We study the microscopic model of electrons in the partially-filled lowest Landau level interacting via the Coulomb potential by the diagrammatic theory within the $GW$ approximation. In a wide range of filling fractions and temperatures, we find a homogeneous non-Fermi liquid (nFL) state similar to that found in the Sachdev-Ye-Kitaev (SYK) model, with logarithmic corrections to the anomalous dimension. In addition, the phase diagram is qualitatively similar to that of SYK: a first-order transition terminating at a critical end-point separates the nFL phase from a band insulator that corresponds to the fully filled Landau level. This critical point, as well as that of the SYK model—whose critical exponents we determine more precisely—are shown to both belong to the Van der Waals universality class. The possibility of a charge density wave (CDW) instability is also investigated, and we find the homogeneous nFL state to extend down to the ground state for fillings $0.2\lesssim \nu \lesssim 0.8$, while a CDW appears outside this range of fillings at sufficiently low temperatures. Our results suggest that the SYK-like nFL state should be a generic feature of the partially filled lowest Landau level at intermediate temperatures.

Ferroelectricity can exist in elemental phases as a result of charge transfers between atoms occupying inequivalent Wyckoff positions. We investigate the emergence of ferroelectricity in two-dimensional elemental materials with buckled honeycomb lattices. Various multibilayer structures hosting ferroelectricity are designed by stacking engineering. Ferroelectric materials candidates formed by group IV and V elements are predicted theoretically. Ultrathin Bi films show layer-stacking-dependent physical properties of ferroelectricity, topology, and metallicity. The two-bilayer Bi film with a polar stacking sequence is found to be an elemental topological ferroelectric material. Three and four bilayers Bi films with polar structures are ferroelectriclike elemental polar metals with topological nontrivial edge states. For Ge and Sn, trivial elemental polar metals are predicted. Our work reveals the possibility of designing two-dimensional elemental topological ferroelectrics and polar metals by stacking engineering.

The interaction between quantum emitters leads to the formation of superradiant and subradiant states with possible applications in quantum technologies. To improve the characterization of light emission from these systems, we present here a systematic theoretical analysis of the intensity correlation from two strongly interacting quantum emitters at cryogenic temperatures as a function of the frequency and intensity of the excitation laser. This analysis effectively accounts for the effect of vibrational modes of the emitters and of phonons of the environment through the combined Debye-Waller/Franck-Condon factor. First, we analyze the color-blind intensity correlation and show that it can be tailored from strong antibunching to strong bunching by tuning the laser from the two-photon resonance to the transition frequency of the superradiant state. We also find a particularly complex behavior of the intensity correlation when the laser frequency is tuned to that of the transition of the subradiant state, giving raise to the possibility of emitting bunched and antibunched light depending on the laser intensity and the detuning between the two emitters. The numerical results are supported by analytical equations that can be used for the experimental characterization of the interacting emitters. Additionally, by selecting photons of particular frequencies, we analyze the rich landscape of frequency-resolved intensity correlations, which also depend on the laser detuning and intensity. The analysis of the frequency-resolved correlations provides further information about the different relaxation processes underlying the photon emission, unveiling one-photon and two-photon emission processes that cannot be resolved neither in the emission spectrum nor in the color-blind intensity correlation. These results show that two interacting emitters are a versatile and practical source of quantum light and highlight the usefulness of the intensity correlation to unveil complex dynamics in this system.

Recent advances in generating well controlled dense arrangements of individual atoms in free space have generated interest in understanding how the extended nature of these systems influences superradiance phenomena. Here, we provide an in-depth analysis on how space-dependent light shifts and decay rates induced by dipole-dipole interactions modify the steady-state properties of coherently driven arrays of quantum emitters. We characterize the steady-state phase diagram, with particular focus on the radiative properties in the steady state. Interestingly, we find that diverging from the well-established Dicke paradigm of equal all-to-all interactions significantly modifies the emission properties. In particular, the prominent quadratic scaling of the radiated light intensity with particle number in the steady state—a hallmark of steady-state Dicke superradiance—is entirely suppressed, resulting in only linear scaling with particle number. We show that this breakdown of steady-state superradiance occurs due to the emergence of additional dissipation channels that populate not only superradiant states but also subradiant ones. The additional contribution of subradiant dark states in the dynamics leads to a divergence in the time scales needed to achieve steady states. Building on this, we further show that measurements taken at finite times for extended atom ensembles reveal properties closely mirroring the idealized Dicke scenario.

