Mechanism of Pseudogap Detected by Electronic Raman Scattering： Phase Fluctuation or Hidden Order？

We study the electronic Raman scattering in the cuprates to distinguish the two possible scenarios of the pseudogap normal state. In one scenario, the pseudogap is assumed to be caused by phase fluctuations of the preformed Cooper pairs. We find that pair-breaking peaks appear in both the B1g and B2g Raman channels, and they axe smeared and tend to shift to the same energy with the increasing strength of phase fluctuations. Thus both channels reflect the same pairing energy scale, irrespectively of the doping level. In another scenario, the pseudogap is assumed to be caused by a hidden order that competes with the superconducting order. As an example, we assume that the hidden order is the d-density-wave （DDW） order. We find analytically and numerically that in the DDW normal state there is no Raman peak in the B2g channel in a tight-binding model up to the second nearest-neighbor hopping, while the Raman peak in the Big channel reflects the energy gap caused by the DDW order. This behavior is in agreement with experiments in the pseudogap normal state. To gain further insights, we also calculate the Raman spectra in the DDW＋SC state. We study the doping and temperature dependence of the peak energy in both channels and find a two-gap behavior, which is in agreement with recent Raman experiments. Therefore, our results shed light on the hidden order scenario for the pseudogap.     

他山之石，可以攻玉——太赫兹二维相干谱学打开探索分数化现象的新窗口

分数化(fractionalization)是一种出现在多体系统中的新奇物理现象^[1]。在出现分数化现象的多体系统中,构成体系的基本物理单元(如电子、自旋等)被相互作用劈裂。这些被劈裂的部分成为体系的元激发。这里以铁磁伊辛自旋链为例说明(图1(a)):该体系中自旋的取向高度受限,其只能沿着或者背对着自旋的易轴(这里假设易轴方向与自旋链方向相同)。