CyberSec Research

We present a topological mechanism for superconductivity emerging from Chern-2 insulators. While, naively, time-reversal symmetry breaking is expected to prevent superconductivity, it turns out that the opposite is the case: An explicit model calculation for a generalized attractive-U Haldane-Hubbard model demonstrates that superconductivity is only stabilized near the quantum anomalous Hall state, but not near a trivial, time-reversal symmetric band insulator. As standard Bardeen-Cooper-Schrieffer-like mean-field theory fails to capture any superconducting state, we explain this using an effective fractionalized field theory involving fermionic chargeons, bosonic colorons and an emergent U(1) gauge field. When the chargeons form a gapped topological band structure, the proliferation of single monopoles of this gauge field is forbidden. However, long-ranged monopole-antimonopole correlations emerge, and we argue that those correspond to superconducting order. Using random phase approximation on top of extensive slave-rotor mean-field calculations we characterize coherence length and stiffness of the superconductor. Thereby, we deduce the phase diagram in parameter space and furthermore discuss the effect of doping, temperature and an external magnetic field. We complement the fractionalized theory with calculations using an effective spin model and Gutzwiller projected wavefunctions. While mostly based on a simple toy model, we argue that our findings contribute to a better understanding of superconductivity emerging out of spin- and valley polarized rhombohedral graphene multilayers in a parameter regime with nearby quantum anomalous Hall insulators.

Localized surface plasmons can confine light within a deep-subwavelength volume comparable to the scale of atoms and molecules, enabling ultrasensitive responses to near-field variations. On the other hand, this extreme localization also inevitably amplifies the unwanted noise from the response of local morphological imperfections, leading to complex spectral variations and reduced consistency across the plasmonic nanostructures. Seeking uniform optical responses has therefore long been a sought-after goal in nanoplasmonics. However, conventional probing techniques by dark-field (DF) confocal microscopy, such as image analysis or spectral measurements, can be inaccurate and time-consuming, respectively. Here, we introduce SPARX, a deep-learning-powered paradigm that surpasses conventional imaging and spectroscopic capabilities. In particular, SPARX can batch-predict broadband DF spectra (e.g., 500-1000 nm) of numerous nanoparticles simultaneously from an information-limited RGB image (i.e., below 700 nm). It achieves this extrapolative inference beyond the camera's capture capabilities by learning the underlying physical relationships among multiple orders of optical resonances. The spectral predictions only take milliseconds, achieving a speedup of three to four orders of magnitude compared to traditional spectral acquisition, which may take from hours to days. As a proof-of-principle demonstration for screening identical resonances, the selection accuracy achieved by SPARX is comparable to that of conventional spectroscopy techniques. This breakthrough paves the way for consistent plasmonic applications and next-generation microscopies.

Exceptional points at which eigenvalues and eigenvectors of non-Hermitian matrices coalesce are ubiquitous in the description of a wide range of platforms from photonic or mechanical metamaterials to open quantum systems. Here, we introduce a class of Hopf exceptional points (HEPs) that are protected by the Hopf invariants (including the higher-dimensional generalizations) and which exhibit phenomenology sharply distinct from conventional exceptional points. Saliently, owing to their $\mathbb{Z}_2$ topological invariant related to the Witten anomaly, three-fold HEPs and symmetry-protected five-fold HEPs act as their own ``antiparticles". Furthermore, based on higher homotopy groups of spheres, we predict the existence of multifold HEPs and symmetry-protected HEPs with non-Hermitian topology captured by a range of finite groups (such as $\mathbb{Z}_3$, $\mathbb{Z}_{12}$, or $\mathbb{Z}_{24}$) beyond the periodic table of Bernard-LeClair symmetry classes.

Properties in crystalline and ordered materials tend to be anisotropic, with their orientation affecting the macroscopic behavior and functionality of materials. The ability to image the orientation of anisotropic material properties in three dimensions (3D) is fundamental for the understanding and functionality-driven development of novel materials. With the development of X ray linear dichroic orientation tomography (XL DOT), it is now possible to non-destructively map three-dimensional (3D) orientation fields in micrometer-sized samples. In this work, we present the iterative, gradient-based reconstruction algorithm behind XL DOT that can be used to map orientations based on linear dichroism in 3D. As linear dichroism can be exhibited by a broad spectrum of materials, XL DOT can be used to map, for example, crystal orientations as well as ferroic alignment, such as ferroelectric and antiferromagnetic order. We demonstrate the robustness of this technique for orientation fields that exhibit smoothly varying and granular configurations, and subsequently identify and discuss optimal geometries for experimental data acquisition and optimal conditions for the reconstruction. We anticipate that this technique will be instrumental in enabling a deeper understanding of the relationship between material structures and their functionality, quantifying, for example, the orientation of charge distributions and magnetic anisotropies at the nanoscale in a wide variety of systems - from functional to energy materials.

