Anomalous transfer of spectral weight in doped strongly correlated systems

Abstract

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 behav-PACS numbers: 74.70.Vy A surprising feature of the high-r^: materials is a strong doping dependence of high-energy spectral distributions, and a shift of spectral intensity from high to low energies. Two of the nicest examples are the electronenergy-loss study [1] and O \s x-ray-absorption study [2] of the La2-xSrxCu04 system. These spectra show a strong decrease with x in the intensity of the upper Hubbard band as the lower-energy structure develops due to holes in the O 2p band. Such behavior has previously been found in Li-doped NiO as well [3].Also, in optical-absorption experiments [4], a transfer of spectral weight from the band-gap transition at about 2 eV in insulating La2Cu04 to the low-energy scale (< 1.0 eV) in the superconductor Lai.8Sro.2Cu04 is observed. The integral of the optical conductivity up to 1 eV increases much more rapidly than the doping concentration would suggest. At the same time the high-energy-scale structure around 2 eV decreases rapidly, whereas the total integrated spectral weight up to 4 eV remains nearly constant. Similar behavior has also been found recently for the electron-doped system Pr2-xCejc04-5 by Cooper et at. [5].That these kinds of effects can be related to strong correlations is evidenced by exact diagonalization studies. A convincing description of the O 1^ spectra is given using the Mott-Hubbard (MH) model [2]. Also, the calculated optical spectra for the MH and t-J models show features quantitatively similar to what is seen experimentally [6][7][8][9].At first glance one would not expect to see such spectral-weight transfers in the hole-doped high-r^ materials, which are generally accepted to be charge-transfer (CT) systems, because the additional holes mainly occupy oxygen 2p states. In a previous paper [10] we found, however, using a multiband cluster (CU2O7) calculation and parameters suitable for the high-r^ compounds and for 50% doping, a substantial transfer of spectral weight. In this paper we describe the physical origin of the observed spectral-weight transfer and its doping dependence. In addition we study both the MH and CT systems and describe the effects of hybridization. Special attention is paid to the asymmetry in electron-and hole-doped systems expected in a CT model. We show that if the hybridization in a CT system is large enough, the spectralweight transfer becomes similar to that of a MH model, but the physical origin is really quite diffe...

X-ray absorption spectroscopy of detwinnedPrxY1−

“…11, we emphasize that if the holes were residing in the ZR state, transitions from the O 1s core level into the UHB would exhibit little spectral weight due to the transfer of spectral weight from the UHB to the ZR state. 21,58,60,61 In contrast to this, the spectral weight of the UHB should be observed with the same strength as for oxygen-depleted 6.91 compared to their oxygen-depleted counterparts ͑Figs. 9 and 10͒ and is, therefore, attributed to a generally higher degree of hybridization in this energy range, independent of the Pr concentration.…”

“…We will also examine to what degree the various existing models, mentioned in the first section, agree with our data. 60,61 This is demonstrated in Fig. 9 where the Eʈa spectra of oxygen-depleted (yϷ0.9) and almost fully oxygenated (yϷ0.1) YBa 2 Cu 3 O 7Ϫy samples are depicted: At higher O content the UHB is reduced while the ZR band is strongly increased.…”

NiO: Correlated Band Structure of a Charge-Transfer Insulator

“…In the present work the O p-orbitals and their hybridization with Ni d-orbitals are explicitly included, thus allowing for a unified description of the full spectrum. Our results reveal a non-trivial effect of the p − d hybridization in strongly correlated system studied so far only in terms of simple models [11,12,13].The application of the standard bandstructure theory to NiO is marked by a failure of LDA to produce an insulating groundstate [14]. The antiferromagnetic order within LDA [15], despite rendering NiO an insulator, does not present much of an improvement since (i) the band gap is severely underestimated, (ii) the experimentally observed lower Hubbard band is completely missing, and (iii) static quantities such as the local magnetic moment do not agree well with experiment.…”

Failure oft−Jmodels in describing doping evolution of spectral weight in x-ray scattering, optical, and photoemission spectra of cuprates

“…In the latter case, for U → ∞, removing one electron creates two low energy holes -one from the lower Hubbard band (LHB), but a second one from the upper Hubbard band (UHB), since without an electron on the atom there is no U -penalty in adding an electron. Paradoxically, as U decreases, the rate of this anomalous spectral weight transfer (ASWT) actually increases 4 . For infinite U double occupancy (DO) is always forbidden, so no matter how few electrons are in the LHB, there will be an equivalent number of holes in the UHB.…”