Anomalous Metalic State in Strongly Correlated Systems

自从高温超导体的发现以来，低维强关联系统中的电子激发引起了人们的广泛关注。在一维系统中，电子的自旋和电荷自由度遵循不同的动力学规律，自旋-电荷分离现象是已知的。自旋和电荷的元激发分别称为自旋子和空穴子。二维系统中自旋自由度和电荷自由度的性质是高温超导体研究的核心问题之一。角分辨光电子能谱（ARPES）的最新进展为强关联系统理论的检验提供了一个契机，我们用解析和数值方法研究了低维强关联系统中的单粒子、自旋和电荷激发。半填充状态下的单粒子谱与一维铜酸盐SrCuO_2的角分辨光电子能谱雅阁，与二维Sr_2CuO_2Cl_2的角分辨光电子能谱形成强烈的对比。在2D中，这些激发光谱的行为强烈依赖于电子浓度。远离半填充，单粒子谱包括自旋和电荷激发，正如费米液体中所预期的那样。另一方面，在半填充附近，谱峰与一维中的谱峰非常相似，表明电荷自由度和自旋自由度服从不同的动力学，我们的研究为从电子内部自由度出发的新的固体量子理论提供了基础.

Electronic excitations in low-dimensional strongly correlated systems have attracted considerable attention since the discocvery of high temperature superconductors. In the one-dimensional (1D) systems, it is known that the spin-charge separation occurs and the spin and charge degress of freedom of electrons obey the different dynamics. The elementary excitations of spin and charge are called spinons and holons, respectively. The nature of spin and charge degrees of freedom in the two-dimensional (2D) systems is one of the central issues in the study of high temperature superconductors. Recent advances in angle-resolved photoemission spectroscopy (ARPES) offers an opportunity of testing the theory of the strongly correlated systems.We have studied the single-particle, spin and charge excitations in strongly correlated low-dimensional systems by using the analytical as well as numerical methods. The single-particle spectrum at half-filling is in accord with the recent angle-resolved photoemission spectrum in the 1D cuprate SrCuO_2 and is in strong contrast with that in the 2D one Sr_2CuO_2Cl_2. In 2D,the behaviors of these excitation spectra are strongly dependent on electron concentration. Far away from half-filling, the single-particle spectrum includes both spin and charge excitations as expected in a Fermi liquid. On the other hand, near half-filling, the apectra are rather similar to those in 1D,indicating that the charge and spin degrees of freedom obey the different dynamics.Our study offers a basis of the new quantum theory of solids which starts with the internal degrees of freedom of electrons.