Exceptional points of order $n$ $\left(\mathrm{EP}n\mathrm{s}\right)$ appear in non-Hermitian systems as points where the eigenvalues and eigenvectors coalesce. They emerge if $2(n-1)$ real constraints are imposed, such that EP2s generically appear in two dimensions (2D). Local symmetries have been shown to reduce this number of constraints. In this work, we provide a complete characterization of the appearance of symmetry-induced higher-order EPs in 2D parameter space. We find that besides EP2s only EP3s, EP4s, and EP5s can be stabilized in 2D. Moreover, these higher-order EPs must always appear in pairs with their dispersion determined by the symmetries. Upon studying the complex spectral structure around these EPs, we find that depending on the symmetry, EP3s are accompanied by EP2 arcs, and two- and three-level open Fermi structures. Similarly, EP4s and closely related EP5s, which arise due to multiple symmetries, are accompanied by exotic EP arcs and open Fermi structures. For each case, we provide an explicit example. We also comment on the topological charge of these EPs, and discuss similarities and differences between symmetry-protected higher-order EPs and EP2s.

Optical near fields are at the heart of various applications in sensing and imaging. We investigate dipole scattering as a parameter estimation problem and show that optical near fields carry more information about the location and the polarizability of the scatterer than the respective far fields. This increase in information originates from, and occurs simultaneously with, the scattering process itself. Our calculations also yield the far-field localization limit for dipoles in free space.

Experiments conducted on two-dimensional twisted materials have revealed a plethora of moiré patterns with different forms and shapes. The formation of these patterns is usually attributed to the presence of small strains in the samples, which typically arise during their fabrication. In this paper we find that the superlattice structure of such systems actually depends crucially on the interplay between twist and strain. For systems composed of honeycomb lattices, we show that this can lead to the formation of practically any moiré geometry, even if each lattice is only slightly distorted. As a result, we show that under strain the moiré Brillouin zone is not a stretched irregular hexagon, but rather a primitive cell that changes according to the geometry of the strained moiré vectors. We identify the conditions for the formation of hexagonal moiré patterns arising solely due to shear or biaxial strain, thus opening the possibility of engineering moiré patterns solely by strain. Moreover, we study the electronic properties in such moiré patterns and find that the strain tends to suppress the formation of the flat moiré bands, even in the strain-induced hexagonal patterns analogous to those obtained by the twist only. Our paper explains the plethora of moiré patterns observed in experiments, and provides a solid theoretical foundation from which one can design moiré patterns by strain.

Topological insulators have been studied intensively over the last decades. Earlier research focused on Hermitian Hamiltonians, but recently, peculiar and interesting properties were found by introducing non-Hermiticity. In this work, we apply a quantum geometric approach to various Hermitian and non-Hermitian versions of the Su-Schrieffer-Heeger (SSH) model. We find that this method allows one to correctly identify different topological phases and topological phase transitions for all SSH models, but only when using the metric tensor containing both left and right eigenvectors. Whereas the quantum geometry of Hermitian systems is Riemannian, introducing non-Hermiticity leads to pseudo-Riemannian and complex geometries, thus significantly generalizing from the quantum geometries studied thus far. One remarkable example of this is the mathematical agreement between topological phase transition curves and lightlike paths in general relativity, suggesting a possibility of simulating space-time in non-Hermitian systems. We find that the metric in non-Hermitian phases degenerates in such a way that it effectively reduces the dimensionality of the quantum geometry by one. This implies that within linear response theory, one can perturb the system by a particular change of parameters while maintaining a zero excitation rate.

Achieving quantum-enhanced performances when measuring unknown quantities requires developing suitable methodologies for practical scenarios, which include noise and the availability of a limited amount of resources. Here, we report on the optimization of substandard quantum limit Bayesian multiparameter estimation in a scenario where a subset of the parameters describes unavoidable noise processes in an experimental photonic sensor. We explore how the optimization of the estimation changes depending on which parameters are either of interest or are treated as nuisance ones. Our results show that optimizing the multiparameter approach in noisy apparata represents a significant tool to fully exploit the potential of practical sensors operating beyond the standard quantum limit for broad resources range.

