arXiv subjects

Y. Xia

Publications and source records attributed to Y. Xia.

At least 19 recordsLinked to original sources

arXiv2026

From Narrow-gap Semiconductor to Metallic Altermagnet: Optical Fingerprints of Co-Doped FeSb2

The realization of bulk metallic altermagnetism has remained elusive despite the growing number of candidate materials. Here, we present evidence that moderate cobalt substitution ($\sim$15%) drives the correlated narrow-gap semiconductor FeSb$_2$ into a metallic altermagnetic state persisting up to room temperature. The infrared optical conductivity reveals low-energy interband transitions near 0.1 eV that emerge upon doping and grow with Co concentration. Density functional theory calculations show that these transitions originate exclusively from altermagnetic spin ordering, with spin split bands ($\sim$0.2 eV) of non-relativistic origin, together with spin-orbit coupling induced band splitting of the order of $\sim$5 meV near the Fermi level. Co substitution further leads to Fano lineshapes and mode mixing in the infrared-active phonons, reflecting enhanced electron-phonon coupling and local inversion symmetry breaking, while leaving the altermagnetic spin symmetry intact. Our results establish carrier-tuned FeSb$_2$ as a platform for exploring metallic $d$-wave altermagnetism and its coupling to lattice degrees of freedom.

cond-mat.mtrl-sci↗

arXiv2023

Blue-detuned molecular magneto-optical trap schemes based on bayesian optimization

Direct laser cooling and trapping of molecules to temperature below Doppler limit and density exceeding $10^8$ are challenging due to the sub-Doppler heating effects of molecular magneto-optical trap (MOT). In our previous paper [1], we presented a general approach to engineering the sub- Doppler force by tuning the AC stark shift with the addition of a blue detuned laser. Here, by employing the Bayesian optimization method to optical Bloch equations, we have identified multiple blue-detuned MOT schemes for the CaF molecule. From the three-dimensional Monte-Carlo simulation, we obtained a MOT temperature and density of 14 $\rm \mu K$ and $\rm 4.5 \times 10^8 cm^{-3}$, respectively. Our findings present a potential avenue for directly loading molecular MOTs into conservative traps, which can capitalize on the high density and low temperature of the MOT

physics.atom-ph↗

arXiv2023

Entropy dissipative higher order accurate positivity preserving time-implicit discretizations for nonlinear degenerate parabolic equations

We develop entropy dissipative higher order accurate local discontinuous Galerkin (LDG) discretizations coupled with Diagonally Implicit Runge-Kutta (DIRK) methods for nonlinear degenerate parabolic equations with a gradient flow structure. Using the simple alternating numerical flux, we construct DIRK-LDG discretizations that combine the advantages of higher order accuracy, entropy dissipation and proper long-time behavior. The implicit time-discrete methods greatly alleviate the time-step restrictions needed for the stability of the numerical discretizations. Also, the larger time step significantly improves computational efficiency. We theoretically prove the unconditional entropy dissipation of the implicit Euler-LDG discretization. Next, in order to ensure the positivity of the numerical solution, we use the Karush-Kuhn-Tucker (KKT) limiter, which couples the positivity inequality constraint with higher order accurate DIRK-LDG discretizations using Lagrange multipliers. In addition, mass conservation of the positivity-limited solution is ensured by imposing a mass conservation equality constraint to the KKT equations. The unique solvability and unconditional entropy dissipation for an implicit first order accurate in time, but higher order accurate in space, KKT-LDG discretizations are proved, which provides a first theoretical analysis of the KKT limiter. Finally, numerical results demonstrate the higher order accuracy and entropy dissipation of the KKT-DIRK-LDG discretizations for problems requiring a positivity limiter.

math.NA↗

arXiv2022

Quantum-improved phase estimation with a displacement-assisted SU(1,1) interferometer

By performing two local displacement operations (LDOs) inside an SU(1,1) interferometer, called as the displacement-assisted SU(1,1) [DSU(1,1)], both the phase sensitivity based on homodyne detection and quantum Fisher information (QFI) with and without photon losses are investigated in this paper. In this DSU(1,1) interferometer, we focus our attention on the extent to which the introduced LDO affects the phase sensitivity and the QFI, even in the realistic scenario. Our analyses show that the estimation performance of DSU(1,1) interferometer is always better than that of SU(1,1) interferometer without the LDO, especially the phase precision of the former in the ideal scenario gradually approaching to the Heisenberg limit via the increase of the LDO strength. More significantly, different from the latter, the robustness of the former can be enhanced markedly by regulating and controlling the LDO. Our findings would open an useful view for quantum-improved phase estimation of optical interferometers.

