Synthesis and Physical Properties of Li_2Pd_3B and Li_2Pt_3B Superconductors

最近在两种含锂化合物Li_2Pd_3B和Li_2Pt_3B中发现了超导电性。它们分别在7.2K和2.6K的温度下表现出超导转变。它们的超导电性是由Li的加入引起的。由于锂在高温下易挥发，在电弧熔炼制备过程中，控制锂的化学计量比是合成高质量样品的关键。这些材料具有相同的立方结构（P4332），其沿着任何方向都不具有镜像对称或反转对称。这是一个非常有趣的特点，这些材料与超导性的对称性。测量了这些化合物的比热，研究了它们的超导性能。测得Li_2Pd_3B的索末菲系数（γn）为9.8mJmol ~（1·K），德拜温度（θD）<-1><-2>为202 K。Δα γnTc值估计为1.6。以相同的方式，对于Li_2Pt_3B，这些参数被描述为9.6mJmol·<01>K·K<-2>、240 K和0.74。由于BCS理论在弱耦合极限下的Δα γnTc为1.43，Li_2Pd_3B的1.6值略高，而Li_2Pt_3B的1.6值较低。电子比热（Ces）通过从整个比热中减去晶格比热来估计。得到的Ces，数据低于Te。Li_2Pd_3B的Ces与BCS理论所预言的有明显的exp（-1/7）依赖关系，而Li_2Pt-3B的Ces似乎不是指数行为而是二次行为。二次行为表明在超导间隙中存在线节点。该结果支持点接触[1]和NMR[2]测量的结果。[1]袁洪庆. F. Agterberg，N. Hayashi，P. Badica，D. Vandervelde，K.托加诺湾Sigrist，M. B。Salamon，Phys.Rev.Lett. 97（2006）017006。[2]M. Nishiyama，Y. Inada，G.- Q. Zheng，Phys.Rev.Lett. 98（2007）047002。

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.