We study the electronic structures and topological properties of ($M+N$)-layer twisted graphene systems. We consider the generic situation that $N$-layer graphene is placed on top of the other $M$-layer graphene and is twisted with respect to each other by an angle $\theta$. In such twisted multilayer graphene systems, we find that there exist two low-energy flat bands for each valley emerging from the interface between the $M$ layers and the $N$ layers. These two low-energy bands in the twisted multilayer graphene system possess valley Chern numbers that are dependent on both the number of layers and the stacking chiralities. In particular, when the stacking chiralities of the $M$ layers and $N$ layers are opposite, the total Chern number of the two low-energy bands for each valley equals $\pm (M+N-2)$ (per spin). If the stacking chiralities of the $M$ layers and the $N$ layers are the same, then the total Chern number of the two low-energy bands for each valley is $\pm (M-N)$ (per spin). The valley Chern numbers of the low-energy bands are associated with large, valley-contrasting orbital magnetizations, suggesting the possible existence of orbital ferromagnetism and anomalous Hall effect once the valley degeneracy is lifted either externally by a weak magnetic field or internally by Coulomb interaction through spontaneous symmetry breaking. Such an orbital ferromagnetic state is characterized by chiral current loops circulating around the $AA$ region of the moiré pattern, which can be experimentally detected.

Galactic rotation curves exhibit diverse behavior in the inner regions while obeying an organizing principle; i.e., they can be approximately described by a radial acceleration relation or the modified Newtonian dynamics phenomenology. We analyze the rotation curve data from the SPARC sample and explicitly demonstrate that both the diversity and uniformity are naturally reproduced in a hierarchical structure formation model with the addition of dark matter self-interactions. The required concentrations of the dark matter halos are fully consistent with the concentration-mass relation predicted by the Planck cosmological model. The inferred stellar mass-to-light ($3.6\mu \mathrm{m}$) ratios scatter around $0.5M_{\odot }/L_{\odot }$, as expected from population synthesis models, leading to a tight radial acceleration relation and a baryonic Tully-Fisher relation. The inferred stellar-halo mass relation is consistent with the expectations from abundance matching. These results provide compelling arguments in favor of the idea that the inner halos of galaxies are thermalized due to dark matter self-interactions.

We report a combined experimental and theoretical investigation that reveals a new mechanism responsible for the enhancement of electron-phonon coupling in doped semiconductors in which multiple inequivalent valleys are simultaneously populated. Using Raman spectroscopy on ionic-liquid-gated monolayer and bilayer ${\mathrm{MoS}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{WSe}}_2$ over a wide range of electron and hole densities, we find that phonons with a dominant out-of-plane character exhibit strong softening upon electron accumulation while remaining unaffected upon hole doping. This unexpected—but very pronounced—electron-hole asymmetry is systematically observed in all monolayers and bilayers. By performing first-principles simulations, we show that the phonon softening occurs when multiple inequivalent valleys are populated simultaneously. Accordingly, the observed electron-hole asymmetry originates from the much larger energy separation between valleys in the valence bands—as compared to the conduction band—that prevents the population of multiple valleys upon hole accumulation. We infer that the enhancement of the electron-phonon coupling occurs because the population of multiple valleys acts to strongly reduce the efficiency of electrostatic screening for those phonon modes that cause the energy of the inequivalent valleys to oscillate out of phase. This robust mechanism is likely to play an important role in several physical phenomena, possibly including the occurrence of superconductivity in different transition metal dichalcogenides.

In prokaryotes, the number of genes in different functional classes shows apparent universal scaling with the total number of genes that can be approximated by a power law, with a sublinear, near-linear, or superlinear scaling exponent. These dependences are gene class specific but hold across the entire diversity of bacteria and archaea. Several models have been proposed to explain these universal scaling laws, primarily based on the specifics of the respective biological functions. However, a population-genetic theory of universal scaling is lacking. We employ a simple mathematical model for prokaryotic genome evolution, which, together with the analysis of 34 clusters of closely related bacterial genomes, allows us to identify the underlying factors that govern the evolution of the genome content. Evolution of the gene content is dominated by two functional class-specific parameters: selection coefficient and genome plasticity. The selection coefficient quantifies the fitness cost associated with deletion of a gene in a given functional class or the advantage of successful incorporation of an additional gene. Genome plasticity reflects both the availability of the genes of a given class in the external gene pool that is accessible to the evolving population and the ability of microbes to accommodate these genes in the short term, that is, the class-specific horizontal gene transfer barrier. The selection coefficient determines the gene loss rate, whereas genome plasticity is the principal determinant of the gene gain rate.

