11 research outputs found
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
Superfluid Density in a Highly Underdoped YBCO Superconductor
The superfluid density rho_s(T) = 1/lambda^2(T) has been measured at 2.64 GHz
in highly underdoped YBCO, at 37 dopings with T_c between 3 K and 17 K. Within
limits set by the transition width Delta T_c ~ 0.4 K, rho_s(T) shows no
evidence of critical fluctuations as T goes to T_c, with a mean-field-like
transition and no indication of vortex unbinding. Instead, we propose that
rho_s displays the behaviour expected for a quantum phase transition in the (3
+ 1)-dimensional XY universality class, with rho_s0 ~ (p - p_c), T_c ~ (p -
p_c)^1/2 and rho_s(T) ~ (T_c - T)^1 as T goes to T_c.Comment: 4 pages, 5 figures; final version of pape
Effective magnetic penetration depth in superconducting cylinders and spheres with highly anisotropic electrodynamics
Effective magnetic penetration depth and microwave surface impedance are
derived for anisotropic layered superconductors in the shape of spheres and
long cylinders, where the external magnetic field is applied in the plane of
the highly conducting layers to induce out-of-plane screening currents. The
results are extended by analytic continuation to highly anisotropic conductors
and to lossy superconductors at high frequency. The electrodynamics for the
general case of a superconductor or metal with arbitrary anisotropy are
presented. The treatment is then specialized to layered materials with
unixaxial anisotropy, in which the penetration depth for currents flowing
perpendicular to the layers, lambda_c, is much greater than that for in-plane
currents, lambda_a. Exact solutions are found in the limit lambda_a goes to
zero, and are expected to provide an accurate representation of many
experiments on cuprates and other layered superconductors, particularly on
grain-aligned powders.Comment: 9 pages, 4 figure
Stability of nodal quasiparticles in underdoped YBa2Cu3O6+y probed by penetration depth and microwave spectroscopy
High resolution measurements of superfluid density and broadband
quasiparticle conductivity have been used to probe the low energy excitation
spectrum of nodal quasiparticles in underdoped YBCO. Penetration depth is
measured to temperatures as low as 0.05 K. Microwave conductivity is measured
from 0.1 to 20 GHz and is a direct probe of zero-energy quasiparticles. The
data are compared with predictions for a number of theoretical scenarios that
compete with or otherwise modify pure d-wave superconductivity, in particular
commensurate and incommensurate spin and charge density waves; d + i s and d +
i d superconductivity; circulating current phases; and the BCS--BEC crossover.
We conclude that the data are consistent with a pure d-wave state in the
presence of a small amount of strong scattering disorder, and are able to rule
out most candidate competing states either completely, or to a level set by the
energy scale of the disorder, ~ 4 K. Commensurate spin and charge density
orders, however, are not expected to alter the nodal spectrum and therefore
cannot be excluded
In-plane superfluid density and microwave conductivity of the organic superconductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br: evidence for d-wave pairing and resilient quasiparticles
We report the in-plane microwave surface impedance of a high-quality single crystal of kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br. In the superconducting state, we find three independent signatures of d-wave pairing: (i) a strong, linear temperature dependence of superfluid density; (ii) deep in the superconducting state the quasiparticle scattering rate Gamma similar to T-3; and (iii) no BCS coherence peak is observed in the quasiparticle conductivity. Above T-c, the Kadowaki-Woods ratio and the temperature dependence of the in-plane conductivity show that the normal state is a Fermi liquid below similar or equal to 23 K, yet resilient quasiparticles dominate the transport up to similar or equal to 50 K
Stability of nodal quasiparticles in underdoped YBa2Cu3O6+y probed by penetration depth and microwave spectroscopy
High resolution measurements of superfluid density and broadband
quasiparticle conductivity have been used to probe the low energy excitation
spectrum of nodal quasiparticles in underdoped YBCO. Penetration depth is
measured to temperatures as low as 0.05 K. Microwave conductivity is measured
from 0.1 to 20 GHz and is a direct probe of zero-energy quasiparticles. The
data are compared with predictions for a number of theoretical scenarios that
compete with or otherwise modify pure d-wave superconductivity, in particular
commensurate and incommensurate spin and charge density waves; d + i s and d +
i d superconductivity; circulating current phases; and the BCS--BEC crossover.
We conclude that the data are consistent with a pure d-wave state in the
presence of a small amount of strong scattering disorder, and are able to rule
out most candidate competing states either completely, or to a level set by the
energy scale of the disorder, ~ 4 K. Commensurate spin and charge density
orders, however, are not expected to alter the nodal spectrum and therefore
cannot be excluded