Synthesis and Physical Properties of Li_2Pd_3B and Li_2Pt_3B Superconductors

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Superconductivity has been recently found in two Li containing compounds, Li_2Pd_3B, and Li_2Pt_3B. They show superconducting transition at the temperatures, 7.2 K and 2.6 K, respectively. Their superconductivity is induced by Li addition. Controlling the Li stoichiometry is a key to synthesize high quality samples in the preparation process of arc-melting because Li is volatile at high temperatures. Those materials have the same cubic structure (P4332) which does not have mirror or inversion symmetry along any directions. It is a very interesting feature of those materials in relation to the symmetry of superconductivity. Specific heat of these compounds was measured for investigating superconducting properties. The Sommerfeld coefficient (γn) and Debye temperature (θD) of Li_2Pd_3B were given as 9.8 mJmo1^<-1>K^<-2> and 202 K respectively. The value of Δα γnTc, was estimated to be 1.6. In the same manner, those parameters were described for Li_2Pt_3B as 9.6 mJmol^<01>K^<-2>, 240 K, and 0.74. Since Δα γnTc in the weak-coupling limit by the BCS theory is 1.43, the value of 1.6 for Li_2Pd_3B is slightly higher, while the value for Li_2Pt_3B is lower. The electronic specific heat (Ces) was estimated by subtracting the lattice one from the entire specific heat. The obtained Ces, data below Te. in Li_2Pd_3B gives clearly exp (-1/7) dependence as predicted by the BCS theory for the superconductivity, while the Ces of Li_2Pt-3B seems not to reflect an exponential behavior but to reflect a quadratic behavior. The quadratic behavior suggests the existence of line nodes in the superconducting gap. This result supports the results of point contact [1] and NMR[2] measurements. [1]H.Q.Yuan D. F. Agterberg, N. Hayashi, P. Badica, D. Vandervelde, K. Togano, M. Sigrist, M. B. Salamon, Phys.Rev.Lett. 97 (2006)017006.[2] M. Nishiyama, Y. Inada, G.-q. Zheng, Phys.Rev.Lett. 98 (2007)047002.