Metasurface-based acoustic wave-front manipulation with broad bandwidth and low transmission loss shows great significance in high-intensity applications such as ultrasonic therapy, acoustic tweezers, and haptics. By taking advantage of the helical-structured metamaterials and their concentrically layered arrangement, we present a systematic strategy to construct two-dimensional transmissive acoustic metasurfaces that possess matched impedance to the background medium and simple governing parameters. As a proof of concept, a concentrically layered circular metalens supporting conversion from spherical wave to plane wave is designed and experimentally demonstrated. It is capable of operating in more than one octave band with high transmission. This work could inspire more intriguing and flexible designs in three-dimensional wave control, which may enhance the practicality of acoustic metasurfaces.

Zirconia $\left({\mathrm{ZrO}}_2\right)$ has been well studied experimentally for decades, but still poses a severe challenge for computational approaches. We present thorough many-electron benchmark calculations within the random-phase approximation framework of the phase stabilities of the most common ${\mathrm{ZrO}}_2$ phases and assess the performance of various density functional theory (DFT) and beyond-DFT methods. We find that the commonly used DFT and hybrid functionals strongly overestimate both the energetic differences of the common phases and the stability of two metastable phases. The many-electron calculations offer a significantly improved description of the predicted bulk properties, especially of the bulk modulus $B_0$. On the DFT level, the van der Waals corrected meta-generalized-gradient approximation (SCAN-rVV10) provides much better agreement with the experimental values than other (semi)local and hybrid approaches.

The reduced dimensionality in two-dimensional materials leads to a wealth of unusual properties, which are currently explored for both fundamental and applied sciences. In order to study the crystal structure, edge states, the formation of defects and grain boundaries, or the impact of adsorbates, high-resolution microscopy techniques are indispensable. Here we report on the development of an electron holography (EH) transmission electron microscopy (TEM) technique, which facilitates high spatial resolution by an automatic correction of geometric aberrations. Distinguished features of EH beyond conventional TEM imaging are gap-free spatial information signal transfer and higher dose efficiency for certain spatial frequency bands as well as direct access to the projected electrostatic potential of the two-dimensional material. We demonstrate these features with the example of $h$-BN, for which we measure the electrostatic potential as a function of layer number down to the monolayer limit and obtain evidence for a systematic increase of the potential at the zig-zag edges.

We propose a mechanism to control the formation of stable obstructions in two-dimensional microchannels of variable sections taking advantage of the peculiar clustering property of active systems. Under the activation of the self-propulsion by external stimuli, the system behaves as a switch according to the following principle: by turning-on the self-propulsion the particles become active and even at very low densities stick to the walls and form growing layers eventually blocking the channel bottleneck, while the obstruction dissolves when the self-propulsion is turned off. We construct the phase diagram distinguishing clogged and open states in terms of density and bottleneck width. The study of the average clogging time, as a function of density and bottleneck width, reveals the marked efficiency of the active clogging that swiftly responds to the self-propulsion turning on. The resulting picture shows a profound difference with respect to the clogging obtained through the slow diffusive dynamics of attractive passive Brownian disks. This numerical work suggests a novel method to use particles with externally tunable self-propulsion to create or destroy plugs in microchannels.

Collective excitations, such as vortex-antivortex and dark solitons, are among the most fascinating effects of macroscopic quantum states. However, two-dimensional (2D) dark solitons are unstable and collapse into vortices due to snake instabilities. Making use of the optical bistability in exciton-polariton microcavities, we demonstrate that a pair of dark solitons can be formed in the wake of an obstacle in a polariton flow resonantly supported by a homogeneous laser beam. Unlike the purely dissipative case where the solitons are gray and spatially separate, here the two solitons are fully dark, rapidly align at a specific separation distance, and propagate parallel as long as the flow is in the bistable regime. Remarkably, the use of this regime allows us to relax the phase fixing constraints imposed by the resonant pumping and to circumvent the polariton decay. Our work opens very wide possibilities for studying new classes of phase-density defects which can form in driven-dissipative quantum fluids of light.

Quantum fluctuations are ubiquitous in physics. Ranging from conventional examples like the harmonic oscillator to intricate theories on the origin of the universe, they alter virtually all aspects of matter, including superconductivity, phase transitions, and nanoscale processes. As a rule of thumb, the smaller the object, the larger its impact. This poses a serious challenge to modern nanotechnology, which aims at total control via atom-by-atom engineered devices. In magnetic nanostructures, high stability of the magnetic signal is crucial when targeting realistic applications in information technology, e.g., miniaturized bits. Here we show that zero-point spin fluctuations play an important role in determining the fundamental magnetic exchange interactions that dictate the nature and stability of the magnetic state. Based on the fluctuation-dissipation theorem, we show that quantum fluctuations correctly account for the large overestimation of the interactions as obtained from conventional static first-principles frameworks, filling in an important gap between theory and experiment [Zhou et al., Nat. Phys. 6, 187 (2010); Khajetoorians et al., Nat. Phys. 8, 497 (2012)]. Our analysis further reveals that zero-point spin fluctuations tend to promote the noncollinearity and stability of chiral magnetic textures such as skyrmions, a counterintuitive quantum effect that inspires practical guidelines for designing disruptive nanodevices.

