66 research outputs found
Development of Density-Functional Theory for Plasmon-Assisted Superconducting State: Application to Lithium Under High Pressures
We extend the density-functional theory for superconductors (SCDFT) to take
account of the dynamical structure of the screened Coulomb interaction. We
construct an exchange-correlation kernel in the SCDFT gap equation on the basis
of the random-phase approximation, where electronic collective excitations such
as plasmons are properly treated. Through an application to fcc lithium under
high pressures, we demonstrate that our new kernel gives higher transition
temperatures (Tc) when the plasmon and phonon cooperatively mediate pairing and
it improves the agreement between the calculated and experimentally observed
Tc. The present formalism opens the door to non-empirical studies on
unconventional electron mechanisms of superconductivity based on density
functional theory.Comment: 5 pages, 4 figures, title has been changed from that of the
previously uploaded version for publication in Phys. Rev. Let
Density Functional Theory for Plasmon-Assisted Superconductivity
We review the recent progress in the density functional theory for
superconductors (SCDFT). Motivated by the long-studied plasmon mechanism of
superconductivity, we have constructed an exchange-correlation kernel entering
the SCDFT gap equation which includes the plasmon effect. For the case of
lithium under high pressures, we show that the plasmon effect substantially
enhances the transition temperature (Tc) by cooperating with the conventional
phonon mechanism and results in a better agreement between the theoretical and
experimentally observed Tc. Our present formalism will be a first step to
density functional theory for unconventional superconductors.Comment: 9 pages, 7 figures; accepted for publication in J. Phys. Soc. Jpn.
Special Topics; conference proceedings of The International Conference on
Strongly Correlated Electron Systems (SCES) 201
Revisiting the homogeneous electron gas in pursuit of the properly normed ab initio Eliashberg theory
We address an issue of how to accurately include the self energy effect of
the screened electron-electron Coulomb interaction in the phonon-mediated
superconductors from first principles. In the Eliashberg theory for
superconductors, self energy is usually decomposed using the Pauli
matrices in the electron-hole space. We examine how the diagonal (
and ) components, which results in the quasiparticle correction to
the normal state, behave in the homogeneous electron gas in order to establish
a norm of treating those components in real metallic systems. Within the
approximation, we point out that these components are non-analytic
near the Fermi surface but their directional derivatives and resulting
corrections to the quasiparticle velocity are nevertheless well defined. In the
low-energy spectrum, we observe large cancellation between effects of these
components and, without the numerically more tedious component,
the effective mass is incorrectly increased. Feasible paths to manage this
cancellation in the ab initio Eliashberg calculations are discussed.Comment: 8 pages, 2 figure
Strong bilayer coupling induced by the symmetry breaking in the monoclinic phase of BiS-based superconductors
We perform first-principles band structure calculations for the tetragonal
and monoclinic structures of LaOFBiS. We find that the Bi
bands on two BiS layers exhibit a sizable splitting at the X =
(, 0, 0) and several other \textbf{k}-points for the monoclinic structure.
We show that this feature originates from the inter-BiS layer coupling
strongly enhanced by the symmetry breaking of the crystal structure. The Fermi
surface also shows a large splitting and becomes anisotropic with respect to
the and directions in the monoclinic structure, whereas it remains
almost flat with respect to the direction.Comment: 10 pages, 7 figure
Application of Coulomb energy density functional for atomic nuclei: Case studies of local density approximation and generalized gradient approximation
We test the Coulomb exchange and correlation energy density functionals of
electron systems for atomic nuclei in the local density approximation (LDA) and
the generalized gradient approximation (GGA). For the exchange Coulomb
energies, it is found that the deviation between the LDA and GGA ranges from
around in to around in , by taking the Perdew-Burke-Ernzerhof (PBE) functional
as an example of the GGA\@. For the correlation Coulomb energies, it is shown
that those functionals of electron systems are not suitable for atomic nuclei.Comment: 22 pages, 9 figures, 2 table
Interference of the Bloch phase in layered materials with stacking shifts
In periodic systems, electronic wave functions of the eigenstates exhibit the
periodically modulated Bloch phases and are characterized by their wave numbers
. We theoretically address the effects of the Bloch phase in general
layered materials with stacking shift. When the interlayer shift and the Bloch
wave vector satisfy certain conditions, interlayer transitions of
electrons are prohibited by the interference of the Bloch phase. We specify the
manifolds in the space where the hybridization of the Bloch states
between the layers is suppressed in accord with the stacking shift. These
manifolds, named stacking-adapted interference manifolds (SAIM), are obviously
applicable to general layered materials regardless of detailed atomic
configuration within the unit cell. We demonstrate the robustness and
usefulness of the SAIM with first-principles calculations for layered boron
nitride, transition-metal dichalcogenide, graphite, and black phosphorus. We
also apply the SAIM to general three-dimensional crystals to derive special
-point paths for the respective Bravais lattices, along which the
Bloch-phase interference strongly suppresses the band dispersion. Our theory
provides a general and novel view on the anisotropic electronic kinetics
intrinsic to the periodic-lattice structure.Comment: 30 pages, 30 figures. v2: Figures added and Reference list corrected.
