Topological insulators are characterized by an insulating bulk with a finite band gap and conducting edge or surface states, where charge carriers are protected against backscattering. These states give rise to the quantum spin Hall effect without an external magnetic field, where electrons with opposite spins have opposite momentum at a given edge. The surface energy spectrum of a threedimensional topological insulator is made up by an odd number of Dirac cones with the spin locked to the momentum. The long-sought yet elusive Majorana fermion is predicted to arise from a combination of a superconductor and a topological insulator. An essential step in the hunt for this emergent particle is the unequivocal observation of supercurrent in a topological phase. Here, we present the first measurement of a Josephson supercurrent through a topological insulator. Direct evidence for Josephson supercurrents in superconductor (Nb) - topological insulator (Bi2Te3) - superconductor e-beam fabricated junctions is provided by the observation of clear Shapiro steps under microwave irradiation, and a Fraunhofer-type dependence of the critical current on magnetic field. The dependence of the critical current on temperature and length shows that the junctions are in the ballistic limit. Shubnikov-de Haas oscillations in magnetic fields up to 30 T reveal a topologically non-trivial two-dimensional surface state. We argue that the ballistic Josephson current is hosted by this surface state despite the fact that the normal state transport is dominated by diffusive bulk conductivity. The lateral Nb-Bi2Te3-Nb junctions hence provide prospects for the realization of devices supporting Majorana fermions

The discovery of materials with improved functionality can be accelerated by rational material design.1 Heusler compounds with tunable magnetic sublattices allow to implement this concept to achieve novel magnetic properties. 2 Here, we have designed a family of Heusler alloys with a compensated ferrimagnetic state. In the vicinity of the compensation composition in Mn-Pt-Ga, a giant exchange bias (EB) of more than 3 T and a similarly large coercivity are established. The large exchange anisotropy originates from the exchange interaction between the compensated host and ferrimagnetic clusters that arise from intrinsic anti-site disorder. We demonstrate the applicability of our design concept on a second material, Mn-1 Fe-Ga, with a magnetic transition above room temperature, exemplifying the universality of the concept and the feasibility of room-temperature applications. Our study points to a new direction for novel magneto-electronic devices. At the same time it suggests a new route for realizing rare-earth free exchange-biased hard magnets, where the second quadrant magnetization can be stabilized by the exchange bias.Exchange bias corresponds to a shift of the hysteresis loop of a ferromagnet along the magnetic field axis due to interfacial exchange coupling with an adjacent antiferromagnet. fully compensated magnet with a compensation point for a particular Mn/Pt ratio. This design scheme is schematically depicted in Fig. 1. From first-principles calculations, it follows that the critical composition with the zero magnetization is achieved in the solid solution Mn 3−x Pt x Ga at a Pt content of about x 0 = 0.59, which is in good agreement with the experimental findings. On optimizing the Mn/Pt ratio in Mn 3−x Pt x Ga, we always find a small lack of compensation in the material, due to the formation of FM clusters by anti-site disorder. This leads to an exceptionally large bulk EB and a large coercivity. In contrast to an artificial antiferromagnet, which is a thin film structure composed of two ferromagnetic layers separated by a coupling layer 9 , here we combine two isostructural ferrimagnetic compounds Mn 3 Ga and Mn 2 PtGa to obtain an intrinsically anisotropic compensated magnetic state on an atomic scale in a bulk material.In order to characterize the magnetic properties of Mn 3−x Pt x Ga we have measured the temperature dependence of the low field magnetization, M(T ). We find a systematic increase in the 4 ferrimagnetic Néel temperature (T N ) with increasing Mn content as shown in Fig. 2. The irreversibility between the ZFC and FC curves reflects the appearance of coercivity. We suggest that FM clusters embedded in the compensated host are the source of this irreversibility. NMR measurements confirm that these clusters originate from random swaps between Pt in the Mn-Pt planes and Mn in the Mn-Ga planes ( Supplementary Fig. 2). The irreversibility between ZFC and FC M(T ) curves increases with increasing magnetic field demonstrating that cooling in higher fields helps the FM clusters to grow in siz...

We have performed high field magnetotransport measurements to investigate the interface electron gas in a high mobility SrTiO3/SrCuO2/LaAlO3/SrTiO3 heterostructure. Shubnikov-de Haas oscillations reveal several 2D conduction subbands with carrier effective masses of 0.9me and 2me, quantum mobilities of order 2000 cm2/V s, and band edges only a few millielectronvolts below the Fermi energy. Measurements in tilted magnetic fields confirm the 2D character of the electron gas, and show evidence of inter-subband scattering.

Shubnikov-de Haas oscillations are observed in Bi 2 Se 3 flakes with high carrier concentration and low bulk mobility. These oscillations probe the protected surface states and enable us to extract their carrier concentration, effective mass, and Dingle temperature. The Fermi momentum obtained is in agreement with angle-resolved photoemission spectroscopy measurements performed on crystals from the same batch. We study the behavior of the Berry phase as a function of magnetic fields. The standard theoretical considerations fail to explain the observed behavior.

The low-temperature resistance of a conducting LaAlO 3 /SrTiO 3 interface with a 10 nm thick LaAlO 3 film decreases by more than 50% after illumination with light of energy higher than the SrTiO 3 band-gap. We explain our observations by optical excitation of an additional high mobility electron channel, which is spatially separated from the photo-excited holes. After illumination, we measure a strongly non-linear Hall resistance which is governed by the concentration and mobility of the photo-excited carriers. This can be explained within a two-carrier model where illumination creates a high-mobility electron channel in addition to a lowmobility electron channel which exists before illumination. 9-11 Several mechanisms are suggested to describe the origin of conductivity at the LAO/STO interface.12-17 However, the relative contribution of each mechanism strongly depends on the LAO film thickness and on the LAO growth conditions such as substrate temperature, oxygen partial pressure and the post annealing treatment.18 In particular, growing 5-10 layers of LAO on STO yields a metallic interface with relatively high mobility and low electron concentration, 19,20 whereas growing 26 LAO layers with the same conditions results in a low mobility, high concentration electron system with interesting magnetic properties.3 Tuning the transport properties at such a complex oxide interface by modulating the carrier density with light can both contribute to the understanding of its physics and open new pathways towards oxide-based optoelectronic device applications. It has been shown previously that interface conductivity in oxide heterostructures can be tuned by light or by an electric field. 21-23In this Letter we report our investigation of the interface of a LAO/STO sample with 26 monolayers of LAO, using low-temperature (4.2 K) magnetotransport experiments under selective illumination. Illuminating the sample with UV light of energy greater than the STO band gap results in a sharp and persistent decrease of electrical resistance. Using Hall effect measurements, we show that before illumination there is a single, low mobility electron conduction band, and that the resistance drop on illumination can be explained by the creation of a parallel conducting channel containing optically excited a) Electronic mail: u.zeitler@science.ru.nl high mobility electrons.Our sample was grown by pulsed laser deposition and has a 10 nm thick (26 unit cells) LAO film on a TiO 2 -terminated single crystal STO [001] substrate (treatment described in Ref. 24). The LAO film was deposited at a substrate temperature of 850• C and an oxygen pressure of 2 × 10 −3 mbar, using a single-crystal LaAlO 3 target. The growth of the LAO film was monitored using in situ reflection high-energy electron diffraction. After the growth, samples were cooled to room temperature in the deposition pressure.The sample was mounted on a ceramic chip carrier and electrical contacts were made with an ultrasonic wirebonder, using aluminium wires. The magnetoresistance an...