arXiv subjects

Zhuquan Zhang

Publications and source records attributed to Zhuquan Zhang.

At least 19 recordsLinked to original sources

arXiv2026

Antiferromagnetism-altered plasmon dynamics

The interaction between plasmons and magnons is a long-sought phenomenon with implications for fundamental physics and spintronics applications. In three-dimensional systems, this coupling is suppressed by the large mismatch in energy scales, but two-dimensional (2D) plasmons with gapless dispersion can overlap with magnons over a broad spectral range. Despite numerous theoretical predictions, experimental observation of magnon-plasmon interaction has remained elusive. In this work, we study a first-of-its-kind hybrid plasmon-magnon platform based on 2D materials. By deploying scattering-type scanning near-field optical microscopy (s-SNOM) with terahertz radiation, we image propagating plasmon wavepackets at a graphene/NiPS$_3$ interface and track their dynamics across the antiferromagnetic transition of NiPS$_3$. We observe a clear renormalization of the plasmon-polariton dispersion concurrent with the onset of antiferromagnetic order. With complementary Raman scattering and nano-terahertz spectroscopy, we unveil spectral weight redistribution and dielectric screening changes, potentially associated with the multi-magnon continuum, as the underlying mechanism. These results provide solid evidence of coupling between plasmon and antiferromagnetic order, marking a cornerstone for a potential platform for hybrid magnon-plasmon interactions in 2D materials, opening avenues for coherent spin-plasmon devices and tunable terahertz spintronic components.

cond-mat.str-el↗

arXiv2026

Observation of Coherent Ferron Emission and Propagation

Excitation of ordered quantum phases gives rise to collective modes and quasiparticles, as exemplified by spin waves and magnons emerging from magnetic order. Extending this paradigm to ferroelectric materials suggests the existence of polarization waves and their fundamental quanta, ferrons. Here, we report the generation and transport of polarization waves, i.e., coherent ferrons, in the van der Waals ferroelectric material NbOI2. Upon excitation by a short laser pulse, the polarization wave emits intense and narrow-band terahertz (THz) radiation at the ferroelectric transverse optical phonon frequency, modulates the ferroelectric order parameter, and propagates uniaxially along the polar axis at hypersonic velocities of ~105 m/s. These long-lived, uniaxial, and dipole-carrying polarization waves may find applications in narrow-band THz emission, ferronic information processing, and coherent electric control.

cond-mat.mtrl-sci↗

arXiv2026

Ultrafast non-thermal suppression of ferroelectricity by carrier screening in LiNbO3

Ferroelectric materials are key to energy-efficient electronics, memory, and optical applications. While charge carriers typically screen and suppress ferroelectricity, their role under nonequilibrium conditions remains elusive. Here, we use femtosecond laser pulses to liberate trapped carriers in LiNbO3 and track the response using time-resolved second-harmonic generation and stimulated Raman scattering. Even dilute photoexcited carriers induce a rapid yet enduring suppression of polarization and Raman susceptibility. Fluence- and temperature-dependent analyses confirm the suppression is non-thermal and arises from transient carrier screening. These findings reveal an efficient, reversible, and symmetry-preserving mechanism to modulate ferroelectricity on ultrafast timescales, offering a new route to control ferroic and competing quantum phases.

cond-mat.mtrl-sci↗

arXiv2026

Dynamically suppressed lattice rotations in SrTiO$_3$ as a basis for photo-induced ferroelectricity

Photo-induced ferroelectricity in the quantum paraelectric SrTiO$_3$ involves the dynamical interplay between a coherently driven Ti-O stretching vibration and multiple structural degrees of freedom, including antiferrodistortive rotations, strain, and the polar mode instability. In the high-temperature cubic phase, in the absence of average antiferrodistortion, time-resolved X-ray diffuse scattering has evidenced a correlation between a photo-induced reduction in antiferrodistortive fluctuations and the emergence of ferroelectric order. Here, we complement these measurements with time-resolved elastic X-ray diffraction in the low-temperature tetragonal phase, in which antiferrodistortive fluctuations are small but a finite average rotation has set in. In this phase, we observe a long-lived reduction of the equilibrium antiferrodistortive rotation angle. A unified theory of the nonlinear lattice dynamics based on first-principles calculations describes the dynamics in both high-temperature cubic and low-temperature tetragonal phases, providing a basis for light-induced ferroelectricity in SrTiO$_3$.

