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Xi Chen

Publications and source records attributed to Xi Chen.

At least 1,333 records · Page 74Linked to original sources
arXiv2011

Electronic beam shifts in monolayer graphene superlattice

Electronic analogue of generalized Goos-Hänchen shifts is investigated in the monolayer graphene superlattice with one-dimensional periodic potentials of square barriers. It is found that the lateral shifts for the electron beam transmitted through the monolayer graphene superlattice can be negative as well as positive near the band edges of zero-$\bar{k}$ gap, which are different from those near the band edges of Bragg gap. These negative and positive beam shifts have close relation to the Dirac point. When the condition $q_A d_A= -q_B d_B= m π$ ($m=1,2,3...$) is satisfied, the beam shifts can be controlled from negative to positive when the incident energy is above the Dirac point, and vice versa. In addition, the beam shifts can be greatly enhanced by the defect mode inside the zero-$\bar{k}$ gap. These intriguing phenomena can be verified in a relatively simple optical setup, and have potential applications in the graphene-based electron wave devices.

cond-mat.mes-hall↗
arXiv2011

Fermi Level Tuning of Epitaxial Sb2Te3 Thin Films on Graphene by Regulating Intrinsic Defects and Substrate Transfer Doping

High-quality Sb2Te3 films are obtained by molecular beam epitaxy on graphene substrate and investigated by in situ scanning tunneling microscopy/spectroscopy. Intrinsic defects responsible for the natural p-type conductivity of Sb2Te3 are identified to be the Sb vacancies and SbTe antisites in agreement with first-principles calculations. By minimizing defect densities, coupled with a transfer doping by the graphene substrate, the Fermi level of Sb2Te3 thin films can be tuned over the entire range of the bulk band gap. This establishes the necessary condition to explore topological insulator behaviors near the Dirac point.

cond-mat.mtrl-sci↗
arXiv2011

Fast transitionless expansions of cold atoms in optical Gaussian beam traps

We study fast expansions of cold atoms in a three-dimensional Gaussian-beam optical trap. Three different methods to avoid final motional excitation are compared: inverse engineering using Lewis-Riesenfeld invariants, which provides the best overall performance, a bang-bang approach, and a fast adiabatic approach. We analyze the excitation effect of anharmonic terms, radial-longitudinal coupling, and radial-frequency mismatch. In the inverse engineering approach these perturbations can be suppressed or mitigated by increasing the laser beam waist.

quant-ph↗
arXiv2011

Graph Homomorphisms with Complex Values: A Dichotomy Theorem

Graph homomorphism has been studied intensively. Given an m x m symmetric matrix A, the graph homomorphism function is defined as \[Z_A (G) = \sum_{f:V->[m]} \prod_{(u,v)\in E} A_{f(u),f(v)}, \] where G = (V,E) is any undirected graph. The function Z_A can encode many interesting graph properties, including counting vertex covers and k-colorings. We study the computational complexity of Z_A for arbitrary symmetric matrices A with algebraic complex values. Building on work by Dyer and Greenhill, Bulatov and Grohe, and especially the recent beautiful work by Goldberg, Grohe, Jerrum and Thurley, we prove a complete dichotomy theorem for this problem. We show that Z_A is either computable in polynomial-time or #P-hard, depending explicitly on the matrix A. We further prove that the tractability criterion on A is polynomial-time decidable.

cs.CC↗
arXiv2011

Optimal trajectories for efficient atomic transport without final excitation

We design optimal harmonic-trap trajectories to transport cold atoms without final excitation, combining an inverse engineering techniqe based on Lewis-Riesenfeld invariants with optimal control theory. Since actual traps are not really harmonic, we keep the relative displacement between the center of mass and the trap center bounded. Under this constraint, optimal protocols are found according to different physical criteria. The minimum time solution has a "bang-bang" form, and the minimum displacement solution is of "bang-off-bang" form. The optimal trajectories for minimizing the transient energy are also discussed.

quant-ph↗
arXiv2011

Domain wall partition function of the eight-vertex model with a non-diagonal reflecting end

With the help of the Drinfeld twist or factorizing F-matrix for the eight-vertex SOS model, we obtain the explicit determinant expression of the partition function of the eight-vertex model with a generic non-diagonal reflecting end and domain wall boundary condition. Our result shows that, contrary to the eight-vertex model without a reflection end, the partition function can be expressed as a single determinant.

