When cell sheets fold during development, their apical or basal surfaces constrict and cell shapes approach the geometric singularity in which these surfaces vanish. Here, we reveal the mechanical consequences of this geometric singularity for tissue folding in a minimal vertex model of an epithelial monolayer. In simulations of the buckling of the epithelium under compression and numerical solutions of the corresponding continuum model, we discover an “unbuckling” bifurcation: at large compression, the buckling amplitude can decrease with increasing compression. By asymptotic solution of the continuum equations, we reveal that this bifurcation comes with a large stiffening of the epithelium. Our results thus provide the mechanical basis for absorption of compressive stresses by tissue folds such as the cephalic furrow during germband extension in Drosophila.

This Letter reports on the first cross-section measurements for the ${}^{94}\mathrm{Sr}(\alpha ,n){}^{97}\mathrm{Zr}$ and ${}^{86}\mathrm{Kr}(\alpha ,n){}^{89}\mathrm{Sr}$ reactions. In particular, our measurement of ${}^{94}\mathrm{Sr}(\alpha ,n){}^{97}\mathrm{Zr}$ is the first weak $r$-process reaction cross section obtained using a radioactive ion beam. This experiment was enabled by the use of novel solid helium targets, comprised of silicon thin films with high helium incorporation obtained via a sputtering technique. Yield measurements were performed at center-of-mass energies of 10.4 and 9.0 MeV for the ${}^{86}\mathrm{Kr}(\alpha ,n){}^{89}\mathrm{Sr}$ reaction, and 9.9 MeV for ${}^{94}\mathrm{Sr}(\alpha ,n){}^{97}\mathrm{Zr}$, extending into the respective Gamow energy windows for a temperature of 5 GK. Reactions were uniquely identified by prompt $\gamma$ rays detected in coincidence with heavy ions selected by a recoil mass spectrometer. The obtained cross sections are smaller than predicted for both reactions. In the case of ${}^{94}\mathrm{Sr}(\alpha ,n){}^{97}\mathrm{Zr}$, the reaction rate found here is lowered by an order of magnitude at temperatures below 5 GK, which is expected to impact the predicted abundance of ruthenium, a signature weak $r$-process element.

Quantum state-resolved spectroscopy of the low energy thorium-229 nuclear transition was recently achieved. The five allowed transitions within the electric quadrupole structure were measured to the kilohertz level in a calcium fluoride host crystal, opening many new areas of research using nuclear clocks. Central to the performance of solid-state clock operation is an understanding of systematic shifts such as the temperature dependence of the clock transitions. In this work, we measure the four strongest transitions of thorium-229 in the same crystal at three temperature values: 150 K, 229 K, and 293 K. We find shifts of the unsplit frequency and the electric quadrupole splittings, corresponding to decreases in the electron density, electric field gradient, and field gradient asymmetry at the nucleus as temperature increases. The $m=\pm 5/2\to \pm 3/2$ line shifts only 62(6) kHz over the temperature range, i.e., approximately $0.4\mathrm{kHz}/\mathrm{K}$, representing a promising candidate for a future solid-state optical clock. Achieving ${10}^{-18}$ precision requires crystal temperature stability of $5\mathrm{\mu }\mathrm{K}$.

Landau levels (LLs) are of great importance for understanding the quantum Hall effect and associated many-body physics. Recently, their three-dimensional (3D) counterparts, i.e., dispersionless 3D LLs with well-defined quantum numbers, have attracted significant attention but have not yet been reported. Here we theoretically propose and experimentally observe 3D LLs with a sharply quantized spectrum in a diamond acoustic lattice, where the eigenstates are characterized by SU(3) quantum numbers. The engineered inhomogeneous hopping strengths not only introduce pseudomagnetic fields that quantize the nodal lines into LLs but also provide three bosonic degrees of freedom, embedding a generic SU(3) symmetry into the LLs. Using a phased array of acoustic sources, we selectively excite distinct eigenstates within the degenerate LL multiplets and visualize their 3D eigenmodes. Importantly, our approach enables the precise reconstruction of SU(3) quantum numbers directly from eigenmode correlations. Our results establish SU(3) LLs as a tractable model in artificial platforms, and pave the way for synthesizing LLs with zero dispersion and countable quantum numbers in arbitrary dimensions.

