Distinct variation of electronic states due to annealing in La$_{1.8}$Eu$_{0.2}$CuO$_4$ and Nd$_{2}$CuO$_4$

Shun Asano; Kenji Ishii; Daiju Matsumura; Takuya Tsuji; Kota Kudo; Takanori Taniguchi; Shin Saito; Toshiki Sunohara; Takayuki Kawamata; Yoji Koike; Masaki Fujita

doi:10.1103/PhysRevB.104.214504

paper

Distinct variation of electronic states due to annealing in La $_{1.8}$ Eu $_{0.2}$ CuO $_{4}$ and Nd $_{2}$ CuO $_{4}$

arXiv:2005.10681 · doi:10.1103/PhysRevB.104.214504

Abstract

We performed Cu {\it K}-edge X-ray absorption fine structure measurements on T'-type La $_{1.8}$ Eu $_{0.2}$ CuO $_{4}$ (LECO) and Nd $_{2}$ CuO $_{4}$ (NCO) to investigate the variation in the electronic state associated with the emergence of superconductivity due to annealing. The X-ray absorption near-edge structure spectra of as-sintered (AS) LECO are quite similar to those of AS NCO, indicating that the ground state of AS T'-type LECO is a Mott insulator. We found a significant variation of the electronic state at the Cu sites in LECO due to annealing. The electron density after annealing ( $n_{AN}$ ) was evaluated for both superconducting LECO and non-superconducting NCO and found to be 0.40 and 0.05 electrons per Cu, respectively. In LECO but not in NCO, extended X-ray absorption fine structure analysis revealed a softening in the strength of the Cu-O bond in the CuO $_{2}$ plane due to annealing, which is consistent with the screening effect on phonons in the metallic state. Since the amounts of oxygen loss due to annealing ( $δ$ ) for LECO and NCO are comparable with each other, these results suggest distinct electron-doping processes in the two compounds. That electron-doping in NCO approximately follows the relation $n_{AN} = 2 δ$ can be understood if electrons are doped through oxygen deficiency, but the anneal-induced metallic nature and large $n_{AN}$ of LECO suggest a variation of the electronic band structure causes self-doping of carriers. The origin of the difference in doping processes due to annealing is discussed in connection with the size of the charge transfer gap.

6 pages, 4 figures, 1 table

Distinct variation of electronic states due to annealing in La1.8​Eu0.2​CuO4​ and Nd2​CuO4​

Abstract

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Distinct variation of electronic states due to annealing in La $_{1.8}$ Eu $_{0.2}$ CuO $_{4}$ and Nd $_{2}$ CuO $_{4}$