skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Electronic structure and electron-phonon coupling in TiH$$_2$$

Abstract

Calculations using first principles methods and strong coupling theory are carried out to understand the electronic structure and superconductivity in cubic and tetragonal TiH$$_2$$. A large electronic density of states at the Fermi level in the cubic phase arises from Ti-$$t_{2g}$$ states and leads to a structural instability against tetragonal distortion at low temperatures. However, constraining the in-plane lattice constants diminishes the energy gain associated with the tetragonal distortion, allowing the cubic phase to be stable at low temperatures. Furthermore, calculated phonon dispersions show decoupled acoustic and optic modes arising from Ti and H vibrations, respectively and frequencies of optic modes to be rather high. The cubic phase has a large electron-phonon coupling parameter $$\lambda$$ and critical temperature of several K. Contribution of the hydrogen sublattice to $$\lambda$$ is found to be small in this material, which we understand from strong coupling theory to be due to the small H-$s$ DOS at the Fermi level and high energy of hydrogen modes at the tetrahedral sites.

Authors:
 [1];  [1];  [1]
  1. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1266003
Grant/Contract Number:
AC05-00OR22725
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Scientific Reports
Additional Journal Information:
Journal Volume: 6; Journal ID: ISSN 2045-2322
Publisher:
Nature Publishing Group
Country of Publication:
United States
Language:
English
Subject:
electronic structure; superconducting properties and materials

Citation Formats

Shanavas, Kavungal Veedu, Lindsay, Lucas R., and Parker, David S. Electronic structure and electron-phonon coupling in TiH$_2$. United States: N. p., 2016. Web. doi:10.1038/srep28102.
Shanavas, Kavungal Veedu, Lindsay, Lucas R., & Parker, David S. Electronic structure and electron-phonon coupling in TiH$_2$. United States. doi:10.1038/srep28102.
Shanavas, Kavungal Veedu, Lindsay, Lucas R., and Parker, David S. 2016. "Electronic structure and electron-phonon coupling in TiH$_2$". United States. doi:10.1038/srep28102. https://www.osti.gov/servlets/purl/1266003.
@article{osti_1266003,
title = {Electronic structure and electron-phonon coupling in TiH$_2$},
author = {Shanavas, Kavungal Veedu and Lindsay, Lucas R. and Parker, David S.},
abstractNote = {Calculations using first principles methods and strong coupling theory are carried out to understand the electronic structure and superconductivity in cubic and tetragonal TiH$_2$. A large electronic density of states at the Fermi level in the cubic phase arises from Ti-$t_{2g}$ states and leads to a structural instability against tetragonal distortion at low temperatures. However, constraining the in-plane lattice constants diminishes the energy gain associated with the tetragonal distortion, allowing the cubic phase to be stable at low temperatures. Furthermore, calculated phonon dispersions show decoupled acoustic and optic modes arising from Ti and H vibrations, respectively and frequencies of optic modes to be rather high. The cubic phase has a large electron-phonon coupling parameter $\lambda$ and critical temperature of several K. Contribution of the hydrogen sublattice to $\lambda$ is found to be small in this material, which we understand from strong coupling theory to be due to the small H-$s$ DOS at the Fermi level and high energy of hydrogen modes at the tetrahedral sites.},
doi = {10.1038/srep28102},
journal = {Scientific Reports},
number = ,
volume = 6,
place = {United States},
year = 2016,
month = 6
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record

Citation Metrics:
Cited by: 1work
Citation information provided by
Web of Science

