arXiv subjects

Shijie Xie

Publications and source records attributed to Shijie Xie.

10 recordsLinked to original sources

arXiv2026

Ellipticity-Controlled Bright-Dark Coherence Transition in Monolayer WSe2

The generation of exciton valley coherence typically requires linearly polarized (LP) light as an external coherent drive, whereas circularly polarized (CP) light fails to induce coherence. Here, we develop a unified, microscopically-grounded open-quantum-system framework within a five-level model incorporating bright-dark exciton interactions in monolayer WSe2, and demonstrate that the polarization ellipticity of the excitation field provides selective control over distinct exciton species contributing to valley coherence. Specifically, LP and CP excitations generate bright and dark coherence, respectively, with continuous ellipticity tuning enabling controlled transitions between these states. We further reveal dual magnetic advantages for manipulating dark coherence even in the absence of initial coherence: (i) an out-of-plane magnetic field suppresses coherence decay and (ii) an in-plane field enables its optical readout, with quantitatively realistic field strengths. These findings provide a powerful mechanism for accessing hidden dark states via ellipticity-driven coherence transfer, and establish a new pathway for harnessing bright-dark valley-coherence transitions in future quantum control.

cond-mat.mes-hall↗

arXiv2024

Laser-field detuning assisted optimization of exciton valley dynamics in monolayer WSe$_2$: Geometric quantum speed limit

Optimizing valley dynamics is an effective instrument towards precisely manipulating qubit in the context of two-dimensional semiconductor. In this work, we construct a comprehensive model, involving both intra- and intervalley channels of excitons in monolayer WSe$_2$, and simultaneously takes the light-matter interaction into account, to investigate the optimal control of valley dynamics with an initial coherent excitonic state. Based on the quantum speed limit (QSL) theory, we propose two optimal control schemes aiming to reduce the evolution time of valley dynamics reaching the target state, along with to boost the evolution speed over a period of time. Further, we emphasize that the implementation of dynamical optimization is closely related to the detuning difference -- the difference of exciton-laser field detunings between the K and K' valleys -- which is determined by the optical excitation mode and magnetically-induced valley splitting. In particular, we reveal that a small detuning difference drives the actual dynamical path to converge towards the geodesic length between the initial and final states, allowing the system to evolve with the least time. Especially, in the presence of valley coherence, the actual evolution time and the calculated QSL time almost coincide, facilitating high fidelity in information transmission based on the valley qubit. Remarkably, we demonstrate an intriguing enhancement in evolution speed of valley dynamics, by adopting a large detuning difference, which induces an emerging valley polarization even without initial polarization. Our work opens a new paradigm for optically tuning excitonic physics in valleytronic applications, and may also offer solutions to some urgent problems such as speed limit of information transmission in qubit.

cond-mat.mes-hall↗

arXiv2023

Magnetically tunable exciton valley coherence in monolayer WS$_2$ mediated by the electron-hole exchange and exciton-phonon interactions

We develop a model, which incorporates both intra- and intervalley scatterings to master equation, to explore exciton valley coherence in monolayer WS$_2$ subjected to magnetic field. For linearly polarized (LP) excitation accompanied with an initial coherence, our determined valley dynamics manifests the coherence decay being faster than the exciton population relaxation, and agrees with experimental data by Hao et al.[Nat. Phys. 12, 677 (2016)]. Further, we reveal that magnetic field may quench the electron-hole (e-h) exchange induced pure dephasing -- a crucial decoherence source -- as a result of lifting of valley degeneracy, allowing to magnetically regulate valley coherence. In particular, at low temperatures for which the exciton-phonon (ex-ph) interaction is weak, we find that the coherence time is expected to attain $τ_{\mathcal{C}}\sim 1$ ps, facilitating full control of qubits based on the valley pseudospin. For dark excitons, we demonstrate an emerging coherence even in the absence of initial coherent state, which has a long coherence time ($\sim 15$ ps) at low temperature. Our work provides an insight into tunable valley coherence and coherent valley control based on dark excitons.

cond-mat.mes-hall↗

arXiv2023

Linkages and removable paths avoiding vertices

We say that a graph $G$ is $(2,m)$-linked if, for any distinct vertices $a_1,\ldots, a_m, b_1,b_2$ in $G$, there exist vertex disjoint connected subgraphs $A,B$ of $G$ such that $\{a_1, \ldots, a_m\}$ is contained in $A$ and $\{b_1,b_2\}$ is contained in $B$. A fundamental result in structural graph theory is the characterization of $(2,2)$-linked graphs, with different versions obtained independently by Robertson and Chakravarty, Seymour, and Thomassen. It appears to be very difficult to characterize $(2,m)$-linked graphs for $m\ge 3$. In this paper, we provide a partial characterization of $(2,m)$-linked graphs by adding an average degree condition. This implies that $(2m+2)$-connected graphs are $(2,m)$-linked. Moreover, if $G$ is a $(2m+2)$-connected graph and $a_1, \ldots, a_m, b_1,b_2$ are distinct vertices of $G$, then there is a path $P$ in $G$ between $b_1$ and $b_2$ and avoiding $\{a_1, \ldots, a_m\}$ such that $G-P$ is connected, improving a previous connectivity bound of $10m$.

