We performed a search for the $K_L\to {\pi }^0\nu \overline{\nu }$ decay using the data taken in 2021 at the J-PARC KOTO experiment. With newly installed counters and new analysis method, the expected background was suppressed to $0.252\pm {0.055}_{\mathrm{stat}}{}_{-{0.067}^{\mathrm{syst}}}^{+0.052}$. With a single event sensitivity of $(9.33\pm 0.06_{\mathrm{stat}}\pm 0.84_{\mathrm{syst}})\times {10}^{-10}$, no events were observed in the signal region. An upper limit on the branching fraction for the decay was set to be $2.2\times {10}^{-9}$ at the 90% confidence level (C.L.), which improved the previous upper limit from KOTO by a factor of 1.4. With the same data, a search for $K_L\to {\pi }^0{X}^0$ was also performed, where $X^0$ is an invisible boson with a mass ranging from 1 to $260\mathrm{MeV}/c^2$. For $X^0$ with a mass of $135\mathrm{MeV}/c^2$, an upper limit on the branching fraction of $K_L\to {\pi }^0{X}^0$ was set to be $1.6\times {10}^{-9}$ at the 90% C.L.

We present a consistent derivation of the complete Wess-Zumino-Witten interactions of axions, including the counter-term necessary to guarantee the gauge invariance of the standard model. By treating the derivative of the axion field as a background gauge field and incorporating auxiliary chiral rotation phases, we ensure consistency in the axion-interaction Lagrangian. This approach allows us to derive basis-independent physical interactions of axions with gauge bosons and vector mesons. These interactions have broad applications in axion phenomenology for both light and heavy axion particles.

The System for Measuring Overlap with Gas (SMOG2) at the LHCb detector enables the study of fixed-target ion-ion collisions at relativistic energies ($\sqrt{s_{\mathrm{NN}}}\sim 100\mathrm{GeV}$ in the center of mass). With input from ab initio calculations of the structure of ${}^{16}\mathrm{O}$ and ${}^{20}\mathrm{Ne}$, we compute $3+1\mathrm{D}$ hydrodynamic predictions for the anisotropic flow of $\mathrm{Pb}+\mathrm{Ne}$ and $\mathrm{Pb}+\mathrm{O}$ collisions to be tested with upcoming LHCb data. This will allow the detailed study of quark-gluon plasma formation as well as experimental tests of the predicted nuclear shapes. Elliptic flow ($v_2$) in $\mathrm{Pb}+\mathrm{Ne}$ collisions is greatly enhanced compared to the $\mathrm{Pb}+\mathrm{O}$ baseline due to the shape of ${}^{20}\mathrm{Ne}$, which is deformed in a bowling-pin geometry. Owing to the large ${}^{208}\mathrm{Pb}$ radius, this effect is seen in a broad centrality range, a unique feature of this collision configuration. Larger elliptic flow further enhances the quadrangular flow ($v_4$) of $\mathrm{Pb}+\mathrm{Ne}$ collisions via nonlinear coupling, and impacts the sign of the kurtosis of the elliptic flow vector distribution ($c_2\{4\}$). Exploiting the shape of ${}^{20}\mathrm{Ne}$ proves thus an ideal method to investigate the formation of quark-gluon plasma in fixed-target experiments at LHCb, and demonstrates the power of System for Measuring Overlap with Gas as a tool to image nuclear ground states.

Generalized thermodynamic relations are introduced into the framework of a relativistic perfect spin hydrodynamics. They allow for consistent treatment of spin degrees of freedom, including the use of spin tensors whose structure follows from microscopic calculations. The obtained results are important for establishing consistency between different formulations of spin hydrodynamics and form the basis for introducing dissipative corrections.

