We identify ${\mathrm{Ba}}^{4+}$ (Te-like) as a promising candidate for a high-accuracy optical clock. The lowest-lying electronic states are part of a ${{}^{3}P}_J$ fine structure manifold with anomalous energy ordering, being nonmonotonic in $J$. We propose a clock based on the 338.8 THz electric quadrupole transition between the ground (${}^3{P}_2$) and first-excited (${}^3{P}_0$) electronic states. We perform relativistic many-body calculations to determine relevant properties of this ion. The lifetime of the excited clock state is found to be several seconds, accommodating low statistical uncertainty with a single ion for practical averaging times. The differential static scalar polarizability is found to be small and negative, providing suppressed sensitivity to blackbody radiation while simultaneously allowing cancellation of Stark and excess micromotion shifts. With the exception of ${\mathrm{Hg}}^{+}$ and ${\mathrm{Yb}}^{+}$, sensitivity to variation of the fine structure constant is greater than other optical clocks thus far demonstrated.

Laser-induced rotational wave packets of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ molecules were experimentally measured in real time by using two sequential 25-fs laser pulses and a reaction microscope. By measuring the time-dependent yields of the above-threshold dissociation and the enhanced ionization of the molecule, we observed a few-femtosecond time delay between the two dissociation channels for both ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$. The delay was interpreted and reproduced by a classical model that considers enhanced ionization and thus additional interaction within the laser pulse. We demonstrate that by accurately measuring the phase of the rotational wave packet in hydrogen molecules we can resolve dissociation dynamics which is occurring within a fraction of a molecular rotation. Such a rotational clock is a general concept applicable to sequential fragmentation processes in other molecules.

Positrons attach to most molecules through Feshbach resonant excitation of fundamental vibrational modes, and this leads to greatly enhanced annihilation rates. In all but the smallest molecules, vibrational energy transfer further enhances these annihilation rates. Evidence is presented that in alkane and cycloalkane molecules, this can occur by the excitation of other than fundamental vibrations and produce roughly comparable annihilation rates. These features are compared to infrared absorption spectra. A possible mechanism is discussed that involves combination and overtone vibrations.

This Letter provides a low-power method for chaos generation that is generally applicable to nonlinear micro- and nanoelectromechanical systems (MNEMS) resonators. The approach taken is independent of the material, scale, design, and actuation of the device in question; it simply assumes a good quality factor and a Duffing type nonlinearity, features that are commonplace to MNEMS resonators. The approach models the rotating-frame dynamics to analytically constrain the parameter space required for chaos generation. By leveraging these common properties of MNEMS devices, a period-doubling route to chaos is generated using smaller forcing than typically reported in the literature.

The dynamics of viscous fluid threads concurrently flowing with miscible solvents is experimentally investigated in square microchannels. Diffusive fluid threads are found to significantly swell at low flow velocities due to large specific interfacial area and hydrodynamic lubrication. An approach based on bounded function analysis of confined thread diameter is developed to model diffusive behavior of viscosity-differing fluids at the small scale. This work shows the determination of a critical flow rate associated with each fluid pair and the use of dynamic similarity to calculate diffusion coefficients between oils and organic solvents. The thread divergence is estimated based on the growth of diffusion layers and related to diffusion-induced buckling instabilities of viscous threads in parallel flows.

When one fluid displaces another in a confined environment, some energy is dissipated in the fluid bulk and the rest is dissipated near the contact line. Here we study the relative strengths of these two sources of dissipation with a novel experimental setup: constant-rate spontaneous imbibition experiments, achieved by introducing a viscous oil slug in front of the invading fluid inside a capillary tube. We show that a large fraction of dissipation can take place near the contact line, and rationalize the observations by means of a theoretical analysis of the dynamic contact angles of the front and back menisci of the oil slug. Our results bear important implications for macroscopic descriptions of multiphase flows in microfluidic systems and porous media.

