K.A. Müller

Evidence for electron-electron correlations in La2CuO4 and La2−xSrxCuO4 superconductors

Abstract We report a study of the magnetic susceptibility of La2−xSrxCuO4for x=0, 0.10, 0.15, 0.20 from 4–350K. Our data suggest that La2CuO4 has a spin-density wave or antiferromagnetic transition near 250K. The Sr doped superconductors have a Pauli susceptibility above Tc=35−40K that is enhanced by electron-electron correlations. The variation in Tc with Sr doping is not directly correlated with the change in electron density of states.

Electron paramagnetic resonance of Mn4+ in BaTiO3

Abstract The Electron Paramagnetic Resonance (EPR) of Mn 4+ in the rhombohedral low- temperature phase of BaTiO 3 has been observed at 19.3 and 13.0 GHz. The spectra have axial symmetry along 〈111〉 directions. Intensity measurements prove that the Mn 4+ EPR reflects {111} domain orientation. The spin-Hamiltonian constants are g ‖ =1.9968(19), g ‖ =2.0015(5), A ‖ =67.48(5)×10 −4 cm −1 , A ‖ =73.04(15)×10 −4 cm −1 , and D=0.65(1)cm −1 at 4.2 K. The very large D value is more than 20 times that found for the isoelectronic Cr 3+ . The latter ion remains centered in its surrounding octahedral oxygen cage, whereas the Mn 4+ is off-center with similar coordinates as Ti 4+ . This result is deduced from a superposition-model analysis.

Monodomain strontium titanate

Abstract Monodomain SrTiO 3 crystals were found in less than 0.3 mm thick (110) plates below the cubic-to-tetragonal phase transition. Up to 98.5% monodomain samples were observed for rectangular plates with edges 7 mm // [001] and 2 mm // [110]. Quantitative results were obtained by monitoring EPR spectra of Fe 3+ impurities. Optical dichroism was also observed. Uniaxial stress experiments to yield large volume monodomain crystals are also reported, and confirm that c/a > 1 below the transition.

Electron spin resonance and optical identification of Fe4+-V0 in SrTiO3

Abstract The electron spin resonance of the minority defect Fe 4+ -V 0 has been observed in SrTiO 3 . It is the first pair center identified with a two-fold positive charge with respect to the lattice. For Fe 4+ it occurs in the unusual high spin S = 2 state. Optically-induced conversion to the neighbor Fe 3+ -V 0 charge state served to identify the acceptor transfer bands, showing the potentiality of the conversion-rate method.

Photochromic Fe5+ (3d3) in SrTiO3 evidence from paramagnetic resonance

Abstract Using paramagnetic resonance it is shown that a photochromic center in SrTiO3 is Fe5+ (3d3). Its resonance parameters are g = 2.0131 ±0.0008, A( 57 Fe) = (8.6 ± 0.1) x 10 −4 cm −1 . As a function of valency the positive g-shift of Fe5+ lies nearly on a straight line joining those of the isoelectronic Cr3+ and Mn4+. The hyperfine fields of V2+, Cr3+, Mn4+ and Fe5+ are close to one another (Geschwinds rule). Simultaneous observation of Fe3+ and Fe4+ proves indirectly the existence of Fe3+ which can act as a hole trap.

Identification of Fe4+ and Fe5+ charge-transfer photochromic absorption bands in SrTiO3

Optical absorption and the enhancement and bleaching of Fe3+ and Fe5+ electron paramagnetic resonance in SrTiO3: Al measured as a function of wavelength and time show that the photochromic absorption bands are due to electron transfer from O2- valence states to Fe4+ and Fe5+. They occur at 2.09 and 2.82 eV for Fe4+ and at 1.99 and 2.53 eV for Fe5+.

Wide-temperature instability in high-Tc superconductors, La1.85Ba0.15CuO4

Ultrasonic attenuation and velocity have been measured in the high-Tc superconductor La1.85Ba0.15CuO4 at 24 MHz. On cooling, a pronounced softening of the lattice is observed, beginning above room temperature. Below 200 K, the lattice remains soft and highly attenuating, suggestive of a near-instability state over the entire range 4 K to 200 K, partial stiffening occurs, but the attenuation remains higher than at room temperature, a feature suggestive of a possible glassy state.

Oxygen vacancy-transition metal-ion impurity association in SrTiO3

Abstract The progressive reduction of SrTiO3:Mn has been monitored by EPR. The results show the conversion of Mn4+ to Mn2+ and Mn2+ to Mn2+-V0 by oxygen-vacancy capture. The latter defect association takes place with the copious oxygen vacancies produced during high-temperature reduction between 1100 - 1300 K. At still higher reduction temperatures, other defects are formed. Analysis of the temperature dependence of the defect association gives a value of 0.7 eV for the enthalpy of association.

Interpretation of charge transfer bands in Fe doped SrTiO3 crystals

Abstract The electronic structures of SrTiO3 crystals doped with Fe3+, Fe4+ and Fe5+ ions have been investigated using the Xα cluster approach. The ground-state eigenvalues show the lower Fe acceptor level, of t2g↓ symmetry, localized inside the SrTiO3 band gap, respectively at 2.8 eV ( Fe 3+ , S = 5 2 ), 1.6 eV ( Fe 4+ , S = 1) or 1.1 eV ( Fe 4+ , S = 0) and 1.1 eV ( Fe 5+ , S = 3 2 ) above the valence band edge. Other acceptor levels, with eg↓ and eg↑ symmetries, appear inside the gap when the Fe nominal ionicity increases. The theoretical Xα excitation energies of O 2p-Fe 3d transitions confirms the experimental interpretations of acceptor charge transfer bands for the optical absorption spectra of SrTiO3:Fe4+ and SrTiO3:Fe5+ crystals. The large optical excitation energies compared with the thermal transitions are partly due to the O 2p band width.

Tunnelling and collective excitations in a microscopic model of ferroelectricity

Abstract Equilibrium properties and collective excitations are studied for a simplified microscopic model of ferroelectricity. The q = 0 mode changes in character from a rest-strahl mode at T = 0 to a tunneling mode at T > T c , and shows the correct Cochran behavior in the transition region.