arXiv subjects

Zhangyuan Chen

Publications and source records attributed to Zhangyuan Chen.

10 recordsLinked to original sources

arXiv2026

Interplay of Quasiperiodic Criticality and the Non-Hermitian Skin Effect

Quasiperiodic lattices can host critical eigenstates, whereas nonreciprocal hopping in non-Hermitian lattices can induce non-Hermitian skin effect. In this work, we investigate localization phenomena in a Hatano--Nelson model with quasiperiodically modulated hopping amplitudes, where nonreciprocity arises from unequal modulation strengths of the right and left hoppings. Using a non-unitary gauge transformation, we map the non-Hermitian system into a Hermitian quasiperiodic system and obtain an exact analytical expression for the Lyapunov exponent in the thermodynamic limit. Under periodic boundary conditions, inverse participation ratios and finite-size scaling analysis are used to identify the quasiperiodic critical regimes. The comparison shows that parameter regimes hosting quasiperiodic critical states under periodic boundary conditions can exhibit the non-Hermitian skin effect under open boundary conditions. Furthermore, the non-Hermitian skin effect associated with quasiperiodic critical regimes is also observed in representative long-range hopping models and multiband extensions. Our results provide an analytically controlled perspective on how quasiperiodicity, modulated nonreciprocity, and boundary conditions jointly shape the non-Hermitian skin effect in critical regimes.

cond-mat.mes-hall↗

arXiv2026

Quasiperiodic Skin Criticality in an Exactly Solvable Non-Hermitian Quasicrystal

Critical states in quasiperiodic systems defy the conventional dichotomy between extended and localized states. In this work, we demonstrate that non-Hermiticity fundamentally reshapes this paradigm by giving rise to an exactly solvable quasiperiodic critical phase with no energy selectivity. We introduce a non-Hermitian quasiperiodic lattice based on a modulated Hatano-Nelson model and uncover a new universality class of quasiperiodic skin criticality, in which all eigenstates share an identical multifractal spatial structure. Through a nonunitary gauge transformation, the system is mapped onto a disorder-free lattice, enabling exact analytical solutions for the full spectrum and eigenstates. As a consequence, the inverse participation ratio is strictly energy-independent and controlled solely by a global phase. We further show that this criticality persists in multiband lattices, establishing a general and analytically controlled framework for non-Hermitian quasiperiodic critical phenomena.

cond-mat.mes-hall↗

arXiv2026

Spin-orbit-driven quarter semimetals in rhombohedral graphene

Semimetals exhibit intriguing characteristics attributed to the coexistence of both electrons and holes. In rhombohedral multilayer graphene, a strong trigonal warping effect gives rise to a semi-metallic state near the Fermi surface, offering unique opportunities to explore the interplay of semi-metallic properties with strong correlations and topologies. Here, the observation of quarter semimetals in rhombohedral multilayer graphene by introducing spin-orbit coupling (SOC) is reported. The semi-metallic characteristics of rhombohedral graphene manifest as nearly vanished Hall resistance and parabolic longitudinal resistance. The strong correlations arising from the surface flat band lead to spontaneous symmetry breaking. SOC proximitized by WSe2 further lifts the valley degeneracy, resulting in the spontaneous time-reversal symmetry breaking, as evidenced by the hysteretic anomalous Hall effect. The coexistence of fully polarized electrons and holes allows for the observation of a non-monotonic temperature dependence of the anomalous Hall resistance. Furthermore, the application of moderate magnetic fields induces a phase transition from quarter semimetals to Chern insulators. These findings establish rhombohedral multilayer graphene as an ideal platform for studying strong correlations and topologies in semimetals.

cond-mat.mes-hall↗

arXiv2026

Layer-engineered quantum anomalous Hall effect in twisted rhombohedral graphene

Realizing programmable topological states in quantum anomalous Hall (QAH) insulators requires the ability to design and dynamically tune their topological invariant, the Chern number C. Here, we report a designer QAH platform based on twisted rhombohedral graphene family, in which C becomes a programmable and electrically tunable degree of freedom. By engineering the layer configuration in twisted monolayer-rhombohedral N-layer graphene, denoted as (1+N)L, we realize QAH states with C=N at moire filling v=1, where the layer number N=3,4,5 directly sets the Chern number. Beyond such static layer programming, we demonstrate in-situ electrical control. In a twisted monolayer-trilayer device, the sign of C (chirality) can be switched by electrostatic doping or displacement field. Most strikingly, in twisted Bernal bilayer-rhombohedral tetralayer graphene denoted as (2+4)L, we drive a displacement-field-induced topological phase transition between two distinct QAH states with C=3 and C=4 in a single device. Our work establishes a layer-engineered and electrically tunable platform that transitions topological quantum matter from discovery to design, opening the way toward on-demand engineering of correlated topological states and reconfigurable topological electronics.

cond-mat.mes-hall↗

arXiv2025

Diverse high-Chern-number quantum anomalous Hall insulators in twisted rhombohedral graphene

Quantum anomalous Hall (QAH) insulators with high Chern number (C) enables multiple dissipationless edge channels for low-power-consumption electronics. We report the realization of multiple high-C QAH insulators including C=3,5,6, and 7 in twisted monolayer-rhombohedral pentalayer graphene. In twist angles of approximately 1.40°, we observe QAH effect with C=5 at a filling of one electron per moiré unit cell, persisting up to 2 Kelvin. Furthermore, incommensurate QAH insulators with C=5,6, and 7 emerge at partial fillings. In twist angles of 0.89°, Chern insulators with C=3 and C=6 appear at fillings of two and three electrons, respectively. Our findings establish twisted rhombohedral multilayer graphene as a highly tunable platform for multichannel, dissipationless electronics and for the exploration of exotic quantum Hall states beyond traditional Landau level paradigm.

