We have performed photoemission and inverse photoemission experiments on a series of 3d-transition-metal oxides with formal ionic configuration from to . The photoemission core-level spectra are analysed in terms of a simple cluster model leading to estimates for the charge-transfer energy , the Coulomb correlation energy , and the hybridization strength V. It is found that the ratio of the correlation energy to the hybridization energy significantly decreases from the late to the early transition metal oxides. This trend is attributed mostly to the increasing number of empty d states in the early transition metals which enhances the effective metal-ligand hybridization. We also compare the experimental valence band spectra with densities of states (DOS) from band-structure calculations. The rather good agreement between the theoretical DOS and the measured single-particle excitation spectra of the early 3d-transition-metal oxides as opposed to the failure of the one-electron description for most of the late transition metal oxides supports the results of the cluster model analysis.

We present x-ray photoemission spectra of the vanadium oxides V 2 O 5 , VO 2 , and V 2 O 3 , and their analysis in terms of a simple cluster model based on the Anderson impurity Hamiltonian. The electronic structure of these materials is characterized by a strong V 3d-O 2p hybridization energy which exceeds the energy scales related to on-site Coulomb correlation and metal-ligand charge transfer. This result is at variance with the usual Mott-Hubbard picture, but agrees with recent studies of other early 3d transition metal compounds. The V 3d groundstate occupations obtained by the cluster-model analysis are considerably higher than the values derived from the formal valencies. Covalency also affects the exchange splitting observed in the V 3s core-hole spectra. X-ray absorption measurements and resonant photoemission spectroscopy at the V 2p-3d threshold provide further evidence for a strong V 3d-O 2p coupling.

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The electronic structure of the 4d transition-metal oxide PdO is investigated by photoemission (UPS and XPS), inverse photoemission (BIS; ), and electron energy loss spectroscopy in reflection geometry (REELS; primary energy, ). The valence band spectra are compared to recent theoretical ab initio band-structure calculations. Good agreement between theory and experiment is found in the occupied part of the band structure down to 8 eV below as well as in the unoccupied part up to 6 eV above . This confirms the common view that the electronic structure of the 4d transition-metal oxides, e.g. PdO, can be explained in terms of a single-electron picture. Nevertheless correlation effects among the Pd 4d electrons are clearly visible in the spectra, as e.g. satellites of the Pd core level spectra. In order to explain the origin of these satellites we performed simple cluster model calculations and as a result we can explain one satellite in a screening picture by means of a charge transfer process. In addition radiation damage effects in PdO during the electron bombardment in the BIS experiments are reported. This is explained by the formation of the Pd -like states connected with oxygen loss due to the electron bombardment.

The anisotropic resistivity components of YBa2Cu408 have been measured from 0 to 900 K. YBa2Cu408 is an underdoped, genuinely untwinned, and stoichiometric high temperature superconductor. The Cu02 plane resistivity pa exhibits a strong correlation with the spin dynamics, i.e., with Knight shift and spin-lattice relaxation time T\. The resistivity along the b axis (plane plus chain) is consistent with Bloch-Griineisen theory. The cotangent of the Hall angle is quadratic in temperature above the spin gap, and linear below.PACS numbers: 74.72.Bk, 72.15.Qm, 72.15.Gd Since the revolution of high temperature superconductivity (HTSC), initiated by the report of Bednorz and Miiller [1], the unusual normal state transport properties of the HTSC's have been suspected of giving clues to the basic mechanism responsible for superconductivity.Especially the normal state resistivity has revealed an anomalous response [2]: The optimal doped samples as, e.g., YBaiCusOy (1:2:3) have shown a linear behavior in temperature from Tc up to some hundreds degrees Kelvin. In the overdoped regime, the exponent a of pec T" changes from a = l to 2; i.e., a Fermi-liquid behavior becomes dominant. The underdoped samples exhibit a nearly T linear dependence with deviations at low temperatures.In the overdoped region, the basic mechanism responsible for superconductivity is hidden due to the excess carrier density. Hence, as concerns the basic mechanism of HTSC, the underdoped region, which is mainly covered by nonstoichiometric samples, should be explored. Unfortunately, nonstoichiometry is accompanied by distortions, nonhomogeneity, oxygen defects, etc., which, in addition, are modestly screened, due to the low carrier density. As a result, the intrinsic conductivity of the low doped Cu02 plane is not easy to uncover (see also Fig. 1).A fingerprint of underdoped samples is an opening of a spin gap above Tc and a magnetic susceptibility increasing with temperature. Both signatures have been found in YBa2Cu408 (1:2:4) by means of the spin Knight shift [3] and the spin-lattice relaxation time T\ [4]. 1:2:4 has a structure similar to 1:2:3 except the single chains along the b direction are replaced by double chains. YBa2-CU4O8 is a bonus from nature since it is simultaneously stoichiometric, underdoped and, in addition, genuinely untwinned. Because of that, the unvarnished conductivity component of the low doped Cu02 plane can be determined.In this Letter, we present the conductivity components for the a axis representing the CUO2 plane as well as for the b axis containing a plane and a chain contribution. Furthermore, we have measured the Hall effect for the Cu02 plane. The results prove the intimate relationship between dynamic spin susceptibility and normal state transport which seems to be the clue to understanding HTSCs [5].1:2:4 single crystals were grown with a high-pressure flux method using a BaO-CuO-rich flux. The growing parameters were 640 bars O2 at a temperature of 1040°C. For transport measurements, gold pads were sputter deposi...