A flavor-tagged time-dependent analysis of $B^0\to K_{\mathrm{S}}^0{\mu }^{+}{\mu }^{-}$ decays is performed across the full dimuon mass range excluding the $J/\psi$ and $\psi (2S)$ resonance regions. The analysis uses proton-proton collision data collected by the LHCb experiment in 2011–2018 at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. The $CP$ violation parameters are determined to be $C=-0.13\pm 0.32\pm 0.04$, $S=+0.82\pm 0.29\pm 0.05$, where the first uncertainties are statistical and the second are systematic. The results are consistent with the standard model prediction. This is the first experimental study of time-dependent $CP$ violation in $b\to s{\ell }^{+}{\ell }^{-}$ processes.

The sensitivity of a mechanical transducer is ultimately limited by its inherent quantum fluctuations. Here, we use an optically levitated nanoparticle to measure impulsive forces smaller than the particle’s zero-point momentum uncertainty. Our approach relies on reversibly squeezing the levitated particle’s center-of-mass motion to coherently amplify the perturbation. We demonstrate an impulsive-force resolution as small as $6.9\mathrm{keV}/\mathrm{c}$, a value 0.6 dB below the sensor’s zero-point value.

Intrinsic responses are of paramount importance in physics research, as they represent the inherent properties of materials, independent of extrinsic factors that vary from sample to sample, and often reveal the intriguing quantum geometry of the band structure. Here, we report the experimental discovery of a new intrinsic response in charge transport, specifically the intrinsic nonlinear planar Hall effect, in the topological semimetal ${\mathrm{TaIrTe}}_4$. This effect is characterized by an induced Hall current that is quadratic in the driving electric field and linear in the in-plane magnetic field. The response coefficient is determined by the susceptibility tensor of the Berry-connection polarizability dipole, which is an intrinsic band geometric quantity. Remarkably, the signal persists up to room temperature. Our theoretical calculations show excellent agreement with the experimental results and further elucidate the significance of a previously unknown orbital mechanism in intrinsic nonlinear planar Hall effect. This finding not only establishes a novel intrinsic material property but also opens a new route toward innovative nonlinear devices capable of operating at room temperature.

Chiral magnons, the quanta of handed spin waves, transport spin angular momentum without energy loss due to Joule heating. The recently discovered altermagnets were proposed to host chiral magnons arising from a nonrelativistic exchange mechanism, similar to that in ferromagnets but without net magnetization, offering a stray-field-free platform for efficient magnon spin-current manipulation. In this Letter, we directly observed chiral magnons in the altermagnetic prototype MnTe using polarized inelastic neutron scattering. Furthermore, the magnon chirality was found to be reversibly switched by magnetic-field control, establishing a robust foundation for functional altermagnetic magnonics.

We demonstrate a three-dimensional magneto-optical trap (MOT) of a metal hydride molecule, CaH. We are able to scatter $\sim {10}^4$ photons with vibrational loss covered up to vibrational quantum number $\nu =2$. This allows us to laser slow the molecular beam near zero velocity with a “white-light” technique and subsequently load it into a radio-frequency MOT. The MOT contains 230(40) molecules, limited by beam source characteristics and predissociative loss of CaH. The temperature of the MOT is below one millikelvin. The predissociative loss mechanism could, in turn, facilitate controlled dissociation of the molecule, offering a possible route to optical trapping of hydrogen atoms for precision spectroscopy.

Due to the Coulomb blockade effect, electrons typically tunnel into a localized state one by one, showing antibunching behavior. However, theory predicts that strong vibronic coupling can generate electron bunching locally, yet direct atomic-scale evidence has remained elusive. In this Letter, we report an atomically resolved shot-noise study probing the transport statistics of single-electron tunneling in an atomic impurity coupled to a vibrational mode. Conventional time-averaged conductance spectroscopy reveals clear signatures of vibron-assisted tunneling in this atomically sized nanoelectromechanical system. Simultaneous shot-noise measurements at the impurity center show super-Poissonian noise, providing direct evidence that vibronic coupling drives electron bunching during tunneling. As a future outlook, if coherence between electrons can be implemented, vibron-mediated electron bunching at single atomic sites may be exploited as a local injection source of $N$-paired electrons.