Superfluorescence, a coherent burst of light from an excited ensemble of emitters, is a crucial quantum optical phenomenon with far-reaching implications in nanophotonics and many-body optical processes. Despite its observation in various systems, realizing superfluorescence in an electron-hole plasma (EHP) at room temperature has remained a formidable challenge, hindering the development of continuous-wave and electrically excited superfluorescence devices. Herein, we address this challenge by condensing the high-density EHP into an electron-hole liquid (EHL) at room temperature, thereby preserving quantum coherence. Using a model system of nanocrystal thin films, we demonstrate the first experimental observation of room temperature superfluorescence from an EHL. Key attributes heralding superfluorescence include a redshift of $\sim 94\mathrm{meV}$ from uncorrelated exciton emission, a fluence-dependent delayed growth of macroscopic coherence with abrupt radiative decay $\sim 1250$ times faster than spontaneous emission, a distinct quadratic fluence dependence with a clear threshold, and Burnham-Chiao ringing. These findings open up exciting possibilities for developing electrically pumped colloidal nanocrystals lasers and quantum technologies operating at room temperature.

We report on the extension of the q-plate concept, a hallmark of spin-orbit optical vortex generation since its introduction in 2006, to the generation of optical skyrmions. Stokes skyrmions of arbitrary order with polarization-controlled Skyrme number and reconfigurable multiskyrmions are obtained. This is done by endowing q-plates with a winding number associated with the radial degree of freedom that adds to the usual one associated with the azimuthal degree of freedom.

The moiré superlattices attract growing interest for holding exotic physics due to their fascinating properties from electronics to photonics. Much attention has been focused on the localization effect for waves in the flat band regime or the delocalization effect from the strongly dispersive band feature. Here, we study the weakly dispersive band in between the two above scenarios in a one-dimensional synthetic frequency moiré superlattice and observe the wave packet distributions therein toward novel frequency comb generation. Mode spacing in the spectral wave packet is reduced compared to the free spectral range of individual rings due to the mode couplings from the unequal sublattice periods of the synthetic moiré lattice. We unveil that the optimal compact frequency comb generation occurs in the weakly dispersive regime holding simultaneously uniform power distribution and broad frequency spanning in our experiment, benefiting from the interplay between the band flatness and power uniformity of mode distribution. Our results study the fundamental physics of the weakly dispersive moiré band in the synthetic frequency dimension and also show a new way for the future compact frequency comb generation in on-chip devices with small footprint size.

Nonlinear photogalvanic effects in two-dimensional materials, particularly the nonlinear circular photocurrents (NCPs) that belong to the helicity-dependent spin photocurrents, have sparked enormous research interest. Although notable progress has been witnessed, the underling origin of NCPs remains elusive. Here, we present systematic photocurrent characteristics, symmetry analysis and theoretical calculations to uncover the physical origin of NCPs in ${\mathrm{MoS}}_2$, a prototypical 2D semiconductor. Our results show that the NCP responses in 2D semiconductor ${\mathrm{MoS}}_2$ result from the circular photon drag effect (CPDE), rather than the generally believed circular photogalvanic effect. Furthermore, we demonstrate that the NCPs are highly tunable with electrostatic doping and increase progressively with ${\mathrm{MoS}}_2$ thickness, evidencing the interlayer constructive nature of CPDE responses. Our Letter unravels the critical role of the previously overlooked CPDE contribution to NCPs, revolutionizing previous understanding and thus providing deep insights into further fundamental studies and technological advances in nonlinear photovoltaic and opto-spintronic devices.

Intrinsic breakdown strength ($F_{\mathrm{bd}}$), as the theoretical upper limit of electric field strength that a material can sustain, plays important roles in determining dielectric and safety performance. The well accepted concept is that a larger band gap ($E_{\mathrm{g}}$) often leads to a larger intrinsic breakdown strength. In this work, we analytically derive a simplified model of $F_{\mathrm{bd}}$, showing a linear relationship between $F_{\mathrm{bd}}$ and the maximum electron density of states (${\mathrm{DOS}}_{\max }$) within the energy range spanning from the conduction band minimum (CBM) to $\mathrm{CBM}+E_{\mathrm{g}}$. Using the Wannier interpolation technique to reduce the cost of calculating the $F_{\mathrm{bd}}$ for various three- and two-dimensional materials, we find that the calculated $F_{\mathrm{bd}}$ did not show any simple relationship with band gap, but it behaves linearly with the ${\mathrm{DOS}}_{\max }$, consistent with our theoretical derivation. Our work shows that the ${\mathrm{DOS}}_{\max }$ is more fundamental than the band gap value in determining the $F_{\mathrm{bd}}$, thus providing useful physical insights into the intrinsic dielectric breakdown strength and opening directions for improving high-power devices. The dimensional effects on $F_{\mathrm{bd}}$ has also been revealed that monolayers tend to have larger $F_{\mathrm{bd}}$ due to reduced screening effects.

