We report an experimental investigation of the two-dimensional $J_{\text{eff}}=1/2$ antiferromagnetic Mott insulator by varying the interlayer exchange coupling in [${({\mathrm{SrIrO}}_3)}_1$, ${({\mathrm{SrTiO}}_3)}_m$] ($m=1$, 2 and 3) superlattices. Although all samples exhibited an insulating ground state with long-range magnetic order, temperature-dependent resistivity measurements showed a stronger insulating behavior in the $m=2$ and $m=3$ samples than the $m=1$ sample which displayed a clear kink at the magnetic transition. This difference indicates that the blocking effect of the excessive ${\mathrm{SrTiO}}_3$ layer enhances the effective electron-electron correlation and strengthens the Mott phase. The significant reduction of the Néel temperature from 150 K for $m=1$ to 40 K for $m=2$ demonstrates that the long-range order stability in the former is boosted by a substantial interlayer exchange coupling. Resonant x-ray magnetic scattering revealed that the interlayer exchange coupling has a switchable sign, depending on the ${\mathrm{SrTiO}}_3$ layer number $m$, for maintaining canting-induced weak ferromagnetism. The nearly unaltered transition temperature between the $m=2$ and the $m=3$ demonstrated that we have realized a two-dimensional antiferromagnet at finite temperatures with diminishing interlayer exchange coupling.

Monolayer transition metal dichalcogenides (TMD) with confined 2D Wannier-Mott excitons are intriguing for the fundamental study of strong light-matter interactions and the exploration of exciton polaritons at high temperatures. However, the research of 2D exciton polaritons has been hindered because the polaritons in these atomically thin semiconductors discovered so far can hardly support strong nonlinear interactions and quantum coherence due to uncontrollable polariton dynamics and weakened coherent coupling. In this work, we demonstrate, for the first time, a precisely controlled hybrid composition with angular dependence and dispersion-correlated polariton emission by tuning the polariton dispersion in TMD over a broad temperature range of 110–230 K in a single cavity. This tamed polariton emission is achieved by the realization of robust coherent exciton-photon coupling in monolayer tungsten disulphide (${\mathrm{WS}}_2$) with large splitting-to-linewidth ratios ($>3.3$). The unprecedented ability to manipulate the dispersion and correlated properties of TMD exciton polaritons at will offers new possibilities to explore important quantum phenomena such as inversionless lasing, Bose-Einstein condensation, and superfluidity.

In the half filled zero-energy Landau level of bilayer graphene, competing phases with spontaneously broken symmetries and an intriguing quantum critical behavior have been predicted. Here we investigate signatures of these broken-symmetry phases in thermal transport measurements. To this end, we calculate the spectrum of spin and valley waves in the $\nu =0$ quantum Hall state of bilayer graphene. The presence of Goldstone modes enables heat transport even at low temperatures, which can serve as compelling evidence for spontaneous symmetry breaking. By varying external electric and magnetic fields, it is possible to determine the nature of the symmetry breaking. Temperature-dependent measurements may yield additional information about gapped modes.

A leading candidate for the experimental confirmation of the nonlocal quantum dynamics of Majorana fermions is the topological Kondo effect, predicted for mesoscopic superconducting islands connected to metallic leads. We identify an anisotropic, Toulouse-like, limit of the topological Kondo problem where the full nonequilibrium conductance and shot noise can be calculated exactly. Near the Kondo fixed point, we find novel asymptotic features including a universal conductance scaling function and fractional charge quantization observable via the Fano factor. In the universal regime, our results apply for generic anisotropy and even away from the Kondo limit as long as the system supports an emergent topological Kondo fixed point. Our approach thus provides key new qualitative insights and exact expressions for quantitative comparisons to future experimental data.