math.CO↗

arXiv2021

Resonant Band Engineering of Ferroelectric Tunnel Junctions

We propose energy band engineering to enhance tunneling electroresistance (TER) in ferroelectric tunnel junctions (FTJs). We predict that an ultrathin dielectric layer with a smaller band gap, embedded into a ferroelectric barrier layer, acts as a switch controlling high and low conductance states of an FTJ depending on polarization orientation. Using first-principles modeling based on density functional theory, we investigate this phenomenon for a prototypical SrRuO3/BaTiO3/SrRuO3 FTJ with a BaSnO3 monolayer embedded in the BaTiO3 barrier. We show that in such a composite-barrier FTJ, ferroelectric polarization of BaTiO3 shifts the conduction band minimum of the BaSnO3 monolayer above or below the Fermi energy depending on polarization orientation. The resulting switching between direct and resonant tunneling leads to a TER effect with a giant ON/OFF conductance ratio. The proposed resonant band engineering of FTJs can serve as a viable tool to enhance their performance useful for device application.

cond-mat.mtrl-sci↗

arXiv2020

Spin soliton of Holstein model with spin-orbit coupling in one-dimensional conjugated polymers

For Holstein model with Rashba spin-orbit coupling (SOC) we establish the nonlinear Schrödinger equations and obtain exact soliton solution analytically. It is found that the soliton is spin polarized determined both by the SOC and the electron-phonon (e-ph) interaction. The soliton can be used to describe the spin transport or spin current in organic semiconductors.

cond-mat.mes-hall↗

arXiv2020

4-Separations in Hajós Graphs

As a natural extension of the Four Color Theorem, Hajós conjectured that graphs containing no $K_5$-subdivision are 4-colorable. Any possible counterexample to this conjecture with minimum number of vertices is called a {\it Hajós graph}. Previous results show that Hajós graphs are 4-connected but not 5-connected. A $k$-separation in a graph $G$ is a pair $(G_1,G_2)$ of edge-disjoint subgraphs of $G$ such that $|V(G_1\cap G_2)|=k$, $G=G_1\cup G_2$, and $G_i\not\subseteq G_{3-i}$ for $i=1,2$. In this paper, we show that Hajós graphs do not admit a 4-separation $(G_1,G_2)$ such that $|V(G_1)|\ge 6$ and $G_1$ can be drawn in the plane with no edge crossings and all vertices in $V(G_1\cap G_2)$ incident with a common face. This is a step in our attempt to reduce Hajós' conjecture to the Four Color Theorem.

math.CO↗

arXiv2019

Wheels in planar graphs and Hajós graphs

It was conjectured by Hajós that graphs containing no $K_5$-subdivision are 4-colorable. Previous results show that any possible minimum counterexample to Hajós' conjecture, called Hajós graph, is 4-connected but not 5-connected. In this paper, we show that if a Hajós graph admits a 4-cut or 5-cut with a planar side then the planar side must be small or contains a special wheel. This is a step in our effort to reduce Hajós' conjecture to the Four Color Theorem.

math.CO↗

arXiv2019

Realization of Lieb Lattice in Covalent-organic Frameworks with Tunable Topology and Magnetism

Lieb lattice, a two-dimensional edge-depleted square lattice, has been predicted to host various exotic electronic properties due to its unusual band structure, i.e., Dirac cone intersected by a flat band (Dirac-flat bands). Until now, although a few artificial Lieb lattices have been discovered in experiments, the realization of a Lieb lattice in a real material is still unachievable. In this article, based on tight-binding modeling and first-principles calculations, we predict that the two covalent organic frameworks (COFs), i.e., sp2C-COF and sp2N-COF, which have been synthesized in the recent experiments, are actually the first two material realizations of organic-ligand-based Lieb lattice. It is found that the lattice distortion can govern the bandwidth of the Dirac-flat bands and in turn determine its electronic instability against spontaneous spin-polarization during carrier doping. The spin-orbit coupling effects could drive these Dirac-flat bands in a distorted Lieb lattice presenting nontrivial topological properties, which depend on the position of Fermi level. Interestingly, as the hole doping concentration increases, the sp2C-COF can experience the phase transitions from a paramagnetic state to a ferromagnetic one and then to a Néel antiferromagnetic one. Our findings not only confirm the first material realization of Lieb lattice in COFs, but also offer a possible way to achieve tunable topology and magnetism in d- (f-) orbital-free organic lattices.

cond-mat.mtrl-sci↗

arXiv2018

Graph-Based Radio Resource Management for Vehicular Networks

This paper investigates the resource allocation problem in device-to-device (D2D)-based vehicular communications, based on slow fading statistics of channel state information (CSI), to alleviate signaling overhead for reporting rapidly varying accurate CSI of mobile links. We consider the case when each vehicle-to-infrastructure (V2I) link shares spectrum with multiple vehicle-to-vehicle (V2V) links. Leveraging the slow fading statistical CSI of mobile links, we maximize the sum V2I capacity while guaranteeing the reliability of all V2V links. We propose a graph-based algorithm that uses graph partitioning tools to divide highly interfering V2V links into different clusters before formulating the spectrum sharing problem as a weighted 3-dimensional matching problem, which is then solved through adapting a high-performance approximation algorithm.

cs.IT↗