstrates the feasibility of the approach, which has the advantage of not requiring a separate strain gauge, i.e. the strain gauge is integral part of the pressure sensor membrane. Hence, graphene membrane based pressure sensors can in principle be scaled quite aggressively in size.

  • 75.
    Smith, Anderson
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Niklaus, Frank
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Paussa, A.
    DIEGM, University of Udine, Via delle Scienze 206, 33100 Udine, Italy.
    Vaziri, Sam
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Fischer, Andreas C.
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Sterner, Mikael
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Forsberg, Fredrik
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Delin, Anna
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Esseni, D.
    DIEGM, University of Udine, Via delle Scienze 206, 33100 Udine, Italy.
    Palestri, P.
    DIEGM, University of Udine, Via delle Scienze 206, 33100 Udine, Italy.
    Östling, Mikael
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Lemme, Max
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits. University of Siegen, Hölderlinstrasse 3, 57076 Siegen, Germany.
    Electromechanical Piezoresistive Sensing in Suspended Graphene Membranes2013In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 13, no 7, p. 3237-3242Article in journal (Refereed)
    Abstract [en]

    Monolayer graphene exhibits exceptional electronic and mechanical properties, making it a very promising material for nanoelectromechanical devices. Here, we conclusively demonstrate the piezoresistive effect in graphene in a nanoelectromechanical membrane configuration that provides direct electrical readout of pressure to strain transduction. This makes it highly relevant for an important class of nanoelectromechanical system (NEMS) transducers. This demonstration is consistent with our simulations and previously reported gauge factors and simulation values. The membrane in our experiment acts as a strain gauge independent of crystallographic orientation and allows for aggressive size scalability. When compared with conventional pressure sensors, the sensors have orders of magnitude higher sensitivity per unit area.

  • 76. Smogunov, A.
    et al.
    Dal Corso, A.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Weht, R.
    Tosatti, E.
    Colossal magnetic anisotropy of monatomic free and deposited platinum nanowires2008In: Nature Nanotechnology, ISSN 1748-3387, Vol. 3, no 1, p. 22-25Article in journal (Refereed)
    Abstract [en]

    Whenever a nanosystem such as an adatom, a cluster or a nanowire spontaneously magnetizes, a crucial parameter is its magnetic anisotropy, the intrinsic preference of magnetization to lie along an easy axis(1). Anisotropy is important in nanosystems because it helps reduce the magnitude of thermal ( superparamagnetic) fluctuations, it can modify the flow of current, and it can induce new phenomena, such as the quantum tunnelling of magnetization(2). We discuss here, on the basis of density functional calculations, the novel and unconventional feature of colossal magnetic anisotropy-the strict impossibility of magnetization to rotate from the parallel to the orthogonal direction-which, owing to a quantum mechanical selection rule, the recently predicted Pt nanowire magnetism should exhibit. Model calculations suggest that the colossal magnetic anisotropy of a Pt chain should persist after weak adsorption on an inert substrate or surface step.

  • 77. Souvatzis, P.
    et al.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Eriksson, O.
    Calculation of the equation of state of fcc Au from first principles2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 73, no 5Article in journal (Refereed)
    Abstract [en]

    The electron and phonon contributions to the free energy of gold are calculated from a first-principles method. From the free energy the equation of state (EOS) along with the shock Hugoniot with initial conditions at ambient pressure and room temperature are calculated. The results show good agreement with the corresponding experimental EOS and Hugoniot up to compressions of V/V-0=0.65-i.e., up to the solid-liquid phase transition in the Hugoniot. Optimal agreement between experiment and theory is obtained after a small adjustment of the calculated equilibrium lattice constant to the experimental lattice constant. (the correction is of the order of a few percent). The current calculations are consistent with recent suggestions of an underestimation of the pressures obtained with the ruby pressure scale R-1 line.

