NaxCoO2中钠空位有序化及定域自旋和巡游电荷的共存
Sodium vacancy ordering and the co-existence of localized spins and itinerant charges in NaxCoO2
摘要：
钴酸钠家族（NaxCoO 2）在过渡金属氧化物中是独特的，因为Co位于三角形晶格上，并且其价态可以通过改变Na浓度x在宽范围内进行调节。到目前为止，对丰富的现象学（从非常规的超导性到增强的热电性）的详细建模一直受到控制纯相的困难的阻碍。我们发现，某些钠浓度是特别稳定的，并与超晶格有序的钠簇。这自然会导致局部自旋和巡回载流子共存的图片。当x = 0.84时，我们发现费米能非常小，为87 K.我们的图片为从核磁共振到光学到热传输的各种测量带来了一致性。我们的研究结果也使我们朝着模拟x = 0.71时神秘的“居里-魏斯”金属态迈出了第一步。我们认为局域矩可能形成一个量子自旋液体态，并对我们的假设进行了实验检验。
英文摘要：
The sodium cobaltate family (NaxCoO2) is unique among transition metal oxides because the Co sits on a triangular lattice and its valence can be tuned over a wide range by varying the Na concentration x. Up to now detailed modeling of the rich phenomenology (which ranges from unconventional superconductivity to enhanced thermopower) has been hampered by the difficulty of controlling pure phases. We discovered that certain Na concentrations are specially stable and are associated with superlattice ordering of the Na clusters. This leads naturally to a picture of co-existence of localized spins and itinerant charge carriers. For x = 0.84 we found a remarkably small Fermi energy of 87 K. Our picture brings coherence to a variety of measurements ranging from NMR to optical to thermal transport. Our results also allow us to take the first step towards modeling the mysterious ``Curie-Weiss'' metal state at x = 0.71. We suggest the local moments may form a quantum spin liquid state and we propose experimental test of our hypothesis.
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