We have investigated the illumination effect on the magnetotransport properties of a two-dimensional electron system at the LaAlO$_3$/SrTiO$_3$ interface. The illumination significantly reduces the zero-field sheet resistance, eliminates the Kondo ef
fect at low-temperature, and switches the negative magnetoresistance into the positive one. A large increase in the density of high-mobility carriers after illumination leads to quantum oscillations in the magnetoresistance originating from the Landau quantization. The carrier density ($sim 2 times 10^{12}$ cm$^{-2}$) and effective mass ($sim 1.7 ~m_e$) estimated from the oscillations suggest that the high-mobility electrons occupy the d$_{xz/yz}$ subbands of Ti:t$_{2g}$ orbital extending deep within the conducting sheet of SrTiO$_3$. Our results demonstrate that the illumination which induces additional carriers at the interface can pave the way to control the Kondo-like scattering and study the quantum transport in the complex oxide heterostructures.
We report the synthesis and physical properties of the single crystals of TaC, which are proposed to hold topological band structure as a topological superconductor (TSC) candidate. Magnetization, resistivity and specific heat measurements are perfor
med and indicate that TaC is bulk superconductor with critical temperature of 10.3 K. TaC is a strongly coupled type-II superconductor and the superconducting state can be well described by s-wave Bardeen-Cooper-Schrieffer (BCS) theory with a single gap. The upper critical field (Hc2) of TaC shows linear temperature dependence, which is quite different from most conventional superconductors and isostructural NbC, which is proposed to manifest topological nodal-loops or type-II Dirac points as well as superconductivity. Our results suggest that TaC would be a new candidate for further research of TSCs.
Disorder-induced magnetoresistance (MR) effect is quadratic at low perpendicular magnetic fields and linear at high fields. This effect is technologically appealing, especially in the two-dimensional (2D) materials such as graphene, since it offers p
otential applications in magnetic sensors with nanoscale spatial resolution. However, it is a great challenge to realize a graphene magnetic sensor based on this effect because of the difficulty in controlling the spatial distribution of disorder and enhancing the MR sensitivity in the single-layer regime. Here, we report a room-temperature colossal MR of up to 5,000% at 9 T in terraced single-layer graphene. By laminating single-layer graphene on a terraced substrate, such as TiO2 terminated SrTiO3, we demonstrate a universal one order of magnitude enhancement in the MR compared to conventional single-layer graphene devices. Strikingly, a colossal MR of >1,000% was also achieved in the terraced graphene even at a high carrier density of ~1012 cm-2. Systematic studies of the MR of single-layer graphene on various oxide- and non-oxide-based terraced surfaces demonstrate that the terraced structure is the dominant factor driving the MR enhancement. Our results open a new route for tailoring the physical property of 2D materials by engineering the strain through a terraced substrate.
We study the possibility of transferring fermions from a trivial system as particle source to an empty system but at topological phase as a mold for casting a stable topological insulator dynamically. We show that this can be realized by a non-Hermit
ian unidirectional hopping, which connects a central system at topological phase and a trivial flat-band system with a periodic driving chemical potential, which scans over the valence band of the central system. The near exceptional-point dynamics allows a unidirectional dynamical process: the time evolution from an initial state with full-filled source system to a stable topological insulating state approximately. The result is demonstrated numerically by a source-assistant QWZ model and SSH chain in the presence of random perturbation. Our finding reveals a classical analogy of quench dynamics in quantum matter and provides a way for topological quantum state engineering.
We study the topological nature of a class of lattice systems, whose Bloch vector can be expressed as the difference of two independent periodic vector functions (knots) in an auxiliary space. We show exactly that each loop as a degeneracy line gener
ates a polarization field, obeying the Biot-Savart law: The degeneracy line acts as a current-carrying wire, while the polarization field corresponds to the generated magnetic field. Applying the Amperes circuital law on a nontrivial topological system, we find that two Bloch knots entangle with each other, forming a link with the linking number being the value of Chern number of the energy band. In addition, two lattice models, an extended QWZ model and a quasi-1D model with magnetic flux, are proposed to exemplify the application of our approach. In the aid of the Biot-Savart law, the pumping charge as a dynamic measure of Chern number is obtained numerically from quasi-adiabatic processes.