  • 78.
    Ulman, Kanchan
    et al.
    Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scienti¯c Research Jakkur.
    Narasimhan, Shobana
    Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scienti¯c Research Jakkur.
    Rungger,, Ivan
    School of Physics, Trinity College.
    Sanvito, Stefano
    School of Physics, Trinity College.
    Odell, Anders
    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.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Spin-polarized conduction in a photoswitching dithienylethene molecule attached to Ni leadsArticle in journal (Other academic)
    Abstract [en]

    The finite-bias conduction properties of the photoswitching dithienyle the molecule attached to spin-polarized nickel electrodes are investigated. We find that the ratio of conduction between the open and closed isomers is around 60 at low bias, decreasing to 50 at a bias of 1 V. The I ¡ Vcharacteristics for the spin up and spin down channels are very similar, despite the fact that the Ni electrode density of states is very different for the two cases - Ni is known to be a so-called strong ferromagnet meaning that it is almost completely spin polarized at the Fermi level.

  • 79. Ulman, Kanchan
    et al.
    Narasimhan, Shobhana
    Delin, Anna
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Tuning spin transport properties and molecular magnetoresistance through contact geometry2014In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 140, no 4, p. 044716-Article in journal (Refereed)
    Abstract [en]

    Molecular spintronics seeks to unite the advantages of using organic molecules as nanoelectronic components, with the benefits of using spin as an additional degree of freedom. For technological applications, an important quantity is the molecular magnetoresistance. In this work, we show that this parameter is very sensitive to the contact geometry. To demonstrate this, we perform ab initio calculations, combining the non-equilibrium Green's function method with density functional theory, on a dithienylethene molecule placed between spin-polarized nickel leads of varying geometries. We find that, in general, the magnetoresistance is significantly higher when the contact is made to sharp tips than to flat surfaces. Interestingly, this holds true for both resonant and tunneling conduction regimes, i.e., when the molecule is in its "closed" and "open" conformations, respectively. We find that changing the lead geometry can increase the magnetoresistance by up to a factor of similar to 5. We also introduce a simple model that, despite requiring minimal computational time, can recapture our ab initio results for the behavior of magnetoresistance as a function of bias voltage. This model requires as its input only the density of states on the anchoring atoms, at zero bias voltage. We also find that the non-resonant conductance in the open conformation of the molecule is significantly impacted by the lead geometry. As a result, the ratio of the current in the closed and open conformations can also be tuned by varying the geometry of the leads, and increased by similar to 400%.

  • 80.
    Ulman, Kanchan
    et al.
    Jawaharlal Nehru Ctr Adv Sci Res, Theoret Sci Unit, Bangalore 560064, Karnataka, India..
    Narasimhan, Shobhana
    Jawaharlal Nehru Ctr Adv Sci Res, Theoret Sci Unit, Bangalore 560064, Karnataka, India.;Jawaharlal Nehru Ctr Adv Sci Res, Sheikh Saqr Lab, ICMS, Bangalore 560064, Karnataka, India..
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Tuning spin transport properties and molecular magnetoresistance through contact geometry (vol 140, 044716, 2014)2014In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 140, no 22, article id 229903Article in journal (Refereed)
  • 81. Vaitheeswaran, G.
    et al.
    Kanchana, V.
    Alouani, M.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ab initio calculated X-ray magnetic circular dichroism of Sr2CrReO62008In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 84, no 4Article in journal (Refereed)
    Abstract [en]

    Ab initio calculated X-ray absorption and X-ray magnetic circular dichroism (XMCD) spectra at the Cr and Re L-2,L-3 edges for the double perovskite Sr2CrReO6 are compared with experiment. The main features of the experimental spectra are reproduced by the calculations, however, the amplitudes of several structures are significantly over- or underestimated. The calculated spin and orbital magnetic moments for Cr and Re agree with those obtained by means of XMCD sum rules, except for the Cr orbital moment, illustrating that numerical uncertainties can be large for small moments. It is also argued that the Re moments are induced and not intrinsic.

