We propose a general necessary condition for a spin chain with SO(3) spin-rotation symmetry to be gapped. Specifically, we prove that the ground state(s) of an SO(3)-symmetric gapped spin chain must be spin singlet(s), and the expectation value of a twisting operator asymptotically approaches unity in the thermodynamic limit, where finite-size corrections are inversely proportional to the system size. This theorem provides (i) supporting evidence for various conjectured gapped phases and (ii) a sufficient criterion for identifying gapless spin chains. We verify our theorem by numerical simulations for a variety of spin models and show that it offers a novel efficient way to identify gapless phases in spin chains with spin-rotation symmetry.

Many-body excitons in CrSBr are attracting intense interest in view of their highly anisotropic magneto-optical coupling and their potential for novel optical interfaces within spintronic and magnonic devices. Characterizing the orbital character and propagation of these electronic excitations is crucial for understanding and controlling their behavior; however, this information is challenging to access. High resolution resonant inelastic x-ray scattering is a momentum-resolved technique that can address these crucial questions. We present measurements collected at the Cr $L_3$-edge which show a rich spectrum of excitations with a variety of spin-orbital characters. While most of these excitations appear to be localized, the dispersion of the lowest energy dark exciton indicates that it is able to propagate along both the $a$ and $b$ directions within the planes of the crystal. This two-dimensional character is surprising as it contrasts with electrical conductivity and the behavior of the bright exciton, both of which are strongly one dimensional. The discovery of this propagating dark exciton highlights an unusual coexistence of one- and two-dimensional electronic behaviors in CrSBr.

We report the discovery of a new form of inductance in the bulk ferrimagnet ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$, which features strong spin-orbit coupling, large magnetic anisotropy, and pronounced magnetoelastic interactions. Below its Curie temperature ($T_C\approx 78\mathrm{K}$), ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ hosts chiral orbital currents (COC) that circulate within the crystal lattice and give rise to collective electronic behavior [Control of chiral orbital currents in a colossal magnetoresistance material, Nature (London) 611, 467 (2022).]. By applying a magnetic field along the hard $c$ axis and driving the system with low-frequency currents, we uncover a giant inductive response up to millhenry scale, originating from first-order reconfigurations of COC domains. These domains act as coherent mesoscopic inductive elements that resist reversal upon current reduction, producing a large electromotive force and sharply increasing voltage. This emergent inductance defies classical models, occurs without superconductivity or engineered nanostructures, and opens a new frontier in orbital-based quantum functionality and device concepts.

Helical trilayer graphene realizes a versatile moiré system for exploring correlated topological states emerging from high Chern bands. Motivated by recent experimental observations of anomalous Hall effects at fractional fillings of magic-angle helical trilayers, we focus on the higher Chern number $|C_{\text{band}}|=2$ band and explore gapped many-body Hall states beyond the conventional Landau level paradigm. Through extensive exact diagonalization, we predict novel phases unattainable in a single $|C_{\text{band}}|=1$ band. At filling $\nu =2/3$ and $\nu =1/3$, a $\sqrt{3}\times \sqrt{3}$ charge-ordered quantum Hall crystal and a Halperin fractional Chern insulator with Hall conductance $|{\sigma }_H|=2e^2/3h$ are predicted, respectively, indicating strong particle-hole asymmetry of the system. At half-filling $\nu =1/2$, an extensively degenerate pseudospin Hall ferromagnet featuring emergent SU(2) symmetry is found without the band being flat. Inspired by striking robustness of the ferromagnetic degeneracy, we develop a method to unveil and quantify the emergent symmetry via pseudospin operator construction in the presence of band dispersion and Coulomb interaction and demonstrate persistence of the SU(2) quantum numbers even far away from the chiral limit. Incorporating spin-valley degrees of freedom, we identify an optimal filling regime ${\nu }_{\text{total}}=3+\nu$ for realizing the above states. Notably, interflavor interactions renormalize the bandwidth and stabilize all the gapped phases even in realistic sublattice corrugation parameter regimes.

Band renormalizations comprise crucial insights for understanding the intricate roles of electron-boson coupling and electron correlation in emergent phenomena such as superconductivity. In this Letter, by combining high-resolution angle-resolved photoemission spectroscopy and theoretical calculations, we systematically investigate the electronic structure of the trilayer nickelate superconductor ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ at ambient pressure. We reveal a dichotomy in the electronic band renormalizations of ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ in comparison to cuprate superconductors. At a high energy scale of hundreds of meV, its band structure is strongly renormalized by an electron correlation effect enhanced by Hund’s coupling. The resultant waterfall-like dispersions resemble the high-energy kinks in cuprate superconductors. However, at low-energy scales of tens of meV, the dispersive bands are nearly featureless and devoid of any resolvable electron-boson interactions, in drastic contrast to the low-energy kinks observed in cuprates and other correlated $3d$ transition-metal compounds. The dichotomic band renormalizations highlight the disparity between nickelate and cuprate superconductors and emphasize the importance of strong electron correlation in the superconductivity of Ruddlesden-Popper phase nickelates.

