Spin-bottleneck due to spin-charge separation in a superconductor

An experimental device was designed to measure the effect of the injection of spin-polarized carriers on the superconductive gap and density of states (DOS). Quasiparticles were injected from a ferromagnet $\left({\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2}\right)$ through a tunnel junction into a conventional superconductor (Nb), while charge neutrality was maintained by a supercurrent. The DOS of the superconductor was measured through a second tunnel junction with a normal paramagnetic metal. No significant decrease of the superconductive gap was observed while a noticeable heating of the quasiparticles of the superconductor was measured. A similar experiment performed with current injected from a paramagnet (Al or Ag) showed no heating of quasiparticles. These observations are consistent with spin-charge separation of Bogoliubov quasiparticles and spin-bottleneck due to the enhanced recombination time of pure spin excitations.

Figure 1

(Color online) Sketch of the device. a) Side view (substrate is omitted). b) Top view (on a different scale). All layers are 50 nm thick. Junctions are 750 $\mu m$ wide. Py denotes the permalloy ${\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2}$.

Figure 2

(Color online) $dI∕dV$ curves of the detector in the case of ferromagnet injection, for different values of the bias current through the injector. $T_{\mathit{\text{bath}}}=1.48\mathrm{K}$. The spectrum is displayed only for positive voltages but was symmetric for negative voltages. The open gray squares correspond to a situation where the current flows through the ferromagnet layer but not through the junction, allowing to rule out spurious Joule effects through the circuitry.

Figure 3

(Color online) $dI∕dV$ curves of the detector in the case of paramagnet injection, for different values of the injection current. All curves are superimposed. $T_{\mathit{\text{bath}}}=1.7\mathrm{K}$.

Figure 4

(Color online) Variation of $dI∕dV$ at zero voltage of the detector for a ferromagnet injector (black dots) and a paramagnet injector (open red squares). The conductance was normalized to its zero bias current value. The resistances of the injector junctions are, respectively, 2.6 and $1.5\Omega$.

Figure 5

(Color online) Fit using BCS theory of the spectrum measured under a spin-polarized current of $25\mathrm{mA}$. Blue circles: experimental data; line, BCS fit using $T^{*}=2.38\mathrm{K}$ and $\Delta =1.37\mathrm{meV}$; red squares, attempt using Dynes model[6] with $\Gamma =110\mu \mathrm{eV}$, $T^{*}=T=1.48\mathrm{K}$, and $\Delta =1.38\mathrm{meV}$.