Structural and Magnetic Instabilities of La2−x_{2-x}Srx_xCaCu2_2O6_6

A neutron scattering study of nonsuperconducting La$_{2-x}$Sr$_x$CaCu$_2$O$_6$ (x=0 and 0.2), a bilayer copper oxide without CuO chains, has revealed an unexpected tetragonal-to-orthorhombic transition with a doping dependent transition temperature. The predominant structural modification below the transition is an in-plane shift of the apical oxygen. In the doped sample, the orthorhombic superstructure is strongly disordered, and a glassy state involving both magnetic and structural degrees of freedom develops at low temperature. The spin correlations are commensurate.Comment: published versio

Theory of the first-order isostructural valence phase transitions in mixed valence compounds YbIn_{x}Ag_{1-x}Cu_{4}

For describing the first-order isostructural valence phase transition in mixed valence compounds we develop a new approach based on the lattice Anderson model. We take into account the Coulomb interaction between localized f and conduction band electrons and two mechanisms of electron-lattice coupling. One is related to the volume dependence of the hybridization. The other is related to local deformations produced by f- shell size fluctuations accompanying valence fluctuations. The large f -state degeneracy allows us to use the 1/N expansion method. Within the model we develop a mean-field theory for the first-order valence phase transition in YbInCu_{4}. It is shown that the Coulomb interaction enhances the exchange interaction between f and conduction band electron spins and is the driving force of the phase transition. A comparison between the theoretical calculations and experimental measurements of the valence change, susceptibility, specific heat, entropy, elastic constants and volume change in YbInCu_{4} and YbAgCu_{4} are presented, and a good quantitative agreement is found. On the basis of the model we describe the evolution from the first-order valence phase transition to the continuous transition into the heavy-fermion ground state in the series of compounds YbIn_{1-x}Ag_{x}Cu_{4}. The effect of pressure on physical properties of YbInCu_{4} is studied and the H-T phase diagram is found.Comment: 17 pages RevTeX, 9 Postscript figures, to be submitted to Phys.Rev.

Magneto-transport and magnetic susceptibility of SmFeAsO1-xFx (x = 0.0 and 0.20)

Bulk polycrystalline samples, SmFeAsO and the iso-structural superconducting SmFeAsO0.80F0.20 are explored through resistivity with temperature under magnetic field {\rho}(T, H), AC and DC magnetization (M-T), and Specific heat (Cp) measurements. The Resistivity measurement shows superconductivity for x = 0.20 sample with Tc(onset) ~ 51.7K. The upper critical field, [Hc2(0)] is estimated ~3770kOe by Ginzburg-Landau (GL) theory. Broadening of superconducting transition in magneto transport is studied through thermally activated flux flow in applied field up to 130 kOe. The flux flow activation energy (U/kB) is estimated ~1215K for 1kOe field. Magnetic measurements exhibited bulk superconductivity with lower critical field (Hc1) of ~1.2kOe at 2K. In normal state, the paramagnetic nature of compound confirms no trace of magnetic impurity which orders ferromagnetically. AC susceptibility measurements have been carried out for SmFeAsO0.80F0.20 sample at various amplitude and frequencies of applied AC drive field. The inter-granular critical current density (Jc) is estimated. Specific heat [Cp(T)] measurement showed an anomaly at around 140K due to the SDW ordering of Fe, followed by another peak at 5K corresponding to the antiferromagnetic (AFM) ordering of Sm+3 ions in SmFeAsO compound. Interestingly the change in entropy (marked by the Cp transition height) at 5K for Sm+3 AFM ordering is heavily reduced in case of superconducting SmFeAsO0.80F0.20 sample.Comment: 18 pages text + Figs: comments/suggestions welcome ([email protected]

Hall effect in YbXCu4 and the role of carrier density in the YbInCu4 valence transition

An unrealistically large value of the Gruneisen parameter is required to explain the valence transition which occurs at 42 K in YbInCu4 as due to a Kondo Volume Collapse. A hint as to the origin of the transition lies in the large change in carrier density which occurs at the transition, from trivalent semimetallic behavior at high temperature to mixed valent metallic behavior at low temperature. In this paper we report measurements of the Hall coefficient RH for temperatures in the range 15-300 K for a series of RXCu4 compounds (R = Yb, Lu and X = Au, Zn, Cd, Mg, Tl) that form in the cubic C15b structure. For all X the Hall coefficients are small (∼10-10 m3/C) so that the transport appears to be metallic. The observation that low carrier density is unique to RInCu4 leads us to hypothesize that the valence transition (which is also unique to YbInCu4) is connected with the existence of a quasigap, which is a common feature of the band structure of RXCu4. The quasigap allows for two competing hybridization states of the 4f electrons: a small TK semimetallic state and a large TK metallic state. © 1998 Elsevier Science Ltd. All rights reserved

Testing the influence of acceleration on time dilation using a rotating Mössbauer absorber

The aim of the experiment series was to test the influence of acceleration on time dilation by measuring the relative spectral shift between the resonance spectra of a rotating Mossbauer absorber with acceleration anti-parallel and parallel to the direction of the incident beam. Based on the experiences and know-how acquired in our previous experiments, We collected data for rotation frequencies up to 510Hz in both directions of rotation and also used different slits. For each run with high rotation, we observed a stable statistically significant relative shift between the spectra of the two states with opposite acceleration. This indicates the influence of acceleration on time dilation. However, we found that this shift also depends on the choice of the slit, and on the direction of rotation. These new unexpected findings, resulting from the loss of symmetry in obtaining the resonant lines in the two states, could overshadow the relative shift due to acceleration. This loss of the symmetry is caused by the deflection of the radiative decay due to the Nuclear Lighthouse effect from the rotating Mossbauer absorber. We also found that it is impossible to keep the alignment (between the optical and the dynamical rotor systems) with accuracy needed for such experiment, for long runs, which resulted in the reduction of the accuracy of the observed relative shift. These prevent us to claim with certainty the influence of acceleration on time dilation using the currently available technology. An improved KB optics with focal spot of less than 1 micron to avoid the use of a slit and a more rigid mounting of the rotor system, are necessary for the success of such experiment. Hopefully, these findings together with the indispensable plan for a conclusive experiment presented in the paper, will prove useful to future experimentalists wishing to pursue such an experiment