Two-dimensional (2D) material-based van der Waals heterostructures (vdWHs) have been identified as an excellent platform from which to expand various device applications for light-emitting, photovoltaic, and field-effect transistors because of their different band alignments, including type-I, type-II, and type-III. However, it is difficult to achieve transformations between types of band alignment in a single heterostructure for diverse applications. In this study, the effects of a vertical electric field on the band alignment transition and electronic properties of 2D $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{S}}_2$ (${\mathrm{Sn}\mathrm{Se}}_2$) vdWHs are investigated systematically using density functional theory calculations. The study shows that the electric field can modulate not only the band gap but also the band alignment to produce multifunctional device applications. A positive electric field can control the band alignment transformation from type-II to type-I for electric field values of approximately 0.39 V/Å (0.12 V/Å), while a negative electric field can transform the type-II to type-III band alignments for electric field values of about −0.2 V/Å (−0.16 V/Å) in $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{S}}_2$ (${\mathrm{Sn}\mathrm{Se}}_2$) vdWHs. We trace these surprising results to the conduction band and valence band edge position movements for the linear decrease of $\mathrm{Ga}\mathrm{Se}$, while the linear increase of ${\mathrm{Sn}\mathrm{S}}_2$ (${\mathrm{Sn}\mathrm{Se}}_2$) occurs with the applied electric field. The present work may provide a direction for tunable multiple-band alignments in 2D vdWHs and help achieve multifunctional device applications.

Seismic metamaterials have been the subject of much recent interest, with notable conceptual advances and the demonstration of possible full-scale effectiveness in earthquake protection. Here, through a fine analysis of the dispersion properties and behavior of resonators introduced in a laboratory scale setup, we propose a strategy for the design of an efficient metaseismic barrier. When positioned onto an unconsolidated granular layer, rigid masses of nontrivial shape become elastic resonators by coupling with the relatively soft propagation medium. Additionally, they affect the mechanical boundary conditions and subsurface medium properties through the added weight. We demonstrate that all these ingredients can be rationally combined to improve the shielding effect and control the seismic waves.

A high-electron-mobility ${\mathrm{Ga}\mathrm{As}/\mathrm{Al}}_{0.36}{\mathrm{Ga}}_{0.64}\mathrm{As}$ heterostructure is processed by electron-beam lithography to fabricate samples with a separation of the current-supplying Ohmic contacts of between $28\mu \mathrm{m}$ and 1.3 mm. Voltage pulses of 20-ms duration, with a 4% filling factor and an amplitude up to 30 V, stimulate terahertz emission from samples cooled to 4 K. The radiation spectrum, centered at about 390 GHz (about 1.6 meV), registered with a Fabry-Perot interferometer, shows almost no dependence on the applied voltage or on the shape of the sample. The emission appears in a threshold manner and is accompanied by a jump of current by more than 2 orders of magnitude. The energy of the emitted photons is about three times smaller than in the case of any previously reported impurity-related emission from either bulk $\mathrm{Ga}\mathrm{As}$ or $\mathrm{Ga}\mathrm{As}$-based quantum wells. The emission is interpreted as resulting from a two-step ionization-recombination process involving shallow donors in the ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ barrier, which create bound states of electrons in the $\mathrm{Ga}\mathrm{As}$ quantum well. Thus, the frequency of emission is argued to be tuned by the width of the spacer layer. We present these as small convenient sources that can be used for calibration and check-up operation of cooled bolometric systems used in astronomical observations.

Spin-dependent transport in full van der Waals (vdW) giant magnetoresistance (GMR) junctions with the structure of ${\mathrm{Fe}}_3{\mathrm{Ge}\mathrm{Te}}_2/X{\mathrm{Te}}_2/{\mathrm{Fe}}_3{\mathrm{Ge}\mathrm{Te}}_2$ (X = $\mathrm{Pt}$, $\mathrm{Pd}$) is investigated by using first-principles calculations. The ballistic conductance, magnetoresistance (MR), and resistance-area product (RA) are calculated in a current-perpendicular-to-plane (CPP) geometry. A giant magnetoresistance of around 2000% and RA less than 0.3 Ω µ${\mathrm{m}}^2$ are found in the proposed vdW CPP GMR. In addition, the spin-orbit-coupling effect on transport and anisotropy magnetoresistance (AMR) are also investigated. The calculated AMR is found to be around 20% in the ${\mathrm{Fe}}_3{\mathrm{Ge}\mathrm{Te}}_2/$trilayer ${\mathrm{Pd}\mathrm{Te}}_2/{\mathrm{Fe}}_3{\mathrm{Ge}\mathrm{Te}}_2$ CPP GMR. Both GMR and AMR in the proposed vdW CPP GMR mainly originate from the bulk electronic structure properties of ${\mathrm{Fe}}_3{\mathrm{Ge}\mathrm{Te}}_2$. This work demonstrates a vdW CPP GMR with superior advantages, including perpendicular magnetic anisotropy, large GMR, low RA, and sizable AMR, which may stimulate future experimental explorations and should be appealing for its application in spintronic devices, including magnetic sensors and memory.

