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.
In the search for Majorana fermions in proximity-induced topological superconducting junctions, we happened to find a signature of same-spin triplet superconductivity which appears to dominate these elusive elementary excitations. Thin-film junctions and bilayers of the doped topological insulator and the s-wave superconductor NbN exhibit conductance spectra with coexisting prominent zero-bias and coherence peaks. Various tunneling models with different pair potentials have failed to fit our data, except for the triplet pair potential, which breaks time-reversal symmetry, that yielded reasonably good fits. This provides supporting evidence for proximity-induced triplet superconductivity in the layer near the interface with the NbN film.
Measurements of conductance spectra in a superconductor -topological insulator -normal metal thin film junctions of NbN-Bi 2 Se 3 -Au are reported. Junctions with ex-situ and in-situ prepared NbN − Bi 2 Se 3 interfaces were studied. At low temperatures, all the ex-situ junctions showed coherence peaks in their conductance spectra, but imbedded robust zero bias conductance peaks were observed only in junctions with a metallic or a metal to insulator transition below T c of the NbN electrode. The in-situ junctions which had about two orders of magnitude lower resistance at low temperatures, generally showed flat conductance spectra at low bias, with no coherence or broad Andreev peaks, since the critical current of the NbN electrode was reached first, at voltage bias below the energy gap of the superconductor. A weak zero bias conductance peak however, was observed in one of these junctions. We conclude that significant tunneling barriers, as in the ex-situ prepared junctions, are essential for the observation of coherence peaks and the zero energy bound states. The later seem to originate in the Bi 2 Se 3 -NbN interface, as they are absent in reference Au-NbN junctions without the topological layer sandwiched in between.
The long range proximity effect in high-Tc c-axis Josephson junctions with a high-Tc barrier of lower Tc is still a puzzling phenomenon. It leads to supercurrents in junctions with much thicker barriers than would be allowed by the conventional proximity effect. Here we measured the T − x (Temperature-doping level) phase diagram of the barrier coherence length ξN(T, x), and found an enhancement of ξN at moderate under-doping and high temperatures. This indicates that a possible origin of the long range proximity effect in the cuprate barrier is the conjectured pre-formed pairs in the pseudogap regime, which increase the length scale over which superconducting correlations survive in the seemingly normal barrier. In more details, we measured the supercurrents Ic of Superconducting - Normal - Superconducting SNS c-axis junctions, where S was optimally doped Y Ba2Cu3O7−δ below Tc (90 K) and N was La2−xSrxCuO4 above its Tc (<25 K) but in the pseudogap regime. From the exponential decay of Ic(T) ∝ exp[−d/ξN(T)], where d is the barrier thickness, the ξN(T) values were extracted. By repeating these measurements for different barrier doping levels x, the whole phase diagram of ξN(T, x) was obtained.
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