In some Dirac systems with time-reversal ($\mathcal{T}$) and glide ($\mathcal{G}$) symmetries, multihelicoid surface states (MHSSs) appear, as discussed in various systems such as electronic and photonic ones. However, the topological nature and the conditions for the appearance of the MHSSs have not been understood. Here we show that MHSSs result from bulk-surface correspondence for the $Z_2$ monopole charge $\mathcal{Q}$, which cannot be defined as a local quantity associated with the Dirac point, unlike the $Z$ monopole charge characterizing Weyl points. The previously known formula of $\mathcal{Q}$ turns out to be non-gauge-invariant and thus cannot characterize the MHSSs. This shortcoming of the definition of $\mathcal{Q}$ is amended by redefining $\mathcal{Q}$ as a global topological invariant in $k$-space. Surprisingly, the newly defined $\mathcal{Q}$, characterizing $\mathcal{GT}$ invariant gapless systems, is equal to the $\mathcal{G}$-protected $Z_2$ topological invariant $\nu$, which is nontrivial only in $\mathcal{T}$-breaking gapped systems. This global definition of $\mathcal{Q}$ automatically guarantees the appearance of MHSSs even when the Dirac point splits into Weyl points or a nodal ring by lowering the symmetry, as long as the $\mathcal{GT}$ symmetry is preserved. $\mathcal{Q}$ can be simplified to symmetry-based indicators when two vertical $\mathcal{G}\mathrm{s}$ are preserved, and filling-enforced topological crystalline insulators are diagnosed in several cases when a $\mathcal{T}$-breaking perturbation is induced. Material candidate ${\mathrm{Li}}_2{\mathrm{B}}_4{\mathrm{O}}_7$ together with a list of space groups preserving MHSSs are also proposed.

The bismuth-based half-Heusler materials host a nontrivial topological band structure, unconventional superconductivity, and large spin-orbit coupling in a system with very low electron density. In particular, the inversion of $p$-orbital-derived bands with an effective angular momentum $j$ of up to 3/2 is thought to play a central role in anomalous Cooper pairing in the cubic half-Heusler semimetal YPtBi, which is thought to be the first “high-spin” superconductor. Here, we report an extensive study of the angular dependence of quantum oscillations (QOs) in the electrical conductivity of YPtBi, revealing an anomalous Shubnikov–de Haas effect consistent with the presence of a coherent $j=3/2$ Fermi surface. The QO signal in YPtBi manifests an extreme anisotropy upon rotation of the magnetic field from the [100] to [110] crystallographic direction, where the QO amplitude vanishes. This radical anisotropy for such a highly isotropic system cannot be explained by trivial scenarios involving changes in effective mass or impurity scattering, but rather is naturally explained by the warping feature of the $j=3/2$ Fermi surface of YPtBi, providing direct proof of active high angular momentum quasiparticles in the half-Heusler compounds.

We investigate twisted bilayer graphene near charge neutrality using a generalized Bistritzer-MacDonald continuum model accounting for corrugation effects. The Fermi velocity vanishes for particular twist angles properly reproducing the physics of the celebrated magic angles. Using group representation theory, we identify all contact interaction potentials compatible with the symmetries of the model. This enables us to identify two classes of quartic interactions leading to either the opening of a gap or to nematic ordering. We then implement a renormalization group analysis to study the effect of quantum fluctuations on the appearance of these states for twist angles approaching the first magic value. This combined group theory-renormalization study reveals that the proximity to the first magic angle favors the occurrence of a layer-polarized, gapped state with a spatial modulation of interlayer correlations, which we call a nematic insulator.

For the efficient implementation of quantum algorithms, practical ways to generate many-body entanglement are a basic requirement. Specifically, coupling multiple qubit pairs at once can be advantageous and may provide multiqubit operations useful in the construction of hardware-tailored algorithms. Here we extend the theory of fractional state transfer and harness the simultaneous coupling of qubits on a chain to engineer a set of nonlocal parity-dependent quantum operations suitable for a wide range of applications. The resulting effective long-range couplings directly implement a parametrizable Trotter-step for Jordan-Wigner fermions, and they can be used for simulations of quantum dynamics, efficient state generation in variational quantum eigensolvers, parity measurements for error-correction schemes, and the generation of efficient multiqubit gates. Moreover, we present numerical simulations of the gate operation in a superconducting quantum circuit architecture, which show a high gate fidelity for realistic experimental parameters.

