We demonstrate theoretically that acoustic forces acting on inhomogeneous fluids can be used to pattern and manipulate solute concentration fields into spatiotemporally controllable configurations stabilized against gravity. A theoretical framework describing the dynamics of concentration fields that weakly perturb the fluid density and speed of sound is presented and applied to study manipulation of concentration fields in rectangular-channel acoustic eigenmodes and in Bessel-function acoustic vortices. In the first example, methods to obtain horizontal and vertical multilayer stratification of the concentration field at the end of a flow-through channel are presented. In the second example, we demonstrate acoustic tweezing and spatiotemporal manipulation of a local high-concentration region in a lower-concentration medium, thereby extending the realm of acoustic tweezing to include concentration fields.

In the present study, we demonstrate ferroelectricity in stuffed aluminate sodalites $({\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{)}_8[{\mathrm{AlO}}_2{]}_{12}({\mathrm{WO}}_4{)}_2$ ($x\le 0.2$) (${\mathrm{C}}_{1-x}{\mathrm{S}}_x\mathrm{AW}$). Pyroelectric measurements clarify switchable spontaneous polarization in polycrystalline ${\mathrm{C}}_{1-x}{\mathrm{S}}_x\mathrm{AW}$, whose polarization values are on the order of ${10}^{-2}\mu \mathrm{C}/{\mathrm{cm}}^2$ at room temperature. A weak anomaly in the dielectric permittivity at temperatures near the ferroelectric transition temperature suggests improper ferroelectricity of ${\mathrm{C}}_{1-x}{\mathrm{S}}_x\mathrm{AW}$ for all investigated values of $x$. A comprehensive study involving synchrotron x-ray powder diffraction measurements, molecular dynamics simulations, and first-principles calculations clarifies that the ferroelectric phase transition of ${\mathrm{C}}_{1-x}{\mathrm{S}}_x\mathrm{AW}$ is driven by the freezing of the fluctuations of ${\mathrm{WO}}_4$ tetrahedra in the voids of an $[{\mathrm{AlO}}_2{{]}_{12}}^{12-}$ framework. The voltage response and electromechanical coupling factor of ${\mathrm{C}}_{1-x}{\mathrm{S}}_x\mathrm{AW}$ estimated from the present results indicate that this material exhibits excellent performance as a pyroelectric energy harvester, suggesting that aluminate sodalites exhibit great promise as a class of materials for highly efficient energy-harvesting devices.

Wavefront aberrations are identified as a major limitation in quantum sensors. They are today the main contribution in the uncertainty budget of the best cold-atom interferometers based on two-photon laser beam splitters and constitute an important limit for their long-term stability, impeding these instruments from reaching their full potential. Moreover, they will also remain a major obstacle in future experiments based on large-momentum beam splitters. In this article, we tackle this issue by using a deformable mirror to control actively the laser wavefronts in atom interferometry. In particular, we demonstrate in an experimental proof of principle the efficient correction of wavefront aberrations in an atomic gravimeter.

Voltage- and temperature-tuned ferromagnetic hysteresis is investigated by a superconducting quantum-interference device and Kerr magnetometry in a thin-film heterostructure of a perpendicular anisotropic Co/Pd ferromagnet exchange coupled to the magnetoelectric antiferromagnet ${\mathrm{Cr}}_2{\mathrm{O}}_3$. An abrupt disappearance of exchange bias with a simultaneous more than twofold increase in coercivity is observed and interpreted as a competition between the effective anisotropy of ${\mathrm{Cr}}_2{\mathrm{O}}_3$ and the exchange-coupling energy between boundary magnetization and the adjacent ferromagnet. The effective anisotropy energy is given by the intrinsic anisotropy energy density multiplied by the effective volume separated from the bulk through a horizontal antiferromagnetic domain boundary. Kerr measurements show that the anisotropy of the interfacial ${\mathrm{Cr}}_2{\mathrm{O}}_3$ can be tuned isothermally and in the absence of an external magnetic field by application of an electric field. A generalized Meiklejohn-Bean model accounts for the change in exchange bias and coercivity as well as the asymmetric evolution of the hysteresis loop. In support of this model, the reversal of the boundary magnetization is experimentally confirmed as a contribution to the magnetic hysteresis loop.