Gauge field theory provides the mathematical and conceptual framework to describe and understand topological singularities such as Weyl points and magnetic monopoles. While singularities associated with vector electromagnetic gauge fields have been well-studied, those of higher-form tensor gauge fields, like the four-dimensional (4D) tensor monopoles predicted by string theory, have remained largely theoretical or limited to experimental demonstration in pure synthetic dimensions, thereby not allowing investigations of the associated boundary effects. Here, we present a 4D system with tensor monopoles using engineered acoustic metamaterials. Our momentum space combines three real momentum dimensions and a geometric parameter as the fourth. By varying this fourth momentum, we experimentally reveal two distinct topological surface states in 3D subsystems: Fermi-arc surface states in a gapless subsystem and Dirac-cone surface states in a gapped subsystem. Our work introduces a novel platform for exploring new topological structures associated with tensor gauge field and topological phenomena in higher dimensions.

Massless Dirac particles are characterized by an effective pseudospin-momentum locking, which is the origin of the peculiar scattering properties of Dirac particles through potential barriers. This pseudospin-momentum locking also governs the quantum geometric properties (such as the Berry phase and Berry curvature) of Dirac particles. In the present work, we demonstrate that a domain wall separating two regions with distinct quantum geometric properties can serve as an alternative to potential barriers. Specifically, using the three-band $\alpha-T_3$ model of two-dimensional Dirac particles, we show that a Berry phase domain wall results in partial reflection and transmission of the Dirac particles, despite the fact that the incident and refracted momenta are identical.

The topology of WTe2, a transition metal dichalcogenide with large spin-orbit interactions, is thought to combine type II Weyl semimetal and second-order topological insulator (SOTI) character. The SOTI character should endow WTe2 multilayer crystals with topologically protected helical states at its hinges, and, indeed, 1D states have been detected thanks to Josephson interferometry. However, the immunity to backscattering conferred to those states by their helical nature has so far not been tested. To probe the topological protection of WTe2 edge states, we have fabricated Superconducting Quantum Interference Devices (SQUIDs) in which the supercurrent through a junction on the crystal edge interferes with the supercurrent through a junction in the bulk of the crystal. We find behaviors ranging from a Symmetric SQUID pattern to asymmetric SQUID patterns, including one in which the modulation by magnetic field reveals a sawtooth-like supercurrent versus phase relation for the edge junction, demonstrating that the supercurrent at the edge is carried by ballistic channels over 600 nm, a tell-tale sign of the SOTI character of WTe2.

Bands away from the Fermi energy do not influence the electrical conduction. In typical rare-earth lanthanide compounds, the localized 4$\textit{f}$-electrons have a weak effect on the electrical conduction, limiting their influence on the Berry curvature and, hence, the intrinsic anomalous Hall effect. However, a comprehensive study of the magnetic, thermodynamic, and transport properties of single-crystalline NdGaSi, guided by first-principles calculations, reveals a ferromagnetic ground state that induces a splitting of quasi-flat 4$\textit{f}$-electronic bands and positions them near the Fermi energy. The observation of an extraordinarily large intrinsic anomalous Hall conductivity of 1165 $\Omega^{-1}$cm$^{-1}$ implies the direct involvement of localized states in the generation of non-trivial band crossings around the Fermi energy. These results are remarkable when compared to ferrimagnetic NdAlSi, which differs only in a non-magnetic atom (a change in the principal quantum number $\textit{n}$ of the outer $\textit{p}$ orbital) with the same number of valence electrons and does not exhibit any measurable anomalous Hall conductivity.

Two-dimensional massless Dirac fermions exhibit Dirac cones, which are classified into three types: type-I, type-II, and type-III. In both type-I and type-II cones, the energy dispersion is linear in all momentum directions. Type-I cones are characterized by a non-overtilted structure, where the Dirac point serves as a local minimum (maximum) for the upper (lower) band. In contrast, type-II cones exhibit overtilted dispersions, leading to the coexistence of electron and hole pockets. At the critical tilt, the linear energy dispersion vanishes in one momentum direction, corresponding to a type-III Dirac cone. We further define a special case, termed the "narrow-sense" type-III cone, where not only the linear term but also quadratic and higher-order terms vanish, resulting in a completely flat dispersion along one direction. In this work, we numerically investigate the temperature ($T$) -dependence of the electronic specific heat ($C$), as the Dirac cone is continuously tilted from type-I to narrow-sense type-III. A model with particle-hole symmetry is employed to ensure that the chemical potential ($\mu$) remains temperature independent. Our results reveal a notable crossover in $C$ near narrow-sense type-III, where $C$ changes from $C \propto T^{2}$ below the crossover temperature ($T_{\rm co}$) to $C \propto T^{\frac{1}{2}}$ above $T_{\rm co}$. This crossover is attributed to the energy-dependent structure of the density of states. The present findings suggest a feasible approach for experimentally probing the degree of Dirac cone tilting near the narrow-sense type-III limit.

