An in situ measurement of spin transport in a graphene nonlocal spin valve is used to quantify the spin current absorbed by a small $(250\times 750{\mathrm{nm}}^2)$ metallic island. The experiment allows for successive depositions of either $\mathrm{Fe}$ or $\mathrm{Cu}$ without breaking vacuum, so that the thickness of the island is the only parameter that is varied. Furthermore, by measuring the effect of the island using separate contacts for injection and detection, we isolate the effect of spin absorption from any change in the spin injection and detection mechanisms. As inferred from the thickness dependence, the effective spin current $j_e=(2e/\hslash )j_s$ absorbed by $\mathrm{Fe}$ is as large as ${10}^8\mathrm{A}/{\mathrm{m}}^2$. The maximum value of $j_e$ is limited by the resistance-area product of the graphene/$\mathrm{Fe}$ interface, which is as small as $3\Omega \mu {\mathrm{m}}^2$. The spin current absorbed by the same thickness of $\mathrm{Cu}$ is smaller than that for $\mathrm{Fe}$, as expected given the longer spin-diffusion length and larger spin resistance of $\mathrm{Cu}$ compared to $\mathrm{Fe}$. These results allow for a quantitative assessment of the prospects for achieving spin-transfer-torque switching of a nanomagnet using a graphene-based nonlocal spin valve.

We report on the growth and study of $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Mg}\mathrm{O}/\mathrm{Ge}(001)$ spin photodiode by using a combination of both molecular beam epitaxy (MBE) and sputtering methods. An epitaxial growth of $\mathrm{Mg}\mathrm{O}$ on $\mathrm{Ge}$ in MBE is achieved by the deposition of $\mathrm{MgO}$ at room temperature (RT) followed by a post-growth annealing at 300 °C. The spin detector, which consists of 1.1-nm ultrathin $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ layer capped with 5-nm $\mathrm{Ta}$ is subsequently grown by sputtering at RT. After a post-growth annealing of the whole structure at 250 °C, we obtain clear evidence of a strong perpendicular magnetic anisotropy in the temperature range 10‐300 K. $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{MgO}/\mathrm{Ge}(001)$ heterojunctions are then processed into spin photodiodes demonstrating at zero magnetic field a photocurrent helicity asymmetry of about 0.9% at 9 K and 0.1% at RT at the telecommunication wavelength of 1310 nm. The demonstration of a spin photodiode working at a telecommunication wavelength with zero applied magnetic field is of great interest for future applications of the optical transport of spin information.

The reconfigurability of integrated photonic chips plays a key role in current experiments in the area of linear-optical quantum computing. We demonstrate a reconfigurable multiport interferometer implemented as a femtosecond-laser-written integrated photonic device. The device includes a femtosecond-laser-written $4\times 4$ multiport interferometer equipped with 12 thermo-optical phase shifters, making it a universal programmable linear-optical circuit. We achieve a fast switching time for a single nested Mach-Zender interferometer of $\sim 10$ ms and quantitatively analyse the reconfigurability of the optical circuit. We believe that our results will improve the current state of quantum-optical experiments utilizing femtosecond-laser-written photonic circuits.

An important development in modern electronics is the realization of band-structure engineering for the design of novel materials and devices. One possible way to realize band-structure engineering is by addition of periodic potentials to two-dimensional (2D) materials, such as graphene, to form a superstructure, known as a “superlattice.’ Unlike the band gap of graphene, the band gap of silicene can be tuned by an out-of-plane electric field owing to its unusual buckled structure. In this work, we use the designable band gap of silicene and the band structure of superlattices, together with the spin and valley degrees of freedom, to propose a design principle for optimizing the performance of spin- and valley-dependent negative-differential-resistance (NDR) devices using silicene superlattices. On the basis of the effective Hamiltonian formalism, we predict that the peak-to-valley current ratio could be larger than most recently reported results achieved by 2D materials, suggesting that the silicene superlattice is a good candidate for realizing NDR devices. The design principle proposed in this work could also be extended to other layered materials with tunable band gaps. This could pave the way for advanced material and device designs based on band-gap engineering of 2D materials.

