336 research outputs found

### Theory of reduced singlet pairing without the underlying state of charge stripes in the high-temperature superconductor YBa_2Cu_3O_6.45

Recently, a strongly enhanced xy anisotropy of magnetic excitations was
observed in YBa_2Cu_3O_y (YBCO_y) with y=6.45 and Tc=35 K [Science 319, 597
(2008)]. Unlike the observation in YBCO_6.6 and YBCO_6.85, the anisotropy grows
to be pronounced at lower temperature and at lower energy, and is not
suppressed by the onset of superconductivity. We propose that the effect of
singlet pairing is substantially reduced in YBCO_6.45. This reduction
concomitantly enhances an order competing with singlet pairing, a strong
tendency of the so-called d-wave Pomeranchuk instability, leading to the
magnetic excitations observed experimentally.Comment: 11 pages, 4 figures, published versio

### Quasi-particle functional Renormalisation Group calculations in the two-dimensional half-filled Hubbard model at finite temperatures

We present a highly parallelisable scheme for treating functional
Renormalisation Group equations which incorporates a quasi-particle-based
feedback on the flow and provides direct access to real-frequency self-energy
data. This allows to map out the boundaries of Fermi-liquid regimes and to
study the effect of quasi-particle degradation near Fermi liquid instabilities.
As a first application, selected results for the two-dimensional half-filled
perfectly nested Hubbard model are shown

### Superconductivity in the attractive Hubbard model: functional renormalization group analysis

We present a functional renormalization group analysis of superconductivity
in the ground state of the attractive Hubbard model on a square lattice.
Spontaneous symmetry breaking is treated in a purely fermionic setting via
anomalous propagators and anomalous effective interactions. In addition to the
anomalous interactions arising already in the reduced BCS model, effective
interactions with three incoming legs and one outgoing leg (and vice versa)
occur. We accomplish their integration into the usual diagrammatic formalism by
introducing a Nambu matrix for the effective interactions. From a random-phase
approximation generalized through use of this matrix we conclude that the
impact of the 3+1 effective interactions is limited, especially considering the
effective interactions important for the determination of the order parameter.
The exact hierarchy of flow equations for one-particle irreducible vertex
functions is truncated on the two-particle level, with higher-order self-energy
corrections included in a scheme proposed by Katanin. Using a parametrization
of effective interactions by patches in momentum space, the flow equations can
be integrated numerically to the lowest scales without encountering
divergences. Momentum-shell as well as interaction-flow cutoff functions are
used, including a small external field or a large external field and a
counterterm, respectively. Both approaches produce momentum-resolved order
parameter values directly from the microscopic model. The size of the
superconducting gap is in reasonable agreement with expectations from other
studies.Comment: 22 pages, 16 figures, references added, some changes in the
introductio

### 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

### 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

### Flow to strong coupling in the two-dimensional Hubbard model

We extend the analysis of the renormalization group flow in the
two-dimensional Hubbard model close to half-filling using the recently
developed temperature flow formalism. We investigate the interplay of d-density
wave and Fermi surface deformation tendencies with those towards d-wave pairing
and antiferromagnetism. For a ratio of next nearest to nearest neighbor
hoppings, t'/t=-0.25, and band fillings where the Fermi surface is inside the
Umklapp surface, only the d-pairing susceptibility diverges at low
temperatures. When the Fermi surface intersects the Umklapp surface close to
the saddle points, d-wave pairing, d-density wave, antiferromagnetic and, to a
weaker extent, d-wave Fermi surface deformation susceptibilities grow together
when the interactions flow to strong coupling. We interpret these findings as
indications for a non-trivial strongly coupled phase with short-ranged
superconducting and antiferromagnetic correlations, in close analogy with the
spin liquid ground state in the well-understood two-leg Hubbard ladder.Comment: 8 pages, to appear in European Physical Journal

### Landau-Fermi liquid analysis of the 2D t-t' Hubbard model

We calculate the Landau interaction function f(k,k') for the two-dimensional
t-t' Hubbard model on the square lattice using second and higher order
perturbation theory. Within the Landau-Fermi liquid framework we discuss the
behavior of spin and charge susceptibilities as function of the onsite
interaction and band filling. In particular we analyze the role of elastic
umklapp processes as driving force for the anisotropic reduction of the
compressibility on parts of the Fermi surface.Comment: 10 pages, 16 figure

### Orbital Dependence of Quasiparticle Lifetimes in Sr2RuO4

Using a phenomenological Hamiltonian, we investigate the quasiparticle
lifetimes and dispersions in the three low energy bands, gamma, beta, and alpha
of Sr2RuO4. Couplings in the Hamiltonian are fixed so as to produce the mass
renormalization as measured in magneto-oscillation experiments. We thus find
reasonable agreement in all bands between our computed lifetimes and those
measured in ARPES experiments by Kidd et al. [1] and Ingle et al. [2]. In
comparing computed to measured quasiparticle dispersions, we however find good
agreement in the alpha-band alone.Comment: 7 pages, 5 figure

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