Save / Share:
  • On the basis of electron transfer from the divalent element to the PdH{sub 3} entities, the perovskite structure {ital M}PdH{sub 3} ({ital M}=Sr, Eu, Yb) are expected to have similar electronic states at the Fermi energy as superconducting Pd-H{sub {ital x}} and Pd{endash}noble-metal{endash}H{sub {ital x}} and thus could be potential candidates for superconductivity. With this motivation, we have investigated the electronic structure and some aspects of the electron-phonon coupling in these hydrides using the {ital ab} {ital initio} augmented plane-wave method, and the results of the energy bands, the total and partial wave analysis of the density of states, andmore » the electronic contribution, {eta}{sub H} to the electron-phonon coupling are presented. Similar studies for the hydrogen defective material CaPdH{sub 2} are also included. We have found that all these compounds are metallic with essentially filled Pd-{ital d} bands and a small density of states at Fermi level as in PdH. Nevertheless, the calculated values of {eta}{sub H}, which govern the contribution of the hydrogen atoms to the electron-optical phonon coupling parameter {lambda}, although sizeable, are lower than in PdH. This would indicate that these compounds, if superconducting, would have lower values of the superconducting transition temperatures {ital T}{sub {ital c}} than in PdH. {copyright} {ital 1996 The American Physical Society.}« less
  • In addition to the record high superconducting transition temperature (T{sub c}), high temperature cuprate superconductors are characterized by their unusual superconducting properties below T{sub c}, and anomalous normal state properties above T{sub c}. In the superconducting state, although it has long been realized that superconductivity still involves Cooper pairs, as in the traditional BCS theory, the experimentally determined d-wave pairing is different from the usual s-wave pairing found in conventional superconductors. The identification of the pairing mechanism in cuprate superconductors remains an outstanding issue. The normal state properties, particularly in the underdoped region, have been found to be at oddmore » with conventional metals which is usually described by Fermi liquid theory; instead, the normal state at optimal doping fits better with the marginal Fermi liquid phenomenology. Most notable is the observation of the pseudogap state in the underdoped region above T{sub c}. As in other strongly correlated electrons systems, these unusual properties stem from the interplay between electronic, magnetic, lattice and orbital degrees of freedom. Understanding the microscopic process involved in these materials and the interaction of electrons with other entities is essential to understand the mechanism of high temperature superconductivity. Since the discovery of high-T{sub c} superconductivity in cuprates, angle-resolved photoemission spectroscopy (ARPES) has provided key experimental insights in revealing the electronic structure of high temperature superconductors. These include, among others, the earliest identification of dispersion and a large Fermi surface, an anisotropic superconducting gap suggestive of a d-wave order parameter, and an observation of the pseudogap in underdoped samples. In the mean time, this technique itself has experienced a dramatic improvement in its energy and momentum resolutions, leading to a series of new discoveries not thought possible only a decade ago. This revolution of the ARPES technique and its scientific impact result from dramatic advances in four essential components: instrumental resolution and efficiency, sample manipulation, high quality samples and well-matched scientific issues. The purpose of this treatise is to go through the prominent results obtained from ARPES on cuprate superconductors. Because there have been a number of recent reviews on the electronic structures of high-T{sub c} materials, we will mainly present the latest results not covered previously, with a special attention given on the electron-phonon interaction in cuprate superconductors. What has emerged is rich information about the anomalous electron-phonon interaction well beyond the traditional views of the subject. It exhibits strong doping, momentum and phonon symmetry dependence, and shows complex interplay with the strong electron-electron interaction in these materials. ARPES experiments have been instrumental in identifying the electronic structure, observing and detailing the electron-phonon mode coupling behavior, and mapping the doping evolution of the high-T{sub c} cuprates. The spectra evolve from the strongly coupled, polaronic spectra seen in underdoped cuprates to the Migdal-Eliashberg like spectra seen in the optimally and overdoped cuprates. In addition to the marked doping dependence, the cuprates exhibit pronounced anisotropy with direction in the Brillouin zone: sharp quasiparticles