arXiv subjects

D. A. Papaconstantopoulos

Publications and source records attributed to D. A. Papaconstantopoulos.

At least 19 recordsLinked to original sources

arXiv2019

Pressure-induced high temperature superconductivity in H3X (X=As, Se, Br, Sb, Te and I)

The discovery of high critical temperature T_{c} superconductivity in highly compressed H_{3}S has opened up the question of searching for strong electron-phonon coupling in the hydrides outside the transition metal series. The specific objective of this work is to explore the possibility of discovering a material that exceeds the superconducting transition temperature of H_{3}S. Our study includes the materials H_{3}X (X=As, Se, Br, Sb, Te, and I), is limited to the Im\overline{3}m crystal structure. The procedure we adopt involves performing linearized augmented plane wave calculations for many different volumes to compute the electronic densities of states and their pressure variation. This is combined with Quantum-ESPRESSO calculations from which we obtain the phonon frequencies and the electron-phonon coupling constant λ, and followed by applying the multiple scattering-based theory of Gaspari and Gyorffy to obtain the Hopfield parameters and the McMillan-Allen-Dynes theory. It should be stressed that the GG approach decouples the electronic contribution to λfrom the corresponding phonon contribution, and provides additional insights for the understanding of superconductivity in these materials. Based on our analysis, the hydrogen is the main contributor to the T_{c} in these materials as it makes up 75\sim80 % of the total λ. Our calculations for H_{3}Se and H_{3}Br give a T_{c}{}{}\sim100 K. For the other materials in our study we find that H_{3}As is unstable and H_{3}Sb, H_{3}Te and H_{3}I have small values of the McMillan-Hopfield paramters which makes it unlikely to give high T_{c}. However, according to both of our rigid band model and virtual crystal calculations, we predict a T_{c}\sim150 K for H_{3}Br with a small amount of hydrogen doping.

cond-mat.supr-con↗

arXiv2017

Possible High-Temperature Superconductivity in Hygrogenated Fluorine

Recent computational studies confirmed by experiment have established the occurrence of superconducting temperatures, $T_c$, near 200 K when the pressure is close to 200 GPa in the compound H$_3S$. Motivated by these findings we investigate in this work the possibility of discovering high-temperature superconductivity in the material H$_3$F. We performed linearized augmented plane wave(LAPW) calculations followed by the determination of the angular momentum components of the density of states, the scattering phase shifts at the Fermi level and the electron-ion matrix element known as the Hopfield parameter. Our calculated Hopfield parameters are much larger than those found in H$_3$S suggesting that they may lead to large electron-phonon coupling constant and hence a large Tc similar or even larger than that of H$_3$S. However, calculations of elastic constants are inconclusive regarding the stability of this material.

cond-mat.supr-con↗

arXiv2015

Cubic H$_3$S around 200 GPa: an atomic hydrogen superconductor stabilized by sulfur

The multiple scattering-based theory of Gaspari and Gyorffy for the electron-ion matrix element in close packed metals is applied to Im-3m H3S, which has been predicted by Duan {\it et al.} and Bernstein {\it et al} to be the stable phase at this stoichiometry around 190 GPa, thus is the leading candidate to be the phase observed to superconduct at 190K by Drozdov, Eremets, and Troyan. The nearly perfect separation of vibrational modes into those of S and of H character provides a simplification that enables identification of contributions of the two atoms separately. The picture that arises is basically that of superconducting atomic H stabilized by strong covalent mixing with S 3p and "3d" character. The reported isotope shift is much larger than the theoretical one,suggesting there is large anharmonicity in the H vibrations. Given the relative unimportance of sulfur, hydrides of lighter atoms at similarly high pressures may also lead to high temperature superconductivity.

cond-mat.supr-con↗

arXiv2010

Tight-binding Hamiltonian for LaOFeAs

First-principles electronic structure calculations have been very useful in understanding some of the properties of the new iron-based superconductors. Further explorations of the role of the individual atomic orbitals in explaining various aspects of research in these materials, including experimental work, would benefit from the availability of a tight-binding(TB) Hamiltonian that reproduces accurately the first-principles band structure results. In this work we have used the NRL-TB method to construct a TB Hamiltonian from Linearized Augmented Plane Wave(LAPW) results. Our TB model includes the Fe d-orbitals, and the p-orbitals from both As and O for the prototype material LaOFeAs. The resulting TB band structure agrees well with that of the LAPW calculations in from 2.7 eV below to 0.8 eV above the Fermi level, epsilon_F, and the Fermi surface matches perfectly to that of the LAPW. The TB densities of states(DOS) are also in very good agreement with those from the LAPW in the above energy range, including the per orbital decomposition. We use our results to provide insights on the existence of a pseudogap in the DOS just above the Fermi level. We have also performed a separate TB fit to a database of LAPW results as a function of volume and with variations of the As positions. This fit although less accurate regarding the band structure near epsilon_F, reproduces the LAPW total energies very well and has transferability to non-fitted energies.

