Wenlong Yu

Ultrafast carrier and phonon dynamics in Bi2Se3 crystals

in this material, 11 particularly the electron‐electron, electron‐phonon, and phonon‐phonon interactions. In this letter, we report the ultrafast time-resolved optical spectroscopy study of Bi2Se3 crystals in both the time domain and the energy domain. Our measurements reveal three underlying relaxation processes in the transient response of Bi2Se3, each associated with different physical mechanisms. It is also shown that the relative strength of these processes is sensitive to air exposure of the samples. The observed charge trapping and air doping effects are likely due to the presence of Se vacancies, a major issue material scientists working to use the properties of Bi2Se3 will face in the near term. The Bi2Se3 single crystals studied in this work were synthesized via the Bridgman method at Purdue University and Fudan University. During crystal growth, the mixture of high purity elements was first deoxidized and purified by multiple vacuum distillations, and then heated to 850‐900 °C for 15 h, followed by a slow cool down under a controlled pressure of Se to compensate for possible Se vacancies. Afterwards, the samples were zone refined at a speed of 0.5‐1.5 mm/hour with a linear temperature gradient set to 4‐5 °C /cm, until a temperature of 670 °C was reached. The as-grown Bi2Se3 crystals from both groups are naturally n-doped due to remnant Se vacancies. 4 Hall mea

Magnetoplasmons in Quasi-Neutral Epitaxial Graphene Nanoribbons.

We present an infrared transmission spectroscopy study of the inter-Landau-level excitations in quasineutral epitaxial graphene nanoribbon arrays. We observed a substantial deviation in energy of the L(0(-1)) → L(1(0)) transition from the characteristic square root magnetic-field dependence of two-dimensional graphene. This deviation arises from the formation of an upper-hybrid mode between the Landau-level transition and the plasmon resonance. In the quantum regime, the hybrid mode exhibits a distinct dispersion relation, markedly different from that expected for conventional two-dimensional systems and highly doped graphene.

Probing terahertz surface plasmon waves in graphene structures

Epitaxial graphene mesas and ribbons are investigated using terahertz (THz) near-field microscopy to probe surface plasmon excitation and THz transmission properties on the sub-wavelength scale. The THz near-field images show variation of graphene properties on a scale smaller than the wavelength, and excitation of THz surface waves occurring at graphene edges, similar to that observed at metallic edges. The Fresnel reflection at the substrate SiC/air interface is also found to be altered by the presence of graphene ribbon arrays, leading to either reduced or enhanced transmission of the THz wave depending on the wave polarization and the ribbon width.

Giant supercurrent states in a superconductor-InAs/GaSb-superconductor junction

Superconductivity in topological materials has attracted a great deal of interest in both electron physics and material sciences since the theoretical predictions that Majorana fermions can be realized in topological superconductors. Topological superconductivity could be realized in a type II, band-inverted, InAs/GaSb quantum well if it is in proximity to a conventional superconductor. Here, we report observations of the proximity effect induced giant supercurrent states in an InAs/GaSb bilayer system that is sandwiched between two superconducting tantalum electrodes to form a superconductor-InAs/GaSb-superconductor junction. Electron transport results show that the supercurrent states can be preserved in a surprisingly large temperature-magnetic field (T – H) parameter space. In addition, the evolution of differential resistance in T and H reveals an interesting superconducting gap structure.

McMillan-Rowell like oscillations in a superconductor-InAs/GaSb-superconductor junction

We have fabricated a superconductor (Ta)-InAs/GaSb bilayer-superconductor (Ta) junction device that has a long mean free path and can preserve the wavelike properties of particles (electrons and holes) inside the junction. Differential conductance measurements were carried out at low temperatures in this device, and McMillan-Rowell like oscillations (MROs) were observed. Surprisingly, a much larger Fermi velocity, compared to that from Shubnikov-de Haas oscillations, was obtained from the frequency of MROs. Possible mechanisms are discussed for this discrepancy.

THz near-field microscopy of graphene nano-ribbon arrays

We use THz near-field microscopy with broadband THz pulses to image graphene nano-ribbons and to probe surface plasmon excitation and uniformity of graphene response. 3.3nm and 7nm thick graphene layers induce (~10%) absorption from 0.5-2.5THz.

Magnetoinfrared spectroscopic study of thin Bi 2 Te 3 single crystals

Ultrathin Bi

Magnetoinfrared spectroscopic study of thin Bi2Te3 single crystals

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Magneto-Infrared Spectroscopic Study of Ultrathin Bi

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