254 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
Exotic pairing state in quasicrystalline superconductors under magnetic field
We theoretically study the effect of a magnetic field on quasicrystalline
superconductors, by modelling them as the attractive Hubbard model on the
Penrose-tiling structure. We find that at low temperatures and under a high
magnetic field there appears an exotic superconducting state with the order
parameter changing its sign in real space. We discuss the state in comparison
with the Fulde-Ferrell-Larkin-Ovchinnikov state proposed many years ago for
periodic systems, clarifying commonalities and differences. It is remarkable
that, even in the absence of periodicity, the electronic system finds a way to
keep a coherent superconducting state with a spatially sign-changing order
parameter compatible with the underlying quasiperiodic structure.Comment: 7 pages, 8 figure
Ab initio downfolding for electron-phonon coupled systems: constrained density-functional perturbation theory (cDFPT)
We formulate an ab initio downfolding scheme for electron-phonon coupled
systems. In this scheme, we calculate partially renormalized phonon frequencies
and electron-phonon coupling, which include the screening effects of
high-energy electrons, to construct a realistic Hamiltonian consisting of
low-energy electron and phonon degrees of freedom. We show that our scheme,
which we call constrained density-functional perturbation theory (cDFPT), can
be implemented by slightly modifying the conventional DFPT, which is one of the
standard methods to calculate phonon properties from first principles. Our
scheme can be applied to various phonon-related problems, such as
superconductivity, electron and thermal transport, thermoelectricity,
piezoelectricity, dielectricity and multiferroicity. We believe that the cDFPT
provides a firm basis for the understanding of the role of phonons in strongly
correlated materials. Here, we apply the scheme to the fullerene
superconductors and discuss how the realistic low-energy Hamiltonian is
constructed.Comment: 18 pages, 5 figures, 4 table
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
Development of a two-particle self-consistent method for multi-orbital systems and its application to unconventional superconductors
We extend the two-particle self-consistent method proposed by Vilk and
Tremblay (J. Phys. I France 7, 1309-1368 (1997)) to study superconductivity in
multi-orbital systems. Starting with the sum rules for the spin and charge
susceptibilities, we derive self-consistent equations to determine the
renormalized effective interactions. We apply this method to the two-orbital
- model for LaCuO and the five-orbital
-model for LaFeAsO. Comparing the results with those of the random phase
approximation or the fluctuation exchange approximation in which vertex
corrections are ignored, we discuss how the vertex corrections affect the
pairing instability of LaCuO and the dominant pairing symmetry of
LaFeAsO.Comment: 11 pages, 14 figures, published by Phys. Rev.
Ab initio Derivation of Correlated Superatom Model for Potassium Loaded Zeolite A
We derive an effective low-energy Hamiltonian for potassium loaded zeolite A,
a unique ferromagnet from non-magnetic elements. We perform ab initio density
functional calculations and construct maximally localized Wannier functions for
low-energy states made from potassium s electrons. The resulting Wannier
orbitals, spreading widely in the alminosilicate cage, are found to be the
superatomic s and p orbitals in the confining potential formed by the host
cage. We then make a tight-binding model for these superatomic orbitals and
introduce interaction parameters such as the Hubbard U. After mean-field
calculations for the effective model, we find that ab initio spin density
functional results are well reproduced by choosing appropriate sets of the
interaction parameters. The interaction parameters turn out to be as large as
the band width, 0.5 eV, indicating the importance of electron
correlation, and that the present system is an interesting analog of correlated
multi-orbital transition metal oxides.Comment: 9 pages, 6 figures, and the top margin was adjuste
Ab initio Derivation of Low-Energy Model for Alkali-Cluster-Loaded Sodalites
An effective low-energy model describing magnetic properties of
alkali-cluster-loaded sodalites is derived by {\em ab initio} downfolding. We
start with constructing an extended Hubbard model for maximally localized
Wannier functions. {\em Ab initio} screened Coulomb and exchange interactions
are calculated by constrained random phase approximation. We find that the
system resides in the strong coupling regime and thus the Heisenberg model is
derived as a low-energy model of the extended Hubbard model. We obtain
antiferromagnetic couplings (10 K), being consistent with the
experimental temperature dependence of the spin susceptibility. Importance of
considering the screening effect in the derivation of the extended Hubbard
model is discussed.Comment: 9 pages, 5 figures, 2 table
Physical properties of weak-coupling quasiperiodic superconductors
We numerically study the physical properties of quasiperiodic superconductors
with the aim of understanding superconductivity in quasicrystals. Considering
the attractive Hubbard model on the Penrose tiling as a simple theoretical
model, we calculate various basic superconducting properties and find
deviations from the universal values of the Bardeen-Cooper-Schrieffer theory.
In particular, we find that the jump of the specific heat at the
superconducting transition is about 10-20% smaller than that universal value,
in consistency with the experimental results obtained for the superconducting
Al-Mg-Zn quasicrystalline alloy. Furthermore, we calculate current-voltage
characteristics and find that the current gradually increases with the voltage
on the Penrose tiling in contrast to a rapid increase in the periodic system.
These distinctions originate from the nontrivial Cooper pairing characteristic
to the quasiperiodic system.Comment: 8 pages, 7 figure
Control of Dzyaloshinskii-Moriya interaction in MnFeGe: a first-principles study
Motivated by the recent experiment on the size and helicity control of
skyrmions in MnFeGe [K. Shibata et al., Nature Nanotechnology 8,
732 (2013)], we study how the Dzyaloshinskii-Moriya (DM) interaction changes
its size and sign in metallic helimagnets. By means of first-principles
calculations, we successfully reproduce the non-trivial sign change of the DM
interaction observed in the experiment. While the DM interaction sensitively
depends on the carrier density or the detail of the electronic structure such
as the size of the exchange splitting, its behavior can be systematically
understood in terms of the distribution of anticrossing points in the band
structure. By following this guiding principle, we can even induce gigantic
anisotropy in the DM interaction by applying a strain to the system. These
results pave the new way for skyrmion crystal engineering in metallic
helimagnets.Comment: 7 pages, 8 figures, 1 tabl
First-Principles Evaluation of the Dzyaloshinskii--Moriya Interaction
We review recent developments of formulations to calculate the
Dzyaloshinskii--Moriya (DM) interaction from first principles. In particular,
we focus on three approaches. The first one evaluates the energy change due to
the spin twisting by directly calculating the helical spin structure. The
second one employs the spin gauge field technique to perform the derivative
expansion with respect to the magnetic moment. This gives a clear picture that
the DM interaction can be represented as the spin current in the equilibrium
within the first order of the spin-orbit couplings. The third one is the
perturbation expansion with respect to the exchange couplings and can be
understood as the extension of the Ruderman--Kittel--Kasuya--Yosida (RKKY)
interaction to the noncentrosymmetric spin-orbit systems. By calculating the DM
interaction for the typical chiral ferromagnets MnFeGe and
FeCoGe, we discuss how these approaches work in actual systems.Comment: invited review pape
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