Although the interlayer interaction is weak, it exhibits an important influence on the electronic behaviors in two-dimensional layered semiconductors. Using first-principles calculations, we find that tuning of lone-pair electron properties by carrier injection breaks time symmetry in SnO nanosheets, which is responsible for the ferromagnetic (FM) phase transition. Upon hole doping, the most stable FM state of monolayer SnO occurs at $3.2\times {10}^{14}/{\mathrm{cm}}^2$ with an average magnetic moment of $1.0{\mu }_B/\text{carrier}$. In multilayer SnO nanosheets, lone-pair electrons are redistributed due to the interaction of Sn atoms in adjacent layers, which cancels out the FM coupling mediated by holes. A half-metallic FM ground state is recovered when the interlayer lone-pair electron interaction is weakened. The itinerant magnetism originates from an exchange splitting of lone-pair electron states at the top of the valence band where the density of states exhibits a sharp van Hove singularity in this system.

We report on high-resolution photoluminescence (PL) and photocurrent (PC) spectroscopies of a single self-assembled InAs/GaAs quantum dot (QD) embedded in an $n\text{−}i$-Schottky device with an applied magnetic field in Faraday and Voigt geometries. The single-QD PC spectrum of neutral exciton ($X^0$) is obtained by sweeping the bias-dependent $X^0$ transition energy to achieve resonance with a fixed narrow-bandwidth laser through the quantum-confined Stark effect. With a magnetic field applied in Faraday geometry, the diamagnetic effect and the Zeeman splitting of $X^0$ are observed both in PL and PC spectra. When the magnetic field is applied in Voigt geometry, the mixture of bright and dark states results in an observation of dark-exciton states, which are confirmed by the polarization-resolved PL and PC spectra.

Stochastic spiking neural networks based on nanoelectronic spin devices can be a possible pathway to achieving “brainlike” compact and energy-efficient cognitive intelligence. The computational model attempt to exploit the intrinsic device stochasticity of nanoelectronic synaptic or neural components to perform learning or inference. However, there has been limited analysis on the scaling effect of stochastic spin devices and its impact on the operation of such stochastic networks at the system level. This work attempts to explore the design space and analyze the performance of nanomagnet-based stochastic neuromorphic computing architectures for magnets with different barrier heights. We illustrate how the underlying network architecture must be modified to account for the random telegraphic switching behavior displayed by magnets with low barrier heights as they are scaled into the superparamagnetic regime. We perform a device-to-system-level analysis on a deep neural-network architecture for a digit-recognition problem on the MNIST data set.

Actuated by alternating stimulation, dielectric elastomers (DEs) show a behavior of complicated nonlinear vibration, implying a potential application as dynamic electromechanical actuators. As is well known, for a vibrational system, including the DE system, the dynamic properties are significantly affected by the geometrical sizes. In this article, a nonlinear dynamical model is deduced to investigate the geometrical effects on dynamic properties of viscoelastic DEs. The DEs with square and arbitrary rectangular geometries are considered, respectively. Besides, the effects of tensile forces on dynamic performances of rectangular DEs with comparably small and large geometrical sizes are explored. Phase paths and Poincaré maps are utilized to detect the periodicity of the nonlinear vibrations of DEs. The resonance characteristics of DEs incorporating geometrical effects are also investigated. The results indicate that the dynamic properties of DEs, including deformation response, vibrational periodicity, and resonance, are tuned when the geometrical sizes vary.

Nanoelectronic devices embedded in the two-dimensional electron system (2DES) of a $\mathrm{GaAs}/(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ heterostructure enable a large variety of applications ranging from fundamental research to high-speed transistors. Electrical circuits are thereby commonly defined by creating barriers for carriers by the selective depletion of a preexisting 2DES. We explore an alternative approach: we deplete the 2DES globally by applying a negative voltage to a global top gate and screen the electric field of the top gate only locally using nanoscale gates placed on the wafer surface between the plane of the 2DES and the top gate. Free carriers are located beneath the screen gates, and their properties can be controlled by means of geometry and applied voltages. This method promises considerable advantages for the definition of complex circuits by the electric-field effect, as it allows us to reduce the number of gates and simplify gate geometries. Examples are carrier systems with ring topology or large arrays of quantum dots. We present a first exploration of this method pursuing field effect, Hall effect, and Aharonov-Bohm measurements to study electrostatic, dynamic, and coherent properties.

