Berezinskii–Kosterlitz–Thouless Transition in the Type-I Weyl Semimetal PtBi₂

Abstract

Symmetry breaking in topological matter has become in recent years a key concept in condensed matter physics to unveil novel electronic states. In this work, we predict that broken inversion symmetry and strong spin–orbit coupling in trigonal PtBi2 lead to a type-I Weyl semimetal band structure. Transport measurements show an unusually robust low dimensional superconductivity in thin exfoliated flakes up to 126 nm in thickness (with T c ∼ 275–400 mK), which constitutes the first report and study of unambiguous superconductivity in a type-I Weyl semimetal. Remarkably, a Berezinskii-Kosterlitz-Thouless transition with T BKT ∼ 310 mK is revealed in up to 60 nm thick flakes, which is nearly an order of magnitude thicker than the rare examples of two-dimensional superconductors exhibiting such a transition. This makes PtBi2 an ideal platform to study low dimensional and unconventional superconductivity in topological semimetals.

Enhanced weak superconductivity in trigonal γ-PtBi2

“…The MR is temperature independent below 4 K and it is drastically suppressed above 25 K. No Shubnikov-de Haas oscillations are observed for this H ∥ [0 0 1] field direction. However, they emerge (not shown here) as the field is tilted away from this direction, in agreement with previous reports [18].…”

“…The lowest temperature data point at T ≲ 0.2 K are determined from the 50% criterion applied to the ρ(H) data of figure 5, yielding µ 0 H ab c2 ≈ 1.25 T and µ 0 H c c2 ≈ 1.85 T. Assuming a minor role of surface superconductivity (H c3 ) on the determination of H ab c2 , we obtain an anisotropy coefficient Γ = H ab c2 /H c c2 ≈ 0.7 at the lowest T. The corresponding in-plane and out of plane coherence lengths are ξ ab = √ ϕ 0 /(2π H c c2 ) ≈ 13 nm and ξ c = ξ ab /Γ ≈ 19 nm, where ϕ 0 = 2.07 × 10 −15 Wb is the flux quantum. We note that these critical field values are much larger than previously reported for bulk γ−PtBi 2 crystals [5] and thin exfoliated flakes [18].…”

“…This is not the case for γ-PtBi 2 , for which the magnetic response is weakly diamagnetic (see figure 2) and ρ ab /ρ c ∼ 0.01 [9]. Despite this large transport anisotropy, we point out that a previous report on a bulk single crystal finds that the H c2 anisotropy is weak, with a value Γ ∼ 1.4 [18,36].…”

“…In these samples, the transition remains broad and it is considerably dependent on the electrical current [5]. Robust SC, with 0.3 K ⩽ T c ⩽ 0.4 K, has been also observed in γ-PtBi 2 thin flakes with anisotropic critical fields reaching µ 0 H c2 (0) ∼ 0.3 T [18]. The analysis of this anisotropy suggested that SC is 2D rather than surface SC.…”

“…On the other hand, detailed DFT calculations predict a type-I Weyl semimetal band structure due to a broken inversion symmetry and strong spin-orbit coupling. Were it confirmed, γ-PtBi 2 would be the first type-I Weyl semimetal with superconducting properties at ambient pressure [18].…”