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Broadband microwave spectroscopy of YBa[sub 2]Cu[sub 3]O[sub 6+y] Turner, Patrick James


A novel low temperature bolometric method has been devised and implemented for high-precision measurements of the microwave surface resistance of small singlecrystal platelet samples having very low absorption, as a continuous function of frequency from 0.5 GHz to 21 GHz. The key to the success of this non-resonant method is the in-situ use of a normal metal reference sample that calibrates the absolute rf field strength. The sample temperature can be controlled independently of the 1.2 K liquid helium bath, allowing for measurements of the temperature evolution of the absorption. Using this method, the minimum detectable power at 1.3 K is 1.5 pW, corresponding to a surface resistance sensitivity of approximately 1 μΩ for a typical 1 mm x 1 mm platelet sample. Using this apparatus, the zero-field electron spin resonance absorption spectrum of a dilute array of Gd ions substituted for Y in YBa₂Cu₃O[sub 6+y] was used as a unique means of accessing the absolute value of the low temperature magnetic penetration depth λ(T → 0) in the Meissner state. The result is a new set of precise values of λ for screening currents along the three principal crystallographic orientations in samples of Gd[sub x]Y[sub 1-x]Ba₂Cu₃O[sub 6+y] for three different oxygen-ordered phases having T[sub c] values of 89 K, 75 K and 56 K. The in-plane values are found to depart substantially from the widely-reported relation T[sub c] ∝ 1/ λ² inferred mainly from muon spin relaxation measurements on polycrystalline materials. The broadband microwave spectrometer has allowed measurements in unprecedented detail of the in-plane microwave absorption in high-purity YBa₂Cu₃O[sub 6+y] single crystals. This has permitted, for the first time, the observation of the cuspshaped conductivity spectra characteristic of weak-limit scattering from impurities in a d-wave superconductor. A comparison of the detailed measurements for samples of ortho-II ordered YBa₂Cu₃O₆.₅₂ (T[sub c] =56 K) and fully-doped YBa₂Cu₃O₆.₉₉₃ (T[sub c] =89 K) with theoretical calculations indicate that intermediate scattering strengths may best describe the behaviour. However, the data reveal a residual quasiparticle spectral weight that is not accounted for within the standard theoretical framework for nodal quasiparticles.

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