Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H6: The Response of Extended Systems to Electrical and Magnetic Fields: Novel, Theoretical, and Computational Methods |
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Sponsoring Units: DCOMP Chair: Nicola Marzari, Massachusetts Institute of Technology Room: Baltimore Convention Center 310 |
Tuesday, March 14, 2006 11:15AM - 11:51AM |
H6.00001: Orbital Magnetization in Periodic Insulators Invited Speaker: Recent years have seen a surge of interest in issues of charge and spin transport in magnetic materials and nanostructures, including anomalous Hall and spin Hall effects. In this context, it is quite surprising that the theory of orbital magnetization has remained in a condition similar to that of the polarization before the early 1990s, when the problem of computing finite polarization changes was solved by the introduction of the Berry-phase theory.\footnote{R.\,D.~King-Smith and D.~Vanderbilt, Phys. Rev. B {\bf 47}, 1651 (1993).} The essential difficulty, that the matrix elements of the position operator $\bf r$ are not well-defined in the Bloch representation, could be overcome by reformulating the problem in the Wannier representation. In order to derive an analogue theory for the orbital magnetization, we again work in the Wannier representation and assume a periodic insulator with broken time-reversal symmetry, vanishing (or commensurate) magnetic field, and zero Chern numbers. We show that by replacing the dipole operator $\bf r$ with the circulation operator ${\bf r}\times {\bf v}$, only one contribution to the magnetization is found, i.e., the magnetization associated with the internal circulation of bulk-like Wannier functions. The missing contribution arises from net currents carried by the Wannier functions at the boundary of the sample. We prove that both contributions can be expressed as bulk properties in terms of Bloch functions in a gauge-invariant way.\footnote{T. Thonhauser, Davide Ceresoli, David Vanderbilt, and R. Resta, Phys.\ Rev.\ Lett. {\bf 95}, 137\,205 (2005).} Our expression for the orbital magnetization is verified by comparing numerical tight-binding calculations for finite and periodic samples. Possible extensions to metals or insulators with non-zero Chern numbers will also be discussed. [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:27PM |
H6.00002: Spin Transport in Metals and Semiconductors Invited Speaker: I will discuss a number of recent issues connected with spin-dependent transport in metals, semiconductors, and molecules with an emphasis on circumstances in which sophisticated electronic structure calculations can help achieve qualitative understanding. The specific topics that I will mention the anomalous Hall effect in ferromagnetic metals and semiconductors, the spin Hall effect, giant magnetoresistance and spin-torques in circuits containing antiferromagnetic metals, and current induced magnetization dynamics in transport through magnetic molecules. [Preview Abstract] |
Tuesday, March 14, 2006 12:27PM - 1:03PM |
H6.00003: Ab Initio Calculations in a Uniform Magnetic Field Using Periodic Supercells Invited Speaker: |
Tuesday, March 14, 2006 1:03PM - 1:39PM |
H6.00004: Linear and Non-Linear Dielectric Response of Periodic Systems from Quantum Monte Carlo Invited Speaker: We present a novel approach that allows to calculate the dielectric response of periodic systems in the quantum Monte Carlo formalism. We employ a many-body generalization for the electric enthalpy functional, where the coupling with the field is expressed via the Berry-phase formulation for the macroscopic polarization. A self-consistent local Hamiltonian then determines the ground-state wavefunction, allowing for accurate diffusion quantum Monte Carlo calculations where the polarization's fixed point is estimated from the average on an iterative sequence. The polarization is sampled through forward-walking. This approach has been validated for the case of the polarizability of an isolated hydrogen atom, and then applied to a periodic system. We then calculate the linear susceptibility and second-order hyper-susceptibility of molecular-hydrogen chains whith different bond-length alternations, and assess the quality of nodal surfaces derived from density-functional theory or from Hartree-Fock. The results found are in excellent agreement with the best estimates obtained from the extrapolation of quantum-chemistry calculations.\\ P. Umari, A.J. Williamson, G. Galli, and N. Marzari\\ Phys.\ Rev.\ Lett.\ {\bf 95}, 207602 (2005). [Preview Abstract] |
Tuesday, March 14, 2006 1:39PM - 2:15PM |
H6.00005: Non-linear response of infinite periodic solids to homogenous electric fields and collective atomic displacements Invited Speaker: The non-linear response of infinite periodic solids to homogenous electric fields and cooperative atomic displacements will be discussed in the framework of density functional perturbation theory. The approach is based on the “2n + 1” theorem applied to an electric field dependent energy functional. We will focus on the non-linear optical susceptibilities, Raman scattering efficiencies and electrooptic coefficients. Different formulations of third-order energy derivatives will be examined and their convergence with respect to the k-point sampling will be discussed. The method will be applied to conventional semiconductors and to ferroelectric oxides. In the latter case, we will also describe how the first- principles results can be combined to an effective Hamiltonian approach in order to provide access to the temperature dependence of the optical properties. This work was done in collabration with M. Veithen and X. Gonze and was supported by the VolkwagenStiftung, FNRS-Belgium and the FAME-NoE. [Preview Abstract] |
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