The use of magnetic insulators is attracting a lot of interest due to the rich variety of spin-dependent phenomena with potential applications for spintronic devices. Here, we report ultrathin yttrium iron garnet (YIG)/gadolinium iron garnet ($\mathrm{Gd}\mathrm{IG}$) insulating bilayers on gadolinium gallium garnet (GGG). From spin Hall magnetoresistance (SMR) and x-ray magnetic circular dichroism measurements, we show that the YIG and $\mathrm{Gd}\mathrm{IG}$ magnetically couple antiparallel even in moderate in-plane magnetic fields. The results demonstrate an all-insulating equivalent of a synthetic antiferromagnet in a garnet-based thin film heterostructure and could open venues for insulators in magnetic devices. As an example, we demonstrate a memory element with orthogonal magnetization switching that can be read by SMR.

Artificial composite multiferroic materials can be created by the interfacing of magnetostrictive and piezoelectric materials, allowing electric field control of magnetic properties. For multilayer-device applications, addition of weakly magnetostrictive layers is a potentially important means of tuning functionality. For such a device, the length scale over which weakly magnetostrictive layer(s) can be coupled to an applied electric field is of critical importance. In this work, we use polarized neutron reflectometry to characterize this distance for a model composite multiferroic multilayer system (${\mathrm{Pb}\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3$-${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3/$galfenol/permalloy). For a superlattice more than 100 nm thick, we observe that the magnetizations of all layers rotate coherently with the applied electric field. Further, we observe electric-field-induced rotation across the entirety of a 46-nm permalloy layer anchored by a single galfenol interface. The confirmation of long-range magnetic coupling in these strain-coupled multiferroic composites opens extensive opportunities for designer technological applications.

The development of photoemission electron sources to specifically address the competing and increasingly stringent requirements of advanced light sources such as x-ray free-electron lasers (XFELs) motivates a comprehensive material-centric approach that integrates predictive computational physics models, advanced nanosynthesis methods, and sophisticated surface-science characterization with in situ correlated study of photoemission performance and properties. Related efforts in material science are adopting various forms of nanostructure (such as compositionally graded stoichiometry in heterostructured architectures, and quantum features) allowing for tailored electronic structure to control and enhance optoelectronic properties. These methods influence the mechanisms of photoemission (absorption, transport, and emission) but have not, as yet, been systematically considered for use in photocathode applications. Recent results and near-term opportunities are described to exploit controlled functionality of nanomaterials for photoemission. An overview of the requirements and status is also provided.

We demonstrate the development of a high-speed neutron-imaging system via a solid-state superconducting detector. Our developed system, a current-biased kinetic-inductance detector (CBKID), operates in a delay-line mode to identify the position of a local hot spot in terms of four different arrival time stamps at the ends of the delay lines. Our detector comprises $X$ and $Y$ superconducting $\mathrm{Nb}$ meander lines, which are fed by a weak dc current, and a ${}^{10}\mathrm{B}$ conversion layer, which converts a neutron into two charged particles. The high-energy light ions (${}^4\mathrm{He}$ or ${}^7\mathrm{Li}$) in the ${}^{10}\mathrm{B}(n,\alpha {)}^7\mathrm{Li}$ reaction are able to create two hot spots simultaneously in the $X$ and $Y$ meanders. A pair of electromagnetic-wave pulses are generated at the hot spot and signals of different polarities start propagating on each of the $X$ and $Y$ meanders toward the ends. The position of the original hot spot is determined by the difference in arrival time of the two pulses at the two ends. To measure the arrival times of the pulse signals, we developed a set of analog signal discriminators from the preset threshold levels using a time-to-digital converter (TDC), which is able to record the timing of the propagated pulses with 1-ns resolution over a time frame of 4.2 s. We successfully reconstructed the neutron image of a test pattern with a spatial resolution of 22 $\mu \mathrm{m}$ and a detection rate tolerance of a few 10 ${\mathrm{MHz}/\mathrm{cm}}^2$.

