Full superconducting gap and type-I to type-II superconductivity transition in single crystalline NbGe2

TL;DR

This study investigates the superconducting properties of NbGe2, revealing a full s-wave gap consistent with BCS theory and a transition from type-I to type-II superconductivity at low temperatures.

Contribution

First detailed spectroscopic analysis of NbGe2 showing a single s-wave gap and identifying a transition from type-I to type-II superconductivity.

Findings

Superconducting gap follows BCS behavior with Δ0 ≈ 0.32 meV.

Superconductivity persists up to Hc2 ≈ 350 Oe.

NbGe2 transitions from type-I to type-II superconductor at low temperature.

Abstract

We report a mechanical point-contact spectroscopy study on the single crystalline NbGe$_2$ with a superconducting transition temperature $T\rm_c$ = 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 $\Delta_0 \sim$ 0.32 meV and 2$\Delta_0$/$k\rm_{B}$$T\rm_{c}$ = 3.62 in the weak coupling limit. In magnetic field, the superconducting gap at 0.3 K keeps constant up to $H_{c1}\sim$150 Oe and gradually decreases until $H_{c2}\sim$350 Oe, indicating NbGe$_2$ going through a transition from type-I to type-II (possible type-II/1) superconductor at low temperature.