The spin-liquid candidate $\kappa -{\left(\mathrm{ET}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$ [ET: bis(ethylenedithio)tetrathiafulvalene] does not exhibit magnetic ordering down to a very low temperature, but shows a mysterious anomaly at 6 K. The origin of the so-called 6-K anomaly is still under debate. We carried out nuclear quadrupole resonance (NQR) measurements on the copper sites of the insulating layers, which are sensitive to the charge dynamics unlike conventional spin-$1/2$ nuclear magnetic resonance (NMR). The main finding of this Rapid Communication is that the observation of a sharp peak behavior in the nuclear spin-lattice relaxation rate $T_1^{-1}$ of ${}^{63}\mathrm{Cu}$ NQR at 6 K while $T_1^{-1}$ of both ${}^{13}\mathrm{C}$ and ${}^1\mathrm{H}$ NMR show no clear anomaly. This behavior can be understood as a second-order phase transition related to charge disproportionation in the ET layers.

There is growing interest in the $U\left(1\right)$ Coulomb liquid in both quantum materials in pyrochlore ice and cluster Mott insulators and cold-atom systems. We explore a paired hardcore boson model on a pyrochlore lattice. This model is equivalent to the XYZ spin model that was proposed for rare-earth-metal pyrochlores with “dipole-octupole” doublets. Since our model has no sign problem for quantum Monte Carlo (QMC) simulations in a large parameter regime, we carry out both analytical and QMC calculations. We find that the $U\left(1\right)$ Coulomb liquid is quite stable and spans a rather large portion of the phase diagram with boson pairing. Moreover, we numerically find a thermodynamic evidence that the boson pairing could induce a possible ${\mathbb{Z}}_2$ liquid in the vicinity of the phase boundary between Coulomb liquid and ${\mathbb{Z}}_2$ symmetry-broken phase. Besides the materials' relevance with quantum spin ice, we point to quantum simulation with cold atoms on optical lattices.

Localized excitons play a vital role in the optical response of monolayers of transition metal dichalcogenides and can be exploited as single photon sources for quantum information technology. While the optical properties of such localized excitons are vastly studied, the ultrafast capture process of delocalized excitons into localized potentials is largely unexplored. We perform quantum kinetic calculations of exciton capture via acoustic and optical phonons showing that efficient capture takes place on an ultrafast time scale. The polaron formation in the low-temperature limit leads to higher-energy excitons which can then be efficiently trapped. We demonstrate that the interplay of acoustic and optical phonons leads to an efficient broadening of energy-selection rules. Our studies provide a deep understanding of the carrier trapping from two-dimensional materials into zero-dimensional potentials.

In frustrated quantum magnetism, chiral spin liquids are a particularly intriguing subset of quantum spin liquids in which the fractionalized parton degrees of freedom form a Chern insulator. Here we study an exactly solvable spin-3/2 model, which harbors not only chiral spin liquids but also spin liquids with higher-order parton band topology—a trivial band insulator, a Chern insulator with gapless chiral edge modes, and a second-order topological insulator with gapless corner modes. With a focus on the thermodynamic precursors and thermal phase transitions associated with these distinct states, we employ numerically exact quantum Monte Carlo simulations to reveal a number of unconventional phenomena. This includes heightened thermal stability of the ground state phases, the emergence of a partial flux ordering of the associated ${\mathbb{Z}}_2$ lattice gauge field, and the formation of a thermal Majorana metal regime extending over a broad temperature range.

We propose a sequential minimal optimization method for quantum-classical hybrid algorithms, which converges faster, robust against statistical error, and hyperparameter-free. Specifically, the optimization problem of the parameterized quantum circuits is divided into solvable subproblems by considering only a subset of the parameters. In fact, if we choose a single parameter, the cost function becomes a simple sine curve with period $2\pi$, and hence we can exactly minimize with respect to the chosen parameter. Furthermore, even in general cases, the cost function is given by a simple sum of trigonometric functions with certain periods and hence can be minimized by using a classical computer. By repeatedly performing this procedure, we can optimize the parameterized quantum circuits so that the cost function becomes as small as possible. We perform numerical simulations and compare the proposed method with existing gradient-free and gradient-based optimization algorithms. We find that the proposed method substantially outperforms the existing optimization algorithms and converges to a solution almost independent of the initial choice of the parameters. This accelerates almost all quantum-classical hybrid algorithms readily and would be a key tool for harnessing near-term quantum devices.

