The optimal amount of dysprosium in the highly magnetostrictive rare-earth compounds ${\mathrm{Tb}}_{1-x}{\mathrm{Dy}}_x{\mathrm{Fe}}_2$ for room-temperature applications has long been known to be $x=0.73$ (Terfenol-D). Here, we derive this value from first principles by calculating the easy magnetization direction and magnetostriction as a function of composition and temperature. We use crystal-field coefficients obtained within density-functional theory to construct phenomenological anisotropy and magnetoelastic constants. The temperature dependence of these constants is obtained from disordered-local-moment calculations of the rare-earth magnetic order parameter. Our calculations find the critical $\mathrm{Dy}$ concentration required to switch the magnetization direction at room temperature to be $x_c=0.78$, with magnetostrictions ${\lambda }_{111}=2700$ and ${\lambda }_{100}=-430$ ppm, close to the Terfenol-D values.

We study electric dipole spin resonance caused by subterahertz radiation in a multilevel finite-size quantum dot formed in a nanowire, focusing on the range of driving electric field amplitudes where a strong interplay between the Rabi spin oscillations and tunneling from the dot to continuum states can occur. A strong effect of the tunneling on the spin evolution in this regime occurs due to the formation of mixed spin states. As a result, the tunneling strongly limits possible spin manipulation times. We demonstrate a backaction of the spin dynamics on the tunneling and position of the electron. The analysis of the efficiency of the spin manipulation in terms of the system energy shows that tunneling decreases this efficiency. Fourier spectra of the time-dependent expectation value of the electron position show a strong effect of the spin-orbit coupling on their low-frequency components. This result can be applied to operational properties of spin-based nanodevices and can extend the range of possible spin resonance frequencies to the subterahertz domain.

The rapid development of active acoustic metamaterials provides an alternative degree of freedom to manipulate scattering fields and acoustic radiation forces. With the multipole expansion method, acoustic radiation forces can be expanded in a series of coupling between adjacent pairs of scattering-field coefficients. Here, systems from two dimensional to one dimensional are explicitly considered. When shining a single acoustic plane wave, a necessary requirement to achieve negative force is that the particle has to be sufficiently active. A criterion of achieving the most negative or positive force by tuning the adjacent terms “in phase” or “out of phase” is derived. Thus, with the required scattering coefficients, the effective density and modulus corresponding to the largest pulling-pushing forces can be retrieved directly and function as a guide in the design of active metamaterials in one-dimensional systems. Our study can bridge the theoretical requirements of radiation force with the practical design of metamaterials.

We investigate surface-acoustic-wave- (SAW) induced ferromagnetic-resonance- (FMR) assisted spin-transfer-torque (STT) switching of perpendicular magnetic tunnel junctions (PMTJs) with inhomogeneities using micromagnetic simulations that include the effect of thermal noise. With suitable frequency excitation, the SAW can induce ferromagnetic resonance in magnetostrictive materials, and the magnetization can precess in a cone with high deflection from the perpendicular direction. With incorporation of inhomogeneity via lateral anisotropy variation as well as room-temperature thermal noise, the magnetization precession in different gains can be significantly incoherent. Interestingly, the precession in different grains are found to be in phase, even though the precession amplitude (angle of deflection from the perpendicular direction) varies across grains of different anisotropy. Nevertheless, the high mean deflection angle due to acoustically induced FMR can complement the STT switching by reducing the STT current significantly; even though the applied stress induced change in anisotropy is much lower than the total anisotropy barrier. This work indicates that SAW-induced FMR-assisted switching can improve energy efficiency while being scalable to very small dimensions, which is technologically important for STT random-access memory and elucidates the physical mechanism for the potential robustness of this paradigm in realistic scenarios with thermal noise and material inhomogeneity.

