X-ray optical activity in underdoped bi-based high-Tc superconductor

We performed X-ray optical activity (XOA) measurements at Cu K absorption edge for the underdoped Bi2212 (T-c = 80 K). X-ray natural circular dichroism (XNCD) appears below T* (similar to 200 K) and becomes larger with decreasing temperature, whereas nonreciprocal X-ray linear dichroism (NRXLD) is not seen. It is revealed that time reversal symmetry is preserved in the pseudogap phase of the underdoped Bi2212. The results demonstrate that XOA is a powerful tool to clarify electronic structure in terms of time reversal and/or chiral symmetry.