Fig. 3: Non-Fermi–Dirac quasiparticle distribution.
From: Evidence for spin-dependent energy transport in a superconductor
The magnetic field (H) is zero for all panels in this figure. a Current (\({I}_{\det }\)) as a function of voltage (\({V}_{\det }\)) across the closest SIS' detector junction (Jdet1) for injection currents Iinj = 0 nA (black) and Iinj = 120 nA (red). On the right vertical scale, Iinj as a function of voltage Vinj across the NIS injector junction Jinj (green). b Differential conductance (\({G}_{\det }\)), in units of the normal state value, as a function of (\({V}_{\det }\)) across Jdet1 for Iinj = 0 nA (black), Iinj ≈ 13 nA (blue, blue dot in a), and Iinj = 120 nA (red). The vertical dashed line indicates \(e{V}_{\det }=\Delta -{\Delta }_{\det }\) : \({G}_{\det }\) at this voltage is proportional to the number of quasiparticles in the superconducting wire at E = Δ. An attempted fit with an effective temperature T* ≈ 1.1 K in S reproduces the peak at Iinj = 13 nA, but grossly overestimates the QP population at higher energies (dashed blue line). In this fit, we use the experimentally determined values Δ = 245 μeV and \({\Delta }_{\det }=180\) μeV, \({T}_{\det }=90\) mK and a phenomenological depairing α ≈ 0.01Δ. c \({G}_{\det }\) at Jdet1 as a function of \({V}_{\det }\) and Iinj with the slice at Iinj = 0 subtracted from all data. The black lines show the measurement of ±Iinj(Vinj) from a shifted downwards by \({\Delta }_{\det }/e\). The black lines fall at the location of a step-like feature in the colour map, as expected: as shown in Fig. 1b, QPs in S are created up E ≈ eVinj + kBT, leading to a step-like cutoff in the distribution function. The dashed line again indicates \(e{V}_{\det }=\Delta -{\Delta }_{\det }\), where the QP density is maximal due to the coherence peak in the DOS of S. d Theoretical prediction of c, with Δ, \({\Delta }_{\det }\) and α as in b.
