We show that the engineering of tunnel barriers forming at the interfaces of a one-dimensional spin valve provides a viable path to a strong gate-voltage tunability of the magnetoresistance effect. Specifically, we investigate theoretically a carbon nanotube (CNT) spin valve in terms of the influence of the CNT-contact interface on the performance of the device. The focus is on the strength and the spin selectivity of the tunnel barriers that are modeled as Dirac-$\delta$ potentials. The scattering matrix approach is used to derive the transmission coefficient that yields the tunneling magnetoresistance (TMR). We find a strong nontrivial gate-voltage response of the TMR in the absence of spin-orbit coupling when the energy of the incident electrons matches the potential energy of the barrier. Analytic expressions for the TMR in various limiting cases are derived. These expressions are used to explain previous experimental results, and also to predict prospective ways for device optimization with respect to the size and tunability of the TMR effect in the ballistic transport regime by means of engineering the tunnel barriers at the CNT-contact interfaces.

We present the design, characterization, and theoretical and experimental demonstration of multiband asymmetric transmission of airborne sound using an ultrathin coded metasurface formed by an alternating arrangement of the coding elements 0 and 1. The asymmetric transmission effect can be easily controlled to selectively achieve off and on by coding different patterns. Both frequency- and angle-selective transmission is discussed. The proposed multiband asymmetric transmission stems from the constructive and destructive interferences of acoustic-wave coupling between the coded elements. The experimental results are in relative agreement with numerical simulations. This work opens an alternative path for ultrathin acoustic-device design and shows promise for application in acoustic rectification and noise control.

We report the results of a comparative study of the magnetic microstructure of textured and isotropic ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}/\alpha \text{−}\mathrm{Fe}$ nanocomposites using magnetometry, transmission electron microscopy, synchrotron x-ray diffraction, and, in particular, magnetic small-angle neutron scattering (SANS). Analysis of the magnetic neutron data of the textured specimen and computation of the correlation function of the spin-misalignment SANS cross section suggests the existence of inhomogeneously magnetized regions on an intraparticle nanometer length scale, about 40–50 nm in the remanent state. Possible origins for this spin disorder are discussed: it may originate in thin-grain-boundary layers (where the material parameters are different than in the ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ grains), or it may reflect the presence of crystal defects (introduced via hot pressing), or the dispersion in the orientation distribution of the magnetocrystalline anisotropy axes of the ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ grains. X-ray powder diffraction data reveal a crystallographic texture in the direction perpendicular to the pressing direction—a finding which might be related to the presence of a texture in the magnetization distribution, as inferred from the magnetic SANS data.

Acoustic-wave devices provide a promising chip-scale platform for efficiently coupling radio frequency (rf) and optical fields. Here, we use an integrated piezo-optomechanical circuit platform that exploits both the piezoelectric and photoelastic coupling mechanisms to link 2.4-GHz rf waves to 194-THz (1550 nm) optical waves, through coupling to propagating and localized 2.4-GHz acoustic waves. We demonstrate acousto-optic modulation, resonant in both the optical and mechanical domains, in which waveforms encoded on the rf carrier are mapped to the optical field. We also show optoacoustic gating, in which the application of modulated optical pulses interferometrically gates the transmission of propagating acoustic pulses. The time-domain characteristics of this system under both pulsed rf and pulsed optical excitation are considered in the context of the different physical pathways involved in driving the acoustic waves, and modeled through the coupled mode equations of cavity optomechanics.

Contactless manipulation of microparticles is demonstrated with single-beam acoustical tweezers based on precursor swirling Rayleigh waves. These surface waves degenerate into acoustical vortices when crossing a stack made of a fluid layer and its solid support, hence creating a localized acoustical trap in a fluid cavity. They can be synthesized with a single interdigitated transducer whose spiraling shape encodes the phase of the field like a hologram. For applications, these tweezers have many attractive features: they are selective, flat, easily integrable, and compatible with disposable substrates.