Maximum entropy null models of networks come in different flavors that depend on the type of constraints under which entropy is maximized. If the constraints are on degree sequences or distributions, we are dealing with configuration models. If the degree sequence is constrained exactly, the corresponding microcanonical ensemble of random graphs with a given degree sequence is the configuration model per se. If the degree sequence is constrained only on average, the corresponding grand-canonical ensemble of random graphs with a given expected degree sequence is the soft configuration model. If the degree sequence is not fixed at all but randomly drawn from a fixed distribution, the corresponding hypercanonical ensemble of random graphs with a given degree distribution is the hypersoft configuration model, a more adequate description of dynamic real-world networks in which degree sequences are never fixed but degree distributions often stay stable. Here, we introduce the hypersoft configuration model of weighted networks. The main contribution is a particular version of the model with power-law degree and strength distributions, and superlinear scaling of strengths with degrees, mimicking the properties of some real-world networks. As a byproduct, we generalize the notions of sparse graphons and their entropy to weighted networks.

Extended single-domain growth of h-BN is observed on Pt(110), if the precursor molecules are deposited at sufficiently high temperatures. We examined the electronic structure of the h-BN/Pt(110) system by angle-resolved photoemission (ARPES), work-function measurements, and density-functional theory (DFT) calculations. van der Waals forces dominate the h-BN/Pt(110) interaction by far, although DFT analysis of the local density of states reveals the existence of a local covalent interaction of some N atoms with Pt surface atoms. The local bonding contributions cause the appearance of a $(1\times n)$ missing-row reconstruction $(n=5 \mathrm{or} 6)$ of the Pt (110) surface, if the system reverts to room temperature after h-BN adlayer formation at 1120 K. This unique phenomenon of the template adapting to the adlayer structure mitigates differences in the thermal-expansion coefficient upon cooling. The h-BN π bands hybridize with Pt $d$ bands. Nevertheless, the dispersion of π and σ bands as measured by ARPES is overall well represented by the free-standing monolayer band structure except for the appearance of replica bands induced by the Moiré structure. A comparison between the experimentally measured π bands and the band structure obtained from DFT slab calculations suggest the existence of significant correlation effects in photoemission from h-BN/Pt(110). The locally varying distribution of N-Pt hybrid states straddling the Fermi level indicates a corresponding spatial variation of the chemical reactivity.

Artificial lattices have served as a platform to study the physics of unconventional superconductivity. We study semiconductor artificial graphene—a honeycomb superlattice imposed on a semiconductor heterostructure—which hosts the Dirac physics of graphene but with a tunable periodic potential strength and lattice spacing, allowing control of the strength of the electron-electron interactions. We demonstrate a new mechanism for superconductivity due to repulsive interactions which requires a strong lattice potential and a minimum doping away from the Dirac points. The mechanism relies on the Berry phase of the emergent Dirac fermions, which causes oppositely moving electron pairs near the Dirac points to interfere destructively, reducing the Coulomb repulsion and thereby giving rise to an effective attraction. The attractive component of the interaction is enhanced by a novel antiscreening effect which, in turn, increases with doping; as a result, there is a minimum doping beyond which superconducting order generically ensues. The dominant superconducting state exhibits a spatially modulated gap with chiral $p$-wave symmetry. Microscopic calculations suggest that the possible critical temperatures are large relative to the low carrier densities, for a range of experimentally realistic parameters.

We study the evolution and persistence of quantum and classical correlations between spatially separated sites in the disordered XXZ spin chain in its ergodic and many-body-localized phases using exact diagonalization and matrix product state simulations. We show that in the many-body-localized phase, quantum entanglement survives in long-time unitary dynamics of the system, even though it is more fragile than the classical correlations. We study the dependence of the residual entanglement and correlations on the system size and on the disorder strength. Finally, we demonstrate the robustness of entanglement and correlations with respect to local dissipation in the bulk of the system.

Sampson's theory for hydrodynamic resistance across a zero-length orifice was developed over a century ago. Although a powerful theory for entrance/exit resistance in nanopores, it lacks accuracy for relatively small-radius pores since it does not account for the molecular interface chemistry. Here, Sampson's theory is revisited for the finite slippage and interfacial viscosity variation near the pore wall. The corrected Sampson's theory can accurately predict the hydrodynamic resistance from molecular dynamics simulations of ultrathin nanopores.

We show how to quantify tripartite entanglement using entropies derived from experimental correlations. We use a multipartite generalization of the entanglement of formation that is greater than zero if and only if the state is genuinely multipartite entangled. We develop an entropic witness for tripartite entanglement, and we show that the degree of violation of this witness places a lower limit on the tripartite entanglement of formation. We test our results in the three-qubit regime using the GHZ-Werner state and the W-Werner state, and in the high-dimensional pure-state regime using the triple-Gaussian wavefunction describing the spatial and energy-time entanglement in photon triplets generated in third-order spontaneous parametric down-conversion. In addition, we discuss the challenges in quantifying the entanglement for progressively larger numbers of parties, and give both entropic and target-state-based witnesses of multipartite entanglement that circumvent this issue.

