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Despite the broadband response, limited optical absorption at a particular wavelength hinders the development of optoelectronics based on Dirac fermions. Heterostructures of graphene and various semiconductors have been explored for this purpose, while non-ideal interfaces often limit the performance. The topological insulator is a natural hybrid system, with the surface states hosting high-mobility Dirac fermions and the small-bandgap semiconducting bulk state strongly absorbing light. In this work, we show a large photocurrent response from a field effect transistor device based on intrinsic topological insulator Sn-Bi1.1Sb0.9Te2S. The photocurrent response is non-volatile and sensitively depends on the initial Fermi energy of the surface state, and it can be erased by controlling the gate voltage. Our observations can be explained with a remote photo-doping mechanism, in which the light excites the defects in the bulk and frees the localized carriers to the surface state. This photodoping modulates the surface state conductivity without compromising the mobility, and it also significantly modify the quantum Hall effect of the surface state. Our work thus illustrates a route to reversibly manipulate the surface states through optical excitation, shedding light into utilizing topological surface states for quantum optoelectronics.

InN superconductivity is very special among III-V semiconductors, because other III-V semiconductor (like GaAs, GaN, InP, InAs etc.) usually lacks strong covalent bonding and seldom shows superconductivity at low-temperature. In this paper, via current-voltage(I-V) measurement, we probe the superconducting phase transitions in InN. The possible connection with those chemical-unstable phase separated inclusions, like metallic indium or In2O3, was removed by HCl acid etching. It finds InN samples can show different phase transition behaviors. The vortex-glass (VG) to liquid transition, which is typical in type-II superconductors, is observed in the sample with large InN grain size. In contrast, the small grain-sized sample's superconducting properties are sensitive to acid etching, shows a transition into a non-zero resistance state at the limit of temperature approaches zero. Our work suggests that the grain size and inter-grain coupling may be two key factors for realizing InN superconductivity. InN superconductivity can become robust and chemical stable if the grain size and inter-grain coupling both are large enough., Comment: 6 pages, 5 figures

The scaling physics of quantum Hall transport in optimized topological insulators with a plateau precision of ~1/1000 e2/h is considered. Two exponential scaling regimes are observed in temperature-dependent transport dissipation, one of which accords with thermal activation behavior with a gap of 2.8 meV (> 20 K), the other being attributed to variable range hopping (1-20 K). Magnetic field-driven plateau-to-plateau transition gives scaling relations of (dR$_{xy}$/dB)$^{max}$ \propto T$^{-\kappa}$ and \DeltaB$^{-1}$ \propto T$^{-\kappa}$ with a consistent exponent of \kappa ~ 0.2, which is half the universal value for a conventional two-dimensional electron gas. This is evidence of percolation assisted by quantum tunneling, and reveals the dominance of electron-electron interaction of the topological surface states.

Most of our current knowledge on condensed matters contains a default assumption: the matters are neutral charged. On the other hand, the two-dimension(2D) vortex-Coulomb gas charge analogy is a very successful theoretical tool in explaining superfluid, type-II superconductor and fractional quantum Hall effect(FQHE), because the interaction among vortices show similarity with 2D Coulomb potential. Here, by breaking the 'neutral charged' assumption, we suggest the positively charged 2D semiconductor system can possess non-trivial particle-like charge centers(called 'quasihole'), which is the charge version of 'vortex'. Using the hypernetted-chain (HNC) approximation, the structure of quasihole is elucidated. Numerical calculations show that the quasihole can be projected onto another 2D layer, producing an electric field configuration characterized by 3/2 topological charge., Comment: 6 pages, 3 figures

The screening of an individual low-dimensional object can be strongly influenced by the objects nearby. We propose that such environment's influence can be absorbed into an effective polarizability, instead of its intrinsic polarizability. Using a toy system consists of two spatially separated 2DEG layers gas an example, this picture is analytically deduced via a multi-component RPA theoretical method. We show that the resultant effective polarizability of backside layer is just the dielectric function describing the system's collective plasmon excitations. Furthermore, several interesting topics are discussed, e.g. the mutual modulation of Friedel oscillation, the ultimate screening limit imposed by metal layer, smoothing potential inhomogeneity in back screening configuration., Comment: 6 pages, 4 figures

Charge-density-wave (CDW) is a modulation of the conduction electron density in a conductor. Under low temperature, it can spontaneously happen in some compounds that consist of anisotropic one-dimensional crystal structures, via a strong electron-lattice interaction mechanism. Many celebrated phenomena, e.g. non-linear transport, narrow-band noise, mode-locking5 and chaos under AC voltage, etc., have been reported in CDW. However, evaluating the application potential of CDW conductors has been hampered by the inconvenient shapes and sizes of CDW single crystals. Although modern fabrication technology can partly resolve those troubles, (for example, cleaved film and nanowire NbSe3 device), the imperfections induced by fabrication that corrupt measured properties are not easy to control and estimate. Here we demonstrate a convenient CDW conductor fabricated by semiconductor double quantum wells (DQW) in a field-effect transistor (FET) configuration: a modulated electron density in one QW resulted from the charged QW nearby. The electric field dependent depinning transport characteristic of CDW is clearly present. This "artificial" CDW, capable of integrating with semiconductor industry, may give fresh impetus to revive the interests in CDW.

An electrically isolated quantum well (QW) island can be positively charged by incoming infrared photon, because its electrons absorb photon energy via intersubband transition and acquire enough energy to escape it. This process has been used in a double QW photon-detector. Here, we present the observation of so-called negative photon-response in such detector. Its origin is clarified to be an electron mobility reduction phenomenon resulted from the photon induced charges., Comment: 8 pages, 4 figures