Using a kinetic-equation approach, we study the dynamics of electrons and phonons in current-carrying superconducting nanostrips after the absorption of a single photon of the near-infrared or optical range. We find that the larger the $C_e/C_{\mathrm{ph}}{|}_{T_c}$ ratio (where $T_c$ is the critical temperature of a superconductor and $C_e$ and $C_{\mathrm{ph}}$ are specific heat capacities of electrons and phonons, respectively), the larger the portion of the photon’s energy goes to electrons. The electrons become more strongly heated and hence can thermalize faster during the initial stage of hot-spot formation. The thermalization time ${\tau }_{\mathrm{th}}$ can be less than 1 ps for superconductors with $C_e/C_{\mathrm{ph}}{|}_{T_c}\gg 1$ and a small diffusion coefficient of $D\simeq 0.5{\mathrm{cm}}^2/\mathrm{s}$ when thermalization occurs, mainly due to electron-phonon and phonon-electron scattering in a relatively small volume of approximately ${\xi }^{2}d$ ($\xi$ is a superconducting coherence length, while $d<\xi$ is a thickness of the strip). For longer time spans, due to diffusion of hot electrons’ effective temperature inside the hot spot decreases, the size of the hot spot increases, the superconducting state becomes unstable, and the normal domain spreads in the strip at a current larger than the so-called detection current. We find the dependence of the detection current on the photon’s energy, the location of its absorption in the strip, the width of the strip, and the magnetic field, and we compare this dependence with existing experiments. Our results demonstrate that materials with $C_e/C_{\mathrm{ph}}{|}_{T_c}\ll 1$ are bad candidates for single-photon detectors due to a small transfer of the photon’s energy to electronic system and a large ${\tau }_{\mathrm{th}}$. We also predict that even a several-micron-wide dirty superconducting bridge is able to detect a single near-infrared or optical photon if its critical current exceeds 70% of the depairing current and $C_e/C_{\mathrm{ph}}{|}_{T_c}\gtrsim 1$.

Neural networks have proven effective for solving many difficult computational problems, yet implementing complex neural networks in software is computationally expensive. To explore the limits of information processing, it is necessary to implement new hardware platforms with large numbers of neurons, each with a large number of connections to other neurons. Here we propose a hybrid semiconductor-superconductor hardware platform for the implementation of neural networks and large-scale neuromorphic computing. The platform combines semiconducting few-photon light-emitting diodes with superconducting-nanowire single-photon detectors to behave as spiking neurons. These processing units are connected via a network of optical waveguides, and variable weights of connection can be implemented using several approaches. The use of light as a signaling mechanism overcomes fanout and parasitic constraints on electrical signals while simultaneously introducing physical degrees of freedom which can be employed for computation. The use of supercurrents achieves the low power density ($1\mathrm{mW}/{\mathrm{cm}}^2$ at 20-MHz firing rate) necessary to scale to systems with enormous entropy. Estimates comparing the proposed hardware platform to a human brain show that with the same number of neurons ($10^{11}$) and 700 independent connections per neuron, the hardware presented here may achieve an order of magnitude improvement in synaptic events per second per watt.

Transition-metal dichalcogenides (TMDs) such as ${\mathrm{MoS}}_2$, ${\mathrm{MoSe}}_2$, and ${\mathrm{WSe}}_2$ have emerged as promising two-dimensional semiconductors. Many anticipated applications of these materials require both $p$-type and $n$-type TMDs with long-term doping stability. Here, we report on the synthesis of substitutionally doped ${\mathrm{WSe}}_2$ crystals using Nb and Re as $p$- and $n$-type dopants, respectively. Hall coefficient and gate-dependent transport measurements reveal drastically different doping properties between nominally 0.5% Nb- and 0.5% Re-doped ${\mathrm{WSe}}_2$. While 0.5% Nb-doped ${\mathrm{WSe}}_2$ (${\mathrm{WSe}}_2:\mathrm{Nb}$) is degenerately hole doped with a nearly temperature-independent carrier density of approximately ${10}^{19}{\mathrm{cm}}^{-3}$, electrons in 0.5% Re-doped ${\mathrm{WSe}}_2$ (${\mathrm{WSe}}_2:\mathrm{Re}$) are largely trapped in localized states below the mobility edge and exhibit thermally activated behavior. Charge transport in both ${\mathrm{WSe}}_2:\mathrm{Nb}$ and ${\mathrm{WSe}}_2:\mathrm{Re}$ is found to be limited by Coulomb scattering from ionized impurities. Furthermore, we fabricate vertical van der Waals–junction diodes consisting of multilayers of ${\mathrm{WSe}}_2:\mathrm{Nb}$ and ${\mathrm{WSe}}_2:\mathrm{Re}$. Finally, we demonstrate reverse rectifying behavior as a direct proof of band-to-band tunneling in our ${\mathrm{WSe}}_2:\mathrm{Nb}/{\mathrm{WSe}}_2:\mathrm{Re}$ diodes.