cond-mat.mtrl-sci↗

arXiv2026

Cavity control of multiferroic order in single-layer NiI$_2$

Controlling materials through their interactions with electromagnetic vacuum fluctuations is an emergent frontier in material engineering. Although recent experiments have demonstrated dark cavity effects for electronic material phases, like superconductivity, ferroelectricity and charge density waves, a smoking gun experiment for magnetic systems is lacking. Largely, this comes from the focus on phase transitions, where a large critical light-matter coupling is needed to observe cavity modifications. Here, we propose spiral magnets, where even a small cavity-mediated change in magnetic interactions is reflected in a change of the spiral wavelength, as a promising platform to observe cavity effects. We focus on the single-layer multiferroic NiI$_2$, interacting with electric field fluctuations from surface phonon polaritons of the paraelectric substrate SrTiO$_3$. With decreasing substrate-material distance, the ratio of nearest and third nearest neighbor exchange interactions reduces, leading to an increase of the spiral wavelength and an eventual transition into a ferromagnetic state. Our work identifies a realistic platform to observe cavity vacuum renormalization effects in magnetic systems.

cond-mat.mtrl-sci↗

arXiv2025

Sliding Disassembly of van der Waals Heterostructures

Many recent advances in our understanding of two-dimensional (2D) electron systems stem from van der Waals (vdW) heterostructures. The assembly process relies on the weak bonding across interfaces between layered vdW compounds, making it possible to construct exceptionally clean heterostructures from chemically and structurally distinct materials - a challenging task for traditional thin-film growth techniques. Here we demonstrate an additional, dynamic degree of freedom afforded by vdW interfaces, wherein we use microstructured polymer stamps to disassemble and reconfigure vdW heterostructures by sliding. We apply this technique to alter the dielectric environment of monolayer graphene, perform scanning tunneling microscopy on semiconducting and air-sensitive monolayers, and manipulate strain-sensitive moiré materials. Together these demonstrations suggest a new paradigm for assembling and dynamically modifying van der Waals heterostructures, with the potential to reveal new insights into 2D electron systems.

cond-mat.mes-hall↗

arXiv2025

Terahertz field-induced metastable magnetization near criticality in FePS3

Controlling the functional properties of quantum materials with light has emerged as a frontier of condensed-matter physics, leading to the discovery of various light-induced phases of matter, such as superconductivity, ferroelectricity, magnetism and charge density waves. However, in most cases, the photoinduced phases return to equilibrium on ultrafast timescales after the light is turned off, limiting their practical applications. Here we use intense terahertz pulses to induce a metastable magnetization with a remarkably long lifetime of more than 2.5 milliseconds in the van der Waals antiferromagnet FePS3. The metastable state becomes increasingly robust as the temperature approaches the antiferromagnetic transition point, suggesting that critical order parameter fluctuations play an important part in facilitating the extended lifetime. By combining first-principles calculations with classical Monte Carlo and spin dynamics simulations, we find that the displacement of a specific phonon mode modulates the exchange couplings in a manner that favours a ground state with finite magnetization near the Néel temperature. This analysis also clarifies how the critical fluctuations of the dominant antiferromagnetic order can amplify both the magnitude and the lifetime of the new magnetic state. Our discovery demonstrates the efficient manipulation of the magnetic ground state in layered magnets through non-thermal pathways using terahertz light and establishes regions near critical points with enhanced order parameter fluctuations as promising areas to search for metastable hidden quantum states.