Both polycrystalline and single-crystal films of iron-substituted strontium titanate, $\mathrm{Sr}({\mathrm{Ti}}_{0.65}{\mathrm{Fe}}_{0.35}){\mathrm{O}}_{3-\delta }$, prepared by pulsed laser deposition, show room-temperature magnetism and Faraday rotation, with the polycrystalline films exhibiting higher saturation magnetization and Faraday rotation. The magnetic properties vary with the oxygen pressure at which the films are grown, showing a maximum at pressures of approximately $4\mu \mathrm{Torr}$ at which the unit-cell volume is largest. The results are discussed in terms of the oxygen stoichiometry and corresponding Fe valence states, the structure and strain state, and the presence of small-volume fractions of metallic Fe in single-crystal films grown at the optimum deposition pressure. Integration of magneto-optical polycrystalline films on an optical-waveguide device demonstrates a nonreciprocal phase shift.

In this paper, a theoretical approach comprising the nonequilibrium Green’s function method for electronic transport and the Landau-Khalatnikov equation for electric polarization dynamics is presented to describe polarization-dependent tunneling electroresistance (TER) in ferroelectric tunnel junctions. Using appropriate contact, interface, and ferroelectric parameters, the measured current-voltage characteristic curves in both inorganic ($\mathrm{Co}/\mathrm{BaTi}{\mathrm{O}}_3/{\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$) and organic ($\mathrm{Au}/\mathrm{PVDF}/\mathrm{W}$) ferroelectric tunnel junctions can be well described by the proposed approach. Furthermore, under this theoretical framework, the controversy of opposite TER signs observed experimentally by different groups in $\mathrm{Co}/\mathrm{BaTi}{\mathrm{O}}_3/{\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ systems is addressed by considering the interface termination effects using the effective contact ratio defined through the effective screening length and dielectric response at the metal-ferroelectric interfaces. Finally, our approach is extended to investigate the role of a ${\mathrm{CoO}}_x$ buffer layer at the $\mathrm{Co}/\mathrm{BaTi}{\mathrm{O}}_3$ interface in a ferroelectric tunnel memristor. It is shown that in order to have a significant memristor behavior not only the interface oxygen vacancies but also the ${\mathrm{CoO}}_x$ layer thickness may vary with the applied bias.

Polar semiconductor materials enable a variety of classic and quantum-light sources, which are optimized continuously. However, one key problem—the inherent electric crystal polarization of such materials—remains unsolved and deteriorates the radiative exciton decay rate. We suggest a sequence of reverse interfaces to compensate these polarization effects, while the polar, natural crystal growth direction is maintained. Former research approaches, like growth on less-polar crystal planes or even the stabilization of unnatural phases, never reached industrial maturity. In contrast, our concept provides a way for the development of ultrafast devices based on established growth processes for polar materials, while the electric potential landscape becomes adjustable.

The use of nanosecond-duration-pulsed voltages with high-intensity electric fields ($\sim 100\mathrm{kV}/\mathrm{cm}$) is a promising development with many biomedical applications. Electroporation occurs in this regime, and has been attributed to the high fields. However, here we focus on temperature gradients. Our numerical simulations based on molecular dynamics predict the formation of nanopores and water nanowires, but only in the presence of a temperature gradient. Our results suggest a far greater role of temperature gradients in enhancing biophysical responses, including possible neural stimulation by infrared lasers.

$\mathrm{CrN}/\mathrm{MgO}(111)$ multilayers modeled via ab initio calculations give rise to nanoscale, scalable, spatially separated two-dimensional electron and hole gases, each confined to its own CrN interface. Because of the Cr $3d^3$ configuration, both electron and hole gases are based on correlated transition-metal layers involving bands of $3d$ character. Transport calculations predict each subsystem will have a large thermopower, on the order of $250\mu \mathrm{V}/\mathrm{K}$ at room temperature. These heterostructures combine a large thermoelectric efficiency with scalable nanoscale conducting sheets; for example, operating at a temperature difference of 50 K, 40 bilayers could produce a 1-V voltage with a film thickness of 100 nm.

The ${\mathrm{Ar}}^{+}$-irradiated molybdenum-disulfide (${\mathrm{MoS}}_2$) surface is studied by means of Raman spectroscopy and first-principles calculation. This experimental study reveals that ${\mathrm{Ar}}^{+}$ irradiation gives rise to satellite peaks at the lower-frequency side of the Raman-active $E_{2g}^1$ and $A_{1g}$ modes of ${\mathrm{MoS}}_2$ and a new peak at approximately $450{\mathrm{cm}}^{-1}$. We calculate the phonon modes and Raman spectra of defective ${\mathrm{MoS}}_2$ systems from first principles, and show that Mo and S vacancies give rise to such satellite peaks. These satellite peaks are a modulation of the $E_{2g}^1$ and $A_{1g}$ modes, described in terms of localization and scattering of vibration modes. The new peak at $450{\mathrm{cm}}^{-1}$, however, is a unique signature of the S vacancy. At low irradiation doses, the S vacancy is the dominant defect, whereas for large irradiation doses, the satellite peaks overshadow the ${\mathrm{MoS}}_2$ peaks, which we show to be typical for the Mo vacancy and ${\mathrm{MoS}}_6$ vacancy cluster. We thus show that Raman spectroscopy can be used not only to observe defects in two-dimensional materials, but also to identify the type of the defects.

