arXiv subjects

Matthieu J. Verstraete

Publications and source records attributed to Matthieu J. Verstraete.

49 records · Page 3Linked to original sources

arXiv2019

Long-range dispersion forces between molecules subject to ultra-short optical pulses from ab initio calculations

London-van der Waals dispersion forces are a fundamental component of condensed matter systems, biological processes, and self-assembly. In this letter we propose a method to calculate the C6 coefficients that characterize dispersion forces in the non-retarded regime for molecules in a coherent superposition of excited states. Several ultrafast femtosecond pump-probe schemes are investigated. We apply the method to LiH molecules and show that their London-van der Waals interaction can change dramatically after the interaction with the pump pulse. The pulse modulates the C6 coefficients, and the interplay between polarization, orientation of the molecules, and the dipole fields gives rise to a rich variety of combinations.

physics.chem-ph↗

arXiv2019

Low energy phases of bilayer Bi predicted by structure search in two dimensions

We employ an ab-initio structure search algorithm to explore the configurational space of Bi in quasi two dimensions. A confinement potential restricts the movement of atoms within a pre-defined thickness during structure search calculations within the minima hopping method to find the stable and metastable forms of bilayer Bi. In addition to recovering the two known low-energy structures (puckered monoclinic and buckled hexagonal), our calculations predict three new structures of bilayer Bi. We call these structures the $α$, $β$, and $γ$ phases of bilayer Bi, which are, respectively, 63, 72, and 83 meV/atom higher in energy than that of the monoclinic ground state, and thus potentially synthesizable using appropriate substrates. We also compare the structural, electronic, and vibrational properties of the different phases. The puckered monoclinic, buckled hexagonal, and $β$ phases exhibit a semiconducting energy gap, whereas $α$ and $γ$ phases are metallic. We notice an unusual Mexican-hat type band dispersion leading to a van Hove singularity in the buckled hexagonal bilayer Bi. Notably, we find symmetry-protected topological Dirac points in the electronic spectrum of the $γ$ phase. The new structures suggest that bilayer Bi provides a novel playground to study distortion-mediated metal-insulator phase transitions.

cond-mat.mtrl-sci↗

arXiv2019

Density functional perturbation theory within non-collinear magnetism

We extend the density functional perturbation theory formalism to the case of non-collinear magnetism. The main problem comes with the exchange-correlation (XC) potential derivatives, which are the only ones that are affected by the non-collinearity of the system. Most of the present XC functionals are constructed at the collinear level, such that the off-diagonal (containing magnetization densities along $x$ and $y$ directions) derivatives cannot be calculated simply in the non-collinear framework. To solve this problem, we consider here possibilities to transform the non-collinear XC derivatives to a local collinear basis, where the $z$ axis is aligned with the local magnetization at each point. The two methods we explore are i) expanding the spin rotation matrix as a Taylor series, ii) evaluating explicitly the XC for the local density approximation through an analytical expression of the expansion terms. We compare the two methods and describe their practical implementation. We show their application for atomic displacement and electric field perturbations at the second order, within the norm-conserving pseudopotential methods.

physics.comp-ph↗

arXiv2018

Electron-Beam Manipulation of Silicon Dopants in Graphene

The direct manipulation of individual atoms in materials using scanning probe microscopy has been a seminal achievement of nanotechnology. Recent advances in imaging resolution and sample stability have made scanning transmission electron microscopy a promising alternative for single-atom manipulation of covalently bound materials. Pioneering experiments using an atomically focused electron beam have demonstrated the directed movement of silicon atoms over a handful of sites within the graphene lattice. Here, we achieve a much greater degree of control, allowing us to precisely move silicon impurities along an extended path, circulating a single hexagon, or back and forth between the two graphene sublattices. Even with manual operation, our manipulation rate is already comparable to the state-of-the-art in any atomically precise technique. We further explore the influence of electron energy on the manipulation rate, supported by improved theoretical modeling taking into account the vibrations of atoms near the impurities, and implement feedback to detect manipulation events in real time. In addition to atomic-level engineering of its structure and properties, graphene also provides an excellent platform for refining the accuracy of quantitative models and for the development of automated manipulation.