cond-mat.mtrl-sci↗

arXiv2025

Discovery of magnon self-interaction in a strongly driven antiferromagnet

Nonlinear dynamics govern a wide array of natural phenomena and are essential for understanding nonequilibrium behaviors in condensed matter systems. In magnetically ordered materials, magnons - the quanta of spin waves - exhibit intrinsic nonlinearities that are of great interest in fundamental research and practical applications. Despite progress in the nonlinear control of magnon modes in antiferromagnetic materials, the transition from perturbative to non-perturbative regimes of magnon coherences has remained elusive. Here, we explore the nonlinear dynamics of a magnon mode in an antiferromagnet using two-dimensional terahertz spectroscopy with waveguide-enhanced terahertz fields. By driving the magnon mode far from equilibrium, we demonstrate the emergence of high-order magnon coherences and delineate a distinct transition into non-perturbative magnon nonlinearities. This behavior originates from the intrinsic anharmonicity of the magnetic potential and marks a regime dominated by magnon self-interactions at large spin deflection angles. These findings provide fundamental mechanistic insights that might be exploited for ultrafast switching and other advanced magnonic applications.

cond-mat.mtrl-sci↗

arXiv2025

Nanoscale Ultrafast Lattice Modulation with Hard X-ray Free Electron Laser

Understanding and controlling microscopic dynamics across spatial and temporal scales has driven major progress in science and technology over the past several decades. While ultrafast laser-based techniques have enabled probing nanoscale dynamics at their intrinsic temporal scales down to femto- and attoseconds, the long wavelengths of optical lasers have prevented the interrogation and manipulation of such dynamics with nanoscale spatial specificity. With advances in hard X-ray free electron lasers (FELs), significant progress has been made developing X-ray transient grating (XTG) spectroscopy, aiming at the coherent control of elementary excitations with nanoscale X-ray standing waves. So far, XTGs have been probed only at optical wavelengths, thus intrinsically limiting the achievable periodicities to several hundreds of nm. By achieving sub-femtosecond synchronization of two hard X-ray pulses at a controlled crossing angle, we demonstrate the generation of an XTG with spatial periods of 10 nm. The XTG excitation drives a thermal grating that drives coherent monochromatic longitudinal acoustic phonons in the cubic perovskite, SrTiO3 (STO). With a third X-ray pulse with the same photon energy, time-and-momentum resolved measurement of the XTG-induced scattering intensity modulation provides evidence of ballistic thermal transport at nanometer scale in STO. These results highlight the great potential of XTG for studying high-wave-vector excitations and nanoscale transport in condensed matter, and establish XTG as a powerful platform for the coherent control and study of nanoscale dynamics.

physics.optics↗

arXiv2025

Electromagnon signatures of a metastable multiferroic state

Magnetoelectric multiferroic materials, particularly type-II multiferroics where ferroelectric polarizations arise from magnetic order, offer significant potential for the simultaneous control of magnetic and electric properties. However, it remains an open question as to how the multiferroic ground states are stabilized on the free-energy landscape in the presence of intricate competition between the magnetoelectric coupling and thermal fluctuations. In this work, by using terahertz time-domain spectroscopy in combination with an applied magnetic field, photoexcitation, and single-shot detection, we reveal the spectroscopic signatures of a magnetic-field-induced metastable multiferroic state in a hexaferrite. This state remains robust until thermal influences cause the sample to revert to the original paraelectric state. Our findings shed light on the emergence of metastable multiferroicity and its interplay with thermal dynamics.

cond-mat.str-el↗

arXiv2024

Terahertz stimulated parametric downconversion of a magnon mode in an antiferromagnet