math-ph↗
arXiv2011

Phase Separation and Magnetic Order in K-doped Iron Selenide Superconductor

Alkali-doped iron selenide is the latest member of high Tc superconductor family, and its peculiar characters have immediately attracted extensive attention. We prepared high-quality potassium-doped iron selenide (KxFe2-ySe2) thin films by molecular beam epitaxy and unambiguously demonstrated the existence of phase separation, which is currently under debate, in this material using scanning tunneling microscopy and spectroscopy. The stoichiometric superconducting phase KFe2Se2 contains no iron vacancies, while the insulating phase has a \surd5\times\surd5 vacancy order. The iron vacancies are shown always destructive to superconductivity in KFe2Se2. Our study on the subgap bound states induced by the iron vacancies further reveals a magnetically-related bipartite order in the superconducting phase. These findings not only solve the existing controversies in the atomic and electronic structures in KxFe2-ySe2, but also provide valuable information on understanding the superconductivity and its interplay with magnetism in iron-based superconductors.

cond-mat.supr-con↗
arXiv2011

A 95 GHz Class I Methanol Maser Survey Toward GLIMPSE Extended Green Objects (EGOs)

We report the results of a systematic survey for 95 GHz class I methanol masers towards a new sample of 192 massive young stellar object (MYSO) candidates associated with ongoing outflows (known as extended green objects or EGOs) identified from the Spitzer GLIMPSE survey. The observations were made with the Australia Telescope National Facility (ATNF) Mopra 22-m radio telescope and resulted in the detection of 105 new 95 GHz class I methanol masers. For 92 of the sources our observations provide the first identification of a class I maser transition associated with these objects (i.e. they are new class I methanol maser sources). Our survey proves that there is indeed a high detection rate (55%) of class I methanol masers towards EGOs. Comparison of the GLIMPSE point sources associated with EGOs with and without class I methanol maser detections shows they have similar mid-IR colors, with the majority meeting the color selection criteria -0.6<[5.8]-[8.0]<1.4 and 0.5<[3.6]-[4.5]<4.0. Investigations of the IRAC and MIPS 24 um colors and the associated millimeter dust clump properties (mass and density) of the EGOs for the sub-samples based on which class of methanol masers they are associated with suggests that the stellar mass range associated with class I methanol masers extends to lower masses than for class II methanol masers, or alternatively class I methanol masers may be associated with more than one evolutionary phase during the formation of a high-mass star.

astro-ph.SR↗
arXiv2011

A Smoothing Stochastic Gradient Method for Composite Optimization

We consider the unconstrained optimization problem whose objective function is composed of a smooth and a non-smooth conponents where the smooth component is the expectation a random function. This type of problem arises in some interesting applications in machine learning. We propose a stochastic gradient descent algorithm for this class of optimization problem. When the non-smooth component has a particular structure, we propose another stochastic gradient descent algorithm by incorporating a smoothing method into our first algorithm. The proofs of the convergence rates of these two algorithms are given and we show the numerical performance of our algorithm by applying them to regularized linear regression problems with different sets of synthetic data.

math.OC↗
arXiv2011

Power-Law Decay of Standing Waves on the Surface of Topological Insulators

We propose a general theory on the standing waves (quasiparticle interference pattern) caused by the scattering of surface states off step edges in topological insulators, in which the extremal points on the constant energy contour of surface band play the dominant role. Experimentally we image the interference patterns on both Bi$_2$Te$_3$ and Bi$_2$Se$_3$ films by measuring the local density of states using a scanning tunneling microscope. The observed decay indices of the standing waves agree excellently with the theoretical prediction: In Bi$_2$Se$_3$, only a single decay index of -3/2 exists; while in Bi$_2$Te$_3$ with strongly warped surface band, it varies from -3/2 to -1/2 and finally to -1 as the energy increases. The -1/2 decay indicates that the suppression of backscattering due to time-reversal symmetry does not necessarily lead to a spatial decay rate faster than that in the conventional two-dimensional electron system. Our formalism can also explain the characteristic scattering wave vectors of the standing wave caused by non-magnetic impurities on Bi$_2$Te$_3$.

cond-mat.mes-hall↗
arXiv2011

Controllable Goos-Hänchen shifts and spin beam splitter for ballistic electrons in a parabolic quantum well under a uniform magnetic field

