Zhang R, Fu Q S, Yin C Y, Li C L, Chen X H, Qian G Y, Lu C L, Yuan S L, Zhao X J, Tao H Z
School of Physics, Huazhong University of Science and Technology, Wuhan, 430074, P. R. China.
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, 430070, P. R. China.
Sci Rep. 2018 Nov 20;8(1):17093. doi: 10.1038/s41598-018-35490-5.
The metal-insulator transition temperature T in VO is experimentally shown to be almost the same as a magnetic transition temperature T characterized by an abrupt decrease in susceptibility, suggesting the evidence of the same underlying origin for both transitions. The measurement of susceptibility shows that it weakly increases on cooling for temperature range of T > T, sharply decreases near T and then unusually increases on further cooling. A theoretical approach for such unusual observations in susceptibility near T or below is performed by modeling electrons from each two adjacent V ions distributed along V-chains as a two-electron system, which indicates that the spin exchange between electrons could cause a level splitting into a singlet (S = 0) level of lower energy and a triplet (S = 1) level of higher energy. The observed abrupt decrease in susceptibility near T is explained to be due to that the sample enters the singlet state in which two electrons from adjacent V ions are paired into dimers in spin antiparallel. By considering paramagnetic contribution of unpaired electrons created by the thermal activation from singlet to triplet levels, an expression for susceptibility is proposed to quantitatively explain the unusual temperature-dependent susceptibility observed at low temperatures. Based on the approach to magnetic features, the observed metal-insulator transition is explained to be due to a transition from high-temperature Pauli paramagnetic metallic state of Vions to low-temperature dimerized state of strong electronic localization.
实验表明，VO中的金属 - 绝缘体转变温度T与以磁化率突然下降为特征的磁转变温度T几乎相同，这表明这两种转变具有相同的潜在起源。磁化率测量结果表明，在T＞T的温度范围内冷却时，磁化率微弱增加，在T附近急剧下降，然后在进一步冷却时异常增加。通过将沿V链分布的每两个相邻V离子的电子建模为双电子系统，对T附近或低于T时磁化率的这种异常观测进行了理论分析，这表明电子之间的自旋交换会导致能级分裂为能量较低的单重态（S = 0）能级和能量较高的三重态（S = 1）能级。T附近观察到的磁化率突然下降被解释为是由于样品进入单重态，其中相邻V离子的两个电子以自旋反平行的方式配对成二聚体。通过考虑由单重态到三重态能级的热激活产生的未配对电子的顺磁贡献，提出了一个磁化率表达式，以定量解释在低温下观察到的异常温度依赖性磁化率。基于对磁性特征的分析方法，观察到的金属 - 绝缘体转变被解释为是由于从V离子的高温泡利顺磁金属态到强电子局域化的低温二聚态的转变。