quant-ph↗

arXiv2022

Lagrangian-based methods in convex optimization: prediction-correction frameworks with ergodic convergence rates

We study the convergence rates of the classical Lagrangian-based methods and their variants for solving convex optimization problems with equality constraints. We present a generalized prediction-correction framework to establish $O(1/K^2)$ ergodic convergence rates. Under the strongly convex assumption, based on the presented prediction-correction framework, some Lagrangian-based methods with $O(1/K^2)$ ergodic convergence rates are presented, such as the augmented Lagrangian method with the indefinite proximal term, the alternating direction method of multipliers (ADMM) with a larger step size up to $(1+\sqrt{5})/2$, the linearized ADMM with the indefinite proximal term, and the multi-block ADMM type method (under an alternative assumption that the gradient of one block is Lipschitz continuous).

math.OC↗

arXiv2018

Limit Cycles in a Model of Olfactory Sensory Neurons

We propose an approach to study small limit cycle bifurcations on a center manifold in analytic or smooth systems depending on parameters. We then apply it to the investigation of limit cycle bifurcations in a model of calcium oscillations in the cilia of olfactory sensory neurons and show that it can have two limit cycles: a stable cycle appearing after a Bautin (generalized Hopf) bifurcation and an unstable cycle appearing after a subcritical Hopf bifurcation.

math.DS↗

arXiv2017

Engineering the coupling between Majorana bound states

We study the coupling between Majorana bound states (CMBS), which is mediated by a topologically trivial chain in the presence of pairing coupling and long-range coupling. The results show that CMBS can be enhanced by the pairing coupling and long-range coupling of the trivial chain. When driving the trivial chain by periodic driving field, we deduce the analytical expressions of CMBS in the high-frequency limit, and demonstrate that CMBS can be modulated by the frequency and amplitude of driving field. Finally we exhibit the application of tunable CMBS in realizing quantum logic gates.

quant-ph↗

arXiv2017

Nucleation of dislocations and their dynamics in layered oxides cathode materials during battery charging

Defects and their interactions in crystalline solids often underpin material properties and functionality as they are decisive for stability, result in enhanced diffusion, and act as a reservoir of vacancies. Recently, lithium-rich layered oxides have emerged among the leading candidates for the next-generation energy storage cathode material, delivering 50 % excess capacity over commercially used compounds. Oxygen-redox reactions are believed to be responsible for the excess capacity, however, voltage fading has prevented commercialization of these new materials. Despite extensive research the understanding of the mechanisms underpinning oxygen-redox reactions and voltage fade remain incomplete. Here, using operando three-dimensional Bragg coherent diffractive imaging, we directly observe nucleation of a mobile dislocation network in nanoparticles of lithium-rich layered oxide material. Surprisingly, we find that dislocations form more readily in the lithium-rich layered oxide material as compared with a conventional layered oxide material, suggesting a link between the defects and the anomalously high capacity in lithium-rich layered oxides. The formation of a network of partial dislocations dramatically alters the local lithium environment and contributes to the voltage fade. Based on our findings we design and demonstrate a method to recover the original high voltage functionality. Our findings reveal that the voltage fade in lithium-rich layered oxides is reversible and call for new paradigms for improved design of oxygen-redox active materials.

cond-mat.mtrl-sci↗

arXiv2016

Fluid-solid-electric energy transport along piezoelectric flags

The fluid-solid-electric dynamics of a flexible plate covered by interconnected piezoelectric patches in an axial steady flow are investigated using numerical simulations based on a reduced-order model of the fluid loading for slender structures. Beyond a critical flow velocity, the fluid-solid instability results in large amplitude flapping of the structure. Short piezoelectric patches positioned continuously along the plate convert its local deformation into electrical currents that are used within a single internal electrical network acting as an electric generator for the external output circuit. The relative role of the internal and external impedance on the energy harvesting of the system is presented and analyzed in the light of a full modeling of the electric and mechanical energy exchanges and transport along the structure.

