We investigate Josephson coupling through a ferromagnetic thin film using superconductor-insulator-ferromagnet-superconductor planar junctions. Damped oscillations of the critical current are observed as a function of the ferromagnetic layer thickness. We show that they result from the exchange energy gained or lost by a quasiparticle Andreev-reflected at the ferromagnet-superconductor interface. The critical current cancels out at the transition from positive ("0") to negative ("pi") coupling, in agreement with theoretical calculations.

Planar tunneling spectroscopy reveals damped oscillations of the superconducting order parameter induced into a ferromagnetic thin film by the proximity effect. The oscillations are due to the finite momentum transfer provided for Cooper pairs by the splitting of the spin-up and spin-down bands in the ferromagnet. As a consequence, for negative values of the superconducting order parameter the tunneling spectra are capsized ("pi state"). The oscillations' damping and period are set by the same length scale, which depends on the spin polarization.

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What happens to spin-polarized electrons when they enter a superconductor? Superconductors at equilibrium and at finite temperature contain both paired particles (of opposite spin) in the condensate phase as well as unpaired, spin-randomized quasiparticles. Injecting spin-polarized electrons into a superconductor (and removing pairs) thus creates both spin and charge imbalances 1-7 , which must relax when the injection stops, but not necessarily over the same time (or length) scale. These different relaxation times can be probed by creating a dynamic equilibrium between continuous injection and relaxation; this leads to constant-in-time spin and charge imbalances, which scale with their respective relaxation times and with the injection current. Whereas charge imbalances in superconductors have been studied in great detail both theoretically 8 and experimentally 9 , spin imbalances have not received much experimental attention 6,10,11 despite intriguing theoretical predictions of spin-charge separation effects 12,13 . Here we present evidence for an almost-chargeless spin imbalance in a mesoscopic superconductor.A pure spin imbalance in a superconductor can be understood in the following manner: imagine injecting spin-randomized electrons continuously into a small superconducting volume and taking out Cooper pairs. The number of electron-like quasiparticles increases, that is, their chemical potential µ QP rises whereas that of the Cooper pairs µ P drops by the same amount to conserve particle number. This charge imbalance was first observed in a pioneering experiment, where µ QP − µ P was measured 1,2,14 . (Hereafter µ P ≡ 0, that is, all chemical potentials are measured with respect to that of the condensate.) If the injected electrons are (or become) spin-polarized, in general µ QP↑ = µ QP↓ = µ P , we can define a charge imbalance µ C ≡ (µ QP↑ + µ QP↓ )/2 and spin imbalance µ S ≡ (µ QP↑ − µ QP↓ )/2 (ref. 13). If charge relaxes faster than spin, a situation may arise in which µ C = 0 while µ S = 0. This is our chargeless spin imbalance. (See Supplementary Information for more details.) In the experiment, µ QP↑ − µ P and µ QP↓ − µ P are measured as a voltage drop between a spin-sensitive electrode and the superconductor.We implement a mesoscopic version of an experiment proposed in refs 12,13; this offers two practical advantages: the detector can be placed within a spin relaxation length λ S of the injection point and all out-of-equilibrium signals are enhanced by the small injection volume. In diffusive transport, λ S = (Dτ S2 ) 1/2 , where τ S2 is the spin relaxation time and D the diffusion constant (∼5 × 10 −3 m 2 s −1 in our samples 15 ). Our samples are FISIF lateral spin valves 16 , where the F are ferromagnets (Co), the I are insulators (Al 2 O 3 ) and S is the superconductor (Al), as shown in Fig. 1a. The SIF junctions have sheet resistances of ∼1.6 × 10 −6 cm 2 (corresponding to a barrier transparency T ∼ 5 × 10 −5 ) and tunnelling is the main transport mechanism through the insulator. By sweeping an...

Data from ab-oriented YBa 2 Cu 3 O 7 ͞I͞Cu tunnel junctions are presented. Self-assembled monolayers form the insulating tunnel barrier, I. The YBa 2 Cu 3 O 7 features in the tunneling conductance match those of low-leakage ab-oriented YBa 2 Cu 3 O 7 ͞Pb junctions. Results show that the zero-bias conductance peak is an Andreev bound state (ABS) of a d-wave order parameter. In zero magnetic field, the ABS splits below ϳ7 K, consistent with the presence of a subdominant order parameter at the surface. An applied magnetic field induces further splitting that grows nonlinearly with increasing field.[S0031-9007 (97)03529-1] PACS numbers: 74.50. + r, 74.72.Bk

We present magnetization measurements of mesoscopic superconducting niobium loops containing a ferromagnetic (PdNi) pi junction. The loops are prepared on top of the active area of a micro-Hall sensor based on high mobility GaAs/AlGaAs heterostructures. We observe asymmetric switching of the loop between different magnetization states when reversing the sweep direction of the magnetic field. This provides evidence for a spontaneous current induced by the intrinsic phase shift of the pi junction. In addition, the presence of the spontaneous current near zero applied field is directly revealed by an increase of the magnetic moment with decreasing temperature, which results in half integer flux quantization in the loop at low temperatures.

Tunnel junctions, an established platform for high resolution spectroscopy of superconductors, require defect-free insulating barriers; however, oxides, the most common barrier, can only grow on a limited selection of materials. We show that van der Waals tunnel barriers, fabricated by exfoliation and transfer of layered semiconductors, sustain stable currents with strong suppression of sub-gap tunneling. This allows us to measure the spectra of bulk (20 nm) and ultrathin (3- and 4-layer) NbSe2 devices at 70 mK. These exhibit two distinct superconducting gaps, the larger of which decreases monotonically with thickness and critical temperature. The spectra are analyzed using a two-band model incorporating depairing. In the bulk, the smaller gap exhibits strong depairing in in-plane magnetic fields, consistent with high out-of-plane Fermi velocity. In the few-layer devices, the large gap exhibits negligible depairing, consistent with out-of-plane spin locking due to Ising spin–orbit coupling. In the 3-layer device, the large gap persists beyond the Pauli limit.

We have observed the unconventional photon blockade effect for microwave photons using two coupled superconducting resonators. As opposed to the conventional blockade, only weakly nonlinear resonators are required. The blockade is revealed through measurements of the second order correlation function g^{(2)}(t) of the microwave field inside one of the two resonators. The lowest measured value of g^{(2)}(0) is 0.4 for a resonator population of approximately 10^{-2} photons. The time evolution of g^{(2)}(t) exhibits an oscillatory behavior, which is characteristic of the unconventional photon blockade.