A combination of digitized shortcut to adiabaticity (STA) and sequential digitized adiabaticity is implemented in a superconducting quantum device to determine the electronic states in two example systems, the ${\mathrm{H}}_2$ molecule and the topological Bernevig-Hughes-Zhang (BHZ) model. For ${\mathrm{H}}_2$, a short internuclear distance is chosen as a starting point, at which the ground and excited states are obtained via digitized STA. From this starting point, a sequence of internuclear distances is built. The eigenstates at each distance are sequentially determined from those at the previous distance via the digitized adiabaticity, leading to the potential energy landscapes of ${\mathrm{H}}_2$. The same approach is applied to the BHZ model and the valence and conduction bands are obtained with a high degree of accuracy along the $X$-$\mathrm{\Gamma }$-$X$ line cut of the first Brillouin zone. Furthermore, a numerical simulation of this method is performed to successfully extract the ground states of hydrogen chains with lengths of between three and six atoms.

The stability of structures with nontrivial topology is a subject of great interest, both from fundamental and technological perspectives. The topology of a magnetic vortex, for example, results in high stability of the vortex core polarity, which is attractive for applications. This high stability, however, greatly impedes the required deterministic polarity switching, which is typically only achieved with large magnetic fields or strong dynamic driving. Here, we show that the interaction between the vortex core and manufactured nanoscale defects in the magnetic material lowers the required driving strength for core polarity reversal by more than an order of magnitude. We excite vortex dynamics in thin permalloy disks, and map the two-dimensional (2D) vortex core trajectory using 3D time-resolved Kerr microscopy. In pristine samples, we observe normal gyrotropic motion of the vortex core. After laser-induced generation of defects, however, we observe repeated vortex core reversal at much-reduced driving strength. Micromagnetic simulations reveal how local reduction of exchange coupling and saturation magnetization can create vortex core reversal sites for deterministic vortex core switching.

We present the results of a three-dimensional kinetic Monte Carlo (3D KMC) simulation study of the current density and efficiency of a prototypical green phosphorescent organic light-emitting diode within which the TPBi [2,$2^{\prime }$,$2^{″}$-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)] molecules that comprise the electron transport layer have a large (7 D) static dipole moment. By explicitly calculating the dipole-induced electrostatic field distribution the additional energetic disorder and the additional internal electrostatic field due to the small net dipole-moment orientation in TPBi are included in a mechanistic manner. The simulation results are compared with the results of an experimental study by B. Sim et al. [ACS Appl. Mater. Interf. 8, 33010 (2016)]. Using a set of simulation parameters that provides a fair agreement with the experimental voltage-dependent current density, external quantum efficiency, and emission profiles, the simulations are used to study the sensitivity to the size and net orientation of the dipole moments. We show how the dipole-induced disorder and the dipole orientation affect the current density, the electron injection efficiency, the blocking of holes by the TPBi layer, the shape of the emission profile, and the quantum efficiency. A key element of the validation of the simulations is a comparison with measured emission profiles, which Sim et al. obtained from sense layer experiments. Explicit KMC simulations of these experiments are used to investigate the expected accuracy with which the emission profiles are probed.

We report direct visualization of gigahertz-frequency Lamb-wave propagation in aluminum nitride phononic circuits by transmission-mode microwave impedance microscopy (TMIM). Consistent with finite-element modeling, the acoustic eigenmodes in both a horn-shaped coupler and a subwavelength waveguide are revealed in the TMIM images. Using fast Fourier-transform filtering, we quantitatively analyze the acoustic loss of individual Lamb modes along the waveguide and the power-coupling coefficient between the waveguide and the parabolic couplers. Our work provides insightful information on the propagation, mode conversion, and attenuation of acoustic waves in piezoelectric nanostructures, which is highly desirable for designing and optimizing phononic devices for microwave-signal processing and quantum-information transduction.