The quantum Goos-Hänchen shift for ballistic electrons is investigated in a parabolic potential well under a uniform vertical magnetic field. It is found that the Goos-Hänchen shift can be negative as well as positive, and becomes zero at transmission resonances. The beam shift depends not only on the incident energy and incidence angle, but also on the magnetic field and Landau quantum number. Based on these phenomena, we propose an alternative way to realize the spin beam splitter in the proposed spintronic device, which can completely separate spin-up and spin-down electron beams by negative and positive Goos-Hänchen shifts.

cond-mat.mes-hall↗
arXiv2011

Crossover between weak localization and weak antilocalization in magnetically doped topological insulator

Topological insulators (TI) are a new class of quantum materials with insulating bulk enclosed by topologically protected metallic boundaries. The surface states of three-dimensional TIs have spin helical Dirac structure, and are robust against time reversal invariant perturbations. This extraordinary property is notably exemplified by the absence of backscattering by nonmagnetic impurities and the weak antilocalization (WAL) of Dirac fermions. Breaking the time reversal symmetry (TRS) by magnetic element doping is predicted to create a variety of exotic topological magnetoelectric effects. Here we report transport studies on magnetically doped TI Cr-Bi2Se3. With increasing Cr concentration, the low temperature electrical conduction exhibits a characteristic crossover from WAL to weak localization (WL). In the heavily doped regime where WL dominates at the ground state, WAL reenters as temperature rises, but can be driven back to WL by strong magnetic field. These complex phenomena can be explained by a unified picture involving the evolution of Berry phase with the energy gap opened by magnetic impurities. This work demonstrates an effective way to manipulate the topological transport properties of the TI surface states by TRS-breaking perturbations.

cond-mat.mtrl-sci↗
arXiv2011

Fast transport of Bose-Einstein condensates

We propose an inverse method to accelerate without final excitation the adiabatic transport of a Bose Einstein condensate. The method, applicable to arbitrary potential traps, is based on a partial extension of the Lewis-Riesenfeld invariants, and provides transport protocols that satisfy exactly the no-excitation conditions without constraints or approximations. This inverse method is complemented by optimizing the trap trajectory with respect to different physical criteria and by studying the effect of noise.

quant-ph↗
arXiv2011

Lewis-Riesenfeld invariants and transitionless tracking algorithm

Different methods have been recently put forward and implemented experimentally to inverse engineer the time dependent Hamiltonian of a quantum system and accelerate slow adiabatic processes via non-adiabatic shortcuts. In the "transitionless tracking algorithm" proposed by Berry, shortcut Hamiltonians are designed so that the system follows exactly, in an arbitrarily short time, the approximate adiabatic path defined by a reference Hamiltonian. A different approach is based on designing first a Lewis-Riesenfeld invariant to carry the eigenstates of a Hamiltonian from specified initial to final configurations, again in an arbitrary time, and then constructing from the invariant the transient Hamiltonian connecting these boundary configurations. We show that the two approaches, apparently quite different in form and so far in results, are in fact strongly related and potentially equivalent, so that the inverse-engineering operations in one of them can be reinterpreted and understood in terms of the concepts and operations of the other one. We study as explicit examples the expansions of time-dependent harmonic traps and state preparation of two level systems.

quant-ph↗
arXiv2011

Magnetic phase diagram of interacting nanoparticle systems under the mean field model

The disordered random-anisotropy magnetic nanoparticle systems with competing dipolar interactions and ferromagnetic exchange couplings are investigated by Monte Carlo simulations. Superspin glass (SSG) and superferromagnetic (SFM) behaviors are found at low temperatures depending on the interactions. Based on the mean field approximation, the Curie-Weiss temperature TCW = 0 is suggested as the phase boundary between the SSG systems and the SFM systems, which is convinced by the spontaneous magnetizations and relaxations. The magnetic phase diagram is plotted.

cond-mat.dis-nn↗
arXiv2011

A Turing model of pattern formation in atmospheric pressure gas discharges

In this letter we propose a Turing model of the formation of patterns of visible light emission intensity in atmospheric pressure gas discharges. The electron density and the electron temperature take the roles of activator and inhibitor respectively in a two-reactant Turing model, with the activator diffusion coeffcient being much smaller than that of the inhibitor, and ionization and excitation from excited state atoms considered as the dominant reaction processes. The model predicts striations in a 1D system, which quantitatively agree with experimental results in terms of the spatial variation scale lengths. Additionally, the model also predicts changes in the discharge structure observed experimentally when input power and gas pressure are varied.

physics.plasm-ph↗