Exotic Phenomena Research Articles

Luttinger liquid under non-Hermitian quantum sensing

The interplay between dissipation and interactions could give rise to exotic phenomena. In this work, we apply the non-Hermitian (NH) linear response theory to investigate the density response of a spinless Luttinger liquid under local NH perturbations. We show that NH backward scattering results in interference between correlation functions arising from dissipation and Langevin noise terms, leading to fluctuations in the induced density rate of change as the Luttinger parameter K varies. For odd integer values of K, this rate vanishes, indicating immunity to dissipation. We demonstrate that the steady-state induced density is predominantly determined by Langevin noise, underscoring the necessity of including Langevin noise terms in NH Hamiltonians. For attractive interactions, the response to NH perturbations grows exponentially and exhibits a π/2 phase shift, indicating stronger sensitivity to NH perturbations compared to Hermitian ones. Additionally, we show that NH forward scattering leads to a decay of the induced density. We refer to these peculiar behaviors of density fluctuations under NH perturbations as anomalous Friedel oscillations. Our findings offer new prospects for sensing and understanding dissipation and interactions.

Elf autoencoder for unsupervised exploration of flat-band materials using electronic band structure fingerprints

Two-dimensional materials with flat electronic bands are promising for realising exotic quantum phenomena such as unconventional superconductivity and nontrivial topology. However, exploring their vast chemical space is a significant challenge. Here we introduce elf, an unsupervised convolutional autoencoder that encodes electronic band structure images into fingerprint vectors, enabling the autonomous clustering of materials by electronic properties beyond traditional chemical paradigms. Unsupervised visualisation of the fingerprint space then uncovers hidden chemical trends and identifies promising candidates based on similarities to well-studied exemplars. This approach complements high-throughput ab initio methods by rapidly screening candidates and guiding further investigations into the mechanisms underlying flat-band physics. The elf autoencoder is a powerful tool for autonomous discovery of unexplored flat-band materials, enabling unbiased identification of compounds with desirable electronic properties across the 2D chemical space.

Barcode encryption based on reflected spatial shifts on the surface of twisted black phosphorus/α-MoO3 heterostructure

In this work, we propose a heterostructure composed of multilayer twisted black phosphorous (BP) and α-phase molybdenum trioxide (α-MoO3) since the twist angles among BP layers may result in exotic phenomena. We theoretically investigate the impact of the primary physical parameters on the Goos–Hänchen (GH) and Imbert–Fedorov (IF) shifts in or near the reststrahlen bands, including the twisted angle, carrier density, and layer numbers of the twisted BP film. The optimal twisted BP/α-MoO3 heterostructure is selected for the different crystalline structure of α-MoO3, where the maximum of GH-shifts can achieve 11704.5λ0 (λ0 is the vacuum wavelength), resulting in a direct measurement. On the other hand, the IF-shift caused by the anisotropy of the twisted BP layer is increased at 1892.1λ0. Based on the tunable GH- and IF-shifts, information processing through two or four separate channels for barcode encryption is constructed and examined. The outcomes can serve as a guide for using GH- and IF-shift in optical encoder design.

Dynamical suppression of many-body non-Hermitian skin effect in anyonic systems

The non-Hermitian skin effect (NHSE) is a fascinating phenomenon in nonequilibrium systems where eigenstates massively localize at the systems’ boundaries, pumping (quasi-)particles loaded in these systems unidirectionally to the boundaries. Its interplay with many-body effects has been widely explored recently, and inter-particle repulsion or Fermi degeneracy pressure have been shown to limit the boundary accumulation induced by the NHSE both in their eigensolutions and dynamics. However, in this work we find that anyonic statistics can even more profoundly affect the NHSE dynamics, suppressing or even reversing the state dynamics against the localizing direction of the NHSE. This phenomenon is found to be more pronounced when more particles are involved. The spreading of quantum information in this system shows even more exotic phenomena, where NHSE affects only the information dynamics for a thermal ensemble, but not that for a single initial state. Our results open up a new avenue on exploring novel non-Hermitian phenomena arisen from the interplay between NHSE and anyonic statistics, and can potentially be demonstrated in ultracold atomic quantum simulators and quantum computers.

