15 research outputs found
Coexistence of spin-triplet superconductivity with magnetic ordering in an orbitally degenerate system: Hartree-Fock-BCS approximation revisited
The Hund's-rule-exchange induced and coexisting spin-triplet paired and
magnetic states are considered within the doubly degenerate Hubbard model with
interband hybridization. The Hartree-Fock approximation combined with the
Bardeen-Cooper-Schrieffer (BCS) approach is analyzed for the case of square
lattice. The calculated phase diagram contains regions of stability of the
spin-triplet superconducting phase coexisting with either ferromagnetism or
antiferromagnetism, as well as a pure superconducting phase. The influence of
the inter-site hybridization on the stability of the considered phases, as well
as the temperature dependence of both the magnetic moment and the
superconducting gaps, are also discussed. Our approach supplements the well
known phase diagrams containing only magnetic phases with the paired triplet
states treated on the same footing. We also discuss briefly how to include the
spin fluctuations within this model with real space pairing
Stability of the coexistent superconducting-nematic phase under the presence of intersite interactions
We analyze the effect of intersite-interaction terms on the stability of the
coexisting superconucting-nematic phase (SC+N) within the extended Hubbard and
-- models on the square lattice. In order to take into account the
correlation effects with a proper precision, we use the approach based on the
\textit{diagrammatic expansion of the Gutzwiller wave function} (DE-GWF), which
goes beyond the renormalized mean field theory (RMFT) in a systematic manner.
As a starting point of our analysis we discuss the stability region of the SC+N
phase on the intrasite Coulomb repulsion-hole doping plane for the case of the
Hubbard model. Next, we show that the exchange interaction term enhances
superconductivity while suppresses the nematicity, whereas the intersite
Coulomb repulsion term acts in the opposite manner. The competing character of
the SC and N phases interplay is clearly visible throughout the analysis. A
universal conclusion is that the nematic phase does not survive within the
-- model with the value of integral typical for the high-T
cuprates (eV). For the sake of completeness, the effect of the
correlated hopping term is also analyzed. Thus the present discussion contains
all relevant two-site interaction terms which appear in the parametrized
one-band model within the second quantization scheme. At the end, the influence
of the higher-order terms of the diagrammatic expansion on the rotational
symmetry breaking is also shown by comparing the DE-GWF results with those
corresponding to the RMFT
Even-parity spin-triplet pairing for orbitally degenerate correlated electrons by purely repulsive interactions
We demonstrate the stability of a spin-triplet paired s-wave (with an
admixture of extended s-wave) state for the case of purely repulsive
interactions in a degenerate two-band Hubbard model. We further show that near
half-filling the considered kind of superconductivity can coexist with
antiferromagnetism. The calculations have been carried out with the use of the
so-called statistically consistent Gutzwiller approximation for the case of a
square lattice. The absence of a stable paired state when analyzed in the
Hartree-Fock-BCS approximation allows us to claim that the electron
correlations in conjunction with the Hund's rule exchange play the crucial role
in stabilizing the spin-triplet superconducting state. A sizable hybridization
of the bands suppresses the paired state
Fulde-Ferrell state induced purely by the orbital effect in a superconducting nanowire
We demonstrate that the Fulde-Ferrell (FF) phase can be induced uniquely by
the orbital effect in a cylindrical metallic nanowire. In the external magnetic
field the two-fold degeneracy with respect to the orbital quantum number is
lifted what leads to a Fermi wave vector mismatch between the subbands with
opposite orbital momenta in the paired state. This mismatch can be compensated
by the nonzero total momentum of the Cooper pairs created by electrons from
split subbands what results in the formation of the FF phase. With increasing
magnetic field a series of FF stability regions appear in between which the
standard BCS superconducting phase is stable.Comment: 8 pages, 6 figure
Effect of interlayer processes on the superconducting state within t-J-U model: Full Gutzwiller wave-function solution and relation to experiment
The Gutzwiller wave function solution of the -- model is considered
for the bilayer high-T superconductor by using the so-called diagrammatic
expansion method. The focus is on the influence of the interlayer effects on
the superconducting state. The chosen pairing symmetry is a mixture of
symmetry within the layers and the so-called symmetry
for the interlayer contribution. The analyzed interlayer terms reflect the
interlayer electron hopping, the interlayer exchange coupling, and the
interlayer pair hopping. The obtained results are compared with selected
experimental data corresponding to the copper-based compound Bi-2212 with two
Cu-O planes in the unit cell. For the sake of comparison, selected results for
the case of the bilayer Hubbard model are also provided. This paper complements
our recent results obtained for the single-plane high temperature cuprates [cf.
J. Spa{\l}ek, M. Zegrodnik, and J. Kaczmarczyk, Phys. Rev. B {\bf 95}, 024506
(2017)
Spontaneous appearance of nonzero momentum Cooper pairing: Possible application to the iron-pnictides
We suggest that an inhomogeneous (non-zero momentum) paired phase can appear
in the absence of an external magnetic field in the system with a predominant
interband pairing and with separate Fermi-surface sheets. The Fermi wave vector
mismatch which appears in such situation can be compensated by nonzero
center-of-mass momentum of the Cooper pairs, what can lead to a spontaneous
appearance of the Fulde-Ferrell type of superconducting state. The idea is
examined using a tight-binding model which emulates the hole-like and the
electron-like bands of iron based superconductor. The state can appear for the
case of both spin-singlet and -triplet pairing channels
Spin-triplet paired state induced by Hund's rule coupling and correlations : a fully statistically consistent Gutzwiller approach
The intrasite and intersite spin-triplet pairing gaps induced by interband Hund's rule coupling and their correlations are analyzed in the doubly degenerate Hubbard Hamiltonian. To include the effect of correlations, the statistically consistent Gutzwiller approximation is used. In this approach the consistency means that the averages calculated from the self-consistent equations and those determined variationally coincide with each other. Emphasis is put on the solution for which the average particle number is conserved when carrying out the Gutzwiller projection. This method leads to a stable equal-spin paired state in the so-called repulsive interactions limit (U > 3J) in the regime of moderate correlations. The interband hybridization introduces an inequivalence of the bands which, above a critical magnitude, suppresses the paired state due to both the Fermi-wavevector mismatch for the Cooper pair and the interband hopping allowed by the Pauli principle
Spontaneous appearance of the spin-triplet Fulde-Ferrell-Larkin-Ovchinnikov phase in a two-band model : possible application to LaFeAsO_{1-x}F_{x}
The possibility of a spontaneous spin-triplet paired phase of the Fulde-Ferrell-Larkin-Ovchinnikov type is studied. As it is shown in a system with the dominant interband pairing and two distinct Fermi surface sheets, the Fermi wave-vector mismatch can be compensated by a nonzero center-of-mass momentum of the Cooper pairs. This idea is examined with the use of a model which describes the two hole-like bands in the iron-based superconductor. It is shown that for the proper range of model parameters, the minima of the free energy appear which correspond to a nonzero Cooper pair momentum. Different superconducting gap symmetries are analyzed, and the corresponding phase diagrams are shown