In this article, we devise and demonstrate experimentally a polarization-dependent diffractive bifocal vortex lens operating via the Pancharatnam-Berry phase. The interaction between the incident beam and the bifocal vortex lens establishes nonseparable spin and orbital angular-momentum photon states. The components of the nonseparable state associated with different couplings of spin and orbital angular momentum can be sorted by the bifocality of the lens. A theoretical model of the device is developed using Fresnel’s diffraction. The device is simply, efficiently, and economically realized from the optical setup using the underlying physics of Pancharatnam-Berry-phase polarization holography. The measured transmittance and diffraction efficiency of the fabricated device is high—up to 90% and 91%, respectively. Various applications of the polarization bifocal vortex lens in the field of orbital angular-momentum lasing and optical manipulation are discussed. Thus, the bifocal vortex lens can have significant impact on classical and quantum optics, as well as theoretical physics.

We perform in situ two-cycle thermal-annealing studies for a transferred CVD-grown monolayer ${\mathrm{MoS}}_2$ on a ${\mathrm{SiO}}_2/\mathrm{Si}$ substrate, using spatially resolved micro-Raman and photoluminescence spectroscopy. The evolution in film morphology and film-substrate bonding is continuously monitored by Raman spectroscopy. After the thermal cycling and being annealed at $305°\mathrm{C}$ twice, the film morphology and film-substrate bonding are significantly modified, which together with the removal of polymer residues causes major changes in the strain and doping distribution over the film, and thus the optical properties. Before annealing, the strain associated with ripples in the transferred film dominates the spatial distributions of the photoluminescence peak position and intensity over the film; after annealing, the variation in film-substrate bonding, affecting both strain and doping, becomes the leading factor. This work reveals that the film-substrate bonding, and thus the strain and doping, is nonstationary under thermal stress, which is important for understanding the substrate effects on the optical and transport properties of the 2D material and their impact on device applications.

Tuning of the electronic state of correlated materials is key to their eventual use in advanced electronics and photonics. The prototypical correlated oxide (${\mathrm{VO}}_2$) is insulating at room temperature and transforms to a metallic state when heated to $67°\mathrm{C}$ (340 K). We report the emergence of a metallic state that is preserved down to 1.8 K by annealing thin films of ${\mathrm{VO}}_2$ at an ultralow oxygen partial pressure ($P_{{\mathrm{O}}_2}\sim {10}^{-24}\mathrm{atm}$). The films can be reverted back to their original state by annealing in oxygen, and this process can be iterated multiple times. The metallic phase created by oxygen deficiency has a tetragonal rutile structure and contains a large number of oxygen vacancies far beyond the solubility at equilibrium (greater than approximately 50 times). The oxygen starvation reduces the oxidation state of vanadium from ${\mathrm{V}}^{4+}$ to ${\mathrm{V}}^{3+}$ and leads to the metallization. The extent of resistance reduction (concurrent with tuning of optical properties) can be controlled by the time-temperature envelope of the annealing conditions since the process is diffusionally driven. This experimental platform, which can extensively tune oxygen vacancies in correlated oxides, provides an approach to study emergent phases and defect-mediated adaptive electronic and structural phase boundary crossovers.

Density-functional calculations are carried out to understand and tailor the electrochemical profile—diffusivity, band gap, and open-circuit voltage—of transition-metal-doped olivine phosphate: ${\mathrm{LiFe}}_{1-x}{M}_x{\mathrm{PO}}_4$ ($M=\mathrm{V}$, Cr, Mn, Co, and Ni). Diffusion and, hence, the ionic conductivity is studied by calculating the activation barrier $V_{\text{act}}$ experienced by the diffusing ${\mathrm{Li}}^{+}$ ion. We show that the effect of dopants on diffusion is both site dependent and short ranged, and thereby it paves ways for microscopic control of ionic conductivity via selective dopants in olivine phosphates. Dopants with lower-valence electrons (LVEs) compared to Fe repel the ${\mathrm{Li}}^{+}$ ion to facilitate its outward diffusion, whereas higher-valence-electron (HVE) dopants attract the ${\mathrm{Li}}^{+}$ ion to facilitate the inward diffusion. From the electronic structure calculation, we establish that irrespective of the dopant $M$, except Mn, the band gap is reduced since the $Md$ states always lie within the pure band gap. Atomically localized $d$ states of HVE dopants lie above the Fermi energy and that of LVE lie below it. Half-filled $\mathrm{Mn}d$ states undergo a large spin-exchange split to bury the dopant states in the valence and conduction bands of the pristine system, and, in turn, the band gap remains unchanged in ${\mathrm{LiFe}}_{1-x}{\mathrm{Mn}}_x{\mathrm{PO}}_4$. Baring Mn, the open-circuit voltage increases with HVE dopants and decreases with LVE dopants.