Spin polarization detection via chirality-induced tunnelling currents in indium selenide.

Chirality, a basic property of symmetry breaking, is crucial for fields such as biology and physics. Recent advances in the study of chiral systems have stimulated interest in the discovery of symmetry-breaking states that enable exotic phenomena such as spontaneous gyrotropic order and superconductivity. Here we examine the interaction between light chirality and electron spins in indium selenide and study the effect of magnetic field on emerging tunnelling photocurrents at the Van Hove singularity. Although the effect is symmetric under linearly polarized light excitation, a non-symmetric signal emerges when the excitation is circularly polarized, making it possible to electrically detect light's chirality. Our study shows a negligible out-of-plane g-factor for few-layer indium selenide at the valence band edge, resulting in an unbalanced Zeeman splitting in hexagonal boron nitride spin bands. This finding allows us to measure the change in energy barrier height with exceptional resolution (~15 μeV). Furthermore, we confirm the long-standing theoretical prediction of spin-polarized hole accumulation in the flat valence band at increasing laser powers.

Magnetochiral charge pumping due to charge trapping and skin effect in chirality-induced spin selectivity

Chirality-induced spin selectivity (CISS) generates giant spin polarization in transport through chiral molecules, paving the way for novel spintronic devices and enantiomer separation. Unlike conventional transport, CISS magnetoresistance (MR) violates Onsager’s reciprocal relation, exhibiting significant resistance changes when reversing electrode magnetization at zero bias. However, its underlying mechanism remains unresolved. In this work, we propose that CISS MR originates from charge trapping that modifies the electron tunneling barrier and circumvents Onsager’s relation, distinct from previous spin polarization-based models. Charge trapping is governed by the non-Hermitian skin effect, where dissipation leads to exponential wavefunction localization at the ferromagnet-chiral molecule interface. Reversing magnetization or chirality alters the localization direction, changing the occupation of impurity/defect states in the molecule (i.e., charge trapping) – a phenomenon we term magnetochiral charge pumping. Our theory explains why CISS MR can far exceed the ferromagnet spin polarization and why chiral molecules violate the reciprocal relation but chiral metals do not. Furthermore, it predicts exotic phenomena beyond the conventional CISS framework, including asymmetric MR induced by magnetic fields alone (without ferromagnetic electrodes), as confirmed by recent experiments. This work offers a deeper understanding of CISS and opens avenues for controlling electrostatic interactions in chemical and biological systems through the magnetochiral charge pumping.

Quasi-1D, rectangular-like and hexagonal moiré superlattices in exfoliated highly oriented pyrolytic graphite

Understanding the dynamics of structural collapse in hexagonal moiré superlattices is of importance towards the stabilization of exotic superconductive phenomena in quasi-1D superlattices. Here we investigate unusual exfoliation-induced structural transformations of hexagonal moiré superlattices, in a μm-thin lamella of highly oriented pyrolytic graphite (HOPG). By employing a combination of experimental techniques, namely transmission electron microscopy (TEM), high resolution TEM (HRTEM) and selective area electron diffraction (SAED) we systematically investigate the collapse of hexagonal moiré superlattices into rectangular-like and quasi-1D superlattices in an exfoliated lamella. Interestingly, the rectangular-like lattice reveals perpendicular super-periodicities D1 ~ 17 nm and D2 ~ 9 nm. The width of the quasi-1D channels was found to vary from ~5 nm to ~13 nm, with a super-periodicity D ~ 3 nm. Most importantly, systematic SAED acquisitions reveal a splitting of the electron diffraction signals into multiple components, consisting of doubled and tripled diffraction rings. We interpret these as a clear indicator of structural disorder which results from an interplay of exfoliation-induced structural transitions, namely interlayer-sliding, local layer-rotation (twisting) and significant wrinkling (strain) effects.

Correlated Dirac Fermions in Twisted Bilayers of MoS2.