  • 82. Vaitheeswaran, G.
    et al.
    Kanchana, V.
    Heathman, S.
    Idiri, M.
    Le Bihan, T.
    Svane, A.
    Delin, Anna
    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.
    Elastic constants and high-pressure structural transitions in lanthanum monochalcogenides from experiment and theory2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 18Article in journal (Refereed)
    Abstract [en]

    The high-pressure structural behavior of lanthanum monochalcogenides is investigated by theory and experiment. Theory comprises density-functional calculations of LaS, LaSe, and LaTe with the general gradient approximation for exchange and correlation effects, as implemented within the full-potential linear muffin-tin orbital method. The experimental studies consist of high-pressure angle dispersive x-ray-diffraction investigations of LaS and LaSe up to a maximum pressure of 41 GPa. A structural phase transition from the NaCl-type to CsCl-type crystal structure is found to occur in all cases. The experimental transition pressures are 27-28 and 19 GPa for LaS and LaSe, respectively, while the calculated transition pressures are 29, 21, and 10 GPa for LaS, LaSe, and LaTe, respectively. The calculated ground-state properties such as equilibrium lattice constant, bulk modulus and its pressure derivative, and Debye temperatures are in good agreement with experimental results. Elastic constants are predicted from the calculations.

  • 83. Vaitheeswaran, G.
    et al.
    Kanchana, V.
    Kumar, Ravhi S.
    Cornelius, A. L.
    Nicol, M. F.
    Svane, A.
    Delin, Anna
    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.
    High-pressure structural, elastic, and electronic properties of the scintillator host material KMgF32007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 1Article in journal (Refereed)
    Abstract [en]

    The high-pressure structural behavior of the fluoroperovskite KMgF3 is investigated by theory and experiment. Density functional calculations were performed within the local density approximation and the generalized gradient approximation for exchange and correlation effects, as implemented within the full-potential linear muffin-tin orbital method. In situ high-pressure powder x-ray diffraction experiments were performed up to a maximum pressure of 40 GPa using synchrotron radiation. We find that the cubic Pm (3) over barm crystal symmetry persists throughout the pressure range studied. The calculated ground state properties-the equilibrium lattice constant, bulk modulus, and elastic constants-are in good agreement with experimental results. By analyzing the ratio between the bulk and shear moduli, we conclude that KMgF3 is brittle in nature. Under ambient conditions, KMgF3 is found to be an indirect gap insulator, with the gap increasing under pressure.

  • 84. Vaitheeswaran, G.
    et al.
    Kanchana, V.
    Svane, A.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Elastic properties of MgCNi3 - a superconducting perovskite2007In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 19, no 32Article in journal (Refereed)
    Abstract [en]

    The cohesive and elastic properties of the non-oxide perovskite type superconductor MgCNi3 are calculated using the full-potential linear muffin-tin orbital method with the local density approximation as well as the generalized gradient approximation for exchange and correlation. The calculated equation of state and ground state properties ( equilibrium lattice constant, bulk modulus and its pressure derivative) agree well with recent experiments. From the elastic constants the Young's modulus, shear modulus, Poisson's ratio, sound velocities and Debye temperature are obtained. By analysing the ratio between bulk modulus and shear modulus we conclude that MgCNi3 is intermediate between brittle and ductile in nature.

  • 85.
    Vaitheeswaran, Ganapathy
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Kanchana, Venkatakrishnan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Electronic structure of the ferromagnetic double-perovskites Sr2CrReO6, Sr2CrWO6, and Ba2FeReO62006In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 29, no 1, p. 50-53Article in journal (Refereed)
    Abstract [en]

    We have studied the electronic structure of the ferromagnetic double perovskites Sr2CrReO6, Sr2CrWO6 and Ba2FeReO6 by means of a full-potential linear muffin-tin orbital density-functional method. Our scalar-relativistic calculations predict these compounds to be half-metallic with a total magnetic moment of 1, 2, and 3 νB respectively. However, when the spin-orbit coupling is included, the 5d transition Re and W ions exhibit substantial unquenched orbital magnetic moments, resulting in a significant increase of the total magnetic moment. The half-metallic gap turns into a pseudo-gap in Sr 2CrReO6 and Ba2FeReO6 when the spin-orbit coupling is included whereas Sr2CrWO6 remains half-metallic even with spin-orbit coupling. The calculated spin and orbital magnetic moments agrees well with the recent experimental XMCD measurements.

