Lattice instability and enhancement of superconductivity in YB6

The superconducting and normal state characteristics of yttrium hexaboride (YB6) have been investigated for the single crystals with a transition temperatures T-c ranging between 4.2 K and 7.6 K. The extracted set of microscopic parameters [the coherence length xi(0) similar to 320-340 angstrom, the penetration depth lambda(0) similar to 1100-3250 angstrom and the mean free path of charge carriers l = 11-58 angstrom, the Ginzburg-Landau-Maki parameters kappa(1,2)(0) similar to 3.3-9.5, and the superconducting gap Delta(0) similar to 6.2-14.8 K] confirms the type II superconductivity in dirty limit (xi >> l) with a medium to strong electron-phonon interaction (the electron-phonon interaction constant lambda(e-ph) = 0.32-0.96) and s-type pairing of charge carriers in this compound [2 Delta(0)/k(B)T(c) approximate to 3-4]. The comparative analysis of charge transport (resistivity, Hall and Seebeck coefficients) and thermodynamic (heat capacity, magnetization) properties in the normal state in YB6 allowed to assume a transition into the cage-glass state at T* similar to 50 K with a static disorder in the arrangement of the Y3+ ions. We argue that the significant T-c variations in YB6 single crystals are determined by two main factors: (i) the superconductivity enhancement is related with the increase of the number of vacancies, both at yttrium and boron sites, leading to nonstoichiometric composition, which is accompanied by the enhancement of electron-phonon interaction in the hexaboride lattice; (ii) stronger T-c depression is observed in near stoichiometric and more dense crystals and it is induced by the development of bcc lattice instability producing strong distortion, disordering, and formation of defect complexes in the matrix of YB6.