The modes of silicon meta-atoms are investigated, motivated by their use as building blocks of Huygens metasurfaces. A model based on these modes is presented, giving a clear physical explanation of all features in the extinction spectrum. Counterintuitively, this model can show negative contributions to extinction, which are shown to arise from the interference between nonorthogonal modes. The direct and interference contributions to extinction are determined, showing that conservation of energy is preserved. The Huygens condition of matched electric- and magnetic-dipole moments leads to strong forward scattering and suppressed backscattering. It is shown that higher-order modes with appropriate symmetry generalize this condition, leading to multiple bands of directional scattering. The presented results are obtained using a robust approach to find the modes of nanophotonic scatterers, commonly referred to as quasinormal modes. By utilizing an integral formulation of Maxwell’s equations, this work avoids the problem of normalizing diverging far fields, which other approaches require. The model and presented results are implemented in open-source code.

Here, we investigate the mechanism of a random-lasing-based sensor which shows $p\text{H}$ sensitivity exceeding by 2 orders of magnitude that of a conventional fluorescence sensor. We explain the sensing mechanism as related to gain modifications and lasing-threshold nonlinearities. A dispersive diffusive lasing theory matches the experimental results well, and it allows us to predict the optimal sensing conditions and a maximal sensitivity as large as 200 times that of an identical fluorescence-based sensor. The lack of complex alignment and the high sensitivity make this mechanism promising for future biosensing applications.

We perform a first-principles study of the Gilbert damping parameter ($\alpha$) in normal-metal/MgO-cap/ferromagnet/MgO-barrier/ferromagnetic magnetic tunnel junctions. The damping is enhanced by interface spin pumping, which can be parametrized by the spin-mixing conductance ($G^{\uparrow \downarrow }$). The calculated dependence of Gilbert damping on the thickness of the MgO capping layer is consistent with experiment and indicates that the decreases in $\alpha$ with increasing thickness of the MgO capping layer is caused by suppression of spin pumping. Smaller $\alpha$ can be achieved by using a clean interface and alloys. For a thick MgO capping layer, the imaginary part of the spin-mixing conductance nearly equals the real part, and the large imaginary mixing conductance implies that the change in the frequency of ferromagnetic resonance can be observed experimentally. The normal-metal cap significantly affects the Gilbert damping.

Based on the impedance-matching condition, we propose a few configurations that can greatly enhance radiation power transfer from the emitter to the photovoltaic (PV) cell for a near-field-based thermophotovoltaic system. In addition to the emitter and the PV cell, these configurations involve the use of additional materials that support resonant modes, such as a metallic material whose dielectric function can be described by a Drude model, or a dielectric material whose dielectric function can be approximated by a Lorentz oscillator model. We show that, by coating the PV cell on both the front and back sides with Lorentz materials, the transferred power can be 2.5 times larger than that without any decorations. When Drude metals are included in the configuration, the optimal transferred power can be 3 times larger than the system without additional materials. We find that the key to enhancing transmitted power is to place a thin layer of Drude or Lorentz material on the front side (facing the emitter) of the PV cell.

The temperature dependences of photoluminescence (PL) decay rates and the PL spectrum of a thermally activated delayed-fluorescence emitter, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), are investigated. It is found that not only the lowest singlet ($S_1$) and triplet ($T_1$) excited states but also an additional triplet excited state ($T_n$) lying between $S_1$ and $T_1$ play an important role in the exciton decay process, particularly around 100 K. At around this temperature, some of the triplet excitons are thermally activated into $T_n$ but not up to $S_1$, and they then decay into the ground state ($S_0$) with phosphorescence emission. Therefore, two kinds of phosphorescence, originating from $T_n$ and $T_1$, are observed. The temperature dependence of the PL decay rates of 4CzIPN can be explained by a four-level model consisting of $S_1$, $T_1$, $T_n$, and $S_0$, and its energy gaps between $T_n$ and $T_1$ and between $S_1$ and $T_1$ are determined to be $45\pm 5\mathrm{meV}$ and $135\pm 10\mathrm{meV}$, respectively.

