373 research outputs found
CeCoIn5 - a quantum critical superfluid
We have made the first complete measurements of the London penetration depth
of CeCoIn5, a quantum-critical metal where superconductivity
arises from a non-Fermi-liquid normal state. Using a novel tunnel diode
oscillator designed to avoid spurious contributions to , we have
established the existence of intrinsic and anomalous power-law behaviour at low
temperature. A systematic analysis raises the possibility that the unusual
observations are due to an extension of quantum criticality into the
superconducting state.Comment: 5 pages, 3 figure
Apparatus for high resolution microwave spectroscopy in strong magnetic fields
We have developed a low temperature, high-resolution microwave surface
impedance probe that is able to operate in high static magnetic fields. Surface
impedance is measured by cavity perturbation of dielectric resonators, with
sufficient sensitivity to resolve the microwave absorption of sub-mm-sized
superconducting samples. The resonators are constructed from high permittivity
single-crystal rutile (TiO2) and have quality factors in excess of 10^6.
Resonators with such high performance have traditionally required the use of
superconducting materials, making them incompatible with large magnetic fields
and subject to problems associated with aging and power-dependent response.
Rutile resonators avoid these problems while retaining comparable sensitivity
to surface impedance. Our cylindrical rutile resonators have a hollow bore and
are excited in TE_01(n-d) modes, providing homogeneous microwave fields at the
center of the resonator where the sample is positioned. Using a sapphire
hot-finger technique, measurements can be made at sample temperatures in the
range 1.1 K to 200 K, while the probe itself remains immersed in a liquid
helium bath at 4.2 K. The novel apparatus described in this article is an
extremely robust and versatile system for microwave spectroscopy, integrating
several important features into a single system. These include: operation at
high magnetic fields; multiple measurement frequencies between 2.64 GHz and
14.0 GHz in a single resonator; excellent frequency stability, with typical
drifts < 1 Hz per hour; the ability to withdraw the sample from the resonator
for background calibration; and a small pot of liquid helium separate from the
external bath that provides a sample base temperature of 1.1 K.Comment: 10 pages, 5 figure
Effect of realistic out-of-plane dopant potentials on the superfluid density of overdoped cuprates
Recent experimental papers on hole-doped overdoped cuprates have argued that
a series of observations showing unexpected behavior in the superconducting
state imply the breakdown of the quasiparticle-based Landau-BCS paradigm in
that doping range. In contrast, some of the present authors have argued that a
phenomenological "dirty -wave" theoretical analysis explains essentially all
aspects of thermodynamic and transport properties in the superconducting state,
provided the unusual effects of weak, out-of-plane dopant impurities are
properly accounted for. Here we attempt to place this theory on a more
quantitative basis by performing calculations of dopant
impurity potentials for LSCO and Tl-2201. These potentials are more complex
than the pointlike impurity models considered previously, and require
calculation of forward scattering corrections to transport properties.