We show how photocapacitance spectra can probe and manipulate two dimensional excitonic complexes and Fermi-edge singularities as a function of applied bias even at a temperature of 100 K. For lower density regimes $(<1\times {10}^{11}{\mathrm{cm}}^{-2})$, the appearance of two distinct peaks in the spectra are identified as a signature of coexistence of both excitons and positively charged trions. We find the binding energy of these trions to be approximately 2.0 meV, which matches well with known estimates. For higher density regimes $(>1\times {10}^{11}{\mathrm{cm}}^{-2})$, we observe a sharp spectral transition from trions to asymmetrically shaped Fermi-edge singularities in photocapacitance spectra above a particular reverse bias. However, these signatures of indirect excitonic states are absent from photoluminescence spectra. Such dissimilarities clearly point out that different many-body physics govern these two spectral measurements. We also argue why such quantum-confined dipoles of spatially indirect trions can have thermodynamically finite probability to survive even around 100 K. Finally, our observations demonstrate that photocapacitance spectroscopy, which was rarely used to detect trions in the past, can also be useful to detect the traces of these spatially indirect excitonic complexes as well as Fermi-edge singularities. This is mainly due to the enhanced sensitivity of these capacitive measurements to “dipolar” changes of excitonic complexes in these heterojunctions. Thus, our studies clearly open up future possibilities for electro-optical modulation and detection of trions and Fermi-edge singularities in several other heterostructures for next-generation optoelectronic applications.

Great efforts have been made in the investigation of defects in silicon carbide for their attractive optical and spin properties. However, most research is done at low and room temperature. Little is known about the spin-coherence property at high temperature. Here we experimentally demonstrate coherent control of divacancy defect spins in silicon carbide above 550 K. The spin properties of defects from room temperature to 600 K are investigated, and the zero-field splitting is found to have a polynomial temperature dependence and the spin-coherence time decreases as the temperature increases. Moreover, as an example of an application, we demonstrate thermal sensing using the Ramsey method at about 450 K. Our experimental results would be useful for the investigation of high-temperature properties of defect spins and silicon carbide–based broad-temperature-range quantum sensing.

Electrical manipulation and control of magnetic nanostructures could greatly reduce the power consumption of spintronic devices. We theoretically describe the strain-mediated electric-field control of ferromagnetic resonance (FMR) in thin-film nanomagnets integrated onto active substrates experiencing voltage-induced piezoelectric deformations. To that end, we first consider the general problem of FMR in ferromagnetic films and islands with nanoscale thicknesses mechanically coupled to much thicker passive or active substrates. An analytic relation is derived for the resonance frequency of such nanomagnets, which takes into account the influence of both substrate-induced lattice strains and surface/interface magnetic anisotropy. This general relation is valid for any equilibrium orientation of the magnetization and renders possible to calculate the FMR frequency and resonance magnetic field with the account of strain-driven and size-induced spin reorientation transitions (SRTs). Next, we quantitatively describe the electrical tuning of FMR frequency in ultrathin $\mathrm{Ni}$, $\mathrm{Fe}$, and ${\mathrm{Fe}}_{60}{\mathrm{Co}}_{40}$ layers coupled to various piezoelectrically active substrates, including conventional and relaxor ferroelectrics. The results of numerical calculations are presented in the form of resonance frequency and electrical tunability maps, where the thickness of the ferromagnetic layer and the electric field applied to the substrate are used as two parameters. It is shown that the FMR frequency strongly decreases near the SRT lines on the thickness–field plane, whereas the tunability increases drastically at most SRTs driven by substrate-induced lattice strains. This remarkable feature provides an efficient electric-field control of FMR, which is necessary for the development of signal processing microwave devices with high tuning speed and low energy consumption.

Nanowires (NWs) with embedded zero-dimensional (0D) quantum dots (QDs) have interesting fundamental properties attractive for a variety of applications. The properties of such embedded QDs can be controlled by 0D quantum confinement and also via strain engineering in axial or radial heterostructures of the nanowire system. We evaluate the electronic structure of QDs, which are formed in the $\mathrm{Ga}(\mathrm{N},\mathrm{As})$ shell of the $\mathrm{Ga}\mathrm{As}/\mathrm{Ga}(\mathrm{N},\mathrm{As})$ core-shell NWs due to alloy fluctuations. It is found that the principal quantization axis of the studied QDs is primarily oriented along the NW axis, based on the performed polarization-resolved magneto-photoluminescence measurements. We also show that the QDs exhibit a large spectrally dependent variation of the valence band character, which changes from pure heavy-hole states for the low-energy QD emitters to the mixed light-hole heavy-hole states for the QDs emitting at high energies. We ascribe these changes to combined effects of the uniaxial strain caused by the lattice mismatch between the $\mathrm{Ga}\mathrm{As}$ core and the $\mathrm{Ga}(\mathrm{N},\mathrm{As})$ shell, and the local strain/lattice distortion within the short-range fluctuations in the $\mathrm{N}$ content. The obtained results underline the importance of the local strain for valence band engineering in hybrid NW structures with embedded QDs.