  • 86.
    Vaitheeswaran, Ganapathy
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Kanchana, Venkatakrishnan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Pseudo-half-metallicity in the double perovskite Sr2CrReO6 from density-functional calculations2005In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 86, no 3Article in journal (Refereed)
    Abstract [en]

    The electronic structure of the spintronic material Sr2CrReO6 is studied by means of full-potential linear muffin-tin orbital method. Scalar relativistic calculations predict Sr2CrReO6 to be half-metallic with a magnetic moment of 1 muB. When spin-orbit coupling is included, the half-metallic gap closes into a pseudo-gap, and an unquenched rhenium orbital moment appears, resulting in a significant increase of the total magnetic moment to 1.28 AB. This moment is significantly larger than the experimental moment of 0.9 mu(B). A possible explanation of this discrepancy is that the anti-site disorder in Sr2CrReO6 is significantly larger than hitherto assumed.

  • 87. von Malottki, S.
    et al.
    Dupe, B.
    Bessarab, P. F.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Department of Physics and Astronomy, Materials Theory Division, Uppsala University, Box 516, SE-75120, Uppsala, Sweden.
    Heinze, S.
    Enhanced skyrmion stability due to exchange frustration2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 12299Article in journal (Refereed)
    Abstract [en]

    Skyrmions are localized, topologically non-trivial spin structures which have raised high hopes for future spintronic applications. A key issue is skyrmion stability with respect to annihilation into the ferromagnetic state. Energy barriers for this collapse have been calculated taking only nearest neighbor exchange interactions into account. Here, we demonstrate that exchange frustration can greatly enhance skyrmion stability. We focus on the prototypical film system Pd/Fe/Ir(111) and use an atomistic spin model parametrized from first-principles calculations. We show that energy barriers and critical fields of skyrmion collapse as well as skyrmion lifetimes are drastically enhanced due to frustrated exchange and that antiskyrmions are metastable. In contrast an effective nearest-neighbor exchange model can only account for equilibrium properties of skyrmions such as their magnetic field dependent profile or the zero temperature phase diagram. Our work shows that frustration of long range exchange interactions -a typical feature in itinerant electron magnets -is a route towards enhanced skyrmion stability even in systems with a ferromagnetic ground state.

  • 88. von Malottki, S.
    et al.
    Dupe, B.
    Bessarab, P. F.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Heinze, S.
    Enhanced skyrmion stability due to exchange frustration (vol 7, 12299, 2017)2019In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 9, article id 8158Article in journal (Refereed)
  • 89.
    von Malottki, Stephan
    et al.
    Univ Kiel, Inst Theoret Phys & Astrophys, Leibnizstr 15, D-24098 Kiel, Germany..
    Bessarab, Pavel F.
    Univ Kiel, Inst Theoret Phys & Astrophys, Leibnizstr 15, D-24098 Kiel, Germany.;ITMO Univ, St Petersburg 197101, Russia..
    Haldar, Soumyajyoti
    Univ Kiel, Inst Theoret Phys & Astrophys, Leibnizstr 15, D-24098 Kiel, Germany..
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. KTH, Centres, SeRC - Swedish e-Science Research Centre. Uppsala Univ, Dept Phys & Astron, Uppsala, Sweden.
    Heinze, Stefan
    Univ Kiel, Inst Theoret Phys & Astrophys, Leibnizstr 15, D-24098 Kiel, Germany..
    Skyrmion lifetime in ultrathin films2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 6, article id 060409Article in journal (Refereed)
    Abstract [en]

    We show that thermal stability of skyrmions due to entropic effects can be strongly affected by external control parameters such as magnetic field and interface composition. The lifetimes of isolated skyrmions in atomic Pd/Fe bilayers on Ir(111) and on Rh(111) are calculated in the framework of harmonic transition state theory based on an atomistic spin model parametrized from density functional theory. Depending on the system the attempt frequency for skyrmion collapse can change by up to nine orders of magnitude with the strength of the applied magnetic field. We demonstrate that this effect is due to a drastic change of entropy with skyrmion radius which opens a route toward stabilizing sub-10-nm skyrmions at room temperature.