physics.flu-dyn↗

arXiv2016

A General Theory for Bandgap Estimation in Locally Resonant Metastructures

Locally resonant metamaterials are characterized by bandgaps at wavelengths that are much larger than the lattice size, enabling low-frequency vibration attenuation. Typically, bandgap analyses and predictions rely on the assumption of traveling waves in an infinite medium, and do not take advantage of modal representations typically used for the analysis of the dynamic behavior of finite structures. Recently, we developed a method for understanding the locally resonant bandgap in uniform finite metamaterial beams using modal analysis. Here we extend that framework to general locally resonant metastructures with specified boundary conditions using a general operator formulation. Using this approach, along with the assumption of an infinite number of resonators tuned to the same frequency, the frequency range of the locally resonant bandgap is easily derived in closed form. Furthermore, the bandgap expression is shown to be the same regardless of the type of vibration problem under consideration, depending only on the added mass ratio and target frequency. It is shown that the number of resonators required for the bandgap to appear increases with the target frequency range, i.e. respective modal neighborhood. Furthermore, it is observed that there is an optimal, finite number of resonators which gives a bandgap that is wider than the infinite-absorber bandgap, and that the optimal number of resonators increases with target frequency and added mass ratio. As the number of resonators becomes sufficiently large, the bandgap converges to the derived infinite-absorber bandgap. The derived bandgap edge frequencies are shown to agree with results from dispersion analysis using the plane wave expansion method. Numerical and experimental investigations are performed regarding the effects of mass ratio, non-uniform spacing of resonators, and parameter variations among the resonators.

physics.class-ph↗

arXiv2012

Dirac point spectral weight suppression and surface "gaps" in nonmagnetic and magnetic topological insulators

It is predicted that electrons on the surface of a topological insulator can acquire a mass (massive Dirac fermion) by opening up a gap at the Dirac point when time-reversal symmetry is broken via the out-of-plane magnetization. We report photoemission studies on a series of topological insulator materials focusing on the spectral behavior in the vicinity of the Dirac node. Our results show that the spectral intensity is suppressed resulting in a "gap"-like feature in materials with or without any magnetic impurity or doping. The Zeeman gap in magnetically doped samples, expected to be rather small, is likely masked by the non-magnetic strong spectral weight suppression involving a large energy scale we report. The photoemission spectral weight suppression observed around the Dirac node thus cannot be taken as the sole evidence for a time-reversal symmetry breaking magnetic gap. We discuss a few possible extrinsic and kinematic origins of the Dirac point spectral weight suppression ("gap") observed in many commonly studied topological materials.

cond-mat.mes-hall↗

arXiv2012

Observation of Topological Order in the TlBiSe2 class : Probing the "spin" and "phase" on topological insulator surfaces

A topological insulator is characterized by spin-momentum locking on its boundary. The spin momentum locking on the surface of a three dimensional topological insulator leads to the existence of a non-trivial Berry's phase which leads to exotic transport phenomena on topological surfaces. Using spin-sensitive probes (Mott polarimetry), we observe the spin-momentum coupling and uncover the chiral nature of surface electrons in TlBiSe2. We demonstrate that the surface electrons in TlBiSe2 collectively carry a quantum Berry's phase of $π$ and a definite chirality ($η$ = -1, left-handed) associated with its spin-texture or vortex-structure on the Fermi surface on both the top and the bottom surfaces. Our experimental results for the first time not only prove the existence of Z2 topological-order in the bulk but also reveal the existence of helical quasiparticle modes on the topological surface. Spin-texture calculations would be reported elsewhere.

cond-mat.mes-hall↗

arXiv2012

Effects of iron doping on the topological insulator surfaces

Topological insulators embody a newly discovered state of matter characterized by conducting spin-momentum locked surface states that span the bulk band gap. So far, most of the study on topological insulator surfaces has been limited to understanding their properties without strong Coulomb perturbation or breaking of time reversal symmetry. We have used deposited iron, with a large positive ionization state and significant magnetic moment as a strong probe to modify the surface electronic structure of the Bi2Se3 surface at the gap energy scale. We observe that such perturbation leads to the creation of multiple Dirac fermions consistent with Z2 or Mod(2) symmetry. Taken collectively, these results are a helpful guide in manipulating topological surfaces for probing fundamental physics or developing device applications.

cond-mat.mes-hall↗

arXiv2012

Possible Origin of the Damocloids:the Scattered Disk or a New Region?

The Damocloids are a group of unusual asteroids, recently enrolling a new member of 2010 EJ104. The dynamical evolution for the Damocloids may uncover a connection passage from the Main Belt, the Kuiper Belt and the scattered disk beyond. According to our simulations, two regions may be considered as possible origin of the Damocloids: the scattered disk, or a part of Oort cloud which will be perturbed to a transient region locating between 700 AU and 1000 AU. Based on the potential origin, the Damocloids can be classified into two types, with relation to their semi-major axes, and about 65.5% Damocloids is classified into type I which mainly originate from Oort cloud. Whether the Damocloids is inactive nuclei of Halley Family Comets may rely on their origin.

astro-ph.EP↗

arXiv2011

Topological Phase Transition and Texture Inversion in a Tunable Topological Insulator

