Pressure Dependence of the Ginzburg–Landau Parameter in Superconducting \(\hbox {YB}_{6}\)

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