Understanding nonequilibrium electron-phonon interactions at the microscopic level and on ultrafast timescales is a central goal of modern condensed matter physics. Combining time- and angle-resolved extreme ultraviolet photoemission spectroscopy with constrained density functional perturbation theory, we demonstrate that photoexcited carrier density can serve as a tuning knob to enhance electron-phonon interactions in nonequilibrium conditions. Specifically, nonequilibrium band structure mapping and valley-resolved ultrafast population dynamics in semiconducting transition-metal dichalcogenide $2{\mathrm{H}\text{−}\mathrm{MoTe}}_2$ reveal band-gap renormalizations and reduced population lifetimes as photoexcited carrier densities increase. Through theoretical analysis of photoinduced electron and phonon energy and linewidth renormalizations, we attribute these transient features to nonequilibrium modifications of electron-phonon coupling matrix elements. This Letter advances our understanding of microscopic coupling mechanisms enabling control over relaxation pathways in driven solids.

Efficient coupling of nanolight sources into photonic waveguides is crucial for integrated photonics, quantum technologies, and biosensing. Practical implementations require light sources with simultaneous high brightness and unidirectional emission. However, it is fundamentally incompatible between strong electromagnetic field confinement and directional radiation. Here, we demonstrate the simultaneous giant excitonic photoluminescence (PL) enhancement and unidirectional emission from a two-dimensional InSe film integrated with an asymmetric plasmonic nanocavity. A 3500-fold PL enhancement is achieved by engineering spatial, spectral, and orientational overlap between cavity modes and out-of-plane excitons in InSe. Symmetry breaking within the nanocavity ensures precise control of emission interference, yielding a record-high directivity exceeding 15 dB. The design achieves a high coupling efficiency of 24% and supports guided light propagation of $140\mathrm{\mu }\mathrm{m}$. Our results establish a scalable approach for the ultracompact integration of nanoscale light sources into monolithic photonic circuits and will advance the development of on-chip nanophotonics.

The dynamics of numerous physical systems, such as spins and qubits, can be described as a series of rotation operations, i.e., walks in the manifold of the rotation group. A basic question with practical applications is how likely and under what conditions such walks return to the origin (the identity rotation), which means that the physical system returns to its initial state. In three dimensions, we show that almost every walk in SO(3) or SU(2), even a very complicated one, will preferentially return to the origin simply by traversing the walk twice in a row and uniformly scaling all rotation angles. We explain why traversing the walk only once almost never suffices to return, and comment on the problem in higher dimensions.

The ripening dynamics in passive systems is governed by the theory of Lifshitz-Slyosov-Wagner (LSW). Here, we present an analog theory for reversed ripening in active systems. To derive the dynamic theory for the droplet size distribution, we consider a minimal ternary emulsion with one active reaction, leading to one conserved quantity. Even for cases where single droplets constantly grow, coupling many droplets via the conserved density in the far field leads to a self-organized reversal of ripening and, thus, a monodisperse emulsion. For late times, we find a scaling ansatz leading to the collapse of the rescaled size distributions, different from the LSW theory. This scaling behavior arises from a stable fixed point in the single droplet dynamics and may capture the late-time behavior of many active matter systems exhibiting reversed ripening.

As with many motile microalgae, the freshwater species Chlamydomonas reinhardtii can detect light sources and adapt its motile behavior in response. Here, we show that suspensions of photophobic cells can be unstable to density fluctuations, as a consequence of shading interactions mediated by light absorption. In a circular illumination geometry, this mechanism leads to the complete phase separation of the system into transient branching patterns, providing experimental evidence of finite wavelength selection through a motility-induced phase separation mechanism. The finite wavelength selection is a consequence of a vision-based interaction length scale which is fixed by the illumination geometry. A simple drift-diffusion framework allows us to understand how the wavelength depends on global cell density, light intensity, and medium viscosity. Finally, we show that this active phase separation shields individual cells from the deleterious effects of high light intensity, demonstrating that phototaxis can efficiently contribute to photoprotection through collective behaviors on short timescales.

Information engines produce mechanical work through measurement and adaptive control. For information engines, the principal challenge lies in how to store the generated work to the external load. Here, we report an experimental demonstration where quantized mechanical motion serves as a quantum battery and gets charged in repeated cycles by a single trapped-ion information engine. This is enabled by a key technological advancement in rapid state discrimination, allowing us to suppress measurement-induced disturbances. Consequently, we were able to obtain an information-to-ergotropy conversion efficiency approaching 67% of the theoretical limit at the optimal temperature, along with a maximum information-to-work conversion efficiency of 70%. The experimental results substantiate that this approach can render trapped ions a promising platform for microscopic information engines with potential applications in the future upon scaling up.

