We study the frictional drag between carriers in two bilayer graphene flakes separated by a 2–5 nm thick hexagonal boron nitride dielectric. At temperatures ($T$) lower than $\sim 10\mathrm{K}$, we observe a large anomalous negative drag emerging predominantly near the drag layer charge neutrality. The anomalous drag resistivity increases dramatically with reducing $T$, and becomes comparable to the layer resistivity at the lowest $T=1.5\mathrm{K}$. At low $T$ the drag resistivity exhibits a breakdown of layer reciprocity. A comparison of the drag resistivity and the drag layer Peltier coefficient suggests a thermoelectric origin of this anomalous drag.

We experimentally investigate and utilize electrothermal feedback in a microwave nanobolometer based on a normal-metal (${\mathrm{Au}}_x{\mathrm{Pd}}_{1-x}$) nanowire with proximity-induced superconductivity. The feedback couples the temperature and the electrical degrees of freedom in the nanowire, which both absorbs the incoming microwave radiation, and transduces the temperature change into a radio-frequency electrical signal. We tune the feedback in situ and access both positive and negative feedback regimes with rich nonlinear dynamics. In particular, strong positive feedback leads to the emergence of two metastable electron temperature states in the millikelvin range. We use these states for efficient threshold detection of coherent 8.4 GHz microwave pulses containing approximately 200 photons on average, corresponding to $1.1\times {10}^{-21}\mathrm{J}\approx 7.0\mathrm{meV}$ of energy.

We examine possible low-temperature phases of a repulsively Rydberg-dressed Fermi gas in a three-dimensional free space. It is shown that the collective density excitations develop a roton minimum, which is softened at a wave vector smaller than the Fermi wave vector when the particle density is above a critical value. The mean field calculation shows that, unlike the insulating density wave states often observed in conventional condensed matters, a self-assembled metallic density wave state emerges at low temperatures. In particular, the density wave state supports a Fermi surface and a body-centered-cubic crystal order at the same time with the estimated critical temperature being about one tenth of the noninteracting Fermi energy. Our results suggest the emergence of a fermionic quantum solid that should be observable in the current experimental setup.

Measurements of the mean refractive index of a spherelike nonpolar fluid, octamethytetracylclosiloxane (OMCTS), confined between mica sheets, demonstrate direct and conclusive experimental evidence of the absence of a first-order liquid-to-solid phase transition in the fluid when confined, which has been suggested to occur from previous experimental and simulation results. The results also show that the density remains constant throughout confinement, and that the fluid is incompressible. This, along with the observation of very large increases (many orders of magnitude) in viscosity during confinement from the literature, demonstrate that the molecular motion is limited by the confining wall and not the molecular packing. In addition, the recently developed refractive index profile correction method, which enables the structural perturbation inherent at a solid-liquid interface and that of a liquid in confinement to be determined independently, was used to show that there was no measurable excess or depleted mass of OMCTS near the mica surface in bulk films or confined films of only two molecular layers.

Active matter systems are driven out of thermal equilibrium by a lack of generalized Stokes-Einstein relation between injection and dissipation of energy at the microscopic scale. We consider such a system of interacting particles, propelled by persistent noises, and show that, at small but finite persistence time, their dynamics still satisfy a time-reversal symmetry. To do so, we compute perturbatively their steady-state measure and show that, for short persistent times, the entropy production rate vanishes. This endows such systems with an effective fluctuation-dissipation theorem akin to that of thermal equilibrium systems. Last, we show how interacting particle systems with viscous drags and correlated noises can be seen as in equilibrium with a viscoelastic bath but driven out of equilibrium by nonconservative forces, hence providing energetic insight into the departure of active systems from equilibrium.