With density functional molecular dynamics simulations, we computed the phase diagram of MgO from 50 to 2000 GPa up to 20 000 K. Via thermodynamic integration (TDI), we derive the Gibbs free energies of the B1, B2, and liquid phases and determine their phase boundaries. With TDI and a pseudo-quasi-harmonic approach, we show that anharmonic effects are important and stabilize the B1 phase in particular. As a result, the B1-B2 transition boundary in the pressure-temperature plane exhibits a steep slope. We predict the B1-B2-liquid triple point to occur at approximately $T=10000\mathrm{K}$ and $P=370\mathrm{GPa}$, which is higher in pressure than was inferred with quasiharmonic methods alone. We predict the principal shock Hugoniot curve to enter the B2 phase stability domain but only over a very small range of parameters. This may render it difficult to observe this phase with shock experiments because of kinetic effects.

We show that a conical magnetic field $\mathbf{H}=(1,1,1)H$ can be used to tune the topological order and hence, anyon excitations of the $Z_2$ quantum spin liquid in the isotropic antiferromagnetic Kitaev model. A novel topological order, featured with Chern number $C=4$ and Abelian anyon excitations, is induced in a narrow range of intermediate fields $H_{c1}\le H\le H_{c2}$. On the other hand, the $C=1$ Ising-topological order with non-Abelian anyon excitations, as previously known to be present at small fields, is found here to survive up to $H_{c1}$. The results are obtained by developing and applying a $Z_2$ mean field theory that works at finite fields and is asymptotically exact in the zero field limit and the associated variational quantum Monte Carlo.

Controlling magnetism, essential for a wide range of technologies, is impaired by the impossibility of generating a maximum of magnetic field in free space. Here, we propose a strategy based on negative permeability to overcome this stringent limitation. We experimentally demonstrate that an active magnetic metamaterial can emulate the field of a straight current wire at a distance. Our strategy leads to an unprecedented focusing of magnetic fields in empty space and enables the remote cancellation of magnetic sources, opening an avenue for manipulating magnetic fields in inaccessible regions.

As a result of nonequilibrium forces, purely repulsive self-propelled particles undergo macrophase separation between a dense and a dilute phase. We present a thorough study of the ordering kinetics of such motility-induced phase separation (MIPS) in active Brownian particles in two dimensions, and we show that it is generically accompanied by microphase separation. The growth of the dense phase follows a law akin to the one of liquid-gas phase separation. However, it is made of a mosaic of hexatic microdomains whose size does not coarsen indefinitely, leaving behind a network of extended topological defects from which microscopic dilute bubbles arise. The characteristic length of these finite-size structures increases with activity, independently of the choice of initial conditions.

Ultracold gases provide an unprecedented level of control for the investigation of soliton dynamics and collisions. We present a scheme for deterministically preparing pairs of three-component solitons in a Bose-Einstein condensate. Our method is based on local spin rotations which simultaneously imprint suitable phase and density distributions. This enables us to observe striking collisional properties of the vector degree of freedom which naturally arises for the coherent nature of the emerging multicomponent solitons. We find that the solitonic properties in the quasi-one-dimensional system are quantitatively described by the integrable repulsive three-component Manakov model.

We report the analog simulation of an ergodic-localized junction by using an array of 12 coupled superconducting qubits. To perform the simulation, we fabricated a superconducting quantum processor that is divided into two domains: one is a driven domain representing an ergodic system, while the second is localized under the effect of disorder. Because of the overlap between localized and delocalized states, for a small disorder there is a proximity effect and localization is destroyed. To experimentally investigate this, we prepare a microwave excitation in the driven domain and explore how deep it can penetrate the disordered region by probing its dynamics. Furthermore, we perform an ensemble average over 50 realizations of disorder, which clearly shows the proximity effect. Our work opens a new avenue to build quantum simulators of driven-disordered systems with applications in condensed matter physics and material science.

