5 A ug 2 00 5 Thermal conductivity of R 2 CuO 4 , with R = La , Pr and Gd

We present measurements of the in-plane (κ ab) and out-of-plane (κc) thermal conductivity of Pr2CuO4 and Gd2CuO4 single crystals. The anisotropy gives strong evidence for a large contribution of magnetic excitations to κ ab , i.e. for a heat current within the CuO2 planes. However, the absolute values of κmag are lower than previous results on La2CuO4. These differences probably arise from deviations from the nominal oxygen stoichiometry. This has a drastic influence on κmag, which is shown by an investigation of a La2CuO 4+δ polycrystal. One peculiarity of low-dimensional spin systems is the possibility of large contributions to the heat transport by magnetic excitations. This has been found in 1D systems like spin ladders,[1] whereas the situation in 2D is less clear. The CuO2 planes in La2CuO4 are a good realization of a 2D antiferromagnetic Heisen-berg square lattice with a large exchange constant J ≈ 1400 K.[2] The thermal conductivity of La2CuO4 shows an unusual behavior.[3] For a heat current along the c direction one low-temperature maximum is present, as it is expected for an insulator. The heat is carried by acoustic phonons, and for high temperatures Umklapp scattering yields an approximate 1/T behavior of κc. However, for a heat current parallel to the CuO2 planes a second broad maximum arises in addition to the low-temperature maximum. This can be interpreted in terms of an additional contribution to the heat transport by magnetic excitations. Depending on the crystals, values of κ (300 K) mag ≈ 15. .. 24 W/Km are reported.[3,4,5] However, a double peak of κ could also arise from a suppression acting in a certain temperature window.[6] This requires an additional scattering mechanism, e.g. due to a structural instability, which is present in La2CuO4.[7] Such a structural instability is not present in the nearly iso-structural compound Sr2CuO2Cl2, but still there is a second maximum of κ ab .[8] This gives evidence for a magnetic origin of the second maximum and suggests that a magnetic contribution to κ ab could be a common feature of all single-layered insulating cuprates. In order to investigate this, we studied κ of R2CuO4, with R = Pr, Gd. Both compounds have a very similar CuO2 square lattice and exchange constant as La2CuO4.[9] However, for R = Pr the structure is stable down to lowest temperatures, whereas for R = Gd a structural phase transition occurs at 685 K.[10] Fig. 1a shows κ …