Thermal conductivity of doped La2CuO4\rm\bf La_2CuO_4 as an example for heat transport by optical phonons in complex materials

We investigate the phonon thermal conductivity $\kappa_{\mathrm{ph}}$ of doped $\rm La_2CuO_4$ based on out-of-plane thermal conductivity measurements. When room temperature is approached the temperature dependence of $\kappa_{\mathrm{ph}}$ strongly deviates from the $T^{-1}$-decrease which is usually expected for heat transport by acoustic phonons. Instead, $\kappa_{\mathrm{ph}}$ decreases much weaker or even increases with rising temperature. Simple arguments suggest that such unusual temperature dependencies of $\kappa_{\mathrm{ph}}$ are caused by heat transport via dispersive optical phonons

The direct Cu NQR Study of the Stripe Phase in the Lanthanum Cuprates

Using Cu NQR in Eu-doped La_(2-x)Sr_xCuO_4 we find the evidence of the pinned stripe phase at 1.3K for 0.08<x<0.18. The pinned fraction increases by one order of magnitude near hole doping x=1/8. The NQR lineshape reveals three inequivalent Cu positions. A dramatic change of the NQR signal for x > 0.18 correlating with the onset of bulk superconductivity corresponds to the depinning of the stripe phase.Comment: 4 pages, 3 figures, to appear in Physica C, Proceedings of the 6th International Conference on Materials and Mechanisms of Superconductivity, Houston, February 200

Magnetism of the LTT phase of Eu doped La_{2-x}Sr_xCuO_4

The ESR signal of Gd spin probes (0.5 at %) as well as the static normal state susceptibility of Eu (J(Eu^{3+})=0) doped La_{2-x-y}Sr_xEu_yCuO_4 reveal pronounced changes of the Cu magnetism at the structural transition from the orthorhombic to the low temperature tetragonal phase for all non-superconducting compositions. Both a jumplike decrease of \chi as well as the ESR data show an increase of the in-plane magnetic correlation length in the LTT phase. From the Gd^{3+} ESR linewidth we find that for specific Eu and Sr concentrations in the LTT phase the correlation length increases up to more than 100 lattice constants and the fluctuation frequency of the CuO_2 spin system slows down to 10^{10}- 10^{11}sec^{-1}. However, there is no static order above T ~ 8K in contrast to the LTT phase of Nd doped La_{2-x}Sr_xCuO_4 with pinned stripe correlations.Comment: 7 pages, RevTex, 3 eps figures. To appear in the Proceedings of the International Conference "Stripes, Lattice Instabilities and High Tc Superconductivity", (Rome, Dec. 1996

Lattice Dimerization in the Spin-Peierls Compound CuGeO3_3

The uniaxial pressure dependences of the exchange coupling and the structural distortion in the dimerized phase of CuGeO$_3$ are analyzed. A minimum magnetic dimerization of 3 % is obtained, incompatible with an adiabatic approach to the spin-Peierls transition. Exploring the properties of an Heisenberg spin chain with dynamical spin-phonon coupling, the dimerization dependence of the spin excitation gap is found to be in qualitative agreement with experiment.Comment: 2 pages, 1 figure include

Why TcT_c of (CaFeAs)10_{10}Pt3.58_{3.58}As8_8 is twice as high as (CaFe0.95_{0.95}Pt0.05_{0.05}As)10_{10}Pt3_3As8_8

Recently discovered (CaFe$_{1-x}$Pt$_x$As)$_{10}$Pt$_3$As$_8$ and (CaFeAs)$_{10}$Pt$_{4-y}$As$_8$ superconductors are very similar materials having the same elemental composition and structurally similar superconducting FeAs slabs. Yet the maximal critical temperature achieved by changing Pt concentration is approximately twice higher in the latter. Using angle-resolved photoemission spectroscopy(ARPES) we compare the electronic structure of their optimally doped compounds and find drastic differences. Our results highlight the sensitivity of critical temperature to the details of fermiology and point to the decisive role of band-edge singularities in the mechanism of high-$T_c$ superconductivity