Probing the Superconducting Ground State of Single Crystal Organic Superconductor lambda-(BETS)2GaCl4

lambda-(BETS)2GaCl4 is a non-magnetic anion-based organic superconductor SC attracting interest since its Fe analog becomes superconductor by applying 33 T. Both have similar Tc ~5 K. We measured TF and ZF-mSR down to 0.3K in the powder sample. The temperature dependence of the damping rate s(T) of the muon-spin precession related to the penetration depth was investigated in TF= 30 and 60 G. They indicate that SC is not stabilized even at 0.3 K or that there are nodes in the SC gap. We found a possible signature of time-reversal symmetry breaking (TRSB) since we detect slightly enhancement of relaxation rate below Tc in ZF-mSR. For extracting intrinsic & clear result, we are now planning to measure single crystals which can be aligned to make angular dependence measurement possible. We plan to align crystals in order to measure s(T) and TRSB and discuss SC gap symmetry by TF and ZF-mSR.

lambda-(BETS)2GaCl4 是一类基于非磁性阴离子的有机超导体（organic superconductor, SC），因其铁同系物在施加33特斯拉磁场时可实现超导而广受关注。二者的临界温度（Tc）均约为5开尔文。我们针对该粉末样品开展了横向场（Transverse Field, TF）与零场（Zero Field, ZF）缪子自旋弛豫（muon Spin Relaxation, mSR）测试，测试温度低至0.3开尔文。在TF=30与60高斯的条件下，我们研究了与穿透深度相关的缪子自旋进动阻尼率s(T)的温度依赖性。实验结果表明，在0.3开尔文下仍未形成稳定超导态，或是超导体能隙中存在节点结构。在零场缪子自旋弛豫测试中，我们观测到临界温度以下的弛豫率存在小幅提升，这暗示存在时间反演对称性破缺（time-reversal symmetry breaking, TRSB）的潜在特征。为获取本征且清晰的实验结果，我们目前正计划对单晶体样品进行取向处理，以实现角度依赖性测量。我们计划通过晶体取向来测定阻尼率s(T)并验证时间反演对称性破缺特征，并借助横向场与零场缪子自旋弛豫研究超导体能隙的对称性。