287 research outputs found

### Confined coherence in quasi-one-dimensional metals

We present a functional renormalization group calculation of the effect of
strong interactions on the shape of the Fermi surface of weakly coupled
metallic chains. In the regime where the bare interchain hopping is small, we
show that scattering processes involving large momentum transfers perpendicular
to the chains can completely destroy the warping of the true Fermi surface,
leading to a confined state where the renormalized interchain hopping vanishes
and a coherent motion perpendicular to the chains is impossible.Comment: 4 RevTex pages, 5 figures,final version as published by PR

### Effect of Electron-Electron Interactions on Rashba-like and Spin-Split Systems

The role of electron-electron interactions is analyzed for Rashba-like and
spin-split systems within a tight-binding single-band Hubbard model with
on-site and all nearest-neighbor matrix elements of the Coulomb interaction. By
Rashba-like systems we refer to the Dresselhaus and Rashba spin-orbit coupled
phases; spin-split systems have spin-up and spin-down Fermi surfaces shifted
relative to each other. Both systems break parity but preserve time-reversal
symmetry. They belong to a class of symmetry-breaking ground states that
satisfy: (i) electron crystal momentum is a good quantum number (ii) these
states have no net magnetic moment and (iii) their distribution of `polarized
spin' in momentum space breaks the lattice symmetry. In this class, the
relevant Coulomb matrix elements are found to be nearest-neighbor exchange $J$,
pair-hopping $J'$ and nearest-neighbor repulsion $V$. These ground states lower
their energy most effectively through $J$, hence we name them Class $J$ states.
The competing effects of $V-J'$ on the direct and exchange energies determine
the relative stability of Class $J$ states. We show that the spin-split and
Rashba-like phases are the most favored ground states within Class $J$ because
they have the minimum anisotropy in `polarized spin'. On a square lattice we
find that the spin-split phase is always favored for near-empty bands; above a
critical filling, we predict a transition from the paramagnetic to the
Rashba-like phase at $J_{c1}$ and a second transition to the spin-split state
at $J_{c2}>J_{c1}$. An energetic comparison with ferromagnetism highlights the
importance of the role of $V$ in the stability of Class $J$ states. We discuss
the relevance of our results to (i) the $\alpha$ and $\beta$ phases proposed by
Wu and Zhang in the Fermi Liquid formalism and (ii) experimental observations
of spin-orbit splitting in \emph{Au}(111) surface states

### Spontaneous Fermi surface symmetry breaking in bilayered systems

We perform a comprehensive numerical study of d-wave Fermi surface
deformations (dFSD) on a square lattice, the so-called d-wave Pomeranchuk
instability, including bilayer coupling. Since the order parameter
corresponding to the dFSD has Ising symmetry, there are two stacking patterns
between the layeres, (+,+) and (+,-). This additional degree of freedom gives
rise to a rich variety of phase diagrams. The phase diagrams are classified by
means of the energy scale Lambda_{z}, which is defined as the bilayer splitting
at the saddle points of the in-plane band dispersion. As long as Lambda_{z} ne
0, a major stacking pattern is usually (+,-), and (+,+) stacking is stabilized
as a dominant pattern only when the temperature scale of the dFSD instability
becomes much smaller than Lambda_z. For Lambda_{z}=0, the phase diagram depends
on the precise form of the bilayer dispersion. We also analyze the effect of a
magnetic field on the bilayer model in connection with a possible dFSD
instability in the bilyared ruthenate Sr_3Ru_2O_7.Comment: 18 pages, 7 figure

### Effect of magnetic field on spontaneous Fermi surface symmetry breaking

We study magnetic field effects on spontaneous Fermi surface symmetry
breaking with d-wave symmetry, the so-called d-wave "Pomeranchuk instability''.
We use a mean-field model of electrons with a pure forward scattering
interaction on a square lattice. When either the majority or the minority spin
band is tuned close to the van Hove filling by a magnetic field, the Fermi
surface symmetry breaking occurs in both bands, but with a different magnitude
of the order parameter. The transition is typically of second order at high
temperature and changes to first order at low temperature; the end points of
the second order line are tricritical points. This qualitative picture does not
change even in the limit of a large magnetic field, although the magnetic field
substantially suppresses the transition temperature at the van Hove filling.
The field produces neither a quantum critical point nor a quantum critical end
point in our model. In the weak coupling limit, typical quantities
characterizing the phase diagram have a field-independent single energy scale
while its dimensionless coefficient varies with the field. The field-induced
Fermi surface symmetry breaking is a promising scenario for the bilayer
ruthenate Sr3Ru2O7, and future issues are discussed to establish such a
scenario.Comment: 28 pages, 9 figure