cond-mat.supr-con↗

arXiv2010

Electronic structure and superconductivity of Europium

We have calculated the electronic structure of Eu for the bcc, hcp, and fcc crystal structures for volumes near equilibrium up to a calculated 90 GPa pressure using the augmented-plane wave method in the local-density approximation. The frozen-core approximation was used with a semi-empirical shift of the f-states energies in the radial Schr$\ddot{o}$dinger equation to move the occupied 4f valence states below the $Γ_1$ energy and into the core. This shift of the highly localized f-states yields the correct europium phase ordering with lattice parameters and bulk moduli in good agreement with experimental data. The calculated superconductivity properties under pressure for the $\it bcc$ and $\it hcp$ structures are also found to agree with and follow a $T_c$ trend similar to recent measurement by Debessai et al.$^1$

cond-mat.supr-con↗

arXiv2005

Evolution of magnetism of Cr nanoclusters on a Au(111) surface

We have carried out collinear and non-collinear electronic structure calculations to investigate the structural, electronic and magnetic properties of isolated Cr atoms, dimers and compact trimers. We find that the Cr monomer prefers to adsrob on the fcc hollow site with a binding energy of 3.13 eV and a magnetic moment of of 3.93 $μ_B$. The calculated Kondo temperature of 0.7 K for the monomer is consistent with the lack of a Kondo peak in scanning tunneling microscopy (STM) experiments at 7 K. The compact Cr dimer orders antiferromagnetically and its bond length contracts to 1.72 $Å$ close to the value for the free-standing Cr dimer. The very low magnetic moment of 0.005 $μ_B$ for the Cr atoms in the dimer is due to the strong $d-d$ hybridization between the Cr adatoms. Thus, these calculations reveal that the absence of the Kondo effect observed in STM experiments is due to the small local moments rather than the Kondo quenching of the local moments suggested experimentally. The Cr compact trimer exhibits non-collinear co-planar magnetism with vanishing net magnetic moment in agreement with experiment.

cond-mat.mtrl-sci↗

arXiv2004

A Tight-Binding Investigation of the NaxCoO2 Fermi Surface

We perform an orthogonal basis tight binding fit to an LAPW calculation of paramagnetic Na$_x$CoO$_2$ for several dopings. The optimal position of the apical oxygen at each doping is resolved, revealing a non-trivial dependence of the band structure and Fermi surface on oxygen height. We find that the small e$_{g'}$ hole pockets are preserved throughout all investigated dopings and discuss some possible reasons for the lack of experimental evidence for these Fermi sheets.

cond-mat.supr-con↗

arXiv2004

Modifications and Extensions to Harrison's Tight-Binding Theory

Harrison's tight-binding theory provides an excellent qualitative description of the electronic structure of the elements across the periodic table. However, the resulting band structures are in significant disagreement with those found by standard methods. We developed a new procedure to generate both the prefactors of Harrison's hopping parameters and the onsite energies. Our approach gives an impressive improvement and puts Harrison's theory on a quantitative basis. Our method retains the most attractive aspect of the theory, in using a revised set of universal prefactors for the hopping integrals. In addition, a new form of onsite parameters allows us to describe the lattice constant dependence of the bands and the total energy, predicting the correct ground state for all transition metals. This work represents not only a useful computational tool but also an important pedagogical enhancement for Harrison's books.

cond-mat.mtrl-sci↗

arXiv2004

Tight-binding study of structure and vibrations of amorphous silicon

We present a tight-binding calculation that, for the first time, accurately describes the structural, vibrational and elastic properties of amorphous silicon. We compute the interatomic force constants and find an unphysical feature of the Stillinger-Weber empirical potential that correlates with a much noted error in the radial distribution function associated with that potential. We also find that the intrinsic first peak of the radial distribution function is asymmetric, contrary to usual assumptions made in the analysis of diffraction data. We use our results for the normal mode frequencies and polarization vectors to obtain the zero-point broadening effect on the radial distribution function, enabling us to directly compare theory and a high resolution x-ray diffraction experiment.

cond-mat.mtrl-sci↗

arXiv2004

Description of the Chemical Reaction Path in the HCO Molecule: A Combined Configuration Interaction and Tight-Binding Approach

It is demonstrated that the reaction path for a polyatomic molecule (applied to the HCO molecule) is easily calculated via ab-initio configuration interaction (CI) total energies. This Hamiltonian not only reproduces the CI calculations accurately and efficiently, but also effectively corrects any CI energies happening to erroneously converge to excited states.

