arXiv subjects

Roberto Car

Publications and source records attributed to Roberto Car.

At least 109 records · Page 6Linked to original sources

arXiv2009

Tunneling and delocalization in hydrogen bonded systems: a study in position and momentum space

Novel experimental and computational studies have uncovered the proton momentum distribution in hydrogen bonded systems. In this work, we utilize recently developed open path integral Car-Parrinello molecular dynamics methodology in order to study the momentum distribution in phases of high pressure ice. Some of these phases exhibit symmetric hydrogen bonds and quantum tunneling. We find that the symmetric hydrogen bonded phase possesses a narrowed momentum distribution as compared with a covalently bonded phase, in agreement with recent experimental findings. The signatures of tunneling that we observe are a narrowed distribution in the low-to-intermediate momentum region, with a tail that extends to match the result of the covalently bonded state. The transition to tunneling behavior shows similarity to features observed in recent experiments performed on confined water. We corroborate our ice simulations with a study of a particle in a model one-dimensional double well potential that mimics some of the effects observed in bulk simulations. The temperature dependence of the momentum distribution in the one-dimensional model allows for the differentiation between ground state and mixed state tunneling effects.

cond-mat.stat-mech↗

arXiv2009

Theory of tunneling transport in periodic chains

We present an extended discussion of a recently proposed theoretical approach for off-resonance tunneling transport. The proofs and the arguments are explained at length and simple analogies and illustrations are used where possible. The result is an analytic formula for the asymptotic tunneling conductance which involves the overlap of three well defined physical quantities. We argue that the formula can be used to gain fresh insight into the tunneling transport characteristics of various systems. The formalism is applied here to molecular devices consisting of planar phenyl chains connected to gold electrodes via amine linkers.

cond-mat.mes-hall↗

arXiv2008

Multipole Representation of the Fermi Operator with Application to the Electronic Structure Analysis of Metallic Systems

We propose a multipole representation of the Fermi-Dirac function and the Fermi operator, and use this representation to develop algorithms for electronic structure analysis of metallic systems. The new algorithm is quite simple and efficient. Its computational cost scales logarithmically with $βΔ\eps$ where $β$ is the inverse temperature, and $Δ\eps$ is the width of the spectrum of the discretized Hamiltonian matrix.

cond-mat.mtrl-sci↗

arXiv2008

Charge Transfer in Partition Theory

The recently proposed Partition Theory (PT) [J.Phys.Chem.A 111, 2229 (2007)] is illustrated on a simple one-dimensional model of a heteronuclear diatomic molecule. It is shown that a sharp definition for the charge of molecular fragments emerges from PT, and that the ensuing population analysis can be used to study how charge redistributes during dissociation and the implications of that redistribution for the dipole moment. Interpreting small differences between the isolated parts' ionization potentials as due to environmental inhomogeneities, we gain insight into how electron localization takes place in H2+ as the molecule dissociates. Furthermore, by studying the preservation of the shapes of the parts as different parameters of the model are varied, we address the issue of transferability of the parts. We find good transferability within the chemically meaningful parameter regime, raising hopes that PT will prove useful in chemical applications.

cond-mat.other↗

arXiv2008

Order-N implementation of exact exchange in extended systems

Exact (Hartree Fock) exchange is needed to overcome some of the limitations of local and semilocal approximations of density functional theory (DFT). So far, however, computational cost has limited the use of exact exchange in plane wave calculations for extended systems. We show that this difficulty can be overcome by performing a unitary transformation from Bloch to Maximally Localized Wannier functions in combination with an efficient technique to compute real space Coulomb integrals. The resulting scheme scales linearly with system size. We validate the scheme with representative applications.

cond-mat.str-el↗

arXiv2008

Role of dipolar correlations in the IR spectra of water and ice

We report simulated infrared spectra of deuterated water and ice using Car-Parrinello molecular dynamics with maximally localized Wannier functions. Experimental features are accurately reproduced within the harmonic approximation. By decomposing the lineshapes in terms of intra and intermolecular dipole correlation functions we find that short-range intermolecular dynamic charge fluctuations associated to hydrogen bonds are prominent over the entire spectral range. Our analysis reveals the origin of several spectral features and identifies network bending modes in the far IR range.