cond-mat.mes-hall↗

arXiv2025

High-quality metalens enables minimally invasive CFB endoscopy

Metalenses, owing to their ultra-thin planar structures, present a promising solution for reducing endoscopic invasiveness. However, achieving high-quality imaging with minimal invasiveness (short focal length of metalens) remains a critical challenge. This paper presents a deep learning assisted metalens with a 1mm focal length tailored for coherent fiber bundle, constituting the least invasive metalens-CFB system reported to date. To overcome the increased chromatic dispersion and aberrations associated with high NA metalens, we compensate for lateral etching at the base of the nanopillars by adjusting the thickness of a sacrificial hard mask. This approach enables the fabrication of nanopillars with small cross sections and high aspect ratios, featuring nearly vertical sidewalls (~90°), thereby enhancing the phase accuracy of the metalens. Experimental validation using the metalens-CFB system demonstrates that the metalens achieves an expanded field of view of 48.3° and a depth of field exceeding 125 mm. This work establishes a new paradigm for ultra-minimally invasive endoscopic imaging.

physics.optics↗

arXiv2025

1-Tb/s/λ Transmission over Record 10714-km AR-HCF

We present the first single-channel 1.001-Tb/s DP-36QAM-PCS recirculating transmission over 73 loops of 146.77-km ultra-low-loss & low-IMI DNANF-5 fiber, achieving a record transmission distance of 10,714.28 km.

physics.optics↗

arXiv2024

Highly coherent two-color laser with stability below 3E-17 at 1 second

Two-color lasers with high coherence are paramount in precision measurement, accurate light-matter interaction, and low-noise photonic microwave generation. However, conventional two-color lasers often suffer from low coherence, particularly when these two colors face large frequency spacings. Here, harnessing the Pound-Drever-Hall technique, we synchronize two lasers to a shared ultra-stable optical reference cavity to break through the thermal noise constraint, achieving a highly coherent two-color laser. With conquering these non-common mode noises, we demonstrate an exceptional fractional frequency instability of 2.7E-17 at 1 second when normalized to the optical frequency. Characterizing coherence across large frequency spacings poses a significant challenge. To tackle this, we employ electro-optical frequency division to transfer the relative stability of a 0.5 THz spacing two-color laser to a 25 GHz microwave signal. As its performance surpasses the sensitivity of the current apparatus, we establish two independent systems for comparative analyses. The resulting 25 GHz signals exhibit exceptional phase noise of -74 dBc/Hz at 1 Hz and -120 dBc/Hz at 100 Hz, demonstrating the two-color laser's performance approaching the quantum noise limit of its synchronization system. It also sets a new record for the two-point frequency division method in photonic microwave generation. Our achievement in highly coherent two-color lasers and low-noise microwave signals will usher in a new era for precision measurements and refine the accuracy of light-matter and microwave-matter interactions to their next decimal place.

physics.optics↗

arXiv2024

Loading-effect-based 3-D microfabrication empowers on-chip Brillouin optomechanics

The acousto-optic interaction known as stimulated Brillouin scattering (SBS) has emerged as fundamental principles for realizing crucial components and functionalities in integrated photonics. However, the main challenge of integrated Brillouin devices is how to effectively confine both optical and acoustic waves. Apart from that, the manufacturing processes for these devices need to be compatible with standard fabrication platforms, and streamlined to facilitate their large-scale integration. Here, we demonstrate a novel suspended nanowire structure that can tightly confine photons and phonons. Furthermore, tailored for this structure, we introduce a loading-effect-based three-dimensional microfabrication technique, compatible with complementary metal-oxide-semiconductor (CMOS) technology. This innovative technique allows for the fabrication of the entire structure using a single-step lithography exposure, significantly streamlining the fabrication process. Leveraging this structure and fabrication scheme, we have achieved a Brillouin gain coefficient of 1100 1/W/m on the silicon-on-insulator platform within a compact footprint. It can support a Brillouin net gain over 4.1 dB with modest pump powers. We believe that this structure can significantly advance the development of SBS on chip, unlocking new opportunities for the large-scale integration of Brillouin-based photonic devices.

physics.optics↗

arXiv2024

Anti-resonant acoustic waveguides enabled tailorable Brillouin scattering on chip

Empowering independent control of optical and acoustic modes and enhancing the photon-phonon interaction, integrated photonics boosts the advancements of on-chip stimulated Brillouin scattering (SBS). However, achieving acoustic waveguides with low loss, tailorability, and easy fabrication remains a challenge. Here, inspired by the optical anti-resonance in hollow-core fibers, we propose suspended anti-resonant acoustic waveguides (SARAWs) with superior confinement and high selectivity of acoustic modes, supporting both forward and backward SBS on chip. Furthermore, this structure streamlines the design and fabrication processes. Leveraging the advantages of SARAWs, we have showcased a series of record-breaking results for SBS within a compact footprint on the silicon-on-insulator platform. For forward SBS, a centimeter-scale SARAW supports a large net gain exceeding 6.4 dB. For backward SBS, we have observed an unprecedented Brillouin frequency shift of 27.6 GHz and a mechanical quality factor of up to 1,960 in silicon waveguides. This paradigm of acoustic waveguide propels SBS into a new era, unlocking new opportunities in the fields of optomechanics, phononic circuits, and hybrid quantum systems.

physics.optics↗