Lattice dynamics of the superconducting Bi₂Sr₂Ca(Cu, Sn)₂O_8−y

Abstract

Phonon anomaly was observed below 140 K for the absorption area (lnA) and the isomer shift (δ) plotted against the temperature (T) in the¹¹⁹Sn Mössbauer study of the Bi₂Sr₂Ca(Cu_0.995Sn_0.005)₂O_8−y superconductor (glass-ceramic,T _c(on)=86 K), which was prepared by heat treatment of the glassy sample. The anomaly suggests that the softening (quenching) of the lattice vibration causes the superconducting transition, as was observed in several YBa₂(Cu, Sn)₃O_7−y superconductors. Fitting of the theoretical curve for the normal phonon of the “combined Debye and Einstein” model to the lnA versusT andδ versusT plots yieldedΘ _D andΘ _E of 240 and 140 K, respectively. These results suggest that the probe (Sn⁴⁺) was located at the interstitial site close to the Cu site, and the softening of the Cu site vibration could be detected before the superconducting transition took place. Disordered structure seems to be favorable for the detection of phonon anomaly in the Bi system.