The presence of trap states plays an important role in the performance of optoelectronic devices. In the present investigation, charge-carrier dynamics in amorphous and crystalline ${\mathrm{In}}_2{\mathrm{Se}}_3$ thin films is studied using broadband femtosecond pump-probe spectroscopy in the temperature range of 5–300 K. Amorphous ${\mathrm{In}}_2{\mathrm{Se}}_3$ thin films (approximately 300 nm thick) are deposited using a thermal-evaporation method, and undergo a transition to a crystalline phase under vacuum annealing at $250{}^{\circ }\mathrm{C}$. Ultrafast transient absorption studies suggest that the charge-carrier dynamics is dominated by the presence of inter-band-gap defect states in both types of film, and it is found to be slower in the amorphous than in the crystalline material due to the presence of deep defect states in the amorphous material. The carrier dynamics in the amorphous film is faster at 5 K than at 300 K, as the probability of free carriers becoming trapped in defect states is much higher at high temperatures due to their higher mobility. The change in resistance under light and with temperature is directly correlated with ultrafast spectroscopic data.

We study, both experimentally and theoretically, the modification of ${\mathrm{Er}}^{3+}$ photoluminescence properties in $\mathrm{Si}$ dielectric nanoslots. The ultrathin nanoslot (down to 5-nm thickness), filled with $\mathrm{Er}$ in ${\mathrm{Si}\mathrm{O}}_2$, boosts the electric and magnetic local density of states via coherent near-field interaction. We report an experimental 20-fold enhancement of the radiative decay rate with negligible losses. Moreover, via modifying the geometry of the all-dielectric nanoslot, the outcoupling of the emitted radiation to the far field can be strongly improved, without affecting the strong decay-rate enhancement given by the nanoslot structure. Indeed, for a periodic square array of slotted nanopillars an almost one-order-of-magnitude-higher ${\mathrm{Er}}^{3+}$ PL intensity is measured with respect to the unpatterned structures. This has a direct impact on the design of more efficient CMOS-compatible light sources operating at telecom wavelengths.

There are diverse interdisciplinary applications for nanoscale-resolution electrometry of elementary charges under ambient conditions. These include characterization of two-dimensional electronics, charge transfer in biological systems, and measurement of fundamental physical phenomena. The nitrogen-vacancy center in diamond is uniquely capable of such measurements, however electrometry thus far has been limited to charges within the same diamond lattice. It has been hypothesized that the failure to detect charges external to diamond is due to quenching and surface screening, but no proof, model, or design to overcome this has yet been proposed. In this work we affirm this hypothesis through a comprehensive theoretical model of screening and quenching within a diamond electrometer and propose a solution using controlled nitrogen doping and a fluorine-terminated surface. We conclude that successful implementation requires further work to engineer diamond surfaces with lower surface-defect concentrations.

Second-order topological insulators, which exhibit capability of hosting topologically protected zero-dimensional corner states distinct from the well-studied topological edge states, unveil a horizon beyond the conventional bulk-edge correspondence. Motivated by recent experimental observation of Wannier-type second-order corner states in acoustic structures, we investigate numerically and demonstrate experimentally the in-plane edge and corner states in a mechanical kagome lattice. By manipulating simply lattice geometry and quantized characterization, we exploit that the emerging corner states are topologically robust against disorders. We further present a second-order topological insulator with multi-interfaces such that the mechanical energy can be localized in multiple locations, which provides the possibility of practical application in energy harvesting devices. The present study is the physical observation to extend the second-order topological insulator to in-plane elastic dynamics, and modal coupling of in-plane elasticity makes it more challenging to be measured experimentally.

Theoretical investigations of carrier transport in type-II superlattice detectors have been mostly limited to simplified semiclassical treatments, due to the computational challenges posed by quantum kinetic approaches. For example, interband tunneling in broken-gap configurations calls for a multiband description of the electronic structure, and spatially indirect optical transitions in superlattice absorbers require fully nonlocal carrier-photon self-energies. Moreover, a large number of iterations is needed to achieve self-consistency between Green’s functions and self-energies in the presence of strongly localized states not directly accessible from the contacts. We demonstrate an accurate, yet computationally feasible nonequilibrium Green’s function model of superlattice detectors by formulating the kinetic equations in terms of problem-matched maximally localized basis functions, numerically generated from few modes representing the main conductive channels of the nanostructure. The contribution of all the remaining modes is folded in an additional self-energy to ensure current conservation. Inspection of spatially and energetically resolved single particle properties offers insight into the complex nature of carrier transport in type-II superlattice detectors, and the connection to semiclassical approaches enables the interpretation of mobility experiments.

