The mixed spin chain compound, $\mathrm{Li}\mathrm{Cu}{\mathrm{Fe}}_2({\mathrm{VO}}_4{)}_3$, reaches a magnetically ordered state in two steps at $T_{\mathrm{N}2}=9.8\mathrm{K}$ and $T_{\mathrm{N}1}=8.2\mathrm{K}$ detected in magnetic-susceptibility χ and specific-heat $C_p$ measurements. Dielectric permittivity ε evidences these transitions by a sharp peak at $T_{\mathrm{N}2}$ and a steplike anomaly at $T_{\mathrm{N}1}$, both easily suppressed by an external magnetic field. These features in permittivity are preceded by a frequency-dependent relaxation-type anomaly at $T^{\ast }$ insensitive to the magnetic field. Mössbauer spectroscopy reveals a wide distribution of a hyperfine field between $T_{\mathrm{N}2}$ and $T_{\mathrm{N}1}$ while the first-principles calculations provide values of magnetic-exchange-interaction parameters. For $\mathrm{Cu}$, the orbital moment aligned in the same direction as the spin moment is substantial. This large orbital moment is important in driving the polarization through an inverse Dzyaloshinksii-Moriya interaction in a situation where the spatial symmetry gets broken in the magnetic structure. The data obtained suggest the spin-order-induced ferroelectricity is inherent for improper multiferroics.

We report very efficient spin-current generation by the spin Hall effect in the alloy ${\mathrm{Au}}_{0.25}{\mathrm{Pt}}_{0.75}$, which, as determined by two different direct spin-orbit torque measurements, exhibits a giant internal spin Hall ratio of ≥0.58 (antidamping spin-orbit torque efficiency of approximately 0.35 in bilayers with $\mathrm{Co}$), a relatively low resistivity of approximately 83 µΩ cm, an exceptionally large spin Hall conductivity of $\ge 7.0\times {10}^5{\mathrm{\Omega }}^{-1}{\mathrm{m}}^{-1}$, and a spin diffusion length of 1.7 nm. This work establishes ${\mathrm{Au}}_{0.25}{\mathrm{Pt}}_{0.75}$ as a milestone spin-current generator that provides greater energy efficiency than that yet obtained with other heavy metals or with the topological insulators ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and $(\mathrm{Bi},\mathrm{Se}{)}_2{\mathrm{Te}}_3$. Our findings should advance spin-orbit torque-based fundamental research and benefit the development of fast, efficient spin-orbit torque-driven magnetic memories, skyrmion and chiral domain wall devices, and microwave and terahertz emitters.

A key ingredient for mechanical information processing with resonator networks is the possibility to tune the relevant mode frequencies independently and to operate the resonators in the strong coupling regime. We investigate a proof-of-principle realization of such a network based on two coupled highly tensile-stressed silicon nitride nanostring resonators. We demonstrate that the fundamental mode frequencies of both nanostrings can be tuned individually by a strong drive tone resonant with one of their higher-harmonic modes. Using this frequency tuning concept, we investigate the coherent dynamics of the two strongly coupled nanostring resonators acting as an effective classical two-level system. We show classical Rabi oscillations as well as Landau-Zener dynamics, enabling the controlled spacial transfer of mechanical excitation.

Scalable quantum computing is based on realizable accurate quantum gates. For neutral atoms, it is an outstanding challenge to design a high-fidelity two-qubit entangling gate without resorting to difficult techniques such as shaping laser pulses or cooling atoms to motional ground states. By using spin echo to suppress the blockade error, we propose an easily realizable controlled-phase Rydberg quantum gate of high intrinsic fidelity. In the context of spin echo, we show that the fundamental blockade error of the traditional Rydberg gate, of the order of $ϵ\sim {10}^{-3}$, actually results from two “clockwise” rotations of Rabi frequencies ${\overline{\mathrm{\Omega }}}_{\pm }=V\pm \sqrt{V^2+{\mathrm{\Omega }}^2}$. In our “echo” sequence, such an error can be suppressed to the order of ${ϵ}^2$ by adding two “anticlockwise” rotations with frequencies $-{\overline{\mathrm{\Omega }}}_{\pm }$. With the blockade error effectively removed, the error caused by Rydberg-state decay becomes the final fundamental limit to the gate accuracy, which, in principle, can be reduced beyond the level of ${10}^{-5}$. Furthermore, due to the small population involved in the “echo” process, the spin-echo gate is robust against the variation of Rydberg blockade caused by the drift of the qubits, so that it can still be much more accurate than that of a traditional Rydberg gate even for qubits cooled only to the submillikelvin regime.