Kitaev quantum spin liquids (KSLs) in layered honeycomb magnets are known to host Majorana quasiparticles, whose excitations depend strongly on the direction of the applied magnetic field. In the high-field phase of $\alpha \text{−}{\text{RuCl}}_3$, specific heat measurements have revealed characteristic field-angle dependence of low-energy excitations consistent with the Kitaev model, providing bulk evidence for the KSL state. Here we present low-temperature measurements of specific heat $C(T)$ for another KSL candidate ${\mathrm{Na}}_2{\mathrm{Co}}_2\mathrm{Te}{\mathrm{O}}_6$ (NCTO) under field rotation within the honeycomb plane. Above the critical field of antiferromagnetic order, the field-angle dependence of $C/T$ exhibits minima along the bond directions, contrasting with the maxima observed in the KSL state of $\alpha \text{−}{\text{RuCl}}_3$. Our analysis indicates nodeless, fully gapped excitations, which are inconsistent with the angle-dependent Majorana excitations with gapless nodes predicted by the Kitaev model. These findings suggest that low-energy excitations in NCTO are governed by gapped magnon excitations rather than Majorana quasiparticles, providing thermodynamic evidence against a KSL state.

Magnetoelectric coupling is crucial for uncovering fundamental phenomena and advancing technologies in high-density data storage and energy-efficient devices. The emergence of altermagnets, which unify the advantages of ferromagnets and antiferromagnets, offers unprecedented opportunities for magnetoelectric coupling. However, electrically tuning altermagnets remains an outstanding challenge. Here, we demonstrate how this challenge can be overcome by using antiferroelectricity and ferroelectricity to modulate the spin splitting in altermagnets, employing a universal, symmetry-based design principle supported by an effective model. We introduce an unexplored class of multiferroics: antiferroelectric altermagnets (AFEAM), where antiferroelectricity and altermagnetism coexist in a single material. From first-principles calculations, we validate the feasibility of AFEAM in well-established van der Waals metal thio(seleno)phosphates and perovskite oxides. We reveal the design of AFEAM ranging from two-dimensional monolayers to three-dimensional bulk structures. Remarkably, even a weak electric field can effectively toggle spin polarization in the AFEAM by switching between antiferroelectric and ferroelectric states. Our findings not only enrich the understanding of magnetoelectric coupling but also pave the way for electrically controlled spintronic and multiferroic devices.

We propose a novel ferroelectric switchable altermagnetism effect: the reversal of ferroelectric polarization is coupled to the switching of altermagnetic spin splitting. We demonstrate the design principles for the ferroelectric altermagnets and the additional symmetry constraints necessary for switching the spin splitting through flipping the electric polarization based on the state-of-the-art spin-group symmetry techniques. We find 22 ferroelectric altermagnets by screening through the 2001 experimental reported magnetic structures in the MAGNDATA database and identify two of them as ferroelectric switchable altermagnets. Using the hybrid improper ferroelectric material $[\mathrm{C}({\mathrm{NH}}_2{)}_3]\mathrm{Cr}{(\mathrm{HCOO})}_3$ as an example, we show how the altermagnetic spin splitting is tightly coupled to the ferroelectric polarization, providing an ideal platform for designing electric-field-controllable multiferroic devices. Finally, we find that such manipulation of altermagnetism can be detected by monitoring the physical quantities that are related to the nonvanishing Berry curvature dipole, such as the linearly polarized photogalvanic spin current.