The Airy beam, known to present particular unique features, such as diffraction-free, self-healing, and self-bending, has attracted considerable interest in recent years. Here, an electronically controlled reflective coding metasurface is exploited to generate and dynamically manipulate the Airy beam over a wide frequency band in the microwave regime. By judiciously controlling the external dc bias voltage applied to the programmable metasurface, two distinct control states, “0” and “1”, are engineered as digital codes for −π/2 and +π/2 phase responses, respectively. The programmable binary-phase-coding metasurface enables the generation and active reshaping of the Airy beam to be numerically simulated and experimentally validated. Moreover, the metasurface allows the bandwidth limitation of passive counterparts to be overcome and is able to modulate electromagnetic waves over a broad frequency range, spanning from 9 to 12 GHz, to realize diffraction-free Airy beams.

The negative-capacitance effect in devices based on combined ferroelectric-dielectric gate oxides is thought to be a potential solution to break free from the so-called Boltzmann tyranny. To lower the power consumption in field-effect transistors, the subthreshold swing factor $S$ should be reduced below the thermodynamic limit of $60\mathrm{mV}$ per decade. Yet, despite numerous studies dedicated to this effect in the past decade, its origin in ferroelectric capacitors or ferroelectric-based superlattices remains unclear, being considered either a transitory product of polarization switching or an intrinsic phenomenon related to the presence of ferroelectric polarization. In this study it is shown, starting from simple electrostatic considerations, that negative capacitance is present during polarization switching and is accompanied by a significant increase of the current flowing through the ferroelectric capacitor. Coupled with piezo-force microscopy results, it is shown that the polarization orientation suddenly changes at the coercive voltage, accompanied by a complete reconfiguration of the potential barriers at the Schottky-like contacts present at the electrode-ferroelectric interfaces. A method to estimate the polarization-switching time, as the time associated with the presence of the negative-capacitance effect, is proposed. Values in the range from 100 to 1000 ns are obtained for epitaxial $\mathrm{Pb}{\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3$ films. These findings suggest that negative capacitance may be an intrinsic effect in ferroelectrics but that it is a transitory effect, present only when ferroelectric polarization passes through zero (switching).

We show how an exceptional point of degeneracy (EPD) is formed in a system composed of an electron beam interacting with an electromagnetic mode guided in a slow wave structure (SWS) with distributed power extraction from the interaction zone. Based on this kind of EPD, a regime of operation is devised for backward-wave oscillators (BWOs) as a synchronous and degenerate regime between a backward electromagnetic mode and the charge wave modulating the electron beam. Degenerate synchronization under this EPD condition means that two complex modes of the interactive system do not share just the wave number, but they rather coalesce in both their wave numbers and eigenvectors (polarization states). In principle, this condition guarantees full synchronization between the electromagnetic wave and the beam’s charge wave for any amount of output power extracted from the beam, setting the threshold of this EPD-BWO to any arbitrary, desired, value. Indeed, we show that the presence of distributed radiation in the SWS results in having high-threshold electron-beam current to start oscillations, which implies higher power generation. These findings have the potential to lead to highly efficient BWOs with very high output power and excellent spectral purity.

An understanding of the damping mechanism in finite-size systems and its dependence on temperature is a critical step in the development of magnetic nanotechnologies. In this work, nanosized materials are modeled via atomistic spin dynamics, the damping parameter being extracted from ferromagnetic resonance (FMR) simulations applied for FePt systems, generally used for heat-assisted magnetic recording media (HAMR). We find that the damping increases rapidly close to $T_C$ and the effect is enhanced with decreasing system size, which is ascribed to scattering at the grain boundaries. Additionally, FMR methods provide the temperature dependence of both damping and the anisotropy, which are important for the development of HAMR. Semianalytical calculations show that, in the presence of a grain-size distribution, the FMR line width can decrease close to the Curie temperature due to a loss of inhomogeneous line broadening. Although FePt has been used in this study, the results presented in the current work are general and valid for any ferromagnetic material.