cond-mat.mtrl-sci↗

arXiv2018

Vibrational and dielectric properties of the bulk transition metal dichalcogenides

Interest in the bulk transition metal dichalcogenides for their electronic, photovoltaic, and optical properties has grown and led to their use in many technological applications. We present a systematic investigation of their interlinked vibrational and dielectric properties, using density functional theory and density functional perturbation theory, studying the effects of the spin-orbit interaction and of the long-range e$^-$- e$^-$ correlation as part of our investigation. This study confirms that the spin-orbit interaction plays a small role in these physical properties, while the direct contribution of dispersion corrections is of crucial importance in the description of the interatomic force constants. Here, our analysis of the structural and vibrational properties, including the Raman spectra, compare well to experimental measurement. Three materials with different point groups are showcased and data trends on the full set of fifteen existing hexagonal, trigonal, and triclinic materials are demonstrated. This overall picture will enable the modeling of devices composed of these materials for novel applications.

cond-mat.mtrl-sci↗

arXiv2017

BoltzTraP2, a program for interpolating band structures and calculating semi-classical transport coefficients

BoltzTraP2 is a software package for calculating a smoothed Fourier expression of periodic functions and the Onsager transport coefficients for extended systems using the linearized Boltzmann transport equation. It uses only the band and $k$-dependent quasi-particle energies, as well as the intra-band optical matrix elements and scattering rates, as input. The code can be used via a command-line interface and/or as a Python module. It is tested and illustrated on a simple parabolic band example as well as silicon. The positive Seebeck coefficient of lithium is reproduced in an example of going beyond the constant relaxation time approximation.

cond-mat.mtrl-sci↗

arXiv2017

Strain-induced effects in the electronic and spin properties of a monolayer of ferromagnetic GdAg2

We report on the structural, electronic and magnetic properties of a monolayer of GdAg2, forming a moiré pattern on Ag(111). Combining scanning tunneling microscopy and ab-initio spin-polarized calculations, we show that the electronic band structure can be shifted linearly via thermal dependent strain of the intra-layer atomic distance in a range between 1-7%, leading to lateral hetero-structuring. Furthermore, the coupling of the incommensurable GdAg2 alloy layer to the Ag(111) substrate leads to spatially varying atomic relaxation causing subsurface layer buckling, texturing of the electronic and spin properties, and inhomogeneity of the magnetic anisotropy energy across the layer. These results provide perspectives for a control of electronic properties and magnetic ordering in atomically-thin layers.

cond-mat.mtrl-sci↗

arXiv2017

Origin of the Counterintuitive Dynamic Charge in the Transition-Metal Dichalcogenides

We investigate the chemical bonding characteristics of the transition metal dichalcogenides based on their static and dynamical atomic charges within Density Functional Theory. The dynamical charges of the trigonal transition metal dichalcogenides are anomalously large, while in their hexagonal counterparts, their sign is even counterintuitive i.e. the transition metal takes the negative charge. This phenomenon cannot be understood simply in terms of a change in the static atomic charge as it results from a local change of polarization. We present our theoretical understanding of these phenomena based on the perturbative response of the system to a static electric field and by investigating the hybridization of the molecular orbitals near the Fermi level. Furthermore, we establish a link between the sign of the Born effective charge and the $π$-backbonding in organic chemistry and propose an experimental procedure to verify the calculated sign of the dynamical charge in the transition metal dichalcogenides.

cond-mat.mtrl-sci↗

arXiv2016

Non-monotonic anisotropy in charge conduction induced by antiferrodistortive transition in metallic SrTiO$_{3}$

