We study narrow ballistic Josephson weak links in a InAs quantum wells contacted by Nb electrodes and find a dramatic magnetic-field suppression of the Andreev reflection amplitude, which occurs even for in-plane field orientation with essentially no magnetic flux through the junction. Our observations demonstrate the presence of a Doppler shift in the energy of the Andreev levels, which results from diamagnetic screening currents in the hybrid Nb/InAs banks. The data for conductance, excess, and critical currents can be consistently explained in terms of the sample geometry and the McMillan energy, characterizing the transparency of the Nb/InAs interface. DOI: 10.1103/PhysRevB.80.220507 PACS number͑s͒: 74.45.ϩc In recent years a detailed microscopic understanding of the proximity effect has emerged. There is now agreement that in highly transparent Josephson junctions formed by a metallic weak link in good contact with two superconducting ͑SC͒ banks the supercurrent is carried by Andreev bound states ͑ABSs͒. 1 These come in pairs corresponding to the opposite directions of Cooper-pair transfer mediated by multiple Andreev reflection ͑MAR͒ at the SC/metal interfaces provided that the acquired quasiparticle phase is a multiple of 2 . 2 At currents exceeding the critical current I C ͑T , B͒ MAR between the SC banks manifests itself in the currentvoltage characteristics as subharmonic gap structures at voltages eV n =2⌬ / n, where ⌬ is the SC energy gap and n =1,2,.... At higher voltage eV ӷ 2⌬, I͑V͒ becomes linear with an excess current I exc = I͑V͒ − G N V determined by a single Andreev reflection ͑AR͒ probability ͉a͑ ͉͒ 2 ͑G N is the normal-state conductance͒. 3,4 Weak links formed by a two-dimensional ͑2D͒ electron gas ͑2DEG͒ in semiconductor quantum wells 5 are of particular interest because here the ballistic transport can be studied. In very high magnetic field perpendicular to the 2DEG, theory 6 and experiments 7,8 have demonstrated Andreev transport via edge states. Indirect evidence for a strong magneticfield effect on AR was experimentally found in antidot billiards. 9 The case of parallel field, with respect to the 2DEG, is equally intriguing: as ideally no magnetic flux threads the 2DEG, one may naively expect the Josephson current to survive up to the critical fields of the SC leads. This is not the case, but the underlying mechanism of the supercurrent suppression is still unclear. This question, also relevant for other 2D hybrid systems, 10 is among the issues this Rapid Communication focuses on.In this Rapid Communication, Nb/InAs Josephson junctions of different width are studied in a four-terminal lead configuration within the 2DEG. This allows us to separately determine the transparencies of the InAs weak link and Nb/ InAs interfaces and identify an additional energy scale in the electronic spectrum of the hybrid SC terminals. We observe a very strong suppression of both the AR probability and supercurrent in weak magnetic fields of 4 and 100 mT for perpendicular and parallel orientations, r...

Granular particles vibrated in a fluid have been found to exhibit self-organization with attractive and repulsive interactions between the particles. These interactions have been attributed to the steady streaming flow around oscillating particles. Here we examine the steady streaming flow surrounding a vertically oscillating sphere using three dimensional particle tracking velocimetry. We present measurements of the flow with the sphere far from boundaries, close to the bottom wall of the tank, and near another oscillating sphere. The steady velocity field is found to disagree with available analytic calculations. When the sphere is oscillated near the bottom wall the entire topology of the flow changes, resulting in a larger repulsive region than expected. Previous experiments saw attraction between particles in the region where the flow around a single particle is repulsive. We conclude that advection in the streaming flow due to a single particle cannot explain the observed attractive and repulsive interactions, rather non-linear interactions between the flows around two or more spheres must be responsible.