The domain wall problem in the axion solution of strong CP violation has condensed-matter-based analogy in nafen-distorted superfluid ${}^3\mathrm{He}$. The Kibble-Lazarides-Shafi (KLS) domain wall, which appears during the temperature of the early universe cooling down to QCD scale, attaches to the string defect that appeared in the first time symmetry breaking phase transition. Recent experiments in rotating superfluid ${}^3\mathrm{He}$ produced the network of the KLS string walls in a human controllable system. In this system, the half quantum vortices (HQVs) appear the first time symmetry breaking from a normal phase vacuum to a polar phase, while the KLS domain walls appear and attach on the HQVs in the phase transition from the polar phase to polar-distorted $B$-phase. Based on the method of relative homotopy group, the KLS string walls have turned out to be the descendants of HQVs of the polar phase. Here we further show the KLS string walls smoothly connect to spin solitons with a length scale around ${\xi }_D$ when the spin orbital coupling is taken into account. This means HQVs are one-dimensional (1D) nexuses, which connect the spin solitons and the KLS domain walls. This is because the subgroup $G={\pi }_1(S_S^1,{\tilde{R}}_2)$ of relative homotopy group describing the spin solitons is isomorphic to the group describing the half spin vortices—the textures of the spin degree of freedom of the KLS string wall. In the nafen-distorted ${}^3\mathrm{He}$ system, 1D nexus objects and the spin solitons with topological invariant $2/4$ have two different types of networks, which are named as pseudorandom lattices of inseparable and separable spin solitons. These two types of pseudorandom lattices correspond to two different representations of $G$. We discuss the condition under which the pseudorandom lattice model works. The equilibrium configuration and surface densities of free energies of pseudorandom lattices are calculated by numeric minimization. Based on the equilibrium spin textures, we calculate their transverse spin dynamic response of NMR, the resulted frequency shifts, and $\sqrt{\mathrm{\Omega }}$ scaling of the ratio intensity exactly coincide with the experimental measurements. We also discuss the mirror symmetry in the presence of the KLS domain wall and its explicit breaking. Our discussions and considerations can be applied to the composite defects in other condensed matter and cosmological systems.

Starting from several copies of bipartite noisy entangled states, we design a global and optimal local measurement-based protocol in one- and two-dimensional lattices by which any two or more prefix sites can be connected via entanglement. Production of bipartite as well as multipartite entangled states in a network is verified in a device-independent way through the violation of Bell inequalities with two settings per site and with continuous range of settings. We also note that if the parties refuse to perform local measurements, the entanglement distribution scheme fails. We obtain critical values of noise allowed in the initial state so that the resulting output state show nonlocal correlation in different networks with arbitrary number of connections. We report that by employing our method, it is possible to create a multipartite entangled state, violating Bell inequality and having a continuous range of settings, from bipartite states which do not violate Clauser-Horne-Shimony-Halt Bell inequalities in an one-dimensional lattice with the minimal coordination number being six. Such a feature of superadditivity in violation can also be observed in a triangular two-dimensional lattice but not in a square lattice.

The temperature dependence of the perpendicular upper critical field of superconducting granular aluminum films has been measured for samples approaching the metal-to-insulator transition. Analysis of the results shows a shift from an orbital to a Pauli-limited critical field, which we propose is made possible by electron mass renormalization. In that regime, the critical field transition becomes of the first order, as predicted by Fulde, Ferrel, Larkin, and Ovchinikov. The phase-coherence length ${\xi }_{\mathrm{phase}}$ and the superconducting gap $\mathrm{\Delta }$ obtained from the analysis are consistent with a Bardeen-Cooper-Schrieffer–Bose-Einstein condensate crossover region, which we propose is triggered by the Mott transition of the granular films.