We demonstrate a single-photon detector operating in the microwave domain, based on photoassisted quasiparticle tunneling events that poison a superconducting island. The detection relies on continuously monitoring the island’s charge parity using microwave reflectometry. This scheme achieves 10% detection efficiency with sub 50 ns time resolution and short dead time ($\sim 1\mathrm{\mu s}$), for microwave photons at 10 GHz. The detector features three junctions connected to a superconducting island, which together carry out photoelectric conversion and charge readout. The enhanced light-matter coupling, crucial to photon-to-quasiparticle conversion, is provided by a granular aluminum-based high-impedance microwave resonator. The time-resolved detection of itinerant microwave photon opens up new perspectives in quantum sensing, microwave quantum optics, and mesoscopic physics.

Magnetic and polar skyrmions exhibit topologically protected quasiparticle behavior, including emergent fields, deformation, and the formation of a densely packed skyrmion lattice, beyond conventional domain configurations described by Kittel’s law. Analogous to atomic crystals, lattice defects, especially dislocations and their associated strain fields are crucial for understanding the lattice behavior of skyrmions; however, their features and roles remain insufficiently understood. Here, we show that magnetic skyrmion dislocations develop a core-split structure due to a significant skyrmion elongation up to 180% of their original length, reaching a topological transition from a single skyrmion to two half-skyrmions. Despite such a distinct structure, the long-range strain fields around the dislocation perfectly obey conventional Volterra’s elasticity theory, in contrast to polar skyrmion lattices, where skyrmion deformations cause a breakdown of the elasticity theory. Furthermore, an energetic analysis shows that Dzyaloshinskii-Moriya interaction drives the large skyrmion deformation of the dislocation core. Our findings not only clarify the coexistence of topological core reconstruction and a robust long-range elastic field of dislocations in magnetic skyrmion lattices, but also reveal that magnetic and electric domains, long regarded as analogous, exhibit fundamental differences when extended into the regime of collective topological quasiparticles.

The equations of classical mechanics can be used to model the time evolution of countless physical systems, from the astrophysical to the atomic scale. Accurate numerical integration requires small time steps, which limits the computational efficiency—especially in cases such as molecular dynamics that span wildly different timescales. Using machine-learning (ML) algorithms to predict trajectories allows one to greatly extend the integration time step, at the cost of introducing artifacts such as lack of energy conservation and loss of equipartition between different degrees of freedom of a system. We propose learning data-driven structure-preserving (symplectic and time-reversible) maps to generate long-time-step classical dynamics and show that this method is equivalent to learning the mechanical action of the system of interest. These models can be learned based on short reference trajectories and be transferred across thermodynamic conditions and chemical composition. We show that an action-derived ML integrator eliminates the pathological behavior of non-structure-preserving ML predictors and that the method can be applied iteratively, serving as a correction to computationally cheaper direct predictors.

Achieving resonant detection and coherent control of phonons with simultaneous nanometer spatial and femtosecond temporal precision is of great importance for developing phonon-based technologies, yet remains a long-standing challenging task. In this Letter, we succeed to fabricate nanoscale coherent phonons with broadband frequency tunability using a femtosecond laser-combined scanning tunneling microscope. An ultrafast photocurrent, enhanced by localized surface plasmons, was used to track the high-frequency mechanical motion of nanoclusters at their intrinsic length and time scales. We demonstrate that the coherent acoustic phonon modes of the nanoclusters can be launched by the impulsive pressure of plasmon-induced hot electrons and that their frequency can be tuned by the size of the nanoparticles. Through tip manipulation, we generated a series of on-tip coherent phonons with an ultrabroad frequency range spanning from 15 GHz to 1 THz in a highly controlled manner. Such a tip with broadband-tunable coherent phonons holds great promise for detecting high-frequency mechanical and electromagnetic fluctuations in resonant modes.

