arXiv subjects

Nicola Marzari

Publications and source records attributed to Nicola Marzari.

261 records · Page 15Linked to original sources

arXiv2003

Ab-initio transport properties of nanostructures from maximally-localized Wannier functions

We present a comprehensive first-principles study of the ballistic transport properties of low dimensional nanostructures such as linear chains of atoms (Al, C) and carbon nanotubes in presence of defects. A novel approach is introduced where quantum conductance is computed from the combination of accurate plane-wave electronic structure calculations, the evaluation of the corresponding maximally-localized Wannier functions, and the calculation of transport properties by a real-space Green's function method based on the Landauer formalism. This approach is computationally very efficient, can be straightforwardly implemented as a post-processing step in a standard electronic-structure calculation, and allows to directly link the electronic transport properties of a device to the nature of the chemical bonds, providing insight onto the mechanisms that govern electron flow at the nanoscale.

cond-mat.mes-hall↗

arXiv2002

Maximally-localized Wannier Functions in Antiferromagnetic MnO within the FLAPW Formalism

We have calculated the maximally-localized Wannier functions of MnO in its antiferromagnetic (AFM) rhombohedral unit cell, which contains two formula units. Electron Bloch functions are obtained with the linearized augmented plane-wave method within both the LSD and the LSD+U schemes. The thirteen uppermost occupied spin-up bands correspond in a pure ionic scheme to the five Mn 3d orbitals at the Mn_1 (spin-up) site, and the four O 2s/2p orbitals at each of the O_1 and O_2 sites. Maximal localization identifies uniquely four Wannier functions for each O, which are trigonally-distorted sp^3-like orbitals. They display a weak covalent bonding between O 2s/2p states and minority-spin d states of Mn_2, which is absent in a fully ionic picture. This bonding is the fingerprint of the interaction responsible for the AFM ordering, and its strength depends on the one-electron scheme being used. The five Mn Wannier functions are centered on the Mn_1 site, and are atomic orbitals modified by the crystal field. They are not uniquely defined by the criterion of maximal localization and we choose them as the linear combinations which diagonalize the r^2 operator, so that they display the D_3d symmetry of the Mn_1 site.

cond-mat.mtrl-sci↗

arXiv2001

Maximally-localized Wannier functions for entangled energy bands

We present a method for obtaining well-localized Wannier-like functions (WFs) for energy bands that are attached to or mixed with other bands. The present scheme removes the limitation of the usual maximally-localized WFs method (N. Marzari and D. Vanderbilt, Phys. Rev. B 56, 12847 (1997)) that the bands of interest should form an isolated group, separated by gaps from higher and lower bands everywhere in the Brillouin zone. An energy window encompassing N bands of interest is specified by the user, and the algorithm then proceeds to disentangle these from the remaining bands inside the window by filtering out an optimally connected N-dimensional subspace. This is achieved by minimizing a functional that measures the subspace dispersion across the Brillouin zone. The maximally-localized WFs for the optimal subspace are then obtained via the algorithm of Marzari and Vanderbilt. The method, which functions as a postprocessing step using the output of conventional electronic-structure codes, is applied to the s and d bands of copper, and to the valence and low-lying conduction bands of silicon. For the low-lying nearly-free-electron bands of copper we find WFs which are centered at the tetrahedral interstitial sites, suggesting an alternative tight-binding parametrization.

cond-mat.mtrl-sci↗

arXiv2000

Wannier-function description of the electronic polarization and infrared absorption of high-pressure hydrogen

We have constructed maximally-localized Wannier functions for prototype structures of solid molecular hydrogen under pressure, starting from LDA and tight-binding Bloch wave functions. Each occupied Wannier function can be associated with two paired protons, defining a ``Wannier molecule''. The sum of the dipole moments of these ``molecules'' always gives the correct macroscopic polarization, even under strong compression, when the overlap between nearby Wannier functions becomes significant. We find that at megabar pressures the contributions to the dipoles arising from the overlapping tails of the Wannier functions is very large. The strong vibron infrared absorption experimentally observed in phase III, above ~ 150 GPa, is analyzed in terms of the vibron-induced fluctuations of the Wannier dipoles. We decompose these fluctuations into ``static'' and ``dynamical'' contributions, and find that at such high densities the latter term, which increases much more steeply with pressure, is dominant.

