SEARCH · Searcharxiv

Results for “cond-mat.mtrl-sci”

Search indexed arXiv papers on artificial intelligence, large language models, computer vision and robotics. Read source abstracts and follow links to arXiv.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 91 records · Page 5Linked to original sources

arXiv2007

Screening and impurity ionization energy in semiconductors

Usually microscopic electrostatic field around charged impurity ions is neglected when the ionization energy is concerned. The ionization energy is considered to be equal to that of a lonely impurity atom. Here the energy of the electrostatic field around charged impurity ions in semiconductor is taken into account. It is shown that the energy of this field contributes to decrease in the effective ionization energy. At high enough current carriers concentration the effective ionization energy becomes zero.

cond-mat.mtrl-sci↗

arXiv2008

Extended Lagrangian formulation of time-reversible Born-Oppenheimer molecular dynamics for higher-order symplectic integration

A Lagrangian generalization of time-reversible Born-Oppenheimer molecular dynamics [Niklasson et al., Phys. Rev. Lett. vol. 97, 123001 (2006)] is proposed. The Lagrangian includes extended electronic degrees of freedom as auxiliary dynamical variables in addition to the nuclear coordinates and momenta. While the nuclear degrees of freedom propagate on the Born-Oppenheimer potential energy surface, the extended auxiliary electronic degrees of freedom evolve as a harmonic oscillator centered around the adiabatic propagation of the self-consistent ground state. The formulation enables the application of higher-order symplectic or geometric integration schemes that are stable and energy conserving even under incomplete self-consistency convergence. It is demonstrated how the extended Born-Oppenheimer molecular dynamics improves the accuracy by over an order of magnitude compared to previous formulations at the same level of computational cost.

cond-mat.mtrl-sci↗

arXiv2014

Electronic structures and lattice dynamics of BaTiO3 and BiFeO3 : a comparative first-principles study

First-principles calculations were performed to investigate the ferroelectric properties of barium titanate and bismuth ferrite, as well as phonon dispersion of BaTiO3, using density functional theory and density functional perturbation theory. Results show that the strong hybridization of Ti-O and Bi-O lead to the corresponding mechanisms for stabilizing the distorted structure. The spontaneous polarization of 59.4 μC/cm2 and 27.6 μC/cm2 were calculated for BiFeO3 and BaTiO3 respectively, using berry phase method within the modern theory of polarization. The stereochemical activity of Bi-6s long-pair, which was the driven mechanism for ferroelectricity in BiFeO3, was able to produce greater polarization than the Ti off-centring displacement in BaTiO3. New multiferroic perovskite type materials combined with these two ferroelectric instabilities were predicted to have a better ferromagnetic ordering in comparison with BiFeO3.

cond-mat.mtrl-sci↗

arXiv2008

Subwavelength imaging of light by arrays of metal-coated semiconductor nanoparticles: a theoretical study

Rigourous calculations of the imaging properties of metamaterials consisting of metal-coated semiconductor nanoparticles are presented. In particular, it is shown that under proper choice of geometric and materials parameters, arrays of such particles exhibit negative refractive index within the region of the excitonic resonance of the semiconductor. The occurrence of negative refractive index is predicted by the extended Maxwell-Garnett theory and confirmed by a layer-multiple scattering method for electromagnetic waves. By using the same method it is shown that within the negative refractive-index band, arrays of such nanoparticles amplify the transmitted near-field emitted while simultaneously narrow down its spatial profile leading to subwavelength resolution. The effect of material losses to the imaging properties of the arrays is also addressed.

cond-mat.mtrl-sci↗

arXiv2007

Polymer nanoparticles to decrease thermal conductivity of phase change materials

Microparticles including paraffin are currently used for textiles coating in order to deaden thermal shocks. We will show that polymer nanoparticles embedded in those microsized capsules allow for decreasing the thermal conductivity of the coating and enhance the protection in the stationary regime. A reasonable volume fraction of polymer nanoparticles reduces the conductivity more than predicted by Maxwell mixing rules. Besides, measurements prove that the polymer nanoparticles do not affect the latent heat and even improve the phase change behaviour as well as the mechanical properties.

cond-mat.mtrl-sci↗

arXiv2008

Cooperative electron-phonon interaction in molecular chains

Using a controlled analytic treatment, we derive a model that generically describes cooperative strong electron-phonon interaction (EPI) in one-band and two-band Jahn-Teller (JT) systems. The model involves a {\em next-nearest-neighbor} hopping and a nearest-neighbor repulsion. Rings with odd number of sites (o-rings) belong to a different universality class compared to rings with even number (e-rings). The e-rings, upon tuning repulsion, undergo a dramatic discontinuous transition to a {\em conducting commensurate} charge density wave (CDW) state with a period independent of filling.

