arXiv subjects

Kang Lan

Publications and source records attributed to Kang Lan.

5 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↗

arXiv2026

Strain-Driven "Sinusoidal" Valley Control of Hybridized $\Gamma -\mathrm{K}$ Excitons

The photoluminescence (PL) of momentum-indirect $\rm \Gamma- K$ excitons in monolayer WS$_2$ under biaxial strain was recently observed by Blundo et al. [Phys. Rev. Lett. 129, 067402 (2022)], yet its microscopic origin remains elusive. Here we develop a unified framework that reproduces the measured PL and reveals its fundamental excitonic mechanism. We reveal that: (i) the PL originates from genuinely hybridized direct-indirect excitonic eigenstates, rather than nominally mixed species with fixed dominant character; (ii) the direct exciton converts into the indirect one via a previously unrecognized two-step pathway -- exchange-interaction-driven exciton transfer followed by a spin flip; and (iii) a higher-energy indirect exciton, absent from prior studies, acts as a crucial intermediate mediating this conversion. Beyond explaining experiment, our theory predicts a striking strain-driven "sinusoidal'' valley response, furnishing a continuously tunable valley dial that far exceeds binary control schemes. This unified picture of strain-engineered direct-indirect exciton dynamics introduces a new paradigm for manipulating long-lived valley degrees of freedom, opening a pathway toward programmable valley pseudospin engineering and next-generation valleytronic quantum technologies.

cond-mat.mes-hall↗

arXiv2025

Quantum speed limits in dephasing dynamics of a qubit system coupled to thermal environments

We theoretically study the quantum speed limits (QSLs) of a qubit system coupled to a thermal dephasing environment with an Ohmic-like spectral density. Based on the geometric QSLs time bound, which is derived by employing the trace distance to quantify the geodesic between two distinguishable states in dynamical evolution, we study the influences of the temperature and spectral density of the environment on the QSLs time of the dephasing qubit. We also investigate the interplay between the QSLs time, the environmental temperature, and the spectral density of the environment. It has been demonstrated that, the QSLs time closely depends on the transition frequency and the dynamical behavior (e.g., coherence trapping) of the dephasing qubit. For a fixed Ohmicity parameter of the environmental spectral density, the increase of environmental temperature can enhance the QSLs time bound. In addition, when the environmental temperature remains constant, the increase in the Ohmicity parameter initially leads to a reduction in the QSLs time bound, which is then followed by an increase of the time bound of QSLs. Our results can help to better understand the QSLs in the dynamics of open quantum systems and have potential application in the modulation of QSLs time in the dephasing qubit by engineering the spectral density of the environment.

quant-ph↗

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.

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