arXiv subjects

Zebin Zhu

Publications and source records attributed to Zebin Zhu.

6 recordsLinked to original sources

arXiv2026

Time-Reversal-Invariant Altermagnetic Acoustic Crystals

Altermagnets have emerged as a new class of magnetic materials that combine spin-split electronic bands with zero net magnetization. Extending this paradigm to classical-wave systems has, however, been fundamentally challenging because conventional realizations require broken time-reversal symmetry (TRS). Here, we overcome this limitation by introducing two pseudospin degrees of freedom and constructing a pseudo-time-reversal operator that faithfully reproduces the action of its physical counterpart while preserving actual TRS. Building on this framework, we theoretically propose and experimentally realize the first time-reversal-invariant altermagnetic acoustic crystal. Acoustic measurements directly reveal pseudospin-dependent band splitting--a defining hallmark of altermagnetism--under strictly TRS-preserving conditions. Moreover, the altermagnetic acoustic crystal exhibits sublattice-pseudospin locking, enabling flexible control over acoustic pseudospin splitting and filtering. Our work establishes acoustic crystals as a versatile platform for exploring altermagnetic physics and opens new avenues for spin-inspired wave manipulation in nonmagnetic devices.

cond-mat.mes-hall↗

arXiv2026

Orbital Altermagnetic Photonic Crystal

Altermagnetism features momentum-dependent spin splitting without net magnetization, extending spintronics beyond conventional ferromagnetism and antiferromagnetism. However, the photonic realization of altermagnetism has remained a formidable challenge due to the fundamental differences between fermionic electrons and bosonic photons. Here, we report the first experimental realization of an orbital altermagnetic photonic crystal, based on an antiunitary $C_{4z}\mathcal{T}$ symmetry enforced correspondence between a local $p$-orbital $\sigma/\pi$ doublet and crystal momentum. We experimentally demonstrate that the resulting system exhibits momentum-dependent spin splitting with alternating pseudospin polarization and a $d_{xy}$-wave form factor, as confirmed by measured band structures and iso-frequency contours. Moreover, we show that the orbital altermagnetic photonic crystal supports unique pseudospin-selective transport of electromagnetic waves, including photonic pseudospin splitting and pseudospin filtering. Our results extend the field of alternagnetism to photonic systems, opening a new avenue for designing spinphotonic devices.

physics.optics↗

arXiv2026

Observation of full momentum bandgap in photonic time crystals

The hallmark feature of photonic time crystals (PTCs) is the momentum bandgap, yet opening such a gap is extremely challenging, as it demands strong and rapid temporal modulation of the material properties. Recent theoretical advances have shown that resonance effects can substantially expand the momentum bandgap, and even give rise to a full (infinite) momentum bandgap spanning the entire momentum space. Despite these predictions, a full momentum bandgap has yet to be observed experimentally. Here, we report the first experimental observation of full momentum bandgaps in a microwave PTC. By enhancing the resonant effect, we demonstrate that the momentum bandgap can be drastically widened in a dynamically modulated microwave surface plasmon transmission-line metamaterial, leading to tighter spatiotemporal field confinement and greater robustness against temporal disorder. Remarkably, using a dynamically modulated microwave coupled resonator metamaterial characterized by coupled-resonator optical waveguide dispersion, we achieve a full momentum bandgap spanning the entire momentum space, thereby enabling arbitrary spatial localization and temporal amplification of microwave fields. Our findings establish a unified experimental framework for expanding momentum bandgaps up to an infinite extent with minimal requirements on modulation strength and speed, thus paving a viable route toward the first experimental realization of PTCs at optical frequencies.

physics.optics↗

arXiv2026

Space and space-time topologies in a type-II hyperbolic lattice

Recent breakthroughs in hyperbolic lattices have expanded the study of topological phases of matter from Euclidean to non-Euclidean spaces. However, prior work has mostly focused on spatial topological states at the single outer edge of type-I hyperbolic lattices. The dynamic transfer of hyperbolic topological states across multiple edges, as well as the emergence of spatiotemporal topological phenomena, remains largely unexplored. Here, we establish both spatial and spatiotemporal topologies in a newly discovered type-II hyperbolic lattice possessing outer and inner edges. Using electric circuits, we experimentally realize a type-II hyperbolic Chern insulator and directly observe degenerate chiral edge states of opposite chirality at its outer and inner edges. Furthermore, by coupling these counter-propagating chiral edge states, we demonstrate an anti-time-parity phase transition, enabling dynamic transfer between them in arbitrary proportions. Finally, we propose a novel paradigm for constructing a (2+1)-dimensional hyperbolic space-time crystal, which hosts an intertwined topology of spatial Chern and temporal winding numbers, resulting in a unique space-time topological string state. Our work expands the frontier of hyperbolic topological physics, paving the way for the spatiotemporal dynamic manipulation of hyperbolic topological states.

physics.optics↗

arXiv2025

Spatiotemporal topological phase transitions in photonic spacetime crystals

Topological phase transitions, characterized by the closing and reopening of band gaps and a concomitant change in topological invariants, have played a central role in topological physics. However, such transitions have so far been restricted to spatial crystals, relying solely on energy band gaps and spatial interfaces. Here, we transcend this conventional framework and report, for the first time, spatiotemporal topological phase transition in photonic spacetime crystals - structures that are periodically modulated in both space and time. Using a dynamically modulated transmission line metamaterial, we theoretically propose and experimentally demonstrate complete spatiotemporal topological phase transitions characterized by the closing and reopening of both energy and momentum band gaps, alongside changes in spatiotemporal topological invariants and topological phases. Furthermore, in a genuine photonic spacetime crystal that possesses a complete energy-momentum band gap, we directly observe a space-time topological event that localizes in both space and time, exhibiting relativistic-causality-governed excitation and robustness against spatiotemporal disorders. Our findings reveal the interplay among space, time, and topology, establishing a unified framework that provides a comprehensive picture of the emerging topological space-time physics and opening new avenues for robust spatiotemporal topological wave manipulations.

physics.optics↗

arXiv2024

Realization of type-II double-zero-index photonic crystals

Some photonic crystals (PCs) with Dirac-like conical dispersions exhibit the property of double zero refractive index (that is, both epsilon and mu near zero (EMNZ)), wherein the electromagnetic waves have an infinite effective wavelength and do not experience any spatial phase change. The Dirac-like cones that support EMNZ are previously thought to present only at the center of the Brillouin zone ($\Gamma$ point) with a zero wavevector (we refer to as type-I EMNZ), which is constrained by the proportional relationship between phase refractive index and wavevector ($n=kc/\omega$). Here, we demonstrate the existence of an anomalous type-II EMNZ in PCs, which is associated with the Dirac-like point at off-$\Gamma$ points. By introducing a wave modulation approach, we theoretically elucidate its physical mechanism, and resolve the paradox of type-II EMNZ with non-zero wavevectors. We then fabricate a type-II EMNZ PC operating at the X point, and experimentally demonstrate that both its effective permittivity and permeability are zero at the Dirac-like point. Type-II EMNZ PCs exhibit a range of intriguing phenomena, including angle-selective transmission, wavefront flattening, a 180$^{\circ}$ phase shift upon transmission, and waveguiding with natural zero radiation loss. The extraordinary properties of type-II EMNZ PCs may open new avenues for the development of angle-selective optical filters, directional light sources, phase-controlled optical switches, ultracompact photonic circuits, nanolasers, and on-chip nonlinear enhancement.

physics.optics↗