Search results for "electrons"
showing 10 items of 1325 documents
Application of the Density Matrix Renormalization Group in momentum space
2001
We investigate the application of the Density Matrix Renormalization Group (DMRG) to the Hubbard model in momentum-space. We treat the one-dimensional models with dispersion relations corresponding to nearest-neighbor hopping and $1/r$ hopping and the two-dimensional model with isotropic nearest-neighbor hopping. By comparing with the exact solutions for both one-dimensional models and with exact diagonalization in two dimensions, we first investigate the convergence of the ground-state energy. We find variational convergence of the energy with the number of states kept for all models and parameter sets. In contrast to the real-space algorithm, the accuracy becomes rapidly worse with increa…
Green functions for nearest- and next-nearest-neighbor hopping on the Bethe lattice
2005
We calculate the local Green function for a quantum-mechanical particle with hopping between nearest and next-nearest neighbors on the Bethe lattice, where the on-site energies may alternate on sublattices. For infinite connectivity the renormalized perturbation expansion is carried out by counting all non-self-intersecting paths, leading to an implicit equation for the local Green function. By integrating out branches of the Bethe lattice the same equation is obtained from a path integral approach for the partition function. This also provides the local Green function for finite connectivity. Finally, a recently developed topological approach is extended to derive an operator identity whic…
A general approach for the calculation of the energy levels and the inelastic neutron scattering cross-section of highly nuclear magnetic clusters
1997
Abstract We develop here a general approach to calculate in an efficient way the spin levels as well as the spin eigenfunctions and the INS intensities of clusters formed by large numbers of exchange-coupled magnetic metal ions. The approach is based on the successive use of the irreducible tensor operator techniques and takes into account all kinds of magnetic exchange interactions between the metal ions. The potentialities of this approach are illustrated from an example comprising nine exchange-coupled Ni (II) ions.
The generalized Kadanoff-Baym ansatz with initial correlations
2018
Within the non-equilibrium Green's function (NEGF) formalism, the Generalized Kadanoff-Baym Ansatz (GKBA) has stood out as a computationally cheap method to investigate the dynamics of interacting quantum systems driven out of equilibrium. Current implementations of the NEGF--GKBA, however, suffer from a drawback: real-time simulations require {\em noncorrelated} states as initial states. Consequently, initial correlations must be built up through an adiabatic switching of the interaction before turning on any external field, a procedure that can be numerically highly expensive. In this work, we extend the NEGF--GKBA to allow for {\em correlated} states as initial states. Our scheme makes i…
Correlation in the transition-metal-based Heusler compoundsCo2MnSiandCo2FeSi
2006
Half-metallic ferromagnets, such as the Heusler compounds with formula ${X}_{2}YZ$, are expected to show an integer value for the spin magnetic moment. In contrast to experiments, calculations give noninteger values in certain cases where the compounds are based on $X=\mathrm{Co}$. In order to explain deviations of the magnetic moment calculated for such compounds, the dependence of the electronic structure on the lattice parameter was studied theoretically. In the local density approximation (LDA), the minimum total energy of ${\mathrm{Co}}_{2}\mathrm{FeSi}$ is found for the experimental lattice parameter, but the calculated magnetic moment is approximately 12% too low. In addition, half-m…
Double Exchange in Orbitally Degenerate Mixed Valence Clusters: Magnetic Anisotropy, Vibronic Effects
2001
In this paper we consider the vibronic problem of the double exchange in mixed-valence dimers containing transition metal ions in orbitally degenerate ground states. The vibronic model includes interaction with the breathing local modes (Piepho-Krausz-Schatz-PKS) as well as the modulation of metal-metal distances as suggested by Piepho. The double exchange in orbitally degenerate systems is shown to produce strong magnetic anisotropy of orbital nature. PKS interaction is expected to suppress the magnetic anisotropy of the system, while the intercenter vibrations tend to enhance it. The roles of spin-orbit coupling and temperature are revealed for the systems with different geometries.
Dynamics of topological spin structures
2015
Topological spin structures that emerge from the Dzyaloshinskii-Moriya interaction (DMI), such as chiral domain walls and skyrmions have become the focus of intense investigations due to exciting physics and possible applications [1].
Weak itinerant ferromagnetism and electronic and crystal structures of alkali-metal iron antimonides: NaFe4Sb12andKFe4Sb12
2004
The synthesis, chemical, structural, and magnetic properties of alkali-metal compounds with filled-skutterudite structure, $\mathrm{Na}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ and $\mathrm{K}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$, are described. X-ray and neutron diffraction and elemental analysis established the crystal structure without defects and disorder on the cation site. The temperature and pressure dependence of the cubic unit cell of $\mathrm{Na}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ and the displacement parameter of Na are investigated. The electronic structure is calculated by density functional methods (LMTO, FPLO). Quantum chemical calculations (electron localization function) reveal …
Temperature-Dependent Change of the Electronic Structure in the Kondo Lattice System $YbRh_{2}Si_{2}$
2021
Seminar, Deutschland; Journal of physics / Condensed matter 00(00), 1-20 (2021). doi:10.1088/1361-648X/abe479
Roles of chiral renormalization on magnetization dynamics in chiral magnets
2018
In metallic ferromagnets, the interaction between local magnetic moments and conduction electrons renormalizes parameters of the Landau-Lifshitz-Gilbert equation such as the gyromagnetic ratio and the Gilbert damping, and makes them dependent on the magnetic configurations. Although the effects of the renormalization for nonchiral ferromagnets are usually minor and hardly detectable, we show that the renormalization does play a crucial role for chiral magnets. Here the renormalization is chiral and as such we predict experimentally identifiable effects on the phenomenology of magnetization dynamics. In particular, our theory for the self-consistent magnetization dynamics of chiral magnets a…