We report the generation of twin beams by the parametric amplification four-wave mixing process and triplet beams by the parametric amplification six-wave mixing (PA SWM) process associated with the multiorder fluorescence signals in a ${\mathrm{Pr}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal. The intensity noise correlation and intensity-difference squeezing result from the nonlinear gain, which can be well controlled by the polarized dressing effect. The correlation value at the resonant position increases due to the double dressing effect; however, such correlation decreases if the triple dressing effect works. Specifically, correlation and squeezing between Stokes and anti-Stokes signals can be also switched by the relative nonlinear phase shift. The generated triplet beams from the PA SWM process have potential applications in three-mode all-optical information processing that can be used in on-chip photonic devices.

We propose a radio-frequency nano-oscillator based on feedback of spin current into a magnetic tunnel junction (MTJ) with an in-plane magnetized pinned layer and an out-of-plane magnetized free layer. The MTJ is connected to a “feedback” strip of a material like tungsten with a giant spin Hall effect. On passing a dc current through the MTJ, the thermal fluctuations of its free layer produce an oscillatory voltage across itself owing to the magnetoresistance effect. This oscillatory voltage drives an oscillatory current into the tungsten strip which converts this charge current into spin current via the spin Hall effect and feeds it back to the MTJ. We show that this feedback can amplify the fluctuations further and drive the free layer into periodic precessional states. We also propose a way of implementing spin-current feedback by using a nanomagnet coupled to the free layer of the MTJ by dipolar magnetic field.

We theoretically and experimentally introduce the concept of the local density of states (LDOS) associated with propagative surface plasmons (PSPs) launched along a structured thin gold film (a concept we call PSP LDOS). The alternative method couples a near-field optical microscope, in either the classical or the quantum regime of excitation, to a far-field leakage-radiation microscope. This method allows for selecting and collecting a very narrow portion of the directional SP wave vectors, thereby offering sufficient resolution to probe the collimation efficiency of a SP beam for a source near the focal point of a Bragg parabolic reflector. We are able to build and image the PSP LDOS in a fully integrated quantum SP launcher by depositing a diamond nanocrystal hosting nitrogen-vacancy centers at the focal point of the mirror. Our demonstration of the PSP LDOS with quantized SPs offers alternative prospects in the field of quantum plasmonics.

Ferroelectric diodes with polarization-modulated Schottky barriers are promising for applications in resistive switching (RS) memories. However, they have not achieved satisfactory performance reliability as originally hoped. The physical origins underlying this issue have not been well studied, although they deserve much attention. Here, by means of scanning Kelvin probe microscopy we show that the electrical poling of ferroelectric diodes can cause significant charge injection and trapping besides polarization switching. We further show that the reproducibility and stability of switchable diode-type RS behavior are significantly affected by the interfacial traps. A theoretical model is then proposed to quantitatively describe the modifications of Schottky barriers by charge injection and trapping. This model is able to reproduce various types of hysteretic current-voltage characteristics as experimentally observed. It is further revealed that the charge injection and trapping can significantly modify the electroresistance ratio, RS polarity, and high- or low-resistance states initially defined by the polarization direction. Several approaches are suggested to suppress the effect of charge injection and trapping so as to realize high-performance polarization-reversal-induced RS. This study, therefore, reveals the microscopic mechanisms for the RS behavior comodulated by polarization reversal and charge trapping in ferroelectric diodes, and also provides useful suggestions for developing reliable ferroelectric RS memories.

