Pair interactions between active particles need not follow Newton’s third law. In this work, we propose a continuum model of pattern formation due to nonreciprocal interaction between multiple species of scalar active matter. The classical Cahn-Hilliard model is minimally modified by supplementing the equilibrium Ginzburg-Landau dynamics with particle-number-conserving currents, which cannot be derived from a free energy, reflecting the microscopic departure from action-reaction symmetry. The strength of the asymmetry in the interaction determines whether the steady state exhibits a macroscopic phase separation or a traveling density wave displaying global polar order. The latter structure, which is equivalent to an active self-propelled smectic phase, coarsens via annihilation of defects, whereas the former structure undergoes Ostwald ripening. The emergence of traveling density waves, which is a clear signature of broken time-reversal symmetry in this active system, is a generic feature of any multicomponent mixture with microscopic nonreciprocal interactions.

There are several FeSe based superconductors, including the bulk FeSe, monolayer FeSe thin film, intercalated ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ and ${\mathrm{Li}}_{1-x}{\mathrm{Fe}}_x\mathrm{OHFeSe}$, etc. Their normal states all show metallic behavior. The key player here is the FeSe layer, which exhibits the highest superconducting transition temperature in the form of monolayer thin film. Recently, a new FeSe based compound, ${\mathrm{CsFe}}_{4-x}{\mathrm{Se}}_4$, with the space group of $Bmmm$ was found. Interestingly, the system shows a strong insulatorlike behavior, although it shares the same FeSe planes as other relatives. Density functional theory calculations indicate that it should be a metal, in sharp contrast with the experimental observations. Here, we report the emergence of unconventional superconductivity by applying pressure to suppress this insulatorlike behavior. At ambient pressure, the insulatorlike behavior cannot be modeled as a band insulator, but it can be described by the variable-range-hopping model for correlated systems. Furthermore, the specific heat down to 400 mK has been measured, and a significant residual coefficient ${\gamma }_0=C/T{{|}_T}_{\to 0}$ is observed, which contrasts the insulatorlike state and suggests some quantum freedom of spin dynamics. By applying pressure, the insulatorlike behavior is gradually suppressed, and the system becomes a metal; finally, superconductivity is achieved at about 5.1 K. The superconducting transition strongly depends on magnetic field and applied current, indicating a fragile superfluid density. Our results suggest that the superconductivity is established by diluted Cooper pairs on top of a strong correlation background in ${\mathrm{CsFe}}_{4-x}{\mathrm{Se}}_4$.

Topologically ordered phases of matter, although stable against local perturbations, are usually restricted to relatively small regions in phase diagrams. Thus, their preparation requires a precise fine-tunning of the system’s parameters, a very challenging task in most experimental setups. In this work, we investigate a model of spinless fermions interacting with dynamical ${\mathbb{Z}}_2$ gauge fields on a cross-linked ladder and show evidence of topological order throughout the full parameter space. In particular, we show how a magnetic flux is spontaneously generated through the ladder due to an Aharonov-Bohm instability, giving rise to topological order even in the absence of a plaquette term. Moreover, the latter coexists here with a symmetry-protected topological phase in the matter sector, which displays fractionalized gauge-matter edge states and intertwines with it by a flux-threading phenomenon. Finally, we unveil the robustness of these features through a gauge frustration mechanism, akin to geometric frustration in spin liquids, allowing topological order to survive to arbitrarily large quantum fluctuations. In particular, we show how, at finite chemical potential, topological solitons are created in the gauge field configuration, which bound to fermions and form ${\mathbb{Z}}_2$ deconfined quasiparticles. The simplicity of the model makes it an ideal candidate for 2D gauge theory phenomena, as well as exotic topological effects, to be investigated using cold-atom quantum simulators.

The cyclotron resonance of monolayer graphene, encapsulated in hexagonal boron nitride and with a graphite backgate, is explored via infrared transmission magnetospectroscopy as a function of the filling factor at fixed magnetic fields. The impact of many-particle interactions in the regime of broken spin and valley symmetries is observed spectroscopically. As the occupancy of the zeroth Landau level is increased from half-filling, a nonmonotonic progression of multiple cyclotron resonance peaks is seen for several interband transitions, revealing the evolution of underlying many-particle-enhanced gaps. Analysis of the peak energies shows significant exchange enhancements of spin gaps both at and below the Fermi energy, a strong filling-factor dependence of the substrate-induced Dirac mass, and also the smallest particle-hole asymmetry reported to date in graphene cyclotron resonance.

