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Register a new userNMR study on the local structure in La-based high-Tc cuprates
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To date, there has been no evidence for the macroscopic structural phase transition to the low temperature tetragonal structure (LTT) with a space group P42/ncm in high-TC cuprate of rare earth-free La2-xSrxCuO4 (LSCO). By investigating Cu-NMR on single crystals, we have found that spatially incoherent LTT structure emerges below 50 K in the sample with x=0.12. This incoherent structure is considered to play a key role for the slight depression of the superconductivity around x=1/8.
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We present a self-consistent RVB theory which unifies the metallic (superconducting) phase with the half-filling antiferromagnetic (AF) phase. Two crucial factors in this theory include the RVB condensation which controls short-range AF spin correlations and the phase string effect introduced by hole hopping as a key doping effect. We discuss both the uniform and non-uniform mean-field solutions and show the unique features of the characteristic spin energy scale, superconducting transition temperature, and the phase diagram, which are all consistent with the experimental measurements of high-$T_c$ cuprates.
The charge dynamics in weakly hole doped high temperature superconductors is studied in terms of the accurate numerical solution to a model of a single hole interacting with a quantum lattice in an antiferromagnetic background, and accurate far-infrared ellipsometry measurements. The experimentally observed two electronic bands in the infrared spectrum can be identified in terms of the interplay between the electron correlation and electron-phonon interaction resolving the long standing mystery of the mid-infrared band.
We revisit the problem of the spectra of two holes in a CuO$_{2}$ layer, modeled as a Cu-d$^{8}$ impurity with full multiplet structure coupled to a full O-2p band as an approximation to the local electronic structure of a hole doped cuprate. Unlike previous studies that treated the O band as a featureless bath, we describe it with a realistic tight binding model. While our results are in qualitative agreement with previous work, we find considerable quantitative changes when using the proper O-2p band structure. We also find (i) that only the ligand O-2p orbitals play an essential role, within this impurity model; (ii) that the three-orbital Emery model provides an accurate description for the subspace with $^{1}!A_1$ symmetry, which includes the ground-state in the relevant region of the phase diagram; (iii) that this ground-state has only $sim 50%$ overlap with a Zhang-Rice singlet; (iv) that there are other low-energy states, in subspaces with different symmetries, that are absent from the three-orbital Emery model and its one-band descendants. These states play an important role in describing the elementary excitations of doped cuprates.
In a recent work [M. Jiang, M. Moeller, M. Berciu, and G. A. Sawatzky, Phys. Rev. B textbf{101}, 035151 (2020)], the authors solved a model with a Cu impurity in an O-2p band as an approximation to the local electronic structure of a hole doped cuprate. One of their conclusions is that the ground-state has only $sim 50$ % overlap with a Zhang-Rice singlet (ZRS). This claim is based on the definition of the ZRS in a different representation, in which the charge fluctuations at the Cu site have been eliminated by a canonical transformation. The correct interpretation of the results, based on known low-energy reduction procedures for a multiband model including 3d$^8$ and 3d$^{10}$ configurations of Cu, indicates that this overlap is near 94 %.
Our recent study has revealed that the mixture of the dz2 orbital component into the Fermi surface suppresses Tc in the cuprates such as La2CuO4. We have also shown that applying hydrostatic pressure enhances Tc due to smaller mixing of the Cu4s component. We call these the orbital distillation effect. In our previous study, the 4s orbital was taken into account through the hoppings in the dx2-y2 sector, but here we consider a model in which of the dx2-y2, dz2 and 4s orbitals are all considered explicitly. The present study reinforces our conclusion that smaller 4s hybridization further enhances Tc.
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