Pure spin current is of central importance in spintronics. Here, we propose a two-dimensional (2D) spin-battery system that delivers pure spin current without an accompanying charge current to the outside world at zero bias. The principle of the spin battery is rooted in the photogalvanic effect (PGE) and the system has good operational stability against structural perturbation, photon energy, and other material characteristics. The principle of the device is numerically implemented in the 2D material phosphorene as an example, and first-principles calculations give excellent qualitative agreement with the physics of the PGE. The 2D spin battery is interesting as it is both a device that generates pure spin currents and also an energy source that harvests photons. Given the versatile operational space, the spin battery should be experimentally feasible.

Ruthenium dioxide ($\mathrm{Ru}{\mathrm{O}}_2$) is an important metal widely used in nanoelectronic devices. It plays indispensable roles in applications such as catalysts and supercapacitors. A good understanding of the origin of the flicker or $1/f$ noise in $\mathrm{Ru}{\mathrm{O}}_2$ will advance the design and efficiency of these applications. We demonstrate in a series of sputtered $\mathrm{Ru}{\mathrm{O}}_2$ polycrystalline films that the $1/f$ noise originates from fluctuating oxygen vacancies which act as dynamical structural defects, i.e., moving scattering centers. Reducing the number of oxygen vacancies by adjusting thermal annealing conditions significantly reduces the noise magnitude γ, the Hooge parameter. We quantify the activation energy distribution function, g(E), and calculate the oxygen vacancy density, $n_{\mathrm{TLS}}$, from the measured γ value. We show that g(E) can be explicitly expressed in terms of γ(T) and the electronic parameters of the metal, where T denotes temperature. The inferred $n_{\mathrm{TLS}}$ value is in line with the oxygen content determined from the x-ray photoelectron spectroscopy studies.

We describe a fast method for estimating flow through a porous medium with a heterogeneous permeability distribution. The main application is to contaminant transport in aquifers and recovery of oil by waterflooding, where such geological heterogeneities can result in regions of bypassed contaminants or oil. The extent of this bypassing is normally assessed by a numerical flow simulation that can take many hours of computer time. Ideally the impact of uncertainty in the geological description is then evaluated by the performing of many such simulations using different realizations of the permeability distribution. Obviously, a proper Monte Carlo evaluation may be impossible when the flow simulations are so computationally intensive. Consequently, methods from statistical mechanics, such as percolation theory and random walkers (such as diffusion-limited aggregation), have been proposed; however, these methods are limited to geological heterogeneities where the correlation lengths are smaller than the system size or to continuous permeability distributions. Here we describe a growth model that can be used to estimate the breakthrough time of the water (and hence the sweep efficiency) in most types of geologically heterogeneous rocks. We show how the model gives good estimates of the breakthrough time of water at the production well in a fraction of the time needed to perform a full flow simulation.

We investigate here the asymmetric domain-wall conductivity (DWC) of neighboring domain walls (DWs) in z-cut periodically poled $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ single crystals. Asymmetric domain-wall conductivity is observed by scanning-probe-microscopy-based methods and the DW inclination α is measured by Cherenkov second-harmonic-generation microscopy. A generalizable dipole-tunneling current model is proposed to elucidate the phenomenon, which also accounts for DW roughness. Our research provides a valuable perspective on how to influence DWC on the microscopic and macroscopic length scales, and the required nanoscopic insights.

