A. Krapf

Experimental electronic structure of In2O3 and Ga2O3

Transparent conducting oxides (TCOs) pose a number of serious challenges. In addition to the pursuit of high-quality single crystals and thin films, their application has to be preceded by a thorough understanding of their peculiar electronic structure. It is of fundamental interest to understand why these materials, transparent up to the UV spectral regime, behave also as conductors. Here we investigate In2O3 and Ga2O3, two binary oxides, which show the smallest and largest optical gaps among conventional n-type TCOs. The investigations on the electronic structure were performed on high-quality n-type single crystals showing carrier densities of ~1019?cm?3 (In2O3) and ~1017?cm?3 (Ga2O3). The subjects addressed for both materials are: the determination of the band structure along high-symmetry directions and fundamental gaps by angular resolved photoemission (ARPES). We also address the orbital character of the valence- and conduction-band regions by exploiting photoemission cross sections in x-ray photoemission (XPS) and by x-ray absorption (XAS). The observations are discussed with reference to calculations of the electronic structure and the experimental results on thin films.

Evolution of the density of states at the Fermi level ofBi2−yPbySr2−xLaxCuO6+δandBi2Sr2−xLaxCuO6+δcuprates with hole doping

Bi2Sr2-xLaxCuO6+d and Bi2-yPbySr2-xLaxCuO6+d high-Tc superconductors in a wide doping range from overdoped to heavily underdoped were studied by X-ray absorption and photo-emission spectroscopy. The hole concentration p was determined by an analysis of the Cu L3-absorption edge. Besides the occupied density of states derived from photoemission, the un-occupied density of states was determined from the prepeak of the O K-absorption edge. Both, the occupied as well as the unoccupied density of states reveal the same dependence on hole doping, i.e. a continuous increase with increasing doping in the hole underdoped region and a constant density in the hole overdoped region. By comparing these results of single-layer BSLCO with previous results on single-layer LSCO it could be argued that besides the localized holes on Cu sites the CuO2-planes consist of two types of doped holes, from which the so-called mobile holes determine the intensity of the prepeak of the O 1s absorption edge.

Fine-structure in the low-energy excitation spectrum of a high-Tcsuperconductor by polarization-dependent photoemission

Angle-resolved photoemission spectroscopy is performed on single crystals of the single-layer high-Tc superconductor Bi(2)Sr(2-x)La(x)CuO(6+d) at optimal doping (x=0.4) in order to study in great detail the Zhang-Rice (ZR) singlet band at the Fermi level. Besides the high crystal quality the advantages of a single-layer material are the absence of bilayer effects and the distinct reduction of thermal broadening. Due to the high energy and angle resolution and, most important, due to the controlled variation of the polarization vector of the synchrotron radiation the emission from the ZR singlet band reveals a distinct fine-structure. It consists of two maxima, the first showing only weak and the second at EF extremely strong polarization dependence. However, our observation has enormous consequences for line shape analyses and the determination of pseudo gaps by photoemission.

Hole doping in the CuO2-plane of Bi-cuprates studied by XAS: polycrystals and single crystals

The exact shape of the superconducting dome in the Bi-cuprates depends on the proper determination of the hole content in the CuO2 layers. Single crystals of Bi(Pb)-2201 were studied over a wide doping range from the overdoped to the heavily underdoped regime by x-ray absorption spectroscopy (XAS) in order to determine the hole content of the compounds. By analysing the Cu-LIII edge this technique proved to be successful for polycrystalline oxide-based high-Tc superconductors, while for single crystals an unexpected variation of the absorption intensity within the ab-plane on a scale of 10–15% with respect to the angle of the incoming linearly polarized light was observed. Due to it, the procedure of hole content determination on polycrystals cannot be adopted straight forward to single crystals. We will show that this technique also works for single crystals, but is in need of higher experimental effort due to the polarization effects.

Systematic X-ray absorption study of hole doping in BSCCO-phases

Abstract X-ray absorption spectroscopy (XAS) on the O 1s threshold was applied to Bi-based, single crystalline high temperature superconductors (HT c s), whose hole densities in the CuO 2 planes were varied by different methods. XAS gives the intensity of the so-called pre-peak of the O 1s line due to the unoccupied part of the Zhang–Rice (ZR) singlet state. The effects of variation of the number n of CuO 2 -planes per unit cell ( n =1,2,3) and the effect of La-substitution for Sr for the n =1 and 2 phases were studied systematically. Furthermore, the symmetry of the states could be probed by the polarization of the impinging radiation.

