77 research outputs found
Time-reversal symmetry breaking phase in the Hubbard model: a VCA study
We study the stability of the time-reversal symmetry breaking staggered flux
phase of a single band Hubbard model, within the Variational Cluster Approach
(VCA). For intermediate and small values of the interaction , we find
metastable solutions for the staggered flux phase, with a maximum current per
bond at . However, allowing for antiferromagnetic and
superconducting long-range order it turns out that in the region at and close
to half filling the antiferromagnetic phase is the most favorable
energetically. The effect of nearest-neighbour interaction is also considered.
Our results show that a negative nearest-neighbour interaction and finite
doping favors the stability of the staggered-flux phase. We also present
preliminary results for the three-band Hubbard model obtained with a restricted
set of variational parameters. For this case, no spontaneous time-reversal
symmetry breaking phase is found in our calculations.Comment: 9 pages,8 figure
Half-metallicity and magnetism in the CoMnAl/CoMnVAl heterostructure
We present a study of the electronic structure and magnetism of CoMnAl,
CoMnVAl and their heterostructure. We employ a combination of
density-functional theory and dynamical mean-field theory (DFT+DMFT). We find
that CoMnAl is a half-metallic ferromagnet, whose electronic and magnetic
properties are not drastically changed by strong electronic correlations,
static or dynamic. Non-quasiparticle states are shown to appear in the minority
spin gap without affecting the spin-polarization at the Fermi level predicted
by standard DFT. We find that CoMnVAl is a semiconductor or a semi-metal,
depending on the employed computational approach. We then focus on the
electronic and magnetic properties of the CoMnAl/CoMnVAl heterostructure,
predicted by previous first principle calculations as a possible candidate for
spin-injecting devices. We find that two interfaces, Co-Co/V-Al and
Co-Mn/Mn-Al, preserve the half-metallic character, with and without including
electronic correlations. We also analyse the magnetic exchange interactions in
the bulk and at the interfaces. At the Co-Mn/Mn-Al interface, competing
magnetic interactions are likely to favor the formation of a non-collinear
magnetic order, which is detrimental for the spin-polarization.Comment: 15 pages, 16 figure
Locally self-consistent embedding approach for disordered electronic systems
We present a new embedding scheme for the locally self-consistent method to
study disordered electron systems. We test this method in a tight-binding basis
and apply it to the single band Anderson model. The local interaction zone is
used to efficiently compute the local Green's function of a supercell embeded
into a local typical medium. We find a quick convergence as the size of the
local interaction zone which reduces the computational costs as expected. This
method captures the Anderson localization transition and accurately predicts
the critical disorder strength. The present work opens the path towards the
development of a typical medium embedding scheme for the multiple
scattering methods.Comment: 7 pages, 5 figure
Superconductivity of Bi-III phase of elemental Bismuth: insights from Muon-Spin Rotation and Density Functional Theory
Using muon-spin rotation the pressure-induced superconductivity in the Bi-III
phase of elemental Bismuth (transition temperature K) was
investigated. The Ginzburg-Landau parameter
( is the magnetic penetration depth, is the coherence length)
was estimated which is the highest among single element superconductors. The
temperature dependence of the superconducting energy gap []
reconstructed from deviates from the weak-coupled BCS
prediction. The coupling strength was
estimated thus implying that Bi-III stays within the strong coupling regime.
The Density Functional Theory calculations suggest that superconductivity in
Bi-III could be described within the Eliashberg approach with the
characteristic phonon frequency meV. An alternative
pairing mechanism to the electron-phonon coupling involves the possibility of
Cooper pairing induced by the Fermi surface nesting.Comment: 5 pages, 4 figure
Electronic Correlations in Vanadium Revealed by Electron-Positron Annihilation Measurements
The electronic structure of vanadium measured by Angular Correlation of
electron-positron Annihilation Radiation (ACAR) is compared with the
predictions of the combined Density Functional and Dynamical Mean-Field Theory
(DMFT). Reconstructing the momentum density from five 2D projections we were
able to determine the full Fermi surface and found excellent agreement with the
DMFT calculations. In particular, we show that the local, dynamic self-energy
corrections contribute to the anisotropy of the momentum density and need to be
included to explain the experimental results
Electron-electron interaction strength in ferromagnetic nickel determined by spin-polarized positron annihilation
The two-photon momentum distribution of annihilating electron-positron pairs
in ferromagnetic nickel (Ni) was determined by measuring the spin-polarized
two-dimensional angular correlation of annihilation radiation (ACAR). The
spectra were compared with theoretical results obtained within LDA+DMFT, a
combination of the local density approximation (LDA) and the many-body
dynamical mean-field theory (DMFT). The self-energy describing the electronic
correlations in Ni is found to make important anisotropic contributions to the
momentum distribution which are not present in LDA. Based on a detailed
comparison of the theoretical and experimental results the strength of the
local electronic interaction U in ferromagnetic Ni is determined as 2.0 +- 0.1
eV
On the superconducting nature of the Bi-II phase of elemental Bismuth
The superconductivity in the Bi-II phase of elemental Bismuth (transition
temperature K at pressure GPa) was studied
experimentally by means of the muon-spin rotation as well as theoretically by
using the Eliashberg theory in combination with Density Functional Theory
calculations. Experiments reveal that Bi-II is a type-I superconductor with a
zero temperature value of the thermodynamic critical field ~mT. The Eliashberg theory approach provides a good agreement
with the experimental and the temperature evolution of .
The estimated value for the retardation (coupling) parameter ( is the logarithmically
averaged phonon frequency) suggests that Bi-II is an intermediately-coupled
superconductor.Comment: 6 pages, 2 figure
Systematic quantum cluster typical medium method for the study of localization in strongly disordered electronic systems
Great progress has been made in the last several years towards understanding
the properties of disordered electronic systems. In part, this is made possible
by recent advances in quantum effective medium methods which enable the study
of disorder and electron-electronic interactions on equal footing. They include
dynamical mean field theory and the coherent potential approximation, and their
cluster extension, the dynamical cluster approximation. Despite their
successes, these methods do not enable the first-principles study of the
strongly disordered regime, including the effects of electronic localization.
The main focus of this review is the recently developed typical medium
dynamical cluster approximation for disordered electronic systems. This method
has been constructed to capture disorder-induced localization, and is based on
a mapping of a lattice onto a quantum cluster embedded in an effective typical
medium, which is determined self-consistently. Here we provide an overview of
various recent applications of the typical medium dynamical cluster
approximation to a variety of models and systems, including single and
multi-band Anderson model, and models with local and off-diagonal disorder. We
then present the application of the method to realistic systems in the
framework of the density functional theory.Comment: 58 pages, 46 figure
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