378 research outputs found
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
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
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
Electronic Structure Calculation and Superconductivity in -(BETS)GaCl
Quasi-two-dimensional molecular conductor -(BETS)GaCl shows
superconductivity (SC) below 5.5K, neighboring the dimer-type Mott insulating
phase. To elucidate the origin of SC and its gap function, we carry out
first-principles band calculation and derive a four-band tight-binding model
from the maximally localized Wannier orbitals. Considering the
spin-fluctuation-mediated mechanism by adding the Hubbard -term to the
model, we analyze the SC gap function by applying the random phase
approximation. We show that the SC gap changes its sign four times along the
Fermi surface (FS) in the unfolded Brillouin zone, suggestive of a
-wave-like SC gap, which only has two-fold symmetry because of the low
symmetry of the crystal structure. Decomposing the SC gap into the pairing
functions along the crystal axes, we compare the result to similar analysis of
the well-studied -type molecular conductors and to the experiments.Comment: 5 pages, 2 figures, 3 table
Doppler shift picture of the Dzyaloshinskii--Moriya interaction
We present a physical picture for the emergence of the Dzyaloshinskii--Moriya
(DM) interaction based on the idea of the Doppler shift by an intrinsic spin
current induced by spin--orbit interaction under broken inversion symmetry. The
picture is confirmed by a rigorous effective Hamiltonian theory, which reveals
that the DM coefficient is given by the magnitude of the intrinsic spin
current. The expression is directly applicable to first principles calculations
and clarifies the relation between the interaction and the electronic band
structures. Quantitative agreement with experimental results is obtained for
the skyrmion compounds MnFeGe and FeCoGe.Comment: 5 pages, 4 figures; v2 references adde
In-plane electric polarization of bilayer graphene nanoribbons by interlayer bias voltage
We theoretically show that an interlayer bias voltage in the AB-stacked
bilayer graphene nanoribbons with armchair edges induces an electric
polarization along the ribbon. Both tight-binding and ab initio calculations
consistently indicate that when the bias voltage is weak, the polarization
shows opposite signs depending on the ribbon width modulo three. This
nontrivial dependence is explained using a two-band effective model. A strong
limit of the bias voltage in the tight-binding model shows either one-third or
zero polarization, which agrees with topological argument.Comment: 11 pages, 9 figures, accepted by PR
Effect of van Hove singularities on high-Tc superconductivity in H3S
One of interesting open questions for the high transition temperature (Tc)
superconductivity in sulfur hydrides is why high pressure phases of H3S have
extremely high Tc's. Recently, it has been pointed out that the presence of the
van Hove singularities (vHs) around the Fermi level is crucial. However, while
there have been quantitative estimates of Tc based on the Migdal-Eliashberg
theory, the energy dependence of the density of states (DOS) has been neglected
to simplify the Eliashberg equation. In this study, we go beyond the constant
DOS approximation and explicitly consider the electronic structure over 40eV
around the Fermi level. In contrast with the previous conventional
calculations, this approach with a sufficiently large number of Matsubara
frequencies enables us to calculate Tc without introducing the empirical pseudo
Coulomb potential. We show that while H3S has much higher Tc than H2S for which
the vHs is absent, the constant DOS approximation employed so far seriously
overestimates (underestimates) Tc by ~ 60K (~ 10K) for H3S (H2S). We then
discuss the impact of the strong electron-phonon coupling on the electronic
structure with and without the vHs and how it affects the superconductivity.
Especially, we focus on (1) the feedback effect in the self-consistent
calculation of the self-energy, (2) the effect of the energy shift due to the
zero-point motion, and (3) the effect of the changes in the phonon frequencies
due to strong anharmonicity. We show that the effect of (1)-(3) on Tc is about
10-30K for both H3S and H2S. Eventually, Tc is estimated to be 181K for H3S at
250GPa and 34K for H2S at 140GPa, which explains the pressure dependence of Tc
observed in the experiment. In addition, we evaluate the lowest order vertex
correction beyond the Migdal-Eliashberg theory and discuss the validity of the
Migdal approximation for sulfur hydrides
Formation mechanism of helical Q structure in Gd-based skyrmion materials
Using the ab initio local force method, we investigate the formation
mechanism of the helical spin structure in GdRuSi and GdPdSi.
We calculate the paramagnetic spin susceptibility and find that the Fermi
surface nesting is not the origin of the incommensurate modulation, in contrast
to the naive scenario based on the Ruderman-Kittel-Kasuya-Yosida mechanism. We
then decompose the exchange interactions between the Gd spins into each orbital
component, and show that spin-density-wave type interaction between the Gd-5
orbitals is ferromagnetic, but the interaction between the Gd-4 orbitals is
antiferromagnetic. We conclude that the competition of these two interactions,
namely, the inter-orbital frustration, stabilizes the finite-Q structure.Comment: 5 pages, 4 figure
Local force method for the ab initio tight-binding model with spin-dependent hopping
To estimate the Curie temperature of metallic magnets from first principles,
we develop a local force method for the tight-binding model having
spin-dependent hopping derived from spin density functional theory. While
spin-dependent hopping is crucial for the self-consistent mapping to the
effective spin model, the numerical cost to treat such non-local terms in the
conventional Green's function scheme is formidably expensive. Here, we propose
a formalism based on the kernel polynomial method (KPM), which makes the
calculation dramatically efficient. We perform a benchmark calculation for
bcc-Fe, fcc-Co, and fcc-Ni and find that the effect of the magnetic non-local
terms is particularly prominent for bcc-Fe. We also present several local
approximations to the magnetic non-local terms for which we can apply the
Green's function method and reduce the numerical cost further by exploiting the
intermediate representation of the Green's function. By comparing the results
of the KPM and local methods, we discuss which local method works most
successfully. Our approach provides an efficient way to estimate the Curie
temperature of metallic magnets with a complex spin configuration.Comment: 10 pages, 3 figure
Field-direction control of the type of charge carriers in nonsymmorphic IrO2
In the quest for switching of the charge carrier type in conductive
materials, we focus on nonsymmorphic crystals, which are expected to have
highly anisotropic folded Fermi surfaces due to the symmetry requirements.
Following simple tight-binding model simulation, we prepare nonsymmorphic IrO2
single-crystalline films with various growth orientations by molecular beam
epitaxy, and systematically quantify their Hall effect for the corresponding
field directions. The results clearly demonstrate that the dominant carrier
type can be intrinsically controlled by the magnetic field direction, as also
evidenced by first-principles calculations revealing nontrivial momentum
dependence of the group velocity and mass tensor on the folded Fermi surfaces
and its anisotropic nature for the field direction.Comment: 12 pages, 4 figure
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