cond-mat.mtrl-sci↗

arXiv1999

Thermal Contraction and Disordering of the Al(110) Surface

Al(110) has been studied for temperatures up to 900 K via ensemble density-functional molecular dynamics. The strong anharmonicity displayed by this surface results in a negative coefficient of thermal expansion, where the first interlayer distance decreases with increasing temperature. Very shallow channels of oscillation for the second-layer atoms in the direction perpendicular to the surface support this anomalous contraction, and provide a novel mechanism for the formation of adatom-vacancy pairs, preliminary to the disordering and premelting transition. Such characteristic behavior originates in the free-electron-gas bonding at a loosely packed surface.

cond-mat.mtrl-sci↗

arXiv1998

Maximally-localized Wannier functions for disordered systems: application to amorphous silicon

We use the maximally-localized Wannier function method to study bonding properties in amorphous silicon. This study represents, to our knowledge, the first application of the Wannier-function analysis to a disordered system. Our results show that, in the presence of disorder, this method is extremely helpful in providing an unambiguous picture of the bond distribution. In particular, defect configurations can be studied and characterized with a novel degree of accuracy that was not available before.

cond-mat.mtrl-sci↗

arXiv1998

Maximally-localized Wannier functions in perovskites: Cubic BaTiO3

The electronic ground state of a periodic crystalline solid is usually described in terms of extended Bloch orbitals; localized Wannier functions can alternatively be used. These two representations are connected by families of unitary transformations, carrying a large degree of arbitrariness. We have developed a localization algorithm that allows one to iteratively transform the extended Bloch orbitals of a first-principles calculation into a unique set of maximally localized Wannier functions. We apply this formalism here to the case of cubic BaTiO3. The purpose is twofold. First, a localized-orbital picture allows a meaningful band-by-band decomposition of the whole Bloch band complex. In perovskites, these Wannier functions are centered on the atomic sites and display clearly a s, p, d, or hybrid character. Second, since the centers of the Wannier functions map the polarization field onto localized point charges, the ground state dielectric properties become readily available. We study the Born effective charges of the paraelectric phase of BaTiO3. We are able to identify not only the contributions that come from a given group of bands, but also the individual contributions from the ``atomic'' Wannier functions that comprise each of these groups.

cond-mat.mtrl-sci↗

arXiv1997

Maximally-localized generalized Wannier functions for composite energy bands

We discuss a method for determining the optimally-localized set of generalized Wannier functions associated with a set of Bloch bands in a crystalline solid. By ``generalized Wannier functions'' we mean a set of localized orthonormal orbitals spanning the same space as the specified set of Bloch bands. Although we minimize a functional that represents the total spread sum_n [ _n - _n^2 ] of the Wannier functions in real space, our method proceeds directly from the Bloch functions as represented on a mesh of k-points, and carries out the minimization in a space of unitary matrices U_mn^k describing the rotation among the Bloch bands at each k-point. The method is thus suitable for use in connection with conventional electronic-structure codes. The procedure also returns the total electric polarization as well as the location of each Wannier center. Sample results for Si, GaAs, molecular C2H4, and LiCl will be presented.

cond-mat.mtrl-sci↗

arXiv1997

Ensemble density-functional theory for ab-initio molecular dynamics of metals and finite-temperature insulators

A new method is presented for performing first-principles molecular-dynamics simulations of systems with variable occupancies. We adopt a matrix representation for the one-particle statistical operator Gamma, to introduce a ``projected'' free energy functional G that depends on the Kohn-Sham orbitals only and that is invariant under their unitary transformations. The Liouville equation [ Gamma , H ] = 0 is always satisfied, guaranteeing a very efficient and stable variational minimization algorithm that can be extended to non-conventional entropic formulations or fictitious thermal distributions.

cond-mat.mtrl-sci↗