The mass law is a cornerstone in predicting sound transmission loss, yet it neglects the constraints of causal dispersion. Current causality-based theories, such as the Rozanov limit, are applicable only to one-port reflective absorbers. Here, we derive a universal sum rule governing causal scattering in acoustic systems, establishing a rigorous analogy to the Baldin sum rule in quantum field theory. This relation reveals that the integral of the extinction cross section is fundamentally locked by the scatterer’s static effective mass and stiffness, which is validated numerically using seminal examples of underwater metamaterials. Furthermore, the proposed sum rule predicts an optimal condition for an anomalously broadened transmission loss bandwidth, as experimentally observed through the spectral shaping effect of an acoustic Fano resonator. Our findings open up an unexplored avenue for enhancing the scattering bandwidth of passive metamaterials.

Superconducting altermagnets have proven to be a promising ground for emergent phenomena, but their study has involved two-dimensional systems. In this work, we investigate three-dimensional $d$- and $g$-wave altermagnets with spin-singlet chiral $d$-wave superconductivity and show the formation of crossed surface flat bands due to the interplay between superconducting and altermagnetic symmetries. We find that these crossed flat bands are topologically protected, appear at zero energy in the surface along $z$ due to the superconducting nodal lines in the $xy$ plane, and their number of corners is determined by the crystal symmetry of altermagnets. We also show that the superconducting nodal lines give rise to Bogoliubov-Fermi surfaces, which then affect the appearance of zero-energy arcs in the surface along $x$. Moreover, we demonstrate that the crossed flat bands or surface arcs, and Bogoliubov-Fermi surfaces give rise to the coexistence of three distinct dependences of the charge conductance on the normal transparency, hence offering a solid way for their detection and paving the way for realizing higher-dimensional topological phases using altermagnets.

We report the experimental observation of magnetochiral anisotropy in the longitudinal and transverse ultrasound propagation in $\alpha$ quartz. To perform such measurement, we have built an ultrasound spectrometer with unprecedented experimental resolution of the order of $\mathrm{\Delta }v/v\sim {10}^{-8}$. We present a simple macroscopic Becquerel-like analytical model that accounts for the magnitude of the observed effect and its frequency dependence.

Bound states in the continuum (BICs), spatially localized states embedded in radiating continuum, have been extensively investigated in single-particle systems, also enabling high-performance applications in classical wave systems. Recently, exploring many-body BICs in correlated systems has emerged as a pivotal frontier with increasing research interest. However, the majority of previous studies focus largely on on-site interactions with tailored conditions, incurring significant implementation challenges. Whether off-site interactions, e.g., many-body hoppings, can generate many-body BICs and how to characterize them in experiments remains an open question. Here, we first predict and realize a boundary-localized two-body BIC, the bound pair state in the continuum (BPIC), arising from uniform two-body hoppings. Analytically, we demonstrate not just the formation of bound pairs, but the spectral coexistence of the boundary-localized BPIC and the scattering continuum. Experimentally, developing a phononic crystal platform as a classical-wave simulator, we map the correlated particle dynamics onto acoustic wave propagation and observe the acoustic BPIC. Our findings build a new bridge between the BIC research and many-body physics, and may promote the development of high-$Q$ acoustic devices in applications.

There is currently no rigorous definition of gravitational wave strain at second order in cosmological perturbation theory. The usual association of gravitational waves with transverse and traceless fluctuations of the metric on spatial hypersurfaces becomes ambiguous at second order, as it inherently depends on the spacetime slicing. While this poses no practical issues in linearized gravity, it presents a fundamental problem for secondary gravitational waves, especially notorious for gravitational waves induced by primordial fluctuations. We compute, for the first time, the observable gravitational wave strain at second order as measured by geodesic observers that emit and receive electromagnetic signals, thereby settling the debate on gauge ambiguities. Working in a gauge invariant fashion, we find that the measured gravitational wave strain coincides with the transverse-traceless components in the Newton gauge.

