150 research outputs found

### 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

### Renormalized mean-field t-J model of high-Tc superconductivity: comparison with experiment

Using an advanced version of the renormalized mean-field theory (RMFT) for
the t-J model, we examine spin-singlet superconducting (SC) state of $d_{x^2 -
y^2}$-symmetry. Overall doping dependence of the SC gap magnitude is in good
agreement with experimental results for $\text{Bi}_{2}\text{Sr}_{2}\text{Ca}
\text{Cu}_2 \text{O}_{8 + \delta}$ (BSCCO) and
$\text{La}_{2-x}\text{Sr}_{x}\text{Cu} \text{O}_{4}$ (LSCO) compounds at the
optimal doping and in the overdoped regime. We also calculate the dispersion
relation for the Bogoliubov quasiparticles and compare our findings both with
the angle resolved photoemission data for the cuprates, as well as with the
variational Monte Carlo and other mean-field studies. Within the method
proposed by Fukushima [cf. Phys. Rev. B \textbf{78}, 115105 (2008)], we analyze
different forms of the t-J Hamiltonian, i.e. modifications caused by the form
of exchange interaction, and by the presence of three-site terms. It is shown
that although the former has a small influence, the latter suppresses strongly
the superconductivity. We also analyze the temperature dependence of the gap
magnitude and compare the results with those of the recently introduced
finite-temperature renormalized mean-field theory (TRMFT) of Wang et al. [cf.
Phys. Rev. B \textbf{82}, 125105 (2010)].Comment: 7 pages, 6 figures, 2 tables. Submitted to Physical Review

### Kohn's theorem in a superfluid Fermi gas with a Feshbach resonance

We investigate the dipole mode in a superfluid gas of Fermi atoms trapped in
a harmonic potential. According to Kohn's theorem, the frequency of this
collective mode is not affected by an interaction between the atoms and is
always equal to the trap frequency. This remarkable property, however, does not
necessarily hold in an approximate theory. We explicitly prove that the
Hartree-Fock-Bogoliubov generalized random phase approximation (HFB-GRPA),
including a coupling between fluctuations in the density and Cooper channels,
is consistent with both Kohn's theorem as well as Goldstone's theorem. This
proof can be immediately extended to the strong-coupling superfluid theory
developed by Nozi\'eres and Schmitt-Rink (NSR), where the effect of superfluid
fluctuations is included within the Gaussian level. As a result, the NSR-GRPA
formalism can be used to study collective modes in the BCS-BEC crossover region
in a manner which is consistent with Kohn's theorem. We also include the effect
of a Feshbach resonance and a condensate of the associated molecular bound
states. A detailed discussion is given of the unusual nature of the Kohn mode
eigenfunctions in a Fermi superfluid, in the presence and absence of a Feshbach
resonance. When the molecular bosons feel a different trap frequency from the
Fermi atoms, the dipole frequency is shown to {\it depend} on the strength of
effective interaction associated with the Feshbach resonance.Comment: 29 pages, 1 figure

### Zeeman response of d-wave superconductors: Born approximation for impurity and spin-orbit scattering potentials

The effects of impurity and spin-orbit scattering potentials can strongly
affect the Zeeman response of a d-wave superconductor. Here, both the phase
diagram and the quasiparticle density of states are calculated within the Born
approximation and it is found that the spin-orbit interaction influences in a
qualitatively different way the Zeeman response of d-wave and s-wave
superconductors.Comment: 19 pages, 6 eps figures, submitted to Physica

### Local Spectral Weight of a Luttinger Liquid: Effects from Edges and Impurities

We calculate the finite-temperature local spectral weight (LSW) of a
Luttinger liquid with an "open" (hard wall) boundary. Close to the boundary the
LSW exhibits characteristic oscillations indicative of spin-charge separation.
The line shape of the LSW is also found to have a Fano-like asymmetry, a
feature originating from the interplay between electron-electron interaction
and scattering off the boundary. Our results can be used to predict how edges
and impurities influence scanning tunneling microscopy (STM) of one-dimensional
electron systems at low temperatures and voltage bias. Applications to STM on
single-walled carbon nanotubes are discussed.Comment: 15 pages, 10 figues, The latest version in pdf format is available at
http://www.physik.uni-kl.de/eggert/papers/LSW-LL.pd

### Gapped optical excitations from gapless phases: imperfect nesting in unconventional density waves

We consider the effect of imperfect nesting in quasi-one-dimensional
unconventional density waves in the case, when the imperfect nesting and the
gap depends on the same wavevector component.
The phase diagram is very similar to that in a conventional density wave. The
density of states is highly asymmetric with respect to the Fermi energy.
The optical conductivity at T=0 remains unchanged for small deviations from
perfect nesting. For higher imperfect nesting parameter, an optical gap opens,
and considerable amount of spectral weight is transferred to higher
frequencies. This makes the optical response of our system very similar to that
of a conventional density wave. Qualitatively similar results are expected in
d-density waves.Comment: 8 pages, 7 figure