In condensed matter systems, interactions between collective modes offer avenues for nonlinear coherent manipulation of coupled excitations and quantum phases. Antiferromagnets, with their inherently coupled magnon modes, provide a promising platform for nonlinear control of microscopic spin waves and macroscopic magnetization. However, nonlinear magnon-magnon interactions have been only partially elaborated, leaving key gaps in the prospects for potential ultrahigh-bandwidth magnonic signal processing. Here, we use a pair of intense terahertz pulses to sequentially excite two distinct coherent magnon modes in an antiferromagnet and find that the magnon mode with a lower frequency undergoes amplification when the higher-frequency mode is driven. We unveil the nonlinear excitation pathways of this stimulated parametric downconversion process by using polarization-selective two-dimensional terahertz spectroscopy. Our work provides fundamental insights into nonlinear magnonics in antiferromagnets, laying the groundwork for forthcoming spintronic and magnonic devices based on nonlinear magnon-magnon interactions.

cond-mat.mtrl-sci↗

arXiv2024

Terahertz field-induced nonlinear coupling of two magnon modes in an antiferromagnet

Magnons are quantized collective spin-wave excitations in magnetically ordered materials. Revealing their interactions among these collective modes is crucial for the understanding of fundamental many-body effects in such systems and the development of high-speed information transport and processing devices based on them. Nevertheless, identifying couplings between individual magnon modes remains a long-standing challenge. Here, we demonstrate spectroscopic fingerprints of anharmonic coupling between distinct magnon modes in an antiferromagnet, as evidenced by coherent photon emission at the sum and difference frequencies of the two modes. This discovery is enabled by driving two magnon modes coherently with a pair of tailored terahertz fields and then disentangling a mixture of nonlinear responses with different origins. Our approach provides a route for generating nonlinear magnon-magnon mixing.

cond-mat.mtrl-sci↗

arXiv2024

Keldysh tuning of photoluminescence in a lead halide perovskite crystal

In 1964, Keldysh laid the groundwork for strong-field physics in atomic, molecular, and solid-state systems by delineating a ubiquitous transition from multiphoton absorption to quantum electron tunneling under intense AC driving forces. While both processes in semiconductors can generate carriers and result in photon emission through electron-hole recombination, the low quantum yields in most materials have hindered direct observation of the Keldysh crossover. Leveraging the large quantum yields of photoluminescence in lead halide perovskites, we show that we can not only induce bright light emission from extreme sub-bandgap light excitation but also distinguish between photon-induced and electric-field-induced processes. Our results are rationalized by the Landau-Dykhne formalism, providing insights into the non-equilibrium dynamics of strong-field light-matter interactions. These findings open new avenues for light upconversion and sub-bandgap photon detection, highlighting the potential of lead halide perovskites in advanced optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2023

Optical-pump terahertz-probe spectroscopy in high magnetic fields with kHz single-shot detection

We demonstrate optical pump/THz probe (OPTP) spectroscopy with a variable external magnetic field (0-9 T) in which the time-dependent THz signal is measured by echelon-based single-shot detection at a 1 kHz repetition rate. The method reduces data acquisition times by more than an order of magnitude compared to conventional electro-optic sampling using a scanning delay stage. The approach illustrates the wide applicability of the single-shot measurement approach to nonequilibrium systems that are studied through OPTP spectroscopy, especially in cases where parameters such as magnetic field strength (B) or other experimental parameters are varied. We demonstrate the capabilities of our measurement by performing cyclotron resonance experiments in bulk silicon, where we observe B-field dependent carrier relaxation and distinct relaxation rates for different carrier types. We use a pair of economical linear array detectors to measure 500 time points on each shot, offering equivalent performance to camera-based detection with possibilities for higher repetition rates.

physics.optics↗

arXiv2023

Terahertz field-driven magnon upconversion in an antiferromagnet

Tailored light excitation and nonlinear control of lattice vibrations have emerged as powerful strategies to manipulate the properties of quantum materials out of equilibrium. Generalizing the use of coherent phonon-phonon interactions to nonlinear couplings among other types of collective modes would open unprecedented opportunities in the design of novel dynamic functionalities in solids. For example, the collective excitations of magnetic order -- magnons -- can carry information with little energy dissipation, and their coherent and nonlinear control would provide an attractive route to achieve collective-mode-based information processing and storage in forthcoming spintronics and magnonics. Here, we discover that intense terahertz (THz) fields can initiate processes of magnon upconversion mediated by an intermediate magnetic resonance. By using a suite of advanced spectroscopic tools, including a newly demonstrated two-dimensional (2D) THz polarimetry technique enabled by single-shot detection, we unveil the unidirectional nature of coupling between distinct magnon modes of a canted antiferromagnet. Calculations of spin dynamics further suggest that this coupling is a universal feature of antiferromagnets with canted magnetic moments. These results demonstrate a route to inducing desirable energy transfer pathways and THz-induced coupling between coherent magnons in solids and pave the way for a new era in the development of ultrafast control of magnetism.