Fermi Surface and Superconducting Gap of Triple-Layered Bi2Sr2Ca2Cu3O10 + δ

We present a comprehensive study performed with high-resolution angle-resolved photoemission spectroscopy on triple-layered Bi2Sr2Ca2Cu3O10 + δ single crystals. By measurements above TC the Fermi surface topology defined by the Fermi level crossings of the CuO2-derived band was determined. A hole-like Fermi surface as for single and double-CuO2 layered Bi-based cuprates is found, giving new input to the current debate of the general Fermi surface topology of the high-TC superconductors. Furthermore, we present measurements of the superconducting gap of Bi-2223 and show that there are clear indications for a strong anisotropy of the superconducting gap. The universal properties of this phase in comparison to the other Bi-based cuprates will be discussed.

Unusual electronic ground state of a prototype cuprate: Band splitting of single CuO2-plane Bi2Sr2-XLaxCuO6+δ

The momentum dependence of the split band of Bi2Sr1.6La0.4CuO6 + δ (Manzke R. et al., Phys. Rev. B, 63 (2001) R100504) is investigated by angle-resolved photoemission spectroscopy with very high resolution along the ΓM-line. The splitting is only observable in this direction and at certain experimental conditions with respect to temperature and doping of the sample and polarization of the impinging radiation. Since bilayer effects cannot be present in this single-layer material and other sources producing a splitting are also less plausible, the results point to an unusual ground state of this system. It is discussed whether microscopic phase separation including striped phase formation, critical fluctuations coupled to electrons (hot spots) or spin charge separation within the Luttinger-liquid picture, leading to the appearance of spinon and holon excitations, is capable to describe the observations qualitatively.

Electron Spectroscopy of Single-Layer (n = 1) Bi2Sr2−xLaxCuO6+δ Crystals at Optimal Doping

On single crystals of the single-layer (n = 1) high-Tc superconductor Bi2Sr2−xLaxCuO6+δ at optimal doping (x = 0.4), the electron spectroscopies x-ray absorption (XAS) and high-resolution angle-resolved photoemission (ARPES) were performed. The XAS gives the intensity of the so-called prepeak of the O 1s line what is due to the unoccupied part of the Zhang–Rice (ZR) singlet band. For ARPES, the advantages of single-layer material are the absence of bilayer effects and the possibility to study the electronic properties of the normal state at a sample temperature where the thermal broadening is extremely small (<10 meV). The controlled variation of the polarization vector of the synchrotron radiation made it possible to resolve a distinct fine-structure of the occupied part of the ZR singlet band at the Fermi level. These observations have enormous consequences for line shape analyses and the determination of pseudogaps, and thus the mechanism of high-Tc superconductivity.

Fermi surface of Bi-cuprates with variable number of CuO2 planes

Abstract Since our photoemission knowledge on high- T c compounds especially below T c is established to a great extent on the two-layer material Bi 2 Sr 2 CaCu 2 O 8 this investigation deals with Bi-based high- T c s with emphasis on the one-CuO 2 -layer material Bi 2 Sr 2− x La x CuO 6+ δ and Bi 2 Sr 2 Ca 2 Cu 3 O 10 with three CuO 2 layers. To access the universal properties of this class of materials a number of investigations such as on the Fermi surface (FS) topology defined by the CuO 2 -derived band crossing E F were performed. In the context of the current debate on the overall FS topology our results confirm the established view of a hole-like FS also for n =1 and n =3 material.

Fermi Surface Map of the Single-Layer Bi-Cuprate Bi2Sr2 − xLaxCuO6 + δ at Optimal Doping

A first Fermi surface map of a single-layer high-Tc superconductor is presented. The experiments were carried out on optimally doped Bi2Sr2 − xLaxCuO6 + δ (x = 0.40) with synchrotron radiation which allow to discuss in detail the strong polarization dependence of the emissions near the Fermi edge. For the cuprates, only little is known about the impact of the electron–photon matrix element determining the photoelectron intensity. For the example of the model layered superconductor Bi2Sr2 − xLaxCuO6 + δ, it will be demonstrated that the polarization geometry has significant influence on the energy distribution curves at EF, and consequently also for the determination of the topology and character of the Fermi surface (FS) by angle-resolved photoemission. For further clarification also, a FS map of the n = 2 material Bi-2212 has been measured applying a different polarization geometry as previously used by Saini et al. In the context of the current debate on the character of the FS of Bi-cuprates, our results confirm a hole-like FS for n = 1 as well as for n = 2 material, what might be the universal FS for high-Tc superconductors.