arXiv subjects

Xiaoyang Zhu

Publications and source records attributed to Xiaoyang Zhu.

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

Candidate for a Fractional Topological Insulator in Twisted MoTe2

The interplay among electronic correlation, topology, and time-reversal-symmetry (TRS) often leads to exotic quantum states of matter, as highlighted by the discoveries of fractional Chern insulators (FCIs) in twisted bilayer MoTe2 (tMoTe2). Among the FCIs in tMoTe2, the most robust is at a hole filling factor of v=-2/3 per moiré unit cell. Here, employing pump-probe circular dichroism (CD) measurement on tMoTe2 at twist angles (3.9 and 3.7 degrees), we show that a correlated state at v =-4/3 exhibits an unusual Ising antiferromagnet behavior. The v =-4/3 state with no net magnetization undergoes first order phase transitions at extremely low magnetic fields of ~ 2-6 mT to partially valley polarized (PVP) states. This behavior is notably absent for all other correlated states in tMoTe2 and also disappears for v =-4/3 at higher or lower twist angles (4.0 or 3.3 degree). The observed magnetic signature is consistent with a theoretically proposed fractional topological insulator (FTI), consisting of two copies of v =-2/3 FCIs with opposite chirality in the two K valleys. The experimental results are supported by interacting continuum model calculations that reveal the extreme closeness in energy ( < 1 meV) between the putative FTI and PVP states. Our findings present a candidate FTI with TRS and call for advanced transport and imaging measurements to establish the quantized helical edge modes.

cond-mat.str-el↗

arXiv2026

Ultrafast Charge-Doping via Photo-Thermionic Injection in van der Waals Devices

Van der Waals (vdW) heterostructures of two-dimensional (2D) materials have become a rich playground for the exploration of correlated quantum phases, and recent studies have begun to probe their non-equilibrium dynamics under femtosecond laser excitation. In a time-resolved experiment, optical excitation of the multilayer structure can lead not only to rich dynamic responses from the target layers, such as moiré interfaces, but also to additional device functionality from the layer degree of freedom. Here, we investigate optical excitation in a prototypical moiré device of dual-gated twisted WSe2 bilayers, with few-layer graphite gates and hexagonal boron nitride (hBN) spacers. We establish an ultrafast photodoping mechanism in the moiré bilayer from photo-thermionic emission of the graphite gates. Using transient reflectance experiments, we reveal photo-induced hole injection evidenced by: (i) a shift of gate voltages at which optical signatures of correlated insulators are observed, (ii) a persistent optical signature indicative of charge diffusion at microsecond timescales and local charge buildup from pulse-to-pulse accumulation, and (iii) photoinduced absorption due likely to transient formation of correlated insulators. We further demonstrate that the injected holes can be selectively controlled by tuning the excitation energy, fluence, and gate bias.

cond-mat.str-el↗

arXiv2026

Photoinduced Metal-to-Insulator Transitions in 2D Moiré Devices

Photoexcitation has been utilized to control quantum matter and to uncover metastable phases far from equilibrium. Among demonstrations to date, the most common is the photo-induced transition from correlated insulators to metallic states; however, the reverse process without initial orders has not been observed. Here, we show ultrafast metal-to-insulator transition in gate-doped WS2/WSe2 and WSe2/WSe2 moiré devices using photo-thermionic hole injection from graphite gates. The resulting correlated insulators are metastable, with lifetimes exceeding microseconds. These findings establish an effective mechanism for the ultrafast control of correlated electronic phases in van der Waals heterostructures.

cond-mat.str-el↗

arXiv2026

All-optical switching of nonlinear structured light in crystal-engineered van der Waals materials

The orbital angular momentum (OAM) of light is a discrete, unbounded degree of freedom that underpins mode-multiplexed communications and high-dimensional quantum photonics. Yet, dynamic OAM control remains dependent on bulky free-space optics or cascaded architectures that separate switching from wavefront shaping, hindering nanoscale integration. Here, we engineer artificial van der Waals crystals from rhombohedrally stacked (3R) MoS$_2$, in which spatial control of the local crystal orientation imprints a nonlinear geometric phase onto the second-harmonic (SH) field, enabling background-free generation of SH vortex beams in an ultrathin (46 nm) van der Waals platform. Leveraging the C$_{3v}$ symmetry of 3R-MoS$_2$, we demonstrate monolithic, all-optical switching with sub-optical-cycle precision between Hermite-Gauss-like and Laguerre-Gaussian vortex SH beams with opposite topological charges ($l=\pm1$). Our results establish artificial 3R-MoS$_2$ crystals as a monolithic platform for the generation and all-optical reconfiguration of nonlinear structured light at the nanoscale, advancing active nanophotonic sources for integrated classical and quantum photonic technologies.

