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

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

Crystal Hall and crystal magneto-optical effect in thin films of SrRuO3_3

Motivated by the recently observed topological Hall effect in ultra-thin films of SrRuO$_3$ (SRO) grown on SrTiO$_3$ (STO) [001] substrate, we investigate the magnetic ground state and anomalous Hall response of the SRO ultra-thin films by virtue of spin density functional theory (DFT). Our findings reveal that in the monolayer limit of an SRO film, a large energy splitting of Ru-$t_{2g}$ states stabilizes an anti-ferromagnetic (AFM) insulating magnetic ground state. For the AFM ground state, our Berry curvature calculations predict a large anomalous Hall response upon doping. From the systematic symmetry analysis, we uncover that the large anomalous Hall effect arises due to a combination of broken time-reversal and crystal symmetries caused by the arrangement of non-magnetic atoms (Sr and O) in the SRO monolayer. We identify the emergent Hall effect as a clear manifestation of the so-called crystal Hall effect in terminology of \v{S}mejkal et al. arXiv:1901.00445 (2019), and demonstrate that it persists at finite frequencies which is the manifestation of the crystal magneto-optical effect. Moreover, we find a colossal dependence of the AHE on the degree of crystal symmetry breaking also in ferromagnetic SRO films, which all together points to an alternative explanation of the emergence of the topological Hall effect observed in this type of systems.Comment: 8 pages, 5 figure

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

Magnetoresistance, specific heat and magnetocaloric effect of equiatomic rare-earth transition-metal magnesium compounds

We present a study of the magnetoresistance, the specific heat and the magnetocaloric effect of equiatomic $RET$Mg intermetallics with $RE = {\rm La}$, Eu, Gd, Yb and $T = {\rm Ag}$, Au and of GdAuIn. Depending on the composition these compounds are paramagnetic ($RE = {\rm La}$, Yb) or they order either ferro- or antiferromagnetically with transition temperatures ranging from about 13 to 81 K. All of them are metallic, but the resistivity varies over 3 orders of magnitude. The magnetic order causes a strong decrease of the resistivity and around the ordering temperature we find pronounced magnetoresistance effects. The magnetic ordering also leads to well-defined anomalies in the specific heat. An analysis of the entropy change leads to the conclusions that generally the magnetic transition can be described by an ordering of localized $S=7/2$ moments arising from the half-filled $4f^7$ shells of Eu$^{2+}$ or Gd$^{3+}$. However, for GdAgMg we find clear evidence for two phase transitions indicating that the magnetic ordering sets in partially below about 125 K and is completed via an almost first-order transition at 39 K. The magnetocaloric effect is weak for the antiferromagnets and rather pronounced for the ferromagnets for low magnetic fields around the zero-field Curie temperature.Comment: 12 pages, 7 figures include

Evidence for a large magnetic heat current in insulating cuprates

The in-plane thermal conductivity $k$ of the two-dimensional antiferromagnetic monolayer cuprate Sr$_2$CuO$_2$Cl$_2$ is studied. Analysis of the unusual temperature dependence of $k$ reveals that at low temperatures the heat is carried by phonons, whereas at high temperatures magnetic excitations contribute significantly. Comparison with other insulating layered cuprates suggests that a large magnetic contribution to the thermal conductivity is an intrinsic property of these materials.Comment: 4 pages, 2 figures included, submitted to Phys. Rev.

Contrasting sensitivity of lake sediment n-alkanoic acids and n-alkanes to basin-scale vegetation and regional-scale precipitation δ2H in the Adirondack Mountains, NY (USA)

The hydrogen isotope values of plant waxes (δ2Hwax) primarily reflect plant source water. δ2Hwax preserved in lake sediments has therefore been widely used to investigate past hydroclimate. The processes by which plant waxes are integrated at regional and catchment scales are poorly understood and may affect the δ2Hwax values recorded in sediments. Here, we assess the variability of sedimentary δ2Hwax for two plant wax compound classes (n-alkanes and n-alkanoic acids) across 12 lakes in the Adirondack Mountains that receive similar regional precipitation δ2H but vary at the catchment-scale in terms of vegetation structure and basin morphology. Total long-chain (n-C27 to n-C35) alkane concentrations were similar across all sites (191 ± 53 µg/g TOC) while total long-chain (n-C28 and n-C30) alkanoic acid concentrations were more variable (117 ± 116 µg/g TOC) and may reflect shoreline vegetation composition. Lakes with shorelines dominated by evergreen gymnosperm plants had significantly higher concentrations of long-chain n-alkanoic acids relative to n-alkanes, consistent with our observations that deciduous angiosperms produced more long-chain n-alkanes than evergreen gymnosperms (471 and 33 µg/g TOC, respectively). In sediments, the most abundant chain lengths in each compound class were n-C29 alkane and n-C28 alkanoic acid, which had mean δ2H values of −188 ± 6‰ and −164 ± 9‰, respectively. Across sites, the range in sedimentary n-C29 alkane (22‰) and n-C28 alkanoic acid δ2H (35‰) was larger than expected based on the total range in modeled mean annual precipitation δ2H (4‰). We observed larger mean εapp (based on absolute values) for n-alkanes (−123‰) than for n-alkanoic acids (−97‰). Across sites, the δ2H offset between n-C29 alkane and the biosynthetic precursor n-C30 alkanoic acid (εC29-C30) ranged from −8 to −58‰, which was more variable than expected based on observations in temperate trees (−20 to −30‰). Sediments with greater aquatic organic matter contributions (lower C/N ratios) had significantly larger (absolute) εC29-C30 values, which may reflect long-chain n-alkanoic acids from aquatic sources. Concentration and δ2Hwax data in Adirondack lakes suggest that long-chain n-alkanes are more sensitive to regional-scale precipitation signals, while n-alkanoic acids are more sensitive to basin-scale differences in catchment vegetation and wax sourcing