The fundamental indication of the departure of quantum mechanics from the classical world is the so-called quantum coherence. Typically, we define it as the characteristic of systems which are in a superposition of states yielding interference patterns in certain kinds of experiments. In addition to its captivating philosophical implications, quantum coherence turned out to be a valuable tool in different areas, ranging from quantum information to biology, where it was used to describe several fundamental processes. Here, we go one step further to study how classical uncertainties in a mixture of similar states reduce quantum coherence in quantum scattering theory. To this end, we deal with different examples, all of them with roots in the widely studied Feynman's two-slit thought experiment. We finally propose an operational and intuitive definition of the concept of coherence length whose implications largely transcend the simplicity of the corresponding mathematical development, as it is demonstrated when applied to the analysis of some recent atomic and molecular processes.

The functional properties of transition metal perovskite oxides are known to result from a complex interplay of magnetism, polarization, strain, and stoichiometry. Here, we show that for materials with a cooperative Jahn-Teller distortion, such as ${\mathrm{LaMnO}}_3$ (LMO), the orbital order can also couple to the defect chemistry and induce novel material properties. At low temperatures, LMO exhibits a strong Jahn-Teller distortion that splits the $e_g$ orbitals of the high-spin ${\mathrm{Mn}}^{3+}$ ions and leads to alternating long, short, and intermediate Mn-O bonds. Our $\mathrm{DFT}+U$ calculations show that, as a result of this orbital order, the charge localization in LMO upon oxygen vacancy formation differs from other manganites, such as ${\mathrm{SrMnO}}_3$, where the two extra electrons reduce the two Mn sites adjacent to the vacancy. In LMO, relaxations around the defect depend on which type of Mn-O bond is broken, affecting the $d$-orbital energies and leading to asymmetric and hence polar excess-electron localization with respect to the vacancy. Moreover, we show that the Mn-O bond lengths, the orbital order, and consequently the charge localization and polarity are tunable via strain.

${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$ became one of the most investigated candidate materials for hosting an excitonic insulator ground state. Many studies describe the corresponding phase transition as a condensation of excitons breaking a continuous symmetry. This view got challenged recently pointing out the importance of the loss of two mirror symmetries at a structural phase transition that occurs together with the semiconductor—excitonic insulator transition. For such a scenario an unstable optical zone-center phonon at low energy is proposed to drive the transition. Here we report on the experimental observation of such a soft mode behavior using Raman spectroscopy. In addition we find a novel spectral feature, likely of electronic or joint electronic and phononic origin, that is clearly distinct from the lattice dynamics and that becomes dominant at $T_c$. This suggests a picture of joint structural and electronic orders driving the phase transition.

The basic mechanisms that lead to the creation of complex socioeconomic structures remain poorly understood. A combination of network science and game theory can help explain them by examining how simple networks interact to build complex entities, both when connections among individuals are exclusively guided by self-interest or when they result from a mixture of individual and collective motivations. Here we present a theoretical framework where individuals or human groups from different communities connect to each other only if they increase their own eigenvector centrality, a topological measure of wide applications in many different contexts that quantifies the importance of a node within the network. Our analytical and numerical results show that the emergence of interconnected networks is catalyzed by the self-interest of peripheral agents, who are penalized in the long run but transiently benefit from establishing links with nodes from other communities. Moreover, the interconnection process leads to a hierarchical, assortative, and very efficient structure where links across networks involve nodes of the same importance. These findings are robust to the introduction of moderate levels of collective-oriented behavior and compatible with the interconnection dynamics observed in real-world socioeconomic networks.

High-precision quasisimultaneous collinear/anticollinear laser spectroscopy was used to measure the $4s{}^2{\mathrm{S}}_{1/2}\to 4p{}^2{\mathrm{P}}_{1/2}$ (D1) and the $4s{}^2{\mathrm{S}}_{1/2}\to 4p{}^2{\mathrm{P}}_{3/2}$ (D2) as well as the three $3d\to 4p$ transitions in the naturally abundant ${\mathrm{Ca}}^{+}$ isotopes. Thereby, accuracies of approximately $100\mathrm{kHz}$ were achieved, which enabled an accurate determination of the field-shift ratio in the D2 and D1 lines through a King-plot analysis. Our value of $F_{\mathrm{D}2}/F_{\mathrm{D}1}=1.0010\left(23\right)$ is in excellent agreement with current theoretical predictions, while former experimental results from ion-trap measurements showed a $3\sigma$ to $6\sigma$ deviation from theory introducing the so-called ${\mathrm{Ca}}^{+}$ field-shift puzzle. A successful check of consistency was made by combining the measurements of the D1 and D2 lines with those of the $3d\to 4p$ transitions and precise literature values for the $4s\to 3d$ and $3d{}^2{\mathrm{D}}_{3/2}\to 3d{}^2{\mathrm{D}}_{5/2}$ transitions. Failing for the previous ion-trap measurements, the check of consistency strongly indicates that our results constitute a solution to this puzzle, supporting the results of atomic theory.

