1,721,028 research outputs found
Mapping topological order in coordinate space
No full textThe organization of the electrons in the ground state is classified by means of topological invariants, defined as global properties of the wave function. Here we address the Chern number of a two-dimensional insulator and we show that the corresponding topological order can be mapped by means of a "topological marker," defined in r space, and which may vary in different regions of the same sample. Notably, this applies equally well to periodic and open boundary conditions. Simulations over a model Hamiltonian validate our theory
Many-body effects on polarization and dynamical charges in a partly covalent polar insulator
No full textAb initio calculation of the macroscopic dielectric constant in silicon
No full textWe perform a first-principles calculation of the static dielectric constant of Si in the framework of density-functional theory. The only essential approximation used in this work is the local-density approximation (LDA): norm-conserving pseudopotentials and large plane-wave basis sets are used, numerical roundoff and convergence errors are kept below 1%. The present calculation gives for the first time the ‘‘exact’’ value of the macroscopic dielectric constant at the LDA level. The theoretical value of ε∞ is 12% higher than experiment
Ab initio calculation of the low-frequency Raman cross section in silicon
No full textThe macroscopic polarizability of silicon is calculated from first principles as a function of the lattice distortion induced by a zone-center optical phonon. The electronic response to the electric field is dealt with by dielectric matrices, and the lattice distortion is treated by frozen-phonon techniques. Our results compare quite well with the most recent measurements of the one-phonon Raman cross section
Orbital magnetization in insulators: Bulk versus surface
No full textThe orbital magnetic moment of a finite piece of matter is expressed in terms of the one-body density matrix as a simple trace. We address a macroscopic system, insulating in the bulk, and we show that its orbital moment is the sum of a bulk term and a surface term, both extensive. The latter only occurs when the transverse conductivity is nonzero and it is due to conducting surface states. Simulations on a model Hamiltonian validate our theory
Orbital Magnetization as a Local Property
No full textThe modern expressions for polarization P and orbital magnetization M are k-space integrals. But a genuine bulk property should also be expressible in r space, as an unambiguous function of the ground-state density matrix, "nearsighted'' in insulators, independently of the boundary conditions-either periodic or open. While P-owing to its "quantum'' indeterminacy-is not a bulk property in this sense, M is. We provide its r-space expression for any insulator, even with a nonzero Chern invariant. Simulations on a model Hamiltonian validate our theory. DOI: 10.1103/PhysRevLett.110.08720
Density-functional theory of macroscopic stress - gradient -corrected calculations for crystalline Se
No full textWe generalize the Nielsen-Martin stress theorem beyond the local-density approximation (LDA) and present an alternative derivation of the whole theorem. We show that the exchange-correlation stress becomes anisotropic in the most general case: its explicit form is given within a gradient-corrected (GC) scheme. As a test implementation, we use the generalized theorem to achieve fast structural optimization in crystalline Se. In this material LDA predicts a rather poor structure. Our GC calculation is in much better agreement with the experiment
Electron localization in the insulating state
No full textThe insulating state of matter is characterized by the excitation spectrum, but also by qualitative features of the electronic ground state. The insulating ground wave function in fact (i) sustains macroscopic polarization, and (ii) is localized. We give a sharp definition of the latter concept and we show how the two basic features stem from essentially the same formalism. Our approach to localization is exemplified by means of a two-band Hubbard model in one dimension. In the noninteracting limit, the wave function localization is measured by the spread of the Wannier orbitals
Band-offset in strained Si/Ge superlattices: the role of absolute deformation potentials
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