Devices relying on microwave circuitry form a cornerstone of many classical and emerging quantum technologies. A capability to provide in-situ, noninvasive, and direct imaging of the microwave fields above such devices would be a powerful tool for their function and failure analysis. In this work, we build on recent achievements in magnetometry using ensembles of nitrogen-vacancy centers in diamond, to present a widefield microwave microscope with few-micron resolution over a millimeter-scale field of view, $130\mathrm{nT}{\mathrm{Hz}}^{-1/2}$ microwave-amplitude sensitivity, a dynamic range of 48 dB, and submillisecond temporal resolution. We use our microscope to image the microwave field a few microns above a range of microwave circuitry components, and to characterize an alternative atom-chip design. Our results open the way to high-throughput characterization and debugging of complex multicomponent microwave devices, including real-time exploration of device operation.

Magnetic heterostructure $\mathrm{Mo}/\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Mg}\mathrm{O}$ has strong perpendicular magnetic anisotropy and thermal stability. Through current-induced hysteresis loop shift measurements, we show that the damping-like spin-orbit torque (SOT) efficiency of $\mathrm{Mo}/\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Mg}\mathrm{O}$ heterostructures is ${\xi }_{\mathrm{DL}}\approx -0.003\pm 0.001$ and is fairly independent of the annealing temperature from 300 °C to 400 °C. Although $|{\xi }_{\mathrm{DL}}|$ is small compared to those from $\mathrm{Ta}\text{−}$ or $\mathrm{W}$-based heterostructures, reversible current-induced SOT switching of a thermally stable $\mathrm{Mo}/\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Mg}\mathrm{O}$ heterostruture can still be achieved. Furthermore, we observe field-free current-induced switching from a $\mathrm{Mo}/\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Mg}\mathrm{O}$ structure with the $\mathrm{Mo}$ layer being wedge-deposited. Our results indicate that even for a weak spin-orbit interaction 4d transition metal such as $\mathrm{Mo}$, it is still possible to generate sufficient spin current for conventional SOT switching and to realize field-free current-induced switching by structural engineering.

Magnetotransport (magnetoresistance and anomalous Hall effect) and thermoelectric (Seebeck and anomalous Nernst effect) effects are investigated on epitaxially grown ${\mathrm{Co}}_2\mathrm{Ti}\mathrm{Sn}$ and ${\mathrm{Co}}_2{\mathrm{Ti}}_{0.6}{\mathrm{V}}_{0.4}\mathrm{Sn}$ Heusler thin films. An anomalous Nernst coefficient up to 1.8 $\mu \mathrm{V}/$K is observed in ${\mathrm{Co}}_2{\mathrm{Ti}}_{0.6}{\mathrm{V}}_{0.4}\mathrm{Sn}$ at 220 K, which is almost 12 times larger than in the undoped ${\mathrm{Co}}_2\mathrm{Ti}\mathrm{Sn}$ thin film at 300 K. In analogy to the anomalous Hall angle, we extract the anomalous Nernst angle from experimental results by comparing the anomalous Nernst voltage with the thermopower. The anomalous Nernst angle for ${\mathrm{Co}}_2{\mathrm{Ti}}_{0.6}{\mathrm{V}}_{0.4}\mathrm{Sn}$ is 15% at 220 K, whereas it is only 0.5% for the undoped film. Considering the Mott relation for anomalous Hall and Nernst effects, these experimental results may be accounted for by an enhanced energy derivative of the anomalous Hall conductivity at the Fermi level that is shifted by vanadium doping. These results of a large anomalous Nernst angle provide opportunities to realize spin-caloritronic devices for efficient on-chip energy harvesting based on magnetic Heusler thin films.