${\mathrm{Bi}}_{0.5}{\mathrm{Sb}}_{1.5}{\mathrm{Te}}_3$ is the most widely used $p$-type commercial thermoelectric materials over six decades, yet its complex electronic structure remains uncertain especially in band degeneracy and $k_{\mathrm{z}}$ dispersions. Here we show an unexpected band structure of 18-fold overlapped feathery Fermi pockets through substantial angle-resolved photoelectron spectroscopy data. Complemented with transport tests, we suggest that the high performance originates in the cooperation of four electronic features—momentum overlap of Fermi pockets, 18-fold band degeneracy, ultrasharp $k_{\mathrm{y}}$ dispersions, and heavy $k_{\mathrm{z}}$ bands. This cooperation of band features proposes a new paradigm for promising thermoelectrics.

The lowest Landau level of bilayer graphene has an octet of internal degrees of freedom, composed from spin, valley, and orbital two-level systems. Dominance of $n=0$ orbitals over $n=1$ orbitals in low energy quantum fluctuations leads to distinct fractional quantum Hall characteristics compared dominance of $n=1$ over $n=0$. The competition between $n=0$ and $n=1$ orbitals depends sensitively on particle-hole asymmetry in the single-particle Hamiltonian and on Lamb shifts due to exchange interactions with the negative energy sea, which must be accounted for simultaneously in assessing the orbital competition. We identify the circumstances under which $n=1$, which supports strong even-denominator fractional quantum Hall states with non-Abelian quasiparticles, emerges robustly as the low-energy Landau level.

Simulated cooling is a robust method for preparing low-energy states of many-body Hamiltonians on near-term quantum simulators. In such schemes, a subset of the simulator’s spins (or qubits) are treated as a “bath” that extracts energy and entropy from the system of interest. However, such protocols are inefficient when applied to systems whose excitations are highly nonlocal in terms of the microscopic degrees of freedom, such as topological phases of matter; such excitations are difficult to extract by a local coupling to a bath. We explore a route to overcome this obstacle by encoding the microscopic degrees of freedom into those of the quantum simulator in a nonlocal manner. To illustrate the approach, we show how to efficiently cool the ferromagnetic phase of the quantum Ising model, whose excitations are domain walls, via a “gauged cooling” protocol in which the Ising spins are coupled to a $Z_2$ gauge field that simultaneously acts as a reservoir for removing excitations. We show that our protocol can prepare the ground states of the ferromagnetic and paramagnetic phases equally efficiently. The gauged cooling protocol naturally extends to (interacting) fermionic systems, where it is equivalent to cooling by coupling to a fermionic bath via single-fermion hopping.

A possible spintronic route to hardware implementation for decision-making involves injecting a domain wall into a bifurcated magnetic nanostrip resembling a Y-shaped junction. A decision is made when the domain wall chooses a particular path through the bifurcation. Recently, it was shown that a structure like a nanomagnetic Galton board, which is essentially an array of interconnected Y-shaped junctions, produces outcomes that are stochastic and therefore relevant to artificial neural networks. However, the exact mechanism leading to the robust nature of randomness is unknown. Here, we directly image the decision-making process in nanomagnetic Galton boards using Lorentz transmission electron microscopy. We identify that the stochasticity in nanomagnetic Galton boards arises as a culmination of (1) the topology of the injected domain wall, (2) dissimilarly sized vertices, and (3) the strength of the applied field. Our results pave the way to a detailed understanding of stochasticity in nanomagnetic networks.

We measure the nonlinear magnetic susceptibility ${\chi }^{(3)}$ of the disordered quantum Ising magnet ${\mathrm{LiHo}}_{0.045}{\mathrm{Y}}_{0.955}{\mathrm{F}}_4$ and demonstrate four-wave mixing due to coherent (anti-)Stokes Raman scattering at $\sim 100\mathrm{Hz}$ (peV) energy scales. The temperature dependence of ${\chi }^{(3)}$ approximately follows a $(1/T)$ form, with a high-$T$ cutoff that can be linked to dissipation in the coherent spin clusters. ${\chi }^{(3)}$ also decreases monotonically with a transverse field, approaching a constant offset above a few kOe, suggesting the presence of both coherent and spontaneous Raman scattering.