We present a contact-based model to study the spreading of epidemics by means of extending the dynamic message-passing approach to temporal networks. The shift in perspective from node- to edge-centric quantities enables accurate modeling of Markovian susceptible-infected-recovered outbreaks on time-varying trees, i.e., temporal networks with a loop-free underlying topology. On arbitrary graphs, the proposed contact-based model incorporates potential structural and temporal heterogeneities of the contact network and improves analytic estimations with respect to the individual-based (node-centric) approach at a low computational and conceptual cost. Within this new framework, we derive an analytical expression for the epidemic threshold on temporal networks and demonstrate the feasibility of this method on empirical data.

Understanding the mechanisms by which crystal nuclei form is crucial for many phenomena such as gaining control over crystallization in glass-forming materials or accurately modeling rheological behavior of magma flows. The microscopic nature of such nuclei, however, makes their understanding extremely hard in experiments, while computer simulations have hitherto been hampered by short timescales and small system sizes. Here we use highly efficient graphics processing unit simulation techniques to address these challenges. The larger systems we access reveal a general nucleation mechanism in mixtures. In particular, we find that the supercooled liquid of a prized atomistic model glass former (Kob-Andersen model) is inherently unstable to crystallization, i.e., that nucleation is unavoidable on the structural relaxation timescale, for system sizes of 10 000 particles and larger. This is due to compositional fluctuations leading to regions composed of one species that are larger than the critical nucleus of that species, which rapidly crystallize. We argue that this mechanism provides a minimum rate of nucleation in mixtures in general, and show that the same mechanism pertains to the metallic glass former copper zirconium (CuZr).

The delay-bandwidth limit implies a stringent trade-off between the time delay, bandwidth, and propagation distance of an electromagnetic signal. Here, we show that temporal modulation can overcome this constraint, enabling extremely broadband wave propagation with close-to-zero group velocity dispersion in switched multipath electronic networks. Contrary to time-invariant waveguides, in which wave propagation implies a delicate balance between electric and magnetic stored energies, in such modulated networks the stored energy is largely electrostatic in nature. We show that in this case the phase and group velocities become independent of the properties of their constituent elements, and they are controlled only by the modulation scheme. Based on these findings, we provide practical designs of deeply subwavelength CMOS-compatible reciprocal and nonreciprocal microwave components, such as delay lines, phase shifters, couplers, and circulators. The obtained results also explicitly show that temporally modulated systems are not bound by constraints of time-invariant systems and can achieve arbitrarily large delay-bandwidth products.

Living liquid crystals, a realization of active nematics where a lyotropic liquid crystal is combined with active bacteria, exhibit a plethora of out-of-equilibrium phenomena that range from active turbulence and dynamic spatiotemporal patterns to the creation and annihilation of motile topological defects. Experiments and hydrodynamic simulations are used here to report on the emergence of bend stripes, which arise as spontaneous undulations of the director field in circularly aligned lyotropic liquid crystals doped with bacteria. The interplay between bacterial-induced hydrodynamic flows and elastic forces in the material induces remarkable deformation patterns consisting of branched, radially elongated bands of a high curvature of the director field. The average number of such branches increases with the distance from the center of the circular alignment, leading to the formation of a radial tree of bands that is reminiscent of a snowflake structure. Hydrodynamic simulations, which are in agreement with the experiments, are used to explain the origin of such structures and to provide additional insights into regimes that are beyond the limit of experimental measurements. In particular, it is found that when activity is switched off in the early stages of pattern formation, a pronounced decay of bend-distortion energy ensues, with little change of the splay energy, serving to confirm that the bend stripes are an outcome of activity-driven bend-instability phenomena. Taken together, experiments and simulations demonstrate a system in which strain and geometry can be combined to dynamically manipulate pattern formation in active matter, paving the way to a deeper understanding and finer control of active colloidal systems.

We propose a quantum-information-based scheme to reduce the temperature of quantum many-body systems and access regimes beyond the current capability of conventional cooling techniques. We show that collective measurements on multiple copies of a system at finite temperature can simulate measurements of the same system at a lower temperature. This idea is illustrated for the example of ultracold atoms in optical lattices, where controlled tunnel coupling and quantum gas microscopy can be naturally combined to realize the required collective measurements to access a lower, virtual temperature. Our protocol is experimentally implemented for a Bose-Hubbard model on up to 12 sites, and we successfully extract expectation values of observables at half the temperature of the physical system. Additionally, we present related techniques that enable the extraction of zero-temperature states directly.

