Pressure Dependence of the Ginzburg–Landau Parameter in Superconducting YB6

We present measurements of the superconducting critical temperature T c , the upper critical field H c 2 and the third critical field H c 3 as a function of pressure in BCS type-II superconductor YB 6 ( T c = 7.5 K , H c 2 ( 0 ) = 270 mT and H c 3 ( 0 ) = 450 mT at ambient pressure) up to 3 GPa. Magnetic susceptibility measurements down to 2 K have shown a negative pressure effect on T c as well as on H c 2 with slopes d T c / d p = - 0.531 K/GPa ( d ln T c / d p = - 7.1 % / GPa ) and d H c 2 ( 0 ) / d p = - 37 mT/GPa ( d ln H c 2 / d p = - 14 % / GPa ) , respectively. Parallel magnetoresistance measurements evidenced nearly the same slopes of d ln T c / d p = - 5.9 % / GPa ( d ln H c 3 / d p = - 11 % / GPa ) in the equal pressure range. From these results, the estimated pressure effect on the coherence length d ξ ( 0 ) / d p = 2.05 nm/GPa together with the supposed zero pressure effect on the magnetic penetration depth ( d λ ( 0 ) / d p ≈ 0 ) implies that the Ginzburg–Landau parameter κ ( 0 ) = λ ( 0 ) / ξ ( 0 ) decreases with pressure as d κ ( 0 ) / d p = - 0.31 / GPa . According to this decrease, a transition from type-II to type-I superconductor should be observed in YB 6 at a critical pressure p c ≈ 10 GPa .