Polytypism, polymorphism and superconductivity in TaSe2-xTex

TL;DR

This study explores how different polytypes and polymorphs in TaSe2-xTex affect its superconducting properties, revealing a significant increase in transition temperature in a specific polytype, which suggests stacking sequence impacts superconductivity.

Contribution

It demonstrates the existence of multiple stable polytypes and polymorphs in TaSe2-xTex and shows that polytype variation can dramatically alter superconducting transition temperatures.

Findings

3R polytype has a Tc nearly 17 times higher than 2H polytype

Polytypism significantly influences superconductivity in TaSe2-xTex

Layer stacking sequence affects electronic properties and superconductivity

Abstract

Polymorphism in materials often leads to significantly different physical properties - the rutile and anatase polymorphs of TiO2 are a prime example. Polytypism is a special type of polymorphism, occurring in layered materials when the geometry of a repeating structural layer is maintained but the layer stacking sequence of the overall crystal structure can be varied; SiC is an example of a material with many polytypes. Although polymorphs can have radically different physical properties, it is much rarer for polytypism to impact physical properties in a dramatic fashion. Here we study the effects of polytypism and polymorphism on the superconductivity of TaSe2, one of the archetypal members of the large family of layered dichalcogenides. We show that it is possible to access 2 stable polytypes and 2 stable polymorphs in the TaSe2-xTex solid solution, and find that the 3R polytype shows a superconducting transition temperature that is nearly 17 times higher than that of the much more commonly found 2H polytype. The reason for this dramatic change is not apparent, but we propose that it arises either from a remarkable dependence of Tc on subtle differences in the characteristics of the single layers present, or from a surprising effect of the layer stacking sequence on electronic properties that instead are expected to be dominated by the properties of a single layer in materials of this kind.