along the nodal direction that broaden significantly in the anti-nodal region of the underdoped cuprates, an anisotropic electron-phonon coupling vertex for particular modes identified in the optimal and overdoped compounds, and preferential scattering across the two parallel pieces of Fermi surface in the antinodal region for all doping levels. This also contributes to the pseudogap effect. To the extent that the Migdal-Eliashberg picture applies, the spectra of the cuprates bear resemblance to that seen in established strongly coupled electron-phonon superconductors such as Pb. On the other hand, the cuprates deviate from this conventional picture. In the underdoped regime, the carriers are best understood as small polarons in an antiferromagnetic, highly electron correlated background, while the doped compounds require an anisotropic electron-phonon vertex to detail the prominent mode coupling signatures in the superconducting state. Electronic vertex corrections to the electron-phonon coupling furthermore may enhance, and for certain phonons, determine, the anisotropy of the electron-phonon coupling. A consistent picture emerges of the cuprates, combining strong, anisotropic electron-phonon coupling, particular phonon modes that could give rise to such a coupling, and an electron-electron interaction modifying the el-ph vertex.« less
  • The evidence for sharp structure in the electronic density of states N(epsilon) of certain classes of high T/sub c/ superconductors is based on the normal state temperature dependence of the spin susceptibility chi, electronic specific-heat coefficient ..gamma.., electrical resistivity rho, and nuclear spin-lattice relaxation rate 1/T/sub 1/. Model calculations of chi, ..gamma.., rho, and T/sub 1/ in the rigid N(epsilon) approximation and of chi and ..gamma.. for a temperature dependent N(epsilon) are presented for V/sub 3/Ga and V/sub 3/Si. The model calculations have been used to calculate the explicit temperature dependence of the electron mass enhancement and the renormalized electronicmore » specific heat coefficient. The temperature dependence of the dielectric screening, which can have an important effect on renormalizing the phonon frequencies at low temperatures, is also discussed.« less
  • The electronic structure, optical and x-ray absorption spectra, angledependence of the cyclotron masses and extremal cross sections of the Fermisurface, phonon spectra, electron-phonon Eliashberg and transport spectralfunctions, temperature dependence of electrical resistivity of the MB2 (M=Tiand Zr) diborides were investigated from first principles using the fullpotential linear muffin-tin orbital method. The calculations of the dynamicmatrix were carried out within the framework of the linear response theory. Agood agreement with experimental data of optical and x-ray absorption spectra,phonon spectra, electron-phonon spectral functions, electrical resistivity,cyclotron masses and extremal cross sections of the Fermi surface was achieved.
  • The electronic dephasing (spectral dynamics) and electron{endash}phonon coupling of aluminum phthalocyanine tetrasulphonate (APT) in glassy films of ethanol and methanol were investigated by nonphotochemical hole burning over a broad temperature range, {approximately}5{endash}100 K. Films formed by hyperquenching ({approximately}10{sup 6} Ks{sup {minus}1}) at 4.7 K were studied as well as films that were subsequently annealed at temperatures up to {approximately}170 K. Results are compared against those for APT in glassy water [Kim {ital et al.}, J. Phys. Chem. {bold 99}, 7300 (1995); Reinot {ital et al.}, J. Chem. Phys. {bold 104}, 793 (1996)]. As in the case of water, the linearmore » coupling is weak with a Huang{endash}Rhys factor S{approximately}0.4 but the mean phonon frequencies for ethanol and methanol of 26 and 17 cm{sup {minus}1} are considerably lower than the 38 cm{sup {minus}1} value for water. These modes are assigned as pseudolocalized with significant amplitude (libration) localized on APT. Below about 8 K, the electronic dephasing/spectral diffusion is dominated by coupling to the tunneling intrinsic two-level systems of the glass. At higher temperatures the electronic dephasing is dominated by the exchange coupling mechanism, which derives from diagonal quadratic electron{endash}phonon coupling. Here, for both ethanol and water, a pseudolocalized mode(s) at {approximately}50 cm{sup {minus}1} is operative. This frequency corresponds to a peak in the spectral density of the liquids which for water is due to the transverse acoustic mode. The results show that the modes responsible for linear and quadratic coupling are distinctly different. Implications of this for optical coherence loss in liquids are considered. Novel results from annealing experiments are reported and discussed in terms of the complex phase diagrams of ethanol and methanol. (Abstract Truncated)« less