arXiv subjects

L. Q. Chen

Publications and source records attributed to L. Q. Chen.

At least 19 recordsLinked to original sources

arXiv2026

Polar-vortex-driven interfacial strain coupling in PbTiO3/SrRuO3 Heterostructures

Interfacial coupling in oxide heterostructures is a central problem in condensed-matter physics, as it typically emerges at the atomic scale through local interactions mediated by lattice polarization and strain. In this work, we investigate nanoscale polar-supertexture-driven interfacial strain coupling in (PbTiO3)16/(SrRuO3)9/(PbTiO3)16 heterostructures grown on DyScO3(110) substrates. Under appropriate epitaxial strain conditions, the PbTiO3 layers form polar vortex superstructures with a periodicity of approximately 10 nm. We demonstrate that the resulting in-plane nanoscale strain modulation propagates into the SrRuO3 layer. Using element-specific resonant X-ray reflectivity, we probe the nanoscale strain modulations of the strontium and ruthenium sublattices at the interface, revealing strong interfacial strain coupling between the ferroelectric and ferromagnetic layers. These findings provide new insights into engineering nanoscale magnetic modulations through interfacial strain and polarization control.

cond-mat.mtrl-sci↗

arXiv2026

Achieving 100$\,$MHz Instantaneous Bandwidth in a Broadband Rydberg Microwave Sensor

Rydberg atoms have attracted considerable attention in recent years as a novel platform for microwave sensing, owing to their unique physical merits: large transition dipole moments between Rydberg levels and broad frequency coverage. As a critical figure of merit for Rydberg microwave sensors, instantaneous bandwidth serves as a key benchmark for evaluating their viability in practical applications. Previous studies on instantaneous bandwidth remain limited to single-frequency operation, with typical demonstrated values of only tens of megahertz, a constraint that hampers the real-world deployment of this sensing technology. Here, we experimentally achieve an instantaneous bandwidth of over 100$\,$MHz across a broad frequency range of 2.7-20$\,$GHz and realize a sensitivity in the hundreds of nV$\,$cm$^{-1}\,$Hz$^{-1/2}$ range. The physical mechanism lies in the dressed-state coherence and the interference effect between different transition channels. Our work substantially broadens the instantaneous bandwidth of Rydberg microwave sensors and paves the way for their practical deployment in fields such as radar and wireless communications.

physics.atom-ph↗

arXiv2026

Broadband Rydberg Atomic Microwave Sensing with 44.6$\,$MHz Instantaneous Bandwidth

Rydberg atoms have become a promising novel type of microwave sensor due to their excellent physical properties -- broad frequency coverage and large electric dipole moments. High sensitivity and broad instantaneous bandwidth are two indispensable requirements for deployable Rydberg microwave sensors. However, enabling broadband operation while retaining high sensitivity has been a longstanding barrier limiting their applications. We propose and experimentally demonstrate a Rydberg microwave sensor whose instantaneous bandwidth is significantly enhanced via an auxiliary microwave field. By finely modulating the Rydberg energy levels with this field, we broaden the bandwidth substantially while retaining the sensor's inherent high sensitivity. An instantaneous bandwidth of 44.6$\,$MHz ($\pm$22.3$\,$MHz) with a sensitivity of 225.7$\,$nV$\,$cm$^{-1}\,$Hz$^{-1/2}$ is realized in a thermal $^{87}$Rb vapor with the local microwave frequency of 16.03$\,$GHz. Our work delivers concurrent broad instantaneous bandwidth and high sensitivity for Rydberg microwave sensors, paving a technically viable path for their practical deployment in broadband microwave metrology, radar, and wireless communication.

physics.atom-ph↗

arXiv2026

Quantum phase-field model: vortices and THz-induced gap dynamics in superconductors

The ability to simulate the spatial and temporal ordering dynamics of quantum phases in inhomogeneous systems, particularly in the low-temperature regime, is crucial for understanding condensate physics and for enabling emerging device technologies. However, fully microscopic kinetic approaches are often computationally prohibitive for macroscopic spatiotemporal simulations in realistic device geometries, while the widely used phenomenological Ginzburg-Landau formulation is, in principle, valid only near the critical transition temperature $T_c$ and becomes inaccurate in the technologically relevant low-temperature regime. To describe the dynamics of quantum phase ordering, we propose a phase-field formulation derived from a microscopic many-body description using the superconducting order as an example. It leads to a compact dynamical evolution equation for the superconducting order parameter, enabling real-space and real-time simulations of phase ordering dynamics. The simulations successfully capture key dynamical phenomena, including vortex nucleation and motion under static magnetic fields, as well as ultrafast gap oscillations driven by THz fields, over the full temperature range from 0 K up to the critical transition temperature. Beyond superconductivity, this method can be extended to a broad class of quantum condensates and ordered systems, providing a practical computational approach to studying low-temperature ordering dynamics, topological defect evolution, and ultrafast electromagnetic responses of quantum phases in realistic geometries.