Cubic SrTiO$_{3}$ becomes tetragonal below 105 K. The antiferrodistortive (AFD) distortion leads to clockwise and counter-clockwise rotation of adjacent TiO$_{6}$ octahedra. This insulator becomes a metal upon the introduction of extremely low concentration of n-type dopants. However, signatures of the structural phase transition in charge conduction have remained elusive. Employing the Montgomery technique, we succeed in resolving the anisotropy of charge conductivity induced by the AFD transition, in the presence of different types of dopants. We find that the slight lattice distortion ($<6 \times 10^{-4}$) gives rise to a twenty percent anisotropy in charge conductivity, in agreement with the expectations of band calculations. Application of uniaxial strain amplifies the detectable anisotropy by disfavoring one of the three possible tetragonal domains. In contrast with all other known anisotropic Fermi liquids, the anisotropy has opposite signs for elastic and inelastic scattering. Increasing the concentration of dopants leads to a drastic shift in the temperature of the AFD transition either upward or downward. The latter result puts strong constraints on any hypothetical role played by the AFD soft mode in the formation of Cooper pairs and the emergence of superconductivity in SrTiO$_3$.

cond-mat.mtrl-sci↗

arXiv2014

On the room temperature ferroelectricity of hydrogen-bonded charge transfer crystals

We present a theoretical investigation of the anomalous ferroelectricity of mixed-stack charge transfer molecular crystals, based on the Peierls-Hubbard model, and first principles calculations for its parameterization. This approach is first validated by reproducing the temperature-induced transition and the electronic polarization of TTF-CA, and then applied to a novel series of hydrogen-bonded crystals, for which room temperature ferroelectricity has recently been claimed. Our analysis shows that the hydrogen-bonded systems present a very low degree of charge transfer and hence support a very small polarization. A critical re-examination of experimental data supports our findings, shedding doubts on the ferroelectricity of these systems. More generally, our modelling allows the rationalization of general features of the ferroelectric transition in charge transfer crystals, and suggests design principles for materials optimization.

cond-mat.mtrl-sci↗

arXiv2013

First Principles explanation of the positive Seebeck coefficient of lithium

Lithium is one of the simplest metals, with negative charge carriers and a close reproduction of free electron dispersion. Experimentally, however, Li is one of a handful of elemental solids (along with Cu, Ag, Au etc.) where the sign of the Seebeck coefficient (S) is opposite to that of the carrier. This counterintuitive behavior still lacks a satisfactory interpretation. We calculate S fully from first-principles, within the framework of P.B. Allen's formulation of Boltzmann transport theory. Here it is crucial to avoid the constant relaxation time approximation, which gives a sign for S which is necessarily that of the carriers. Our calculated S are in excellent agreement with experimental data, up to the melting point. In comparison with another alkali metal Na, we demonstrate that within the simplest non-trivial model for the energy dependency of the electron lifetimes, the rapidly increasing density of states (DOS) across the Fermi energy is related to the sign of S in Li. The exceptional energy dependence of the DOS is beyond the free-electron model, as the dispersion is distorted by the Brillouin Zone edge, a stronger effect in Li than other Aliki metals. The electron lifetime dependency on energy is central, but the details of the electron-phonon interaction are found to be less important, contrary to what has been believed for several decades. Band engineering combined with the mechanism exposed here may open the door to new "ambipolar" thermoelectric materials, with a tunable sign for the thermopower even if either n- or p-type doping is impossible.

cond-mat.mtrl-sci↗

arXiv2009

On the phases of Polonium

The thermodynamical properties of the main phases of metallic polonium are examined using Density Functional Theory. The exceptional nature of the solid-solid phase transition of $α$ to $β$ Po is underlined: it induces a lowering in symmetry, from cubic to rhombohedral, with increasing temperature. This is explained as the result of a delicate balance between relativistic and entropic effects. Overall agreement with existing experimental data is very good by state-of-the-art standards. The phonons of Po present Kohn anomalies, and it is shown that the effect of spin-orbit interactions is the inverse of that in normal metals: due to the non-spherical nature of the Fermi Surface, spin-orbit effects reduce nesting and harden most phonon frequencies.

cond-mat.mtrl-sci↗

arXiv2008

First-Principles conductance of nanoscale junctions from the polarizability of finite systems

A method for the calculation of the conductance of nanoscale electrical junctions is extended to ab-initio electronic structure methods and applied to realistic models of metallic wires and break-junctions of sodium and gold. The method is systematically controllable and convergeable, and can be straightforwardly extended to include more complex processes and interactions. Important issues about the order in which are taken both the thermodynamic and the static (small field) limits are clarified, and characterized further through comparisons to model systems.

cond-mat.mtrl-sci↗