37 research outputs found
Coexistence of Nematic Order and Superconductivity in the Hubbard Model
We study the interplay of nematic and superconducting order in the
two-dimensional Hubbard model and show that they can coexist, especially when
superconductivity is not the energetically dominant phase. Due to a breaking of
the symmetry, the coexisting phase inherently contains admixture of the
-wave pairing components. As a result, the superconducting gap exhibits very
non-standard features including changed nodal directions. Our results also show
that in the optimally doped regime the superconducting phase is typically
unstable towards developing nematicity (breaking of the symmetry). This
has implications for the cuprate high- superconductors, for which in this
regime the so-called intertwined orders have recently been observed. Namely,
the coexisting phase may be viewed as a precursor to such more involved
patterns of symmetry breaking.Comment: 5 pages, 3 figure
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
Approximation schemes for the study of multi-band Gutzwiller wave functions
The minimum of the Gutzwiller energy functional depends on the number of
parameters considered in the variational state. For a three-orbital Hubbard
model we find that the frequently used diagonal Ansatz is very accurate in
high-symmetry situations. For lower symmetry, induced by a crystal-field
splitting or the spin-orbit coupling, the discrepancies in energy between the
most general and a diagonal Gutzwiller Ansatz can be quite significant. We
discuss approximate schemes that may be employed in multi-band cases where a
minimization of the general Gutzwiller energy functional is too demanding
numerically.Comment: 8 pages, 8 figure
Gutzwiller theory of band magnetism in LaOFeAs
We use the Gutzwiller variational theory to calculate the ground-state phase
diagram and quasi-particle bands of LaOFeAs. The Fe3d--As4p Wannier-orbital
basis obtained from density-functional theory defines the band part of our
eight-band Hubbard model. The full atomic interaction between the electrons in
the iron orbitals is parameterized by the Hubbard interaction U and an average
Hund's-rule interaction J. We reproduce the experimentally observed small
ordered magnetic moment over a large region of (U,J) parameter space. The
magnetically ordered phase is a stripe spin-density wave of quasi-particles.Comment: 4 pages, 5 figure
Exchange couplings for Mn ions in CdTe: validity of spin models for dilute magnetic II-VI semiconductors
We employ density-functional theory (DFT) in the generalized gradient
approximation (GGA) and its extensions GGA+ and GGA+Gutzwiller to calculate
the magnetic exchange couplings between pairs of Mn ions substituting Cd in a
CdTe crystal at very small doping. DFT(GGA) overestimates the exchange
couplings by a factor of three because it underestimates the charge-transfer
gap in Mn-doped II-VI semiconductors. Fixing the nearest-neighbor coupling
to its experimental value in GGA+, in GGA+Gutzwiller, or by a simple
scaling of the DFT(GGA) results provides acceptable values for the exchange
couplings at 2nd, 3rd, and 4th neighbor distances in Cd(Mn)Te, Zn(Mn)Te,
Zn(Mn)Se, and Zn(Mn)S. In particular, we recover the experimentally observed
relation . The filling of the Mn 3-shell is not integer which
puts the underlying Heisenberg description into question. However, using a
few-ion toy model the picture of a slightly extended local moment emerges so
that an integer -shell filling is not a prerequisite for equidistant
magnetization plateaus, as seen in experiment.Comment: 12 pages, 10 figure
Variational study of Fermi-surface deformations in Hubbard models
We study the correlation-induced deformation of Fermi surfaces by means of a
new diagrammatic method which allows for the analytical evaluation of
Gutzwiller wave functions in finite dimensions. In agreement with
renormalization-group results we find Pomeranchuk instabilities in
two-dimensional Hubbard models for sufficiently large Coulomb interactions.Comment: 6 pages, 6 figure