Computable information density (CID), the ratio of the length of a losslessly compressed data file to that of the uncompressed file, is a measure of order and correlation in both equilibrium and nonequilibrium systems. Here we show that correlation lengths can be obtained by decimation, thinning a configuration by sampling data at increasing intervals and recalculating the CID. When the sampling interval is larger than the system’s correlation length, the data becomes incompressible. The correlation length and its critical exponents are thus accessible with no a priori knowledge of an order parameter or even the nature of the ordering. The correlation length measured in this way agrees well with that computed from the decay of two-point correlation functions $g_2(r)$ when they exist. But the CID reveals the correlation length and its scaling even when $g_2(r)$ has no structure, as we demonstrate by “cloaking” the data with a Rudin-Shapiro sequence.

We present the results of an all-sky search for continuous gravitational-wave signals with frequencies in the 500–1700 Hz range targeting neutron stars with ellipticity of ${10}^{-8}$. The search is done on LIGO O2 data using the Falcon analysis pipeline. The results presented here double the sensitivity over any other result on the same data [B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), Phys. Rev. D 100, 024004 (2019), C. Palomba et al., Phys. Rev. Lett. 123, 171101 (2019)]. The search is capable of detecting low-ellipticity sources up to 170 pc. We establish strict upper limits which hold for worst-case signal parameters. We list outliers uncovered by the search, including several which we cannot associate with any known instrumental cause.

The parameter ${\epsilon }_K$ describes $CP$ violation in the neutral kaon system and is one of the most sensitive probes of new physics. The large uncertainties related to the charm-quark contribution to ${\epsilon }_K$ have so far prevented a reliable standard-model prediction. We show that Cabibbo-Kobayashi-Maskawa unitarity enforces a unique form of the $|\mathrm{\Delta }S=2|$ weak effective Lagrangian in which the short-distance theory uncertainty of the imaginary part is dramatically reduced. The uncertainty related to the charm-quark contribution is now at the percent level. We present the updated standard-model prediction ${\epsilon }_K=2.16(6)(8)(15)\times {10}^{-3}$, where the errors in parentheses correspond to QCD short-distance, long-distance, and parametric uncertainties, respectively.

We present phenomenological results for the inclusive cross section for the production of a lepton pair via virtual photon exchange at next-to-next-to-next-to-leading order in perturbative QCD. In line with the case of Higgs production, we find that the hadronic cross section receives corrections at the percent level, and the residual dependence on the perturbative scales is reduced. However, unlike in the Higgs case, we observe that the uncertainty band derived from scale variation is no longer contained in the band of the previous order.

The electronic bands of twisted bilayer graphene (TBLG) with a large-period moiré superlattice fracture to form narrow Bloch minibands that are spectrally isolated by forbidden energy gaps from remote dispersive bands. When these gaps are sufficiently large, one can study a band-projected Hamiltonian that correctly represents the dynamics within the minibands. This inevitably introduces nontrivial geometrical constraints that arise from the assumed form of the projection. Here we show that this choice has a profound consequence in a low-energy experimentally observable signature that therefore can be used to tightly constrain the analytic form of the appropriate low-energy theory. We find that this can be accomplished by a careful analysis of the electron density produced by backscattering of Bloch waves from an impurity potential localized on the moiré superlattice scale. We provide numerical estimates of the effect that can guide experimental work to clearly discriminate between competing models for the low-energy band structure.

Single layers of transition metal dichalcogenides are of interest for emergent properties; an often-neglected issue is substrate effects. Our experiments show that the charge density wave in a single-layer ${\mathrm{TiTe}}_2$ grown on ${\mathrm{PtTe}}_2$ films is strongly suppressed by increasing the ${\mathrm{PtTe}}_2$ substrate thickness. Given that the interfacial bonding remains of the weak incommensurate van der Waals type, the observed changes are correlated with a thickness-dependent metallicity transformation in the ${\mathrm{PtTe}}_2$ substrate. The results illustrate the crucial role of the substrate in single-layer physics.