cond-mat.soft↗

arXiv2008

Band Alignment in Molecular Devices: Influence of Anchoring Group and Metal Work Function

We present periodic Density Functional Theory calculations of the electronic properties of molecular junctions formed by amine-, and thiol-terminated alkane chains attached to two metal (Au, Ag) electrodes. Based on extensive analysis that includes molecular monolayers of varying densities, we establish a relationship between the alignment of the molecular energy levels and the interface dipoles, which shows that the band alignment (BA) in the limit of long, isolated chains is independent of the link group and can be computed from a reference system of non interacting molecule + metal electrodes. The main difference between the amine and thiol linkers is the effective dipole moment at the contact. This is very large, about 4.5 D, for amine linkers, leading to a strong dependence of the BA on the monolayer density and a slow convergence to the isolated molecule limit. Instead, this convergence is fast for S anchors due to the very small, ~ 0.2 D, effective dipoles at the contacts.

cond-mat.mes-hall↗

arXiv2008

Nuclear quantum effects in water

In this work, a path integral Car-Parrinello molecular dynamics simulation of liquid water is performed. It is found that the inclusion of nuclear quantum effects systematically improves the agreement of first principles simulations of liquid water with experiment. In addition, the proton momentum distribution is computed utilizing a recently developed open path integral molecular dynamics methodology. It is shown that these results are in good agreement with neutron Compton scattering data for liquid water and ice.

cond-mat.soft↗

arXiv2008

Tunneling conductance of amine linked alkyl chains

The tunneling transport theory developed in Phys. Rev. B {\bf 76}, 115102 (2007) is applied to molecular devices made of alkyl chains linked to gold electrodes via amine groups. Using the analytic expression of the tunneling conductance derived in our previous work, we identify the key physical quantities that characterize the conductance of these devices. By investigating the transport characteristics of three devices, containing 4, 6, and 8 methyl groups, we extract the dependence of the tunneling conductance on the chain's length, which is an exponential decay law in close agreement with recent experimental data.

cond-mat.mes-hall↗

arXiv2007

Density functional calculations of nanoscale conductance

Density functional calculations for the electronic conductance of single molecules are now common. We examine the methodology from a rigorous point of view, discussing where it can be expected to work, and where it should fail. When molecules are weakly coupled to leads, local and gradient-corrected approximations fail, as the Kohn-Sham levels are misaligned. In the weak bias regime, XC corrections to the current are missed by the standard methodology. For finite bias, a new methodology for performing calculations can be rigorously derived using an extension of time-dependent current density functional theory from the Schroedinger equation to a Master equation.

cond-mat.mes-hall↗

arXiv2005

First principles study of density, viscosity, and diffusion coefficients of liquid MgSiO3 at conditions of the Earth's deep mantle

Constant-pressure constant-temperature {\it ab initio} molecular dynamics simulations at high temperatures have been used to study MgSiO$_3$ liquid, the major constituent of the Earth's lower mantle to conditions of the Earth's core-mantle boundary (CMB). We have performed variable-cell {\it ab initio} molecular dynamic simulations at relevant thermodynamic conditions across one of the measured melting curves. The calculated equilibrium volumes and densities are compared with the simulations using an orthorhombic perovskite configuration under the same conditions. For molten MgSiO$_3$, we have determined the diffusion coefficients and shear viscosities at different thermodynamic conditions. Our results provide new constraints on the properties of molten MgSiO$_3$ at conditions near the core-mantle boundary. The volume change on fusion is positive throughout the pressure-temperature conditions examined and ranges from 5% at 88 GPa and 3500 K to 2.9% at 120 GPa and 5000 K. Nevertheless, neutral or negatively buoyant melts from (Mg,Fe)SiO$_3$ perovskite compositions at deep lower mantle conditions are consistent with existing experimental constraints on solid-liquid partition coefficients for Fe. Our simulations indicate that MgSiO$_3$ is liquid at 120 GPa and 4500 K, consistent with the lower range of experimental melting curves for this material. Linear extrapolation of our results indicates that the densities of liquid and solid perovskite MgSiO$_3$ will become equal near 180 GPa.