Spin-transfer-torque nano-oscillators are potential candidates for replacing the traditional inductor-based voltage-controlled oscillators in modern communication devices. Typical oscillator designs are based on trilayer magnetic tunnel junctions, which have the disadvantages of low power outputs and poor conversion efficiencies. We theoretically propose using resonant spin filtering in pentalayer magnetic tunnel junctions as a possible route to alleviate these issues and present viable device designs geared toward a high microwave output power and an efficient conversion of the dc input power. We attribute these robust qualities to the resulting nontrivial spin-current profiles and the ultrahigh tunnel magnetoresistance, both of which arise from resonant spin filtering. The device designs are based on the nonequilibrium Green’s-function spin-transport formalism self-consistently coupled with the stochastic Landau-Lifshitz-Gilbert-Slonczewski equation and Poisson’s equation. We demonstrate that the proposed structures facilitate oscillator designs featuring a large enhancement in microwave power of around 1150% and an efficiency enhancement of over 1100% compared to typical trilayer designs. We rationalize the optimum operating regions via an analysis of the dynamic and static device resistances. We also demonstrate the robustness of our structures against device design fluctuations and elastic dephasing. This work sets the stage for pentalyer spin-transfer-torque nano-oscillator device designs that ameliorate major issues associated with typical trilayer designs.

The realization of an elementary quantum network that is intrinsically secure and operates over long distances requires the interconnection of several quantum modules performing different tasks. In this work, we report the realization of a communication network functioning in a quantum regime, consisting of four different quantum modules: (i) a random polarization qubit generator, (ii) a free-space quantum-communication channel, (iii) an ultralow-noise portable quantum memory, and (iv) a qubit decoder, in a functional elementary quantum network possessing all capabilities needed for quantum-information distribution protocols. We create weak coherent pulses at the single-photon level encoding polarization states $|H⟩$, $|V⟩$, $|D⟩$, and $|A⟩$ in a randomized sequence. The random qubits are sent over a free-space link and coupled into a dual-rail room-temperature quantum memory and after storage and retrieval are analyzed in a four-detector polarization analysis akin to the requirements of the BB84 protocol. We also show ultralow noise and fully portable operation, paving the way towards memory-assisted all-environment free-space quantum cryptographic networks.

Electrically tunable bandpass filters based on liquid crystals are designed, built, and characterized using a vector network analyzer. The filters are composed of half-wavelength open stubs and quarter-wavelength connecting lines in an inverted microstrip geometry. The filters are modeled using computational electromagnetics software utilizing the finite integration technique. Photolithography and thin-film deposition processes are employed, and standard liquid-crystal cell-assembly techniques are used to make the final filter structures. The three-stub filters with passband central frequencies of 30, 50, and 85 GHz are filled with the nematic liquid crystal, LC1917, and tested. 10% tuning of the central frequency is achieved with a 14-volt peak-to-peak ac bias across the $38\text{−}\mu \mathrm{m}$ liquid-crystal layer (electric field of $0.19\mathrm{V}/\mu \mathrm{m}$). At 50 GHz, the insertion loss is $-3.76\mathrm{dB}$, while the return loss ranges from $-9$ to $-25\mathrm{dB}$, indicating a good impedance match for a proof-of-concept device. The passband widths of the 30-, 50-, and 85-GHz filters are 5, 9, and 14 GHz, respectively, resulting in a $Q$ factor of 6. The filter devices presented in this study, although intended for microwave signal-processing applications, furnish an effective methodology for characterizing the dielectric properties of liquid-crystal materials (and fluids or solids in general) up to the terahertz frequency range.

In this paper we demonstrate the injection locking of a recently demonstrated spintronic feedback nano-oscillator to microwave magnetic fields at integers ($n=1$, 2, 3) as well as fractional multiples ($f=1/2$, $3/2$, and $5/2$) of its auto-oscillation frequency. Feedback oscillators have delay as a new “degree of freedom” which is absent for spin-transfer torque-based oscillators, which gives rise to side peaks along with a main peak. We show that it is also possible to lock the oscillator on its sideband peaks, which opens an alternative avenue to phase-locked oscillators with large frequency differences. We observe that for low driving fields, sideband locking improves the quality factor of the main peak, whereas for higher driving fields the main peak is suppressed. Further, measurements at two field angles provide some insight into the role of the symmetry of oscillation orbit in determining the fractional locking.

We propose a high-resolution microscopy technique for enantiospecific detection of chiral samples down to sub-100-nm size based on force measurement. We delve into the differential photoinduced optical force $\mathrm{\Delta }F$ exerted on an achiral probe in the vicinity of a chiral sample when left and right circularly polarized beams separately excite the sample-probe interactive system. We analytically prove that $\mathrm{\Delta }F$ is entangled with the enantiomer type of the sample enabling enantiospecific detection of chiral inclusions. Moreover, we demonstrate that $\mathrm{\Delta }F$ is linearly dependent on both the chiral response of the sample and the electric response of the tip and is inversely related to the quartic power of probe-sample distance. We provide physical insight into the transfer of optical activity from the chiral sample to the achiral tip based on a rigorous analytical approach. We support our theoretical achievements by several numerical examples highlighting the potential application of the derived analytic properties. Lastly, we demonstrate the sensitivity of our method to enantiospecify nanoscale chiral samples with chirality parameter on the order of 0.01 and discuss how the sensitivity of our proposed technique can be further improved.