Machine-learning algorithms can be fooled by small well-designed adversarial perturbations. This is reminiscent of cellular decision-making where ligands (called antagonists) prevent correct signaling, like in early immune recognition. We draw a formal analogy between neural networks used in machine learning and models of cellular decision-making (adaptive proofreading). We apply attacks from machine learning to simple decision-making models and show explicitly the correspondence to antagonism by weakly bound ligands. Such antagonism is absent in more nonlinear models, which inspires us to implement a biomimetic defense in neural networks filtering out adversarial perturbations. We then apply a gradient-descent approach from machine learning to different cellular decision-making models, and we reveal the existence of two regimes characterized by the presence or absence of a critical point for the gradient. This critical point causes the strongest antagonists to lie close to the decision boundary. This is validated in the loss landscapes of robust neural networks and cellular decision-making models, and observed experimentally for immune cells. For both regimes, we explain how associated defense mechanisms shape the geometry of the loss landscape and why different adversarial attacks are effective in different regimes. Our work connects evolved cellular decision-making to machine learning and motivates the design of a general theory of adversarial perturbations, both for in vivo and in silico systems.

In the context of quantum theories of spacetime, one overarching question is how quantum information in the bulk spacetime is encoded holographically in boundary degrees of freedom. It is particularly interesting to understand the correspondence between bulk subregions and boundary subregions in order to address the emergence of locality in the bulk quantum spacetime. For the AdS/CFT correspondence, it is known that this bulk information is encoded redundantly on the boundary in the form of an error-correcting code. Having access only to a subregion of the boundary is as if part of the holographic code has been damaged by noise and rendered inaccessible. In quantum-information science, the problem of recovering information from a damaged code is addressed by the theory of universal recovery channels. We apply and extend this theory to address the problem of relating bulk and boundary subregions in AdS/CFT, focusing on a conjecture known as entanglement wedge reconstruction. Existing work relies on the exact equivalence between bulk and boundary relative entropies, but these are only approximately equal in bulk effective field theory, and in similar situations it is known that predictions from exact entropic equalities can be qualitatively incorrect. We show that the framework of universal recovery channels provides a robust demonstration of the entanglement wedge reconstruction conjecture as well as new physical insights. Most notably, we find that a bulk operator acting in a given boundary region’s entanglement wedge can be expressed as the response of the boundary region’s modular Hamiltonian to a perturbation of the bulk state in the direction of the bulk operator. This formula can be interpreted as a noncommutative version of Bayes’s rule that attempts to undo the noise induced by restricting to only a portion of the boundary. To reach these conclusions, we extend the theory of universal recovery channels to finite-dimensional operator algebras and demonstrate that recovery channels approximately preserve the multiplicative structure of the operator algebra.

The extraordinary electronic properties of Dirac materials, the two-dimensional partners of Weyl semimetals, arise from the linear crossings in their band structure. When the dispersion around the Dirac points is tilted, one can predict the emergence of intricate transport phenomena such as modified Klein tunneling, intrinsic anomalous Hall effects, and ferrimagnetism. However, Dirac materials are rare, particularly with tilted Dirac cones. Recently, artificial materials whose building blocks present orbital degrees of freedom have appeared as promising candidates for the engineering of exotic Dirac dispersions. Here we take advantage of the orbital structure of photonic resonators arranged in a honeycomb lattice to implement photonic lattices with semi-Dirac, tilted, and, most interestingly, type-III Dirac cones that combine flat and linear dispersions. Type-III Dirac cones emerge from the touching of a flat and a parabolic band when synthetic photonic strain is introduced in the lattice, and they possess a nontrivial topological charge. This photonic realization provides a recipe for the synthesis of orbital Dirac matter with unconventional transport properties and, in combination with polariton nonlinearities, opens the way to study Dirac superfluids in topological landscapes.

