980 research outputs found
Nernst effect of iron pnictide and cuprate superconductors: signatures of spin density wave and stripe order
The Nernst effect has recently proven a sensitive probe for detecting unusual
normal state properties of unconventional superconductors. In particular, it
may sensitively detect Fermi surface reconstructions which are connected to a
charge or spin density wave (SDW) ordered state, and even fluctuating forms of
such a state. Here we summarize recent results for the Nernst effect of the
iron pnictide superconductor , whose ground state evolves
upon doping from an itinerant SDW to a superconducting state, and the cuprate
superconductor which exhibits static stripe
order as a ground state competing with the superconductivity. In , the SDW order leads to a huge Nernst response, which allows
to detect even fluctuating SDW precursors at superconducting doping levels
where long range SDW order is suppressed. This is in contrast to the impact of
stripe order on the normal state Nernst effect in . Here, though signatures of the stripe order are
detectable in the temperature dependence of the Nernst coefficient, its overall
temperature dependence is very similar to that of ,
where stripe order is absent. The anomalies which are induced by the stripe
order are very subtle and the enhancement of the Nernst response due to static
stripe order in as compared to that of the
pseudogap phase in , if any, is very small.Comment: To appear in: 'Properties and applications of thermoelectric
materials - II', V. Zlatic and A. Hewson, editors, Proceedings of NATO
Advanced Research Workshop, Hvar, Croatia, September 19 -25, 2011, NATO
Science for Peace and Security Series B: Physics and Biophysics, (Springer
Science+Business Media B.V. 2012
Inductive Lusternik-Schnirelmann category in a model category
We introduce the notion of inductive category in a model category and prove
that it agrees with the Ganea approach given by Doeraene. This notion also
coincides with the topological one when we consider the category of (well-)
pointed topological spaces.Comment: 14 page
Phonon thermal conductivity in doped : Relevant scattering mechanisms
Results of in-plane and out-of-plane thermal conductivity measurements on
() single crystals are
presented. The most characteristic features of the temperature dependence are a
pronounced phonon peak at low temperatures and a steplike anomaly at ,
i.e., at the transition to the low temperature tetragonal phase (LTT-phase),
which gradually decrease with increasing Sr-content. Comparison of these
findings with the thermal conductivity of and clearly reveals that in the most effective
mechanism for phonon scattering is impurity-scattering (dopants), as well as
scattering by soft phonons that are associated with the lattice instability in
the low temperature orthorhombic phase (LTO-phase). There is no evidence that
stripe correlations play a major role in suppressing the phonon peak in the
thermal conductivity of .Comment: 7 pages, 4 figure
Pseudogap Formation in the Symmetric Anderson Lattice Model
We present self-consistent calculations for the self-energy and magnetic
susceptibility of the 2D and 3D symmetric Anderson lattice Hamiltonian, in the
fluctuation exchange approximation. At high temperatures, strong f-electron
scattering leads to broad quasiparticle spectral functions, a reduced
quasiparticle band gap, and a metallic density of states. As the temperature is
lowered, the spectral functions narrow and a pseudogap forms at the
characteristic temperature at which the width of the quasiparticle
spectral function at the gap edge is comparable to the renormalized activation
energy. For , the pseudogap is approximately equal to the
hybridization gap in the bare band structure. The opening of the pseudogap is
clearly apparent in both the spin susceptibility and the compressibility.Comment: RevTeX - 14 pages and 7 figures (available on request),
NRL-JA-6690-94-002
Decoding 3D search coil signals in a non-homogeneous magnetic field
We present a method for recording eye-head movements with the magnetic search coil technique in a small external magnetic field. Since magnetic fields are typically non-linear, except in a relative small region in the center small field frames have not been used for head-unrestrained experiments in oculomotor studies. Here we present a method for recording 3D eye movements by accounting for the magnetic non-linearities using the Biot-Savart law. We show that the recording errors can be significantly reduced by monitoring current head position and thereby taking the location of the eye in the external magnetic field into account
Nernst Effect of stripe ordering LaEuSrCuO
We investigate the transport properties of
LaEuSrCuO (, 0.08, 0.125, 0.15, 0.2) with a
special focus on the Nernst effect in the normal state. Various anomalous
features are present in the data. For and 0.15 a kink-like anomaly is
present in the vicinity of the onset of charge stripe order in the LTT phase,
suggestive of enhanced positive quasiparticle Nernst response in the stripe
ordered phase. At higher temperature, all doping levels except exhibit
a further kink anomaly in the LTO phase which cannot unambiguously be related
to stripe order. Moreover, a direct comparison between the Nernst coefficients
of stripe ordering LaEuSrCuO and superconducting
LaSrCuO at the doping levels and reveals
only weak differences. Our findings make high demands on any scenario
interpreting the Nernst response in hole-doped cuprates
Atomic Bose Gas with Negative Scattering Length
We derive the equation of state of a dilute atomic Bose gas with an
interatomic interaction that has a negative scattering length and argue that
two continuous phase transitions, occuring in the gas due to quantum degeneracy
effects, are preempted by a first-order gas-liquid or gas-solid transition
depending on the details of the interaction potential. We also discuss the
consequences of this result for future experiments with magnetically trapped
spin-polarized atomic gasses such as lithium and cesium.Comment: 16 PAGES, REVTEX 3.0, ACCEPTED FOR PUBLICATION IN PHYS. REV.
Thermal conductivity via magnetic excitations in spin-chain materials
We discuss the recent progress and the current status of experimental
investigations of spin-mediated energy transport in spin-chain and spin-ladder
materials with antiferromagnetic coupling. We briefly outline the central
results of theoretical studies on the subject but focus mainly on recent
experimental results that were obtained on materials which may be regarded as
adequate physical realizations of the idealized theoretical model systems. Some
open questions and unsettled issues are also addressed.Comment: 17 pages, 4 figure
Proximity effect and strong coupling superconductivity in nanostructures built with an STM
We present high resolution tunneling spectroscopy data at very low
temperatures on superconducting nanostructures of lead built with an STM. By
applying magnetic fields, superconductivity is restricted to length scales of
the order of the coherence length. We measure the tunneling conductance and
analyze the phonon structure and the low energy DOS. We demonstrate the
influence of the geometry of the system on the magnetic field dependence of the
tunneling density of states, which is gapless in a large range of fields. The
behavior of the features in the tunneling conductance associated to phonon
modes are explained within current models.Comment: 4 figures, 4 page
A model for the atomic-scale structure of a dense, nonequilibrium fluid: the homogeneous cooling state of granular fluids
It is shown that the equilibrium Generalized Mean Spherical Model of fluid
structure may be extended to nonequilibrium states with equation of state
information used in equilibrium replaced by an exact condition on the two-body
distribution function. The model is applied to the homogeneous cooling state of
granular fluids and upon comparison to molecular dynamics simulations is found
to provide an accurate picture of the pair distribution function.Comment: 29 pages, 11 figures Revision corrects formatting of the figure
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