We analyze the presence of nonreciprocal critical currents, the so-called superconducting diode effect, in chiral superconductors within a generalized Ginzburg-Landau framework. After deriving its key symmetry conditions we illustrate the basic mechanism for two examples, the critical current in a thin film and a Josephson junction. The appearance of spontaneous edge currents and the energy bias for the formation of Josephson vortices play an essential part in establishing a splitting of the critical currents running in opposite directions. Eventually this allows us to interpret a superconducting diode effect observed in the 3-Kelvin phase of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ as evidence for spontaneously broken time-reversal symmetry in the superconducting phase.

Wavelength division multiplexing (WDM), the technology that utilizes different wavelengths of carrier signal as independent information channels and helps to realize enhanced information density and functional diversity, is desired in various acoustic applications for the growing demand of higher transmission efficiency and lower cost. In this paper, we marry the rising concept of acoustic metagrating with the technology of WDM and demonstrate that interesting and distinct wavelength-dependent functionalities with unitary efficiency in prespecified frequency ranges can be realized by a simple grating structure. To this end, we integrate the particle swarm optimization with the deep learning technique to resolve the uncertainty issue and multi-objective problem and develop both deterministic and probabilistic neural networks for the inverse design of a metagrating-based WDM device. As a result, the computational cost is greatly reduced, and the probabilistic model reveals the sensitivity and flexibility of the parameters of the metagrating with respect to the desired functionality. In this paper, we not only provide an intelligent inverse design paradigm of high-performance WDM devices for multiple manipulation purposes but also present a feasible solution for the development of integrated acoustic devices for wavelength-dependent applications.

In confined nanoscale structures, such as nanopores or nanotubes, the polar characteristic of water can be evident; while in bulk or droplet form, it is masked by the statistical molecular arrangements dictated by free energy minimization. In this paper, we show that, under an applied electric field, the polar character of water molecules enables the formation of macroscopic scale columns of water vapor penetrating through a silicone oil bath. These columns are up to 60 μm in length and tens of microns in cross-sectional dimension. We have directly visualized such columns by using water-soluble fluorescent molecules dispersed in silicone oil. The polar nature of columns is evidenced by the measured giant electrorheological effect that correlates with the application of an electric field. We present a phenomenological theory to support our observations and show that the experimental data are consistent with a state of highly aligned water molecular dipoles within each column.

We demonstrate the ability of low energy electron microscopy (LEEM) to extract fundamental information in surface phase transitions during in situ observations of complex semiconductor surfaces. We utilize established LEEM techniques and develop a methodology that enables us to calculate the surface entropy difference using only LEEM measurements without the need for external characterization. We demonstrate the effectiveness of the unified method by monitoring the phase coexistence during the first-order transition between the $c(8\times 2)$ and $(6\times 6)$ phases on the surface of GaAs(001) at a range of temperatures relevant for epitaxy. The coexistence behavior with temperature and the fluctuations of phase boundaries are measured and analyzed to obtain the entropy difference and stress difference between the phases. The calculated values show that the entropy difference is not large enough to stabilize the $(6\times 6)$ phase with respect to the $c(8\times 2)$ by itself, suggesting that the elastic relaxation during the coexistence between the two phases is necessary to stabilize the $(6\times 6)$ phase.

The size of the Hilbert space for a multiqubit state scales exponentially with the number of constituent qubits. Often this leads to a similar exponential scaling of the experimental resources required to characterize the state. Contrary to this, we propose a physically motivated method for experimentally assessing the fidelity of an important class of entangled states known as cluster states. The proposed method always yields a lower bound of the fidelity with a number of measurement settings scaling only linearly with the system size, and is tailored to correctly account for the errors most likely to occur in experiments. For one-dimensional cluster states, the constructed fidelity measure is tight to lowest order in the error probability for experimentally realistic noise sources and thus closely matches the true fidelity. Furthermore, it is tight for the majority of higher-order errors, except for a small subset of certain nonlocal multiqubit errors irrelevant in typical experimental situations. The scheme also performs very well for higher-dimensional cluster states, assessing correctly the majority of experimentally relevant errors.