In this work, we propose the use of power-flow–conformal metasurfaces for the creation of dual-physics devices that can simultaneously control waves of different natures. In particular, we introduce metasurface devices that perform specified operations on both electromagnetic and acoustic waves at the same time. Using a purely analytical model based on surface impedances, we introduce metasurfaces that perform the same functionality for electromagnetic and acoustic waves and, even more challenging, different functionalities for electromagnetics and acoustics. We provide realistic topologies for practical implementations of the proposed metasurfaces and confirm the results with numerical simulations.

Current paradigm suggests that the Ohmic electrical response of nanochannel-microchannel systems is determined solely by the nanochannel while the effects of the adjacent microchannels are negligible. However, recent works have challenged this paradigm and have shown that at low concentrations the microchannels contribute in a non-negligible manner. As such, the two favored models used to explain experiments are inadequate in describing realistic nanochannel-microchannel systems. To partially reconcile some of these issues, two newer nanochannel-microchannel models were derived and suggested as a suitable replacement for the nanochannel-dominant models. Unfortunately, these two models are limited to either very low or very high concentrations. In this work, we review these four leading models. We discuss their key assumptions, advantages, shortcomings, and present a knowledge gap between all models pertaining to the effects of the microchannel resistance for all concentrations. To overcome this gap, we derive an analytical solution that accounts for the effects of the microchannels and holds for all concentrations. This solution unifies three of the existing models where we show that they are limiting cases of our more general solution. We are also able to disqualify the fourth model. Our derived solution shows remarkable correspondence to simulations and experiments. The insights from this unifying model can be used to improve the design of any nanofluidic based systems.

A promising method to leverage near-field power densities without the use of nanoscale vacuum gaps is through hyperbolic metamaterial (HMM) waveguides. When placed between a hot and cold reservoir, an ideal HMM can transmit surface waves across several microns, enabling an extension of near-field enhancements. However, when accounting for transmission loss due to realistic levels of absorption within the waveguide, previous studies have shown that the enhancements are significantly curtailed at wide separations. In our study, we investigate the role of internal sources within nonisothermal HMMs. We demonstrate that, in some cases, the emission from the HMM accounts for over 90% of the total radiative heat transfer to the receiver, and that these additional sources can largely compensate for optical losses associated with decreased transmission from the emitter to the receiver. Lastly, we investigate the spectral transport in a realistic three-body system that has mismatching optical properties between the boundaries (emitter and receiver) and the waveguide (HMM). Our model shows that the near-field thermal transport remains spectrally selective to the boundaries, even as major radiative contributions come from the waveguide. This work may enable the design of nonisothermal emitter-waveguide-receiver systems that transmit near-field power levels over wider separations.

Layer-dependent electronic and structural properties of emerging graphitic carbon-boron compound ${\mathrm{C}}_3\mathrm{B}$ are investigated using both density-functional theory and the GW approximation. We discover that, in contrast to a moderate quasiparticle band gap of 2.55 eV for monolayer ${\mathrm{C}}_3\mathrm{B}$, the calculated quasiparticle band gap of perfectly stacked bulk phase ${\mathrm{C}}_3\mathrm{B}$ is as small as 0.17 eV. Therefore, our results suggest that layered material ${\mathrm{C}}_3\mathrm{B}$ exhibits a remarkably large band-gap renormalization of over 2.3 eV due to the interlayer coupling and screening effects, providing a single material with an extraordinary band-gap tunability. The quasiparticle band gap of monolayer ${\mathrm{C}}_3\mathrm{B}$ is also over 1.0 eV larger than that of ${\mathrm{C}}_3\mathrm{N}$, a closely related two-dimensional semiconductor. Detailed inspections of the near-edge electronic states reveal that the conduction- and valence-band edges of ${\mathrm{C}}_3\mathrm{B}$ are formed by out-of-plane and in-plane electronic states, respectively, suggesting an interesting possibility of tuning the band edges of such layered material separately by modulating the in-plane and out-of-plane interactions.