We investigate Feshbach resonances in collisions of high-spin atoms such as Er and Dy with closed-shell atoms such as Sr and Yb, using coupled-channel scattering and bound-state calculations. We consider both low-anisotropy and high-anisotropy limits. In both regimes, we find many resonances with a wide variety of widths. The wider resonances are suitable for tuning interatomic interactions, while some of the narrower resonances are highly suitable for magnetoassociation to form high-spin molecules. These molecules might be transferred to short-range states, where they would have large magnetic moments and electric dipole moments that can be induced with very low electric fields. The results offer the opportunity to study mixed quantum gases where one species is dipolar and the other is not and open up important prospects for a new field of ultracold high-spin polar molecules.

Covariance mapping [L. J. Frasinski, K. Codling, and P. A. Hatherly, Science 246, 1029 (1989)] is a well-established technique used for the study of mechanisms of laser-induced molecular ionization and decomposition. It measures statistical correlations between fluctuating signals of pairs of detected species (ions, fragments, electrons). A positive correlation identifies pairs of products originating from the same dissociation or ionization event. A major challenge for covariance-mapping spectroscopy is accessing decompositions of large polyatomic molecules, where true physical correlations are overwhelmed by spurious signals of no physical significance induced by fluctuations in experimental parameters. As a result, successful applications of covariance mapping have so far been restricted to low-mass systems, e.g., organic molecules of around 50 daltons (Da). Partial-covariance mapping was suggested to tackle the problem of spurious correlations by taking into account the independently measured fluctuations in the experimental conditions. However, its potential has never been realized for the decomposition of large molecules, because in these complex situations, determining and continuously monitoring multiple experimental parameters affecting all the measured signals simultaneously becomes unfeasible. We introduce, through deriving theoretically and confirming experimentally, a conceptually new type of partial-covariance mapping—self-correcting partial-covariance spectroscopy—based on a parameter extracted from the measured spectrum itself. We use the readily available total ion count as the self-correcting partial-covariance parameter, thus eliminating the challenge of determining experimental parameter fluctuations in covariance measurements of large complex systems. The introduced self-correcting partial covariance enables us to successfully resolve correlations of molecules as large as ${10}^3–{10}^4\mathrm{Da}$, 2 orders of magnitude above the state of the art. This opens new opportunities for mechanistic studies of large molecule decompositions through revealing their fragment-fragment correlations. Moreover, we demonstrate that self-correcting partial covariance is applicable to solving the inverse problem: reconstruction of a molecular structure from its fragment spectrum, within two-dimensional partial-covariance mass spectrometry.

Recent studies on superconductivity in ${\mathrm{NbSe}}_2$ have demonstrated a large anisotropy in the superconducting critical field when the material is reduced to a single monolayer. Motivated by this recent discovery, we use density-functional theory (DFT) calculations to quantitatively address the superconducting properties of bulk and monolayer ${\mathrm{NbSe}}_2$. We demonstrate that ${\mathrm{NbSe}}_2$ is close to a ferromagnetic instability, and analyze our results in the context of experimental measurements of the spin susceptibility in ${\mathrm{NbSe}}_2$. We show how this magnetic instability, which is pronounced in a single monolayer, can enable sizable singlet-triplet mixing of the superconducting order parameter, contrary to contemporary considerations of the pairing symmetry in monolayer ${\mathrm{NbSe}}_2$, and discuss approaches as to how this degree of mixing can be addressed quantitatively within our DFT framework. Our calculations also enable a quantitative description of the large anisotropy of the superconducting critical field, using DFT calculations of monolayer ${\mathrm{NbSe}}_2$ in the normal state.

The predominant Ni-multiorbital nature of infinite-layer neodynium nickelate at stoichiometry and with doping is revealed. We investigate the correlated electronic structure of ${\mathrm{NdNiO}}_2$ at lower temperatures and show that first-principles many-body theory may account for Kondo(-lattice) features. Yet, those features are not only based on localized $\mathrm{Ni}\text{−}{d}_{x^2-y^2}$ and a Nd-dominated self-doping band, but they heavily build on the participation of $\mathrm{Ni}\text{−}{d}_{z^2}$ in a Hund-assisted manner. In a tailored three-orbital study, the half-filled regime of the former in-plane Ni orbital remains surprisingly robust even for substantial hole doping $\delta$. Reconstructions of the interacting Fermi surface designate the superconducting region within the experimental phase diagram. Furthermore, they provide clues to recent Hall measurements, as well as to the astounding weakly insulating behavior at larger experimental $\delta$. Finally, a strong asymmetry between electron and hole doping, with a revival of Ni single-orbital features in the former case, is predicted. Unlike cuprates, superconductivity in ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$ is of distinct multiorbital kind, building up on nearly localized $\mathrm{Ni}\text{−}{d}_{x^2-y^2}$ and itinerant $\mathrm{Ni}\text{−}{d}_{z^2}$.