cond-mat.supr-con↗

arXiv2026

Multi-Dressed-State Engineered Rydberg Electrometry

Rydberg atoms, with their giant transition electric dipole moments and abundant energy-level transitions, offer exceptional potential for microwave (MW) electric field sensing, combining high sensitivity and broad frequency coverage. However, simultaneously achieving high sensitivity and broad instantaneous bandwidth in a Rydberg-based MW sensor remains a critical challenge. Here, we propose a multi-dressed-state engineered superheterodyne detection scheme for Rydberg electrometry to overcome this challenge. It is found that the key to simultaneously achieving large instantaneous bandwidth and high sensitivity lies in the coherence of dressed states and the interference between transition channels of dressed states. By strategically engineering the multiple dressed states of Rydberg atoms, we demonstrate a thermal $\mathrm{^{87}Rb}$ vapor-based sensor with a sensitivity of 222.6$\,$nV$\,$cm$^{-1}\,$Hz$^{-1/2}$ and a record instantaneous bandwidth of 76.8$\,$MHz with the local microwave frequency 16.03$\,$GHz. This advancement paves the way for Rydberg-atom technologies in radar, wireless communication, and spectrum monitoring.

physics.atom-ph↗

arXiv2026

A Self-Consistent Computational Framework for Displacive Ferroelectrics from the Condensed Ground State

Quantitative description of finite-temperature properties of displacive ferroelectrics, and in particular the critical behavior, is of fundamental importance to both theory and device design, going beyond the Landau-Ginzburg approach, which requires known knowledge of critical behaviors and temperature-dependent parameter fitting. Here within quantum statistic description of polarization fluctuations, we develop a self-consistent, microscopically based computationalframework for finite-temperature thermodynamics and phase transitions in displacive ferroelectrics. It enables one to use only the ground-state properties to predict the finite-temperature properties and in particular, the criticality of phase transitions of various displacive ferroelectrics. Its applications to the classical ferroelectric PbTiO$_3$, quantum paraelectrics SrTiO$_3$ and KTaO$_3$, and recently fabricated ferroelectric strained SrTiO$_3$, demonstrate remarkable quantitative agreements with the experimentally measured dielectric/ferroelectric properties throughout the entire temperature ranges of the phases, including the critical behaviors of phase transitions. The proposed computational framework offers a tractable quantitative basis for bridging microscopic ground-state modeling and macroscopic device-level design in a broad range of ferroelectric systems under diverse thermodynamic and external conditions.

cond-mat.mtrl-sci↗

arXiv2026

Microscopic phase-transition theory of charge density waves: revealing hidden crossovers of phason and amplitudon

We develop a self-consistent phase-transition theory of charge density waves (CDWs), starting from a purely microscopic model. Specifically, we derive a microscopic CDW gap equation $|Δ_0(T)|$, taking into account of thermal phase fluctuations (i.e., thermal excitation of phason) and their influence on CDW pinning (i.e., the phason mass) and CDW gap. We demonstrate that as temperature increases from zero, the phason gradually softens, leading to a thermal depinning crossover (where the phason becomes gapless) at $T_d$ and a subsequent first-order CDW phase transition at $T_c>T_d$. The predicted values of $T_d$, $T_c$ as well as the large ratio of $|Δ_0(T=0)|/(k_BT_c)$ for the quasi-1D CDW material (TaSe$_4$)$_2$I show quantitative agreements with experimental measurements and explain many of the previously observed key thermodynamic features and unresolved issues in literature. To further validate the theory, we calculate the energy gap of CDW amplitudon and its lifetime, and reveal a crossover of amplitudon from a lightly damped to a heavily damped excitation during pinning-depinning crossover while its energy gap is nearly unchanged throughout the entire CDW phase. This finding quantitatively captures and explains the recently observed coherent signal in ultrafast THz emission spectroscopy on (TaSe$_4$)$_2$I.

cond-mat.str-el↗

arXiv2026

Preformed Cooper pairing and the uncondensed normal-state component in phase-fluctuating monolayer cuprate superconductivity

