Infrared study of Ca-substituted YBa2Cu4O8 high-Tc superconductors

Abstract

A detailed far-infrared reflectivity study on ceramic samples of the high-Tc superconductor Y1−0.75xBa2−0.25xCaxCu4O8 was made as a function of temperature (T=30–300 K) and Ca concentration (x=0.0, 0.025, 0.075, and 0.1) in the frequency range 20–5000 cm−1. Kramers-Kronig analysis of the reflectivity was used to extract the phonon frequencies. Phonons at 114, 130, 192, 294, 501, and 597 cm−1 (at 30 K), which were reported earlier in YBa2Cu4O8 samples, were clearly seen in all the samples studied. The two highest-frequency modes do not shift significantly with Ca substitution; the other modes shift markedly to higher frequencies. The temperature dependence of the phonons is reported; phonons at 294 and 501 cm−1 soften upon cooling while others harden. The pattern of shifts is found to be consistent with the dirty limit of the strong-coupling theory of Zeyher and Zwicknagl if a single gap is assumed to lie near 450 cm−1. However, the dirty limit is likely inappropriate for these materials, so the observed agreement must be treated with caution. Reflectivity ratios of the superconducting state to the normal state show two regions of enhancement that can be qualitatively modeled by an effective-medium theory incorporating a single BCS-like gap near 500 cm−1. Although this approach may not be appropriate for quantitative modeling, it permits a close comparison of YBa2Cu4O8 with YBa2Cu3O7; it is observed that there are strong similarities in the behaviors of the two systems.

Source-reported materials — not catalogue approval

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