8,496 research outputs found
Ab-initio procedure for effective models of correlated materials with entangled band structure
We present a first-principles method for deriving effective low-energy models
of electrons in solids having entangled band structure. The procedure starts
with dividing the Hilbert space into two subspaces, the low-energy part ("
space'') and the rest of the space (" space''). The low-energy model is
constructed for the space by eliminating the degrees of freedom of the
space. The thus derived model contains the strength of electron correlation
expressed by a partially screened Coulomb interaction, calculated in the
constrained random-phase-approximation (cRPA) where screening channels within
the space, , are subtracted. One conceptual problem of this
established downfolding method is that for entangled bands it is not clear how
to cut out the space and how to distinguish from the total
polarization. Here, we propose a simple procedure to overcome this difficulty.
In our scheme, the subspace is cut out from the Hilbert space of the Kohn
Sham eigenfunctions with the help of a procedure to construct a localized
Wannier basis. The subspace is constructed as the complementary space
orthogonal to the subspace. After this disentanglement, becomes well
defined. Using the disentangled bands, the effective parameters are uniquely
determined in the cRPA. The method is successfully applied to 3 transition
metals.Comment: 14 pages, 4 figure
Realistic many-body models for Manganese Monoxide under pressure
In materials like transition metals oxides where electronic Coulomb
correlations impede a description in terms of standard band-theories, the
application of genuine many-body techniques is inevitable. Interfacing the
realism of density-functional based methods with the virtues of Hubbard-like
Hamiltonians, requires the joint ab initio construction of transfer integrals
and interaction matrix elements (like the Hubbard U) in a localized basis set.
In this work, we employ the scheme of maximally localized Wannier functions and
the constrained random phase approximation to create effective low-energy
models for Manganese monoxide, and track their evolution under external
pressure. We find that in the low pressure antiferromagnetic phase, the
compression results in an increase of the bare Coulomb interaction for specific
orbitals. As we rationalized in recent model considerations [Phys. Rev. B 79,
235133 (2009)], this seemingly counter-intuitive behavior is a consequence of
the delocalization of the respective Wannier functions. The change of screening
processes does not alter this tendency, and thus, the screened on-site
component of the interaction - the Hubbard U of the effective low-energy system
- increases with pressure as well. The orbital anisotropy of the effects
originates from the orientation of the orbitals vis-a-vis the deformation of
the unit-cell. Within the high pressure paramagnetic phase, on the other hand,
we find the significant increase of the Hubbard U is insensitive to the orbital
orientation and almost exclusively owing to a substantial weakening of
screening channels upon compression.Comment: 13 pages, 6 figure
Supersymmetric Matrix model on Z-orbifold
We find that the IIA Matrix models defined on the non-compact ,
and orbifolds preserve supersymmetry where the fermions
are on-mass-shell Majorana-Weyl fermions. In these examples supersymmetry is
preserved both in the orbifolded space and in the non-orbifolded space at the
same time. The Matrix model on orbifold has the same
supersymmetry as the case of
orbifold which was pointed out previously.
On the other hand the Matrix models on and orbifold have
a half of the supersymmetry. We further find that the Matrix model
on orbifold with a parity-like identification preserves
supersymmetry.Comment: 21 pages, no figur
Theoretical evidence for strong correlations and incoherent metallic state in FeSe
The role of electronic Coulomb correlations in iron-based superconductors is
an important open question. We provide theoretical evidence for strong
correlation effects in the FeSe compound, based on dynamical mean field
calculations. A lower Hubbard band is found in the spectral properties.