cond-mat.str-el↗

arXiv2009

Resonant carrier dynamics in strongly biassed superlattices

We study coherent electron dynamics in a biassed undriven ideal semiconductor superlattice coupled to the continuum, near energy level anticrossings. In particular, we examine the dependence of wavepacket dynamical characteristics on electric field detuning, and investigate mixed regimes involving a superposition of energy level anticrossings showing both Rabi oscillations and resonant tunnelling. In earlier work [Phys. Rev. B 75 165421 (2007)], Rabi and Zener resonances were shown to have a common origin, and a criteria for the occurrence of either was proposed. The present results allow a better understanding of the nature of an interminiband resonance, which can be useful in the areas of microwave radiation generation and matter manipulation on the particle level, as well as demonstrate an alternative approach to examining electron level structure of a finite superlattice.

cond-mat.mtrl-sci↗

arXiv2007

A first-principles study of the electronic structure and stability of Be(BH4)2

Alanates and boranates are studied intensively because of their potential use as hydrogen storage materials. In this paper we present a first-principles study of the electronic structure and the energetics of beryllium boranate, Be(BH4)2. From total energy calculations we show that - in contrast to the other boranates and alanates - hydrogen desorption directly to the elements is likely, and is at least competitive with desorption to the elemental hydride (BeH2). The formation enthalpy of Be(BH4)2 is only -0.12 eV/H2 (at T=0K). This low value can be rationalized by the participation of all atoms in the covalent bonding, in contrast to the ionic bonding observed in other boranates. From calculations of thermodynamic properties at finite temperature we estimate a decomposition temperature of 162 K at a pressure of 1 bar.

cond-mat.mtrl-sci↗

arXiv2009

Momentum-dependent resonant inelastic X-ray scattering at the Si K edge of 3C-SiC: A theoretical study on a relation between spectra and valence band dispersion

We theoretically demonstrate that a resonant inelastic x-ray scattering (RIXS) with a sizable momentum transfer can be utilized to study valence band dispersion for broad band materials. We take RIXS at the Si K edge of 3C-SiC as a typical example. The RIXS spectra are calculated by systematically changing the transferred momentum, an incident photon polarization and an incident photon energy, on the basis of an ab initio calculation. We find that the spectra depend heavily on both the transferred momentum and the incident photon polarization, and the peaks in the spectra correspond to the energies of the valence bands. We conclude that the information on the energy dispersion of valence bands can be extracted from the transferred momentum dependence of the RIXS spectra. These findings lead to further application for RIXS when investigating the band structure of broad band materials.

cond-mat.mtrl-sci↗

arXiv2009

Excitation energies from ground-state density-functionals by means of generator coordinates

The generator-coordinate method is a flexible and powerful reformulation of the variational principle. Here we show that by introducing a generator coordinate in the Kohn-Sham equation of density-functional theory, excitation energies can be obtained from ground-state density functionals. As a viability test, the method is applied to ground-state energies and various types of excited-state energies of atoms and ions from the He and the Li isoelectronic series. Results are compared to a variety of alternative DFT-based approaches to excited states, in particular time-dependent density-functional theory with exact and approximate potentials.

physics.chem-ph↗

arXiv2007

Dynamical Symmetry Breaking of a Relativistic Model in Quasi-(1+1)-Dimensions. I. Formulation

The dynamical symmetry breaking in a quasi-(1+1)-dimensional relativistic model is investigated. The motions of particles in intrachain are described as a relativistic electron-hole gas, while the interchain hopping term is introduced as a 0th-component of vector in (1+1)-dimensions, a kind of chemical potential of the system. The gauge symmetry of the model is chosen as U(1) suitable for a possible situation of a real substance in condensed matter physics. We consider the BCS-type contact interactions for the s-wave fermion-pair condensates, while employ the nonlocal interactions of the generalized BCS framework to generate the $p$-, $d$- and $f$-wave condensations in the system. Especially we examine the dynamical generation of a Dirac mass term and superconductivity in the model. The phenomenon is interpreted as metal-insulator/metal-superconductor phase transitions.

cond-mat.supr-con↗

arXiv2008

Large magnetocaloric effect in Gd4Co3

We report a large entropy change (DeltaS) below 300 K, peaking near TC= 220 K, due to isothermal change of magnetic field, for Gd4Co3, with a refrigeration capacity higher than that of Gd. Notably, the isothermal magnetization is nonhysteretic - an important criterion for magnetic refrigeration without loss. DeltaS behavior is also compared with that of magnetoresistance.