Entanglement in high-dimensional quantum systems, where one or more degrees of freedom of light are involved, offers increased information capacities and enables new quantum protocols. Here, we demonstrate a functional source of high-dimensional, noise-resilient hyperentangled states encoded in time-frequency and vector vortex structured modes, which in turn carry single-particle entanglement between polarization and orbital angular momentum. Pairing nonlinearity-engineered parametric downconversion in an interferometric scheme with spin-to-orbital-angular-momentum conversion, we generate highly entangled photon pairs at telecom wavelength that we characterize via two-photon interference and quantum state tomography, achieving near-unity visibilities and fidelities. While hyperentanglement has been demonstrated before in photonic qubits, here we present a rich entanglement structure involving spectrally and spatially structured light, where three different forms of entanglement coexist in the same biphoton state.

Simple plasmonic multilayers are found to perfectly trap electromagnetic waves around specific incidence directions, regardless of the sample thickness. Such a counterintuitive feature is acquired via increasing unboundedly the effective path length of the ray into the film with proper tilt of the optical axis. The obtained optimal setups support wide-band operation while being substantially robust with respect to fabrication defects. Accordingly, the reported designs may exceptionally serve a range of objectives, from photovoltaic conversion and sensing to light modulation and detection, as parts of photonic devices requiring high efficiency combined with compact packaging.

Photodissociation dynamics of the NH molecule excited from the triplet ground $\mathrm{X}{}^3{\mathrm{\Sigma }}^{-}$ to $2{}^3\mathrm{\Pi }$ state by resonant intense VUV pulses have been comprehensively investigated by the recently proposed theoretical treatment involving the electron-rotation coupling. The rotational dynamics are described by the Wigner $\mathrm{D}$ functions, and the electronic transition operators are refined with respect to the changes of the projection of the total orbital angular momentum onto the molecular axis $\mathrm{\Delta }\mathrm{\Lambda }$. The kinetic energy release (KER) spectra and the angular distribution of photofragments are obtained by averaging over nine degenerate initial angular states, computed separately. We have shown that the KER spectra are not very sensitive to the electron-rotation coupling for different pulse intensity and energy detuning from the resonance. The angular distribution of the photofragments, on the contrary, is strongly affected by the electron-rotation coupling at the small angles between the molecular axis and laser polarization. The influence of the electron-rotational coupling shows different trends for positive and negative detunings from the resonance at variation of the pulse intensity, which is explained by sufficient changes of the rotational wave packet dynamics caused by different phases of the initial rotational states.

The classical electromagnetic self-force on an arbitrary time-dependent electric or magnetic dipole moving with constant velocity in vacuum, and in a medium, is considered. Of course, in vacuum there is no net force on such a particle. Rather, because of loss of mass by the particle due to radiation, the self-force precisely cancels this inertial effect, and thus the spectral distribution of the energy radiated by dipole radiation is deduced without any consideration of radiation fields or of radiation reaction, in both the nonrelativistic and relativistic regimes. If the particle is moving in a homogeneous medium faster than the speed of light in the medium, Vavilov-Čerenkov radiation results. This is derived for the different polarization states, in agreement with the earlier results of Frank. The friction experienced by a point (time-independent) dipole moving parallel to an imperfectly conducting surface is examined. Finally, the relativistic quantum/thermal Einstein-Hopf effect is rederived. We obtain a closed form for the spectral distribution of the force and demonstrate that, even if the atom and the blackbody background have independent temperatures, the force is indeed a drag when the imaginary part of the polarizability is proportional to a power of the frequency. The unifying theme of these investigations is that friction on an atom requires a dissipative mechanism, be it through radiation or resistivity in the environment.

In this paper, we suggest an extension of the Rosenzweig-Porter (RP) model, the LN-RP model, in which the off-diagonal matrix elements have a wide, log-normal distribution. We argue that this model is more suitable to describe a generic many-body localization problem. In contrast to RP model, in LN-RP model, a fragile weakly ergodic phase appears that is characterized by broken basis-rotation symmetry which the fully ergodic phase, also present in this model, strictly respects in the thermodynamic limit. Therefore, in addition to the localization and ergodic transitions in LN-RP model, there exists also the transition between the two ergodic phases (FWE transition). We suggest new criteria of stability of the nonergodic phases that give the points of localization and ergodic transitions and prove that the Anderson localization transition in LN-RP model involves a jump in the fractal dimension of the egenfunction support set. We also formulate the criterion of FWE transition and obtain the full phase diagram of the model. We show that truncation of the log-normal tail shrinks the region of weakly ergodic phase and restores the multifractal and the fully ergodic phases.