physics.geo-ph↗

arXiv2004

Point defect dynamics in bcc metals

We present an analysis of the time evolution of self-interstitial atom and vacancy (point defect) populations in pure bcc metals under constant irradiation flux conditions. Mean-field rate equations are developed in parallel to a kinetic Monte Carlo (kMC) model. When only considering the elementary processes of defect production, defect migration, recombination and absorption at sinks, the kMC model and rate equations are shown to be equivalent and the time evolution of the point defect populations is analyzed using simple scaling arguments. We show that the typically large mismatch of the rates of interstitial and vacancy migration in bcc metals can lead to a vacancy population that grows as the square root of time. The vacancy cluster size distribution under both irreversible and reversible attachment can be described by a simple exponential function. We also consider the effect of highly mobile interstitial clusters and apply the model with parameters appropriate for vanadium and $α-$iron.

cond-mat.mtrl-sci↗

arXiv2004

Density functional theory of dissipative systems

Time-dependent density functional theory is extended to include dissipative systems evolving under a master equation, providing a Hamiltonian treatment for molecular electronics. For weak electric fields, the isothermal conductivity is shown to match the adiabatic conductivity, thereby recovering the Landauer result.

cond-mat.mtrl-sci↗

arXiv2004

Kinetic theory of quantum transport at the nanoscale

We present a quantum-kinetic scheme for the calculation of non-equilibrium transport properties in nanoscale systems. The approach is based on a Liouville-master equation for a reduced density operator and represents a generalization of the well-known Boltzmann kinetic equation. The system, subject to an external electromotive force, is described using periodic boundary conditions. We demonstrate the feasibility of the approach by applying it to a double-barrier resonant tunneling structure.

cond-mat.mtrl-sci↗

arXiv2004

Current in open quantum systems

We show that a dissipative current component is present in the dynamics generated by a Liouville-master equation, in addition to the usual component associated with Hamiltonian evolution. The dissipative component originates from coarse graining in time, implicit in a master equation, and needs to be included to preserve current continuity. We derive an explicit expression for the dissipative current in the context of the Markov approximation. Finally, we illustrate our approach with a simple numerical example, in which a quantum particle is coupled to a harmonic phonon bath and dissipation is described by the Pauli master equation.

cond-mat.mtrl-sci↗

arXiv2004

Strongly Non-Arrhenius Self-Interstitial Diffusion in Vanadium

We study diffusion of self-interstitial atoms (SIAs) in vanadium via molecular dynamics simulations. The <111>-split interstitials are observed to diffuse one-dimensionally at low temperature, but rotate into other <111> directions as the temperature is increased. The SIA diffusion is highly non-Arrhenius. At T<600 K, this behavior arises from temperature-dependent correlations. At T>600 K, the Arrhenius expression for thermally activated diffusion breaks down when the migration barriers become small compared to the thermal energy. This leads to Arrhenius diffusion kinetics at low T and diffusivity proportional to temperature at high T.

cond-mat.mtrl-sci↗

arXiv2002

Theory of Quantum Annealing of an Ising Spin Glass

Probing the lowest energy configuration of a complex system by quantum annealing was recently found to be more effective than its classical, thermal counterpart. Comparing classical and quantum Monte Carlo annealing protocols on the random two-dimensional Ising model we confirm the superiority of quantum annealing relative to classical annealing. We also propose a theory of quantum annealing, based on a cascade of Landau-Zener tunneling events. For both classical and quantum annealing, the residual energy after annealing is inversely proportional to a power of the logarithm of the annealing time, but the quantum case has a larger power which makes it faster

cond-mat.dis-nn↗

arXiv1997

Amorphous indium phosphide from first principles

We report detailed and extensive first-principles molecular-dynamics (MD) simulations of the structure and electronic properties of amorphous InP produced by rapid quenching from the liquid. The structure of the material is found to be strongly ordered chemically, even though there is a significant number of coordination defects and despite the presence of odd-membered rings. We find, as a consequence, that there exists ``wrong bonds'' in the system, in an amount of about 8%; these result from the presence of coordination defects, not of local composition fluctuations, as has been conjectured. The system, in fact, is found to be over-coordinated, which might be the reason for the observed higher density of a-InP compared to c-InP. We have also investigated the possibility of pressure-amorphizing InP. Our calculations indicate that the cost of a transformation of the compressed zinc-blende crystal into an amorphous phase is so large that it is very unlikely that it would take place.

cond-mat.mtrl-sci↗