用NQR研究层状二硫族化合物CuS（铜蓝）的相变和反常电子行为
Phase transition and anomalous electronic behavior in layered dichalcogenide CuS (covellite) probed by NQR
摘要：
利用铜核核四极共振（NQR）研究了准二维低温超导体CuS（铜蓝）在1.47 ~ 290 K温度范围内的电子性质。发现了对应于结构中铜的两个非等效位置Cu（1）和Cu（2）的两个NQR信号，铜四极频率、线宽和自旋晶格弛豫率的温度依赖性，迄今为止从未对该材料进行过如此精确的研究，共同证明了结构在55 K附近的相变。这伴随着电子光谱的转变，而这种转变对于简单金属来说并不典型。对NQR结果的分析及其与文献数据的比较表明，在这两个网站上的铜离子的价态是一价和二价状态之间的中间特征与前者占主导地位。人们发现，在低温下，Cu（1）和Cu（2）导带存在强烈的杂交，这表明即使在2D区域，这些离子之间也会发生电荷离域。根据我们的实验数据，讨论了CuS中能隙、电荷涨落和电荷密度波的出现以及相变的本质。结果表明，硫化铜的某些物理性质与正常状态下的高温超导体相似。
英文摘要：
Nuclear quadrupole resonance (NQR) on copper nuclei has been applied for studies of the electronic properties of quasi-two-dimensional low-temperature superconductor CuS (covellite) in the temperature region between 1.47 and 290 K. Two NQR signals corresponding to two non-equivalent sites of copper in the structure, Cu(1) and Cu(2), has been found. The temperature dependences of copper quadrupole frequencies, line-widths and spin-lattice relaxation rates, which so far had never been investigated so precisely for this material, altogether demonstrate the structural phase transition near 55 K, which accompanies transformations of electronic spectrum not typical for simple metals. The analysis of NQR results and their comparison with literature data show that the valence of copper ions at both sites is intermediate in character between monovalent and divalent states with the dominant of the former. It has been found that there is a strong hybridization of Cu(1) and Cu(2) conduction bands at low temperatures, indicating that the charge delocalization between these ions takes place even in 2D regime. Based on our data, the occurrence of energy gap, charge fluctuations and charge-density waves, as well as the nature of phase transition in CuS are discussed. It is concluded that some physical properties of CuS are similar to those of high-temperature superconductors (HTSC) in normal state.
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