Tunable spin and charge Seebeck effects in magnetic molecular junctions

Orateur : Pablo Cornaglia (Bariloche Atomic Center, Argentina)

The increasing interest in the thermoelectric properties of materials and the quest for high Seebeck coefficients is motivated by the promise of more efficient solid state refrigerators and the conversion of waste heat into electricity. The Seebeck effect refers to the generation of a charge current (or a voltage drop) by a temperature gradient applied across a metal, and the spin-Seebeck effect, concerns the thermal generation of pure spin currents. The recent experimental observation of the Seebeck effect in different nano-structures, in particular in molecular junctions and quantum dots, opened new routes to study these phenomena. On the one hand, the high sensitivity of these systems to external fields, their scalability and tunability make them potential candidates for a variety of technological applications. On the other hand, thermoelectric and thermomagnetic effects provide a unique probe of electron correlation effects and are a useful tool to gain further insight on fundamental problems like the Kondo regime where the energy transfer is dominated by spin fluctuations. I’ll present results for the charge and spin Seebeck effects of a spin-1 molecular junction as a function of temperature (T), applied magnetic field (H), and molecular magnetic anisotropy (D) obtained using Wilson’s numerical renormalization group [1]. A hard-axis magnetic anisotropy produces a large enhancement of the charge Seebeck coefficient Sc ( kB/|e|) whose value only depends on the residual interaction between quasiparticles in the low temperature Fermi-liquid regime. In the underscreened spin-1 Kondo regime, the high sensitivity of the system to magnetic fields makes it possible to obtain a sizable value for the spin Seebeck coefficient even for magnetic fields much smaller than the Kondo temperature. Similar effects can be obtained in C60 junctions where the control parameter is the gap between a singlet and a triplet molecular state. I’ll also discuss briefly the thermoelectric properties of an SU(4) Kondo resonance, that describes the low temperature transport through clean C nanotubes [2].

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