The performance of solar-cell devices is conventionally analyzed electrically by current-voltage measurements. To access the physics of the current generation process at the operating point, the analysis of fast optical responses from devices is highly beneficial. However, the optical responses from $p\text{−}n$ junctions exhibit a complex photoluminescence (PL) decay behavior due to their time-dependent electric fields. Here, we propose a method to systematically assign the physical meanings of the PL decay time constants by recording the complete excitation power dependence and voltage dependence of the time-resolved PL from a GaAs single junction. The experimentally obtained PL curves are in agreement with numerical predictions of dominant charge separation. We conclude that the charge separation can be directly observed in state-of-the-art devices. The experimental data set enables assignment of the effective separation time constant for the maximum output power condition, which is the most important number for understanding the carrier dynamics during device operation. The technique developed in this work constitutes a contactless characterization technique for solar cells.

We demonstrate a vapor-cell atomic-clock prototype based on a continuous-wave interrogation and double-modulation coherent population trapping (DM-CPT) technique. The DM-CPT technique uses a synchronous modulation of polarization and the relative phase of a bichromatic laser beam in order to increase the number of atoms trapped in a dark state, i.e., a nonabsorbing state. The narrow resonance, observed in the transmission of a Cs vapor cell, is used as a narrow frequency discriminator in an atomic clock. A detailed characterization of the CPT resonance versus numerous parameters is reported. A short-term fractional-frequency stability of $3.2\times {10}^{-13}{\tau }^{-1/2}$ up to a 100-s averaging time is measured. These performances are more than one order of magnitude better than industrial Rb clocks and are comparable to those of the best laboratory-prototype vapor-cell clocks. The noise-budget analysis shows that the short- and midterm frequency stability is mainly limited by the power fluctuations of the microwave used to generate the bichromatic laser. These preliminary results demonstrate that the DM-CPT technique is well suited for the development of a high-performance atomic clock, with the potential compact and robust setup due to its linear architecture. This clock could find future applications in industry, telecommunications, instrumentation, or global navigation satellite systems.

On a solid surface, a droplet of liquid will stick due to the capillary adhesion, and this causes low droplet mobility. To reduce contact line pinning, surface chemistry can be coupled to micro- and/or nanostructures to create superhydrophobic surfaces on which a droplet balls up into an almost spherical shape, thus, minimizing the contact area. Recent progress in soft matter has now led to alternative lubricant-impregnated surfaces capable of almost zero contact line pinning and high droplet mobility without causing droplets to ball up and minimize the contact area. Here we report an approach to surface-acoustic-wave- (SAW) actuated droplet transportation enabled using such a surface. These surfaces maintain the contact area required for efficient energy and momentum transfer of the wave energy into the droplet while achieving high droplet mobility and a large footprint, therefore, reducing the threshold power required to induce droplet motion. In our approach, we use a slippery layer of lubricating oil infused into a self-assembled porous hydrophobic layer, which is significantly thinner than the SAW wavelength, and avoid damping of the wave. We find a significant reduction (up to 85%) in the threshold power for droplet transportation compared to that using a conventional surface-treatment method. Moreover, unlike droplets on superhydrophobic surfaces, where interaction with the SAW induces a transition from a Cassie-Baxter state to a Wenzel state, the droplets on our liquid-impregnated surfaces remain in a mobile state after interaction with the SAW.

We use transient absorption spectroscopy to measure carrier lifetimes in the multiband semiconductor ${\mathrm{GaP}}_y{\mathrm{As}}_{1-x-y}{\mathrm{N}}_x$. These measurements probe the electron populations in the conduction band, intermediate band, and valence band as a function of time after an excitation pulse. Following photoexcitation of ${\mathrm{GaP}}_{0.32}{\mathrm{As}}_{0.67}{\mathrm{N}}_{0.01}$, we find that the electron population in the conduction band decays exponentially with a time constant ${\tau }_{\mathrm{CB}}=23\mathrm{ps}$. The electron population in the intermediate band exhibits bimolecular recombination with recombination constant $r=2\times {10}^{-8}{\mathrm{cm}}^3/\mathrm{s}$. In our experiment, an optical pump pulse excites electrons from the valence band to the intermediate and conduction bands, and the change in interband absorption due to absorption saturation and induced absorption is probed with a delayed white-light pulse. We model the optical properties of our samples using the band anticrossing model to extract carrier densities as a function of time. These results not only identify the short minority-carrier lifetime as a key factor affecting the performance of ${\mathrm{GaP}}_y{\mathrm{As}}_{1-x-y}{\mathrm{N}}_x$-based intermediate-band solar cells but also provide guidance on ways to address this issue.