We develop a self-consistent microscopic framework beyond mean-field theory for monolayer cuprate superconductivity. It couples fermionic quasiparticles with collective phase dynamics to treat the gap and superfluid stiffness. The phase sector explicitly incorporates both smooth bosonic Nambu-Goldstone phase fluctuations, renormalized by long-range Coulomb interactions, and topological BKT-type vortex-antivortex fluctuations. The required input is the correlated single-particle spectral function, enabling direct interfacing with Hubbard-type models. The theory provides access to key superconducting observables, including $T$-dependent gap and phase stiffness, gap-closing temperature $T_{\rm os}$, and transition temperature $T_c$, across wide ranges of doping. Using a solvable interaction model as input, our simulations reveal several important features consistent with experimental observations in cuprate superconductors: a $d$-wave superconducting dome in $T$-$p$ phase diagram with a shoulder-like anomaly in underdoped regime, a pronounced separation between $T_c$ and $T_{\rm os}$ signaling preformed Cooper pairing, a finite uncondensed normal component persisting even at $T=0$, and the onset temperature $T_{\rm on,vortex}$ of vortex signals, offering a consistent understanding of how strong correlations and phase fluctuations cooperate to shape high-$T_c$ superconductivity.

cond-mat.str-el↗

arXiv2026

Superconducting Decoherence and Thermal Quenching of the Josephson Diode Effect in Low-Dimensional Josephson Systems

Motivated by recent studies on superconducting (SC) diode nonreciprocity, we uncover a generic smooth SC-phase decoherence mechanism in low-dimensional Josephson structures. Contrary to the conventional single-energy-scale paradigm where Josephson coherence and diode nonreciprocity vanish simultaneously only at the SC gap-closing temperature, we demonstrate, within a fully self-consistent microscopic framework beyond mean-field theory, that SC phase fluctuations generically split these phenomena into distinct energy scales. As a result, rather than a single SC-normal transition, the system exhibits a sequence of distinct thermal crossovers upon heating: the diode effect disappears first at $T_η$, Josephson coherence is subsequently lost at $T_c$, and the SC gap collapses only at a higher temperature $T_s$. Using a bilayer SC system as a concrete example, we show that the separation between these temperature scales is not solely dictated by Josephson coupling, but is instead strongly and counterintuitively shaped by the in-plane disorder and carrier density. These findings reveal that smooth SC phase decoherence introduces a distinct and more fragile energy scale, with potential implications for layered superconductors such as cuprates and recently discovered nickelates, as well as for SC qubit platforms.

cond-mat.supr-con↗

arXiv2026

A tractable framework for phase transitions in phase-fluctuating disordered 2D superconductors: applications to bilayer MoS$_2$ and disordered InO$_x$ thin films

Starting from the purely microscopic model, we go beyond conventional mean-field theory and develop a self-consistent microscopic thermodynamic framework for disordered 2D superconductors. It incorporates the fermionic Bogoliubov quasiparticles, bosonic Nambu-Goldstone (NG) quantum and thermal phase fluctuations in the presence of long-range Coulomb interactions, and topological Berezinskii-Kosterlitz-Thouless (BKT) vortex-antivortex fluctuations on an equal footing, to self-consistently treat the superconducting gap and superfluid density. This unified phase-fluctuating description naturally recovers the previously known limiting results: the superconducting gap in the 2D limit can remain robust against long-wavelength NG phase fluctuations at $T=0^+$ due to Coulomb-induced regularization, while the gradual proliferation of BKT fluctuations as the system approaches criticality drives a separation between the global superconducting transition temperature $T_c$ and the gap-closing temperature $T^*$. In contrast to mean-field theory, which predicts 2D superconductivity to be independent of carrier density and non-magnetic disorder (Anderson theorem), the incorporation of phase fluctuations generates a density- and disorder-dependent zero-point gap $Δ(0)$ and consequently $T_c$ and $T^*$. Remarkably, applications to bilayer MoS$_2$ [Nat. Nanotechnol. 14, 1123 (2019)] and disordered InO$_x$ thin films [Nat. Phys. 21, 104 (2025)] quantitatively reproduce key experimental observations in excellent agreement. The framework offers a useful theoretical tool for understanding phase-fluctuation-dominated superconductivity.

cond-mat.supr-con↗

arXiv2026

Microscopic Phase-Transition Framework for Gate-Tunable Superconductivity in Monolayer WTe$_2$