Using scanning probe microscopy, we measure the out-of-plane mechanical response of ferroelectric 180° domain walls and observe that, despite separating domains that are mechanically identical, the walls appear mechanically distinct—softer—compared to the domains. This effect is observed in different ferroelectric materials (${\mathrm{LiNbO}}_3$, ${\mathrm{BaTiO}}_3$, and ${\mathrm{PbTiO}}_3$) and with different morphologies (from single crystals to thin films), suggesting that the effect is universal. We propose a theoretical framework that explains the domain wall softening and justifies that the effect should be common to all ferroelectrics. The lesson is, therefore, that domain walls are not only functionally different from the domains they separate, but also mechanically distinct.

Quantum mechanically, photoionization can be fully described by the complex photoionization amplitudes that describe the transition between the ground state and the continuum state. Knowledge of the value of the phase of these amplitudes has been a central interest in photoionization studies and newly developing attosecond science, since the phase can reveal important information about phenomena such as electron correlation. We present a new attosecond-precision interferometric method of angle-resolved measurement for the phase of the photoionization amplitudes, using two phase-locked extreme ultraviolet pulses of frequency $\omega$ and $2\omega$, from a free-electron laser. Phase differences $\mathrm{\Delta }\tilde{\eta }$ between one- and two-photon ionization channels, averaged over multiple wave packets, are extracted for neon $2p$ electrons as a function of the emission angle at photoelectron energies 7.9, 10.2, and 16.6 eV. $\mathrm{\Delta }\tilde{\eta }$ is nearly constant for emission parallel to the electric vector but increases at 10.2 eV for emission perpendicular to the electric vector. We model our observations with both perturbation and ab initio theory and find excellent agreement. In the existing method for attosecond measurement, reconstruction of attosecond beating by interference of two-photon transitions (RABBITT), a phase difference between two-photon pathways involving absorption and emission of an infrared photon is extracted. Our method can be used for extraction of a phase difference between single-photon and two-photon pathways and provides a new tool for attosecond science, which is complementary to RABBITT.

A promising route to realize entangled magnetic states combines geometrical frustration with quantum-tunneling effects. Spin-ice materials are canonical examples of frustration, and Ising spins in a transverse magnetic field are the simplest many-body model of quantum tunneling. Here, we show that the tripod-kagome lattice material ${\mathrm{Ho}}_3{\mathrm{Mg}}_2{\mathrm{Sb}}_3{\mathrm{O}}_{14}$ unites an icelike magnetic degeneracy with quantum-tunneling terms generated by an intrinsic splitting of the ${\mathrm{Ho}}^{3+}$ ground-state doublet, which is further coupled to a nuclear spin bath. Using neutron scattering and thermodynamic experiments, we observe a symmetry-breaking transition at $T^{*}\approx 0.32\mathrm{K}$ to a remarkable state with three peculiarities: a concurrent recovery of magnetic entropy associated with the strongly coupled electronic and nuclear degrees of freedom; a fragmentation of the spin into periodic and icelike components; and persistent inelastic magnetic excitations down to $T\approx 0.12\mathrm{K}$. These observations deviate from expectations of classical spin fragmentation on a kagome lattice, but can be understood within a model of dipolar kagome ice under a homogeneous transverse magnetic field, which we survey with exact diagonalization on small clusters and mean-field calculations. In ${\mathrm{Ho}}_3{\mathrm{Mg}}_2{\mathrm{Sb}}_3{\mathrm{O}}_{14}$, hyperfine interactions dramatically alter the single-ion and collective properties, and suppress possible quantum correlations, rendering the fragmentation with predominantly single-ion quantum fluctuations. Our results highlight the crucial role played by hyperfine interactions in frustrated quantum magnets and motivate further investigations of the role of quantum fluctuations on partially ordered magnetic states.

We carry out an ab initio calculation of the neutrino flux-folded inclusive cross sections measured on ${}^{12}\mathrm{C}$ by the MiniBooNE and T2K Collaborations in the charged-current quasielastic regime. The calculation is based on realistic two- and three-nucleon interactions, and on a realistic nuclear electroweak current with one- and two-nucleon terms that are constructed consistently with these interactions and reproduce low-energy electroweak transitions. Numerically exact quantum Monte Carlo methods are utilized to compute the nuclear weak response functions, by fully retaining many-body correlations in the initial and final states and interference effects between one- and two-body current contributions. We employ a nucleon axial form factor of the dipole form with ${\mathrm{\Lambda }}_A=1.0$ or 1.15 GeV, the latter more in line with a very recent lattice QCD determination. The calculated cross sections are found to be in good agreement with the neutrino data of MiniBooNE and T2K, and antineutrino MiniBooNE data, yielding a consistent picture of nuclei and their electroweak properties across a wide regime of energy and momenta.