Considerable work has recently been directed toward developing resource theories of quantum coherence. In this Letter, we establish a criterion of physical consistency for any resource theory. This criterion requires that all free operations in a given resource theory be implementable by a unitary evolution and projective measurement that are both free operations in an extended resource theory. We show that all currently proposed basis-dependent theories of coherence fail to satisfy this criterion. We further characterize the physically consistent resource theory of coherence and find its operational power to be quite limited. After relaxing the condition of physical consistency, we introduce the class of dephasing-covariant incoherent operations as a natural generalization of the physically consistent operations. Necessary and sufficient conditions are derived for the convertibility of qubit states using dephasing-covariant operations, and we show that these conditions also hold for other well-known classes of incoherent operations.

Quasiparticle excitations in ${\mathrm{UPd}}_2{\mathrm{Al}}_3$ were studied by means of heat-capacity ($C$) measurements under rotating magnetic fields using a high-quality single crystal. The field dependence shows $C(H)\propto H^{1/2}$-like behavior at low temperatures for both two hexagonal crystal axes, i.e., $H\parallel [0001]$ ($c$ axis) and $H\parallel [11\overline{2}0]$ ($a$ axis), suggesting the presence of nodal quasiparticle excitations from heavy bands. At low temperatures, the polar-angle ($\theta$) dependence of $C$ exhibits a maximum along $H\parallel [0001]$ with a twofold symmetric oscillation below 0.5 T, and an unusual shoulder or hump anomaly has been found around 30°–60° from the $c$ axis in $C(\theta )$ at intermediate fields ($1\lesssim {\mu }_{0}H\lesssim 2\mathrm{T}$). These behaviors in ${\mathrm{UPd}}_2{\mathrm{Al}}_3$ purely come from the superconducting nodal quasiparticle excitations, and can be successfully reproduced by theoretical calculations assuming the gap symmetry with a horizontal linear line node. We demonstrate the whole angle-resolved heat-capacity measurements done here as a novel spectroscopic method for nodal gap determination, which can be applied to other exotic superconductors.

We report on a direct measurement of the pairwise antisymmetric exchange interaction, known as the Dzyaloshinsky-Moriya interaction (DMI), in a ${\mathrm{Nd}}^{3+}$-doped ${\mathrm{YVO}}_4$ crystal. To this end, we introduce a broadband electron spin resonance technique coupled with an optical detection scheme which selectively detects only one ${\mathrm{Nd}}^{3+}\text{−}{\mathrm{Nd}}^{3+}$ pair. Using this technique we can fully measure the spin-spin coupling tensor, allowing us to experimentally determine both the strength and direction of the DMI vector. We believe that this ability to fully determine the interaction Hamiltonian is of interest for studying the numerous magnetic phenomena where the DMI interaction is of fundamental importance, including multiferroics. We also detect a singlet-triplet transition within the pair, with a highly suppressed magnetic-field dependence, which suggests that such systems could form singlet-triplet qubits with long coherence times for quantum information applications.

We report the experimental observation of Snell’s law for magnetostatic spin waves in thin ferromagnetic Permalloy films by imaging incident, refracted, and reflected waves. We use a thickness step as the interface between two media with different dispersion relations. Since the dispersion relation for magnetostatic waves in thin ferromagnetic films is anisotropic, deviations from the isotropic Snell’s law known in optics are observed for incidence angles larger than $25°$ with respect to the interface normal between the two magnetic media. Furthermore, we can show that the thickness step modifies the wavelength and the amplitude of the incident waves. Our findings open up a new way of spin wave steering for magnonic applications.

A major limitation of many heat engines is that their functioning demands on-line control and/or an external fitting between the environmental parameters (e.g., temperatures of thermal baths) and internal parameters of the engine. We study a model for an adaptive heat engine, where—due to feedback from the functional part—the engine’s structure adapts to given thermal baths. Hence, no on-line control and no external fitting are needed. The engine can employ unknown resources; it can also adapt to results of its own functioning that make the bath temperatures closer. We determine resources of adaptation and relate them to the prior information available about the environment.

