Type-I superconductivity in noncentrosymmetric NbGe2

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

“…The sharp specific jump in Fig. 1(b) is probably due to a remnant field in PPMS causing a first-order transition, consistent with the reported specific-heat behavior in field 17,18 . Mechanical PCS in a needle-anvil type was employed to study the superconducting gap of NbGe 2 , where a sharp gold tip prepared by the electrochemical etching was gently engaged on the crystal surface by piezoelectric-controlled nanopositioners.…”

“…The Andreev reflection signal is totally suppressed and the curves become flat above its upper critical field H c2 ∼ 350 Oe. The gradual decrease of double-peak distance supports NbGe 2 as a type-II superconductor at low temperature 0.3 K, implying a transition from type-I to type-II superconductivity with decreased temperature as reported for NbGe 2 17,18 . The ZBC curves as a function of field for three different contacts are plotted in Fig.…”

“…In addition, recent magnetization and specific heat measurements on NbGe 2 have confirmed its crossover from type-I to type-II superconductor with decreased temperatures 17,18 , similar to the case of ZrB 12 and LaRhSi 3 19,20 . In general, superconductors can be simply classified into type-I and type-II cases, where the Meissner state becomes normal suddenly at H > H c for type-I superconductors but magnetic field can enter into the sample continuously in the form of quantum flux in type-II superconductors with a transition from the Meiss-ner phase to Shubnikov phase.…”

“…For type-II/1 superconductor, a discontinuous increase of the flux density from zero to a certain value B 0 corresponds to a first-order phase transition from the Meissner state to I-M state at H c1 19,24,25 . The magnetization usually shows an abrupt decrease from the 4πM = −H line at H c1 with a long tail till M = 0 at H c2 , suggesting the I-M and mixed states, respectively 17,19,24 . It is thus desirable to systematically explore the possible topological superconductivity in NbGe 2 and its superconducting nature.…”

“…NbGe 2 single crystals were grown by a two-step vapor transport technique, where iodine is the transport agent with stoichiometric amounts of high-purity niobium (99.99%) and germanium (99.99%) as described elsewhere 17 . The electrical resistivity and specific-heat of NbGe 2 were measured by a Physical Property Measurement System (PPMS) from Quantum Design with a Helium 3 insert cooling down to 0.4 K. Both the specificheat jump and resistive drop at T c ∼ 2 K support the high quality of the NbGe 2 crystals, as shown in Fig.…”