Crystal growth and in-plane optical properties of Tl$_2$Ba$_2$Ca$_{n-1}$Cu$_n$O$_x$ (n=1,2,3) superconductors

TL;DR

This study reports the growth, structural analysis, and in-plane optical properties of Tl-based cuprate superconductors with varying layers, revealing characteristic spectral features below the superconducting transition temperature.

Contribution

It presents the first detailed optical reflectance measurements and analysis of Tl$_2$Ba$_2$Ca$_{n-1}$Cu$_n$O$_x$ crystals across different n-values, highlighting spectral features related to superconductivity.

Findings

Superconducting transition temperatures are 92 K, 109 K, and 119 K for n=1,2,3.

Optical reflectance shows a linear dependence above T$_c$ and distinct features below T$_c$.

Features in Tl2223 appear at higher energy scales than in Tl2212 and Tl2201.

Abstract

Single crystals of thallium-based cuprates with the general formula Tl$_{2}$Ba$_{2}$Ca$_{n-1}$Cu$_{n}$O$_{x}$(n=1,2,3) have been grown by the flux method. The superconducting transition temperatures determined by the ac magnetic susceptibility are 92 K, 109 K, and 119 K for n=1,2,3 respectively. X-ray diffraction measurements and EDX compositional analysis were described. We measured in-plane optical reflectance from room temperature down to 10 K, placing emphasis on Tl-2223. The reflectance roughly has a linear-frequency dependence above superconducting transition temperature, but displays a pronounced knee structure together with a dip-like feature at higher frequency below T$_c$. Correspondingly, the ratio of the reflectances below and above T$_{c}$ displays a maximum and a minimum near those feature frequencies. In particular, those features in Tl2223 appear at higher energy scale than Tl2212, and Tl2201. The optical data are analyzed in terms of spectral function. We discussed the physical consequences of the data in terms of both clean and dirty limit.