v3: Upon publication, data on black phosphorene added, title changed,
derivation of some formulae refined, et
First-principles study of phonon anharmonicity and negative thermal expansion in ScF3
The microscopic origin of the large negative thermal expansion of cubic
scandium trifluorides (ScF3) is investigated by performing a set of anharmonic
free-energy calculations based on density functional theory. We demonstrate
that the conventional quasiharmonic approximation (QHA) completely breaks down
for ScF3 and the quartic anharmonicity, treated nonperturbatively by the
self-consistent phonon theory, is essential to reproduce the observed
transition from negative to positive thermal expansivity and the hardening of
the R4+ soft mode with heating. In addition, we show that the contribution from
the cubic anharmonicity to the vibrational free energy, evaluated by the
improved self-consistent phonon theory, is significant and as important as that
from the quartic anharmonicity for robust understandings of the temperature
dependence of the thermal expansion coefficient. The first-principles approach
of this study enables us to compute various thermodynamic properties of solids
in the thermodynamic limit with the effects of cubic and quartic
anharmonicities. Therefore, it is expected to solve many known issues of the
QHA-based predictions particularly noticeable at high temperature and in
strongly anharmonic materials
Neural-network Kohn-Sham exchange-correlation potential and its out-of-training transferability
We incorporate in the Kohn-Sham self consistent equation a trained
neural-network projection from the charge density distribution to the
Hartree-exchange-correlation potential for possible
numerical approach to the exact Kohn-Sham scheme. The potential trained through
a newly developed scheme enables us to evaluate the total energy without
explicitly treating the formula of the exchange-correlation energy. With a case
study of a simple model we show that the well-trained neural-network achieves accuracy for the charge density and total energy out of the
model parameter range used for the training, indicating that the property of
the elusive ideal functional form of can approximately be
encapsulated by the machine-learning construction. We also exemplify a factor
that crucially limits the transferability--the boundary in the model parameter
space where the number of the one-particle bound states changes--and see that
this is cured by setting the training parameter range across that boundary. The
training scheme and insights from the model study apply to more general
systems, opening a novel path to numerically efficient Kohn-Sham potential.Comment: 7 pages, 6 figure
Density Functional Theory for Superconductors with Particle-hole Asymmetric Electronic Structure
To extend the applicability of density functional theory for superconductors
(SCDFT) to systems with significant particle-hole asymmetry, we construct a new
exchange-correlation kernel entering the gap equation. We show that the kernel
is numerically stable and does not diverge even in the low temperature limit.
Solving the gap equation for model systems with the present kernel analytically
and numerically, we find that the asymmetric component of electronic density of
states, which has not been considered with the previous kernel, systematically
decreases transition temperature (Tc). We present a case where the decrease of
Tc amounts to several tens of percent.Comment: 12 pages, 7 figure
Non-empirical Study of Superconductivity in Alkali-doped Fullerides Based on Density Functional Theory for Superconductors
We apply the density functional theory for superconductors (SCDFT) based on
the local-density approximation (LDA) to alkali-doped fullerides A3C60 with the
face-centered cubic structure. We evaluate the superconducting transition
temperature (Tc) from first principles considering energy dependence of
electron-phonon coupling, the mass renormalization, and the retardation effect.
The calculated Tc=7.5, 9.0 and 15.7 K for A=K, Rb, Cs are approximately 60 %
smaller than the experimentally observed values. Our results strongly suggest
necessity to go beyond the framework of the Migdal-Eliashberg theory based on
the LDA.Comment: 5 pages, 4 figure
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