We study transport in twisted bilayer graphene and show that electrostatic barriers can act as valley splitters, where electrons from the $K$ ($K^{\prime }$) valley are transmitted only to, e.g., the top (bottom) layer, leading to valley-layer locked currents. We show that such a valley splitter is obtained when the barrier varies slowly on the moiré scale and induces a Lifshitz transition across the junction, i.e., a change in the Fermi-surface topology. Furthermore, we show that for a given valley the reflected and transmitted current are transversely deflected, as time-reversal symmetry is effectively broken in each valley separately, resulting in valley-selective transverse focusing at zero magnetic field.

We consider the growth of clusters in disordered media at zero temperature, as exemplified by supercritical generalized percolation and by the $T=0$ random field Ising model. We show that the morphology of such clusters and of their surfaces can be of different types: They can be standard compact clusters with rough or smooth surfaces, but there exists also a completely different “spongy” phase. Clusters in the spongy phase are compact as far as the size-mass relation $M\sim R^D$ is concerned (with $D$ being the space dimension) but have an outer surface (or “hull”) whose fractal dimension is also $D$ and which is indeed dense in the interior of the entire cluster. This behavior is found in all dimensions $D\ge 3$. Slightly supercritical clusters can be of either type in $D=3$, while they are always spongy in $D\ge 4$. Possible consequences for the applicability of Kardar-Parisi-Zhang (KPZ) scaling to interfaces in media with frozen pinning centers are studied in detail. In particular, we find—in contrast to KPZ—a weak-coupling phase in $2+1$ dimensions.

We study the collective decay rates of multidimensional quantum networks in which one-dimensional waveguides form an intersecting hyperrectangular lattice, with qubits located at the lattice points. We introduce and motivate the dimensional reduction of poles (DRoP) conjecture, which identifies all collective decay rates of such networks via a connection to waveguides with a one-dimensional topology (e.g., a linear chain of qubits). Using DRoP, we consider many-body effects such as superradiance, subradiance, and bound states in continuum in multidimensional quantum networks. We find that, unlike one-dimensional linear chains, multidimensional quantum networks have superradiance in distinct levels, which we call multidimensional superradiance. Furthermore, we generalize the $N^{-3}$ scaling of subradiance in a linear chain to $d$-dimensional networks.

We demonstrate the coexistence of pseudospin- and valley-Hall-like edge states in a photonic crystal with $C_{3v}$ symmetry, which is composed of three interlacing triangular sublattices with the same lattice constants. By tuning the geometry of the sublattices, three complete photonic band gaps with nontrivial topology can be created, one of which is due to the band inversion associated with the pseudospin degree of freedom at the $\mathrm{\Gamma }$ point and the other two due to the gapping out of Dirac cones associated with the valley degree of freedom at the $K,K^{\prime }$ points. The system can support triband pseudospin- and valley-momentum locking edge states at properly designed domain-wall interfaces. Furthermore, to demonstrate the novel interplay of the two kinds of edge states in a single configuration, we design a four-channel system, where the unidirectional routing of electromagnetic waves against sharp bends between two routes can be selectively controlled by the pseudospin and valley degrees of freedom. Our work combines the pseudospin and valley degrees of freedom in a single configuration and may provide more flexibility in manipulating electromagnetic waves with promising potential for multiband and multifunctional applications.

Measurements in quantum theory exhibit incompatibility, i.e., they can fail to be jointly measurable. An intuitive way to represent the (in)compatibility relations among a set of measurements is via a hypergraph representing their joint measurability structure: its vertices represent measurements and its hyperedges represent (all and only) subsets of compatible measurements. Projective measurements in quantum theory realize (all and only) joint measurability structures that are graphs. On the other hand, general measurements represented by positive operator-valued measures (POVMs) can realize arbitrary joint measurability structures. Here we explore the scope of joint measurability structures realizable with qubit POVMs. We develop a technique that we term marginal surgery to obtain nontrivial joint measurability structures starting from a set of compatible measurements. We show explicit examples of marginal surgery on a special set of qubit POVMs to construct joint measurability structures such as $N$-cycle and $N$-Specker scenarios for any integer $N\ge 3$. We also show the realizability of various joint measurability structures with $N\in \{4,5,6\}$ vertices. In particular, we show that all possible joint measurability structures with $N=4$ vertices are realizable. We conjecture that all joint measurability structures are realizable with qubit POVMs. This contrasts with the unbounded dimension required in R. Kunjwal et al. [Phys. Rev. A 89, 052126 (2014)]. Our results also render this previous construction maximally efficient in terms of the required Hilbert space dimension. We also obtain a sufficient condition for the joint measurability of any set of binary qubit POVMs which powers many of our results and should be of independent interest.

