Maignan A, Caignaert V, Raveau B, Khomskii D, Sawatzky G
Laboratoire CRISMAT, UMR 6508 CNRS ENSICAEN, 6 Boulevard Maréchal Juin, 14050 Caen, France.
Phys Rev Lett. 2004 Jul 9;93(2):026401. doi: 10.1103/PhysRevLett.93.026401. Epub 2004 Jul 8.
Thermoelectric power measurements have been performed for an ordered oxygen-deficient perovskite, HoBaCo2O5.5, in which the alternative layers of CoO6 octahedra and of CoO(5) bipyramids are occupied by Co3+ species. The T-dependent Seebeck coefficient S shows a clear change of the conduction regime at the metal-insulator (MI) transition (T(MI) approximately 285 K). The sign change of S from S<0 to S>0 can be explained assuming that a spin state transition occurs at T(MI). In the metallic state, Co2+ e(g) electrons are moving in a broad band on the background of high or intermediate spin Co3+ species. In contrast, the insulating behavior may result from the Co3+ spin state transition to a low-spin Co3+ occurring in the octahedra. In this phase the transport would occur by hopping of the low-spin Co(4+)t(2g) holes, whereas the high-spin Co2+ electrons become immobilized due to a spin blockade.
已对有序缺氧钙钛矿HoBaCo2O5.5进行了热电功率测量，其中CoO6八面体和CoO(5)双锥体的交替层被Co3+物种占据。与温度相关的塞贝克系数S在金属-绝缘体（MI）转变（T(MI)约285K）时显示出导电机制的明显变化。S从S<0到S>0的符号变化可以通过假设在T(MI)发生自旋态转变来解释。在金属态下，Co2+ e(g)电子在高自旋或中间自旋Co3+物种的背景下在一个宽带中移动。相比之下，绝缘行为可能是由于八面体中Co3+自旋态转变为低自旋Co3+所致。在这个相中，传输将通过低自旋Co(4+)t(2g)空穴的跳跃发生，而高自旋Co2+电子由于自旋阻塞而固定不动。
