The pressure-enhanced superconducting phase of Sr$$_x$$–Bi$$_2$$Se$$_3$$ probed by hard point contact spectroscopy

Abstract

The superconducting systems emerging from topological insulators upon metal ion intercalation or application of high pressure are ideal for investigation of possible topological superconductivity. In this context, Sr-intercalated Bi$$_2$$ 2 Se$$_3$$ 3 is specially interesting because it displays pressure induced re-entrant superconductivity where the high pressure phase shows almost two times higher $$T_c$$ T c than the ambient superconducting phase ( $$T_C\sim 2.9$$ T C ∼ 2.9 K). Interestingly, unlike the ambient phase, the pressure-induced superconducting phase shows strong indication of unconventional superconductivity. However, since the pressure-induced phase remains inaccessible to spectroscopic techniques, the detailed study of the phase remained an unattained goal. Here we show that the high-pressure phase can be realized under a mesoscopic point contact, where transport spectroscopy can be used to probe the spectroscopic properties of the pressure-induced phase. We find that the point contact junctions on the high-pressure phase show unusual response to magnetic field supporting the possibility of unconventional superconductivity.

Observability of superconductivity in Sr-doped Bi2Se3 at the surface using scanning tunneling microscope

“…This absence of superconductivity on the surface has been a problem in many STM experiments performed on doped Bi 2 Se 3 superconductors, but the present study found that during the prolonged scanning of the tip on the Sr x Bi 2 Se 3 surface, the tip will always accumulate micro-flakes of Sr x Bi 2 Se 3 which show superconductivity. This finding helps to clarify some discrepancies in the observed SC gaps in earlier STM studies [13,14,16], which were inconsistent with the bulk T c . Note that our experimental observation is essentially consistent with those on superconducting Tl-doped Bi 2 Te 3 and Nb-doped Bi 2 Se 3 reported by Wilfert et al [17], who concluded that superconductivity does not extend to the surface in these superconductors.…”

“…Han et al [13] used ARPES to demonstrate that the TSS of their Sr x Bi 2 Se 3 samples was intact and the STS data showed a superconducting gap, but their data are also fully compatible with a superconducting flake having been transferred to the tip. The superconductivity observed on the surface of Sr x Bi 2 Se 3 by STM in the works by Kumar et al [16] and Du et al [14] showed values of ∆, T c , and µ 0 H c2 that are incompatible with the bulk values; this problem can be straightforwardly reconciled if a superconducting flake was present on the tip. Interestingly, Kumar et al [16] also performed hard pointcontact spectroscopy and found an increase in T c with increasing pressure.…”

“…The superconductivity observed on the surface of Sr x Bi 2 Se 3 by STM in the works by Kumar et al [16] and Du et al [14] showed values of ∆, T c , and µ 0 H c2 that are incompatible with the bulk values; this problem can be straightforwardly reconciled if a superconducting flake was present on the tip. Interestingly, Kumar et al [16] also performed hard pointcontact spectroscopy and found an increase in T c with increasing pressure. It would be instructive to clarify if the strain due to the pressure from the tip leads to a destruction of the TSS.…”

“…Note that in the bulk of Sr x Bi 2 Se 3 , µ 0 H c2 amounts only to about 1.5 T [15]. A recent work by Kumar et al [16] reported a superconducting gap of ∆ ≈ 0.19 -0.31 meV on the surface of a Sr x Bi 2 Se 3 crystal with the bulk T c of 2.9 K.…”

Intrinsic finite-energy Cooper pairing in $j=3/2$ superconductors

“…This manifests itself by a pair of indirect finite-energy anti-crossings of BdG bands signaling pairing of quasi-particles having high total angular momenta with quasi-holes possessing lower angular momenta. The phenomenon may be experimentally detectable through tunneling spectroscopy [64][65][66][67] and angle-resolved photoemission spectroscopy [68].…”