The Josephson effect describes the generic appearance of a supercurrent in a weak link between two superconductors. Its exact physical nature deeply influences the properties of the supercurrent. In recent years, considerable efforts have focused on the coupling of superconductors to the surface states of a three-dimensional topological insulator. In such a material, an unconventional induced p-wave superconductivity should occur, with a doublet of topologically protected gapless Andreev bound states, whose energies vary 4π-periodically with the superconducting phase difference across the junction. In this article, we report the observation of an anomalous response to rf irradiation in a Josephson junction made of a HgTe weak link. The response is understood as due to a 4π-periodic contribution to the supercurrent, and its amplitude is compatible with the expected contribution of a gapless Andreev doublet. Our work opens the way to more elaborate experiments to investigate the induced superconductivity in a three-dimensional insulator.

No abstract

We report studies of cyclotron resonance in monolayer graphene. Cyclotron resonance is detected using the photoconductive response of the sample for several different Landau level occupancies. The experiments measure an electron velocity at the K-(Dirac) point of c * K = 1.093 x 10 6 ms −1 which is substantially larger than in thicker graphitic systems. In addition we observe a significant asymmetry between the electron and hole bands, leading to a difference in the electron and hole velocities of 5% by energies of 125 meV away from the Dirac point. The observation of two dimensional electronic systems in monolayer graphene 1 , where the electrons behave as Dirac Fermions and show a variety of novel quantum Hall effects 2 , 3 , 4 , has led to an explosion of interest in this system. As well as new basic science, the exceptionally high electron velocities also mean that graphene has considerable potential for applications in high speed electronics 5 . The basis for this behaviour is the nearly linear dispersion of the energy bands close to the K point, where the dispersion relations cross with the form E = ±c * h k, where c * is the electron velocity. This has been predicted for over 50 years 6 , but has only been measured recently for bulk graphite 7 and ultrathin graphite layers 8 , while the first direct absorption measurements for monolayer graphene have just been reported 9 . We describe here a photoconuctance study of cyclotron resonance in a monolayer of graphene in which the application of a magnetic field leads to the formation of Landau levels given by 10where |N | is the Landau quantum index and B is the magnetic field. This allows us to make a precise measurement of the electron velocity and to examine deviations from exact linear behaviour which show that the electron and hole-like parts of the band structure have significantly different masses and that the velocity is significantly larger than for thicker graphitic material. The experiment studies the photoconductive response from a multiply contacted single monolayer sample of graphene, which was prepared using the techniques which have been described earlier 1,2 . The graphene films were deposited by micromechanical cleavage of graphite with multi-terminal devices produced by conventional microfabrication, with a typical sample displayed in figure 1 (a). Shubnikov-de Haas oscillations were first studied at 1.5K to establish the carrier densities as a function of gate voltage and to ensure that the film studied was a single monolayer of graphene, since bilayers and thicker films are known to have a completely different dispersion relation 11 , 12 , 13 . Cyclotron resonance was measured by detecting the modulation of the conductivity of the samples produced by chopped infrared radiation from a CO 2 laser operating between 9.2 and 10.8 µm. The sample was illuminated normally with unpolarized light parallel to the magnetic field in the Faraday geometry. Typical power densities were ∼3 x 10 4 Wm −2 , corresponding to a total power incident on the sampl...

The coupling of a quantum dot with a BCS-type superconducting reservoir results in an intriguing system where low energy physics is governed by the interplay of two distinct phases, singlet and doublet. In this Letter we show that the spectrum of Andreev energy levels, which capture the properties of the two phases, can be detected in transport measurements with a quantum dot strongly coupled to a superconducting lead and weakly coupled to a normal metal lead. We observe phase transitions between BCS singlet and degenerate magnetic doublet states when the quantum dot chemical potential is tuned with an electrostatic gate, in good qualitative agreement with numerical renormalization group calculations.