Eukaryotic cells are able to sense chemical gradients in a wide range of environments. We show that, if a cell is exposed to a highly variable environment, it may gain chemotactic accuracy by expressing multiple receptor types with varying affinities for the same signal, as found commonly in chemotaxing cells like Dictyostelium. As environment uncertainty is increased, there is a transition between cells preferring a single receptor type and a mixture of types—hedging their bets against the possibility of an unfavorable environment. We predict the optimal receptor affinities given a particular environment. In chemotaxing, cells may also integrate their measurement over time. Surprisingly, time integration with multiple receptor types is qualitatively different from gradient sensing by a single type—cells may extract orders of magnitude more chemotactic information than expected by naive time integration. Our results show when cells should express multiple receptor types to chemotax, and how cells can efficiently interpret the data from these receptors.

High-pressure synthesized quasi-one-dimensional ${\mathrm{NaMn}}_2{\mathrm{O}}_4$ and ${\mathrm{Li}}_{0.92}{\mathrm{Mn}}_2{\mathrm{O}}_4$ are both antiferromagnetic insulators. Here their atomic and magnetic structures are investigated using neutron powder diffraction. The present crystal structural analyses of ${\mathrm{NaMn}}_2{\mathrm{O}}_4$ reveal that a ${\mathrm{Mn}}^{3+}/{\mathrm{Mn}}^{4+}$ charge-ordering state exists even at low temperature (down to 1.5 K). It is evident that one of the Mn sites shows a strongly distorted ${\mathrm{Mn}}^{3+}$ octahedron due to the Jahn-Teller effect. Above $T_N=35$ K, a two-dimensional short-range correlation is observed, as indicated by asymmetric diffuse scattering. Below $T_N$, two antiferromagnetic transitions are observed: (i) a commensurate long-range ${\mathrm{Mn}}^{3+}$ spin ordering below $T_{N1}=35$ K and (ii) an incommensurate ${\mathrm{Mn}}^{4+}$ spin ordering below $T_{N2}=11$ K. Surprisingly, the two antiferromagnetic orders are found to be independent of each other. The commensurate magnetic structure ($k_{\mathrm{C}}=0.5,0.5,0.5$) follows the magnetic anisotropy of the local easy axes of ${\mathrm{Mn}}^{3+}$, while the incommensurate ${\mathrm{Mn}}^{4+}$ one shows a spin-density-wave or a cycloidal order with $k_{\mathrm{IC}}=(0,0,0.216)$. For ${\mathrm{Li}}_{0.92}{\mathrm{Mn}}_2{\mathrm{O}}_4$, on the other hand, the absence of a long-range spin-ordered state is confirmed down to 1.5 K.

Size-disperse mixtures of noncohesive particles segregate, or demix, during flow. For spherical particles, mixture segregation can be predicted based on the relative particle diameters. However, most particle systems in industry and geophysics involve nonspherical particles. Accounting for the immense range of particle shapes introduces additional parameters. As a proxy for nonspherical particles in general, we perform discrete element method simulations of gravity-driven free-surface flows of bidisperse mixtures of mm-sized particles that vary widely in their size and shape (disks, rods, and spheres). Remarkably, the propensity to segregate, measured in terms of a segregation length scale that characterizes the segregation velocity of the two species, can be predicted based on only the volume ratio of the two particle species. The segregation length scale increases linearly with the log of the volume ratio, as it does for bidisperse mixtures of spherical particles, independent of particle shape.

We introduce a class of variational states to study ground-state properties and real-time dynamics in $(2+1)$-dimensional compact QED. These are based on complex Gaussian states which are made periodic to account for the compact nature of the $U\left(1\right)$ gauge field. Since the evaluation of expectation values involves infinite sums, we present an approximation scheme for the whole variational manifold. We calculate the ground-state energy density for lattice sizes up to $20\times 20$ and extrapolate to the thermodynamic limit for the whole coupling region. Additionally, we study the string tension both by fitting the potential between two static charges and by fitting the exponential decay of spatial Wilson loops. As the ansatz does not require a truncation in the local Hilbert spaces, we analyze truncation effects which are present in other approaches. The variational states are benchmarked against exact solutions known for the one plaquette case and exact diagonalization results for a ${\mathbb{Z}}_3$ lattice gauge theory. Using the time-dependent variational principle, we study real-time dynamics after various global quenches, e.g., the time evolution of a strongly confined electric field between two charges after a quench to the weak-coupling regime. Up to the points where finite-size effects start to play a role, we observe equilibrating behavior.