Strain-controlled superconductivity in epitaxially grown thin films of 1T-TaS2

Abstract

Abstract 1T-TaS2 is a prototype layered material with a rich phase diagram that includes multiple charge density wave (CDW) transitions and technologically important metastable states. It also supports a superconducting phase induced by hydrostatic pressure, cation substitution, intercalation, or doping. Thin 1T-TaS2 crystals deposited on various substrates exhibit transition temperatures that are strongly dependent on the substrate-induced strain, and depart from bulk transition temperatures in a way that is not clearly understood at present. Here we show that thin polycrystalline films of 1T-TaS2 grown by molecular beam epitaxy on (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT) substrates have a suppressed CDW transition to a commensurate phase. Instead, resistivity, magnetoresistance, and critical current measurements reveal metallic behavior with an onset to a superconducting state below $$\:{T}_{c}=3.8$$ K. The appearance of superconductivity is suggested to be driven by the in-plane tensile differential strain exerted on the 1T-TaS2 film by the LSAT substrate during cooling, which in turn results in a strongly amplified out-of-plane compressive strain triggered by the Poisson effect, combined with traceable signs of intercalation with La and Sr atoms from the substrate. The experiments suggest that tensile substrate strain may be usefully applied for achieving desirable Functional properties that are otherwise accessible through hydrostatic pressure, and generally for investigating of the effects of anisotropic strain in 2D materials and monolayer stacks or heterostructures.