Full superconducting gap and type-I to type-II superconductivity transition in single crystalline NbGe 2

We report a mechanical point-contact spectroscopy study on the single crystalline ${\mathrm{NbGe}}_{2}$ with a superconducting transition temperature ${T}_{c}=2.0$--2.1 K. The differential conductance curves at 0.3 K can be well fitted by a single-gap $s$-wave Blonder-Tinkham-Klapwijk model and the temperature-dependent gap follows a standard Bardeen-Cooper-Schrieffer behavior, yielding ${\mathrm{\ensuremath{\Delta}}}_{0}\ensuremath{\sim}0.32$ meV and $2{\mathrm{\ensuremath{\Delta}}}_{0}/{k}_{\text{B}}{T}_{c}=3.62$ in the weak-coupling limit. In magnetic field, the superconducting gap at 0.3 K remains constant up to ${H}_{c1}\ensuremath{\sim}150$ Oe and gradually decreases until ${H}_{c2}\ensuremath{\sim}350$ Oe, indicating ${\mathrm{NbGe}}_{2}$ is going through a transition from a type-I to type-II (possible type-II/1) superconductor at low temperature.