The pressure-enhanced superconducting phase of Sr[Formula: see text]-Bi[Formula: see text]Se[Formula: see text] 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[Formula: see text]Se[Formula: see text] is specially interesting because it displays pressure induced re-entrant superconductivity where the high pressure phase shows almost two times higher [Formula: see text] than the ambient superconducting phase ( [Formula: see text] 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.

Figure 1

(a) Magnetization vs temperature (Zero field cooled (ZFC) and Field cooled (FC)). (b) Resistivity ($\rho$) as a function of temperature (T) of Sr${}_{0.1}$Bi${}_2$Se${}_3$. (c) STM topography of the Sr${}_{0.1}$Bi${}_2$Se${}_3$ cleaved surface (20 nm $\times$ 20 nm). (d) Atomic resolution image (10 nm $\times$ 10 nm) of the Sr${}_{0.1}$Bi${}_2$Se${}_3$ surface. (e) Temperature dependence of the STS spectra upto 1.85 K. (f) Normalized STS spectra with varying magnetic fields upto 15 kG.

Figure 2

(a) Normalized (${(dV/dI)}_N$) spectrum obtained in the intermediate regime of transport showing the characteristic signatures of critical current peaks and Andreev reflection (AR) dips. (b) Schematic of point contact setup. Inset shows the assembly of microconstriction of different diameter and the effective electrical circuit. (c) I–V characteristics corresponding to Maxwell's resistance for different values of $I_C$. (d) I–V characteristics corresponding to Sharvin's resistance calculated from the BTK model (red) and effective Maxwell's resistance (blue). (e) Calculated dV/dI vs V spectrum of the modelled electrical circuit. (f) Zero bias resistance (R) vs. T of the point contact in intermediate regime.

Figure 3

(a) Temperature dependence coloured dots) of normalized (dI/dV)_N vs V along with the best theoretical BTK fits (solid black lines) of the spectra shown in Fig. 2a. (b) Temperature dependence of the spectrum shown in Fig. 2a. The inset shows temperature dependence of the current corresponding to the peaks ($I_c$) extracted from the data. (c) $\mathrm{\Delta }$ vs. T plot extracted from (a). The Red line shows the expected BCS line while the black dots show the experimental data (d) Magnetic field (H) dependence of the spectrum shown in Fig. 2a. The inset shows magnetic field dependence of the current corresponding to the peaks ($I_c$) extracted from the data.

Figure 4

(a) Normalized differential resistance (${(dV/dI)}_N$) spectrum in the thermal regime showing the multiple critical current peaks. (b) Schematic of point contact setup with microconstrictions of different diameter and modelled electrical circuit in thermal regime. (c) I–V characteristics of Maxwell's resistance for different values of $I_c$. (d) I–V characteristics corresponding to effective Maxwell's resistance ($R_M$) (blue) and normal point contact resistance ($R_N$)(red). (e) Calculated dV/dI vs V spectrum. (f) Resistance (R) of the point contact vs. T in the thermal regime. The inset shows the method of estimation of $T_c$.

Figure 5

(a) Temperature dependence of the spectrum shown in Fig. 4a. (b) Temperature dependence of the critical current corresponding to the peaks structure ($I_c$) vs. T extracted from the data in (a). Inset shows the critical current peak position marking with coloured arrows. (c) Magnetic field dependence of the spectrum shown in Fig. 4a. (d) Magnetic field dependence of the critical current corresponding to the peaks structure ($I_c$) with H extracted from the data in (c). The inset shows the multiple critical current peaks (marked with coloured arrows).

Figure 6

(a) Magnetic field dependence of the R-T curves. (b) H-T phase diagram. The red line shows the empirical plot for a conventional superconductor. The blue dots are the data points extracted from the curves in (a).