NMR Study of peculiar electronic state in the vortex core of anisotrpic superconducttors

在Fulde-Ferrell-Larkin-Ovchinnkov（FFLO）态中，由于Pauli效应导致的对破缺被超导（SC）态中新的对态（k，-k+q）的形成所减少。结果表明，一个新的具有空间振荡SC序参量和自旋极化的SC态将会出现。[1]CeCoIn_5的发现引起了人们极大的兴趣，最近的测量表明，在H_2附近的SC态发生了二级相变，<c2>其中H平行于ab平面。[2]最近的实验结果使得FFLO方案对于CeCoIn_5非常有吸引力。特别是在磁场平行于ab面时，In ~ 1H<115>-NMR谱在H_（T）附近有明显的变化<c2>。基于对空间调制SC能隙的分析，建立了平行于ab平面的高场FFLO态的明确证据。[3]CeCoIn_5的2D性质被认为是形成FFLO态所必需的， ...更多信息 因为平行场中轨道对断裂的强烈减少和准2D费米表面的嵌套性质被期望稳定FFLO状态。因此，澄清在H//ab的情况下是否可以存在FFLO状态对于理解CeCoIn_5中的FFLO状态的性质是重要的。本文研究了CeCoIn_5的SC态在垂直场（H_xab）和平行场（H//ab）中的情况，并给出了两种情况下FFLO态的证据. [4]在Co，In（1）和In（2）位置的NMR谱已经在宽频率范围内测量了H β ab。在高场低温区，当进入新的SC态时，在H_c和H//ab的情况下，NMR谱在T（H_c）以下发生了剧烈的变化。两个分离位置的NMR信号一直持续到最低温度（70~mK），表明超导序参量的Nordal平面结构的出现。我们发现<FFLO>分离FFLO态和BCS态的临界场H_c在T_c（H）以下4.65 ~ 4.70 T的一个很窄的范围内，并得到了In（1）、In（2）和Co位的Knight位移与场的关系的差异。我们发现，在低温区和H_1一级跃迁附近，强的泡利顺磁效应对Knight位移的温度依赖性和场依赖性都有影响<c2>。少

In the Fulde-Ferrell-Larkin-Ovchinnkov (FFLO) state, a pair-breaking due to a Pauli effect is reduced by the formation of new pairing state(k,-k+q) in the superconducting (SC) state. As a result, a new SC state with spatially-oscillating SC order parameter and spin polarization should appear.[1] A newly-discovered CeCoIn_5 has aroused great interest, as recent measurements revealed that a second order phase transition takes place within the SC state in the vicinity of H_<c2> with H parallel to the ab-plane.[2] Recent experimental results make the FFLO scenario a very appealing one for CeCoIn_5. Particularly, In^<115>-NMR spectra exhibited a dramatic change in the vicinity of H_<c2>(T) and with a magnetic field applied parallel to the ab-plane. Based on an analysis with a spatially-modulated SC energy gap, a clear evidence of the FFLO state in high field parallel to the ab-plane is established.[3] The 2D nature of CeCoIn_5 is believed to be essential for the formation of the FFLO state, … More because the strong reduction of the orbital pair breaking in a parallel field and the nesting properties of the quasi-2D Fermi surface are expected to stabilize the FFLO state. Thus, a clarification whether the FFLO state can exist in the case of H//ab is important for understanding the nature of the FFLO state in CeCoIn_5. We here direct our attention to the SC state of CeCoIn_5 in a perpendicular field (H⊥ab) as well as in a parallel field (H//ab), and present evidences of the FFLO state in both cases as well.[4]NMR spectra at the Co, In(1), and In(2) sites have been measured in a wide frequency range for H⊥ab. Upon entering the new SC state at high field and low temperature region, NMR spectra change dramatically below T (H_c) in the case of H⊥ab as well as H//ab. NMR signals at two well-separated positions persists down to the lowest temperature (70~mK), which indicates the appearance of a nordal plane structure of superconducting order parameter. We find the critical field H_<FFLO> separating the FFLO and the BCS state lies in a very narrow range between 4.65 T and 4.70 T below T_c(H).We have obtained discrepancies of the field dependence of the Knight shift among In(1), In(2) and Co site. We have reveal strong Pauli paramagnetic effects on the temperature and field dependences of the Knight shift both in the region of low temperature and in the vicinity of the first order transition at H_<c2>. Less