Influence of Pressure on Superconductivity in YB_{6}

Abstract

Magnetoresistivity measurements on a superconducting system of YB6 (Tc ≈ 7.5 K) down to 60 mK at hydrostatic pressures up to 47 kbar are presented. The superconducting transition temperature, as well as the third critical eld Hc3 reveal a linear decrease with increasing pressure with slopes of d ln Hc3/dp = −1.1 %/kbar, and d ln Tc/ dp = −0.59 %/kbar. From the latter a critical pressure, pc ≈ 170 kbar, at which Tc vanishes, is determined. DOI: 10.12693/APhysPolA.126.340 PACS: 74.70.Ad, 74.25.F, 62.50.p Among large number of boron-rich binary compounds MB x (x ≥ 6), superconductivity has been observed only in eight systems: MB 6 (M = Y, La, Th, Nd) and MB 12 (M = Sc, Y, Zr, Lu) [1]. Among these, yttrium hexaboride (YB 6 ) exhibits the highest transition tempera-. In recent years some of the properties of YB 6 have been intensely investigated, such as the specic heat, resistivity, magnetic susceptibility and thermal expansion [2], optical properties [3], the electronic band structure [4,5] and point-contact spectra [6]. From these studies we can conclude that YB 6 is a conventional type-II BCS superconductor, where the strong coupling in Cooper pairs with 2∆/k B T c ≈ 4.0 is mediated by the phonon mode of Y atoms located at ≈ 8 meV.To our knowledge there is a single experimental study of the pressure (p) eect on the superconducting properties of YB 6 [7]. In particular, the eect on transition temperature T c , the upper critical eld H c2 , and the magnetic penetration depth λ has been measured up to 9.2 kbar. It was shown that T c , H c2 (0), and the coherence length ξ(0) ∝ H c2 (0) −1/2 change linearly with pressure as follows: dT c /dp = −0.055 K/kbar, dH c2 /dp = −4.84 mT/kbar and dξ(0)/ dp = 0.28 nm/kbar, respectively. No pressure eect on λ(0) = 199.0 nm was observed within the experimental accuracy. This implies that one of the fundamental parameters of superconductors, the Ginzburg-Landau parameter κ(0) = λ(0)/ξ(0), which establishes the border between type-I and type-II, is pressure dependent and decreases linearly with pressure. Thus pressure softens the YB 6 superconductor and drives it towards type-I superconductivity.We present the investigations of the inuence of the hydrostatic pressure on the behavior of magnetoresistivity in YB 6 at signicantly higher pressures, up to 47 kbar. * corresponding author; e-mail: gabani@saske.sk As a result we obtain the pressure dependence of superconducting transition temperature T c and the third critical eld, H c3 (0).