Chirality, or handedness, is a geometrical property denoting a lack of mirror symmetry. Chirality is ubiquitous in nature and is associated with the nonreciprocal interactions observed in complex systems ranging from biomolecules to topological materials. Here, we demonstrate that chiral arrangements of dipole-coupled atoms or molecules can facilitate the helicity-dependent superradiant emission of light. We show that the collective modes of these systems experience an emergent spin-orbit coupling that leads to chirality-dependent photon transport and nontrivial topological properties. These phenomena are fully described within the electric dipole approximation, resulting in very strong optical responses. Our results demonstrate an intimate connection between chirality, superradiance, and photon helicity and provide a comprehensive framework for studying electron transport dynamics in chiral molecules using cold atom quantum simulators.

Magnetic skyrmions are promising building blocks for future spintronic devices. However, the skyrmion Hall effect (SkHE) remains an obstacle for practical applications based on the in-line transport of skyrmions. Here, we numerically study the static properties and current-driven dynamics of synthetic ferrimagnetic skyrmions. Inspired by graded-index magnonics, we introduce a linear gradient of saturation magnetization ($M_{\mathrm{s}}$) in the skyrmion-hosting sample, which effectively modulates the skyrmion Hall angle and suppresses the SkHE. Micromagnetic simulations reveal that ferrimagnetic skyrmions could exhibit greater susceptibility to the variation of $M_{\mathrm{s}}$ as compared to their ferromagnetic counterparts. The Thiele analysis is also applied to support the simulation results, which elucidates that the $M_{\mathrm{s}}$ gradient dynamically modifies the intrinsic normalized size of skyrmions, consequently impacting the SkHE. Our results pave the way to the graded-index skyrmionics, which offers novel insights for designing ferrimagnet-based skyrmionic devices.

The analytical continuation of classical equations of motion to complex times suggests that a tunneling particle spends in the barrier an imaginary duration $i\left|\mathcal{T}\right|$. Does this mean that it takes a finite time to tunnel, or should tunneling be seen as an instantaneous process? It is well known that examination of the adiabatic limit in a small additional AC field points towards $\left|\mathcal{T}\right|$ being the time it takes to traverse the barrier. However, this is only half the story. We probe the transmitted particle's history, and find that it remembers very little of the field's past behavior, as if the transit time were close to zero. The ensuing contradiction suggests that the question is ill posed, and we explain why.

We analyze the formation of multiparticle bound states in ladders with frustrated kinetic energy in two-component bosonic and two-component fermionic systems. We focus on the regime of light doping relative to insulating states at half-filling, spin polarization close to 100%, and strong repulsive interactions. A special feature of these systems is that the binding energy scales with single-particle tunneling $t$ rather than exchange interactions, since effective attraction arises from alleviating kinetic frustration. For two-component Fermi systems on a zigzag ladder we find a bound state between a hole and a flipped spin (magnon) with a binding energy that can be as large as $0.6t$. We demonstrate that magnon-hole attraction leads to formation of clusters comprising several holes and magnons, and we expound on antiferromagentic correlations for the transverse spin components inside the clusters. We identify several many-body states that result from self-organization of multiparticle bound states, including a Luttinger liquid of hole-magnon pairs and a density wave state of two-hole–three-magnon composites. We establish a symmetry between the spectra of Bose and Fermi systems and use it to establish the existence of antibound states in two-component Bose mixtures with SU(2) symmetric repulsion on a zigzag ladder. We also consider Bose and Fermi systems on a square ladder with flux and demonstrate that both systems support bound states. We discuss experimental signatures of multiparticle bound states in both equilibrium and dynamical experiments. We point out intriguing connections between these systems and the quark bag model in QCD.