Exceptional points (EPs), known as non-Hermitian degeneracies featuring missing eigenspace dimensions, have led to a variety of intriguing wave phenomena in various physical platforms. Chiral EPs collapsing in two orthogonal eigenstates can lead to unique effects and applications, such as loss-induced transparency, EP-enhanced sensing, and chirality-reversal electronics, etc. However, in previous experiments, chiral EPs were typically induced in fixed structures with inherent active gains or nearby nanotips, which are unfavorable for on-chip integrations with low-power elements. Here, we demonstrate the active control of EPs chirality in situ with exceptional-line metasurface. Selective chirality inversion can be well achieved by only light-induced loss without altering the metasurface size. We also perform an ultrafast chirality switch in the picosecond level within transient disturbance. In a broader view, our results provide a platform for the investigation of metasurface-based non-Hermitian physics and active EP modulation, which can stimulate exciting works along this line in near future.

The $CP$ asymmetry and branching fraction of the Cabibbo-Kobayashi-Maskawa-suppressed decay $B^{+}\to J/\psi {\pi }^{+}$ are precisely measured relative to the favored decay $B^{+}\to J/\psi K^{+}$ using a sample of proton-proton collision data corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$ recorded at a center-of-mass energy of 13 TeV during 2016–2018. The results of the $CP$ asymmetry difference and branching fraction ratio are $\mathrm{\Delta }{\mathcal{A}}^{CP}\equiv {\mathcal{A}}^{CP}(B^{+}\to J/\psi {\pi }^{+})-{\mathcal{A}}^{CP}(B^{+}\to J/\psi K^{+})=(1.29\pm 0.49\pm 0.08)\times {10}^{-2}$, ${\mathcal{R}}_{\pi /K}\equiv [\mathcal{B}(B^{+}\to J/\psi {\pi }^{+})/\mathcal{B}(B^{+}\to J/\psi K^{+})]=(3.852\pm 0.022\pm 0.018)\times {10}^{-2}$, where the first uncertainties are statistical and the second are systematic. A combination with previous LHCb results based on data collected at 7 and 8 TeV in 2011 and 2012 yields $\mathrm{\Delta }{\mathcal{A}}^{CP}=(1.42\pm 0.43\pm 0.08)\times {10}^{-2}$ and ${\mathcal{R}}_{\pi /K}=(3.846\pm 0.018\pm 0.018)\times {10}^{-2}$. The combined $\mathrm{\Delta }{\mathcal{A}}^{CP}$ value deviates from zero by 3.2 standard deviations, providing the first evidence for direct $CP$ violation in the amplitudes of beauty decays to charmonium final states.

A study of ${\mathrm{\Lambda }}_b^0$ and ${\mathrm{\Xi }}_b^0$ decays to $\mathrm{\Lambda }{h}^{+}{h}^{\prime -}$ ($h^{(\prime )}=\pi ,K$) is performed using $pp$ collision data collected by the LHCb experiment during LHC Runs 1–2, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. The branching fractions for these decays are measured using the ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}(\to \mathrm{\Lambda }{\pi }^{+}){\pi }^{-}$ decay as a control channel. The decays ${\mathrm{\Lambda }}_b^0\to \mathrm{\Lambda }{\pi }^{+}{\pi }^{-}$ and ${\mathrm{\Xi }}_b^0\to \mathrm{\Lambda }{K}^{-}{\pi }^{+}$ are observed for the first time. For decay modes with sufficient signal yields, $CP$ asymmetries are measured in the full and localized regions of the final-state phase space. Evidence is found for $CP$ violation in the ${\mathrm{\Lambda }}_b^0\to \mathrm{\Lambda }{K}^{+}{K}^{-}$ decay, interpreted as originating primarily from an asymmetric ${\mathrm{\Lambda }}_b^0\to N^{*+}{K}^{-}$ decay amplitude. The measured $CP$ asymmetries for other decays are compatible with zero.