We study theoretically the edge fracture instability in sheared complex fluids, by means of linear stability analysis and direct nonlinear simulations. We derive an exact analytical expression for the onset of edge fracture in terms of the shear-rate derivative of the fluid’s second normal stress difference, the shear-rate derivative of the shear stress, the jump in shear stress across the interface between the fluid and the outside medium (usually air), the surface tension of that interface, and the rheometer gap size. We provide a full mechanistic understanding of the edge fracture instability, carefully validated against our simulations. These findings, which are robust with respect to choice of rheological constitutive model, also suggest a possible route to mitigating edge fracture, potentially allowing experimentalists to achieve and accurately measure flows stronger than hitherto possible.

Coherent, large-scale dynamics in many nonequilibrium physical, biological, or information transport networks are driven by small-scale local energy input. Here, we introduce and explore an analytically tractable nonlinear model for compressible active flow networks. In contrast to thermally driven systems, we find that active friction selects discrete states with a limited number of oscillation modes activated at distinct fixed amplitudes. Using perturbation theory, we systematically predict the stationary states of noisy networks and find good agreement with a Bayesian state estimation based on a hidden Markov model applied to simulated time series data. Our results suggest that the macroscopic response of active network structures, from actomyosin force networks to cytoplasmic flows, can be dominated by a significantly reduced number of modes, in contrast to energy equipartition in thermal equilibrium. The model is also well suited to study topological sound modes and spectral band gaps in active matter.

Boolean network models describe genetic, neural, and social dynamics in complex networks, where the dynamics depend generally on network topology. Fixed points in a genetic regulatory network are typically considered to correspond to cell types in an organism. We prove that the expected number of fixed points in a Boolean network, with Boolean functions drawn from probability distributions that are not required to be uniform or identical, is one, and is independent of network topology if only a feedback arc set satisfies a stochastic neutrality condition. We also demonstrate that the expected number is increased by the predominance of positive feedback in a cycle.

Utilizing the Fermi measurement of the $\gamma$-ray spectrum toward the Inner Galaxy, we derive some of the strongest constraints to date on the dark matter (DM) lifetime in the mass range from hundreds of MeV to above an EeV. Our profile-likelihood-based analysis relies on 413 weeks of Fermi Pass 8 data from 200 MeV to 2 TeV, along with up-to-date models for diffuse $\gamma$-ray emission within the Milky Way. We model Galactic and extragalactic DM decay and include contributions to the DM-induced $\gamma$-ray flux resulting from both primary emission and inverse-Compton scattering of primary electrons and positrons. For the extragalactic flux, we also calculate the spectrum associated with cascades of high-energy $\gamma$ rays scattering off of the cosmic background radiation. We argue that a decaying DM interpretation for the 10 TeV–1 PeV neutrino flux observed by IceCube is disfavored by our constraints. Our results also challenge a decaying DM explanation of the AMS-02 positron flux. We interpret the results in terms of individual final states and in the context of simplified scenarios such as a hidden-sector glueball model.

MeV dark matter (DM) particles annihilating or decaying to electron-positron pairs cannot, in principle, be observed via local cosmic-ray (CR) measurements because of the shielding solar magnetic field. In this Letter, we take advantage of spacecraft Voyager 1’s capacity for detecting interstellar CRs since it crossed the heliopause in 2012. This opens up a new avenue to probe DM in the sub-GeV energy/mass range that we exploit here for the first time. From a complete description of the transport of electrons and positrons at low energy, we derive predictions for both the secondary astrophysical background and the pair production mechanisms relevant to DM annihilation or decay down to the MeV mass range. Interestingly, we show that reacceleration may push positrons up to energies larger than the DM particle mass. We combine the constraints from the Voyager and AMS-02 data to get novel limits covering a very extended DM particle mass range, from MeV to TeV. In the MeV mass range, our limits reach annihilation cross sections of order $⟨\sigma v⟩\sim {10}^{-28}{\mathrm{cm}}^3/\mathrm{s}$. An interesting aspect is that these limits barely depend on the details of cosmic-ray propagation in the weak reacceleration case, a configuration which seems to be favored by the most recent $B/C$ data. Though extracted from a completely different and new probe, these bounds have a strength similar to those obtained with the cosmic microwave background—they are even more stringent for $p$-wave annihilation.

