Direct determination of the pseudogap temperature T* of YBCO

The d-wave symmetry of the superconducting gap in high Tc cuprates was visualized and measured by ARPES only in “photoemission friendly” materials, mostly BSCCO. The same technique has uncovered the existence of the pseudogap, i.e., a larger energy gap affecting only a fraction of the Fermi surface up to the temperature T*>>Tc. Less direct signatures of the pseudogap have been found in measurements other than ARPES, but the determination of T* is there not univocal. This is a problem in materials where ARPES cannot be performed, such as YBCO. We propose to use very high resolution RIXS to study the opening of the superconducting gap and of the pseudogap in YBCO. The sensitivity of RIXS to the gap opening has been recently demonstrated for BSCCO, for which ARPES and Raman spectroscopy were used as reference. We propose to use RIXS for the important case of the gaps of YBCO, with the goal of determining T*.