We observe strongly nonlinear spin dynamics in ferro-/antiferro-magnetic multilayers, controlled by the number of bilayers in the system, layer thicknesses, as well as temperature, peaking in magnitude near the N\'eel point of the antiferromagnetic layers just above room temperature. Well above the N\'eel transition, the individual ferromagnetic layers are exchange decoupled and resonate independently. As the temperature is lowered toward the N\'eel point, the ferromagnetic proximity effect through the thin antiferromagnetic spacers transforms the system into a weakly coupled macrospin chain along the film normal, which exhibits pronounced standing spin-wave resonance modes, comparable in intensity to the uniform resonance in the ferromagnetic layers. These findings are supported by our micromagnetic simulations showing clear spin-wave profiles with precessional phase lag along the macrospin chain. Well below the N\'eel transition, the FeMn layers order strongly antiferromagnetically and exchange-pin the ferromagnetic layers to effectively make the multilayer one macrospin. The appearance and intensity of the high-frequency spin-wave modes can thus be conveniently controlled by thermal gating the multilayer. The nonlinearity in the microwave response of the demonstrated material can approach 100\%, large compared to nonlinear materials used in e.g. optics, with second-harmonic generation often at the single percentage level.

Domain-wall skyrmions are magnetic solitons embedded in a domain wall that are topologically equivalent to skyrmions. Here, we theoretically study antiferromagnetic domain-wall skyrmions and their current-driven motion within the Landau-Lifshitz-Gilbert phenomenology, and verify our findings with micromagnetic simulations. While the skyrmion Hall effect is expected to be suppressed in the current-induced motion of antiferromagnetic domain-wall skyrmions, we observe a finite Hall angle, which originates from the anisotropic spin configuration of domain-wall skyrmions. The skyrmion Hall effect is, however, conditionally suppressed and the motion aligns with the current applied in certain directions, which can be interpreted as principal axes of a domain-wall skyrmion that is easily identified from the symmetry of the spin configuration. Our work on antiferromagnetic domain-wall skyrmions shows that the dynamics of spin textures endowed with multiple soliton characteristics can be unconventional, which is envisaged to enrich the field of topological solitons.

Anderson transition in quasiperiodic potentials and the associated mobility edges have been a central focus in quantum simulation across multidisciplinary physical platforms. While these transitions have been experimentally observed in ultracold atoms, acoustic systems, optical waveguides, and superconducting junctions, their interplay between quasiperiodic potential and long-range hopping remains unexplored experimentally. In this work, we report the observation of localization-delocalization transition induced by the hopping between the next-nearest neighboring sites using quasiperiodic photonic waveguides. Our findings demonstrate that increasing the next-nearest hopping strength induces a reentrant phase transition, where the system transitions from an initially extended phase into a localized phase before eventually returning to an extended phase. This remarkable interplay between hopping and quasiperiodic potential in the lattice models provides crucial insights into the mechanism of Anderson transition. Furthermore, our numerical simulation reveals that this phase transition exhibits a critical exponent of $\nu \simeq 1/3$, which is experimentally observable for system sizes $L\sim10^3$ - $10^4$. These results establish a framework for direct observation of the Anderson transition and precise determination of its critical exponents, which can significantly advance our understanding of localization physics in quasiperiodic systems.

In 1978, Wilczek and Weinberg theoretically discovered a new boson-the Axion-which is the coherent oscillation of the $\theta$ field in QCD. Its existence can solve multiple fundamental questions including the strong CP problem of QCD and the dark matter. However, its detection is challenging because it has almost no interaction with existing particles. Similar $\theta$ has been introduced to condensed matter and so far studied as a static, quantized value to characterize topology of materials. But the coherent oscillation of $\theta$ in condensed matter is proposed to lead to new physics directly analogous to the high-energy Axion particle, the dynamical Axion quasiparticle (DAQ). In this paper, we present the direct observation of the DAQ. By combining 2D electronic device with ultrafast pump-probe optics, we manage to measure the magnetoelectric coupling $\theta$ ($\theta\propto\alpha$) of 2D MnBi$_2$Te$_4$ with sub-picosecond time-resolution. This allows us to directly observe the DAQ by seeing a coherent oscillation of $\theta$ at ~44 GHz in real time, which is uniquely induced by the out-of-phase antiferromagnetic magnon. Interestingly, in 2D MnBi$_2$Te$_4$, the DAQ arises from the magnon-induced coherent modulation of Berry curvature. Such ultrafast control of quantum wavefunction can be generalized to manipulate Berry curvature and quantum metric of other materials in ultrafast time-scale. Moreover, the DAQ enables novel quantum physics such as Axion polariton and electric control of ultrafast spin polarization, implying applications in unconventional light-matter interaction and coherent antiferromagnetic spintronics. Beyond condensed matter, the DAQ can serve as a detector of the dark matter Axion particle. We estimate the detection frequency range and sensitivity in the critically-lacking meV regime, contributing to one of the most challenging questions in fundamental physics.