We present an alternative laboratory implementation of x-ray phase-contrast tomography through a beam-tracking approach. A nonmicrofocal rotating anode source is combined with a high-resolution detector and an absorbing mask to obtain attenuation, phase, and ultra-small-angle scattering tomograms of different specimens. A theoretical model is also presented which justifies the implementation of beam tracking with polychromatic sources and provides quantitative values of attenuation and phase, under the assumption of low sample attenuation. The method is tested on a variety of samples featuring both large and small x-ray attenuation, phase, and scattering signals. The complementarity of the contrast channels can enable subtle distinctions between materials and tissue types, which appear indistinguishable to conventional tomography scanners.

In this work, we propose a flexible architecture of microwave resonators with tunable couplings to perform quantum simulations of problems from the field of molecular chemistry. The architecture builds on the experience of the D-Wave design, working with nearly harmonic circuits instead of qubits. This architecture, or modifications of it, can be used to emulate molecular processes such as vibronic transitions. Furthermore, we discuss several aspects of these emulations, such as dynamical ranges of the physical parameters, quenching times necessary for diabaticity, and, finally, the possibility of implementing anharmonic corrections to the force fields by exploiting certain nonlinear features of superconducting devices.

We propose a design of a terahertz-frequency signal generator based on a layered structure consisting of a current-driven platinum (Pt) layer and a layer of an antiferromagnet (AFM) with easy-plane anisotropy, where the magnetization vectors of the AFM sublattices are canted inside the easy plane by the Dzyaloshinskii-Moriya interaction (DMI). The dc electric current flowing in the Pt layer creates due to the spin Hall effect, a perpendicular spin current that, being injected in the AFM layer, tilts the DMI-canted AFM sublattices out of the easy plane, thus exposing them to the action of a strong internal exchange magnetic field of the AFM. The sublattice magnetizations, along with the small net magnetization vector ${\mathbf{m}}_{\mathrm{DMI}}$ of the canted AFM, start to rotate about the hard anisotropy axis of the AFM with the terahertz frequency proportional to the injected spin current and the AFM exchange field. The rotation of the small net magnetization ${\mathbf{m}}_{\mathrm{DMI}}$ results in the terahertz-frequency dipolar radiation that can be directly received by an adjacent (e.g., dielectric) resonator. We demonstrate theoretically that the radiation frequencies in the range $f=0.05–2\mathrm{THz}$ are possible at the experimentally reachable magnitudes of the driving current density, and we evaluate the power of the signal radiated into different types of resonators. This power increases with the increase of frequency $f$, and it can exceed $1\mu \mathrm{W}$ at $f\sim 0.5\mathrm{THz}$ for a typical dielectric resonator of the electric permittivity $ϵ\sim 10$ and a quality factor $Q\sim 750$.

Magnetic nanoarrays with special geometries exhibit nontrivial collective behaviors similar to those observed in spin-ice materials. Here, we present a circuit model to describe the complex magnetotransport phenomena in artificial kagome spin ice. In this picture, the system can be viewed as a resistor network driven by voltage sources that are located at vertices of the honeycomb array. The differential voltages across different terminals of these sources are related to the ice rules that govern the local magnetization ordering. The circuit model relates the transverse Hall voltage of kagome ice to the underlying spin correlations. Treating the magnetic nanoarray as metamaterials, we present a mesoscopic constitutive equation relating the Hall resistance to magnetization components of the system. We further show that the Hall signal is significantly enhanced when the kagome ice undergoes a magnetic-charge-ordering transition. Our analysis can be readily generalized to other lattice geometries, providing a quantitative method for the design of magnetoresistance devices based on artificial spin ice.

Ferroelectric materials have been extensively explored for applications in high-density nonvolatile memory devices because of their ferroelectric-ferroelastic domain-switching behavior under electric loading or mechanical stress. However, the existence of ferroelectric and ferroelastic backswitching would cause significant data loss, which affects the reliability of data storage. Here, we apply in situ transmission electron microscopy and phase-field modeling to explore the unique ferroelastic domain-switching kinetics and the origin of this in relaxor-based $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3\text{−}33\%{\mathrm{PbTiO}}_3$ single-crystal pillars under electrical and mechanical stimulations. Results showed that the electric-mechanical hysteresis loop shifted for relaxor-based single-crystal pillars because of the low energy levels of domains in the material and the constraint on the pillars, resulting in various mechanically reversible and irreversible domain-switching states. The phenomenon can potentially be used for advanced bit writing and reading in nonvolatile memories, which effectively overcomes the backswitching problem and broadens the types of ferroelectric materials for nonvolatile memory applications.

