3,623 research outputs found
Nodal/Antinodal Dichotomy and the Two Gaps of a Superconducting Doped Mott Insulator
We study the superconducting state of the hole-doped two-dimensional Hubbard
model using Cellular Dynamical Mean Field Theory, with the Lanczos method as
impurity solver. In the under-doped regime, we find a natural decomposition of
the one-particle (photoemission) energy-gap into two components. The gap in the
nodal regions, stemming from the anomalous self-energy, decreases with
decreasing doping. The antinodal gap has an additional contribution from the
normal component of the self-energy, inherited from the normal-state pseudogap,
and it increases as the Mott insulating phase is approached.Comment: Corrected typos, 4.5 pages, 4 figure
Signature of antiferromagnetic long-range order in the optical spectrum of strongly correlated electron systems
We show how the onset of a non-Slater antiferromagnetic ordering in a
correlated material can be detected by optical spectroscopy. Using dynamical
mean-field theory we identify two distinctive features: The antiferromagnetic
ordering is associated with an enhanced spectral weight above the optical gap,
and well separated spin-polaron peaks emerge in the optical spectrum. Both
features are indeed observed in LaSrMnO_4 [G\"ossling et al., Phys. Rev. B 77,
035109 (2008)]Comment: 11 pages, 9 figure
Superconductivity in the Two-Band Hubbard Model in Infinite Dimensions
We study a two-band Hubbard model in the limit of infinite dimensions, using
a combination of analytical methods and Monte-Carlo techniques. The normal
state is found to display various metal to insulators transitions as a function
of doping and interaction strength. We derive self-consistent equations for the
local Green's functions in the presence of superconducting long-range order,
and extend previous algorithms to this case. We present direct numerical
evidence that in a specific range of parameter space, the normal state is
unstable against a superconducting state characterized by a strongly frequency
dependent order-parameter.Comment: 12 pages (14 figures not included, available upon request), Latex,
LPTENS Preprint 93/1
k-dependent spectrum and optical conductivity near metal-insulator transition in multi-orbital Hubbard bands
We apply the dynamical mean field theory (DMFT) in the iterative perturbation
theory(IPT) to doubly degenerate eg bands and triply degenerate tg bands on a
simple cubic lattice and calculate the spectrum and optical conductivity in
arbitrary electron occupation. The spectrum simultaneously shows the effects of
multiplet structure and DMFT together with the electron ionization and affinity
levels of different electron occupations, coherent peaks at the Fermi energy in
the metallic phase and a gap at an integer filling of electrons for
sufficiently large Coulomb U. We also calculate the critical value of the
Coulomb U for degenerate orbitals.Comment: 8 pages, 6 figure
Correlation and surface effects in Vanadium Oxides
Recent photoemission experiments have shown strong surface modifications in
the spectra from vanadium oxides as (V,Cr)_2O_3 or
(Sr,Ca)VO_3. The effective mass is enhanced at the surface and the coherent
part of the surface spectrum is narrowed as compared to the bulk. The
quasiparticle weight is more sensitive at the surface than in the bulk against
bandwidth variations. We investigate these effects theoretically considering
the single-band Hubbard model for a film geometry. A simplified dynamical
mean-field scheme is used to calculate the main features of the interacting
layer-dependent spectral function. It turns out that the experimentally
confirmed effects are inherent properties of a system of strongly correlated
electrons. The reduction of the weight and the variance of the coherent part of
the surface spectrum can be traced back to the reduced surface coordination
number. Surface correlation effects can be strongly amplified by changes of the
hopping integrals at the surface.Comment: to appear in PRB; 8 pages, 6 figure
Competition between electron-phonon attraction and weak Coulomb repulsion
The Holstein-Hubbard model is examined in the limit of infinite dimensions.