An amplitude analysis of $B^{-}\to D^{-}{D}^0{K}_S^0$ decays is performed using proton-proton collision data, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$, collected with the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV. A resonant structure of spin-parity $0^{+}$ is observed in the $D^0{K}_S^0$ invariant-mass spectrum with a significance of $5.3\sigma$. The mass and width of the state, modeled with a Breit-Wigner line shape, are determined to be $2883\pm 11\pm 8\mathrm{MeV}/c^2$ and ${87}_{-47}^{+22}\pm 17\mathrm{MeV}$, respectively, where the first uncertainties are statistical and the second systematic. These properties and the quark content are consistent with those of the open-charm tetraquark candidate $T_{cs0}^{*}(2870{)}^0$ observed previously in the $D^{+}{K}^{-}$ final state of the $B^{-}\to D^{-}{D}^{+}{K}^{-}$ decay. This result confirms the existence of the $T_{cs0}^{*}(2870{)}^0$ state in a new decay mode. The $T_{cs1}^{*}(2900{)}^0$ state, reported in the $B^{-}\to D^{-}{D}^{+}{K}^{-}$ decay, is also searched for in the $D^0{K}_S^0$ invariant-mass spectrum of the $B^{-}\to D^{-}{D}^0{K}_S^0$ decay, without finding evidence for it.

In recent years, twisting has emerged as a new degree of freedom that plays an increasingly important role in Bloch bands of various physical systems. However, there is currently a lack of reports on the nontrivial physics of topological degeneracy in twisted systems. In this Letter, we investigated the intrinsic physical correlation between twisting and topological degeneracy. We found that twisting not only breaks the symmetry of the system but also introduces topological degeneracy that does not exist under the original symmetric system without twisting. Furthermore, the topological degeneracy can be easily tuned through twisting. This new twist-induced topological degeneracy gives rise to a unique polarization-degenerate birefringent medium, wherein the twist angle acts as a novel degree of freedom for dispersion and polarization management of interface states. Exhibiting fascinating properties and experimental feasibilities, our Letter points to new possibilities in the research of various topological physics in twisted photonics.

The excess free energy functional of classical density functional theory depends upon the type of fluid model, specifically on the choice of (pair) potential. This functional is unknown in general and is approximated reliably only in special cases. We present a machine learning scheme for training a neural network that acts as a generic metadensity functional for truncated but otherwise arbitrary pair potentials. Automatic differentiation and neural functional calculus then yield, for one-dimensional fluids, accurate predictions for inhomogeneous states and immediate access to the pair distribution function. The approach provides a means of addressing a fundamental problem in the physics of liquids and for soft matter design: “How do we best invert structural data to obtain the pair potential?”

We study dynamical scaling associated with a Kondo-breakdown quantum-critical point (KB QCP) of the periodic Anderson model, treated by two-site cellular dynamical mean-field theory (2CDMFT). In the quantum-critical region, the dynamical staggered-spin susceptibility exhibits $\omega /T$ scaling. We propose a scaling ansatz that describes this behavior and reveals Planckian dissipation for the longest-lived excitations. The current susceptibility follows the same scaling, leading to strange-metal behavior for the optical conductivity and resistivity. Importantly, this behavior is driven by strong short-ranged vertex contributions, not single-particle decay. This suggests that the KB QCP described by 2CDMFT is a novel intrinsic (i.e., disorder-free) strange-metal fixed point. Our results for the optical conductivity match experimental observations on ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$ and ${\mathrm{CeCoIn}}_5$.

The rise of moiré materials has led to experimental realizations of integer and fractional Chern insulators in small or vanishing magnetic fields. At the same time, a set of minimal conditions sufficient to guarantee an Abelian fractional state in a flat band were identified, namely, “ideal” or “vortexable” quantum geometry. Such vortexable bands share essential features with the lowest Landau level (LL), while excluding the need for more fine-tuned aspects such as flat Berry curvature. A natural and important generalization is to ask if such conditions can be extended to capture the quantum geometry of higher Landau levels, particularly the first LL (1LL), where non-Abelian states at $\nu =1/2,2/5$ are known to be competitive. The possibility of realizing these states at zero magnetic field, and perhaps even more exotic ones, could become a reality if we could identify the essential structure of the 1LL in Chern bands. In this work, we introduce a precise definition of 1LL quantum geometry, along with a figure of merit that measures how closely a given band approaches the 1LL. Periodically strained Bernal graphene is shown to realize such a 1LL structure even in zero magnetic field.

