We report on transport measurements of an InAs nanowire coupled to niobium nitride leads at high magnetic fields. We observe a zero-bias anomaly (ZBA) in the differential conductance of the nanowire for certain ranges of magnetic field and chemical potential. The ZBA can oscillate in width with either the magnetic field or chemical potential; it can even split and re-form. We discuss how our results relate to recent predictions of hybridizing Majorana fermions in semiconducting nanowires, while considering more mundane explanations.

Coulomb drag is a process whereby the repulsive interactions between electrons in spatially separated conductors enable a current flowing in one of the conductors to induce a voltage drop in the other. If the second conductor is part of a closed circuit, a net current will flow in that circuit. The drag current is typically much smaller than the drive current owing to the heavy screening of the Coulomb interaction. There are, however, rare situations in which strong electronic correlations exist between the two conductors. For example, double quantum well systems can support exciton condensates, which consist of electrons in one well tightly bound to holes in the other. 'Perfect' drag is therefore expected; a steady transport current of electrons driven through one quantum well should be accompanied by an equal current of holes in the other. Here we demonstrate this effect, taking care to ensure that the electron-hole pairs dominate the transport and that tunnelling of charge between the quantum wells, which can readily compromise drag measurements, is negligible. We note that, from an electrical engineering perspective, perfect Coulomb drag is analogous to an electrical transformer that functions at zero frequency.

Josephson junctions with topological insulator weak links can host low-energy Andreevbound states giving rise to a current-phase relation that deviates from sinusoidal behaviour. Of particular interest are zero-energy Majorana-bound states that form at a phase difference of p. Here we report on interferometry studies of Josephson junctions and superconducting quantum interference devices (SQUIDs) incorporating topological insulator weak links. We find that the nodes in single-junction diffraction patterns and SQUID oscillations are lifted and independent of chemical potential. At high temperatures, the SQUID oscillations revert to conventional behaviour, ruling out asymmetry. The node-lifting of the SQUID oscillations is consistent with low-energy Andreev-bound states exhibiting a nonsinusoidal current-phase relation, co-existing with states possessing a conventional sinusoidal current-phase relation. However, the finite nodal currents in the single-junction diffraction pattern suggest an anomalous contribution to the supercurrent possibly carried by Majorana-bound states, although we also consider the possibility of inhomogeneity.

We demonstrate that counterflowing electrical currents can move through the bulk of the excitonic quantized Hall phase found in bilayer two-dimensional electron systems (2DES) even as charged excitations cannot. These counterflowing currents are transported by neutral excitons which are emitted and absorbed at the inner and outer boundaries of an annular 2DES via Andreev reflection.PACS numbers: 73.43.Nq, 71.35.Lk Bose-Einstein condensation of excitons (electron-hole pairs) was predicted [1-3] nearly a half century ago, in the aftermath of the Bardeen-Cooper-Schrieffer theory of superconductivity. Surprisingly, the first compelling evidence for the phenomenon came from measurements on bilayer two-dimensional electron systems (2DES) in semiconductor heterostructures at high magnetic fields in the quantum Hall effect (QHE) regime [4]. In analogy with the Cooper pair condensate in a conventional superconductor, an exciton condensate is separated from its charged quasiparticle excitations by an energy gap. However, for the quantum Hall exciton condensate this is only true in the bulk of the 2D system. At the edge of the system, where any interface with normal metal contacts must lie, topologically-protected gapless charged excitations are always present. These gapless excitations complicate the interpretation of transport measurements on bilayer 2DESs supporting an exciton condensate. Indeed, while much dramatic evidence for exciton transport in such systems has been reported [5-9] it remains essentially indirect and unable to unambiguously demonstrate that excitons are moving through the bulk of the 2D system. In this paper we report just such a demonstration.The exciton condensate in bilayer quantum Hall systems occurs at Landau level filling factor ν T = n T /(eB/h) = 1, where n T is the total density of electrons in the bilayer and eB/h is the degeneracy of a single spin-resolved Landau level created by the applied magnetic field B. Numerous experiments have shown that when the interlayer separation d in the bilayer is less than a critical value (d 1.8 ℓ with ℓ = ( /eB) 1/2 the magnetic length) several dramatic transport anomalies appear at low temperatures. Prominent among these are a Josephson-like enhancement of interlayer tunneling [5] and the vanishing of the Hall resistance when equal but oppositely directed electrical currents flow through the two layers [7][8][9]. The great majority of these previous transport experiments were performed using simplyconnected geometries in which all ohmic contacts to the 2DES are on the single outside edge of the sample. For example, in the counterflow experiments of Kellogg et al.[7] which demonstrated the vanishing of the Hall effect, a Hall bar geometry was employed. These and related experiments [6-9] found a natural interpretation in the exciton condensation model: counterflowing electrical currents in the two layers might be realized via the unidirectional flow of interlayer excitons. Being neutral, these excitons experience no Lorentz force and hence exhibit no...

Using Coulomb drag as a probe, we explore the excitonic phase transition in quantum Hall bilayers at T ¼ 1 as a function of Zeeman energy E Z . The critical layer separation ðd='Þ c for exciton condensation initially increases rapidly with E Z , but then reaches a maximum and begins a gentle decline. At high E Z , where both the excitonic phase at small d=' and the compressible phase at large d=' are fully spin polarized, we find that the width of the transition, as a function of d=', is much larger than at small E Z and persists in the limit of zero temperature. We discuss these results in the context of two models in which the system contains a mixture of the two fluids. DOI: 10.1103/PhysRevLett.104.016801 PACS numbers: 73.43.Nq, 71.35.Lk Following the development of the Bardeen-CooperSchrieffer theory of superconductivity, physicists [1][2][3][4][5] speculated that excitons in semiconductors, conduction band electrons bound to valence band holes, could undergo a similar pairing transition to a collective state with macroscopic quantum phase coherence. Strong evidence of exciton condensation was ultimately found in a surprising place: double layer two-dimensional electron systems at high perpendicular magnetic field B ? [6][7][8][9][10]. In this, the quantum Hall effect regime, excitons consisting of electrons in the lowest Landau level (LLL) of one layer bound to holes in the LLL of the other layer, condense into a coherent collective state whenever the temperature and layer separation are small enough, and the total density n T of electrons in the double layer system equals the degeneracy eB ? =h of one spin-resolved Landau level [11][12][13][14]. This collective electronic state exhibits several dramatic electrical transport properties including Josephson-like interlayer tunneling [6], quantized Hall drag [7], and vanishing Hall resistance [8-10] when currents are driven in opposition in the two layers.Exciton condensation in bilayer quantum Hall systems at total Landau level occupancy T n T =ðeB ? =hÞ ¼ 1 reflects a spontaneously broken U(1) symmetry in which electrons are no longer confined to one layer or the other but instead reside in coherent linear combinations of the two. This interlayer phase coherence develops only when the effective interlayer separation d=' (with d the centerto-center quantum well separation and ' ¼ ð@=eB ? Þ 1=2 the magnetic length) is less than a critical value, ðd='Þ c . At large d=' the bilayer system behaves qualitatively like two independent two-dimensional electron systems (2DESs). The nature of the quantum phase transition separating these two very different bilayer states remains poorly understood. In particular, while essentially all theoretical work concerning the transition makes the simplifying assumption that both phases are fully spin polarized [11][12][13][14][15][16][17][18][19][20], recent experiments [21,22] convincingly demonstrate that this is not the case in typical samples. Instead, these experiments prove that the two phases have different spin polari...