Berlin 2015 – wissenschaftliches Programm

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TT: Fachverband Tiefe Temperaturen

TT 17: Superconductivity: Properties and Electronic Structure

TT 17.13: Vortrag

Montag, 16. März 2015, 18:15–18:30, H 2053

Umkehr effect observed in the magnetothermoelectric power of graphite — •Santiago Muiños-Landin, Israel Lorite Villalba, Winfried Böhlmann, and Pablo Esquinazi — Division of Superconductivity and Magnetism, Institute for experimental Physics II, Fakultät für Physik und Geowissenschaften, Linnéstrasse 5, 04103 Leipzig, Germany

In thermoelectricity, the Umkehr effect is defined as the difference between the the Seebeck coeffcient values upon reversal of an applied magnetic field. It was shown to be very large for certain samples of the semimetal bismuth. We show here that the effect is also observed in highly oriented pyrolytic graphite(HOPG Union Carbide), while it is not for a HOPG sample of a different provider. The only difference between these two samples is the amount of well deffined two-dimensional interfaces that exist inside them. The contribution to the thermoelectric power (TEP) that comes only from the graphite interfaces, makes non symmetric the response of the Seebeck effect once the sign of the magnetic field is changed. This fact provides an explanation for the Umkehr effect in graphite, in terms of the contribution of the interfaces to the TEP in our samples. Recently published work indicates that granular superconductivity is embedded in some of the interfaces. Therefore, we may now speculate that the extra entropy contribution to the magnetothermoelectric effect may be due to some kind of vortices produced by the applied magnetic field at the superconducting regions. We correlate therefore the field dependence of this contribution and its temperature dependence with that of the magnetoresistance.