Blasting with explosives and crushing with mills are two major processes for extracting ore minerals. Longstanding problems with these processes are “fines” production in blasting and the related energy consumption of mills. Here, we demonstrate, using numerical simulations and comparison with experiments, that both problems emerge from two universal mechanisms: unstable tensile-crack propagation and compressive impact crushing. These lead to a universal mass-passing-fraction function in sieving. Crushing is limited to, and produces almost all the fines, and thereby inherently consumes a lot of fracture energy. Tensile cracks also produce fines, but the majority of the mass is confined in larger fragments. The key to resolving the fines and energy problem thus lies in minimizing crushing while inducing enough tensile load to reach the breakage threshold.

Presently, silicon photonics requires photodetectors that are sensitive in a broad infrared range, can operate at room temperature, and are suitable for integration with the existing $\mathrm{Si}$-technology process. Here, we demonstrate strong room-temperature sub-band-gap photoresponse of photodiodes based on $\mathrm{Si}$ hyperdoped with tellurium. The epitaxially recrystallized Te-hyperdoped $\mathrm{Si}$ layers are developed by ion implantation combined with pulsed-laser melting and incorporate $\mathrm{Te}$-dopant concentrations several orders of magnitude above the solid solubility limit. With increasing $\mathrm{Te}$ concentration, the $\mathrm{Te}$-hyperdoped layer changes from insulating to quasi-metallic behavior with a finite conductivity as the temperature tends to zero. The optical absorptance is found to increase monotonically with increasing $\mathrm{Te}$ concentration and extends well into the mid-infrared range. Temperature-dependent optoelectronic photoresponse unambiguously demonstrates that the extended infrared photoresponsivity from $\mathrm{Te}$-hyperdoped $\mathrm{Si}$ p-n photodiodes is mediated by a $\mathrm{Te}$ intermediate band within the upper half of the $\mathrm{Si}$ band gap. This work contributes to pave the way toward establishing a $\mathrm{Si}$-based broadband infrared photonic system operating at room temperature.

Using a conventional refraction-based optical lens, it is challenging to achieve both high-resolution imaging and long-working-distance conditions. To increase the numerical aperture of a lens, the working distance should be compensated and vice versa. Here we propose and demonstrate a new concept in optical microscopy that can achieve both high-resolution imaging and long-working-distance conditions by utilizing a scattering layer instead of refractive optics. When light passes through a scattering layer, it creates a unique interference pattern. To retrieve the complex amplitude image from the interference pattern, we utilize a speckle-correlation scattering-matrix method. The proposed method enables holographic microscopy without any lens or external reference beam. Importantly, it allows high-resolution imaging with a long working distance beyond that which a conventional objective lens can achieve. As an experimental verification, we image various microscopic samples and compare their performances with off-axis digital holographic microscopy.

We study a circuit, the Josephson sampler, that embeds a real vector into an entangled state of $n$ qubits, and optionally samples from it. We estimate two fidelity measures and a variety of entropies and entanglement measures for this circuit, versus depth, on the 16-qubit ibmqx5 chip. This provides a quantitative assessment of the actual performance of the circuit on a first-generation quantum computer. To assess its expressiveness, we also measure its ability to generate quantum chaos as measured by out-of-time-order correlators (OTOCs). We conjecture that OTOCs can be used to validate a source of pseudorandom unitaries with only a single instance, which has application beyond the Josephson sampler. The circuit requires nearest-neighbor controlled-Z gates on a chain and is especially well suited for first-generation superconducting architectures.