Light nuclei were created during big-bang nucleosynthesis (BBN). Standard BBN theory, using rates inferred from accelerator-beam data, cannot explain high levels of ${}^6\mathrm{Li}$ in low-metallicity stars. Using high-energy-density plasmas we measure the $\mathrm{T}({}^3\mathrm{He},\gamma ){}^6\mathrm{Li}$ reaction rate, a candidate for anomalously high ${}^6\mathrm{Li}$ production; we find that the rate is too low to explain the observations, and different than values used in common BBN models. This is the first data directly relevant to BBN, and also the first use of laboratory plasmas, at comparable conditions to astrophysical systems, to address a problem in nuclear astrophysics.

Using resonant magnetic x-ray scattering we address the unresolved nature of the magnetic ground state and the low-energy effective Hamiltonian of ${\mathrm{Sm}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, a prototypical pyrochlore iridate with a finite temperature metal-insulator transition. Through a combination of elastic and inelastic measurements, we show that the magnetic ground state is an all-in–all-out (AIAO) antiferromagnet. The magnon dispersion indicates significant electronic correlations and can be well described by a minimal Hamiltonian that includes Heisenberg exchange [$J=27.3(6)\mathrm{meV}$] and Dzyaloshinskii-Moriya interactions [$D=4.9(3)\mathrm{meV}$], which provides a consistent description of the magnetic order and excitations. In establishing that ${\mathrm{Sm}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ has the requisite inversion symmetry preserving AIAO magnetic ground state, our results support the notion that pyrochlore iridates may host correlated Weyl semimetals.

Reversible ferroelectric domain (FD) manipulation with a high spatial resolution is critical for memory storage devices based on thin film ferroelectric materials. FD can be manipulated using techniques that apply heat, mechanical stress, or electric bias. However, these techniques have some drawbacks. Here we propose to use an electron beam with an omnidirectional electric field as a tool for erasable stable ferroelectric nanodomain manipulation. Our results suggest that local accumulation of charges contributes to the local electric field that determines domain configurations.

We report evidence for a narrow structure, $X(5568)$, in the decay sequence $X(5568)\to B_s^0{\pi }^{\pm }$, $B_s^0\to J/\psi \varphi$, $J/\psi \to {\mu }^{+}{\mu }^{-}$, $\varphi \to K^{+}{K}^{-}$. This is evidence for the first instance of a hadronic state with valence quarks of four different flavors. The mass and natural width of this state are measured to be $m=5567.8\pm 2.9{(\mathrm{stat})}_{-1.9}^{+0.9}(\mathrm{syst})\mathrm{MeV}/c^2$ and $\mathrm{\Gamma }=21.9\pm 6.4{(\mathrm{stat})}_{-2.5}^{+5.0}(\mathrm{syst})\mathrm{MeV}/c^2$. If the decay is $X(5568)\to B_s^{*}{\pi }^{\pm }\to B_s^0\gamma {\pi }^{\pm }$ with an unseen $\gamma$, $m\mathbf{(}X(5568)\mathbf{)}$ will be shifted up by $m(B_s^{*})-m(B_s^0)\sim 49\mathrm{MeV}/c^2$. This measurement is based on $10.4{\mathrm{fb}}^{-1}$ of $p\overline{p}$ collision data at $\sqrt{s}=1.96\mathrm{TeV}$ collected by the D0 experiment at the Fermilab Tevatron collider.

We have uncovered a giant gyrotropic magneto-optical response for doped ferromagnetic manganite ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_3$ around the near room-temperature paramagnetic-to-ferromagnetic transition. At odds with current wisdom, where this response is usually assumed to be fundamentally fixed by the electronic band structure, we point to the presence of small polarons as the driving force for this unexpected phenomenon. We explain the observed properties by the intricate interplay of mobility, Jahn-Teller effect, and spin-orbit coupling of small polarons. As magnetic polarons are ubiquitously inherent to many strongly correlated systems, our results provide an original, general pathway towards the generation of magnetic-responsive gigantic gyrotropic responses that may open novel avenues for magnetoelectric coupling beyond the conventional modulation of magnetization.