We use first-principles calculations to investigate how deviations from perfect chemical order affect the magnetocrystalline anisotropy energy (MAE) in $L{1}_0$ FeNi. We first analyze the local chemical environment of the Fe atoms in various partially ordered configurations, using the orbital magnetic moment anisotropy (OMA) as proxy for a local contribution to the MAE. We are able to identify a specific nearest neighbor configuration and use this “favorable environment” to successfully design various structures with MAE higher than the perfectly ordered system. However, a systematic analysis of the correlation between local environment and OMA using smooth overlap of atomic positions indicates only a partial correlation, which exists only if the deviation from full chemical order is not too large, whereas in general no such correlation can be identified even using up to third nearest neighbors. Guided by the observation that the identified favorable environment implies an Fe-rich composition, we investigate the effect of randomly inserting additional Fe into the nominal Ni planes of the perfectly ordered structure. We find that the MAE increases with Fe content, at least up to 62.5% Fe. Thus, our paper shows that the perfectly ordered case is not the one with highest MAE and that an increased MAE can be obtained for slightly Fe-rich compositions.

A parallel hybrid quantum-classical algorithm for the solution of the quantum-chemical ground-state energy problem on gate-based quantum computers is presented. This approach is based on the reduced density-matrix functional theory (RDMFT) formulation of the electronic structure problem. For that purpose, the density-matrix functional of the full system is decomposed into an indirectly coupled sum of density-matrix functionals for all its subsystems using the adaptive cluster approximation to RDMFT. The approximations involved in the decomposition and the adaptive cluster approximation itself can be systematically converged to the exact result. The solutions for the density-matrix functionals of the effective subsystems involves a constrained minimization over many-particle states that are approximated by parametrized trial states on the quantum computer similarly to the variational quantum eigensolver. The independence of the density-matrix functionals of the effective subsystems introduces a new level of parallelization and allows for the computational treatment of much larger molecules on a quantum computer with a given qubit count. In addition, for the proposed algorithm techniques are presented to reduce the qubit count, the number of quantum programs, as well as its depth. The evaluation of a density-matrix functional as the essential part of our approach is demonstrated for Hubbard-like systems on IBM quantum computers based on superconducting transmon qubits.

The origin and nature of glassy dynamics presents one of the central enigmas of condensed-matter physics across a broad range of systems ranging from window glass to spin glasses. The spin-ice compound ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, which is perhaps best known as hosting a three-dimensional Coulomb spin liquid with magnetically charged monopole excitations, also falls out of equilibrium at low temperature. How and why it does so remains an open question. Based on an analysis of low-temperature diffuse neutron-scattering experiments employing different cooling protocols alongside recent magnetic noise studies, combined with extensive numerical modeling, we argue that upon cooling, the spins freeze into what may be termed a “structural magnetic glass,” without an $a$ priori need for chemical or structural disorder. Specifically, our model indicates the presence of frustration on two levels, first producing a near-degenerate constrained manifold inside which phase ordering kinetics is in turn frustrated. A remarkable feature is that monopoles act as sole annealers of the spin network and their pathways and history encode the development of glass dynamics, allowing the glass formation to be visualized. Our results suggest that spin ice ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ provides one prototype of magnetic glass formation specifically and a setting for the study of kinetically constrained systems more generally.

We derive a fully covariant theory of the hydrodynamics of nematic and polar active surfaces, subjected to internal and external forces and torques. We study the symmetries of polar and nematic surfaces and find that in addition to five different types of in-plane isotropic surfaces, polar and nematic surfaces can be classified into five polar, two pseudopolar, five nematic and two pseudonematic types of surfaces. We give examples of physical realisations of the different types of surfaces we have identified. We obtain expressions for the equilibrium tensions, moments, and external forces and torques acting on a passive polar or nematic surface. We calculate the entropy production rate using the framework of thermodynamics close to equilibrium and find constitutive equations for polar and nematic active surfaces with different symmetries. We study the instabilities of a confined flat planar-chiral polar active layer and of a confined deformable polar active surface with broken up-down symmetry.