The recently discovered three dimensional or bulk topological insulators are expected to exhibit exotic quantum phenomena. It is believed that a trivial insulator can be twisted into a topological state by modulating the spin-orbit interaction or the crystal lattice via odd number of band inversions, driving the system through a topological quantum phase transition. By directly measuring the topological invariants (for the method to directly measure Fu-Kane {$ν_0$}, see Hsieh \textit{et.al.,} Science 323, 919 (2009) at http://www.sciencemag.org/content/323/5916/919.abstract) we report the observation of a phase transition in a tunable spin-orbit system BiTl(S{1-d}Se{d})2 (which is an analog of the most studied topological insulator Bi2Se3, see Xia \textit{et.al.,} Nature Phys. 5, 398 (2009) at http://www.nature.com/nphys/journal/v5/n6/full/nphys1294.html and Spin-Momentum locking at http://www.nature.com/nature/journal/v460/n7259/full/nature08234.html) where the topological insulator state formation is visualized for the first time. In the topological state, vortex-like polarization states are observed to exhibit 3D vectorial textures, which collectively feature a chirality transition of its topological spin-textures as the spin-momentum locked (Z2 topologically ordered) electrons on the surface go through the zero carrier density point. Such phase transition and texture chirality inversion can be the physical basis for observing \textit{fractional charge} (e/2) and other related fractional topological phenomena.

cond-mat.mes-hall↗

arXiv2011

Direct observation of spin-polarized surface states in the parent compound of topological insulator Bi-Sb using spin-resolved-ARPES in a 3D Mott-polarimetry spin mode

We report high-resolution spin-resolved photoemission spectroscopy (Spin-ARPES) measurements on the parent compound Sb of the first discovered 3D topological insulator Bi{1-x}Sb{x} [D. Hsieh et al., Nature 452, 970 (2008) Submitted 2007]. By modulating the incident photon energy, we are able to map both the bulk and (111) surface band structure, from which we directly demonstrate that the surface bands are spin polarized by the spin-orbit interaction and connect the bulk valence and conduction bands in a topologically non-trivial way. A unique asymmetric Dirac surface state gives rise to a $k$-splitting of its spin polarized electronic channels. These results complement our previously published works on this materials class and re-confirm our discovery of first bulk (3D) topological insulator - topological order in bulk solids. [Invited article for NJP-IOP Focus issue on "Topological Insulators"]

cond-mat.mes-hall↗

arXiv2011

Realization of an isolated Dirac node and strongly modulated Spin Texture in the topological insulator Bi2Te3

The development of spin-based applications of topological insulators requires the knowledge and understanding of spin texture configuration maps as they change via gating in the vicinity of an isolated Dirac node. An isolated (graphene-like) Dirac node, however, does not exist in Bi2Te3. While the isolation of surface states via transport channels has been promisingly achieved in Bi2Te3, it is not known how spin textures modulate while gating the surface. Another drawback of Bi2Te3 is that it features multiple band crossings while chemical potential is placed near the Dirac node (at least 3 not one as in Bi2Se3 and many other topological insulators) and its buried Dirac point is not experimentally accessible for the next generation of experiments which require tuning the chemical potential near an isolated (graphene-like) Dirac node. Here, we image the spin texture of Bi2Te3 and suggest a simple modification to realize a much sought out isolated Dirac node regime critical for almost all potential applications (of topological nature) of Bi2Te3. Finally, we demonstrate carrier control in magnetically and nonmagnetically doped Bi2Te3 essential for realizing giant magneto-optical effects and dissipationless spin current devices involving a Bi2Te3-based platform.

cond-mat.mes-hall↗

arXiv2010

Discovery of several large families of Topological Insulator classes with backscattering-suppressed spin-polarized single-Dirac-cone on the surface

Three dimensional (3D) topological insulators are novel states of quantum matter that feature spin-momentum locked helical Dirac fermions on their surfaces and hold promise to open new vistas in spintronics, quantum computing and fundamental physics. Experimental realization of many of the predicted topological phenomena requires finding multi-variant topological band insulators which can be multiply connected to magnetic semiconductors and superconductors. Here we present our theoretical prediction and experimental discovery of several new topological insulator classes in AB2X4(124), A2B2X5(225), MN4X7(147), A2X2X'(221) [A,B=Pb,Ge,Sb,Bi and M,N=Pb,Bi and X,X'=Chalcogen family]. We observe that these materials feature gaps up to about 0.35eV. Multi-variant nature allows for diverse surface dispersion tunability, Fermi surface spin-vortex or textured configurations and spin-dependent electronic interference signaling novel quantum transport processes on the surfaces of these materials. Our discovery also provides several new platforms to search for topological-superconductivity (arXiv:0912.3341v1 (2009)) in these exotic materials.

cond-mat.mes-hall↗