Thermal Conductivity, Thermopower, and Figure of Merit of La_{1-x}Sr_xCoO_3

We present a study of the thermal conductivity k and the thermopower S of single crystals of La_{1-x}Sr_xCoO_3 with 0<= x <= 0.3. For all Sr concentrations La_{1-x}Sr_xCoO_3 has rather low k values, whereas S strongly changes as a function of x. We discuss the influence of the temperature- and the doping-induced spin-state transitions of the Co ions on both, S and k. From S, k, and the electrical resistivity rho we derive the thermoelectric figure of merit Z=S^2/(k*rho). For intermediate Sr concentrations we find notably large values of Z indicating that Co-based materials could be promising candidates for thermoelectric cooling.Comment: 7 pages, 5 figures included, submitted to Phys. Rev.

Magnetic excitations in two-leg spin 1/2 ladders: experiment and theory

Magnetic excitations in two-leg S=1/2 ladders are studied both experimentally and theoretically. Experimentally, we report on the reflectivity, the transmission and the optical conductivity sigma(omega) of undoped La_x Ca_14-x Cu_24 O_41 for x=4, 5, and 5.2. Using two different theoretical approaches (Jordan-Wigner fermions and perturbation theory), we calculate the dispersion of the elementary triplets, the optical conductivity and the momentum-resolved spectral density of two-triplet excitations for 0.2 <= J_parallel/J_perpendicular <= 1.2. We discuss phonon-assisted two-triplet absorption, the existence of two-triplet bound states, the two-triplet continuum, and the size of the exchange parameters.Comment: 6 pages, 7 eps figures, submitted to SNS 200

Heat transport in SrCu_2(BO_3)_2 and CuGeO_3

In the low dimensional spin systems $SrCu_2(BO_3)_2$ and $CuGeO_3$ the thermal conductivities along different crystal directions show pronounced double-peak structures and strongly depend on magnetic fields. For $SrCu_2(BO_3)_2$ the experimental data can be described by a purely phononic heat current and resonant scattering of phonons by magnetic excitations. A similar effect seems to be important in $CuGeO_3$, too but, in addition, a magnetic contribution to the heat transport may be present.Comment: 4 pages, 2 figures; appears in the proceedings of the SCES2001 (Physica B

Separation of Quasiparticle and Phononic Heat Currents in YBCO

Measurements of the transverse (k_{xy}) and longitudinal (k_{xx}) thermal conductivity in high magnetic fields are used to separate the quasiparticle thermal conductivity (k_{xx}^{el}) of the CuO_2-planes from the phononic thermal conductivity in YBa_2Cu_3O_{7-\delta}. k_{xx}^{el} is found to display a pronounced maximum below T_c. Our data analysis reveals distinct transport (\tau) and Hall (\tau_H) relaxation times below T_c: Whereas \tau is strongly enhanced, \tau_H follows the same temperature dependence as above T_c

Thermal conductivity of R2CuO4, with R = La, Pr and Gd

We present measurements of the in-plane kappa_ab and out-of-plane kappa_c thermal conductivity of Pr2CuO4 and Gd2CuO4 single crystals. The anisotropy gives strong evidence for a large contribution of magnetic excitations to kappa_ab i.e. for a heat current within the CuO2 planes. However, the absolute values of kappa_mag are lower than previous results on La2CuO4. These differences probably arise from deviations from the nominal oxygen stoichiometry. This has a drastic influence on kappa_mag, which is shown by an investigation of a La2CuO4+delta polycrystal.Comment: 2 pages, 1 figure; presented at SCES200

Spin-State Transition and Metal-Insulator Transition in La1−x_{1-x}Eux_xCoO3_3}

We present a study of the structure, the electric resistivity, the magnetic susceptibility, and the thermal expansion of La$_{1-x}$Eu$_x$CoO$_3$. LaCoO$_3$ shows a temperature-induced spin-state transition around 100 K and a metal-insulator transition around 500 K. Partial substitution of La$^{3+}$ by the smaller Eu$^{3+}$ causes chemical pressure and leads to a drastic increase of the spin gap from about 190 K in LaCoO$_3$ to about 2000 K in EuCoO$_3$, so that the spin-state transition is shifted to much higher temperatures. A combined analysis of thermal expansion and susceptibility gives evidence that the spin-state transition has to be attributed to a population of an intermediate-spin state with orbital order for $x<0.5$ and without orbital order for larger $x$. In contrast to the spin-state transition, the metal-insulator transition is shifted only moderately to higher temperatures with increasing Eu content, showing that the metal-insulator transition occurs independently from the spin-state distribution of the Co$^{3+}$ ions. Around the metal-insulator transition the magnetic susceptibility shows a similar increase for all $x$ and approaches a doping-independent value around 1000 K indicating that well above the metal-insulator transition the same spin state is approached for all $x$.Comment: 10 pages, 6 figure

DNA methylotype analysis in colorectal cancer

The methylation status of a gene promoter is considered to be an important mechanism for the development of many tumors, including colorectal cancer. Recent studies have shown that specific patterns of DNA methylation across multiple CpG loci in some human tumors are more informative than the detection of one single CpG locus in tumor genomes. In the present study, multiple CpG methylations of three genes (CDKN2A, DPYD and MLH1) were detected in DNA samples from patients with colorectal cancer using Pyrosequencing(®) technology. The bisulfite-converted DNA was amplified with a nested PCR and five or six CpG loci of each gene were assessed to determine DNA methylotype. Our data showed that 10/49 (20.4%), 6/48 (12.5%) and 14/49 (28.6%) of tumors were methylated with a DNA methylation level >0.2 in CDKN2A, DPYD and MLH1, respectively. Our study indicated a similar DNA methylation level across the multiple CpG loci for all three genes in the methylated tumor DNA samples, demonstrating a dichotomous trait in DNA methylation. The tumor DNA samples had unique DNA methylation patterns, which were high-degree and multiple-site methylation, but the normal DNA samples had no or a low-degree and dispersed single-site methylation. In addition, an inverse correlation in those methylated tumors was observed between DNA methylation and RNA expression for MLH1 (R(S)=−0.62, P=0.003), but not for CDKN2A and DPYD. In conclusion, distinctive DNA methylotypes exist in colorectal cancer and may depict a distinct biology in apparently homogeneous tumors