The kinetic energy of electrons in conductors of all types is generally neglected when computing the intrinsic impedance of structures and devices. Except at very high frequencies this is a good approximation. In superconductors the kinetic energy contributes a strong kinetic inductance contribution to the impedance. We report use of the kinetic inductance to both generate and detect ultrafast voltage transients in superconducting device structures. Superconducting micro-bridge structures are fabricated from Tl 2Ba2CaCu2O10 thin films which are then used to demonstrate ultrafast voltage sampling. A simple model for sampling is explained in terms of hot electron dynamics in the presence of a superconducting energy gap.
We have conducted numerical and experimental studies of a simple metamaterial structure formed from 'C' shaped copper rings. Our study focuses on the investigation of the individual resonant elements by surface current and Q factor. We have also analysed wavelike signal propagation along these structures' axes recently predicted theoretically - so called magneto-inductive waves (MIWs). Computer based finite difference electromagnetic methods have been employed to both visualize the surface currents and investigate the effects of varying coupling within the structures. Experimental work has closely followed the theoretical work with measurements carried out using a Vector Network Analyzer to determine the frequency dependent scattering parameters. Applications of these structures are also considered in our work and the aim is to develop a robust, reliable design tool that enables rapid determination of the appropriate dimensions to enable operation of a magneto-inductive waveguide at a desired frequency. The simulation work demonstrates the possibilities for wave propagation in curved guides formed from stacks and rows of elements. And these may form the basis for a new class of microwave filter offering tunability and flexibility and requiring no direct connections to the driving circuits.
The dynamics of the optical response of BSCCO-2212 measured at 1.5 eV has been investigated using femtosecond spectroscopy. The temperature dependence of the response shows three distinct regimes: (1) T less than Tc: with increasing temperature the differential reflectance, (Delta) R/R greater than 0, decreases to zero at Tc, similar to the behavior of the BCS gap function, and the relaxation rate increases linearly with temperature. (2) Tc less than T less than T* (T* - Tc approximately 35 K): (Delta) R/R less than 0. (3) T greater than T*: a weak, positive (Delta) R/R response is measured in the metallic phase. The intermediate regime observed here coincides with the pseudo gap phase detected in ARPES measurements.
Ultrafast time-resolved optical absorption has been measured as a function of doping and temperature in the high temperature superconductors Bi2Sr2Ca1-yYyCu2O8 (0 less than y less than 1) and YBa2Cu3O7-(delta ) (0.06 less than 1). A fast transient response is observed, which changes sign as a function of y in Bi2Sr2Ca1-yYyCu2O8 and in YBa2Cu3O7-(delta ). Careful consideration of the possible mechanisms for the observed behavior lead us to suggest that two independent mechanisms are operating, whereby the two observed positive photoinduced absorption components are due to the electrodynamic response of photoinduced carriers, while the negative signal arises due to spectral hole burning. In the metallic samples a long-lived component is observed, which we suggest comes from photoinduced localized states intrinsic only to the superconducting species of the high-Tc materials. A very significant increase in the scattering rate (1/(tau) ) at temperatures just above Tc signifies coupling to charge and possibly spin fluctuations just above Tc. A large reduction in scattering rate is observed below Tc, similar to that observed previously in YBa2Cu3O6.9 and Tl2Ba2CaCu2O8.
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