### Mean-field theory for symmetry-breaking Fermi surface deformations on a square lattice

We analyze a mean-field model of electrons with pure forward scattering
interactions on a square lattice which exhibits spontaneous Fermi surface
symmetry breaking with a d-wave order parameter: the surface expands along the
kx-axis and shrinks along the ky-axis (or vice versa). The symmetry-broken
phase is stabilized below a dome-shaped transition line Tc(mu), with a maximal
Tc near van Hove filling. The phase transition is usually first order at the
edges of the transition line, and always second order around its center. The
d-wave compressibility of the Fermi surface is however strongly enhanced even
near the first order transition down to zero temperature. In the weak coupling
limit the phase diagram is fully determined by a single non-universal energy
scale, and hence dimensionless ratios of different characteristic quantities
are universal. Adding a uniform repulsion to the forward scattering
interaction, the two tricritical points at the ends of the second order
transition line are shifted to lower temperatures. For a particularly favorable
choice of hopping and interaction parameters one of the first order edges is
replaced completely by a second order transition line, leading to a quantum
critical point.Comment: 23 pages, 8 figure

### Competition of Fermi surface symmetry breaking and superconductivity

We analyze a mean-field model of electrons on a square lattice with two types
of interaction: forward scattering favoring a d-wave Pomeranchuk instability
and a BCS pairing interaction driving d-wave superconductivity. Tuning the
interaction parameters a rich variety of phase diagrams is obtained. If the BCS
interaction is not too strong, Fermi surface symmetry breaking is stabilized
around van Hove filling, and coexists with superconductivity at low
temperatures. For pure forward scattering Fermi surface symmetry breaking
occurs typically via a first order transition at low temperatures. The presence
of superconductivity reduces the first order character of this transition and,
if strong enough, can turn it into a continuous one. This gives rise to a
quantum critical point within the superconducting phase. The superconducting
gap tends to suppress Fermi surface symmetry breaking. For a relatively strong
BCS interaction, Fermi surface symmetry breaking can be limited to intermediate
temperatures, or can be suppressed completely by pairing.Comment: 14 pages, 10 figure

### Electrical resistivity near Pomeranchuk instability in two dimensions

We analyze the DC charge transport in the quantum critical regime near a
d-wave Pomeranchuk instability in two dimensions. The transport decay rate is
linear in temperature everywhere on the Fermi surface except at cold spots on
the Brillouin zone diagonal. For pure systems, this leads to a DC resistivity
proportional to T^{3/2} in the low-temperature limit. In the presence of
impurities the residual impurity resistance at T=0 is approached linearly at
low temperatures.Comment: 9 pages, no figure

### Interplay between parallel and diagonal electronic nematic phases in interacting systems

An electronic nematic phase can be classified by a spontaneously broken
discrete rotational symmetry of a host lattice. In a square lattice, there are
two distinct nematic phases. The parallel nematic phase breaks $x$ and $y$
symmetry, while the diagonal nematic phase breaks the diagonal $(x+y)$ and
anti-diagonal $(x-y)$ symmetry. We investigate the interplay between the
parallel and diagonal nematic orders using mean field theory. We found that the
nematic phases compete with each other, while they coexist in a finite window
of parameter space. The quantum critical point between the diagonal nematic and
isotropic phases exists, and its location in a phase diagram depends on the
topology of the Fermi surface. We discuss the implication of our results in the
context of neutron scattering and Raman spectroscopy measurements on
La$_{2-x}$Sr$_x$CuO$_4$.Comment: 8 pages, 10 figure

### Meta-nematic transitions in a bilayer system: Application to the bilayer ruthenate

It was suggested that the two consecutive metamagnetic transitions and the
large residual resistivity discovered in Sr$_3$Ru$_2$O$_7$ can be understood
via the nematic order and its domains in a single layer system. However, a
recently reported anisotropy between two longitudinal resistivities induced by
tilting the magnetic field away from the c-axis cannot be explained within the
single layer nematic picture. To fill the gap in our understanding within the
nematic order scenario, we investigate the effects of bilayer coupling and
in-plane magnetic field on the electronic nematic phases in a bilayer system.
We propose that the in-plane magnetic field in the bilayer system modifies the
energetics of the domain formation, since it breaks the degeneracy of two
different nematic orientations. Thus the system reveals a pure nematic phase
with a resistivity anisotropy in the presence of an in-plane magnetic field. In
addition to the nematic phase, the bilayer coupling opens a novel route to a
hidden nematic phase that preserves the x-y symmetry of the Fermi surfaces.Comment: 8 pages, 6 figure

### Order parameter symmetries for magnetic and superconducting instabilities: Bethe-Salpeter analysis of functional renormalization-group solutions

The Bethe-Salpeter equation is combined with the temperature-cutoff
functional renormalization group approach to analyze the order parameter
structure for the leading instabilities of the 2D t-t' Hubbard model. We find
significant deviations from pure s-, d-, or p-wave forms, which is due to the
frustration of antiferromagnetism at small and intermediate t'. With adding a
direct antiferromagnetic spin-exchange coupling the eigenfunctions in the
particle-hole channel have extended s-wave form, while in the particle-particle
singlet pairing channel a higher angular momentum component arises besides the
standard d-wave symmetry, which flattens the angular dependence of the gap. For
t' closer to 1/2 we find a delicate competition of ferromagnetism and triplet
pairing with a nontrivial pair-wavefunction.Comment: 4 pages, 4 figures, RevTe

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