The recently reported gate-tunable superconductivity in monolayer WTe$_2$ [Science 362, 922 (2018); Science 362, 926 (2018); Nat. Phys. 20, 269 (2024); PRR 7, 013224 (2025)] exhibits several striking anomalies beyond the standard paradigm, including a contrasting carrier-density dependence of the transition temperature $T_c$ in weakly and strongly disordered regimes and more surprisingly, the sudden disappearance of superconducting fluctuations below a critical carrier density. To understand these features, we go beyond mean-field theory and develop a microscopic framework that treats the gap and superfluid density by explicitly and self-consistently incorporating both Nambu-Goldstone phase fluctuations and Berezinskii-Kosterlitz-Thouless fluctuations. We show that these fluctuations are minimal in the weak-disorder regime but become crucial under strong disorder, where the zero-temperature gap renormalized by NG quantum fluctuations becomes density-dependent while the BKT fluctuations drive the $T_c$ below the gap-closing temperature. Simulations within this unified framework combining with the density-functional-theory input to account for the excitonic instability quantitatively reproduced nearly all key experimental observations, providing a consistent understanding of reported anomalies.

cond-mat.supr-con↗

arXiv2026

Revealing THz optical signatures of Shiba-state-induced gapped and gapless superconductivity

We report a fully self-consistent calculation of the complex renormalization by exchange interactions and hence the complete phase diagram of conventional $s$-wave superconductors with magnetic impurities as well as the related physical properties including the optical response. We show that a small amount of magnetic disorder can drive the system into a gapless superconducting state, where the single-particle excitation gap vanishes whereas the superconducting order parameter $Δ_0$ remains finite. In this phase, the linear optical conductivity exhibits a finite absorption over the low-frequency regime, particularly for photon energies below the conventional threshold $2|Δ_0|$, even at low temperatures, in sharp contrast to the gapped state. The nonlinear response, however, remains coherent and is dominated by the Higgs-mode dynamics rather than gapless quasiparticle background. These findings reveal a fundamental distinction between dissipative single-particle excitations and coherent collective dynamics of the condensate, a feature likely general to other gapless superconductors, and introduces a fundamentally different detection scheme, using THz spectroscopy to probe the signatures of Shiba states.

cond-mat.supr-con↗

arXiv2026

Coherent Amplifier-Empowered Quantum Interferometer: Preserving Sensitivity and Quantum Advantage under High Loss

Quantum interferometers offer phase measurement capabilities that surpass the standard quantum limit (SQL), with phase sensitivity and quantum enhancement factor serving as key performance metrics. However, practical implementations face severe degradation of both metrics due to unavoidable losses, representing the foremost challenge in advancing quantum interferometry toward real-world applications. To address this challenge, we propose a coherent-amplifier-empowered quantum interferometer. The coherent amplifier dramatically suppresses the decay of both sensitivity and quantum enhancement under high-loss conditions, maintaining phase sensitivity beyond the original SQL even for losses exceeding 90%. Using an injected 4.2 dB squeezed-vacuum state in experimental demonstration, our scheme reduces the quantum enhancement degradation under 90% loss from 3.7 dB in a conventional quantum interferometer (CQI) to only 1.5 dB. More importantly, the phase sensitivity degradation under the same loss is limited to 4.0 dB, markedly outperforming the 11.2 dB degradation observed in a CQI. This improvement is enabled by the coherent amplifier's phase-sensitive photon amplification and its protection of the quantum state. This breakthrough in amplifier-empowered quantum interferometry overcomes the critical barrier to practical deployment, enabling robust quantum-enhanced measurements in lossy environments.

quant-ph↗

arXiv2025

Altermagnetism-induced non-collinear superconducting diode effect and unidirectional superconducting transport

Current studies of non-reciprocal superconducting (SC) transport have centered on the forward-backward asymmetry of the critical current measured along a single axis. In most realizations, this diode effect is achieved via introducing ferromagnetism or applying an external magnetic field, which drives system into an effective Fulde-Ferrell (FF) state but often at the cost of severely suppressing the SC gap and thus compromising device robustness. Here we propose and theoretically demonstrate that coupling a conventional $s$-wave SC thin film to a $d$-wave altermagnet offers a more resilient alternative. The momentum-dependent spin splitting inherent to altermagnets induces a non-collinear SC-diode effect in the BCS state, with the critical-current anisotropy exhibiting a fourfold ($C_4$) symmetry. Upon entering the FF state at large splitting, this anisotropy gradually evolves into a unidirectional ($C_1$) pattern. Crucially, the FF pairing momentum locks to the discrete crystal axes, eliminating the rotational Goldstone mode and preserving a sizable SC gap without any abrupt or significant suppression. These combined features make the altermagnetic proximity an appealing platform to engineer symmetry-protected, energy-efficient and programmable SC diodes for next-generation electronic devices.

cond-mat.supr-con↗

arXiv2025

Ferroelectric Order and Enhanced Interfacial Superconductivity in Lightly-Doped Quantum Paraelectric KTa$_{1-x}$Nb$_x$O$_3$