Celebrated fluctuation-dissipation theorem (FDT) linking the response function to time dependent correlations of observables measured in the reference unperturbed state is one of the central results in equilibrium statistical mechanics. In this Letter we discuss an extension of the standard FDT to the case when multidimensional matrix representing transition probabilities is strictly non-normal. This feature dramatically modifies the dynamics, by incorporating the effect of eigenvector nonorthogonality via the associated overlap matrix of Chalker-Mehlig type. In particular, the rate of entropy production per unit time is strongly enhanced by that matrix. We suggest, that this mechanism has an impact on the studies of collective phenomena in neural matrix models, leading, via transient behavior, to such phenomena as synchronization and emergence of the memory. We also expect, that the described mechanism generating the entropy production is generic for wide class of phenomena, where dynamics is driven by non-normal operators. For the case of driving by a large Ginibre matrix the entropy production rate is evaluated analytically, as well as for the Rajan-Abbott model for neural networks.

Rigorous understanding of assembly in colloidal systems is crucial to the development of tailored nanostructured materials. Despite extensive studies, a mechanistic understanding of the dynamics governing encounters of colloidal particles remains an ongoing challenge. We study colloidal encounter dynamics by inducing assembly through optical tweezers that impose an external attractive field for cubic-phase sodium yttrium fluoride nanocrystals. We show that surface roughness of the nanocrystals is a decisive factor for contact leading to assembly between the nanocrystals, manifested by the roughness-dependent hydrodynamic resistivity. This provides direct evidence that dynamics are equally important to energetics in understanding assembly.

The field of active nematics has traditionally employed descriptions based on dipolar activity. However, it is theoretically predicted that interactions with a substrate, prevalent in most biological systems, lead to novel forms of activity, such as quadrupolar activity, that are governed by hydrodynamic screening. Here, combining experiments and numerical simulations, we show that upon light-induced solidification of the underlying medium, microtubule-kinesin mixtures undergo a transformation that leads to a biphasic active suspension. Using an active lyotropic model, we prove that the transition is governed by screening effects that alter the dominant form of active stress. Specifically, the combined effect of friction and quadrupolar activity leads to a hierarchical folding that follows the intrinsic bend instability of the active nematic layer. Our results demonstrate the dynamics of the collapse of orientational order in active nematics and present a new route for controlling active matter by modifying the activity through changing the surrounding environment.

Extracting the rotational energy from a Kerr black hole (BH) is one of the crucial topics in relativistic astrophysics. Here, we give special attention to the Penrose ballistic process based on the fission of a massive particle ${\mu }_0$ into two particles ${\mu }_1$ and ${\mu }_2$, occurring in the ergosphere of a Kerr BH. Bardeen et al. indicated that for the process to occur, some additional “hydrodynamical forces or superstrong radiation reactions” were needed. Wald and Chandrasekhar further expanded this idea. This animosity convinced Piran and collaborators to move from a simple three-body system characterizing the original Penrose process to a many-body system. This many-body approach was further largely expanded by others, some questionable in their validity. Here, we return to the simplest original Penrose process and show that the solution of the equations of motion, imposing the turning point condition on their trajectories, leads to the rotational energy extraction from the BH expected by Penrose. The efficiency of energy extraction by a single process is quantified for three different single decay processes occurring, respectively, at $r=1.2M$, $r=1.5M$, and $r=1.9M$. An interesting repetitive model has been proposed by Misner et al. [Gravitation (W. H. Freeman, San Francisco, 1973)]. Indeed, it would appear that a repetitive sequence of 246 decays of the above injection process at $r=1.2M$ and the corresponding ones at $r=1.5M$ and $r=1.9M$ could extract 100% of the rotational energy of the BH, so violating energy conservation. The accompanying article, accounting for the existence of the BH irreducible mass, introduces a nonlinear approach that avoids violating energy conservation and leads to a new energy extraction process.