Spectroscopic evidence of mixed angular momentum symmetry in non-centrosymmetric Ru$$_7$$B$$_3$$

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$$_7$$ 7 B$$_3$$ 3 . Our temperature and magnetic field-dependent studies also indicate that the relative amplitudes of the singlet and triplet components change differently with temperature.

Singlet-triplet mixing in the order parameter of the noncentrosymmetric superconductor Ru7B3

“…However, a fit to the extracted value of H c1 from magnetization curves indicated an isotropic s-wave gap [42]. More recently, direct measurement of the gap through scanning tunneling spectroscopy measurements by Datta et al [44] also indicated a mixed structure appropriate for noncentrosymmetric systems. We can therefore conclude that the majority of the evidence is in favor of a mixed gap structure.…”

“…Specific-heat and magnetization measurements on a single crystal of Ru 7 B 3 resulted in Ginzburg-Landau parameters of 21.6 and 25.5 for the [100] and [001] directions, respectively [43], making it a reasonably strong type-II superconductor. Reports on the gap structure for Ru 7 B 3 vary, with some measurements concluding an isotropic s-wave gap [42], some a mixed gap [44], while others are less conclusive [43]. Our previous investigations into the VL structure of Ru 7 B 3 revealed highly unusual behavior for magnetic fields applied along the a axis [45], where instead of the VL having a single or degenerate set of orientations with respect to the crystal axes for each field and temperature, which is typical behavior seen in almost all type II superconductors, it instead showed a field-history-dependent orientation.…”

Singlet-triplet mixing in the order parameter of the noncentrosymmetric superconductor Ru$_{7}$B$_{3}$

“…However, a fit to the extracted value of H c1 from magnetisation curves indicated an isotropic s-wave gap [42]. More recently, direct measurement of the gap through scanning tunneling spectroscopy measurements by Datta et al [44] also indicated a mixed structure appropriate for noncentrosymmetric systems. We can therefore conclude that the majority of the evidence is in favour of a mixed gap structure.…”

“…Specific-heat and magnetisation measurements on a single crystal of Ru 7 B 3 resulted in Ginzburg-Landau parameters of 21.6 and 25.5 for the [100] and [001] directions respectively [43], making it a reasonably strong type-II superconductor. Reports into the gap structure for Ru 7 B 3 vary, with some measurements concluding an isotropic s-wave gap [42], some a mixed gap [44], while others are less conclusive [43]. Our previous investigations into the VL structure of Ru 7 B 3 revealed highly unusual behaviour for magnetic fields applied along the a axis [45], where instead of the VL having a single or degenerate set of orientations with respect to the crystal axes for each field and temperature, which is typical behaviour seen in almost all type II superconductors, it instead showed a field-history dependent orientation.…”