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Zero field ESR of magnetic impurities in superconductors : a novel technique for measuring magnetic penetration depth and application to high Tc YBa2Cu3O7-[delta]

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Title: Zero field ESR of magnetic impurities in superconductors : a novel technique for measuring magnetic penetration depth and application to high Tc YBa2Cu3O7-[delta]
Author: Pereg-Barnea, Tamar
Degree Master of Science - MSc
Program Astronomy
Copyright Date: 2001
Abstract: The Meissner effect in superconductors is characterized by an exponential decay of externally applied magnetic fields into the sample with a length scale A, that is called the magnetic penetration depth. λ is directly related to the superfluid density of the superconductor and is therefore one of the most important probes of the intrinsic properties of superconductors. This work describes the development of a new technique for measuring the penetration depth at microwave frequencies and low temperatures. The surprising combination of two seemingly unrelated experiments has led to a direct method of measuring the magnetic penetration depth in the London limit of high T[sub c] superconductors. Samples of high T[sub c] YBCO with uniform doping of magnetic ions (Gd[sub x]Y[sub 1-x]Ba₂Cu₃0 ₇-[sub δ] where x ~ 1.28%) were prepared. The magnetic ions embedded in the superconductor serve as magnetic field probes through a zero field Electron Spin Resonance (ESR) experiment. Measurement of the imaginary part of the magnetic susceptibility, x"(ω), through the sample's microwave surface resistance, reveals relatively sharp ESR transitions with integrated intensity that is proportional to the number of ions that are exposed to the field. With a uniform ion distribution, the number of ions that are exposed to the field is proportional to the penetration depth λ. The spectrum of x”(ω) is obtained from surface resistance measurement in a bolometric apparatus. Our new method was successfully applied to overdoped and optimally doped YBCO samples and absolute values of the a-b plane penetration depth at T = 1.2 are given.
URI: http://hdl.handle.net/2429/11792
Series/Report no. UBC Retrospective Theses Digitization Project [http://www.library.ubc.ca/archives/retro_theses/]

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