We define dynamical universality classes for many-body systems whose unitary evolution is punctuated by projective measurements. In cases where such measurements occur randomly at a finite rate $p$ for each degree of freedom, we show that the system has two dynamical phases: “entangling” and “disentangling.” The former occurs for $p$ smaller than a critical rate $p_c$ and is characterized by volume-law entanglement in the steady state and “ballistic” entanglement growth after a quench. By contrast, for $p>p_c$ the system can sustain only area-law entanglement. At $p=p_c$ the steady state is scale invariant, and in $1+1\mathrm{D}$, the entanglement grows logarithmically after a quench. To obtain a simple heuristic picture for the entangling-disentangling transition, we first construct a toy model that describes the zeroth Rényi entropy in discrete time. We solve this model exactly by mapping it to an optimization problem in classical percolation. The generic entangling-disentangling transition can be diagnosed using the von Neumann entropy and higher Rényi entropies, and it shares many qualitative features with the toy problem. We study the generic transition numerically in quantum spin chains and show that the phenomenology of the two phases is similar to that of the toy model but with distinct “quantum” critical exponents, which we calculate numerically in $1+1\mathrm{D}$. We examine two different cases for the unitary dynamics: Floquet dynamics for a nonintegrable Ising model, and random circuit dynamics. We obtain compatible universal properties in each case, indicating that the entangling-disentangling phase transition is generic for projectively measured many-body systems. We discuss the significance of this transition for numerical calculations of quantum observables in many-body systems.

Knowledge of the behavior of hydrogen in metal hydrides is the key for understanding their electronic properties. Here, we present an ${}^1\mathrm{H}\text{−}\mathrm{NMR}$ study of cubic FeH up to 202 GPa. We observe a distinct deviation from the ideal metallic behavior between 64 and 110 GPa that suggests pressure-induced H-H interactions. Accompanying ab initio calculations support this result, as they reveal the formation of an intercalating sublattice of electron density, which enhances the hydrogen contribution to the electronic density of states at the Fermi level. This study shows that pressure-induced H-H interactions can occur in metal hydrides at much lower compression and larger H-H distances than previously thought and stimulates an alternative pathway in the search for novel high-temperature superconductors.

During mitosis, human chromosomes are linearly compacted about 1000-fold by loop-extruding motors. Recent experiments have shown that condensins extrude DNA loops but in a “one-sided” manner. This contrasts with existing models, which predict that symmetric, “two-sided” loop extrusion accounts for mitotic chromosome compaction. We explore whether one-sided extrusion, as it is currently seen in experiments, can compact chromosomes by developing a mean-field theoretical model for polymer compaction by motors that actively extrude loops and dynamically turnover. The model establishes a stringent upper bound of only about tenfold for compaction by strictly one-sided extrusion. We confirm this result with stochastic simulations. Thus, strictly one-sided extrusion as it has been observed so far cannot be the sole mechanism of chromosome compaction. However, as shown by the model, other two-sided or effectively two-sided mechanisms can achieve sufficient compaction.

The propagation of information in nonrelativistic quantum systems obeys a speed limit known as a Lieb-Robinson bound. We derive a new Lieb-Robinson bound for systems with interactions that decay with distance $r$ as a power law, $1/r^{\alpha }$. The bound implies an effective light cone tighter than all previous bounds. Our approach is based on a technique for approximating the time evolution of a system, which was first introduced as part of a quantum simulation algorithm by Haah et al., FOCS’18. To bound the error of the approximation, we use a known Lieb-Robinson bound that is weaker than the bound we establish. This result brings the analysis full circle, suggesting a deep connection between Lieb-Robinson bounds and digital quantum simulation. In addition to the new Lieb-Robinson bound, our analysis also gives an error bound for the Haah et al. quantum simulation algorithm when used to simulate power-law decaying interactions. In particular, we show that the gate count of the algorithm scales with the system size better than existing algorithms when $\alpha >3D$ (where $D$ is the number of dimensions).

We report the coexistence of ferroelectricity and a spin liquid state in hexagonal ${\mathrm{TbInO}}_3$ with quasi-two-dimensional triangular spin lattice. Geometrical ferroelectricity associated with In trimerization accompanies topological ferroelectric structural defects. Magnetic susceptibility data show in-plane magnetic anisotropy of Tb spins without any long-range order above 1.8 K, and we also confirm no trace of any phase transition down to 0.15 K from a specific heat measurement, which indicates that this system is highly frustrated and may host a spin liquid ground state. By analyzing the Schottky anomaly in the specific heat results, we propose a model where crystal-field levels are different in each Tb sites, and only one of them has a magnetic ground state and forms a unique honeycomb spin lattice. These observations put forward an interesting possibility where spin liquid and ferroelectric behaviors coexist, and the atomically sharp ferroelectric domain walls may host new magnetic edge states or local spin excitations.

