The interfacial Dzyaloshinskii-Moriya interaction (IDMI) is investigated in ${\mathrm{Co}}_2\mathrm{FeAl}$ (CFA) ultrathin films of various thicknesses ($0.8\mathrm{nm}\le t_{\mathrm{CFA}}\le 2\mathrm{nm}$) grown by sputtering on Si substrates, using Pt, W, Ir, and MgO buffer or/and capping layers. Vibrating sample magnetometry reveals that the magnetization at saturation ($M_s$) for the Pt- and Ir-buffered films is higher than the usual $M_s$ of CFA due to the proximity-induced magnetization (PIM) in Ir and Pt estimated to be 19% and 27%, respectively. The presence of PIM in these materials is confirmed using x-ray resonant magnetic reflectivity. Moreover, while no PIM is induced in W, higher PIM is obtained with Pt when it is used as a buffer layer rather than a capping layer. Brillouin light scattering in the Damon-Eshbach geometry is used to investigate the thickness dependences of the IDMI constants from the spin-wave nonreciprocity and the perpendicular anisotropy field versus the annealing temperature. The IDMI sign is found to be negative for $\mathrm{Pt}/\mathrm{CFA}$ and $\mathrm{Ir}/\mathrm{CFA}$, while it is positive for $\mathrm{W}/\mathrm{CFA}$. The thickness dependence of the effective IDMI constant for stacks involving Pt and W shows the existence of two regimes similar to that of the perpendicular anisotropy constant due to the degradation of the interfaces as the CFA thickness approaches a critical thickness. The surface IDMI and anisotropy constants of each stack are determined for the thickest samples where a linear thickness dependence of the effective IDMI constant and the effective magnetization are observed. The interface anisotropy and IDMI constants investigated for the $\mathrm{Pt}/\mathrm{CFA}/\mathrm{MgO}$ system show different trends with the annealing temperature. The decrease of the IDMI constant with increasing annealing temperature is probably due to the electronic structure changes at the interfaces, while the increase of the interface anisotropy constant is coherent with the interface quality and disorder enhancement.

Nonlinear optical absorption of light by materials is weak due to its perturbative nature, although a strong nonlinear response is of crucial importance to applications in optical limiting and switching. Here we demonstrate experimentally and theoretically an extremely efficient scheme of excited-state absorption by charge transfer between donor and acceptor materials as a method to enhance the nonlinear absorption by orders of magnitude. With this idea, we demonstrate a strong excited-state absorption (ESA) in reduced graphene oxide that otherwise shows an increased transparency at high fluence and enhancement of ESA by one order of magnitude in graphene oxide by attaching gold nanoparticles (Au NP) in the tandem configuration that acts as an efficient charge-transfer pair when excited at the plasmonic wavelength. To explain the unprecedented enhancement, we develop a five-level rate-equation model based on the charge transfer between the two materials and numerically simulate the results. To understand the correlation of interfacial charge transfer with the concentration and type of the functional ligands attached to the graphene oxide sheet, we investigate the Au-NP—graphene oxide interface with various possible ligand configurations from first-principles calculations. By using the strong ESA of our hybrid materials, we fabricate liquid cell-based high-performance optical limiters with important device parameters better than that of the benchmark optical limiters.

One of the fundamental limitations in photonics is the lack of a bidirectional transducer that can convert optical information into electronic signals or vice versa. In acoustics or at microwave frequencies, wave signals can be simultaneously measured and modulated by a single transducer. In optics, however, optical fields are generally measured via reference-based interferometry or holography using silicone-based image sensors, whereas they are modulated using spatial light modulators. Here, we propose a scheme for an optical bidirectional transducer using a spatial light modulator. By exploiting the principle of time-reversal symmetry of light scattering, two-dimensional reference-free measurement and modulation of optical fields are realized. We experimentally demonstrate the optical bidirectional transducer for optical information consisting of $128\times 128$ spatial modes at visible and short-wave infrared wavelengths.

Antiferromagnets (AFMs) have recently gathered a large amount of attention as a potential replacement for ferromagnets (FMs) in spintronic devices due to their lack of stray magnetic fields, invisibility to external magnetic probes, and faster magnetization dynamics. Their development into a practical technology, however, has been hampered by the small number of materials where the antiferromagnetic state can be both controlled and read out. We show that by relaxing the strict criterion on pure antiferromagnetism, we can engineer an alternative class of magnetic materials that overcome these limitations. This is accomplished by stabilizing a noncollinear magnetic phase in ${\mathrm{LaNiO}}_3/{\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ superlattices. This state can be continuously tuned between AFM and FM coupling through varying the superlattice spacing, strain, applied magnetic field, or temperature. By using this alternative “knob” to tune magnetic ordering, we take a nanoscale materials-by-design approach to engineering ferromagneticlike controllability into antiferromagnetic synthetic magnetic structures. This approach can be used to trade-off between the favorable and unfavorable properties of FMs and AFMs when designing realistic resistive antiferromagnetic memories. We demonstrate a memory device in one such superlattice, where the magnetic state of the noncollinear antiferromagnet is reversibly switched between different orientations using a small magnetic field and read out in real time with anisotropic magnetoresistance measurements.

