A decades-long fast-rise-exponential-decay flare in low-luminosity AGN NGC 7213

We analysed the four-decades-long X-ray light curve of the low-luminosity active galactic nucleus (LLAGN) NGC 7213 and discovered a fast-rise-exponential-decay (FRED) pattern, i.e. the X-ray luminosity increased by a factor of $\approx 4$ within 200d, and then decreased exponentially with an $e$-folding time $\approx 8116$d ($\approx 22.2$ yr). For the theoretical understanding of the observations, we examined three variability models proposed in the literature: the thermal-viscous disc instability model, the radiation pressure instability model, and the tidal disruption event (TDE) model. We find that a delayed tidal disruption of a main-sequence star is most favourable; either the thermal-viscous disk instability model or radiation pressure instability model fails to explain some key properties observed, thus we argue them unlikely.Comment: Accepted for publication in MNRAS, updated version after proof correction

Superfluidity and effective mass of magnetoexcitons in topological insulator bilayers: Effect of inter-Landau-level Coulomb interaction

The effective mass and superfluidity-normal phase transition temperature of magnetoexcitons in topological insulator bilayers are theoretically investigated. The intra-Landau-level Coulomb interaction is treated perturbatively, from which the effective magnetoexciton mass is analytically discussed. The inclusion of inter-Landau-level Coulomb interaction by more exact numerical diagonalization of the Hamiltonian brings out important modifications to magnetoexciton properties, which are specially characterized by prominent reduction in the magnetoexciton effective mass and promotion in the superfluidity-normal phase transition temperature at a wide range of external parameters.Comment: 5.6 EPL pages, 4 figure

Probing crossover from analogous weak antilocalization to localization by an Aharonov-Bohm interferometer on topological insulator surface

We propose a scanning tunneling microscopy Aharonov-Bohm (AB) interferometer on the surface of a topological insulator (TI) to probe the crossover from analogous weak antilocalization (WAL) to weak localization (WL) phenomenon via the AB oscillations in spin-resolved local density of states (LDOS). Based on our analytical and numerical results, we show that with increasing the energy gap of TI surface states, the $\Phi_{0}/2$=$hc/2e$ periodic AB oscillations in spin-resolved LDOS gradually transit into the $\Phi_{0}$ periodic oscillations.Comment: 4.2 APL pages, 2 figure

Fractional quantum Hall effect of topological surface states under a strong tilted magnetic field

The fractional quantum Hall effect (FQHE) of topological surface-state particles under a tilted strong magnetic field is theoretically studied by using the exact diagonalization method. The Haldane's pseudopotentials for the Coulomb interaction are analytically obtained. The results show that by increasing the in-plane component of the tilted magnetic field, the FQHE state at $n$=0 Landau level (LL) becomes more stable, while the stabilities of $n$=$\pm1$ LLs become weaker. Moreover, we find that the excitation gaps of the $\nu=1/3$ FQHE states increase as the tilt angle is increased.Comment: 4.2 pages, 4 figure

Aharonov-Bohm oscillations in the local density of topological surface states

We study Aharonov-Bohm (AB) oscillations in the local density of states (LDOS) for topological insulator (TI) and conventional metal Au(111) surfaces with spin-orbit interaction, which can be probed by spin-polarized scanning tunneling microscopy. We show that the spacial AB oscillatory period in the total LDOS is a flux quantum $\Phi_{0}\mathtt{=}hc/e$ (weak localization) in both systems. Remarkably, an analogous weak antilocalization with $\Phi_{0}/2$ periodic spacial AB oscillations in spin components of LDOS for TI surface is observed, while it is absent in Au(111).Comment: 4 APL pages, 3 figure

Quantum Corrals and Quantum Mirages on the Surface of a Topological Insulator

We study quantum corrals on the surface of a topological insulator (TI). Different resonance states induced by nonmagnetic (NM), antiferromagnetic (AFM), and ferromagnetic (FM) corrals are found. Intriguingly, the spin is clearly energy-resolved in a FM corral, which can be effectively used to operate surface carrier spins of TI. We also show that an observable quantum mirage of a magnetic impurity can be projected from the occupied into the empty focus of a FM elliptic corral, while in NM and AFM corrals the mirage signal becomes negligibly weak. In addition, the modulation of the interaction between two magnetic impurities in the quantum corrals is demonstrated. These prominent effects may be measured by spin-polarized STM experiments.Comment: 5 PRB pages, 4 figure