T.S. Plaskett

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.

Raman and infrared results on YBa2Cu3O7−x type materials

Abstract We report both Raman and infrared results on the same semiconducting YBa2Cu3O7-x material. With the appropriate heat treatment, this material can be reversibly changed from an orthorhombic superconductor (x≈0.0) to a semiconductor (x>0.5). In the semiconducting material it is easier to measure the phonon features using either technique, and we observe almost the number of modes allowed. By simultaneously considering the modes observed by both techniques, using group theory to sort out the modes in both phases, and using projection operators to determine the symmetry adapted vectors, we arrive at an understanding of some of the modes. We find few differences when comparing our results to other measurements in the superconducting phase. The two general conclusions that we arrive at are: 1) splittings of the modes due to lowering the symmetry from tetragonal to the superconducting orthorhombic phase are not observed; 2) no frequency renormalizations, of the type that have been discussed theoretically, are observed.

Surface core-level shifts and surface valence change in mixed-valent YbAl2

Abstract High-resolution photoemission studies show that about two surface layers of mixed-valent YbAl2 are divalent. These layers exhibit large surface core-level shifts of Δs1=0.92 eV for the topmost and Δs2=0.35 eV for an underlying surface layer, allowing a separation of the divalent part of the spectrum into bulk and surface contributions. In this way, a mean valence of 2.4±0.1 can be derived for bulk YbAl2. The observed Δs1/Δs2 ratio is consistent with a recent theoretical prediction.

The gunn effect and conduction band structure in GaxIn1−xSb alloys

Abstract The observation of the Gunn effect in GaxIn1−xSb alloys is reported and shown to be consistent with a conduction band structure having a next higher set of minima along the directions in k space above the lowest conduction band minimum at the center of the Brillouin zone for all values of x.

Elastic properties of intermediate valence, Kondo and stable valence RAl2 compounds

Abstract Measurements of ultrasonic velocities and pressure volume behavior show that, compared to integral valence rare earth dialuminides, the intermediate valence compound, YbAl2, is mechanically soft and the Kondo compound, CeAl2, is much less so. These results show that in the case of intermediate valence the degenerate electronic configurations strongly effect the elastic properties while for the Kondo case, where the electronic configurations are nondegenerate, there is little effect.

Interplay of dipolar and random strain effects in the cooperative Jahn-Teller system TmAsO4

Abstract Electron paramagnetic resonance studies of the cooperative Jahn-Teller system TmAsO4 doped with Gd show that above ∼ 140 K dynamic random strains caused by the strong interaction of the first excited electronic doublet of Tm3+ with phonons are the major mechanisms for the line broadenings. At lower temperatures, because of the weaker interactions of the ground electronic doublet with the phonons, the induced dipolar mechanism for the line broadenings becomes important.