Moreover, together with significant orbital-dependent mass enhancements, we
find that the normal state is a bad metal over an extended temperature range,
implying a non-Fermi liquid. Predictions for angle-resolved photoemission
spectroscopy are made.Comment: 5 pages, 5 figures, published versio
Valence Instability and Superconductivity in Heavy Fermion Systems
Many cerium-based heavy fermion (HF) compounds have pressure-temperature
phase diagrams in which a superconducting region extends far from a magnetic
quantum critical point. In at least two compounds, CeCu2Si2 and CeCu2Ge2, an
enhancement of the superconducting transition temperature was found to coincide
with an abrupt valence change, with strong circumstantial evidence for pairing
mediated by critical valence, or charge transfer, fluctuations. This pairing
mechanism, and the valence instability, is a consequence of a f-c Coulomb
repulsion term U_fc in the hamiltonian. While some non-superconducting Ce
compounds show a clear first order valence instability, analogous to the Ce
alpha-gamma transition, we argue that a weakly first order valence transition
may be a general feature of Ce-based HF systems, and both magnetic and critical
valence fluctuations may be responsible for the superconductivity in these
systems.Comment: 11 pages, 16 figure
Roles of Critical Valence Fluctuations in Ce- and Yb-Based Heavy Fermion Metals
The roles of critical valence fluctuations of Ce and Yb are discussed as a
key origin of several anomalies observed in Ce- and Yb-based heavy fermion
systems. Recent development of the theory has revealed that a magnetic field is
an efficient control parameter to induce the critical end point of the
first-order valence transition. Metamagnetism and non-Fermi liquid behavior
caused by this mechanism are discussed by comparing favorably with CeIrIn5,
YbAgCu4, and YbIr2Zn20. The interplay of the magnetic order and valence
fluctuations offers a key concept for understanding Ce- and Yb-based systems.
It is shown that suppression of the magnetic order by enhanced valence
fluctuations gives rise to the coincidence of the magnetic-transition point and
valence-crossover point at absolute zero as a function of pressure or magnetic
field. The interplay is shown to resolve the outstanding puzzle in CeRhIn5 in a
unified way. The broader applicability of this newly clarified mechanism is
discussed by surveying promising materials such as YbAuCu4, beta-YbAlB4, and
YbRh2Si2.Comment: 17 pages, 8 figures, invited paper in special issue on strongly
correlated electron system
The effects of k-dependent self-energy in the electronic structure of correlated materials
It is known from self-energy calculations in the electron gas and sp
materials based on the GW approximation that a typical quasiparticle
renormalization factor (Z factor) is approximately 0.7-0.8. Band narrowing in
electron gas at rs = 4 due to correlation effects, however, is only
approximately 10%, significantly smaller than the Z factor would suggest. The
band narrowing is determined by the frequency-dependent self-energy, giving the
Z factor, and the momentum-dependent or nonlocal self-energy. The results for
the electron gas point to a strong cancellation between the effects of
frequency- and momentum-dependent self-energy. It is often assumed that for
systems with a nar- row band the self-energy is local. In this work we show
that even for narrow-band materials, such as SrVO3, the nonlocal self-energy is
important.Comment: 7 pages, 6 figure
Quantum Valence Criticality as Origin of Unconventional Critical Phenomena
It is shown that unconventional critical phenomena commonly observed in
paramagnetic metals YbRh2Si2, YbRh2(Si0.95Ge0.05)2, and beta-YbAlB4 is
naturally explained by the quantum criticality of Yb-valence fluctuations. We
construct the mode coupling theory taking account of local correlation effects
of f electrons and find that unconventional criticality is caused by the
locality of the valence fluctuation mode. We show that measured low-temperature
anomalies such as divergence of uniform spin susceptibility \chi T^{-\zeta)
with giving rise to a huge enhancement of the Wilson ratio and the
emergence of T-linear resistivity are explained in a unified way.Comment: 5 pages, 3 figures, to be published in Physical Review Letter
A theory of new type of heavy-electron superconductivity in PrOs_4Sb_12: quadrupolar-fluctuation mediated odd-parity pairings
It is shown that unconventional nature of superconducting state of
PrOs_4Sb_12, a Pr-based heavy electron compound with the filled-Skutterudite
structure, can be explained in a unified way by taking into account the
structure of the crystalline-electric-field (CEF) level, the shape of the Fermi
surface determined by the band structure calculation, and a picture of the
quasiparticles in f-configuration with magnetically singlet CEF ground
state. Possible types of pairing are narrowed down by consulting recent
experimental results. In particular, the chiral "p"-wave states such as
p_x+ip_y is favoured under the magnetic field due to the orbital Zeeman effect,
while the "p"-wave states with two-fold symmetery such as p_x can be stabilized
by a feedback effect without the magnetic field. It is also discussed that the
double superconducting transition without the magnetic field is possible due to
the spin-orbit coupling of the "triplet" Cooper pairs in the chiral state.Comment: 12 pages, 2 figures, submitted to J. Phys.: Condens. Matter Lette
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