Artificial honeycomb lattices are essential for understanding exotic quantum phenomena arising from the interplay between Dirac physics and electron correlation. This work shows that the top two moiré valence bands in rhombohedral-stacked twisted MoS2 bilayers (tb-MoS2) form a honeycomb lattice with massless Dirac fermions. The hopping and Coulomb interaction parameters are explicitly determined based on large-scale ab initio calculations. The system exhibits significant nonlocal Coulomb repulsion and can be described by the extended Hubbard model. At half-filling, strong Coulomb repulsion in free-standing tb-MoS2 drives the system away from the semimetal phase, resulting in strongly correlated Dirac fermions. By varying the twist angle and dielectric environment, the Hamiltonian parameters can be tuned in a wide range, enabling transitions between distinct quantum phases. The high tunability makes tb-MoS2 a promising simulator for exploring many-body effects of Dirac fermions.

Band evolution and 2D nonreciprocal wave propagation in strongly nonlinear meta-plates

AbstractNonlinear meta-materials/structures can enable exotic wave phenomena, leading to extraordinary functionalities, but the bandgap properties and nonreciprocal wave manipulation in high-dimensional nonlinear metamaterials, such as plates, are not fully understood. In this study, we examine the band evolution and nonreciprocal wave propagation in a two-dimensional (2D) strongly nonlinear meta-plate based on analytical methods and numerical integration on the infinite model. The 2D dispersion curves are obtained through combining harmonic balance and Shooting-Newton iteration methods. Numerical studies show that the 2D nonlinear meta-plate exhibits band degeneration and self-adaptive propagation, analogous to 1D cases. However, as confirmed analytically, both band degeneration and self-adaptivity are regulated by the spatial wave divergence in the 2D meta-plate. Furthermore, combining nonlinear and linear portions of a meta-plate entails nonreciprocal wave propagation in 2D space with elucidated physical properties and mechanisms specific to 2D wave propagation. In light of the above, this work presents new computational methods as well as the elucidation of new wave phenomena and properties which are conducive to wave manipulation in 2D strongly nonlinear meta-materials/structures.

Topological flat bands in a family of multilayer graphene moiré lattices

Moiré materials host a wealth of intertwined correlated and topological states of matter, all arising from flat electronic bands with nontrivial quantum geometry. A prominent example is the family of alternating-twist magic-angle graphene stacks, which exhibit symmetry-broken states at rational fillings of the moiré band and superconductivity close to half filling. Here, we introduce a second family of twisted graphene multilayers made up of twisted sheets of M- and N-layer Bernal-stacked graphene flakes. Calculations indicate that applying an electric displacement field isolates a flat and topological moiré conduction band that is primarily localized to a single graphene sheet below the moiré interface. Phenomenologically, the result is a striking similarity in the hierarchies of symmetry-broken phases across this family of twisted graphene multilayers. Our results show that this family of structures offers promising new opportunities for the discovery of exotic new correlated and topological phenomena, enabled by using the layer number to fine tune the flat moiré band and its screening environment.

Phase Transition and Multistability in Dicke Dimer.

The hybrid quantum system of cold atomic gas and optical cavity can host many exotic phenomena including phase transitions and multistabilities. In this Letter, we investigate the effect of photon hopping between two Dicke cavities and show rich quantum phases for steady states and dynamic processes. Starting from a generic dimer system where the two cavities are not necessarily identical, we analytically obtain all possible steady-state phases and confirm their existence by numerical calculations. We then focus on the special case where the two cavities are identical, where exact solutions of all phases are obtained. Our results suggest that photon hopping is a convenient and powerful tool to manipulate the quantum phases and induce multistable behavior in this system.

Controllable Synthesis of High-Quality Magnetic Topological Insulator MnBi2Te4 and MnBi4Te7 Multilayers by Chemical Vapor Deposition.

With a nontrivial topological band and intrinsic magnetic order, two-dimensional (2D) MnBi2Te4-family materials exhibit great promise for exploring exotic quantum phenomena and potential applications. However, the synthesis of 2D MnBi2Te4-family materials via chemical vapor deposition (CVD), which is essential for advancing device applications, still remains a significant challenge since it is difficult to control the reactions among multi-precursors and form pure phases. Here, we report a controllable synthesis of high-quality magnetic topological insulator MnBi2Te4 and MnBi4Te7 multilayers via an evaporation-rate-controlled CVD approach. The multilayers are grown on a mica substrate epitaxially, exhibiting a regular triangle shape. By controlling growth temperatures, the thickness and lateral size of the 2D MnBi2Te4 are well regulated. Furthermore, the magneto-transport measurements clearly reveal multistep spin-flop transitions for both odd- and even-number-layered MnBi2Te4 multilayers. Our study marks a significant stride toward future transformative applications in devices based on high-quality, edge- and thickness-controlled 2D magnetic topological quantum materials.

