Point contact Andreev reflection studies of a non-centro symmetric superconductor Re6Zr

Abstract

Re 6 Zr, a non-centrosymmetric superconductor is an interesting system as recent experimental evidence suggests that the superconducting state breaks time reversal symmetry. This implies a mixing of spin singlet-triplet states leading to a complex order parameter in this system. Here, we report point contact Andreev Reflection (PCAR) measurements on a single crystal of Re 6 Zr (superconducting transition temperature (T c ) = 6.78 K). We observe multiple gap features in the PCAR spectra which depends on the type of tip and contact. Spectral features appear at voltages 1.0 ± 0.1 mV, 0.75 ± 0.05 mV and 0.45 ± 0.1 mV suggesting that there are at least more than one band contributing to superconductivity. However, strong surface inter-band scattering is possibly responsible for the uncertainty in observing them together distinctly in a single contact in the PCAR measurements. Interestingly, the bulk gap (Δ = 1.95k B T c = 1.1 meV) is occasionally observed in PCAR spectra, mostly with ferromagnetic tips. The gap features associated with the other two smaller gaps disappear at the bulk T c . In addition, no anisotropy in the upper critical field was observed. Our results suggest an unconventional superconducting order in this compound: Multiband singlet states dominated by inter-band pairing which break the time reversal symmetry or singlet mixed with triplet states.

Single full gap with mixed type-I and type-II superconductivity on surface of the type-II Dirac semimetal PdTe2 by point-contact spectroscopy

“…4(d). We notice that a similar contact-dependence for PCS spectra has been reported in a recent study on the non-centrosymmetric superconductor Re 6 Zr and it is ascribed to the surface superconductivity as well [51].…”

Time-Reversal Symmetry Breaking in Re-Based Superconductors: Recent Developments

“…Clearly, it would be interesting to know if their SC behavior is similar to that of Re-Mo alloys. The nodeless SC scenario in ReT alloys is also supported by other techniques, as the electronic specific heat, the magnetic penetration depth measured via the tunnel-diode-oscillator-based technique, or the point-contact Andreev reflection [19,20,27,53,54,58,[64][65][66]. In addition, some studies have found evidence of twogap superconductivity in Re 0.82 Nb 0.18 and Re 6 Zr [49,66].…”

Recent progress on superconductors with time-reversal symmetry breaking

“…In all cases, the TRS breaking is manifested by the increase of σ ZF below T c . Moreover, while some studies have found evidence for two-gap superconductivity in ReX 126,127 , most measurements were generally well accounted for by a single gap s-wave model [114][115][116][123][124][125][128][129][130][131][132][133][134] . An example of this is displayed in Fig.…”