Institute for Solid State Physics, University of Tokyo
High-temperature superconductivity is one of the most significant discoveries in 20th-century physics. Immediately after its discovery, attracted attention was given to the anomalous behaviours observed in various physical properties above the superconducting transition temperature Tc. The electronic state relevant to these is a pseudogap, which opens above Tc in the density of states close to the Fermi level. The pseudogap was first discovered in the measurement of nuclear magnetic relaxation (NMR) rate 1/T1. 1/T1T at high temperatures follows the Curie-Weiss law, whereas, on cooling, it becomes peaked at a certain temperature much higher than Tc, and then begins to decrease. This phenomenon was called "spin gap" because a gap seems to appear in the spin excitation spectrum, while an energy gap was later found to be opened at temperatures higher than Tc in the density of states near the Fermi level by photoelectron spectroscopy and tunnelling spectroscopy. Since then, this gap has been called “pseudogap”.
The formation of "small fermi pocket" was expected for the electronic structure of the superconducting phase, which occurs by carrier doping to a Mott insulator. However, an arc-shaped Fermi surface, that cannot be called neither a "large Fermi surface" nor a "small Fermi pocket", has been observed, due to the opening of a pseudogap in the electronic structure near (p, 0). The Fermi-arc is an extremely strange electronic state, thus the origin of the pseudogap and its role in the development of superconductivity is crucial in elucidating the mechanism of high-Tc superconductors. A controversy has been continuing between two theories on its origin. The first is that the pseudogap arises due to the pair formation of electrons as a precursor phenomenon of superconductivity. The second is that the pseudogap is, in contrast, generated by an electronic state different from superconductivities, such as charge order or charge density wave; in this case, the pseudogap state competes with superconductivity.
Based on the above background, this talk will introduce my study of the pseudogap state in cuprates, using high-resolution angle-resolved photoelectron spectroscopy (ARPES). I will clearly show the competing behavior between pseudogap and superconducting states. Not only that, we found that the paired electrons develop in competition with the pseudogap state at temperatures relatively lower than the pseudogap temperature (T*) but sufficiently higher than Tc. So far, it has been argued whether the gap that opens above Tc is due to the precursor state of superconductivity or the competing state against it. This is mainly because it has been believed that there is a single gap that opens above Tc, and that gaps sensitive to different experimental probes have been observed separately. By succeeding in selectively observing both the gaps by one experimental probe, we were able to clarify the relationship between the pseudogap and the pairing gap, that both develop above Tc while competing with each other. Furthermore, we report the first successful observation of a "small fermi pocket" near the Mott insulator phase. The properties in the lightly-doped region could lead to the elucidation of the Mott transition, which has been a theoretical difficulty for many years; however, random substitutions of the element associated with carrier doping cause large inhomogeneity in electronic states of the real materials, making it difficult to understand the essential phenomena. The multi-layered cuprates focused on in this study has a "clean" CuO2 plane protected inside the crystal, which can avoid direct contact with charge reservoir layers; this became the key leading to the first observation of "small Fermi pockets".