Electronic structure of the systemLa2−xSr

Abstract

The electronic structure of La2-Sr Cu04+~has been studied by measuring 0 1s absorption edges using high-energy electron-energy-loss spectroscopy in transmission.Upon doping the insulating compound the conduction-band states are reduced and states in the gap are formed. When going from insulating to conducting compounds there is a continuous increase of states at the Fermi level, which is at the bottom of the gap. The insulator-metal transition is probably driven by delocalization of these states.Among the cuprates showing high-T, superconductivity, the system La2 "M"Cu04 (M Ba,Sr), first discovered by Bednorz and Miiller, ' is one of the most suitable to study the electronic structure of high-T, superconductors. The undoped parent compound La2Cu04 is an antiferromagnetic insulator. This is due to strong electronic correlation efl'ects on the Cu sites which force the otherwise metallic compound with a half-filled band to become a charge-transfer insulator.A charge-transfer gap appears between the valence band (ligand band) with mainly 0 2p character and the conduction band (upper Hubbard band) with mainly Cu 3d character. Upon p-type doping with Sr, antiferromagnetism disappears and an insulatormetal transition occurs near x=0.06. For 0.06~x (0.30 superconductivity is observed with a maximum T, near x =0.15. Though numerous experimental and theoretical studies have been carried out on this system, the electronic structure as a function of dopant concentration, the mechanism of high-T, superconductivity, and even the nature of the normal state for x-0.15 still remain unclear.In this Rapid Communication we report on changes of the electronic structure of La2Cu04 upon doping with Sr, which are probably essential to understand the normalstate properties of high-T, superconductors.With increasing dopant concentration there is a continuous increase of the density of states at the Fermi level, which is located at the bottom of the gap between the ligand band and the upper Hubbard band. No discontinuity is observed close to the insulator-metal transition. This points out that this transition is driven by delocalization. The present results probably can be generalized to other cuprate systems showing high-T, superconductivity. 0 ls absorption edges of La2 "Sr"Cu04+s have been measured in the concentration range 0(x (0.30 by electron-energy-loss spectroscopy (EELS) in transmission. These edges probe the local unoccupied density of states with p symmetry at the 0 sites provided the interaction of the excited electron with the core hole is small. The latter assumption is supported, at least for metallic cuprates, by the strong similarity of 0 ls absorption edges and resonant bremsstrahlung isochromat spectroscopy (BIS) spectra on Bi2SrqCaCuqQs, and by the rather good agreement between experimental C and N ls core-level absorption spectra with the calculated total density of states in transition-metal carbides and nitrides. Ceramic samples were prepared as described previously.From these samples about 1000-A-thick films were c...

Evolution of the density of states at the Fermi level ofBi2−yPbySr2−xSchneider1, Unger2, Mitdank3 et al. 2005Phys. Rev. B308311View full textAdd to dashboardCite

“…The focus of these studies on the La 2-x Sr x CuO 4 (LSCO)-system was the underdoped range, where effects due to the pseudogap and the spectral weight renormalization were discussed for the occupied DOS. Interestingly in the overdoped range the DOS at E F was found to converge to a constant value and a similar trend was observed by Romberg et al [4] for the unfilled DOS by EELS. Although the authors did not explicitly comment on this, in a naive view one would expect an increasing DOS with increasing hole concentration.…”

“…It is established that the states at the Fermi energy have primarily O 2p-character [17] and are connected with the prepeak feature of the O 1s → 2p absorption edge. The intensity of the prepeak is mainly interpreted as a direct measure of the hole concentration [4]. In contradiction to this assumption there are hints already in this previous experiment [4] which will be confirmed here by a large number of measured samples that for hole overdoping the intensity of the prepeak is not further increasing but saturating.…”

“…The intensity of the prepeak is mainly interpreted as a direct measure of the hole concentration [4]. In contradiction to this assumption there are hints already in this previous experiment [4] which will be confirmed here by a large number of measured samples that for hole overdoping the intensity of the prepeak is not further increasing but saturating. Therefore an alternative explanation of the origin of the O 1s prepeak consistent with all observations has to be found.…”

Polarized x-ray absorption spectroscopy study of the symmetry of unoccupied electronic states near the Fermi level in the system

“…Peak A is due to the transition from the O 1s levels into the unoccupied O 2p states appearing in the correlation gap with doping holes and has been observed for a number of correlated systems [29]. It has been found that the intensity of this peak is not explicitly dependent on the T c of the material but is dependent on the density of the doping holes [30,31]. The intensity of peak A decreases in going towards the E c polarization.…”

Modelling Cuprate Gaps in a Composite Two-Band Model

“…First, the structure of doped cuprates has been found to be essentially inhomogeneous on the nanoscale (stripes, tweed patterns, granularity) with the associated electronic phase separation [6][7][8][9][10][11][12] in the CuO 2 planes. At the same time it has been found that the hole doping creates a new band of defect-states in the cuprate charge-transfer gap [13][14][15][16][17][18][19][20][21][22][23][24]. Various data have indicated the functioning of itinerant and "defect"-type carriers in the basic physics of cuprate superconductivity.…”