We report point contact measurements in high quality single crystals of Cu0.2Bi2Se3. We observe three different kinds of spectra: 1) Andreev reflection spectra, from which we infer a superconducting gap size of 0.6mV. 2) Spectra with a large gap which closes above Tc at about 10K and 3) Tunnelinglike spectra with Zero Bias Conductance Peaks (ZBCP). These tunneling spectra show a very large gap of about 2meV (2∆/K b Tc ∼14).
Point contact conductance measurements on topological Bi 2 Te 2 Se and Bi 2 Se 3 films reveal a signature of superconductivity below 2-3 K. In particular, critical current dips and a robust zero-bias conductance peak are observed. The latter suggests the presence of zero-energy bound states that could be assigned to Majorana fermions in an unconventional topological superconductor. We attribute these observations to proximity-induced local superconductivity in the films by small amounts of superconducting Bi inclusions or segregation to the surface, and provide supportive evidence for these effects.
The presence of optical polarization anisotropies, such as Faraday/Kerr effects, linear birefringence, and magnetoelectric birefringence are evidence for broken symmetry states of matter. The recent discovery of a Kerr effect using near-IR light in the pseudogap phase of the cuprates can be regarded as a strong evidence for a spontaneous symmetry breaking and the existence of an anomalous long-range ordered state. In this work we present a high precision study of the polarimetry properties of the cuprates in the THz regime. While no Faraday effect was found in this frequency range to the limits of our experimental uncertainty (1.3 milli-radian or 0.07• ), a small but significant polarization rotation was detected that derives from an anomalous linear dichroism. In YBa2Cu3Oy the effect has a temperature onset that mirrors the pseudogap temperature T * and is enhanced in magnitude in underdoped samples. In x = 1/8 La2−xBaxCuO4, the effect onsets above room temperature, but shows a dramatic enhancement near a temperature scale known to be associated with spin and charge ordered states. These features are consistent with a loss of both C4 rotation and mirror symmetry in the electronic structure of the CuO2 planes in the pseudogap state.PACS numbers: 74.25. Gz, 74.72.Kf, An extensive research effort has been carried out over the last two decades on defining the role and origin of the pseudogap phase in the cuprates. The pseudogap, a regime of the phase diagram generally located at higher temperatures than the superconducting state, is characterized by an energy gap in the density of states at the Fermi level as well as various transport and magnetic anomalies. Whether this gap is related to superconductivity or competes with it, and whether it realizes an additional long-range ordered state is controversial [1,2]. Characterization of a stable static order with true broken symmetry in the pseudogap regime could solve the mystery surrounding its origin.Optical polarization anisotropies, such as Faraday/Kerr effects, gyrotropic rotation, linear birefringence, and magneto-electric birefringence can be sensitive tools for the detection of broken symmetry states of matter. For instance, materials with anti-symmetric off-diagonal components in the dielectric tensor can rotate the plane of polarization of linearly polarized light. Such tensor elements are only allowed in a material that breaks either time-reversal or inversion and mirror symmetries. Such effects are referred to as circular, since the eigenmodes of their transmission or reflection matrices are left and right circular polarizations. The most common such "circular" effects are magneto-optical ones arising from time-reversal symmetry breaking from magnetic moments aligned either by applying external magnetic field or spontaneous magnetization. Another circular effect arises in so-called gyrotropic ordered materials that breaks all mirror symmetries. Spiral structures and cholesteric textures have such optical activity and can rotate polarization in the absence o...
Long-ranged superconductor proximity effects recently found in superconductorferromagnetic (S-F) systems are generally attributed to the formation of triplet-pairing correlations due to various forms of magnetic inhomogeneities at the S-F interface. In order to investigate this conjecture within a single F layer coupled to a superconductor, we performed scanning tunneling spectroscopy on bilayers of La2/3Ca1/3MnO3 (LCMO) ferromagnetic thin-films grown on high temperature superconducting films of YBa2Cu3O7- (YBCO) or Pr1.85Ca0.15CuO4 (PCCO) under various magnetic fields. We find a strong correlation between the magnitude of superconductor-related spectral features measured on the LCMO layer and the degree of magnetic inhomogeneity controlled by the external magnetic field. This corroborates theoretical predictions regarding the role played by magnetic inhomogeneities in inducing triplet-pairing at S-F interfaces.
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