Active materials form a class of far-from-equilibrium systems that are driven internally and exhibit self-organization, which can be harnessed to perform mechanical work. Inspired by experiments on synthetic active polymer networks, in this paper we examine limits of work extraction from an active viscoelastic medium by analyzing the transport of a particle. The active viscoelastic material possesses an equilibrium density where the active and passive forces are balanced out. In a one-dimensional system, a gliding activation front (AF), which converts a passive to an active medium, provides active energy at a constant rate, which is injected into the system at one end and propagates to the other. We demonstrate that there exists a maximum velocity of the AF, above which the activated region fails to deliver the transport power. We hypothesize and intuitively argue based on the limit cases that the feasibility and the velocity of transport of the particle can be interpreted in terms of the velocity of an equilibration domain wall of the field, which is set by two parameters: (i) a measure of activity, and (ii) the viscoelastic timescale. The phase diagram is divided into “transport” and “no-transport” sectors, namely, for any pair of the two parameters, there exists a threshold velocity of the AF above which the particle transport becomes impossible. Constructing the phase diagram we find that (1) there are regions of the phase diagram for which the threshold velocity of the AF diverges, and (2) larger viscoelastic timescale makes the transport region more accessible and also increases the transport velocity therein. Furthermore, we find that increasing the velocity of AF, results in larger extracted power but smaller transport coefficient; defined as the ratio of the transport velocity and that of the AF. Our model provides a framework for understanding the energetics of transport phenomena in biology and designing efficient mechanisms of transport in synthetic active materials.

The origin of the linear-in-temperature ($T$-linear) resistivity in cuprate superconductors remains a profound mystery in condensed matter physics. Here, we investigate the dependence of the $T$-linear resistivity coefficient on doping, i.e., $A_1\left(p\right)$, for three typical regions in the temperature versus doping phase diagram of hole-doped cuprates, from which the doping dependence of the scattering rate, i.e., ${\alpha }_1\left(p\right)$, is further derived. It is found that for region I ($p<p^{*}$ and $T>T^{*}$), ${\alpha }_1\left(p\right)$ is almost a constant; for region II ($p>p^{*}$ and $T>T_{\mathrm{coh}}$), ${\alpha }_1\left(p\right)\propto p$; for region III ($p>p^{*}$ and $T<T_{\mathrm{coh}}$), ${\alpha }_1\left(p\right)\propto p(p_c-p)$, where $T^{*}$ is the onset temperature of the pseudogap phase, $p^{*}$ indicates the doping at which $T^{*}$ goes to zero, $T_{\mathrm{coh}}$ marks the onset of antinodal quasiparticle coherence, and $p_{\mathrm{c}}$ is the doping where the low-temperature linear behavior in the overdoped regime vanishes. Moreover, the deduced ${\alpha }_1\left(p\right)$ relations are verified with the experimental data from previous reports. The discovered scattering rate versus doping relationship will shed light on the scattering mechanism underlying the $T$-linear resistivity in cuprate superconductors.

Neural-network state representations of quantum many-body systems are attracting great attention and more rigorous quantitative analysis of their expressibility and complexity is warranted. Our analysis of the restricted Boltzmann machine (RBM) state representation of one-dimensional (1D) quantum spin systems provides new insight into their computational complexity. We define a class of long-range-fast-decay (LRFD) RBM states with quantifiable upper bounds on truncation errors and provide numerical evidence for a large class of 1D quantum systems that may be approximated by LRFD RBMs of at most polynomial complexities. These results lead us to conjecture that the ground states of a wide range of quantum systems may be exactly represented by LRFD RBMs or a variant of them, even in cases where other state representations become less efficient. At last, we provide the relations between multiple typical state manifolds. Our work proposes a paradigm for doing complexity analysis for generic long-range RBMs which naturally yields a further classification of this manifold. This paradigm and our characterization of their nonlocal structures may pave the way for understanding the natural measure of complexity for quantum many-body states described by RBMs and are generalizable for higher-dimensional systems and deep neural-network quantum states.

We report a magneto-induced topological phase transition in inverted InAs/GaSb bilayers from a quantum spin Hall insulator to a normal insulator. We utilize a dual-gated Corbino device in which the degree of band inversion, or equivalently the electron and hole densities, can be continuously tuned. We observe a topological phase transition around the magnetic field where a band crossing occurs, accompanied by a bulk-gap closure characterized by a bulk conductance peak (BCP). In another set of experiments, we study the transition under a tilted magnetic field (tilt angle θ). We observe the characteristic magnetoconductance around BCP as a function of θ, which dramatically depends on the density of the bilayers. In a relatively deep inversion (hence a higher density) regime, where the electron-hole hybridization dominates the excitonic interaction, the BCP grows with θ. On the contrary, in a shallowly inverted (a lower density) regime, where the excitonic interaction dominates the hybridization, the BCP is suppressed indicating a smooth crossover without a gap closure. This suggests the existence of a low-density, correlated insulator with spontaneous symmetry breaking near the critical point. Our highly controllable electron-hole system offers an ideal platform to study interacting topological states as proposed by recent theories.