305 research outputs found
Soft Fermi Surfaces and Breakdown of Fermi Liquid Behavior
Electron-electron interactions can induce Fermi surface deformations which
break the point-group symmetry of the lattice structure of the system. In the
vicinity of such a "Pomeranchuk instability" the Fermi surface is easily
deformed by anisotropic perturbations, and exhibits enhanced collective
fluctuations. We show that critical Fermi surface fluctuations near a d-wave
Pomeranchuk instability in two dimensions lead to large anisotropic decay rates
for single-particle excitations, which destroy Fermi liquid behavior over the
whole surface except at the Brillouin zone diagonal.Comment: 12 pages, 2 figures, revised version as publishe
Fermi surface instabilities at finite Temperature
We present a new method to detect Fermi surface instabilities for interacting
systems at finite temperature. We first apply it to a list of cases studied
previously, recovering already known results in a very economic way, and
obtaining most of the information on the phase diagram analytically. As an
example, in the continuum limit we obtain the critical temperature as an
implicit function of the magnetic field and the chemical potential
. By applying the method to a model proposed to describe reentrant
behavior in , we reproduce the phase diagram obtained
experimentally and show the presence of a non-Fermi Liquid region at
temperatures above the nematic phase.Comment: 10 pages, 10 figure
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
Turning a First Order Quantum Phase Transition Continuous by Fluctuations: General Flow Equations and Application to d-Wave Pomeranchuk Instability
We derive renormalization group equations which allow us to treat order
parameter fluctuations near quantum phase transitions in cases where an
expansion in powers of the order parameter is not possible. As a prototypical
application, we analyze the nematic transition driven by a d-wave Pomeranchuk
instability in a two-dimensional electron system. We find that order parameter
fluctuations suppress the first order character of the nematic transition
obtained at low temperatures in mean-field theory, so that a continuous
transition leading to quantum criticality can emerge
Spin fluctuations and ferromagnetic order in two-dimensional itinerant systems with Van Hove singularities
The quasistatic approach is used to analyze the criterion of ferromagnetism
for two-dimensional (2D) systems with the Fermi level near Van Hove (VH)
singularities of the electron spectrum. It is shown that the spectrum of spin
excitations (paramagnons) is positively defined when the interaction between
electrons and paramagnons, determined by the Hubbard on-site repulsion U, is
sufficiently large. Due to incommensurate spin fluctuations near the
ferromagnetic quantum phase transition, the critical interaction Uc remains
finite at VH filling and exceeds considerably its value obtained from the
Stoner criterion. A comparison with the functional renormalization group
results and mean-field approximation which yields a phase separation is also
performed
Novel in-gap spin state in Zn-doped La_1.85Sr_0.15CuO_4
Low-energy spin excitations of La1.85Sr0.15Cu1-yZnyO4 were studied by neutron
scattering. In y=0.004, the incommensurate magnetic peaks show a well defined
``spin gap'' below Tc. The magnetic signals at omega=3 meV decrease below Tc=27
K for y=0.008, also suggesting the gap opening. At lower temperatures, however,
the signal increases again, implying a novel in-gap spin state. In y=0.017, the
spin gap vanishes and elastic magnetic peaks appear. These results clarify that
doped Zn impurities induce the novel in-gap state, which becomes larger and
more static with increasing Zn.Comment: 4 pages, 4 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
Fermi-surface reconstruction involving two Van Hove singularities across the antiferromagnetic transition in BaFe2As2
We report an angle-resolved photoemission study of BaFe2As2, a parent
compound of iron-based superconductors. Low-energy tunable excitation photons
have allowed the first observation of a saddle-point singularity at the Z
point, as well as the Gamma point. With antiferromagnetic ordering, both of
these two van Hove singularities come down below the Fermi energy, leading to a
topological change in the innermost Fermi surface around the kz axis from
cylindrical to tear-shaped, as expected from first-principles calculation.
These singularities may provide an additional instability for the Fermi surface
of the superconductors derived from BaFe2As2.Comment: 14 pages, 4 figures, 1 tabl
Renormalized perturbation theory for Fermi systems: Fermi surface deformation and superconductivity in the two-dimensional Hubbard model
Divergencies appearing in perturbation expansions of interacting many-body
systems can often be removed by expanding around a suitably chosen renormalized
(instead of the non-interacting) Hamiltonian. We describe such a renormalized
perturbation expansion for interacting Fermi systems, which treats Fermi
surface shifts and superconductivity with an arbitrary gap function via
additive counterterms. The expansion is formulated explicitly for the Hubbard
model to second order in the interaction. Numerical soutions of the
self-consistency condition determining the Fermi surface and the gap function
are calculated for the two-dimensional case. For the repulsive Hubbard model
close to half-filling we find a superconducting state with d-wave symmetry, as
expected. For Fermi levels close to the van Hove singularity a Pomeranchuk
instability leads to Fermi surfaces with broken square lattice symmetry, whose
topology can be closed or open. For the attractive Hubbard model the second
order calculation yeilds s-wave superconductivity with a weakly momentum
dependent gap, whose size is reduced compared to the mean-field result.Comment: 18 pages incl. 6 figure
Van Hove singularity and spontaneous Fermi surface symmetry breaking in Sr3Ru2O7
The most salient features observed around a metamagnetic transition in
Sr3Ru2O7 are well captured in a simple model for spontaneous Fermi surface
symmetry breaking under a magnetic field, without invoking a putative quantum
critical point. The Fermi surface symmetry breaking happens in both a majority
and a minority spin band but with a different magnitude of the order parameter,
when either band is tuned close to van Hove filling by the magnetic field. The
transition is second order for high temperature T and changes into first order
for low T. The first order transition is accompanied by a metamagnetic
transition. The uniform magnetic susceptibility and the specific heat
coefficient show strong T dependence, especially a log T divergence at van Hove
filling. The Fermi surface instability then cuts off such non-Fermi liquid
behavior and gives rise to a cusp in the susceptibility and a specific heat
jump at the transition temperature.Comment: 11 pages, 4 figure
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