We present a scheme to generate continuous-variable bipartite entanglement between two optical modes in a hybrid optical-microwave-plasmonic graphene waveguide system. In this scheme, we exploit the interaction of two light fields coupled to the same microwave mode via plasmonic graphene waveguide to generate two-mode squeezing, which can be used for continuous-variable quantum teleportation of the light signals over large distances. Furthermore, we study the teleportation fidelity of an unknown coherent state. The teleportation protocol is robust against the thermal noise associated with the microwave degree of freedom.

The performances of mode-locked fiber lasers rest with the design and configuration of the cavity, and their temporal morphologies are rarely linked with the modulation of transverse modes. Here, we demonstrate a morphology-controllable ultrafast fiber laser based on the intracavity manipulation of transverse modes, capable of producing dual-color pulses, narrowband picosecond pulses, and broadband femtosecond pulses. Unlike traditional multicolor pulses with unequal group velocities, the dual-color pulse operates in a synchronous state, displaying a multipeak structure where the modulation period relies on the group-delay difference of two modes in the fiber. Simulation results fully reproduce the experimental observations and show that the laser follows the minimum-loss principle with pulse morphologies depending on the mode-interference-induced filtering effect. This work connects transverse-mode modulation with pulse morphology, providing a stable and cost-effective laser source for terahertz-wave generation and nonlinear spectroscopy.

Pure spin currents, generated by nonlocal spin injection and spin-orbit effects, have been widely used to control magnetization reversal and dynamics by current without charge transfer and its side effects in the device’s actual working region. Here we experimentally demonstrate that a single coherent spin-wave mode can be excited by the pure spin current in a nonlocal spin-injection spin-valve device. The microwave spectra show that the observed spin-wave frequency is higher than the ferromagnetic resonance frequency, and almost does not change with the excitation current at moderate magnetic fields, indicating that the observed dynamical mode is a linear propagating spin-wave mode. Furthermore, micromagnetic simulations based on our device geometry generally reproduce the experimentally observed field-dependent and current-dependent oscillation characteristics, and provide us with the additional spatial information that the spin-wave mode exhibits collimated and bidirectional propagation paths in the direction perpendicular to the applied magnetic field. Our simulation results also show that the current-local Oersted field in our nonlocal device is much smaller than in conventional nanocontact magnetic oscillators, and has a minimal impact on the spin-wave dynamics. A near-symmetrically-collimated and bidirectional propagating spin-wave beam with magnetic field–controllable beam direction and current-independent frequency, achieved in our demonstrated nonlocal spin-current nano-oscillator, can be used as a local spin-wave source for magnonic logic devices and can be used to build daisy-chaining oscillatory neural networks with mutual synchronization.

We perform a computational study, based on the molecular-dynamics method, of the shape of Miram curves obtained from microscale planar diodes. We discuss the smooth transition from the source-limited regime to the space-charge-limited regime due to the finite size of the emitter (i.e., the “knee” in the Miram curve). In our model we find that the smoothing occurs mostly due to the increased emission at the external edges of the emitting area, and that the knee becomes softer when the size of the emitting area decreases. We relate this to recent work that has described how a heterogeneous work function similarly affects the Miram curve.

The utility of electrical resistivity as an indicator of magnetoionic performance in stoichiometrically and structurally similar thin-film systems is demonstrated. A series of highly nanocrystalline cobalt nitride ($\mathrm{Co}\text{−}\mathrm{N}$) thin films (85 nm thick) with a broad range of electrical properties exhibit markedly different magnetoionic behaviors. Semiconducting, near stoichiometric $\mathrm{Co}\mathrm{N}$ films show the best performance, better than their metallic and insulating counterparts. Resistivity reflects the interplay between atomic bonding, carrier localization, and structural defects, and in turn determines the strength and distribution of applied electric fields inside the actuated films. This fact, generally overlooked, reveals that resistivity can be used to quickly evaluate the potential of a system to exhibit optimal magnetoionic effects, while also opening interesting challenges.