physics.chem-ph↗

arXiv2004

Nesting, spin-fluctuations, and odd-gap superconductivity in NaxCoO2 yH2O

We have calculated the one-electron susceptibility of hydrated NaxCoO2 and find strong nesting nearly commensurate with a 2X2 superstructure. The nesting involves about 70% of all electrons at the Fermi level and is robust with respect to doping. This nesting creates a tendency to a charge density wave compatible with the charge order often seen at x approx 0.5, which is usually ascribed to electrostatic repulsion of Na ions. In the spin channel, it gives rise to strong spin-fluctuations, which should be important for superconductivity. The superconducting state most compatible with this nesting structure is an odd-gap triplet s-wave state.

cond-mat.supr-con↗

arXiv2003

Calculation of magnetic anisotropy energy in SmCo5

SmCo5 is an important hard magnetic material, due to its large magnetic anisotropy energy (MAE). We have studied the magnetic properties of SmCo5 using density functional theory (DFT) calculations where the Sm f-bands, which are difficult to include in DFT calculations, have been treated within the LDA+U formalism. The large MAE comes mostly from the Sm f-shell anisotropy, stemming from an interplay between the crystal field and the spin-orbit coupling. We found that both are of similar strengths, unlike some other Sm compounds, leading to a partial quenching of the orbital moment (f-states cannot be described as either pure lattice harmonics or pure complex harmonics), an optimal situation for enhanced MAE. A smaller portion of the MAE can be associated with the Co-d band anisotropy, related to the peak in the density of states at the Fermi energy. Our result for the MAE of SmCo5, 21.6 meV/f.u., agrees reasonably with the experimental value of 13-16 meV/f.u., and the calculated magnetic moment (including the orbital component) of 9.4 mu_B agrees with the experimental value of 8.9 mu_B.

cond-mat.mtrl-sci↗

arXiv2001

First-principles study of superconductivity in high-pressure Boron

We study superconductivity in Boron using first-principles LAPW calculations, the rigid-muffin-tin approximation and the McMillan theory. Our results point to an electron-phonon mechanism producing transition temperatures near 10K at high pressures in agreement with recent measurements.

cond-mat.supr-con↗

arXiv2001

Tight-binding study of high-pressure phase transitions in titanium: alpha to omega and beyond

We use a tight-binding total energy method, with parameters determined from a fit to first-principles calculations, to examine the newly discovered gamma phase of titanium. Our parameters were adjusted to accurately describe the alpha Ti-omega Ti phase transition, which is misplaced by density functional calculations. We find a transition from omega Ti to gamma Ti at 102 GPa, in good agreement with the experimental value of 116 GPa. Our results suggest that current density functional calculations will not reproduce the omega Ti-gamma Ti phase transition, but will instead predict a transition from omega Ti to the bcc beta Ti phase.

cond-mat.mtrl-sci↗

arXiv2001

Superconductivity in compressed iron: Role of spin fluctuations

The recent discovery of superconductivity in hexagonal iron under pressure poses a question about whether it is of conventional (phonon) or unconventional (magnetic?) origin. We present first-principles calculations of the electron-phonon coupling in iron at $P\agt 15$ GPa, and argue that a conventional mechanism can explain the appearance of superconductivity, but not its rapid disappearance at $P\agt 30$ GPa. We suggest that spin fluctuations, ferro- and/or antiferromagnetic, play a crucial role in superconductivity in this case.

cond-mat.supr-con↗

arXiv2001

Precise Tight-binding Description of the Band Structure of MgB2

We present a careful recasting of first-principles band structure calculations for MgB2 in a non-orthogonal sp-tight-binding (TB) basis. Our TB results almost exactly reproduce our full potential linearized augmented plane wave results for the energy bands, the densities of states and the total energies. Our procedure generates transferable Slater-Koster parameters which should be useful for other studies of this important material.

cond-mat.supr-con↗

arXiv2001

Effects of C, Cu and Be substitutions in superconducting MgB2

Density functional calculations are used to investigate the effects of partial substitutional alloying of the B site in MgB2 with C and Be alone and combined with alloying of the Mg site with Cu. The effect of such substitutions on the electronic structure, electron phonon coupling and superconductivity are discussed. We find that Be substitution for B is unfavorable for superconductivity as it leads to a softer lattice and weaker electron-phonon couplings. Replacement of Mg by Cu leads to an increase in the stiffness and doping level at the same time, while the carrier concentration can be controlled by partial replacement of B by C. We estimate that with full replacement of Mg by Cu and fractional substitution of B by C, Tc values of 50K may be attainable.

cond-mat.supr-con↗

arXiv2001

Dynamical properties of Au from tight-binding molecular-dynamics simulations

We studied the dynamical properties of Au using our previously developed tight-binding method. Phonon-dispersion and density-of-states curves at T=0 K were determined by computing the dynamical-matrix using a supercell approach. In addition, we performed molecular-dynamics simulations at various temperatures to obtain the temperature dependence of the lattice constant and of the atomic mean-square-displacement, as well as the phonon density-of-states and phonon-dispersion curves at finite temperature. We further tested the transferability of the model to different atomic environments by simulating liquid gold. Whenever possible we compared these results to experimental values.

cond-mat.mtrl-sci↗