Controlling the polarization state of electromagnetic radiation enables the investigation of fundamental symmetry properties of matter through chiroptical processes. Over the past decades, many strategies have been developed to reveal structural or dynamical information about chiral molecules with high sensitivity, from the microwave to the extreme ultraviolet range. Most schemes employ circularly or elliptically polarized radiation, and more sophisticated configurations involve, for instance, light pulses with time-varying polarization states. All these schemes share a common property—the polarization state of light is always considered as constant over one optical cycle. In this study, we focus on the optical cycle in order to resolve and control a subcyle chiroptical process. We engineer an electric field whose instantaneous chirality can be controlled within the optical cycle, by combining two phase-locked orthogonally polarized fundamental and second harmonic fields. While the composite field has zero net ellipticity, it shows an instantaneous optical chirality which can be controlled via the two-color delay. We theoretically and experimentally investigate the photoionization of chiral molecules with this controlled chiral field. We find that electrons are preferentially ejected forward or backward relative to the laser propagation direction depending on the molecular handedness, similarly to the well-established photoelectron circular dichroism process. However, since the instantaneous chirality switches sign from one half-cycle to the next, electrons ionized from two consecutive half-cycles of the laser show opposite forward-backward asymmetries. This chiral signal, the enantiosensitive subcycle antisymmetric response gated by electric-field rotation, provides a unique insight into the influence of instantaneous chirality in the dynamical photoionization process. More generally, our results demonstrate the important role of subcycle polarization shaping of electric fields as a new route to study and manipulate chiroptical processes.

We study a two-dimensional tight-binding model of a topological crystalline insulator (TCI) protected by rotation symmetry. The model is built by stacking two Chern insulators with opposite Chern numbers which transform under conjugate representations of the rotation group, e.g., $p_{\pm }$ orbitals. Despite its apparent similarity to the Kane-Mele model, it does not host stable gapless surface states. Nevertheless, the model exhibits topological responses including the appearance of quantized fractional charge bound to rotational defects (disclinations) and the pumping of angular momentum in response to threading an elementary magnetic flux, which are described by a mutual Chern-Simons coupling between the electromagnetic gauge field and an effective gauge field corresponding to the rotation symmetry. In addition, we show that although the filled bands of the model do not admit a symmetric Wannier representation, this obstruction is removed upon the addition of appropriate atomic orbitals, which implies “fragile” topology. As a result, the response of the model can be derived by representing it as a superposition of atomic orbitals with positive and negative integer coefficients. Following the analysis of the model, which serves as a prototypical example of 2D TCIs protected by rotation, we show that all TCIs protected by point group symmetries which do not have protected surface states are either atomic insulators or fragile phases. Remarkably, this implies that gapless surface states exist in free-electron systems if and only if there is a stable Wannier obstruction. We then use dimensional reduction to map the problem of classifying 2D TCIs protected by rotation to a zero-dimensional problem which is then used to obtain the complete noninteracting classification of such TCIs as well as the reduction of this classification in the presence of interactions.

We propose and numerically demonstrate a new chiral spectroscopy method that is based on a universal system-independent mechanism of dynamical symmetry breaking in high harmonic generation (HHG). The proposed technique relies only on intense electric-dipole transitions and not on their interplay with magnetic dipole transitions. The symmetry breaking results in the emission of otherwise “forbidden” harmonics from chiral media (i.e., that are not emitted from achiral or racemic media), yielding a huge, nearly background-free, chiral-achiral signal that is correlated to the magnitude of the medium’s enantiomeric excess. The handedness of the medium can be directly detected by measuring the polarization helicity of the emitted harmonics. Moreover, the strength of the “allowed” harmonics (that are not related to symmetry breaking) is chirality independent; hence, they can be used as a reference to probe chiral degrees of freedom within a single measurement. We numerically demonstrate up to 99% chiral-achiral signal level (normalized difference between the chiral and achiral HHG spectra) from microscopic gas-phase emission using state-of-the-art models for HHG in bromochlorofluoromethane and propylene oxide. We expect the new method to give rise to precise tabletop characterization of chiral media in the gas phase and for highly sensitive time-resolved probing of dynamical chiral processes with femtosecond-to-attosecond temporal resolution.