358 research outputs found
Weakly correlated electrons on a square lattice: a renormalization group theory
We study the weakly interacting Hubbard model on the square lattice using a
one-loop renormalization group approach. The transition temperature T_c between
the metallic and (nearly) ordered states is found. In the parquet regime, (T_c
>> |mu|), the dominant correlations at temperatures below T_c are
antiferromagnetic while in the BCS regime (T_c << |mu|) at T_c the d-wave
singlet pairing susceptibility is most divergent.Comment: 12 pages, REVTEX, 3 figures included, submitted to Phys. Rev. Let
Phonons and d-wave pairing in the two-dimensional Hubbard model
We analyze the influence of phonons on the d-wave pairing instability in the
Hubbard model on the two-dimensional square lattice at weak to moderate
interaction U, using a functional renormalization group scheme with
frequency-dependent interaction vertices. As measured by the pairing scale, the
B1g buckling mode enhances the pairing, while other phonon modes decrease the
pairing. When various phonon modes are included together, the net effect on the
scale is small. However, in situations where d-wave superconductivity and other
tendencies, e.g. antiferromagnetism, are closely competing, the combined effect
of different phonons may be able to tip the balance towards pairing.Comment: 4 pages, 3 figure
d-wave superconductivity and Pomeranchuk instability in the two-dimensional Hubbard model
We present a systematic stability analysis for the two-dimensional Hubbard
model, which is based on a new renormalization group method for interacting
Fermi systems. The flow of effective interactions and susceptibilities confirms
the expected existence of a d-wave pairing instability driven by
antiferromagnetic spin fluctuations. More unexpectedly, we find that strong
forward scattering interactions develop which may lead to a Pomeranchuk
instability breaking the tetragonal symmetry of the Fermi surface.Comment: 4 pages (RevTeX), 4 eps figure
organic crystals: superconducting versus antiferromagnetic instabilities in an anisotropic triangular lattice Hubbard model
A Hubbard model at half-filling on an anisotropic triangular lattice has been
proposed as the minimal model to describe conducting layers of
organic materials. The model interpolates between the
square lattice and decoupled chains. The materials
present many similarities with cuprates, such as the presence of unconventional
metallic properties and the close proximity of superconducting and
antiferromagnetic phases. As in the cuprates, spin fluctuations are expected to
play a crucial role in the onset of superconductivity. We perform a
weak-coupling renormalization-group analysis to show that a superconducting
instability occurs. Frustration in the antiferromagnetic couplings, which
arises from the underlying geometrical arrangement of the lattice, breaks the
perfect nesting of the square lattice at half-filling. The spin-wave
instability is suppressed and a superconducting instability predominates. For
the isotropic triangular lattice, there are again signs of long-range magnetic
order, in agreement with studies at strong-coupling.Comment: 4 pages, 5 eps figs, to appear in Can. J. Phys. (proceedings of the
Highly Frustrated Magnetism (HFM-2000) conference, Waterloo, Canada, June
2000
Re-entrant hidden order at a metamagnetic quantum critical end point
Magnetization measurements of URu2Si2 in pulsed magnetic fields of 44 T
reveal that the hidden order phase is destroyed before appearing in the form of
a re-entrant phase between ~ 36 and 39 T. Evidence for conventional itinerant
electron metamagnetism at higher temperatures suggests that the re-entrant
phase is created in the vicinity of a quantum critical end point.Comment: 8 pages, including 3 figures (Physical Review Letters, in press) a
systematic error in the field calibration has been fixed since the original
submission of this manuscrip
Orbital quantization in the high magnetic field state of a charge-density-wave system
A superposition of the Pauli and orbital coupling of a high magnetic field to
charge carriers in a charge-density-wave (CDW) system is proposed to give rise
to transitions between subphases with quantized values of the CDW wavevector.
By contrast to the purely orbital field-induced density-wave effects which
require a strongly imperfect nesting of the Fermi surface, the new transitions
can occur even if the Fermi surface is well nested at zero field. We suggest
that such transitions are observed in the organic metal
-(BEDT-TTF)KHg(SCN) under a strongly tilted magnetic field.Comment: 14 pages including 4 figure
Magnetic and superconducting instabilities of the Hubbard model at the van Hove filling
We use a novel temperature-flow renormalization group technique to analyze
magnetic and superconducting instabilities in the two-dimensional t-t' Hubbard
model for particle densities close to the van Hove filling as a function of the
next-nearest neighbor hopping t'. In the one-loop flow at the van Hove filling,
the characteristic temperature for the flow to strong coupling is suppressed
drastically around t'_c approx. -0.33t, suggesting a quantum critical point
between d-wave pairing at moderate t'>t'_c and ferromagnetism for t'<t'_c. Upon
increasing the particle density in the latter regime the leading instability
occurs in the triplet pairing channel.Comment: 4 pages, to appear in Physical Review Letter
Effect of a lattice upon an interacting system of electrons: Breakdown of scaling and decay of persistent currents
For an interacting system of N electrons, we study the conditions under which
a lattice model of size L with nearest neighbor hopping t and U/r Coulomb
repulsion has the same ground state as a continuum model. For a fixed value of
N, one gets identical results when the inter-electron spacing to the Bohr
radius ratio r_s < r_s^*. Above r_s^*, the persistent current created by an
enclosed flux begins to decay and r_s ceases to be the scaling parameter. Three
criteria giving similar r_s^* are proposed and checked using square lattices.Comment: 7 pages, 5 postscript figure
Kohn-Luttinger instability of the t-t' Hubbard model in two dimensions: variational approach
An effective Hamiltonian for the Kohn-Luttinger superconductor is constructed
and solved in the BCS approximation. The method is applied to the t-t' Hubbard
model in two dimensions with the following results: (i) The superconducting
phase diagram at half filling is shown to provide a weak-coupling analog of the
recently proposed spin liquid state in the J_1-J_2 Heisenberg model. (ii) In
the parameter region relevant for the cuprates we have found a nontrivial
energy dependence of the gap function in the dominant d-wave pairing sector.
The hot spot effect in the angular dependence of the superconducting gap is
shown to be quite weak
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