Observation of lattice softening at $T_{\rm c}$ in the FeSe$_{0.5}$Te$_{0.5}$ superconductor

TL;DR

This study investigates lattice softening at the superconducting transition temperature in FeSe$_{0.5}$Te$_{0.5}$, revealing changes in lattice dynamics and electronic properties associated with superconductivity.

Contribution

It provides experimental evidence of lattice softening at $T_c$ in FeSe$_{0.5}$Te$_{0.5}$, linking structural and electronic changes to superconductivity.

Findings

Lattice softening occurs at $T_c$, indicated by Mossbauer spectra.

The Debye temperature decreases by at least 60 K at $T_c$.

Samples exhibit a crossover from n-type to p-type conduction around 135 K.

Abstract

Polycrystalline samples of FeSe$_{0.5}$Te$_{0.5}$ were synthesized using a conventional solid-state reaction method. The onset of bulk superconductivity transition was confirmed by SQUID magnetometry at 12.5~K. $^{57}$Fe M\"ossbauer spectra in transmission geometry were recorded at temperatures between 6.0 and 320 K. Both the isomer shift and the total absorption started to drop about $T_c$, indicating a softening of the lattice. The drop is estimated to correspond to at least 60~K from the original Debye temperature $\theta_{\rm D}\approx 460$~K. Seebeck measurements indicate that the samples are $n$-type conductors at low temperatures with a cross-over to $p$-type conductivity around 135 K. The zero Seebeck coefficient is seen below $10.6$~K.