Including realistic, ARPES-derived bandstructures, Fermi liquid
renormalizations, and vertex corrections, we show that the theory can explain
semiquantitatively the unusual superfluid density measurements of the two most
studied overdoped materials.Comment: 19 page, 13 figure
Electrical transport measurements in the superconducting state of Bi2212 and Tl2201
Precise measurements of the in-plane microwave surface impedance of
high-quality single crystals of Bi2212 and Tl2201 are used to probe the
relaxation time of nodal quasiparticles in the d-wave superconducting state
through a two-fluid analysis of the microwave conductivity. While this analysis
requires us to posit a form for the frequency-dependent quasiparticle
conductivity, we clearly demonstrate that the extraction of the relaxation rate
is quite insensitive to the assumed shape of the quasiparticle spectrum. The
robustness of the analysis is rooted in the oscillator-strength sum rule and
the fact that we simultaneously measure the real and imaginary parts of the
conductivity. In both Bi2212 and Tl2201 we infer a linear temperature
dependence of the transport relaxation rate 1/tau and a small but finite
zero-temperature intercept. The linear temperature dependence of 1/tau is in
accord with expectations for weak elastic scattering in an unconventional
superconductor with line nodes and a small residual density of states. The same
analysis reveals an onset of inelastic scattering at higher temperatures
similar to that seen in the YBCO superconductors. Finally we extrapolate the
two-fluid model over a range of frequencies up to five times the measurement
frequency, where the extrapolation predicts behaviour that is qualitatively
similar to terahertz conductivity data on Bi2212 thin films. While relaxation
rates in Bi2212 and Tl2201 are substantially higher than in YBCO there are
qualitative similarities between all three materials, and the differences can
likely be attributed to varying levels of static disorder. We therefore
conclude that a universal picture of quasiparticle scattering in the cuprates
is emerging.Comment: 10 pages, 9 figure
Phenomenology of a-axis and b-axis charge dynamics from microwave spectroscopy of highly ordered YBa2Cu3O6.50 and YBa2Cu3O6.993
Extensive measurements of the microwave conductivity of highly pure and
oxygen-ordered \YBCO single crystals have been performed as a means of
exploring the intrinsic charge dynamics of a d-wave superconductor. Broadband
and fixed-frequency microwave apparatus together provide a very clear picture
of the electrodynamics of the superconducting condensate and its thermally
excited nodal quasiparticles. The measurements reveal the existence of very
long-lived excitations deep in the superconducting state, as evidenced by sharp
cusp-like conductivity spectra with widths that fall well within our
experimental bandwidth. We present a phenomenological model of the microwave
conductivity that captures the physics of energy-dependent quasiparticle
dynamics in a d-wave superconductor which, in turn, allows us to examine the
scattering rate and oscillator strength of the thermally excited quasiparticles
as functions of temperature. Our results are in close agreement with the
Ferrell-Glover-Tinkham sum rule, giving confidence in both our experiments and
the phenomenological model. Separate experiments for currents along the and directions of detwinned crystals allow us to isolate the role
of the CuO chain layers in \YBCO, and a model is presented that incorporates
both one-dimensional conduction from the chain electrons and two-dimensional
transport associated with the \cuplane plane layers.Comment: 17 pages, 13 figure
Observation of the Transverse Optical Plasmon in SmLa0.8Sr0.2CuO4-d
We present microwave and infrared measurements on SmLa0.8Sr0.2CuO4-d, which
are direct evidence for the existence of a transverse optical plasma mode,
observed as a peak in the c-axis optical conductivity. This mode appears as a
consequence of the existence of two different intrinsic Josephson couplings
between the CuO2 layers, one with a Sm2O2 block layer, and the other one with a
(La,Sr)O block layer. From the frequencies and the intensities of the
collective modes we determine the value of the compressibility of the two
dimensional electron fluid in the copper oxygen planes.Comment: REVTeX, 4 pages, 5 eps-figures, PRL, in pres
Non-Locality and Strong Coupling in the Heavy Fermion Superconductor CeCoIn: A Penetration Depth Study
We report measurements of the magnetic penetration depth in single
crystals of CeCoIn down to 0.14 K using a tunnel-diode based,
self-inductive technique at 28 MHz. While the in-plane penetration depth tends
to follow a power law, , the data are better
described as a crossover between linear ({\it T} ) and
quadratic ({\it T} ) behavior, with the
crossover temperature in the strong-coupling limit. The {\it c}-axis
penetration depth is linear in {\it T}, providing evidence
that CeCoIn is a {\it d}-wave superconductor with line nodes along the
{\it c}-axis. The different temperature dependences of and
rule out impurity effects as the source of .Comment: 4 pages, 3 figure
Microwave Spectroscopy of Thermally Excited Quasiparticles in YBa_2Cu_3O_{6.99}
We present here the microwave surface impedance of a high purity crystal of
measured at 5 frequencies between 1 and 75 GHz. This data
set reveals the main features of the conductivity spectrum of the thermally
excited quasiparticles in the superconducting state. Below 20 K there is a
regime of extremely long quasiparticle lifetimes, due to both the collapse of
inelastic scattering below and the very weak impurity scattering in the
high purity -grown crystal used in this study. Above 20 K, the
scattering increases dramatically, initially at least as fast as .Comment: 13 pages with 10 figures. submitted to Phys Rev
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