Mirror symmetry is among the most fundamental concepts of physics, and its spontaneous breaking at the molecular level allows chiral molecules to exist in two enantiomers that are mirror images of each other. The majority of the chiro-optical effects routinely used to detect enantiomers in mixtures, such as circular dichroism, rely on chiral sensitivity to photon circular polarization, and thus do not harness the full potential of mirror-symmetry breaking, which also involves the spatial profile of the radiation. Here we show that the parity indefiniteness of an electromagnetic field interacting with chiral matter supports mirror optical activity, a chiro-optical effect where a chiral film, when it is probed by the mirror-symmetric field of a nanoemitter, produces a near field whose spatial profile has broken mirror symmetry. The detection of near-field dissymmetry can provide an efficient chiral sensing technique. We specialize the discussion to nanofilms with infrared chirality by using a swift electron in an aloof configuration as the nanoemitter and an off-axis transparent conducting nanoparticle as the near-field probe; the spatial dissymmetry factor of the nanoparticle cathodoluminescence is 1 order of magnitude larger than the dimensionless circular dichroism, and is further enhanced to 2 orders of magnitude if an additional graphene sheet is deposited on the film interface.

Nonground eigenvalues are widely disregarded in magnetic resonance relaxation measurements of porous media due to difficulties involved in their measurement, detection, and the derivation of physically meaningful parameters from them. Such nonground eigenvalues may be experimentally observed in relaxation measurements, such as the relaxation correlation of $T_1-T_2$, and yield information on the pore size and surface relaxivity of porous media without calibration through other independent measurements. Nonground eigenvalue analysis of $T_1-T_2$ measurements on Berea sandstone undertaken at three static magnetic fields produces pore sizes consistent with those obtained through x-ray microtomography and SEM measurements. Similar agreement is found for a Bentheimer sandstone with a more complex pore geometry. A phase-encoding imaging variant of this method measures the imbibition confinement-size profile in Berea sandstone. It is suggested that the existence of nonground eigenmodes may be much more prevalent in simple magnetic resonance relaxation measurements than previously considered. Therefore, it is possible to measure pore size by matching numerical Brownstein-Tarr solutions with those of experiments in a wide variety of samples and magnetic resonance methods.

The 5d transition-metal oxides are an intriguing platform to demonstrate efficient charge-to-spin-current conversion due to a unique electronic structure dominated by strong spin-orbit coupling. Here, we report on the stacking-order effect of spin-orbit torque (SOT), spin-Hall magnetoresistance, and magnetic anisotropy in bilayer ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$-5d iridium oxide, ${\mathrm{Ir}\mathrm{O}}_2$. While all ${\mathrm{Ir}\mathrm{O}}_2$ and $\mathrm{Pt}$ control samples exhibit large dampinglike SOT generation, stemming from the efficient charge-to-spin-current conversion, the magnitude of the SOT is larger in the ${\mathrm{Ir}\mathrm{O}}_2$($\mathrm{Pt})$ bottom sample than in the ${\mathrm{Ir}\mathrm{O}}_2$($\mathrm{Pt})$ top one. The fieldlike SOT has an even more significant stacking-order effect, resulting in an opposite sign in the ${\mathrm{Ir}\mathrm{O}}_2$ samples in contrast to the same sign in the $\mathrm{Pt}$ samples. Furthermore, we observe that the magnetic anisotropy energy density and the anomalous Hall effect are increased in the ${\mathrm{Ir}\mathrm{O}}_2$($\mathrm{Pt})$ bottom sample, suggesting enhanced interfacial perpendicular magnetic anisotropy. Our findings highlight the significant influence of the stacking order on spin transport and the magnetotransport properties of $\mathrm{Ir}$ oxide-ferromagnet systems, providing useful information for the design of SOT devices, including 5d transition-metal oxides.

We investigate temperature-dependent magnetization reversal of $\mathrm{Co}\mathrm{Ru}$ graded films, in which a predefined depth-dependent exchange-coupling strength J follows a V-shaped profile. Magnetometry reveals an extended temperature range below the Curie temperature $T_C$ where the reversal of the magnetization M is not accompanied by the conventionally occurring hysteresis, in stark contrast with homogeneous $\mathrm{Cr}\mathrm{Ru}$ reference films. This is caused by the temperature-driven paramagnetic (PM)-ferromagnetic (FM) phase transition, which does not occur in the entirety of the graded material but only in well-defined nanoscopic regions at any given temperature, enabling the creation of two internal PM/FM interfaces that assist the external magnetic field in reversing the magnetization of the FM graded sample region. Hysteretic reversal is recovered at sufficiently low sample temperatures or by using graded structures with very steep J gradients, so that no PM/FM interfaces form inside the exchange-coupled layer material that could influence the magnetization reversal process. Our findings open interesting material design options, and we envision wide application potential, since we succeed in engineering a temperature gap between the temperature dependent magnetization and hysteresis onsets, a capability of interest for any technology benefitting from field-free magnetization switching.