The mechanical and chemical properties of soft solids are crucial to many applications in biology and surface science. Recent studies use wetting by liquid drops to probe the surface mechanics of reticulated polymer networks, leading to controversial interpretations. This controversy relates to the long-standing paradox of Young’s law for the liquid contact angle, which invokes only a horizontal force balance. Recent work shows that, for very soft materials, the solid’s surface tension plays a key role for the vertical force balance, involving a singular ridgelike deformation exactly at the point where the droplet pulls on the network. A hotly debated question is whether unexpected measurements on this singular deformation can be attributed to nonlinear bulk elasticity or whether these provide evidence for an intricate surface elasticity, known as the Shuttleworth effect. Here, we theoretically reveal the nature of the elastocapillary singularity on a hyperelastic substrate with various constitutive relations for the interfacial energy. First, we finely resolve the vicinity of the singularity using goal-adaptive finite-element simulations. This simulation confirms that bulk elasticity cannot affect the force balance at the contact line. Subsequently, we derive exact solutions of nonlinear elasticity that describe the singularity analytically. These solutions are in perfect agreement with numerics and show that both the angles and stretch at the contact line, as previously measured experimentally, consistently point to a strong Shuttleworth effect. Finally, using Noether’s theorem, we reveal the quantitative link between Young’s law, hysteresis, and the nature of the elastocapillary singularity. Our contribution closes the issue of the missing normal force at the contact line and opens up the development of modern techniques in polymer surface science.

In certain analytically tractable quantum chaotic systems, the calculation of out-of-time-order correlation functions, entanglement entropies after a quench, and other related dynamical observables reduces to an effective theory of an “entanglement membrane” in spacetime. These tractable systems involve an average over random local unitaries defining the dynamical evolution. We show here how to make sense of this membrane in more realistic models, which do not involve an average over random unitaries. Our approach relies on introducing effective pairing degrees of freedom in spacetime, describing a pairing of forward and backward Feynman trajectories, inspired by the structure emerging in random unitary circuits. This viewpoint provides a framework for applying ideas of coarse graining to dynamical quantities in chaotic systems. We apply the approach to some translationally invariant Floquet spin chains studied in the literature. We show that a consistent line tension may be defined for the entanglement membrane and that there are qualitative differences in this tension between generic models and “dual-unitary” circuits. These results allow scaling pictures for out-of-time-order correlators and for entanglement to be taken over from random circuits to nonrandom Floquet models. We also provide an efficient numerical algorithm for determining the entanglement line tension in $1+1\mathrm{D}$.

We report on the cancellation of quantum backaction noise in an optomechanical cavity. We perform measurements of the displacement of the microresonator, one in reflection of the cavity and one in transmission of the cavity. We show that measuring the amplitude quadrature of the light transmitted by the optomechanical cavity allows us to cancel the backaction noise between 2 and 50 kHz as a consequence of the strong optical spring present in the detuned cavity. This cancellation yields a more sensitive measurement of the microresonator’s position with a 2 dB increase in sensitivity. To confirm that the backaction is eliminated, we measure the noise in the transmission signal as a function of circulating power and use a correlation technique between two photodetectors to remove shot noise. Remaining backaction noise would be observable as a power-dependent noise floor, which is not observed. Eliminating the effects of backaction in this frequency regime is an important demonstration of a technique that could be used to mitigate the effects of backaction in interferometric gravitational wave detectors such as Advanced LIGO, VIRGO, and KAGRA.