Giant Topological Hall Effect and Colossal Magnetoresistance in Heusler Ferromagnet near Room Temperature.

Colossal magnetoresistance (CMR) is an exotic phenomenon that allows for the efficient magnetic control of electrical resistivity and has attracted significant attention in condensed matter due to its potential for memory and spintronic applications. Heusler alloys are the subject of considerable interest in this context due to the electronic properties that result from the nontrivial band topology. Here, the observation of CMR near room temperature is reported in the shape memory Heusler alloy Ni2Mn1.4In0.6, which is attributed to the combined effects of magnetic field-induced martensite twin variant reorientation (MFIR) and magnetic field-induced structural phase transformation (MFIPT). This compound undergoes a structural phase transition from a cubic (austenite-L21) ferromagnetic (FM) to a monoclinic (martensite) antiferromagnetic (AFM), which leads to an effective increase in the size of the Fermi surface and consequently in CMR. Additionally, it exhibits significant anomalous Hall conductivity in both antiferromagnetic and ferromagnetic phases. Furthermore, it demonstrates a giant topological Hall resistivity (THR) ≈6µΩ.cm in the vicinity of martensite transition due to the enhanced spin chirality resulting from the formation of magnetic domains with Bloch-type domain walls. The findings contribute to the understanding of the magnetotransport of Ni-Mn-In Heusler alloys, which are prospective candidates for room-temperature spintronic applications.

Three-Dimensional Multiorbital Flat Band Models and Materials.

Flat band (FB) systems are essential for uncovering exotic quantum phenomena associated with strong electron correlations. Here we present a systematic theoretical framework for constructing multiorbital FB models and identifying feasible material candidates. This framework integrates group theory and crystallography into a symmetry-adapted tight-binding model incorporating lattice, site, and orbital degrees of freedom. Using this approach, we unveil a novel three-dimensional (3D) multiorbital FB model in the face-centered-cubic lattice, distinct from well-known single-orbital Lieb and kagome models. Critically, we identify numerous high-quality binary materials with ultraclean 3D FBs near the Fermi level. Furthermore, we explore diverse orbital bases within this model and extend our analysis to other cubic lattices with different space groups, broadening the scope for realizing 3D multiorbital FB systems. Our findings provide a foundational platform for exploring correlated physics in multiorbital FB systems and guiding the discovery of new quantum materials.

Degenerate merging BICs in resonant metasurfaces.

Resonant metasurfaces driven by bound states in the continuum (BIC) offer an intriguing approach to engineering high-Q resonances. Merging multiple BICs in the momentum space could further enhance the Q-factor as well as its robustness to fabrication imperfections. Here, we report the doubly degenerate guided mode resonances (GMR) in a resonant metasurface, whose radiation losses could be totally suppressed due to merging BICs. We show that the GMRs and their associated accidental BICs can evolve into degenerate merging BICs by parametric tuning of the metasurface. Significantly, these two GMRs share the same critical parameter (i.e., lattice constants or thickness) that the merging BICs occur. Interestingly, thanks to the degenerate property of two GMRs, a larger (smaller) period will split one of the merging BICs into eight accidental BICs at an off-Γ point but annihilate the other. Such an exotic phenomenon can be explained by the interaction of GMRs and background Fabry-Perot resonances. Our result provides new, to the best of our knowledge, strategies for engineering high-Q resonances in resonant metasurfaces for light-matter interaction.

