High-pressure synthesis and superconductivity of Ca2-xNaxCuO2Cl2

TL;DR

This study reports the synthesis of Ca2-xNaxCuO2Cl2, a cuprate superconductor analog, demonstrating how synthesis conditions affect Na doping levels and superconducting transition temperatures, with potential for optimizing high-Tc materials.

Contribution

It introduces a new synthesis method for Ca2-xNaxCuO2Cl2 and explores how pressure and temperature influence Na doping and superconductivity.

Findings

Na content depends on synthesis pressure, temperature, and time.

Crystals with Tc up to 13.5 K for x=0.10.

Polycrystalline samples reach Tc of 28.0 K at x=0.20.

Abstract

Ca2-xNaxCuO2Cl2 is a structural analogue to the La2-xSrxCuO4 superconductor. By substituting Na+ for Ca2+ mobile holes are introduced into the CuO2 planes. A series of experiments shows that the Na content x depends not only on the pressure during synthesis but also on the synthesis temperature and time. Ca2-xNaxCuO2Cl2 crystals (x=0.10 and Tc,on=13.5 K) up to 1x1x0.05 mm3 have been grown by a flux method at 35 kbar and the temperature was slowly ramped from 1250 C to 1050 C within 8 h. Applying higher synthesis pressure (up to 55 kbar) and temperature (up to 1700 C) results in polycrystalline samples with highest Tc,on=28.0 K at x=0.20. Further experiments are under way to optimize conditions to grow crystals with extended concentration of Na and Tc characteristic for optomally doped and overdoped regions.