The helicity-dependent recoil proton polarizations $P_x^{\prime }$ and $P_z^{\prime }$ as well as the helicity-independent component $P_y$ have been measured in the $p(\overrightarrow{e},e^{\prime }\overrightarrow{p}){\pi }^0$ reaction at four-momentum transfer $Q^2\simeq 0.1{\mathrm{GeV}}^2$, center-of-mass proton emission angle ${\theta }_p^{*}\simeq 90°$, and invariant mass $W\simeq 1440\mathrm{MeV}$. This first precise measurement of double-polarization observables in the energy domain of the Roper resonance $P_{11}(1440)$ by exploiting recoil polarimetry has allowed for the extraction of its scalar electroexcitation amplitude at an unprecedentedly low value of $Q^2$, establishing a powerful instrument for probing the interplay of quark and meson degrees of freedom in the nucleon.

A strategy for generating entanglement between two separated optomechanical oscillators is analyzed, using entangled radiation produced from down-conversion and stored in an initiating cavity. We show that the use of pulsed entanglement with optimally shaped temporal modes can efficiently transfer quantum entanglement into a mechanical mode, then remove it after a fixed waiting time for measurement. This protocol could provide new avenues for testing for bounds on decoherence in massive systems that are spatially separated, as originally suggested by Furry not long after the discussion by Einstein-Podolsky-Rosen and Schrödinger of entanglement.

A superconducting qubit in the strong dispersive regime of circuit quantum electrodynamics is a powerful probe for microwave photons in a cavity mode. In this regime, a qubit excitation spectrum is split into multiple peaks, with each peak corresponding to an individual photon number in the cavity (discrete ac Stark shift). Here, we measure the qubit spectrum in a cavity that is driven continuously with a squeezed vacuum generated by a Josephson parametric amplifier. By fitting the obtained spectrum with a model which takes into account the finite qubit excitation power, we determine the photon number distribution, which reveals an even-odd photon number oscillation and quantitatively fulfills Klyshko’s criterion for nonclassicality.

We report on the first experimental observations of quasichanneling oscillations, recently seen in simulations and described theoretically. Although above-barrier particles penetrating a single crystal are generally seen as behaving almost as in an amorphous substance, distinct oscillation peaks nevertheless appear for particles in that category. The quasichanneling oscillations were observed at SLAC National Accelerator Laboratory by aiming 20.35 GeV positrons and electrons at a thin silicon crystal bent to a radius of $R=0.15\mathrm{m}$, exploiting the quasimosaic effect. For electrons, two relatively faint quasichanneling peaks were observed, while for positrons, seven quasichanneling peaks were clearly identified.

We present the first laboratory generation of high-Mach-number magnetized collisionless shocks created through the interaction of an expanding laser-driven plasma with a magnetized ambient plasma. Time-resolved, two-dimensional imaging of plasma density and magnetic fields shows the formation and evolution of a supercritical shock propagating at magnetosonic Mach number $M_{\mathrm{ms}}\approx 12$. Particle-in-cell simulations constrained by experimental data further detail the shock formation and separate dynamics of the multi-ion-species ambient plasma. The results show that the shocks form on time scales as fast as one gyroperiod, aided by the efficient coupling of energy, and the generation of a magnetic barrier between the piston and ambient ions. The development of this experimental platform complements present remote sensing and spacecraft observations, and opens the way for controlled laboratory investigations of high-Mach number collisionless shocks, including the mechanisms and efficiency of particle acceleration.

