Spectroscopic evidence of mixed angular momentum symmetry in non-centrosymmetric Ru[Formula: see text]B[Formula: see text]

Abstract

Superconducting crystals with a lack of inversion symmetry can potentially host unconventional pairing. However, till today, no direct conclusive experimental evidence of such unconventional order parameters in non-centrosymmetric superconductors has been reported. In this paper, through direct measurement of the superconducting energy gap by scanning tunnelling spectroscopy, we report the existence of both s-wave (singlet) and p-wave (triplet) pairing symmetries in non-centrosymmetric Ru[Formula: see text]B[Formula: see text]. Our temperature and magnetic field-dependent studies also indicate that the relative amplitudes of the singlet and triplet components change differently with temperature.

Figure 1

(a) Six representative tunneling spectra ($S_1$–$S_6$) plots (color points) along with corresponding numerically generated spectra under single gap s-wave model (black lines). The extracted fitting parameters $\mathrm{\Delta }$ and $\mathrm{\Gamma }$ are also shown for each spectrum. (c) Spectrum $S_4$ with both best single gap ‘s-wave’ and ‘p-wave’ fit along with the corresponding extracted parameters. The temperature (T) $\sim$ 310 mK for all spectra. (b) Bulk magnetization (M) data in both zero field cool warming (ZFCW) and field cool cooling (FCC) condition with 10 G magnetic field. Inset: STM topographic image of the sample.

Figure 2

(a) Temperature (T) dependence of tunneling conductance spectra $S_1$ (color lines) with theoretical fits (black lines) in the absence of any magnetic field. (b) Spectra $S_1$ at 340 mK and also at 960 mK along with corresponding fitting parameters, where better fit at higher temperature is visible. (c) Evolution of $\mathrm{\Delta }$ and $\mathrm{\Gamma }$ with temperature, extracted from plot (a) along with an ideal BCS trend of $\mathrm{\Delta }$ for comparison.

Figure 3

(a) Magnetic field (H) dependence of tunneling conductance spectra $S_3$ (color lines) with theoretical fits (black lines) all measured at T $\sim$ 310 mK. (b) Spectra $S_3$ in the environment of H = 0 and H = 16 kG field, along with corresponding fitting parameters. $H\Vert c$-axis of the crystal and a better fit at higher field is visible. (c) Evolution of $\mathrm{\Delta }$ and $\mathrm{\Gamma }$ with the magnetic field, extracted from plot (a).

Figure 4

(a) Temperature (T) dependence of tunneling conductance spectra $S_6$ incorporating gradual appearance and disappearance of the peak-like feature. (b) Spectrum $S_1$ along with best pure s-wave (red line) and mixed $s+p$-wave (blue line) fits with corresponding extracted parameters $\mathrm{\Delta }$ and $\mathrm{\Gamma }$. (c) Evolution of ${\mathrm{\Delta }}_s$ and ${\mathrm{\Delta }}_p$ with temperature, extracted from the $s+p$-wave fits of spectra $S_1$. Ideal BCS trends of $\mathrm{\Delta }$ are also shown for comparison.