James C. Booth

A broadband method for the measurement of the surface impedance of thin films at microwave frequencies

We present a new technique to measure the complex surface impedance of the mixed state of superconducting thin films over the broad frequency range 45 MHz–20 GHz. The surface impedance is extracted from measurements of the complex reflection coefficient made on the film using a vector network analyzer. The technique takes advantage of a special geometry in which the self‐fields from currents flowing in the film are everywhere parallel to the film surface, making it an ideal configuration in which to study vortex dynamics in superconductors. The broadband nature of the measurement system allows us to explore a region of magnetic field–temperature–frequency parameter space of superconductors previously inaccessible with other measurement techniques. The power of the technique is illustrated by measurements on thin films of the high temperature superconductor YBa2Cu3O7−δ.

Measurements of the Frequency Dependent Microwave Fluctuation Conductivity of Cuprate Thin Film Superconductors

We describe our measurements of the fluctuation lifetime in high-Tc superconducting thin films above Tc. The paper is laid out as follows. Section 1 discusses our our motivation and broadband experimental technique, while sections 2 and 3 present the experimental method in detail. Section 4 is a brief discussion of the electromagnetic fields used in our experiment. Section 5 presents and discusses the data on fluctuation conductivity, and summarizes our observations regarding the nature of fluctuations above Tc in the cuprate superconductors. Finally, section 6 serves as a conclusion.

Microwave device applications and high-frequency electrodynamics of high-temperature superconductors

Despite the numerous advantages of microwave passive devices using high temperature superconductors, use of all-HTSC microwave devices has been limited to weak power signal processing due to the limitations in power handung capabilities of the materials. Moreover, practical applications of the HTSC microwave devices have been focused on frequencies below 10 GHz. These technical barriers can be overcome by understanding the detailed high frequency electrodynamic properties of high temperature superconductors. Our experiments, carried out on samples with various degrees of quality over a broad dynamic range of microwave power, frequency, external magnetic fields and temperatures, suggests that for the improvement of the power handling capability and an increase of the frequency application range, the devices made of HTSC materials should have fewer Josephson junctions and increased materials homogeneity.

Temperature dependence of the microwave conductivity near Tc in YBa2Cu3O7-δ thin films

We present measurements of the temperature-dependent complex conductivity of YBa2Cu3O7-δ (YBCO) thin films near Tc at several different fixed frequencies in the microwave range. Our measurements show a local maximum in σ1(T) in the vicinity of Tc, which becomes smaller and broader as the frequency increases.

Vortex motion at high frequencies in superconductors: insights into microwave power dependence

We present a brief description of the role played by the motion of magnetic vortices in the power dependence and non-linearity of high Tc superconductors at rf and microwave frequencies. We then review the current understanding of vortex motion at rf and microwave frequencies, and present broadband (45 MHz - 50 GHz) experimental results which shows a striking crossover in the behavior of the vortex dynamics from a low-frequency interaction- dominated regime, to a high frequency essentially single-particle regime. Finally, we discuss the impact these different regimes of vortex motion have on the design and operation of high Tc rf and microwave devices.