D. M. Broun

Apparatus for high-resolution microwave spectroscopy in strong magnetic fields

We have developed a low-temperature, high-resolution microwave surface-impedance probe that uses cavity perturbation of dielectric resonators and is able to operate in high static magnetic field. This method has sufficient sensitivity to resolve the microwave absorption of submillimeter-sized superconducting samples. The resonators are constructed from high-permittivity single-crystal rutile (TiO2) and have quality factors in excess of 106. 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 TE01(n−δ) modes, providing homogeneous microwave fields at the center of the resonator where the sample is positioned. Using a sapphire hot-finger technique, measurements ...

Determination of electron-phonon interaction parameters from time-domain terahertz spectroscopy

We present an analytical framework for determining metallic electron-phonon interaction parameters from time-domain terahertz spectroscopy measurements. We apply this analysis to the case of lead, where we obtain values that are consistent with existing estimates. We discuss the statistical and systematic errors that limit the uncertainty in the parameter estimates.

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.

Microwave spectroscopy of vortex dynamics in ortho-II YBa2Cu3O6.52

We present measurements of the vortex-state surface impedance, Z_s = R_s + i X_s, of a high quality, ortho-II-ordered single crystal of the cuprate high temperature superconductor YBa2Cu3O6.52 (T_c = 59K). Measurements have been made at four microwave frequencies (\omega/2\pi = 2.64, 4.51, 9.12 and 13.97 GHz), for magnetic fields ranging from 0 to 7 T. From these data we obtain the field, frequency and temperature dependence of the vortex viscosity, pinning constant, depinning frequency and flux-flow resistivity. The vortex viscosity, \eta(\omega,T), has a surprisingly strong frequency dependence and bears a striking resemblance to the zero-field quasiparticle conductivity, \sigma_qp(\omega,T), suggesting that the dominant dissipative mechanism for the flux lines is induced electric fields coupling to bulk, long-lived d-wave quasiparticles outside the vortex cores. This is in sharp contrast to the conventional Bardeen-Stephen picture, in which dissipation takes place inside quasi-normal vortex cores. The strong frequency dependence of the vortex viscosity in the microwave range requires us to treat it as a complex response function, with an imaginary part that is predicted to contribute to the apparent pinning force on the vortices. Measurements of the frequency dependence of the pinning force confirm that this term is present, and in a form consistent with the requirements of causality. At low temperatures the flux-flow resistivity, \rho_ff \propto 1/\eta, has the form \rho_ff(T) = \rho_0 + \rho_1 \ln(1/T), reminiscent of the DC resistivity of cuprates in the pseudogap regime.

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. We show how the results can be extended by analytical 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 uniaxial anisotropy, in which the penetration depth for currents flowing perpendicular to the layers,

Roughness-induced domain structure in perpendicular Co/Ni multilayers

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Stability of nodal quasiparticles in underdoped YBa 2 Cu 3 O 6 + y probed by penetration depth and microwave spectroscopy

, is much greater than that for in-plane currents,

Fluctuation effects in the microwave conductivity of cuprate superconductors

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Spin transport in tantalum studied using magnetic single and double layers

. Exact solutions are found in the limit

Effects of substitution on the exchange stiffness and magnetization of Co films

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