With use of the techniques of optomechanics, a high-$Q$ mechanical oscillator may serve as a link between electromagnetic modes of vastly different frequencies. This approach has successfully been exploited for the frequency conversion of classical signals and has the potential of performing quantum-state transfer between superconducting circuitry and a traveling optical signal. Such transducers are often operated in a linear regime, where the hybrid system can be described with linear response theory based on the Heisenberg-Langevin equations. While these equations are mathematically straightforward to solve, this approach yields little intuition about the dynamics of the hybrid system to aid the optimization of the transducer. As an analysis and design tool for such electrooptomechanical transducers, we introduce an equivalent-circuit formalism, where the entire transducer is represented by an electrical circuit. Thereby we integrate the transduction functionality of optomechanical systems into the toolbox of electrical engineering, allowing the use of its well-established design techniques. This unifying impedance description can be applied for both static (dc) and harmonically varying (ac) bias fields, accommodates arbitrary linear circuits, and is not restricted to the resolved-sideband regime. Furthermore, by establishing the quantized input-output formalism for the equivalent circuit, we obtain the scattering matrix for linear transducers using circuit analysis, and thereby have a complete quantum-mechanical characterization of the transducer. Hence, this mapping of the entire transducer to the language of electrical engineering both sheds light on how the transducer performs and can at the same time be used to optimize its performance by aiding the design of a suitable electrical circuit.

Continuous terahertz wave sources with narrow bandwidth and wide frequency tunability enable high-resolution terahertz spectroscopy and high-speed information communication. In this study, using the optical nonlinearity of excitons as the source of second-order nonlinear polarization, we realize a continuous terahertz electromagnetic wave demonstrating wide frequency tunability from 0.1 to 18 THz without a decrease in intensity due to phonon scattering. Because of excitation of two exciton states in a $\mathrm{Ga}\mathrm{As}$/$\mathrm{Al}\mathrm{As}$ multiple quantum well using two continuous-wave lasers, terahertz waves are emitted as a result of difference-frequency mixing, where the intensity shows a square dependence on the excitation intensity. Using the inhomogeneous width of exciton lines, we achieve wide frequency tunability without phonon effects.

We develop and characterize an atom-guiding technique that loads $3\times {10}^6$ cold rubidium atoms into a hollow-core optical fiber, an order-of-magnitude greater than previously reported results. This result is possible because it is guided by a physically realistic simulation that can provide the specifications for a loading efficiency of 3.0% and a peak optical depth of 600. The simulation further shows that the loading efficiency is limited solely by the geometric overlap of the atom cloud and the optical guide beam, and is thus open to further improvement with experimental modification. The experimental arrangement allows observation of the real-time effects of light-assisted cold-atom collisions and background-gas collisions by tracking the dynamics of the cold-atom cloud as it falls into the fiber. The combination of these observations, and physical understanding from the simulation, allows estimation of the limits to loading cold atoms into hollow-core fibers.

Phase-estimation protocols provide a fundamental benchmark for the field of quantum metrology. The latter represents one of the most relevant applications of quantum theory, potentially enabling the capability of measuring unknown physical parameters with improved precision over classical strategies. Within this context, most theoretical and experimental studies have focused on determining the fundamental bounds and how to achieve them in the asymptotic regime where a large number of resources are employed. However, in most applications, it is necessary to achieve optimal precision by performing only a limited number of measurements. To this end, machine-learning techniques can be applied as a powerful optimization tool. Here, we implement experimentally single-photon adaptive phase-estimation protocols enhanced by machine learning, showing the capability of reaching optimal precision after a small number of trials. In particular, we introduce an approach for Bayesian estimation that exhibits best performance for a very low number of photons $N$. Furthermore, we study the resilience to noise of the tested methods, showing that the optimized Bayesian approach is very robust in the presence of imperfections. Application of this methodology can be envisaged in the more general multiparameter case, which represents a paradigmatic scenario for several tasks, including imaging or Hamiltonian learning.

A Bloch surface wave (BSW) is a kind of resonant propagating wave sustained at the surface of a truncated dielectric multilayer. Spiral or concentric rings engraved on the top layer of the truncated multilayer, termed a “BSW lens,” will concentrate BSWs into the center. As a dielectric counterpart of a surface plasmon on a metal surface, a BSW propagates farther because of the absence of ohmic loss in a dielectric. Therefore, it is believed that a BSW lens can produce a much stronger focal spot if more rings are engraved. In this paper, we present a comprehensive study on the focusing properties of a BSW lens. We derive the dispersion relation of a BSW, analyze the focal pattern based on the interaction between the spiral-ring structure and light with specific spin and orbital angular momentum, and investigate transmittance and reflectance of the slot for the BSW. We find the reflectance of the slot introduces nonignorable loss for a BSW lens with multiple rings. In addition, we demonstrate that BSW waves can be efficiently concentrated into the focal spot by optimizing the trade-off between increasing number of rings and reflective loss of the ring slot, which is potentially helpful and valuable for integrated nanophotonic devices.