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Physical review. B, Condensed matter

Spectral weight transfer in a doped strongly correlated insulator

Tetsuya Mutou, Dai S. Hirashima

Published Oct 1, 1996• 5 citations
Publisher

Abstract

We study the spectral weight transfer and the chemical potential shift in strongly correlated electron systems caused by carrier doping by applying the dynamical mean-field theory to a two-band Hubbard model. The parameters of the model are chosen so that the undoped system is a charge-transfer-type insulator. We find that in the insulating phase there are finite intensities corresponding to local singlet states formed by hybridized d and p electrons, and that, on doping a part of them, turn into itinerant states. The results are discussed in the light of experimental and theoretical results for high-${\mathit{T}}_{\mathit{c}}$ cuprates. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physrevb.54.9058

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We study the spectral weight transfer and the chemical potential shift in strongly correlated electron systems caused by carrier doping by applying the dynamical mean-field theory to a two-band Hubbard model. The parameters of the model are chosen so that the undoped system is a charge-transfer-type insulator. We find that in the insulating phase there are finite intensities corresponding to local singlet states formed by hybridized d and p electrons, and that, on doping a part of them, turn into itinerant states. The results are discussed in the light of experimental and theoretical results for high-Tc{\mathit{T}}_{\mathit{c}}Tc​ cuprates. \textcopyright{} 1996 The American Physical Society.
We study the formation of itinerant electronic states on carrier doping in a strongly correlated insulator by calculating one-particle-excitation spectral functions. We apply (1) the quantum Monte Carlo method and (2) the exact diagonalization method in the framework of the dynamical mean field theory to a two-band Hubbard model. The parameters of the model are chosen so that the undoped system is a charge-transfer-type insulator. We find that the itinerant states originate from local singlet states formed by hybridized d - and p -holes. We also find asymmetry between hole-doped and electron-doped cases: (1) in the electron-doped case a peak is not observed at the chemical potential in contrast to the hole-doped case; and (2) the mass-enhancement factor in the electron-doped case is smaller than that in the hole-doped case. The results of the one-particle-excitation spectrum and the mass-enhancement factor are discussed in the light of experimental and theoretical results for high- T c cuprates.
Since the discovery of half-filled cuprate to be a Mott insulator, the excitation spectra above the chemical potential for the unoccupied states has attracted much research attention. There were many theoretical works using different numerical techniques to study this problem, but many have reached different conclusions. One of the reasons is the lack of very detailed high-resolution experimental results for the theories to be compared with. Recently, the scanning tunneling spectroscopy [P. Cai et al., Nat. Phys. 12, 1047 (2016); C. Ye et al., Nat. Commun. 4, 1365 (2013)] on lightly doped Mott insulator with an antiferromagnetic order found the presence of in-gap states with energy of order half an eV above the chemical potential. The measured spectral properties with doping are not quite consistent with earlier theoretical works. Although the experiment has disorder and localization effect, but for the energy scale we will study here, a model without disorder is sufficed to illustrate the underlying physics. We perform a diagonalization method on top of the variational Monte Carlo calculation to study the evolution of antiferromagnetic Mott state with doped hole concentration in the Hubbard model. Our results found in-gap states that behave similarly with ones reported by STS. These in-gap states acquire a substantial amount of dynamical spectral weight transferred from the upper Hubbard band. The in-gap states move toward chemical potential with increasing spectral weight as doping increases. Our result also provides information about the energy scale of these in-gap states in relation with the Coulomb coupling strength UUU.
We investigate the metal-insulator transitions at finite temperature for the Hubbard model with diagonal alloy disorder. We solve the dynamical mean-field-theory equations with the noncrossing approximation and we use the coherent potential approximation to handle disorder. The excitation spectrum is given for various correlation strength UUU and disorder. Two successive metal-insulator transitions are observed at integer-filling values as UUU is increased. An important selective transfer of spectral weight arises upon doping. The strong influence of the temperature on the low-energy dynamics is studied in detail.
A qualitative and quantitative description (exact diagonalization of finite clusters) of the doping dependence of one-electron-removal and -addition spectral weights is given. A half-filled Hubbard system in the localized limit has two low-energy electron-addition (removal) states for each hole (electron) created by doping. In contrast, a charge-transfer system in the localized limit is fundamentally asymmetric between hole and electron doping. However, when the hybridization is increased this asymmetry disappears quickly and both holes and electrons introduced by doping show a strongly correlated behavior.
We investigate the strong correlation effect in the spinless electron-hole two-band Hubbard model using the dynamical mean-field theory. At half filling, both the renormalization factor ZZZ and the number of conduction electrons (valence holes) ncn_cnc​ decrease with increasing the interband Coulomb interaction UUU down to Z∼0.15Z\sim 0.15Z∼0.15 and nc∼0.1n_c\sim 0.1nc​∼0.1 for $U_c \sim \mbox{bandwidth}$ at which the first-order Lifshitz transition occurs from a correlated semimetal with a large effective mass m∗/m=Z−1m^*/m=Z^{-1}m∗/m=Z−1 to a band insulator with a finite gap due to the Hartree shift. A slight hole doping xxx in the band insulator with a large U>UcU>U_cU>Uc​ yields a remarkable correlated semimetal with Z∼0.1Z\sim 0.1Z∼0.1 at x∼0.01x\sim 0.01x∼0.01, where in-gap states emerge above the valence band top and those weights increase with increasing xxx together with the increase in ZZZ similar to the in-gap states observed in doped Mott insulators.
We study the evolution of a Mott-Hubbard insulator into a correlated metal upon doping in the two-dimensional Hubbard model using the cellular dynamical mean-field theory. Short-range spin correlations create two additional bands apart from the familiar Hubbard bands in the spectral function. Even a tiny doping into this insulator causes a jump of the Fermi energy to one of these additional bands and an immediate momentum-dependent suppression of the spectral weight at this Fermi energy. The pseudogap is closely tied to the existence of these bands. This suggests a strong-coupling mechanism that arises from short-range spin correlations and large scattering rates for the pseudogap phenomenon seen in several cuprates.