The interaction between free electrons and optical modes underlies a variety of quantum and nanoscale light-matter phenomena, yet the associated momentum exchange with the sample largely remained overlooked. Here, we experimentally demonstrate the momentum transfer from free electrons to planar samples during optical mode excitation using momentum-resolved electron energy-loss spectroscopy. The momentum transfer to the sample modifies the apparent dispersion relation which is significant when the planar sample is tilted. Under specific conditions, the sample receives momentum opposite to the electron beam direction.

Classical liquid–liquid phase separation predicts that droplets grow continuously via Brownian coalescence or Ostwald ripening, yet many nanoscale biomolecular condensates remain stable for hours or days without active regulation. Such condensates often arise through complex coacervation between oppositely charged macromolecules, making this interaction motif broadly relevant in biology and soft matter. Here we combine experiments, theory, and simulations to identify a merging-limited coarsening (MLC) regime in which merging of condensates decreases sharply below a critical droplet size. Chain-length asymmetry between oppositely charged polymers drives entropic interfacial charge separation even at globally neutral stoichiometry, imparting net droplet charges and generating long-range electrostatic repulsion. These size-dependent barriers lead to exponential rather than classical power-law growth and trap droplets in long-lived metastable states. Our framework unifies suppressed MLC and classical Brownian coalescence within a single predictive model and provides a general mechanism for condensate stability in both synthetic systems and living cells.

Decorating the superconducting Pb tip of a scanning tunneling microscope with a Mn atom cluster gives rise to Yu-Shiba-Rusinov resonances that serve as spin-sensitive probes of the tunneling current in junctions of single heptahelicene molecules adsorbed on Pb(111). The signal strength of these states in spectroscopy of the differential conductance depends on the handedness of the probed molecule, the direction of current flow, and the intramolecular site. These findings support the presence of the chirality-induced spin selectivity effect and the role of a single enantiomer as a spin polarizer.

${\text{FeF}}_2$ is a prototypical rutile antiferromagnet recently proposed as an altermagnet, with a magnetic symmetry that permits spin-split electronic bands and chiral magnons. Using very-high-resolution inelastic neutron scattering on a single crystal of ${\mathrm{FeF}}_2$, we show that the dominant source of magnon splitting is in fact the long-range dipolar interaction rather than altermagnetic exchange terms. At momenta where the dipolar splitting vanishes, we observe additional broadening due to altermagnetic chiral splitting and estimate this splitting to be $\sim 35\mathrm{\mu }\mathrm{eV}$. Polarized measurements further reveal that, where dipolar splitting is present, the chiral magnon modes become mixed and the resulting modes are predominantly linearly polarized, with at most a small chiral component. These findings highlight the significant effect of dipolar interactions on magnon chirality, particularly when altermagnetic interactions are weak.

Active semiflexible filament collectives, ranging from motor-driven cytoskeletal filaments to slender organisms such as cyanobacteria and worm aggregates, abound in nature. Yet how activity and flexibility jointly govern their organization, especially isotropic-nematic (I-N) transition, remains poorly understood. Performing large-scale Brownian dynamics simulations of 3D active semiflexible polymers with varying flexibility degrees, we show that tangential active forces systematically shift the I-N transition to higher densities, with the shift controlled by the flexibility degree and activity strength. Strikingly, activity alters the nature of the transition: discontinuous at low strengths, continuous at moderate strengths, and ultimately suppressed at high activity levels. The delayed I-N transition originates from enhanced collective bending fluctuations, resulting in chain shrinkage and enlargement of effective confinement tube. At moderate activity levels, these fluctuations can trigger large-scale excitations that stochastically drive temporal transitions between nematic and isotropic states, indicating an activity-induced instability of the nematic field. We summarize this behavior in nonequilibrium state diagrams of density and activity for different flexibility degrees.