Inspired by moth eyes, nature’s most powerful antireflex, we present a subwavelength SiC micropyramid design, which operates in the reststrahlen band of SiC, namely, the spectral band of strong phonon-photon coupling in the SiC material. While within this band, SiC repels electromagnetic waves, we observe here a broad low-reflectivity window with unique attributes, with distinct characteristics different from typical dielectric moth-eye-like structures. To be specific, while the latter systems are entirely symmetric, the reflection response of our SiC micropyramid system can be highly asymmetric. In particular, the SiC micropyramid system can be near reflectionless for light impinging from the tip side of the micropyramids and can exhibit more than 90% reflection for light impinging from the base side of the micropyramids, over a broad wavelength range in the SiC reststrahlen band. This strongly asymmetric reflection response emanates from the cascaded coupling of vortexlike cavity modes at each of the SiC blocks comprising the micropyramids and translates into a strongly unidirectional absorber response. We discuss how, by virtue of Kirchhoff’s law, this strongly unidirectional superabsorber behavior implies a strongly unidirectional emission profile that is important for one-way infrared sources and passive-radiative-cooling systems.

Nanofocus x-ray diffraction is used to investigate the structure and local strain field of an isolated $(\mathrm{In},\mathrm{Ga})\mathrm{N}/\mathrm{GaN}$ core-shell microrod. Because the high spatial resolution of the x-ray beam is only $80\times 90{\mathrm{nm}}^2$, we are able to investigate several distinct volumes on one individual side facet. Here, we find a drastic increase in thickness of the outer GaN shell along the rod height. Additionally, we performed high-angle annular dark-field scanning-transmission-electron-microscopy measurements on several rods from the same sample showing that (In,Ga)N double-quantum-well and GaN barrier thicknesses also increase strongly along the height. Moreover, plastic relaxation is observed in the top part of the rod. Based on the experimentally obtained structural parameters, we simulate the strain-induced deformation using the finite-element method, which serves as the input for subsequent kinematic scattering simulations. The simulations reveal a significant increase of elastic in-plane relaxation along the rod height. However, at a certain height, the occurrence of plastic relaxation yields a decrease of the elastic strain. Because of the experimentally obtained structural input for the finite-element simulations, we can exclude unknown structural influences on the strain distribution, and we are able to translate the elastic relaxation into an indium concentration which increases by a factor of 4 from the bottom to the height where plastic relaxation occurs.

We examine the possible role of stress-driven surface instability in the failure of electric interconnects found in large-scale integrated circuits. While electromigration is commonly known as the main reason behind interconnect failure, the complex interplay of electromigration-induced mass transport and stress-induced transport has also been studied extensively since the discovery of the Blech effect due to its importance in integrated-circuit design. However, the role of the dielectric medium confining the interconnect has not been properly included in previous analysis of this phenomenon. Here, we examine the classic ATG instability in the presence of dielectric confinement. We propose that thermal stress and surface transport, typically active in all metal interconnects, may trigger a surface instability at the metal-dielectric interface. In particular, we show that there exists a critical thermal stress level below which the stress-driven surface instability cannot be responsible for the failure of interconnects of any length. However, for an interconnect confined by soft low-$k$ dielectric materials, thermal stresses can still be large enough that such stress-driven instability may break the metal conductor even if its length is below the Blech limit.

Single-photon sources are an integral part of various quantum technologies, and solid-state quantum emitters at room temperature appear to be a promising implementation. We couple the fluorescence of individual silicon-vacancy centers in nanodiamonds to a tunable optical microcavity to demonstrate a single-photon source with high efficiency, increased emission rate, and improved spectral purity compared to the intrinsic emitter properties. We use a fiber-based microcavity with a mode volume as small as $3.4{\lambda }^3$ and a quality factor of $1.9\times 10^4$ and observe an effective Purcell factor of up to 9.2. Furthermore, we study modifications of the internal rate dynamics and propose a rate model that closely agrees with the measurements. We observe lifetime changes of up to 31%, limited by the finite quantum efficiency of the emitters studied here. With improved materials, our achieved parameters predict single-photon rates beyond 1 GHz.