84 research outputs found
Development and application of optical anemometers
Imperial Users onl
Two different quasiparticle scattering rates in vortex line liquid phase of layered d-wave superconductors
We carry out a quantum mechanical analysis of the behavior of nodal
quasiparticles in the vortex line liquid phase of planar d-wave
superconductors. Applying a novel path integral technique we calculate a number
of experimentally relevant observables and demonstrate that in the low-field
regime the quasiparticle scattering rates deduced from photoemission and
thermal transport data can be markedly different from that extracted from
tunneling, specific heat, superfluid stiffness or spin-lattice relaxation time.Comment: Latex, 4 pages, no figure
Quantum Criticality and Superconductivity in Quasi-Two-Dimensional Dirac Electronic Systems
We present a theory describing the superconducting (SC) interaction of Dirac
electrons in a quasi-two-dimensional system consisting of a stack of N planes.
The occurrence of a SC phase is investigated both at T=0 and T\neq 0, in the
case of a local interaction, when the theory must be renormalized and also in
the situation where a natural physical cutoff is present in the system. In both
cases, at T=0, we find a quantum phase transition connecting the normal and SC
phases at a certain critical coupling. The phase structure is shown to be
robust against quantum fluctuations. The SC gap is determined for T=0 and T\neq
0, both with and without a physical cutoff and the interplay between the gap
and the SC order parameter is discussed. Our theory qualitatively reproduces
the SC phase transition occurring in the underdoped regime of the high-Tc
cuprates. This fact points to the possible relevance of Dirac electrons in the
mechanism of high-Tc superconductivity.Comment: To be published in Nuclear Physics, Section B. 24 pages, 4 figure
Fractional electric charge of a magnetic vortex at nonzero temperature
An ideal gas of twodimensional Dirac fermions in the background of a
pointlike magnetic vortex with arbitrary flux is considered. We find that this
system acquires fractional electric charge at finite temperatures and determine
the functional dependence of the thermal average and quadratic fluctuation of
the charge on the temperature, the vortex flux, and the continuous parameter of
the boundary condition at the location of the vortex.Comment: 25 pages, 5 figures, journal version, minor changes, Eqs.(3.2)-(3.5)
correcte
Novel anisotropy in the superconducting gap structure of Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} probed by quasiparticle heat transport
Since the nature of pairing interactions is manifested in the superconducting
gap symmetry, the exact gap structure, particularly any deviation from the
simple d_{x^2-y^2} symmetry, would help elucidating the pairing mechanism in
high-T_c cuprates. Anisotropic heat transport measurement in
Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} (Bi-2212) reveals that the quasiparticle
populations are different for the two nodal directions and thus the gap
structure must be uniquely anisotropic, suggesting that pairing is governed by
interactions with a rather complicated anisotropy. Intriguingly, it is found
that the "plateau" in the magnetic-field dependence of the thermal conductivity
is observed only in the b-axis transport.Comment: 4 pages, 5 figures, accepted for publication in Phys. Rev. Let
Theories of Low-Energy Quasi-Particle States in Disordered d-Wave Superconductors
The physics of low-energy quasi-particle excitations in disordered d-wave
superconductors is a subject of ongoing intensive research. Over the last
decade, a variety of conceptually and methodologically different approaches to
the problem have been developed. Unfortunately, many of these theories
contradict each other, and the current literature displays a lack of consensus
on even the most basic physical observables. Adopting a symmetry-oriented
approach, the present paper attempts to identify the origin of the disagreement
between various previous approaches, and to develop a coherent theoretical
description of the different low-energy regimes realized in weakly disordered
d-wave superconductors. We show that, depending on the presence or absence of
time-reversal invariance and the microscopic nature of the impurities, the
system falls into one of four different symmetry classes. By employing a
field-theoretical formalism, we derive effective descriptions of these
universal regimes as descendants of a common parent field theory of
Wess-Zumino-Novikov-Witten type. As well as describing the properties of each
universal regime, we analyse a number of physically relevant crossover
scenarios, and discuss reasons for the disagreement between previous results.
We also touch upon other aspects of the phenomenology of the d-wave
superconductor such as quasi-particle localization properties, the spin quantum
Hall effect, and the quasi-particle physics of the disordered vortex lattice.Comment: 42 Pages, 8 postscript figures, published version with updated
reference
Low temperature superfluid stiffness of d-wave superconductor in a magnetic field
The temperature and field dependence of the superfluid density in
the vortex state of a d-wave superconductor are calculated using a microscopic
model in the quasiclassical approximation. We show that at temperatures below
T^{*} \varpropto \sqrt{H}$, the linear T dependence of rho_s crosses over to a
T^2 dependence differently from the behavior of the effective penetration
depth, lambda_eff^{-2}(T). We point out that the expected dependences could be
probed by a mutual-inductance technique experiment.Comment: 4 pages, RevTeX4, 2 EPS figures; minor revisions made and 1 new
reference added; final version published in PR
Phase fluctuations, dissipation and superfluid stiffness in d-wave superconductors
We study the effect of dissipation on quantum phase fluctuations in d-wave
superconductors. Dissipation, arising from a nonzero low frequency optical
conductivity which has been measured in experiments below , has two
effects: (1) a reduction of zero point phase fluctuations, and (2) a reduction
of the temperature at which one crosses over to classical thermal fluctuations.
For parameter values relevant to the cuprates, we show that the crossover
temperature is still too large for classical phase fluctuations to play a
significant role at low temperature. Quasiparticles are thus crucial in
determining the linear temperature dependence of the in-plane superfluid
stiffness. Thermal phase fluctuations become important at higher temperatures
and play a role near .Comment: Presentation improved, new references added (10 latex pages, 3 eps
figures). submitted to PR
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