cond-mat.mtrl-sci↗

arXiv2023

Discovery of enhanced lattice dynamics in a single-layered hybrid perovskite

Layered hybrid perovskites have attracted much attention in recent years due to their emergent physical properties and exceptional functional performances, but the coexistence of lattice order and structural disorder severely hinders our understanding of these materials. One unsolved problem regards how the lattice dynamics are affected by the dimensional engineering of the inorganic frameworks and the interaction with the molecular moieties. Here, we address this question by using a combination of high-resolution spontaneous Raman scattering, high-field terahertz spectroscopy, and molecular dynamics simulations. This approach enables us to reveal the structural vibrations and disorder in and out of equilibrium and provides surprising observables that differentiate single- and double-layered perovskites. While no distinct vibrational coherence is observed in double-layer perovskites, we discover that an off-resonant terahertz pulse can selectively drive a long-lived coherent phonon mode through a two-photon process in the single-layered system. This difference highlights the dramatic change in the lattice environment as the dimension is reduced. The present findings pave the way for the ultrafast structural engineering of hybrid lattices as well as for developing high-speed optical modulators based on layered perovskites.

cond-mat.mtrl-sci↗

arXiv2022

Snapshots of a light-induced metastable hidden phase driven by the collapse of charge order

Nonequilibrium hidden states, both transient and long-lived, provide a unique window into thermally inaccessible regimes of strong coupling between microscopic degrees of freedom in quantum materials. Understanding the physical origin of these states is of both fundamental and practical significance, allowing the exploration of far-from-equilibrium thermodynamics and the development of optoelectronic devices with on-demand photoresponses. However, mapping the ultrafast formation of a long-lived hidden phase remains a long-standing challenge in physics since the initial state of the system is not recovered rapidly and conventional pump-probe methods are thus not applicable. Here, using a suite of state-of-the-art single-shot spectroscopy techniques, we present a direct ultrafast visualization of the photoinduced phase transition to both transient and long-lived hidden states in an electronic crystal, 1T-TaS2. Capturing the dynamics of this complex phase transformation in a single-shot fashion demonstrates a commonality in microscopic pathways, driven by the collapse of charge order, that the system undergoes to enter the hidden state and provides unambiguous spectral fingerprints that distinguish such state from thermally accessible phases. We present a theory of fluctuation-dominated process that explains both the dynamics and the nature of the metastable state. Our results settle the debate around the origin of this elusive metastable state and pave the way for the discovery of new quantum phases of matter.

cond-mat.str-el↗

arXiv2021

Terahertz Field-Induced Reemergence of Quenched Photoluminescence in Quantum Dots

Continuous and concerted development of colloidal quantum-dot light-emitting diodes over the past two decades has established them as a bedrock technology for the next generation of displays. However, a fundamental issue that limits the performance of these devices is the quenching of photoluminescence due to excess charges from conductive charge transport layers. Although device designs have leveraged various workarounds, doing so often comes at the cost of limiting efficient charge injection. Here we demonstrate that high-field terahertz (THz) pulses can dramatically brighten quenched QDs on metallic surfaces, an effect which persists for minutes after THz irradiation. This phenomenon is attributed to the ability of the THz field to remove excess charges, thereby reducing trion and non-radiative Auger recombination. Our findings show that THz technologies can be used to suppress and control such undesired non-radiative decay, potentially in a variety of luminescent materials for future device applications.

cond-mat.mes-hall↗