We present the results of measurements of the thermal conductivity of the quasi one-dimensional spin S=1/2 chain compound SrCuO2 in the temperature range between 0.4 and 300 K along the directions parallel and perpendicular to the chains. An anomalously enhanced thermal conductivity is observed along the chains. The analysis of the present data and a comparison with analogous recent results for Sr2CuO3 and other similar materials demonstrates that this behavior is generic for cuprates with copper-oxygen chains and strong intrachain interactions. The observed anomalies are attributed to the one-dimensional energy transport by spin excitations (spinons), limited by the interaction between spin and lattice excitations. The energy transport along the spin chains has a non-diffusive character, in agreement with theoretical predictions for integrable models. 66.70.+f, 75.40.Gb, 63.20.Ls

The ab-plane thermal conductivity κ of single-crystalline hexagonal MgB2 has been measured as a function of magnetic field H with orientations both parallel and perpendicular to the c-axis and at temperatures between 0.5 and 300 K. In the mixed state, κ(H) measured at constant temperatures reveals features that are not typical for common type-II superconductors. The observed behavior may be associated with the field-induced reduction of two superconducting energy gaps, significantly different in magnitude. A nonlinear temperature dependence of the electronic thermal conductivity is observed in the field-induced normal state at low temperatures. This behavior is at variance with the law of Wiedemann and Franz, and suggests an unexpected instability of the electronic subsystem in the normal state at T ≈ 1 K.

The thermal conductivity of the spin-1/2 ladder system Sr14-xCaxCu24O41 ( x = 0, 2, and 12) has been measured both along ( kappa(c)) and perpendicular to ( kappa(a)) the ladder direction at temperatures between 5 and 300 K. While the temperature dependence of kappa(a) is typical for phonon heat transport, an unusual double-peak structure is observed for kappa(c)(T). We interpret this unexpected feature as a manifestation of quasi-one-dimensional magnon thermal transport mediated by spin excitations along the ladders.

We report measurements of the specific heat and the thermal conductivity of the model Heisenberg spin-1/2 chain cuprate Sr2CuO3 at low temperatures. In addition to a nearly isotropic phonon heat transport, we find a quasi one-dimensional excess thermal conductivity along the chain direction, most likely associated with spin excitations (spinons). The spinon energy current is limited mainly by scattering on defects and phonons. Analyzing the specific heat data, the intrachain magnetic exchange J/kB is estimated to be ≃2650 K.PACS numbers: 66.70.+f, 75.40.Gb, 74.72.Jt There is a considerable theoretical interest in onedimensional (1D) Heisenberg spin-1/2 systems because they exhibit a number of properties that are entirely dominated by quantum-mechanical behavior and have no analogues in three-dimensional systems. In particular, it has been shown that the Heisenberg S=1/2 chain represents an integrable system characterized by a macroscopic number of conservation laws. 1 One important conserved quantity is the energy current, 1,2 implying an ideal (infinite) thermal conductivity along the chains at nonzero temperatures, if perturbations from impurities, phonons, or an interchain coupling, which always lead to non-integrable models, are negligible. It is an open question, to what extent a real material may be regarded as an ideal integrable system. Probably, the most obvious evidence for the predicted anomalous heat transport is the recent observation of an unusually high quasi-1D magnon thermal conductivity in the series (Sr,Ca,La) 14 Cu 24 O 41 . 3,4 The structure of these materials contains two building blocks with 1D character, namely CuO 2 chains and Cu 2 O 3 ladders, both oriented along the same direction. Unfortunately, the dimerisation within the chains and a non-negligible interchain interaction in this system complicate the analysis of the observed features in terms of an integrable model.In this work, we have searched for anomalies in the thermal transport of Sr 2 CuO 3 , which is often considered as the best physical realization of the 1D Heisenberg S=1/2 model. The crystal structure of Sr 2 CuO 3 contains chains formed by CuO 4 squares sharing oxygen corners. 5 The chains run along the b axis and, as shown in the inset of Fig. 2, the CuO 4 squares lie in the ab plane. The intrachain exchange interaction between neighboring Cu 2+ ions connected via 180 • Cu-O-Cu bonds, measured as J/k B , is between 2150 and 3000 K. 6-9 The ratio k B T N /J, where T N is the 3D Néel temperature, is as small as 2 × 10 −3 , reflecting an extremely small ratio J ′ /J, J ′ representing the interchain interaction.Our observations indicate an excess thermal conductivity along the chain direction, provided by quasi-1D spin excitation (spinons). According to our analysis presented below, its magnitude is limited by scattering of spinons on defects and phonons. We find no evidence for a mutual scattering between spin excitations and hence it seems to be absent or at least negligibly small, in agreement with theoretical predict...

No abstract