The Mpemba effect is a counterintuitive physical phenomenon where a hot system cools faster than a warm one. In recent years, theoretical analyses of the Mpemba effect have been developed for microscopic systems and experimentally verified. However, the conventional theory relies on a specific choice of distance measure to quantify relaxation speed, leading to several theoretical ambiguities. In this Letter, we derive a rigorous quantification of the Mpemba effect based on thermomajorization theory, referred to as the thermomajorization Mpemba effect. This approach resolves all existing ambiguities and provides a unification of the conventional Mpemba effect across all monotone measures. Furthermore, we demonstrate the generality of the thermomajorization Mpemba effect for Markovian dynamics, rigorously proving that it can occur in any temperature regime with fixed energy levels.

The DeepCore subdetector of the IceCube Neutrino Observatory provides access to neutrinos with energies above approximately 5 GeV. Data taken between 2012 and 2021 (3387 days) are utilized for an atmospheric ${\nu }_{\mu }$ disappearance analysis that studied 150 257 neutrino-candidate events with reconstructed energies between 5 and 100 GeV. An advanced reconstruction based on a convolutional neural network is applied, providing increased signal efficiency and background suppression, resulting in a measurement with both significantly increased statistics compared to previous DeepCore oscillation results and high neutrino purity. For the normal neutrino mass ordering, the atmospheric neutrino oscillation parameters and their $1\sigma$ errors are measured to be $\mathrm{\Delta }{\mathrm{m}}_{32}^2={2.40}_{-0.04}^{+0.05}\times {10}^{-3}{\mathrm{eV}}^2$ and ${\sin }^2{\theta }_{23}={0.54}_{-0.03}^{+0.04}$. The results are the most precise to date using atmospheric neutrinos, and are compatible with measurements from other neutrino detectors including long-baseline accelerator experiments.

We report on the formation of multiple quantum droplets in a heteronuclear ${}^{41}\mathrm{K}\text{−}{}^{87}\mathrm{Rb}$ mixture released in an optical waveguide. By a sudden change of the interspecies interaction from the noninteracting to the strongly attractive regime, we initially form a single droplet in an excited compression-elongation mode. The latter axially expands up to a critical length and then splits into two or more smaller fragments, recognizable as quantum droplets. We find that the number of formed droplets increases with decreasing interspecies attraction and increasing atom number. We show, by combining theory and experiment, that this behavior is consistent with capillary instability, which causes the breakup of the stretching droplet due to the surface tension. Our results open new possibilities to explore the properties of quantum liquids and systems of multiple quantum droplets in two-component bosonic mixtures.

Luminescent conjugated polymers are organic semiconductors possessing unique electrical and optical properties. Luminescent polymers have been extensively investigated because of their potential applications in light sources and display devices. Here, we provide the first evidence that a luminescent polymer is also a strong emitter of free electrons and a source of new light. Vivid green light flashes have been observed from a red-emitting polymer $p\text{−}n$ junction under a large reverse bias. The $p\text{−}n$ junctions have a planar configuration, revealing that the green light flashes are emitted into free space, far beyond the confines of electrodes. Moreover, the green flashes can be strongly distorted by a permanent magnet placed nearby, generating a light show resembling an aurora. Like the aurora, the green light flashes are caused by charged particles. By applying a known transverse magnetic field to bend the flashes into circular arcs, a charge to mass ratio is determined for the charged particles. The auroralike green light coincides with the formation of electrical trees in the $n$-doped region of the junction. We postulate that the green light is caused by field-emitted electrons exciting an unknown organic vapor released in the treeing process.

We report Raman forbidden layer-breathing modes (LBMs) in layered semiconductor materials (LSMs). The intensity distribution of all observed LBMs depends on layer number, incident light wavelength, and refractive index mismatch between LSM and underlying substrate. These results are understood by a Raman scattering theory via the proposed spatial interference model, where the naturally occurring optical and phonon cavities in LSMs enable spatially coherent photon-phonon coupling mediated by the corresponding one-dimensional periodic electronic states. Our work reveals the impact of spatial coherence of photon and phonon fields on phonon excitation via photon or phonon cavity engineering.