Conventional folklore states that charge-density-wave (CDW) order is more
strongly affected by Coulomb repulsion than superconducting order because of
the pseudopotential effect. We find that both incommensurate CDW and
superconducting phases are stabilized by the Coulomb repulsion, but,
surprisingly, the commensurate CDW transition temperature is more robust than
the superconducting transition temperature. This puzzling feature is resolved
by a detailed analysis of perturbation theory.Comment: 13 pages in ReVTex including 3 encapsulated postscript files
(embedded in the text). The encapsulated postscript files are compressed and
uuencoded after the TeX file
Electronic correlations and Hund's coupling effects in SrMoO revealed by photoemission spectroscopy
We investigate the electronic structure of a perovskite-type Pauli paramagnet
SrMoO3 (t2g2) thin film using hard x-ray photoemission spectroscopy and compare
the results to the realistic calculations that combine the density functional
theory within the local-density approximation (LDA) with the dynamical-mean
field theory (DMFT). Despite the clear signature of electron correlations in
the electronic specific heat, the narrowing of the quasiparticle bands is not
observed in the photoemission spectrum. This is explained in terms of the
characteristic effect of Hund's rule coupling for partially-filled t2g bands,
which induces strong quasiparticle renormalization already for values of
Hubbard interaction which are smaller than the bandwidth. The interpretation is
supported by additional model DMFT calculations including Hund's rule coupling,
that show renormalization of low-energy quasiparticles without affecting the
overall bandwidth. The photoemission spectra show additional spectral weight
around -2 eV that is not present in the LDA+DMFT. We interpret this weight as a
plasmon satellite, which is supported by measured Mo, Sr and Oxygen core-hole
spectra that all show satellites at this energy.Comment: 8 pages, 7 figure
The RKKY interactions and the Mott Transition
A two-site cluster generalization of the Hubbard model in large dimensions is
examined in order to study the role of short-range spin correlations near the
metal-insulator transition (MIT). The model is mapped to a two-impurity
Kondo-Anderson model in a self-consistently determined bath, making it possible
to directly address the competition between the Kondo effect and RKKY
interactions in a lattice context. Our results indicate that the RKKY
interactions lead to qualitative modifications of the MIT scenario even in the
absence of long range antiferromagnetic ordering.Comment: 10 pages, 10 figures; to appear in Phys. Rev. B (1999
Origin of the spectral linewidth in non linear oscillators based on MgO tunnel junctions
We demonstrate the strong impact of the oscillator agility on the line
broadening by studying spin transfer induced microwave emission in MgO-based
tunnel junctions with current. The linewidth is almost not affected by
decreasing the temperature. At very low currents, a strong enhancement of the
linewidth at low temperature is attributed to an increase of the non linearity,
probably due to the field-like torque. Finally we evidence that the noise is
not dominated by thermal fluctuations but rather by the chaotization of the
magnetization system induced by the spin transfer torque.Comment: 12 pages, 3 figures, published in Phys. Rev. B 80, 060404 (2009
Self-consistency over the charge-density in dynamical mean-field theory: a linear muffin-tin implementation and some physical implications
We present a simple implementation of the dynamical mean-field theory
approach to the electronic structure of strongly correlated materials. This
implementation achieves full self-consistency over the charge density, taking
into account correlation-induced changes to the total charge density and
effective Kohn-Sham Hamiltonian. A linear muffin-tin orbital basis-set is used,
and the charge density is computed from moments of the many body
momentum-distribution matrix. The calculation of the total energy is also
considered, with a proper treatment of high-frequency tails of the Green's
function and self-energy. The method is illustrated on two materials with
well-localized 4f electrons, insulating cerium sesquioxide Ce2O3 and the
gamma-phase of metallic cerium, using the Hubbard-I approximation to the
dynamical mean-field self-energy. The momentum-integrated spectral function and
momentum-resolved dispersion of the Hubbard bands are calculated, as well as
the volume-dependence of the total energy. We show that full self-consistency
over the charge density, taking into account its modification by strong
correlations, can be important for the computation of both thermodynamical and
spectral properties, particularly in the case of the oxide material.Comment: 20 pages, 6 figures (submitted in The Physical Review B
- …