The nature of the pairing states of superconducting ${\mathrm{LaNiC}}_2$ and ${\mathrm{LaNiGa}}_2$ has to date remained a puzzling question. Broken time reversal symmetry has been observed in both compounds and a group theoretical analysis implies a nonunitary triplet pairing state. However, all the allowed nonunitary triplet states have nodal gap functions but most thermodynamic and NMR measurements indicate fully gapped superconductivity in ${\mathrm{LaNiC}}_2$. Here we probe the gap symmetry of ${\mathrm{LaNiGa}}_2$ by measuring the London penetration depth, specific heat, and upper critical field. These measurements demonstrate two-gap nodeless superconductivity in ${\mathrm{LaNiGa}}_2$, suggesting that this is a common feature of both compounds. These results allow us to propose a novel triplet superconducting state, where the pairing occurs between electrons of the same spin, but on different orbitals. In this case the superconducting wave function has a triplet spin component but isotropic even parity gap symmetry, yet the overall wave function remains antisymmetric under particle exchange. This model leads to a nodeless two-gap superconducting state which breaks time reversal symmetry, and therefore accounts well for the seemingly contradictory experimental results.

We demonstrate that a homogeneous porous medium composed of sedimentary particles develops channels due to curvature driven growth of fluid flow coupled with an increase in porosity. While the flux is increased linearly, the evolution of porosity is observed to be intermittent with erosion occurring at the boundaries between low and high porosity regions. Calculating the spatial distribution of the flow within the medium and the fluid stress given by the product of the fluid flux and the volume fraction of the particles, we find that the system organizes itself to be locally near the threshold needed to erode the weakest particles. A statistical model simulating the coupling of the erosion, transport, and deposition of the particles to the local fluid flow and porosity is found to capture the overall development of the observed channels.

We study experimentally the flow and patterning of a granular suspension displaced by air inside a narrow tube. The invading air-liquid interface accumulates a plug of granular material that clogs the tube due to friction with the confining walls. The gas percolates through the static plug once the gas pressure exceeds the pore capillary entry pressure of the packed grains, and a moving accumulation front is reestablished at the far side of the plug. The process repeats, such that the advancing interface leaves a trail of plugs in its wake. Further, we show that the system undergoes a fluidization transition—and complete evacuation of the granular suspension—when the liquid withdrawal rate increases beyond a critical value. An analytical model of the stability condition for the granular accumulation predicts the flow regime.

We study quantum cosmology with conformal matter comprising a perfect radiation fluid and a number of conformally coupled scalar fields. Focusing initially on the collective coordinates (minisuperspace) associated with homogeneous, isotropic backgrounds, we are able to perform the quantum gravity path integral exactly. The evolution describes a “perfect bounce”, in which the Universe passes smoothly through the singularity. We extend the analysis to spatially flat, anisotropic universes, treated exactly, and to generic inhomogeneous, anisotropic perturbations treated at linear and nonlinear order. This picture provides a natural, unitary description of quantum mechanical evolution across a cosmological bounce. We provide evidence for a semiclassical description in which all fields pass “around” the cosmological singularity along complex classical paths.

We report a test of the universality of free fall by comparing the gravity acceleration of the ${}^{87}\mathrm{Rb}$ atoms in $m_F=+1$ versus those in $m_F=-1$, of which the corresponding spin orientations are opposite. A Mach-Zehnder-type atom interferometer is exploited to alternately measure the free fall acceleration of the atoms in these two magnetic sublevels, and the resultant Eötvös ratio is ${\eta }_S=(0.2\pm 1.2)\times {10}^{-7}$. This also gives an upper limit of $5.4\times {10}^{-6}{\mathrm{m}}^{-2}$ for a possible gradient field of the spacetime torsion. The interferometer using atoms in $m_F=\pm 1$ is highly sensitive to the magnetic field inhomogeneity. A double differential measurement method is developed to alleviate the inhomogeneity influence, of which the effectiveness is validated by a magnetic field modulating experiment.