Pseudogaps due to electron fractionalization and electron pairing in cuprates studied by ARPES and RIXS

Atsushi Fujimori

The origin of the pseudogap in cuprate superconductors has been debated for many decades, but no consensus has been reached till now. In this talk, this problem is approached from two points of view, namely, electron fractionalization and electron pairing above Tc. 

First, we demonstrate that the strong electron correlations in doped Mott insulators naturally lead to electron fractionalization and explain the antiferromagnetic pseudogap in the ARPES spectra of electron-dopd cuprates [1]. Electron-hole excitation spectra in hole-doped cuprates observed by RIXS are also interpreted along this line [2]. Here, carrier doping creates an in-gap band which is separated from one of the Hubbard bands by an s-symmetry electron-hole -asymmetric pseudogap of order 100 meV, giving rise to the T* of 100-400 K. 

Next, a d-symmetry pseudogap due to Cooper pairing above Tc, recently identified in double-layer cuprates aobve Tc [3], is studied for triple-layer cuprates. We observed a large d-symmetry electron-hole symmetric pseudogap up to ~2Tc in the inner CuO₂ plane of underdoped samples, demonstrating a robust nodal metallic behavior [4]. 

On the basis of the s-symmetry pseudogap and the d-symmetry pairing gap, which take place in different doping and temperature ranges, the phase diagram of the cuprates is discussed. 

Collaboration with M. Horio, M. Imada, S. Sakai, Y. Koike, T. Adachi, A. Singh, H. Y. Huang, T. Watanabe, D. J. Huang, S. Ideta, T. Yoshida, S. Uchida, W. O. Wang, T. P. Devereaux, and T. K. Lee is gratefully acknowledged.

[1] M. Horio et al., arXiv:1801.04247.

[2] A. Singh et al., Nat. Commun., 13, 7906 (2022).

[3] T. Kondo et al., Nat. Commun., 6, 7699 (2015).

[4] S. Ideta et al., in preparation.