We present a comprehensive first-principles study of the magnetoresistance in ZrSi$X$ ($X=$ S, Se, Te) topological nodal-line semimetals. Our study demonstrates that all primary features of the experimentally measured magnetoresistance in these materials are captured by our calculations, including the unusual butterfly-shaped anisotropic magnetoresistance. This anisotropic magnetoresistance can be accurately reproduced using the semiclassical Boltzmann transport theory without introducing any information on the topological nature of bands or the concepts of topological phase transition. Considering the complex structure of the Fermi surface in these topological materials, we develop a theoretical description explaining the features observed in magnetoresistance measurements. Additionally, the atypical Hall resistance can be interpreted by the same semiclassical approach. Our findings establish magnetotransport as a powerful tool for analyzing the geometry of the Fermi surface, complementing angle-resolved photoemission spectroscopy and quantum oscillation measurements. This approach is demonstrated to be particularly useful for determining the role of non-trivial topology in transport properties.

The motion of a quantum particle in a one-dimensional periodic potential can be described in terms of Bloch wave packets. Like free-particle wave packets, they can propagate without attenuation. Here, we examine this similarity more closely by investigating whether Bloch wave packets can maintain a definite -- or nearly definite -- momentum direction, a property inherent to free-particle wave packets. This question is particularly relevant to the feasibility of using solid-state-based systems in the search for the first experimental realization of quantum backflow, a quantum interference effect in which a particle's probability density flows in a direction opposite to its momentum.

This paper deals with the spectral densities of a dispersive dielectric object in the framework of macroscopic quantum electrodynamics based on the modified Langevin noise formalism. In this formalism, the electromagnetic field in the presence of a dielectric object has two contributions, one taking into account the polarization current fluctuations of the object and the other taking into account the vacuum field fluctuations scattered by the object. The combined effect of these fields on the dynamics of a quantum emitter can be described by means of two independent continuous bosonic reservoirs, a medium-assisted reservoir and a scattering-assisted reservoir, each described by its own spectral density. Therefore, for initial thermal states of the reservoirs having different temperatures, the common approach based on the dyadic Green function of the dielectric object cannot be employed. We study the interaction of a quantum emitter with these two reservoirs introducing a temperature-dependent effective spectral density of the electromagnetic environment, focusing on the case of a homogeneous dielectric sphere. We derive analytical expressions for the medium-assisted, scattering-assisted, and effective spectral densities in this setting. We then study the dynamics of the quantum emitter for initial thermal states of the two reservoirs, adopting a non-perturbative approach.

Quantum dots in SiGe/Si/SiGe heterostructures host coherent electron spin qubits, which are promising for future quantum computers. The silicon quantum well hosts near-degenerate electron valley states, creating a low-lying excited state that is known to reduce spin qubit readout and control fidelity. The valley energy splitting is dominated by the microscopic disorder in the SiGe alloy and at the Si/SiGe interfaces, and while Si devices are compatible with large-scale semiconductor manufacturing, achieving a uniformly large valley splitting energy across a many-qubit device spanning mesoscopic distances is an outstanding challenge. In this work we study valley splitting variations in a 1D quantum dot array manufactured by Intel. We observe correlations in valley splitting, at both sub-100nm (single gate) and >1{\mu}m (device) lengthscales, that are consistent with alloy disorder-dominated theory and simulation. Our results develop the mesoscopic understanding of Si/SiGe heterostructures necessary for scalable device design.

We present a detailed investigation of an overlooked symmetry structure in non-collinear antiferromagnets that gives rise to an emergent quantum number for magnons. Focusing on the triangular-lattice Heisenberg antiferromagnet, we show that its spin order parameter transforms under an enlarged symmetry group, $\mathrm{SO(3)_L \times SO(3)_R}$, rather than the conventional spin-rotation group $\mathrm{SO(3)}$. Although this larger symmetry is spontaneously broken by the ground state, a residual subgroup survives, leading to conserved Noether charges that, upon quantization, endow magnons with an additional quantum number -- \emph{isospin} -- beyond their energy and momentum. Our results provide a comprehensive framework for understanding symmetry, degeneracy, and quantum numbers in non-collinear magnetic systems, and bridge an unexpected connection between the paradigms of symmetry breaking in non-collinear antiferromagnets and chiral symmetry breaking in particle physics.