Anderson transition and mobility edges on hyperbolic lattices with randomly connected boundaries

Hyperbolic lattices, formed by tessellating the hyperbolic plane with regular polygons, exhibit a diverse range of exotic physical phenomena beyond conventional Euclidean lattices. Here, we investigate the impact of disorder on hyperbolic lattices and reveal that the Anderson localization occurs at strong disorder strength, accompanied by the presence of mobility edges. Taking the hyperbolic {p, q} = {3, 8} and {p, q} = {4, 8} lattices as examples, we employ finite-size scaling of both spectral statistics and the inverse participation ratio to pinpoint the transition point and critical exponents. Our findings indicate that the transition points tend to increase with larger values of {p, q} or curvature. In the limiting case of {∞, q}, we further determine its Anderson transition using the cavity method, drawing parallels with the random regular graph. Our work lays the cornerstone for a comprehensive understanding of Anderson transition and mobility edges on hyperbolic lattices.

Candidate detection of GRB221009A by Yangbajing muon telescope

Long gamma-ray bursts (GRBs) can be generated by the collapse of a rapidly rotating massive star into a black hole and are the brightest known explosive events in the Universe. GRB221009A is the most energetic gamma-ray burst ever recorded, providing delayed detection of very high energy photons that challenge the standard propagation mechanisms, possibly suggesting exotic physics phenomena. An excess, synchronous with the ignition of GRB221009A, is detected in the rate of the Yangbajing muon telescope. A preliminary assessment of the significance of this excess limits the probability of a chance coincidence to less than 10−3. The hypothetical mechanisms for producing a muon excess induced by the GRB are exotic/puzzling. If confirmed by further investigations, this excess would add another feature to the many anomalies observed in this extreme astrophysical event.

Spin-valley locked topological phase transitions in reversible strain-tailoring honeycomb motifs

Using an effective low-energy k·p model on the frontier px,y orbitals, we establish a general phase diagram of spin-valley locked band inversion by introducing a mechanical strain field into nonmagnetic honeycomb motifs with robust spin–orbit coupling and intrinsically broken inversion symmetry. Using first-principles calculations, we realize such multiple topological phase transitions in a strained InTe monolayer within experimental reach with the Weyl semimetal as the nontrivial boundary state at two critical strains. The massless Weyl fermions endow the spin and valley Hall effects with ultrafast and dissipationless transport over a broad low-energy window. The valley selective circular dichroism can be regulated by strain-induced band inversion. A crossover between the topologically trivial and nontrivial regimes with sizable bandgaps makes InTe suitable for room-temperature (RT) topological strain-effect transistors. Our work not only demonstrates a fundamental mechanism for exploring tunable topological states and valley physics but also provides a potential platform for realizing many exotic phenomena and RT quantum devices.

Diverse electronic landscape of the kagome metal YbTi3Bi4

Kagome lattices have emerged as an ideal platform for exploring exotic quantum phenomena in materials. Here, we report the discovery of Ti-based kagome metal YbTi3Bi4 which we characterize using angle-resolved photoemission spectroscopy (ARPES) and magneto-transport, in combination with density functional theory calculations. Our ARPES results reveal the complex fermiology of YbTi3Bi4 and provide spectroscopic evidence of four flat bands. Our measurements also show the presence of multiple van Hove singularities originating from Ti 3d orbitals and a linearly-dispersing gapped Dirac-like bulk state at the K¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\overline{\\,{\\mbox{K}}\\,}$$\\end{document} point in accord with our theoretical calculations. Our study establishes YbTi3Bi4 as a platform for exploring exotic phases in the wider LnTi3Bi4 (Ln = lanthanide) family of materials.

Dirac-Like Fermions Anomalous Magneto-Transport in a Spin-Polarized Oxide 2D Electron System.

In a 2D electron system (2DES) the breaking of the inversion, time-reversal and bulk crystal-field symmetries is interlaced with the effects of spin-orbit coupling (SOC) triggering exotic quantum phenomena. Here, epitaxial engineering is used to design and realize a 2DES characterized simultaneously by ferromagnetic order, large Rashba SOC and hexagonal band warping at the (111) interfaces between LaAlO3, EuTiO3, and SrTiO3 insulators. The 2DES displays anomalous quantum corrections to the magneto-conductance driven by the time-reversal-symmetry breaking occurring below the magnetic transition temperature. The results are explained by the emergence of a non-trivial Berry phase and competing weak anti-localization/weak localization back-scattering of Dirac-like fermions, mimicking the phenomenology of gapped topological insulators. These findings open perspectives for the engineering of novel spin-polarized functional 2DES holding promises in spin-orbitronics and topologicalelectronics.