  • 90. Wierzbowska, M.
    et al.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Tosatti, E.
    Effect of electron correlations in Pd, Ni, and Co monowires2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 3Article in journal (Refereed)
    Abstract [en]

    We investigated the effect of mean-field electron correlations on the band electronic structure of Co, Ni, and Pd ultrathin monatomic nanowires, at the breaking point, by means of density-functional calculations in the self-interaction corrected (SIC) local density approximation (LDA) approach and alternatively by the LDA+U scheme. We find that adding static electron correlations increases the magnetic moment in Pd monowires, but has negligible effect on the magnetic moment in Co and Ni. Furthermore, the number of d-dominated conductance channels decreases somewhat compared to the LDA value, but the number of s-dominated channels is unaffected, and remains equal to one per spin.

  • 91. Wills, J. M.
    et al.
    Eriksson, O.
    Delin, Anna
    Andersson, P. H.
    Joyce, J. J.
    Durakiewicz, T.
    Butterfield, M. T.
    Arko, A. J.
    Moore, D. P.
    Morales, L. A.
    A novel electronic configuration of the 5f states in delta-plutonium as revealed by the photo-electron spectra2004In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 135, no 2-3, p. 163-166Article in journal (Refereed)
    Abstract [en]

    We present a theoretical model, the mixed-level model, aiming at describing metals with very complex, strongly correlated, electronic structures. As a demonstration, it is applied to the electronic structure of delta-Pu. The model reproduces the electronic-structure related properties of this complex metal; in particular, the theory is capable of reproducing the valence band photoemission spectrum of delta-Pu. We also report new experimental photoemission spectra at several photon energies. Taken together, our results provide strong evidence that the electronic structure of delta-Pu involves a 5f shell with four electrons in a localized multiplet hybridizing with valence states, and approximately one 5f electron forming a completely delocalized band state.

  • 92.
    Wills, John M.
    et al.
    LANL, Los Alamos, USA.
    Eriksson, Olle
    Uppsala University.
    Andersson, Per
    FOI.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Grechnyev, Oleksiy
    Natl Acad Sci Ukraine, BI Verkin Inst Low Temp Phys & Engn, UA-61103 Kharkov, Ukraine.
    Alouani, Mebarek
    UdS CNRS, UMR 7504, Inst Phys & Chim Mat Strasbourg, F-67034 Strasbourg 2, France.
    Full-Potential Electronic Structure Method: Energy and Force Calculations with Density Functional and Dynamical Mean Field Theory2010Book (Other academic)
  • 93. Yin, Yuli
    et al.
    Pan, Fan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Ahlberg, Martina
    Ranjbar, Mojtaba
    Duerrenfeld, Philipp
    Houshang, Afshin
    Haidar, Mohammad
    Bergqvist, Lars
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Zhai, Ya
    Dumas, Randy K.
    Delin, Anna
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Åkerman, Johan
    University of Gothenburg, Sweden.
    Tunable permalloy-based films for magnonic devices2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 2, article id 024427Article in journal (Refereed)
    Abstract [en]

    Using both broadband ferromagnetic resonance (FMR) spectroscopy and ab initio calculations, we study the magnetodynamic properties of permalloy (Py, Ni80Fe20) and Py100-x M-x films with M as platinum (Pt), gold (Au), or silver (Ag). From the uniform FMR mode, we extract the saturation magnetization (M-S), damping (alpha), and inhomogeneous broadening (Delta H-0); from the first perpendicular standing spin-wave (PSSW) mode, we extract the exchange stiffness (A). M-S and A are found to decrease with increasing alloying, most strongly for Au and less so for Pt. On the other hand, alpha increases rapidly with both Pt and Au content, while being virtually independent of Ag content. The physical origins of the observed trends in alpha, M-S, and A are analyzed and explained using density functional theory calculations in the coherent potential approximation. The calculated trends quantitatively agree with the experimental observations. The drastically different impacts of Pt, Au, and Ag on the various fundamental magnetodynamic properties will allow for significant design freedom, where different properties can be varied independently of others through careful combinations of the Pt, Au, and Ag contents of Py100-x M-x films. By empirical approximations of each property's concentration dependence, we can dial in any desired combination of magnetodynamic properties within this parameter space. As a proof-of-principle demonstration we design a set of Py100-x-yPtxAgy films, where the saturation magnetization stays constant throughout the set and the damping can be tuned by a factor of 4.

12 51 - 93 of 93
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