1,535 research outputs found
Competing phases in the extended U-V-J Hubbard model near the van Hove fillings
The phase diagram of the two-dimensional extended one-band U-V-J Hubbard
model is considered within a mean-field approximation and two- and many-patch
renormalization group (RG) approaches near the van Hove band fillings. At small
t' and J>0 mean-field and many-patch RG approaches give similar results for the
leading spin-density-wave (SDW) instability, while the two-patch RG approach,
which predicts a wide region of charge-flux (CF) phase becomes unreliable due
to nesting effect. At the same time, there is a complex competition between
SDW, CF phases, and d-wave superconductivity in two- and many-patch RG
approaches. While the spin-flux (SF) phase is not stable at the mean-field
level, it is identified as a possible ground state at J<0 in both RG
approaches. With increasing t' the results of all three approaches merge:
d-wave superconductivity at J>0 and ferromagnetism at J<0 become the leading
instabilities. For large enough V the charge-density-wave (CDW) state occurs.Comment: This is the extended version of the paper, which includes both two-
and many-patch RG analyse
Renormalization group analysis of magnetic and superconducting instabilities near van Hove band fillings
Phase diagrams of the two-dimensional one-band t-t' Hubbard model are
obtained within the two-patch and the temperature-cutoff many-patch
renormalization group approach. At small t' and at van Hove band fillings
antiferromagnetism dominates, while with increasing t' or changing filling
antiferromagnetism is replaced by d-wave superconductivity. Near t'=t/2 and
close to van Hove band fillings the system is unstable towards ferromagnetism.
Away from van Hove band fillings this ferromagnetic instability is replaced by
a region with dominating triplet p-wave superconducting correlations. The
results of the renormalization-group approach are compared with the mean-field
results and the results of the T-matrix approximation.Comment: 29 pages, 17 figure
Anisotropy of the Energy Gap in the Insulating Phase of the U-t-t' Hubbard Model
We apply a diagrammatic expansion method around the atomic limit (U >> t) for
the U-t-t' Hubbard model at half filling and finite temperature by means of a
continued fraction representation of the one-particle Green's function. From
the analysis of the spectral function A(\vec{k},\omega) we find an energy
dispersion relation with a (cos k_x-cos k_y)^2 modulation of the energy gap in
the insulating phase. This anisotropy is compared with experimental ARPES
results on insulating cuprates.Comment: 4 pages Revtex, 6 embedded eps figures; Figures 5 and 6 were in error
and have been replaced including the discussion of the figure
Raman Scattering in Cuprate Superconductors
A theory for electronic Raman scattering in the cuprate superconductors is
presented with a specific emphasis on the polarization dependence of the
spectra which can infer the symmetry of the energy gap. Signatures of the
effects of disorder on the low frequency and low temperature behavior of the
Raman spectra for different symmetry channels provide detailed information
about the magnitude and the phase of the energy gap. Properties of the theory
for finite T are discussed and compared to recent data concerning the doping
dependence of the Raman spectra in cuprate superconductors, and remaining
questions are addressed.Comment: 27 pages, 11 figures, style file include
Quartett formation at (100)/(110)-interfaces of d-wave superconductors
Across a faceted (100)/(110) interface between two d-wave-superconductors the
structure of the superconducting order parameter leads to an alternating sign
of the local Josephson coupling. Describing the Cooper pair motion along and
across the interface by a one-dimensional boson lattice model, we show that a
small attractive interaction between the bosons boosts boson binding at the
interface -- a phenomenon, which is intimately tied to the staggered sequence
of 0- and Pi-junction contacts along the interface. We connect this finding to
the recently observed h/4e oscillations in (100)/(110) SQUIDS of cuprate
superconductors.Comment: 13 pages, 2 figure
High energy constraints in the octet SS-PP correlator and resonance saturation at NLO in 1/Nc
We study the octet SS-PP correlator within resonance chiral theory up to the
one-loop level, i.e., up to next-to-leading order in the 1/Nc expansion. We
will require that our correlator follows the power behaviour prescribed by the
operator product expansion at high euclidian momentum. Nevertheless, we will
not make use of short-distance constraints from other observables. Likewise,
the high-energy behaviour will be demanded for the whole correlator, not for
individual absorptive channels. The amplitude is progressively improved by
considering more and more complicated operators in the hadronic lagrangian.
Matching the resonance chiral theory result with chiral perturbation theory at
low energies produces the estimates L_8(mu)^{SU(3)} = (1.0+-0.4)10^-3 and
C_{38}(mu)^{SU(3)} = (8+-5) 10^-6 for mu=770 MeV. The effect of alternative
renormalization schemes is also discussed in the article.Comment: 40 pages, 18 figure
Emergence of charge order in a staggered loop-current phase of cuprate high-temperature superconductors
We study the emergence of charge ordered phases within a pi-loop current
(piLC) model for the pseudogap based on a three-band model for underdoped
cuprate superconductors. Loop currents and charge ordering are driven by
distinct components of the short-range Coulomb interactions: loop currents
result from the repulsion between nearest-neighbor copper and oxygen orbitals,
while charge order results from repulsion between neighboring oxygen orbitals.
We find that the leading piLC phase has an antiferromagnetic pattern similar to
previously discovered staggered flux phases, and that it emerges abruptly at
hole dopings p below the van Hove filling. Subsequent charge ordering
tendencies in the piLC phase reveal that diagonal d-charge density waves (dCDW)
are suppressed by the loop currents while axial order competes more weakly. In
some cases we find a wide temperature range below the loop-current transition,
over which the susceptibility towards an axial dCDW is large. In these cases,
short-range axial charge order may be induced by doping-related disorder. A
unique feature of the coexisting dCDW and piLC phases is the emergence of an
incommensurate modulation of the loop currents. If the dCDW is biaxial
(checkerboard) then the resulting incommensurate current pattern breaks all
mirror and time-reversal symmetries, thereby allowing for a polar Kerr effect
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