We demonstrate that small quantum memories, realized via quantum error correction in multiqubit devices, can benefit substantially by choosing a quantum code that is tailored to the relevant error model of the system. For a biased noise model, with independent bit and phase flips occurring at different rates, we show that a single code greatly outperforms the well-studied Steane code across the full range of parameters of the noise model, including for unbiased noise. In fact, this tailored code performs almost optimally when compared with 10 000 randomly selected stabilizer codes of comparable experimental complexity. Tailored codes can even outperform the Steane code with realistic experimental noise, and without any increase in the experimental complexity, as we demonstrate by comparison in the observed error model in a recent seven-qubit trapped ion experiment.

Increasing the maximum cooling effect of a Peltier cooler can be achieved through material and device design. The use of inhomogeneous, functionally graded materials may be adopted in order to increase maximum cooling without improvement of the $ZT$ (figure of merit); however, these systems are usually based on the assumption that the local optimization of the $ZT$ is the suitable criterion to increase thermoelectric performance. We solve the heat equation in a graded material and perform both analytical and numerical analysis of a graded Peltier cooler. We find a local criterion that we use to assess the possible improvement of graded materials for thermoelectric cooling. A fair improvement of the cooling effect (up to 36%) is predicted for semiconductor materials, and the best graded system for cooling is described. The influence of the equation of state of the electronic gas of the material is discussed, and the difference in term of entropy production between the graded and the classical system is also described.

Porpoises are small-toothed whales, and they can produce directional acoustic waves to detect and track prey with high resolution and a wide field of view. Their sound-source sizes are rather small in comparison with the wavelength so that beam control should be difficult according to textbook sonar theories. Here, we demonstrate that the multiphase material structure in a porpoise’s forehead is the key to manipulating the directional acoustic field. Computed tomography (CT) derives the multiphase (bone-air-tissue) complex, tissue experiments obtain the density and sound-velocity multiphase gradient distributions, and acoustic fields and beam formation are numerically simulated. The results suggest the control of wave propagations and sound-beam formations is realized by cooperation of the whole forehead’s tissues and structures. The melon size significantly impacts the side lobes of the beam and slightly influences the main beams, while the orientation of the vestibular sac mainly adjusts the main beams. By compressing the forehead complex, the sound beam can be expanded for near view. The porpoise’s biosonar allows effective wave manipulations for its omnidirectional sound source, which can help the future development of miniaturized biomimetic projectors in underwater sonar, medical ultrasonography, and other ultrasonic imaging applications.

We have developed a hybrid quantum Hall midinfrared (QHMIR)–quantum Hall far-infrared (QHFIR) photodetector by the use of graphene-$\mathrm{GaAs}/(\mathrm{Al},\mathrm{Ga})\mathrm{As}$–layered composite material. Both MIR and FIR photoresistance are observed in a single chip by utilizing cyclotron resonance in the quantum Hall regimes of graphene and two-dimensional electron gas (2DEG) in $\mathrm{GaAs}/(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ heterostructure, respectively. By cooperatively operating 2DEG as a back-gate electrode to change the carrier density of graphene or graphene as a top-gate electrode to modulate the carrier density of 2DEG with an applied gate voltage less than 1 V and applying the magnetic field to tune cyclotron resonance, we achieve a wide frequency selectivity, covering $640–790{\mathrm{cm}}^{-1}$ for the graphene-QHMIR detector and $24–89{\mathrm{cm}}^{-1}$ for the 2DEG-QHFIR detector. Moreover, our design integrates a log-periodic antenna with the detector to minimize the device size, while preserving high sensitivity. Our results pave the way for implementing a highly tunable MIR-to-FIR photodetector and a dual-band (MIR-FIR) imaging array.

Combined with liquid crystals, plasmonic nanostructures acquire voltage-driven optical tunability. We report that light-transmitting interdigitated aluminum grating covered with a nematic liquid crystal exhibits fast electro-optical switching: for the visible light wavelengths above the surface-plasmon resonance, its switching is thresholdless and, being driven by a voltage of a few volts, occurs on a time scale of tens of microseconds. We relate these features to the fast liquid crystal realignment within a thin surface layer that controls the plasmon excitation and light transmission.