cond-mat.str-el↗

arXiv2007

The Origin and Control of the Sources of AMR in (Ga,Mn)As Devices

We present details of our experimental and theoretical study of the components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As. We develop experimental methods to yield directly the non-crystalline and crystalline AMR components which are then independently analyzed. These methods are used to explore the unusual phenomenology of the AMR in ultra thin (5nm) (Ga,Mn)As layers and to demonstrate how the components of the AMR can be engineered through lithography induced local lattice relaxations. We expand on our previous [Phys. Rev. Lett. \textbf{99}, 147207 (2007)] theoretical analysis and numerical calculations to present a simplified analytical model for the origin of the non-crystalline AMR. We find that the sign of the non-crystalline AMR is determined by the form of spin-orbit coupling in the host band and by the relative strengths of the non-magnetic and magnetic contributions to the impurity potential.

cond-mat.mes-hall↗

arXiv2007

Diamagnetic expansions for perfect quantum gases II: uniform bounds

Consider a charged, perfect quantum gas, in the effective mass approximation, and in the grand-canonical ensemble. We prove in this paper that the generalized magnetic susceptibilities admit the thermodynamic limit for all admissible fugacities, uniformly on compacts included in the analyticity domain of the grand-canonical pressure. The problem and the proof strategy were outlined in \cite{3}. In \cite{4} we proved in detail the pointwise thermodynamic limit near $z=0$. The present paper is the last one of this series, and contains the proof of the uniform bounds on compacts needed in order to apply Vitali's Convergence Theorem.

math-ph↗

arXiv2008

Random sequential adsorption model of damage and crack accumulation: Exact one-dimensional results

The random sequential adsorption (RSA) model is modified to describe damage and crack accumulation. The exclusion for object deposition (for damaged region formation) is not for the whole object, as in the standard RSA, but only for the initial point (or higher-dimensional defect) from which the damaged region or crack initiates. The one-dimensional variant of the model is solved exactly.

cond-mat.stat-mech↗

arXiv2008

One-magnon (electromagnon) light scattering in BiFeO3 single crystals

We observed Raman scattering from magnon in frequency range from 10 to 65 cm-1 in BiFeO3 single crystals at cryogenic temperatures; the temperature dependence of the magnon frequency at 18.2 cm-1 approximates an S=5/2 Brillouin function up to the temperature (280 K) at which the magnon becomes overdamped. The diverging cross-section and the frequency-shift at 140K and 200 K implies a magnon-reorientation transition as in orthoferrites. Magnons in polar materials such as BiFeO3 are often termed electromagnons meaning that they possess an electric dipole moment due to magnetoelectric coupling.

cond-mat.mtrl-sci↗

arXiv2007

Half-Metallicity in Undoped and Boron Doped Graphene Nanoribbons in Presence of Semi-local Exchange-Correlation Interactions

We perform density functional calculations on one-dimensional zigzag edge graphene nano-ribbons (ZGNRs) of different widths, with and without edge doping including semi-local exchange-correlations. Our study reveals that, although the ground state of edge passivated (with hydrogen) ZGNRs prefers to be anti-ferromagnetic, the doping of both the edges with Boron atoms stabilizes the system in a ferromagnetic ground state. Both the local and semi-local exchange-correlations result in half-metallicity in edge passivated ZGNRs at a finite cross-ribbon electric field. However, the ZGNR with Boron edges shows half-metallic behavior irrespective of the ribbon-width even in absence of electric field and this property sustains for any field strength, opening a huge possibility of applications in spintronics.

cond-mat.mtrl-sci↗

arXiv2007

Effects of electron-phonon coupling in angle-resolved photoemission spectra of SrTiO3

We have studied the O 2p valence-band structure of Nb-doped SrTiO3, in which a dilute concentration of electrons are doped into the d0 band insulator, by angle-resolved photoemission spectroscopy (ARPES) measurements. We found that ARPES spectra at the valence band maxima at the M [k = (pi/a, pi/a, 0)]and R [k = (pi/a, pi/a, pi/a)] points start from ~ 3.3 eV below the Fermi level (EF), consistent with the indirect band gap of 3.3 eV and the EF position at the bottom of the conduction band. The peak position of the ARPES spectra were, however, shifted toward higher binding energies by ~ 500 meV from the 3.3 eV threshold. Because the bands at M and R have pure O 2p character, we attribute this ~ 500 meV shift to strong coupling of the oxygen p hole with optical phonons in analogy with the peak shifts observed for d-electron photoemission spectra in various transition-metal oxides.

cond-mat.mtrl-sci↗