physics.optics↗

arXiv2026

Nonequilibrium dynamics of high energy transitions in monolayer WSe$_{2}$

High-energy optical transitions in monolayer transition-metal dichalcogenides exhibit characteristics that are markedly distinct from those of lower-lying band-edge excitons. These differences arise from the involvement of electronic states located at regions of the Brillouin zone that are displaced from the $K$ valleys. In this work, we investigate the ultrafast dynamics of these high-energy excitations by employing broadband ultrafast transient absorption spectroscopy spanning the visible to ultraviolet spectral range. We observe that the formation and relaxation dynamics of one of the high energy transitions display a distinct behavior compared to the lower-energy excitonic resonances, developing on a significantly slower timescale. First-principles calculations of the excitonic landscape allow us to account for this delayed response and attribute it to the phonon-mediated formation of momentum-dark excitons.

cond-mat.mtrl-sci↗

arXiv2026

Giant optical spin-orbit interactions in ferroelectric van der Waals waveguides

Optical spin-orbit interactions (SOI) link photonic spin to momentum, offering a route toward on-chip polarization control and beam steering. Nevertheless, achieving sufficient optical SOI and nonlinearities on sub-micrometer scales - a prerequisite for dense photonic integration - remains an outstanding challenge. Here, we show that highly birefringent van der Waals (vdW) waveguides provide an ideal, chip-compatible platform to address this limitation. We focus on the ferroelectric semiconductor NbOI2, which exhibits record optical nonlinearities and dielectric anisotropy. Using femtosecond optical microscopy, we image light propagation and harmonic conversion beyond the total internal reflection barrier over tens of micrometers in NbOI2 slab waveguides. We report giant optical spin-splitting through the optical spin Hall effect, which facilitates spatial separation of optical spin currents on sub-micrometer scales, in quantitative agreement with a microscopic light-matter interaction model. We further leverage optical spin-momentum locking to realize polarization-controlled waveguide steering. We generalize these observations across various vdW waveguides and empirically confirm a scaling law linking dielectric anisotropy to geometric spin-splitting. Our results establish highly anisotropic vdW waveguides as an ideal platform for densely integrated opto-spintronic technologies.

cond-mat.mtrl-sci↗

arXiv2026

Finite-momentum Cooper plasmons in superconducting terahertz microcavities

The phase mode of a superconductor's order parameter encodes fundamental information about pairing and dissipation, but is typically inaccessible at low frequencies due to the Anderson-Higgs mechanism. Superconducting samples thinner than the London penetration depth, however, support a gapless phase mode whose dispersion can be reshaped by a proximal screening layer. Here, we theoretically and experimentally show that this screened phase mode in a superconducting thin film integrated into on-chip terahertz circuitry naturally forms a superconducting microcavity that hosts resonant finite-momentum standing waves of supercurrent density, which we term Cooper plasmons. We measure two Cooper plasmons in a superconducting NbN microcavity and demonstrate that their resonance frequencies and linewidths independently report the density of participating carriers and plasmon's dissipation at finite momenta. Our results reveal an emergent collective mode of an integrated superconductor-circuit system and establish design principles for engineering or suppressing Cooper plasmons in superconducting terahertz devices and circuits.

cond-mat.supr-con↗

arXiv2025

Tunable Narrowband Terahertz Radiation from van der Waals Ferroelectrics

The terahertz (THz) spectral range is central to high-speed communication, precision metrology, sensing technologies, and a range of fundamental scientific investigations. Achieving these capabilities in practical systems increasingly demands chip-scale integration of THz photonic components that are typically bulky. In this context, van der Waals (vdW) materials provide a unique platform for integrated nonlinear photonics in the visible and near-infrared regimes, and extending this framework into the THz domain would constitute a significant advance. Here, we report tunable, intense, and narrowband THz radiation from ferroelectric niobium oxyhalides. Through halogen substitution and alloying, we achieve continuous and precise control over the emission frequency from 3.1 to 5.8 THz. We show that the narrowband THz radiation is driven by phonons associated with the ferroelectric polarization. We further demonstrate dynamic and nonvolatile control of the polarity of the coherent THz wave with external electric field. This work demonstrates efficient narrowband THz emission from vdW ferroeletrics and provides microscopic insight into its origin, paving the way for on-chip THz technology for a broad range of applications.

physics.optics↗

arXiv2025

Tracking the photoinduced dynamics of a dark excitonic state in single-layer WS$_2$ via resonant Autler-Townes splitting