When the spins on the frustrated pyrochlore lattice obey the celebrated 2-in-2-out ice rule, they stay in a correlated disordered phase and break the third law of thermodynamics. Similarly, if the atomic ions on the pyrochlore lattice move in and outward of the tetrahedra, they may obey a constraint resembling the ice rule. We discover that a model for pyrochlore molybdates $A_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ ($A=\mathrm{Y}$, Dy, Tb) exhibits a “supercooled ice” state of the displacement degrees of freedom of ${\mathrm{Mo}}^{4+}$ ions, when we take account of the Jahn-Teller (JT) effect. The JT effect occurs when the lattice distortions reduce the symmetry of the local crystal field, resulting in the orbital-energy splitting that causes the local energy gain. Unlike the standard JT effect that leads to periodic long-range ordering, the displacements of ${\mathrm{Mo}}^{4+}$ ions are disordered following the icelike rule. We microscopically derive a model that describes this situation by having the 2nd- and 3rd-neighbor interactions between in-out lattice displacements comparably as strong as the nearest-neighbor interactions of standard ice. There, the well-known nearly flat energy landscape of the ice state is altered to a metastable highly quasidegenerate icelike liquid state coexisting with a crystalline-like ground state. Our Monte Carlo simulations show that this liquid remains remarkably stable down to low temperatures by avoiding the putative first-order transition. The relaxation in the supercooled JT ice state exhibits glassy dynamics with a plateau structure. They fit the feature of a “good glass former” very often found in molecular liquids but that has never been observed in material solids. The high glass-forming ability of the interacting lattice degrees of freedom will play a key role in the spin-glass transition of the material.

Competition between different cell types plays a crucial role in bacterial ecology, developmental biology, and tumor growth. We study how cells compete in 2D by using both a mean field theory and particle-based simulations. We employ a mechanical model that incorporates a stylized form of cell cycle regulation to control cell division events. This is extended to treat multiple cell types with different rates of programed cell death (apoptosis) and characteristic cell-cycle control pressures. Analytic predictions for the invasion speed and the coexistence line are in agreement with simulations. Synchronization in the cell division/apoptosis events can emerge, leading to oscillations in pressure and cell-cycle activity. We also study the invasion or elimination of small (pre)cancerous colonies. We show how a Laplace pressure at the colony interface, here controlled by differential cell adhesion, shifts the coexistence line; there are cell types that can invade when starting from a large colony but will be eliminated if the colony is small.

The maximum entropy principle determines the values of thermodynamic variables in thermally isolated equilibrium systems. This paper extends the principle to a variational principle that applies to liquid–gas coexistence in heat conduction. We show the uniqueness of the extension under the assumption that the variational principle and the fundamental thermodynamic relation are simultaneously extended in the linear response regime with the total energy fixed. Using the extended variational principle, we calculate the thermodynamic quantities in this steady state and find that the temperature of the liquid–gas interface deviates from the equilibrium transition temperature, which should be verified in experiments.

Recent advances in quantum computing devices have brought attention to hybrid quantum-classical algorithms like the variational quantum eigensolver (VQE) as a potential route to practical quantum advantage in chemistry. However, it is not yet clear whether such algorithms, even in the absence of device error, could actually achieve quantum advantage for systems of practical interest. We have performed an exhaustive analysis to estimate the number of qubits and number of measurements required to compute the combustion energies of small organic molecules and related systems to within chemical accuracy of experimental values using the VQE. We consider several key modern improvements to the VQE, including low-rank factorizations of the Hamiltonian. Our results indicate that, although these techniques are useful, they will not be sufficient to achieve practical quantum computational advantage for our molecular set, or for similar molecules. This suggests that novel approaches to operator estimation leveraging quantum coherence, such as enhanced likelihood functions, may be required.

We develop a theory for a two-component miscible dipolar condensate in a planar trap. Using numerical solutions and a variational theory we solve for the excitation spectrum and identify regimes where density- and spin-roton excitations are favored. We characterize the various instabilities that can emerge in this system over a wide parameter regime and present results for the stability phase diagram. Importantly this allows us to identify the parameter regimes where a novel roton-immiscibility transition can occur, driven by the softening of the spin-roton excitation.

Pattern formation via nonequilibrium dynamical paths has been the focus of much recent attention. An example is phase separation under the action of a nonstationary temperature field. Under directional quenching (DQ), where a region at some “quenched” temperature spreads in one direction over time, it has been found that a variety of patterns (random droplet, lamellar, and columnar patterns) may be formed by controlling the dynamics of the quenching front in two dimensions. Although a similarly rich phenomenology may be expected in three dimensions (3D), DQ in 3D would require temperature control in bulk, which is difficult to realize. Instead of DQ, we fix the temperature gradient through the slab, and control of the surface temperature of both sides of the slab (gradient cooling, GC). It seems similar to DQ and it is significantly easier to realize experimentally. Here, we compare the mechanism of the phase separation in DQ with that in GC. It is found that no columnar pattern is formed in GC. It is also revealed that a pattern formation mechanism in GC is clearly different from conventional DQ.