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Broadband spectrometer for measuing the low-temperature microwave absorption of unconventional superconductors Bobowski, Jake Stanley

Abstract

A highly sensitive spectrometer has been designed and built to measure the ultralow temperature surface resistance of unconventional superconducting single crystals continuously over a broad range of microwave frequencies. The technique uses bolometric detection to measure the temperature rise of samples that have been exposed to a microwave magnetic field. An in-situ normal metal reference sample is used to calibrate the field strength at the sample site and is vital to the success of the measurement. This broadband technique has already been applied by researches at UBC to make detailed measurements of the conductivity spectrum of YBa₂Cu₃O[sub 6+y] and have revealed, for the first time, the cusp-like line shape expected for a d-wave superconductor. Thus far these measurements have been limited to temperatures above 1 K and there remain several key issues regarding low-energy quasiparticle dynamics in these materials that this new spectrometer will address. A new broadband apparatus has been designed to be used with a ³He/⁴He.dilution refrigerator that can achieve base temperatures less than 50 mK. The main obstacle encountered in adapting this technique for use at lower temperatures is achieving the necessary temperature stability required to reliably perform the measurements. Low-pass thermal filters have been designed and constructed that will substantially suppress any temperature instabilities before they are able to reach the sample thermal stages. An equivalent circuit model was developed and implemented to numerically evaluate the filter performance. This low-temperature microwave spectrometer will be used to uncover new physics regarding low-energy quasiparticle dynamics in YBa₂Cu₃O[sub 6+y] and other materials, such as heavy fermion compounds, that is inaccessible by any other measurement technique.

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