Excitons in a monolayer transition metal dichalcogenide (1L-TMD) are highly bound states characterized by a Rydberg-like spectrum of discrete energy levels. Among these, states with odd-parity are known as dark excitons due to selection rules, which make their stationary and transient characterization challenging using linear optical techniques. Here, we demonstrate that the dynamics of a 2p dark excitonic state in 1L-WS$_2$ can be directly retrieved by measuring the Autler-Townes splitting of bright states in a three-pulse experiment. The splitting of the bright 1s excitonic state, observed by detuning a mid-infrared control field across the 1s-2p transition, provides an accurate characterization of the 2p state. Following carrier photoinjection, we observe a qualitatively different dynamics of the 1s and 2p levels, which is indicative of symmetry-dependent screening and exciton-exciton interactions. These findings provide new insights into many-body effects in TMDs, offering potential avenues for advancing the next generation optoelectronics.

cond-mat.mes-hall↗

arXiv2025

Dissecting intervalley coupling mechanisms in monolayer transition metal dichalcogenides

Monolayer (1L) transition metal dichalcogenides (TMDs) provide a unique opportunity to control the valley degree of freedom of optically excited charge carriers due to the spin-valley locking effect. Despite extensive studies of the valley-contrasting physics, stimulated by perspective valleytronic applications, a unified picture of competing intervalley coupling processes in 1L-TMDs is lacking. Here, we apply broadband helicity-resolved transient absorption to explore exciton valley polarization dynamics in 1L-WSe${}_2$. By combining experimental results with microscopic simulations, we dissect individual intervalley coupling mechanisms and reveal the crucial role of phonon-assisted scattering in the fast decay of the A exciton circular dichroism and the formation of the dichroism of opposite polarity for the B exciton. We further provide a consistent description of the valley depolarization driven by the combined action of Coulomb scattering processes and indicate the presence of efficient single spin-flip mechanisms. Our study brings us closer to a complete understanding of exciton dynamics in TMDs.

cond-mat.mes-hall↗

arXiv2025

Purcell enhancement of photogalvanic currents in a van der Waals plasmonic self-cavity

Cavities provide a means to manipulate the optical and electronic responses of quantum materials by selectively enhancing light-matter interaction at specific frequencies and momenta. While cavities typically involve external structures, exfoliated flakes of van der Waals (vdW) materials can form intrinsic self-cavities due to their small finite dimensions, confining electromagnetic fields into plasmonic cavity modes, characterized by standing-wave current distributions. While cavity-enhanced phenomena are well-studied at optical frequencies, the impact of self-cavities on nonlinear electronic responses--such as photogalvanic currents--remains largely unexplored, particularly in the terahertz regime, critical for emerging ultrafast optoelectronic technologies. Here, we report a self-cavity-induced Purcell enhancement of photogalvanic currents in the vdW semimetal WTe$_2$. Using ultrafast optoelectronic circuitry, we measured coherent near-field THz emission resulting from nonlinear photocurrents excited at the sample edges. We observed enhanced emission at finite frequencies, tunable via excitation fluence and sample geometry, which we attribute to plasmonic interference effects controlled by the cavity boundaries. We developed an analytical theory that captures the cavity resonance conditions and spectral response across multiple devices. Our findings establish WTe$_2$ as a bias-free, geometry-tunable THz emitter and demonstrate the potential of self-cavity engineering for controlling nonlinear, nonequilibrium dynamics in quantum materials.

cond-mat.mes-hall↗

arXiv2025

Flat band excitons in a three-dimensional supertwisted spiral transition metal dichalcogenide

A new frontier in van der Waals twistronics is the development of three-dimensional (3D) supertwisted materials, where each successive atomic layer rotates by the same angle. While two-dimensional (2D) moire systems have been extensively studied, the unique phenomena arising from 3D twistronics remain largely unexplored. In this work, we report the discovery of flat-band excitons in 3D supertwisted WS2, revealed by systematic photoluminescence (PL) experiments and electronic structure calculations. These excitons retain key features of 2D moire transition metal dichalcogenides (TMDs)-such as layer confinement, moire-driven localization, and strong Coulomb interactions-while also offering advantages in scalability and enhanced optical responses in three dimensions. Beyond the PL signatures reminiscent of 2D A excitons, we observe novel direct and indirect exciton emission uniquely tied to the supertwist geometry. Using generalized Bloch band theory and local density of states calculations that incorporate screw rotational symmetry, we uncovered the coexistence of 2D and 3D flatband gaps. These flat-band excitons serve as sensitive probes of the electronic properties of 3D supertwisted semiconductors and open new pathways for applications in quantum optoelectronics.

physics.app-ph↗

arXiv2025

Van der Waals waveguide quantum electrodynamics probed by infrared nano-photoluminescence

