Bereziskii-Kosterlitz-Thouless transition in the Weyl system PtBi2

Abstract

Symmetry breaking in topological matter became, in the last decade, a key concept in condensed matter physics to unveil novel electronic states. In this work, we reveal that broken inversion symmetry and strong spin-orbit coupling in trigonal PtBi2 lead to a Weyl semimetal band structure, with unusually robust two-dimensional superconductivity in thin fims. Transport measurements show that high-quality PtBi2 crystals are three-dimensional superconductors (Tc≈600 mK) with an isotropic critical field (Bc≈50 mT). Remarkably, we evidence in a rather thick flake (60 nm), exfoliated from a macroscopic crystal, the two-dimensional nature of the superconducting state, with a critical temperature Tc≈370 mK and highly-anisotropic critical fields. Our results reveal a Berezinskii-Kosterlitz-Thouless transition with TBKT≈310 mK and with a broadening of Tc due to inhomogenities in the sample. Due to the very long superconducting coherence length ξ in PtBi2, the vortex-antivortex pairing mechanism can be studied in unusually-thick samples (at least five times thicker than for any other two-dimensional superconductor), making PtBi2 an ideal platform to study low dimensional superconductivity in a topological semimetal.