We have performed specific-heat measurements on the heavy-fermion superconductor URu 2 Si 2 in magnetic fields up to 17.5 T. A sharp peak in the specific heat signals the antiferromagnetic transition at T N ϭ17.5 K, which shifts to lower temperatures in applied magnetic fields. In order to describe the specific heat below T N , we have used the characteristic features of the excitation spectrum measured by neutron scattering. The relative field dependence of the antiferromagnetic transition temperature T N and the energy gap ⌬ in the magnetic excitation spectrum can be described by a single scaling relation of the form ͓1Ϫ(B/B 0 ) 2 ͔. The scaling field of 48.5 T is close to the metamagnetic transition field B*ϭ40 T, where the heavy-fermion state is suppressed.
The in-plane thermal conductivity k of the two-dimensional antiferromagnetic monolayer cuprate Sr2CuO2Cl2 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.There is growing experimental evidence that spin excitations may contribute significantly to the heat current in low-dimensional spin systems. This seems to be well established for one-dimensional (1D) systems 1,2,3,4,5 . For example, in the insulating spin-ladder material Sr 14−x Ca x Cu 24 O 41 a large magnetic contribution k m to the thermal conductivity k can be derived from a pronounced double-peak structure of k along the ladder direction 2,4 . The situation is less clear in twodimensional (2D) spin systems. These are, however, of particular importance due to their relevance for hightemperature superconductivity 6,7 . A double-peak structure comparable to that in 1D systems is found in the in-plane thermal conductivity of insulating 2D cuprates such as La 2 CuO 4 (LCO) and YBa 2 Cu 3 O 6 (YBCO) (Ref. 8,9,10). This may indicate a sizable magnetic contribution to the heat current at high temperatures 8 . However, the phononic thermal conductivity k ph may show a double-peak structure also, as a result of pronounced (resonant) scattering in a narrow temperature range. Such scattering may arise from the presence of local magnetic excitations, as was recently shown for the 2D spindimer system SrCu 2 (BO 3 ) 2 (Ref. 11), or it may arise from the presence of soft phonon modes 9 . The latter was suggested for LCO and YBCO, in which soft modes e.g. associated with tilt distortions of the CuO polyhedra are known to be present 9,12 . An additional complication arises from a strong sensitivity of the double-peak structure to light oxygen doping 9 .A material of particular interest in this context is Sr 2 CuO 2 Cl 2 (SCOC). It is structurally very similar to LCO: It contains CuO 2 -layers as in LCO, but the out-ofplane oxygen ions at the apices of the CuO 6 -octahedra are replaced by Cl and La by Sr. The material has several advantages compared to LCO and YBCO (see e.g. Ref.6): (1) SCOC does not exhibit any distortion from tetragonal symmetry down to at least 10K so that there is no structural instability associated with soft tilting modes.(2) Because of the absence of tilt distortions the magnetic properties are simpler than those of LCO. For example, there is no Dzyaloshinski-Moriya exchange interaction. Thus, SCOC is believed to represent the best realization of a two-dimensional square-lattice S = 1/2 Heisenberg antiferromagnet. (3) In contrast to LCO and YBCO, SCOC cannot be doped easily with charge carriers.In this paper we present measurements of the in-plane thermal conductivity k of SCOC. We identify a doublepeak structure from a pronounced ...
Giant magnetoresistance effects in intermetallic compoundsSechovsky, V.; Havela, L.; Prokes, K.; Nakotte, H.; de Boer, F.R.; Bruck, E.H. General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Giant magnetoresistance (GMR) effects are observed in several classes of bulk magnetic materials.The resistance changes at metamagnetic transitions connected with reorientation of 4f moments are only moderate due to the relatively weak coupling of the 4f and conduction electrons. Much larger GMR effects can be achieved by mechanisms involving the d states (RhFe, RCo&, though the most spectacular resistance variations are connected with metamagnetic transitions in U-intermetallic antiferromagnets. This phenomenon can 'be interpreted as due to Fermi surface gapping (due to magnetic superzones) and/or due to spin-dependent scattering in analogy with magnetic multilayers.
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