Atomically layered van der Waals (vdW) materials exhibit remarkable properties, including highly-confined infrared waveguide modes and the capacity for infrared emission in the monolayer limit. Here, we engineered structures that leverage both of these nano-optical functionalities. Specifically, we encased a photoluminescing atomic sheet of MoTe2 within two bulk crystals of WSe2, forming a vdW waveguide for the embedded light-emitting monolayer. The modified electromagnetic environment offered by the WSe2 waveguide alters MoTe2 spontaneous emission, a phenomenon we directly image with our interferometric nano-photoluminescence technique. We captured spatially-oscillating nanoscale patterns prompted by spontaneous emission from MoTe2 into waveguide modes of WSe2 slabs. We quantify the resulting Purcell-enhanced emission rate within the framework of a waveguide quantum electrodynamics (QED) model, relating the MoTe2 spontaneous emission rate to the measured waveguide dispersion. Our work marks a significant advance in the implementation of all-vdW QED waveguides.

physics.optics↗

arXiv2025

Charge Density Wave and Ferromagnetism in Intercalated CrSBr

In materials with one-dimensional electronic bands, electron-electron interactions can produce intriguing quantum phenomena, including spin-charge separation and charge density waves (CDW). Most of these systems, however, are non-magnetic, motivating a search for anisotropic materials where the coupling of charge and spin may affect emergent quantum states. Here, chemical intercalation of the van der Waals magnetic semiconductor CrSBr yields $Li_{0.17(2)} (tetrahydrofuran)_{0.26(3)} CrSBr$, which possess an electronically driven quasi-1D CDW with an onset temperature above room temperature. Concurrently, electron doping increases the magnetic ordering temperature from 132 K to 200 K and switches its interlayer magnetic coupling from antiferromagnetic to ferromagnetic. The spin-polarized nature of the anisotropic bands that give rise to this CDW enforces an intrinsic coupling of charge and spin. The coexistence and interplay of ferromagnetism and charge modulation in this exfoliatable material provides a promising platform for studying tunable quantum phenomena across a range of temperatures and thicknesses.

cond-mat.mtrl-sci↗

arXiv2025

Engineering robust strain transmission in van der Waals heterostructure devices

Atomically thin van der Waals materials provide a highly tunable platform for exploring emergent quantum phenomena in solid state systems. Due to their remarkable mechanical strength, one enticing tuning knob is strain. However, the weak strain transfer of graphite and hBN, which are standard components of high-qualityvdW devices, poses fundamental challenges for high-strain experiments. Here, we investigate strain transmission in less-explored orthorhombic crystals and find robust transmission up to several percent at cryogenic temperatures. We further show that strain can be efficiently transferred through these crystals to other 2D materials in traditional heterostructure devices. Using this capability, we demonstrate in-situ strain and gate control of the optical properties of monolayer WS2 utilizing the high-\k{appa} dielectric insulator Bi2SeO5 as a substrate. These results enable the exploration of combined cryo-strain and gate tuning in a variety of layered systems such as moiré heterostructures, air-sensitive 2D magnets and superconductors, and any gated 2D device.

cond-mat.mes-hall↗

arXiv2024

Real-Space Imaging of the Band Topology of Transition Metal Dichalcogenides

The topological properties of Bloch bands are intimately tied to the structure of their electronic wavefunctions within the unit cell of a crystal. Here, we show that scanning tunneling microscopy (STM) measurements on the prototypical transition metal dichalcogenide (TMD) semiconductor WSe$_2$ can be used to unambiguously fix the location of the Wannier center of the valence band. Using site-specific substitutional doping, we first determine the position of the atomic sites within STM images, establishing that the maximum electronic density of states at the $K$-point lies between the atoms. In contrast, the maximum density of states at the $Γ$ point is at the atomic sites. This signifies that WSe$_2$ is a topologically obstructed atomic insulator, which cannot be adiabatically transformed to the trivial atomic insulator limit.

cond-mat.mtrl-sci↗

arXiv2024

Exciton Dressing by Extreme Nonlinear Magnons in a Layered Semiconductor

Collective excitations presenting nonlinear dynamics are fundamental phenomena with broad applications. A prime example is nonlinear optics, where diverse frequency mixing processes are central to communication, sensing, wavelength conversion, and attosecond physics. Leveraging recent progress in van der Waals magnetic semiconductors, we demonstrate nonlinear opto-magnonic coupling by presenting exciton states dressed by up to 20 harmonics of magnons, resulting from their nonlinearities, in the layered antiferromagnetic semiconductor CrSBr. We also create tunable optical side bands from sum- and difference-frequency generation between two optically bright magnon modes under symmetry breaking magnetic fields. Moreover, the observed difference-frequency generation mode can be continuously tuned into resonance with one of the fundamental magnons, resulting in parametric amplification of magnons. These findings realize the modulation of the optical frequency exciton with the extreme nonlinearity of magnons at microwave frequencies, which could find applications in magnonics and hybrid quantum systems, and provide new avenues for implementing opto-magnonic devices.

cond-mat.mes-hall↗