Ferroelectric quantum criticality in perovskite oxides offers a fertile ground for emergent collective phenomena. Here we develop a first-principles-inspired quantum-statistics-based theoretical analysis of the ferroelectric order and interfacial superconductivity in lightly-doped quantum paraelectric, niobium (Nb)-doped KTaO$_3$. We demonstrate that local distortions induced by the doped Nb atoms beyond its quantum critical composition induce a long-range ferroelectric order. The predicted dielectric properties quantitatively agree with the experimental measurements over the entire temperature range from the symmetry-broken ferroelectric phase across the phase transition to the paraelectric region. As the same soft phonon mode that governs dielectric behavior provides the essential pairing channel for interfacial superconductivity of KTaO$_3$, we predict a pronounced enhancement of this superconductivity on (111) surface when the system is tuned to its quantum-critical composition via Nb doping, providing a concrete avenue for experimental verification. This finding establishes ferroelectric quantum criticality as a unique design principle for engineering enhanced superconductivity and discovering emergent quantum phases in polar oxide heterostructures, explicitly suggesting that similar materials-tuning strategies (e.g., epitaxial strain) could be exploited to enhance superconductivity in quantum paraelectric systems.

cond-mat.mtrl-sci↗

arXiv2025

AI-assisted hyper-dimensional broadband quantum memory with efficiency above 90% in warm atoms

High-dimensional broadband quantum memory significantly expands quantum information processing capabilities, but the memory efficiency becomes insufficient when extended to high dimensions. We demonstrate an efficient quantum memory for hyper-dimensional photons encoded with orbital angular momentum (OAM) and spin angular momentum (SAM). OAM information is encoded from -5 to +5, combined with SAM encoding, enabling up to 22 dimensions. To ensure high memory efficiency, an artificial intelligence algorithm, a modified Differential Evolution (DE) algorithm using Chebyshev sampling, is developed to obtain a perfect signal-control waveform matching. Memory efficiency is experimentally achieved at 92% for single-mode Gaussian signal, 91% for information dimension of 6 and 80% for dimensional number to 22. The fidelity is achieved up to 99% for single-mode Gaussian signal, 95.5% for OAM information, 97.4% for SAM information, and 92% for whole hyper-dimensional signal, which is far beyond no-cloning limitation. Our results demonstrate superior performance and potential applications in high-dimensional quantum information processing. This achievement provides a crucial foundation for future quantum communication and quantum computing.

quant-ph↗

arXiv2025

Role of Ferrons in the Heat Capacity and Thermal Transport of Displacive Ferroelectrics

The collective amplitude mode of the order parameter in displacive ferroelectrics, termed the ferron, represents the amplitude fluctuations of long-range ordered polarization. At temperatures well below phase transition temperature $T_c$, the energy of ferron excitation is significantly gapped in the long-wavelength limit. As $T_c$ is approached, this gap softens dramatically to minimal or gapless values, thereby should lead to a substantial contribution to thermal properties. In this context, we explore the role of ferrons in heat capacity and thermal transport by incorporating a microscopic self-consistent phase-transition theory for displacive ferroelectricity in contrast to the conventional treatment of attributing thermal properties solely to acoustic phonons. Using ferroelectric $\rm{PbTiO}_{3}$ as a case study, we show that the softening of ferrons near the phase transition is essential to accurately capturing the experimental temperature and electric-field dependencies of thermal properties.

cond-mat.mtrl-sci↗

arXiv2025

Terahertz-induced second-harmonic generation in quantum paraelectrics: hot-phonon effect

Recent terahertz-pump second-harmonic-generation(SHG)-probe measurements of quantum paraelectrics observed a significant long-lived non-oscillatory SHG component following an ultrafast resonant excitation of the soft mode, which was interpreted as a signature of terahertz-induced transient ferroelectric order. Here we propose a temperature-dependent dynamic model incorporating the hot-phonon effect to simulate the soft-mode behaviors under ultrafast terahertz excitation. Its application to paraelectric KTaO3 produces quantitatively most of the features exhibited in our time-resolved SHG measurements and those in existing literature, including a long-lived non-oscillatory SHG response, SHG oscillations at twice the soft-mode frequency, SHG dampings as well as temperature and field-strength dependencies. We conclude that the observed terahertz-induced non-oscillatory SHG response in quantum paraelectrics is a consequence of the induced nonequilibrium hot-phonon effect, offering an alternative to its existing interpretation as a signature of transient ferroelectric order.

cond-mat.mtrl-sci↗