In this paper, we report an experimental and first-principles calculation study on the anomalous Hall effect (AHE) in $\mathrm{Pt}/{\mathrm{Y}}_3{\mathrm{Fe}}_{5-x}{\mathrm{Al}}_x{\mathrm{O}}_{12}$ bilayer structures. Using a combinatorial pulsed-laser deposition method, we doped ${\mathrm{Al}}^{3+}$ ions in the ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ lattice with different concentrations, which preferentially substitute the tetrahedral ${\mathrm{Fe}}^{3+}$, leading to significant variation of AHE. A monotonic decrease of anomalous Hall resistance $R_{\mathrm{AHE}}$ with increasing ${\mathrm{Al}}^{3+}$ doping concentration is observed at a temperature of 5 K, which scales linearly with the magnetic proximity effect- (MPE-)induced magnetic moments in $\mathrm{Pt}$, as calculated using first-principles calculations. Temperature-dependent $R_{\mathrm{AHE}}$ characterizations indicate a sign change in $R_{\mathrm{AHE}}$ at around 100 K, which can be explained by a predominant contribution from spin Hall effect-(SHE-)induced AHE at higher temperatures. Our study demonstrates that the band hybridization and exchange coupling between tetrahedral ${\mathrm{Fe}}^{3+}$ ions and $\mathrm{Pt}$ are the major contributors to the magnetic proximity effect in $\mathrm{Pt}/\mathrm{YIG}$ heterojunctions.

Solid-state single-photon emitters are attractive for the realization of integrated quantum systems due to their experimental convenience and scalability. Unfortunately, however, their complex mesoscopic environments cause photons from different emitters to be spectrally distinguishable. Here we demonstrate spectral alignment of two solid-state single-photon emitters by utilizing the strain gradient. Multiple germanium-vacancy (GeV) color centers in diamond are created in fabricated diamond microcantilevers using focused ion-beam implantation. The strain response of the electronic energy levels of the GeV is measured by inducing an electrically controlled strain in the device. Leveraging the large strain gradient, we tune two GeVs in the spot to emit at the same optical wavelength. Simultaneous resonant excitation of two spectrally aligned GeVs is demonstrated using a narrow linewidth laser. This strain gradient tuning can be used to realize spectrally identical quantum emitters for on-chip integrated quantum systems.

In nuclear nonproliferation and safeguards, detecting and accurately characterizing special nuclear material remains one of the greatest challenges. Uranium enrichment determination is typically achieved by measuring the ratio of characteristic $\gamma$-ray emissions from ${}^{235}\mathrm{U}$ and ${}^{238}\mathrm{U}$. Fission also produces $\beta$-delayed neutrons, which have been used in the past to determine uranium enrichment from the time dependence of the long-lived delayed-neutron emission rate. Such measurements typically use moderated ${}^3\mathrm{He}$-tube detectors. We demonstrate an alternative measurement technique that employs a fast neutron active-interrogation probe and a scintillation detector to measure the enrichment of uranium using both the buildup and decay of $\beta$-delayed-neutron emission. Instead of ${}^3\mathrm{He}$ tubes, a capture-based heterogeneous composite detector consisting of scintillating $\mathrm{Li}$ glass and polyvinyl toluene is constructed and used, offering a prospect to scale delayed-neutron measurements to larger detector sizes. Since the technique relies on the existing tabulated nuclear data, no calibration standards are required. It is shown that the buildup of delayed-neutron emission can be used to distinguish between uranium samples and infer the uranium enrichment level, with accuracy that rivals the method that employs the time-dependent decay of delayed-neutron emission.

Defect dipoles, strain gradients, and the electric boundary conditions at interfaces and surfaces often impose internal bias fields in acceptor-doped ferroelectrics, ferroelectric films, nanocomposites, and multilayers. In this work, we study the impact of internal bias fields on the electrocaloric effect (ECE), utilizing an analytical model and ab-initio-based molecular-dynamics simulations. We reveal the complex dependency of the ECE on field protocol and the relative strength of internal and external fields. The internal fields may even reverse the sign of the response (inverse or negative ECE). We explore the transition between conventional and inverse ECE and discuss reversible and irreversible contributions to the field-induced specific entropy change. Most importantly, we predict design routes to optimize the cooling and heating response for small external fields by the combination of internal field strengths and the field-loading protocol.

