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DDC: 530 × Subject: temperature dependence ×

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    Large magneto-Seebeck effect in magnetic tunnel junctions with half-metallic Heusler electrodes
    (London : Nature Publishing Group, 2017) Boehnke, A.; Martens, U.; Sterwerf, C.; Niesen, A.; Huebner, T.; Von Der Ehe, M.; Meinert, M.; Kuschel, T.; Thomas, A.; Heiliger, C.; Münzenberg, M.; Reiss, G.
    Spin caloritronics studies the interplay between charge-, heat- and spin-currents, which are initiated by temperature gradients in magnetic nanostructures. A plethora of new phenomena has been discovered that promises, e.g., to make wasted heat in electronic devices useable or to provide new read-out mechanisms for information. However, only few materials have been studied so far with Seebeck voltages of only some microvolt, which hampers applications. Here, we demonstrate that half-metallic Heusler compounds are hot candidates for enhancing spin-dependent thermoelectric effects. This becomes evident when considering the asymmetry of the spin-split density of electronic states around the Fermi level that determines the spin-dependent thermoelectric transport in magnetic tunnel junctions. We identify Co2FeAl and Co2FeSi Heusler compounds as ideal due to their energy gaps in the minority density of states, and demonstrate devices with substantially larger Seebeck voltages and tunnel magneto-Seebeck effect ratios than the commonly used Co-Fe-B-based junctions.
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    Evolution of the Kondo lattice and non-Fermi liquid excitations in a heavy-fermion metal
    (London : Nature Publishing Group, 2018) Seiro, S.; Jiao, L.; Kirchner, S.; Hartmann, S.; Friedemann, S.; Krellner, C.; Geibel, C.; Si, Q.; Steglich, F.; Wirth, S.
    Strong electron correlations can give rise to extraordinary properties of metals with renormalized Landau quasiparticles. Near a quantum critical point, these quasiparticles can be destroyed and non-Fermi liquid behavior ensues. YbRh2Si2 is a prototypical correlated metal exhibiting the formation of quasiparticle and Kondo lattice coherence, as well as quasiparticle destruction at a field-induced quantum critical point. Here we show how, upon lowering the temperature, Kondo lattice coherence develops at zero field and finally gives way to non-Fermi liquid electronic excitations. By measuring the single-particle excitations through scanning tunneling spectroscopy, we find the Kondo lattice peak displays a non-trivial temperature dependence with a strong increase around 3.3 K. At 0.3 K and with applied magnetic field, the width of this peak is minimized in the quantum critical regime. Our results demonstrate that the lattice Kondo correlations have to be sufficiently developed before quantum criticality can set in.
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    Unexpected differences between surface and bulk spectroscopic and implied Kondo properties of heavy fermion CeRh2Si2
    (2020) Poelchen, Georg; Schulz, Susanne; Mende, Max; Güttler, Monika; Generalov, Alexander; Fedorov, Alexander V.; Caroca-Canales, Nubia; Geibel, Christoph; Kliemt, Kristin; Krellner, Cornelius; Danzenbächer, Steffen; Usachov, Dmitry Y.; Dudin, Pavel; Antonov, Victor N.; Allen, James W.; Laubschat, Clemens; Kummer, Kurt; Kucherenko, Yuri; Vyalikh, Denis V.
    Ultra-violet angle-resolved photoemission spectroscopy (UV-ARPES) was used to explore the temperature dependence of the Ce-4f spectral responses for surface and bulk in the antiferromagnetic Kondo lattice CeRh2Si2. Spectra were taken from Ce- and Si-terminated surfaces in a wide temperature range, and reveal characteristic 4f patterns for weakly (surface) and strongly (bulk) hybridized Ce, respectively. The temperature dependence of the Fermi level peak differs strongly for both cases implying that the effective Kondo temperature at the surface and bulk can be rather distinct. The greatly reduced crystal–electric-field (CEF) splitting at the surface gives reason to believe that the surface may exhibit a larger effective Kondo temperature because of a higher local-moment effective degeneracy. Further, the hybridization processes could strongly affect the 4f peak intensity at the Fermi level. We derived the k-resolved dispersion of the Kondo peak which is also found to be distinct due to different sets of itinerant bands to which the 4f states of surface and bulk Ce are coupled. Overall our study brings into reach the ultimate goal of quantitatively testing many-body theories that link spectroscopy and transport properties, for both the bulk and the surface, separately. It also allows for a direct insight into the broader problem of Kondo lattices with two different local-moment sublattices, providing some understanding of why the cross-talking between the two Kondo effects is weak.