We study nonlocal vortex transport in mesoscopic amorphous Nb 0.7 Ge 0.3 samples. A dc current I is passed through a wire connected via a perpendicular channel, of a length L =2-5 m, with a pair of voltage probes where a nonlocal response V nl ϰ I is measured. The maximum of R nl = V nl / I for a given temperature occurs at an L-independent magnetic field and is proportional to 1 / L. The results are interpreted in terms of the dissipative vortex motion along the channel driven by a remote current and can be understood in terms of a simple model. DOI: 10.1103/PhysRevB.74.220510 PACS number͑s͒: 74.78.Na, 74.78.Db, 74.25.Qt, 74.25.Fy In a pioneering work Giaver measured a magnetic-fluxtransformer effect in type-II superconductors. 1 He applied a magnetic field B perpendicularly to a sample comprising two superconducting sheets separated by a thin insulator, passed a current I through one of the superconductors, and measured a voltage developed over the other one-where no current was flowing. The induced voltage was a consequence of an electromagnetic coupling of vortices in the two layers. In their recent experiment Grigorieva et al. 2 demonstrated a complementary flux-transformer phenomenon associated with vortices. They produced mesoscopic amorphous MoGe structures of a double-cross shape, consisting of two parallel wires connected at a right angle by a channel of width w = 0.07-2 m and a length L = 0.5-12 m. In a perpendicular B and with I through one of the parallel wires a nonlocal voltage V nl appeared over the second, current-free wire. This novel, transversal flux-transformer effect originated in the in-plane vortex-vortex repulsion, which conveyed the driving force from the current-carrying wire to the vortices in the channel. The effect disappeared not only for L exceeding 6 -7 m but also for w larger than ϳ0.5-1 m. When w was sufficiently small the force on the vortices in the channel was transferred over many intervortex distances and, moreover, V nl was proportional to I. The efficiency of the transversal flux-transformer effect can be quantified by a nonlocal resistance R nl = V nl / I.In the experiment of Grigorieva et al. 2 the local mixedstate dissipation was characterized on separate mm-sized films, whereas V nl was measured by a low-frequency ac method during B sweeps at constant temperatures T. An ac method was used because V nl was in nV range-i.e., R nl Ͻ 5 m⍀-thus being too small for dc detection. In our work we focused on dc probing of the transversal fluxtransformer effect and measuring V nl and the local voltage V l on the same sample, which was possible in multiterminal amorphous ͑a − ͒Nb 0.7 Ge 0.3 structures of the geometry shown in the inset to Fig. 1. The weak pinning, characteristic of the a −Nb 0.7 Ge 0.3 material used, resulted in a dc-measurable V nl and R nl ϳ 1 ⍀ even at very low temperatures. The measured nonlocal resistance was hence two orders of magnitude larger than in Ref. 2. In this study we investigate the transversal fluxtransformer effect in samples of different...

We investigate nonlocal vortex motion in weakly pinning a-NbGe nanostructures, which is driven by a transport current I and remotely detected as a nonlocal voltage V{nl}. At a high I of a given polarity, V{nl} changes sign dramatically. This is followed by V{nl} becoming even in I, with the opposite sign at low and high temperatures T. These findings can be explained by a Nernst-like effect resulting from local electron overheating (low T), and a magnetization enhancement due to a nonequilibrium quasiparticle distribution that leads to a gap enhancement near the vortex core (high T).

In this follow up to our recent letter ͓F. Otto et al., Phys. Rev. Lett. 104, 027005 ͑2010͔͒, we present a more detailed account of the superconducting transversal flux transformer effect ͑TFTE͒ in amorphous ͑a-͒NbGe nanostructures in the regime of strong nonequilibrium in local vortex motion. Emphasis is put on the relation between the TFTE and local vortex dynamics, as the former turns out to be a reliable tool for determining the microscopic mechanisms behind the latter. By this method, a progression from electron heating at low temperatures T to the Larkin-Ovchinnikov effect close to the transition temperature T c is traced over a range 0.26Յ T / T c Յ 0.95. This is represented by a number of relevant parameters such as the vortex transport entropy related to the Nernst-type effect at low T and a nonequilibrium magnetization enhancement close to T c . At intermediate T, the Larkin-Ovchinnikov effect is at high currents modified by electron heating, which is clearly observed only in the TFTE.