Many applications of quantum simulation require one to prepare and then characterize quantum states by efficiently estimating $k$-body reduced density matrices ($k$-RDMs), from which observables of interest may be obtained. For instance, the fermionic 2-RDM contains the energy, charge density, and energy gradients of an electronic system, while the qubit 2-RDM contains the spatial correlation functions of magnetic systems. Naive estimation of such RDMs requires repeated state preparations for each matrix element, which makes for prohibitively large computation times. However, commuting matrix elements may be measured simultaneously, allowing for a significant cost reduction. In this work, we design schemes for such a parallelization with near-optimal complexity in the system size $N$. We first describe a scheme to sample all elements of a qubit $k$-RDM using only $\mathcal{O}(3^k{\log }^{k-1}N)$ unique measurement circuits, an exponential improvement over prior art. We then describe a scheme for sampling all elements of the fermionic 2-RDM using only $\mathcal{O}(N^2)$ unique measurement circuits, each of which requires only a local $\mathcal{O}(N)$-depth measurement circuit. We prove a lower bound of $\mathrm{\Omega }({\epsilon }^{-2}{N}^k)$ on the number of state preparations, Clifford circuits, and measurement in the computational basis required to estimate all elements of a fermionic $k$-RDM, making our scheme for sampling the fermionic 2-RDM asymptotically optimal. We finally construct circuits to sample the expectation value of a linear combination of $\omega$ anticommuting two-body fermionic operators with only $\mathcal{O}(\omega )$ gates on a linear array. These circuits allows for sampling any linear combination of fermionic 2-RDM elements in $\mathcal{O}(N^4/\omega )$ time, with a significantly lower measurement circuit complexity than prior art. Our results improve the viability of near-term quantum simulation of molecules and strongly correlated material systems.

We present a new method for the spectral characterization of pulsed twin-beam sources in the high-gain regime, using cascaded stimulated emission. We show an implementation of this method for a periodically poled potassium titanyl phosphate spontaneous parametric down-conversion source generating up to 60 photon pairs per pulse, and we demonstrate excellent agreement between our experiments and our theory. This work enables the complete and accurate experimental characterization of high-gain effects in parametric down-conversion, including self- and cross-phase modulation. Moreover, our theory allows the exploration of designs with the goal of improving the specifications of twin-beam sources for application in quantum information, computation, sampling, and metrology.

An ever-increasing number of strong-field applications, such as ultrafast coherent control over matter and light, require driver light pulses that are both high power and spectrally tunable. The realization of such a source in the terahertz (THz) band has long been a formidable challenge. Here, we demonstrate, via experiment and theory, efficient production of terawatt (TW)-level THz pulses from high-intensity picosecond laser irradiation on a metal foil. It is shown that the THz spectrum can be manipulated effectively by tuning the laser pulse duration or target size. A general analytical framework for THz generation is developed, involving both the high-current electron emission and a time-varying electron sheath at the target rear, and the spectral tunability is found to stem from the change of the dominant THz generation mechanism. In addition to being an ultrabright source (brightness temperature of about ${10}^{21}\mathrm{K}$) for extreme THz science, the THz radiation presented here also enables a unique in situ laser-plasma diagnostic. Employing the THz radiation to quantify the escaping electrons and the transient sheath shows good agreement with experimental measurements.

In one-dimensional (1D) metallic systems, the diverging electron susceptibility and electron-phonon coupling collaboratively drive the electrons into a charge density wave (CDW) state. However, a strictly 1D system is unstable against perturbations, whose effect on CDW order requires clarification ideally with altered coupling to surroundings. Here, we fabricate such a system with nanowires of ${\mathrm{Mo}}_6{\mathrm{Se}}_6$ bundles, which are either attached to edges of monolayer ${\mathrm{MoSe}}_2$ or isolated freely, by postannealing the preformed ${\mathrm{MoSe}}_2$. Using scanning tunneling microscopy, we visualize charge modulations and CDW gaps with prominent coherent peaks in the edge-attached nanowires. Astonishingly, the CDW order becomes suppressed in the isolated nanowires, showing CDW correlation gaps without coherent peaks. The contrasting behavior, as revealed with theoretical modeling, is interpreted as the effect of phason polarons on the 1D CDW state. Our work elucidates a possibly unprecedented many-body effect that may be generic to strictly 1D system but undermined in a quasi-1D system.

Optically addressable spins are actively investigated in quantum communication, processing, and sensing. Optical and spin coherence lifetimes, which determine quantum operation fidelity and storage time, are often limited by spin-spin interactions, which can be decreased by polarizing spins. Spin polarization can be achieved using optical pumping, large magnetic fields, or mK-range temperatures. Here, we show that optical pumping of a small fraction of ions with a fixed-frequency laser, coupled with spin-spin interactions and spin diffusion, leads to substantial spin polarization in a paramagnetic rare-earth doped crystal, ${{}^{171}\mathrm{Yb}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$. Indeed, more than 90% spin polarization has been achieved at 2 K and zero magnetic field. Using this spin polarization mechanism, we further demonstrate an increase in optical coherence lifetime from 0.3 ms to 0.8 ms, due to a strong decrease in spin-spin interactions. This effect opens the way to new schemes for obtaining long optical and spin coherence lifetimes in various solid-state systems such as ensembles of rare-earth ions or color centers in diamond, which are of interest for a broad range of quantum technologies.