### Correlated bosons on a lattice: Dynamical mean-field theory for Bose-Einstein condensed and normal phases

We formulate a bosonic dynamical mean-field theory (B-DMFT) which provides a
comprehensive, thermodynamically consistent framework for the theoretical
investigation of correlated lattice bosons. The B-DMFT is applicable for
arbitrary values of the coupling parameters and temperature and becomes exact
in the limit of high spatial dimensions d or coordination number Z of the
lattice. In contrast to its fermionic counterpart the construction of the
B-DMFT requires different scalings of the hopping amplitudes with Z depending
on whether the bosons are in their normal state or in the Bose-Einstein
condensate. A detailed discussion of how this conceptual problem can be
overcome by performing the scaling in the action rather than in the Hamiltonian
itself is presented. The B-DMFT treats normal and condensed bosons on equal
footing and thus includes the effects caused by their dynamic coupling. It
reproduces all previously investigated limits in parameter space such as the
Beliaev-Popov and Hartree-Fock-Bogoliubov approximations and generalizes the
existing mean-field theories of interacting bosons. The self-consistency
equations of the B-DMFT are those of a bosonic single-impurity coupled to two
reservoirs corresponding to bosons in the condensate and in the normal state,
respectively. We employ the B-DMFT to solve a model of itinerant and localized,
interacting bosons analytically. The local correlations are found to enhance
the condensate density and the Bose-Einstein condensate (BEC) transition
temperature T_{BEC}. This effect may be used experimentally to increase T_{BEC}
of bosonic atoms in optical lattices.Comment: 17 pages, 4 figures, extended versio

### Anomalous impurity effects in nonadiabatic superconductors

We show that, in contrast with the usual electron-phonon Migdal-Eliashberg
theory, the critical temperature Tc of an isotropic s-wave nonadiabatic
superconductor is strongly reduced by the presence of diluted non-magnetic
impurities. Our results suggest that the recently observed Tc-suppression
driven by disorder in K3C60 [Phys. Rev. B vol.55, 3866 (1997)] and in
Nd(2-x)CexCuO(4-delta) [Phys. Rev. B vol.58, 8800 (1998)] could be explained in
terms of a nonadiabatic electron-phonon coupling. Moreover, we predict that the
isotope effect on Tc has an impurity dependence qualitatively different from
the one expected for anisotropic superconductors.Comment: 10 pages, euromacr.tex, europhys.sty, 6 figures. Replaced with
accepted version (Europhysics Letters

### Bulk inhomogeneous phases of anisotropic particles: A fundamental measure functional study of the restricted orientations model

The phase diagram of prolate and oblate particles in the restricted
orientations approximation (Zwanzig model) is calculated. Transitions to
different inhomogeneous phases (smectic, columnar, oriented, or plastic solid)
are studied through minimization of the fundamental measure functional (FMF) of
hard parallelepipeds. The study of parallel hard cubes (PHC's) as a particular
case is also included motivated by recent simulations of this system. As a
result a rich phase behavior is obtained which include, apart from the usual
liquid crystal phases, a very peculiar phase (called here discotic smectic)
which was already found in the only existing simulation of the model, and which
turns out to be stable because of the restrictions imposed on the orientations.
The phase diagram is compared at a qualitative level with simulation results of
other anisotropic particle systems.Comment: 11 pages, 10 figure

### Electrodynamics of superconductors

An alternate set of equations to describe the electrodynamics of
superconductors at a macroscopic level is proposed. These equations resemble
equations originally proposed by the London brothers but later discarded by
them. Unlike the conventional London equations the alternate equations are
relativistically covariant, and they can be understood as arising from the
'rigidity' of the superfluid wave function in a relativistically covariant
microscopic theory. They predict that an internal 'spontaneous' electric field
exists in superconductors, and that externally applied electric fields, both
longitudinal and transverse, are screened over a London penetration length, as
magnetic fields are. The associated longitudinal dielectric function predicts a
much steeper plasmon dispersion relation than the conventional theory, and a
blue shift of the minimum plasmon frequency for small samples. It is argued
that the conventional London equations lead to difficulties that are removed in
the present theory, and that the proposed equations do not contradict any known
experimental facts. Experimental tests are discussed.Comment: Small changes following referee's and editor's comments; to be
published in Phys.Rev.

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