During a magnetopause crossing the Magnetospheric Multiscale spacecraft encountered an electron diffusion region (EDR) of asymmetric reconnection. The EDR is characterized by agyrotropic beam and crescent electron distributions perpendicular to the magnetic field. Intense upper-hybrid (UH) waves are found at the boundary between the EDR and magnetosheath inflow region. The UH waves are generated by the agyrotropic electron beams. The UH waves are sufficiently large to contribute to electron diffusion and scattering, and are a potential source of radio emission near the EDR. These results provide observational evidence of wave-particle interactions at an EDR, and suggest that waves play an important role in determining the electron dynamics.

A novel type of temporal and spatial self-organization in a heterogeneous catalytic reaction is described for the first time. Using in situ x-ray photoelectron spectroscopy, gas chromatography, and mass spectrometry, we show that, under certain conditions, self-sustained reaction-rate oscillations arise in the oxidation of propane over Ni foil because of reversible bulk oxidation of Ni to NiO, which can be observed even with the naked eye as chemical waves propagating over the catalyst surface.

${\mathrm{SrRu}\mathrm{O}}_3$ heterostructures grown in the (111) direction are a rare example of thin film ferromagnets. By means of density functional theory plus dynamical mean field theory we show that the half-metallic ferromagnetic state with an ordered magnetic moment of $2{\mu }_B/\mathrm{Ru}$ survives the ultimate dimensional confinement down to a bilayer, even at elevated temperatures of 500 K. In the minority channel, the spin-orbit coupling opens a gap at the linear band crossing corresponding to $\frac{3}{4}$ filling of the $t_{2g}$ shell. We predict that the emergent phase is Haldane’s quantum anomalous Hall state with Chern number $C=1$, without an external magnetic field or magnetic impurities.

A recent experiment indicated that a ferromagnetic EuS film in contact with a topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ might show a largely enhanced Curie temperature and perpendicular magnetic anisotropy [F. Katmis et al., Nature (London) 533, 513 (2016).]. Through systematic density functional calculations, we demonstrate that in addition to the factor that ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ has a strong spin orbit coupling, the topological surface states are crucial to make these unusual behaviors robust as they hybridize with EuS states and extend rather far into the magnetic layers. The magnetic moments of Eu atoms are nevertheless not much enhanced, unlike what was reported in the experiment. Our results and model analyses provide useful insights for how these quantities are linked, and pave a way for the control of properties of magnetic films via contact with topological insulators.

A general approach to model vibrational electron energy loss spectra obtained using an electron beam positioned away from the specimen is presented. The energy-loss probability of the fast electron is evaluated using first-principles quantum mechanical calculations (density functional theory) of the dielectric response of the specimen. The validity of the method is assessed using recently measured anhydrous $\beta$-guanine, an important molecular solid used by animals to produce structural colors. The good agreement between theory and experiments lays the basis for a quantitative interpretation of this spectroscopy in complex systems.

Random sequential adsorption (RSA) of particles of a particular shape is used in a large variety of contexts to model particle aggregation and jamming. A key feature of these models is the observed algebraic time dependence of the asymptotic jamming coverage $\sim t^{-\nu }$ as $t\to \infty$. However, the exact value of the exponent $\nu$ is not known apart from the simplest case of the RSA of monodisperse spheres adsorbed on a line (Renyi’s seminal “car parking problem”), where $\nu =1$ can be derived analytically. Empirical simulation studies have conjectured on a case-by-case basis that for general nonspherical particles, $\nu =1/(d+\tilde{d})$, where $d$ denotes the dimension of the domain, and $\tilde{d}$ the number of orientational degrees of freedom of a particle. Here, we solve this long-standing problem analytically for the $d=1$ case—the “Paris car parking problem.” We prove, in particular, that the scaling exponent depends on the particle shape, contrary to the original conjecture and, remarkably, falls into two universality classes: (i) $\nu =1/(1+\tilde{d}/2)$ for shapes with a smooth contact distance, e.g., ellipsoids, and (ii) $\nu =1/(1+\tilde{d})$ for shapes with a singular contact distance, e.g., spherocylinders and polyhedra. The exact solution explains, in particular, why many empirically observed scalings fall in between these two limits.