441 research outputs found
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
Field Redefinition Invariance in Quantum Field Theory
The issue of field redefinition invariance of path integrals in quantum field
theory is reexamined. A ``paradox'' is presented involving the reduction to an
effective quantum-mechanical theory of a -dimensional free scalar field
in a Minkowskian spacetime with compactified spatial coordinates. The
implementation of field redefinitions both before and after the reduction
suggests that operator-ordering issues in quantum field theory should not be
ignored.Comment: 7 page
Instabilities at [110] Surfaces of d_{x^2-y^2} Superconductors
We compare different scenarios for the low temperature splitting of the
zero-energy peak in the local density of states at (110) surfaces of
d_{x^2-y^2}-wave superconductors, observed by Covington et al.
(Phys.Rev.Lett.79 (1997), 277). Using a tight binding model in the
Bogolyubov-de Gennes treatment we find a surface phase transition towards a
time-reversal symmetry breaking surface state carrying spontaneous currents and
an s+id-wave state. Alternatively, we show that electron correlation leads to a
surface phase transition towards a magnetic state corresponding to a local spin
density wave state.Comment: 4 pages, 5 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
Truncated unity functional renormalization group for multiband systems with spin-orbit coupling
Although the functional renormalization group (fRG) is by now a
well-established method for investigating correlated electron systems, it is
still undergoing significant technical and conceptual improvements. In
particular, the motivation to optimally exploit the parallelism of modern
computing platforms has recently led to the development of the
"truncated-unity" functional renormalization group (TU-fRG). Here, we review
this fRG variant, and we provide its extension to multiband systems with
spin-orbit coupling. Furthermore, we discuss some aspects of the implementation
and outline opportunities and challenges ahead for predicting the ground-state
ordering and emergent energy scales for a wide class of quantum materials.Comment: consistent with published version in Frontiers in Physics (2018
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
Antiferromagnetically Driven Electronic Correlation in Iron Pnictides and Cuprates
The iron pnictides and the cuprates represent two families of materials,
where strong antiferromagnetic correlation drives three other distinct ordering
tendencies: (1) superconducting pairing, (2) Fermi surface distortion, and (3)
orbital current order. We propose that (1)-(3) and the antiferromagnetic
correlation are the hallmarks of a class of strongly correlated materials to
which the cuprates and pnictides belong. In this paper we present the results
of the functional renormalization group studies to support the above claim. In
addition, we show that as a function of the interlayer hopping parameter, the
double layer Hubbard model nicely interpolates between the cuprate and the iron
pnictide physics. Finally, as a check, we will present the renormalization
group study of a ladder version of the iron pnictide, and compare the results
to those of the two-dimensional model.Comment: 18 pages, 20 figures, revised version, one more figure added and
references update
Mean encounter times for cell adhesion in hydrodynamic flow: analytical progress by dimensional reduction
For a cell moving in hydrodynamic flow above a wall, translational and
rotational degrees of freedom are coupled by the Stokes equation. In addition,
there is a close coupling of convection and diffusion due to the
position-dependent mobility. These couplings render calculation of the mean
encounter time between cell surface receptors and ligands on the substrate very
difficult. Here we show for a two-dimensional model system how analytical
progress can be achieved by treating motion in the vertical direction by an
effective reaction term in the mean first passage time equation for the
rotational degree of freedom. The strength of this reaction term can either be
estimated from equilibrium considerations or used as a fit parameter. Our
analytical results are confirmed by computer simulations and allow to assess
the relative roles of convection and diffusion for different scaling regimes of
interest.Comment: Reftex, postscript figures include
Near-equilibrium measurements of nonequilibrium free energy
A central endeavor of thermodynamics is the measurement of free energy
changes. Regrettably, although we can measure the free energy of a system in
thermodynamic equilibrium, typically all we can say about the free energy of a
non-equilibrium ensemble is that it is larger than that of the same system at
equilibrium. Herein, we derive a formally exact expression for the probability
distribution of a driven system, which involves path ensemble averages of the
work over trajectories of the time-reversed system. From this we find a simple
near-equilibrium approximation for the free energy in terms of an excess mean
time-reversed work, which can be experimentally measured on real systems. With
analysis and computer simulation, we demonstrate the accuracy of our
approximations for several simple models.Comment: 5 pages, 3 figure
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