We investigate a non-Hermitian extension of Kitaev chain by considering imaginary $p$-wave pairing amplitudes. The exact solution shows that the phase diagram consists two phases with real and complex Bogoliubov-de-gens spectra, associated with $mat
hcal{PT}$-symmetry breaking, which is separated by a hyperbolic exceptional line. The exceptional points (EPs) correspond to a specific Cooper pair state $( 1+c_{k}^{dagger }c_{-k}^{dagger }) leftvert 0rightrangle $ with movable $k$ when the parameters vary along the exceptional line. The non-Hermiticity around EP supports resonant generation of such a pair state from the vacuum state $% leftvert 0rightrangle $ of fermions via the critical dynamic process. In addition, we propose a scheme to generate a superconducting state through a dynamic method.
We investigate localization properties in a two-coupled uniform chains with quasiperiodic modulation on interchain coupling strength. We demonstrate that this ladder is equivalent to a Aubry-Andre (AA) chain when two legs are symmetric. Analytical an
d numerical results indicate the appearance of mobility edges for asymmetric ladder. We also propose an easily engineered quasiperiodic ladder system which is a moir{e} superlattice system consisting of two-coupled uniform chains. An irrational lattice constant difference results in quasiperiodic structure. Numerical simulations show that such a system supports mobility edge. Additionally, we find that the mobility edge can be detected by a dynamic method, which bases on the measurement of surviving probability in the presence of a single imaginary negative potential as a leakage. The result provides insightful information about the localization transitions and mobility edge in experiment.
The Hubble Space Telescope (HST) has obtained multi-epoch observations providing the opportunity for a comprehensive variability search aiming to uncover new variables. We have therefore undertaken the task of creating a catalog of variable sources b
ased on version 3 of the Hubble Source Catalog (HSC), which relies on publicly available images obtained with the WFPC2, ACS, and WFC3 instruments onboard the HST. We adopted magnitude-dependent thresholding in median absolute deviation (a robust measure of light curve scatter) combined with sophisticated preprocessing techniques and visual quality control to identify and validate variable sources observed by Hubble with the same instrument and filter combination five or more times. The Hubble Catalog of Variables (HCV) includes 84,428 candidate variable sources (out of 3.7 million HSC sources that were searched for variability) with $V leq 27$ mag; for 11,115 of them the variability is detected in more than one filter. The data points in the light curves of the variables in the HCV catalog range from five to 120 points (typically having less than ten points); the time baseline ranges from under a day to over 15 years; while $sim$8% of all variables have amplitudes in excess of 1 mag. Visual inspection performed on a subset of the candidate variables suggests that at least 80% of the candidate variables that passed our automated quality control are true variable sources rather than spurious detections resulting from blending, residual cosmic rays, and calibration errors. The HCV is the first, homogeneous catalog of variable sources created from archival HST data and currently is the deepest catalog of variables available. The catalog includes variable stars in our Galaxy and nearby galaxies, as well as transients and variable active galactic nuclei. (abbreviated)
Electronic correlation is believed to play an important role in exotic phenomena such as insulator-metal transition, colossal magneto resistance and high temperature superconductivity in correlated electron systems. Recently, it has been shown that e
lectronic correlation may also be responsible for the formation of unconventional plasmons. Herewith, using a combination of angle-dependent spectroscopic ellipsometry, angle resolved photoemission spectroscopy and Hall measurements all as a function of temperature supported by first-principles calculations, the existence of low-loss high-energy correlated plasmons accompanied by spectral weight transfer, a fingerprint of electronic correlation, in topological insulator (Bi$_{0.8}$Sb$_{0.2}$)$_2$Se$_3$ is revealed. Upon cooling, the density of free charge carriers in the surface states decreases whereas those in the bulk states increase, and that the newly-discovered correlated plasmons are key to explaining this phenomenon. Our result shows the importance of electronic correlation in determining new correlated plasmons and opens a new path in engineering plasmonic-based topologically-insulating devices.
Knot theory provides a powerful tool for the understanding of topological matters in biology, chemistry, and physics. Here knot theory is introduced to describe topological phases in the quantum spin system. Exactly solvable models with long-range in
teractions are investigated, and Majorana modes of the quantum spin system are mapped into different knots and links. The topological properties of ground states of the spin system are visualized and characterized using crossing and linking numbers, which capture the geometric topologies of knots and links. The interactivity of energy bands is highlighted. In gapped phases, eigenstate curves are tangled and braided around each other forming links. In gapless phases, the tangled eigenstate curves may form knots. Our findings provide an alternative understanding of the phases in the quantum spin system, and provide insights into one-dimension topological phases of matter.
