Observation of Thermoelectric Currents in High-Field Superconductor-Ferromagnet Tunnel Junctions

We report on the experimental observation of spin-dependent thermoelectric currents in superconductor-ferromagnet tunnel junctions in high magnetic fields. The thermoelectric signals are due to a spin-dependent lifting of the particle-hole symmetry, and are found to be in excellent agreement with recent theoretical predictions. The maximum Seebeck coefficient inferred from the data is about $-100\mu \mathrm{V}/\mathrm{K}$, much larger than commonly found in metallic structures. Our results directly prove the coupling of spin and heat transport in high-field superconductors.

Figure 1

(a) False-color scanning electron microscopy image of a region of sample FIS1 with the measurement configuration for the thermoelectric measurements. (b) Sketch of the measurement configuration for the conductance and heater calibration measurements.

Figure 2

(a) Differential conductance $g$ of junction $\alpha$ of sample FIS1 as a function of bias $V$ for different magnetic fields $B$. (b) Zero-bias conductance $g$ as a function of magnetic field $B$ for different base temperatures $T_0$. (c) Differential conductance $g$ at fixed base temperature and magnetic field for different heater currents $I_{\text{heat}}$. (d) Temperature $T_{\mathit{F}}$ of the ferromagnet as a function of $I_{\text{heat}}$ for different $T_0$.

Figure 3

(a) Differential conductance $g$ of contact $\beta$ of sample FIS2 at fixed magnetic field $B$ and temperature $T_0$ for different heater currents $I_{\text{heat}}$. (b) Pair potential $\mathrm{\Delta }$ as a function of $I_{\text{heat}}$ obtained by fitting the differential conductance shown in (a). (c) Increase $\delta T_{\mathit{S}}$ of the temperature of the superconductor as a function of $I_{\text{heat}}$ extracted by inverting $\mathrm{\Delta }(T_{\mathit{S}})$, together with $\delta T_{\mathit{F}}$.

Figure 4

(a) Thermoelectric current $I_{\mathrm{th}}$ as a function of magnetic field $B$ at base temperature $T_0=100\mathrm{mK}$, together with theoretical calculations (see text). (b) Thermoelectric coefficient $\eta$ of sample FIS1 for different temperatures $T_0$. (c) Thermoelectric coefficient $\eta$ at $T_0=100\mathrm{mK}$ for all three samples. (d) Seebeck coefficient $S$ as a function of $B$ calculated as described in the text. The error bar indicates the estimated error for the maximum at $T_0=250\mathrm{mK}$ [38].