${\mathrm{Ga}}_2{\mathrm{O}}_3$ thin films are grown on $(01\overline{1}2)$-oriented ($r$-plane) $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ by plasma-assisted MBE (PAMBE). The metastable $\alpha \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ phase is isomorphically stabilized up to a thickness of more than 200 nm, exceeding the maximum layer thickness reported for PAMBE growth on $c$-plane ${\mathrm{Al}}_2{\mathrm{O}}_3$. The thickness is limited by nucleation of $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ on the exposed $c$ facets of the $\alpha \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$. The growth process is characterized by in situ reflection of high-energy electron diffraction and ex situ high-resolution x-ray diffraction measurements. Heteroepitaxial growth of $r$-plane $\alpha \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ further allows for the determination of the dielectric function’s anisotropy. Both the ordinary and the extraordinary elements of the dielectric tensor are determined by spectroscopic ellipsometry measurements. The extraordinary absorption onset is found to be at lower photon energy than the ordinary one. Lineshape analysis for light polarizations perpendicular and parallel to the optic axis reveals band gaps of 5.62 and 5.58 eV, respectively.

The transfer of high-precision optical frequency signals over free-space links, particularly between ground stations and satellites, will enable advances in fields ranging from coherent optical communications and satellite Doppler ranging to tests of General Relativity and fundamental physics. We present results for the actively stabilized coherent phase transfer of a 193-THz continuous-wave optical frequency signal over horizontal free-space links 150 and 600 m in length. Over the 600-m link, we achieve a fractional frequency stability of $8.9\times {10}^{-18}$ at 1 s of integration time, improving to $1.3\times {10}^{-18}$ at an integration time of 64 s, suitable for transmission of optical atomic clock signals. The achievable transfer distance is limited by deep fading of the transmitted signal due to atmospheric turbulence. We also estimate the expected additional degradation in stability performance for frequency transfer to Low Earth Orbit.

We propose, analyze, and experimentally verify an optimized gradient-index (GRIN) phononic-crystal lens (PCL) for increased focusing efficiency. The proposed GRIN PCL is formed from a nonsquare lattice of void inclusions in a host plate with transverse variations of the inclusion diameters. With use of a Bloch-based multiple-scattering approach, the refractive index of the lens is made to satisfy a sech profile for flexural waves and then optimized for enhanced harvesting. Under harmonic-plane-wave excitation, the resulting wavefield is analyzed with a ray approximation to characterize the focal points of the lens, followed by representative experiments that demonstrate the validity and effectiveness of the optimized GRIN structure. Furthermore, a piezoelectric energy harvester, located at the first focus of the GRIN PCL, is used to illustrate increases in power gain compared with baseline harvesting.

Traditional microwave-induced thermoacoustic imaging is regarded as a structural imaging method that can recover the water content of biological tissues. Here, an alternative noninvasive functional imaging system based on microwave-pumped electric-dipole resonance absorption is reported. On the basis of the resonance principle, microwave radiation of a specific frequency can be used to selectively excite the polar molecules of interest. The microwave energy deposited creates an ultrasound disturbance through thermoelastic expansion. By detection of the ultrasound field, in vivo imaging of the target polar molecules with ultrasound resolution at the submillimeter level is realized. We show the thermoacoustic signal intensity of biological polar molecules with different electric dipole moments. We perform large-field-of-view thermoacoustic imaging of arterial and venous blood vessels in vivo, and find the thermoacoustic signal intensity increases synchronously on injection of exogenous polar molecules into the venous blood vessels of a rabbit ear. We also demonstrate the functional imaging capability of the thermoacoustic system in a study of the sensitivity of microwave absorption to changes in the concentration of polar molecules in the blood vessels of a rabbit ear. This imaging technique may find application in the detection of abnormal concentrations of biological polar molecules with large electric-dipole moments.