We present a formula for the universal anomalous scaling of the multipole moments of a generic gravitating source in classical general relativity. We derive this formula in two independent ways using effective field theory methods. First, we use the absorption of low-frequency gravitational waves by a black hole to identify the total multipole scaling dimension as the renormalized angular momentum of black hole perturbation theory. More generally, we show that the anomalous dimension is determined by phase shifts of gravitational waves elastically scattering off generic source multipole moments, which reproduces the renormalized angular momentum in the particular case of black holes. The effective field theory approach thus clarifies the role of the renormalized angular momentum in the multipole expansion. The universality of the point particle effective description of compact gravitating systems further allows us to extract the universal part of the anomalous dimension, which is the same for any object, including black holes, neutron stars, and binary systems. As an application, we propose a novel resummation of the universal short-distance logarithms (“tails”) in the gravitational waveform of binary systems, which may improve the modeling of signals from current and future gravitational wave experiments.

The double copy relates gauge and gravitational theories, with widespread application to quantum scattering amplitudes and classical perturbative results. It also connects exact classical solutions of Abelian gauge and gravitational theories in a small number of specific examples, known as classical double copies. These have a number of special properties, such as being algebraically special, and it remains an open question whether examples exist for algebraically general spacetimes or with nontrivial dynamics. Here, we provide a novel framework for understanding classical double copies at the level of the metric, both organizing the known examples and exploring their properties under a new lens. Utilizing Killing vectors as natural gauge fields on a spacetime, we propose a procedure for generating new classical double copies. As a proof of concept, we provide a flat-space single copy for the Kasner metric, an algebraically general spacetime. This example is also a double copy at the level of its curvature and provides the first type-I Weyl double copy on flat spacetime, confirming expectations from perturbation theory. This provides a promising avenue for extending exact double copies to a broader class of general, physically relevant spacetimes.

We study the boundary criticality for the Gross-Neveu-Yukawa (GNY) models. Employing interacting Dirac fermions on a honeycomb lattice with armchair boundaries, we use determinant quantum Monte Carlo simulation to uncover rich boundary criticalities at the quantum phase transition to a charge density wave (CDW) insulator, including the ordinary, special, and extraordinary transitions. The Dirac fermions satisfy a Dirichlet boundary condition, while the boson field, representing the CDW order, obeys Dirichlet and Neumann conditions at the ordinary and special transitions, respectively, thereby enriching the critical GNY model. We develop a perturbative $4-\epsilon$ renormalization group approach to compute the boundary critical exponents. Our framework generalizes to other GNY universality class variants and provides theoretical predictions for experiments.

In a seminal paper now a decade old, it was shown that dark-matter detectors geared at probing interactions with nucleons could also be used to probe dark-matter interactions with electrons. In this Letter, we show that new detector concepts designed to probe dark-matter–electron interactions at low masses can similarly be used to probe new parameter space for dark-matter–nucleon interactions. We demonstrate the power of this approach by using existing data from superconducting detectors to place new limits on the interactions of nuclei with MeV-scale dark matter. Further, we show that advances in detector technology that have been anticipated for electronic interactions will automatically extend sensitivity deep into uncharted territory for nuclear interactions. This doubles the effective science output of future low-threshold experiments.

The properties of bound states are fundamental to hadronic spectroscopy and play a central role in the transition from hadronic matter to a quark-gluon plasma (QGP). In a strongly coupled QGP (sQGP), the interplay of temperature, binding energy, and large collisional widths of the partons poses formidable challenges in evaluating the in-medium properties of hadronic states and their eventual melting. In particular, the existence of heavy quarkonia in the QGP is a long-standing problem that is hard to solve by considering their spectral properties on the real-energy axis. We address this problem by analyzing in-medium thermodynamic quarkonium $T$ matrices in the complex energy plane. We first validate this method in vacuum, where the $T$-matrix poles of observed states are readily identified. When deploying this approach to recent self-consistently calculated $T$ matrices in the QGP, we find that poles in the complex energy plane can persist to surprisingly large temperatures, depending on the strength of the in-medium interactions. While the masses and widths of the pole positions are precisely defined, the notion of a binding energy is not due to the absence of thresholds caused by the (large) widths of the underlying quark or anti-quark spectral functions. Our method thus provides a new and definitive quantum-mechanical criterion to determine the melting temperature of hadronic states in the sQGP while increasing the accuracy in the theoretical determination of transport parameters.

A novel projective measurement technique utilizing conventional photoelectron velocity-map imaging to probe the spatial part of the wave function by projecting it into momentum space is demonstrated. Oscillations between the states $|m_l=0,m_s=\pm 1/2⟩$ and $|m_l=\pm 1,m_s=\mp 1/2⟩$ are observed with no detectable decoherence over 170 cycles with a speed of around 1.7 THz driven purely by spin-orbit interaction. Our measurements achieve a quantum operation fidelity of approximately 90%, limited by the photoionization detection scheme. This Letter establishes a new platform for high-fidelity quantum operations driven by the intrinsic spin-orbit coupling.