Atomic force microscopy is an analytical characterization method that is able to image a sample’s surface topography at high resolution while simultaneously probing a variety of different sample properties. Such properties include tip-sample interactions, the local measurement of which has gained much popularity in recent years. To this end, either the oscillation frequency or the oscillation amplitude and phase of the vibrating force-sensing cantilever are recorded as a function of tip-sample distance and subsequently converted into quantitative values for the force or interaction potential. Here, we theoretically and experimentally show that the force law obtained from such data acquired under vacuum conditions using the most commonly applied methods may deviate more than previously assumed from the actual interaction when the oscillation amplitude of the probe is of the order of the decay length of the force near the surface, which may result in a non-negligible error if correct absolute values are of importance. Caused by approximations made in the development of the mathematical reconstruction procedures, the related inaccuracies can be effectively suppressed by using oscillation amplitudes sufficiently larger than the decay length. To facilitate efficient data acquisition, we propose a technique that includes modulating the drive amplitude at a constant height from the surface while monitoring the oscillation amplitude and phase. Ultimately, such an amplitude-sweep-based force spectroscopy enables shorter data acquisition times and increased accuracy for quantitative chemical characterization compared to standard approaches that vary the tip-sample distance. An additional advantage is that since no feedback loop is active while executing the amplitude sweep, the force can be consistently recovered deep into the repulsive regime.

Electronic phase separation has been used to realize exotic functionalities in complex oxides with external stimuli, such as magnetic field, electric field, current, light, strain, etc. Using the ${\mathrm{Nd}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3/0.7\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3\text{−}0.3{\mathrm{PbTiO}}_3$ multiferroic heterostructure as a model system, we investigate the electric field and light cocontrol of phase separation in resistive switching. The electric-field-induced nonvolatile electroresistance response is achieved at room temperature using reversible ferroelastic domain switching, which can be robustly modified on illumination of light. Moreover, the electrically controlled ferroelastic strain can effectively enhance the visible-light-induced photoresistance effect. These findings demonstrate that the electric-field- and light-induced effects strongly correlate with each other and are essentially driven by electronic phase separation. Our work opens a gate to design electrically tunable multifunctional storage devices based on multiferroic heterostructures by adding light as an extra control parameter.

A tribopolymer formed on the contacts of microelectromechanical and nanoelectromechanical system (MEMS-NEMS) devices is a major concern hampering their practical use in information technology. Conductive metal oxides, such as ${\mathrm{RuO}}_2$ and ${\mathrm{ReO}}_3$, have been regarded as promising candidate materials for MEMS-NEMS contacts due to their conductivity, hardness, and relatively chemically inert surfaces. However, recent experimental works demonstrate that trace amounts of a polymer could still form on ${\mathrm{RuO}}_2$ surfaces. We demonstrate the mechanism of this class of unexpected tribopolymer formation by conducting density-functional-theory-based computational compression experiments with benzene as the contamination gas. First, mechanical force during compression changes the benzene molecules from slightly physisorbed to strongly chemisorbed. Further compression causes deformation and chemical linkage of the benzene molecules. Finally, the two contacts detach, with one having a complex organic molecule attached and the other a more reactive surface. The complex organic molecule, which has an oxabicyclic segment, can be viewed as the rudiment of a tribopolymer, and the more reactive surface can trigger the next adsorption-reaction-tribopolymer formation cycle. Based on these results, we also predict tribopolymer formation rates by using transition-state theory and the second-order rate law. We promote a deeper understanding of tribopolymer formation (especially on metal oxides) and provide strategies for suppressing tribopolymerization.

We provide direct experimental evidence for the effect of a phonon bottleneck on exciton and spin generation in self-assembled ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ quantum dots (QDs). With the aid of tunable laser spectroscopy, we resolve and identify efficient exciton generation channels in the QDs mediated by longitudinal-optical (LO) phonons from an otherwise inhomogeneously broadened QD emission background that suffers from the phonon bottleneck effect in exciton generation. Spin-generation efficiency is found to be enhanced under the LO-assisted excitation condition due to suppressed spin relaxation accompanying accelerated exciton generation. These findings underline the importance of fine-tuning QD energy levels that will benefit potential spin-optoelectronic applications of QDs by reducing spin loss due to the phonon bottleneck.

Simulating strongly correlated fermionic systems is notoriously hard on classical computers. An alternative approach, as proposed by Feynman, is to use a quantum computer. We discuss simulating strongly correlated fermionic systems using near-term quantum devices. We focus specifically on two-dimensional (2D) or linear geometry with nearest-neighbor qubit-qubit couplings, typical for superconducting transmon qubit arrays. We improve an existing algorithm to prepare an arbitrary Slater determinant by exploiting a unitary symmetry. We also present a quantum algorithm to prepare an arbitrary fermionic Gaussian state with $O(N^2)$ gates and $O(N)$ circuit depth. Both algorithms are optimal in the sense that the numbers of parameters in the quantum circuits are equal to those describing the quantum states. Furthermore, we propose an algorithm to implement the 2D fermionic Fourier transformation on a 2D qubit array with only $O(N^{1.5})$ gates and $O(\sqrt{N})$ circuit depth, which is the minimum depth required for quantum information to travel across the qubit array. We also present methods to simulate each time step in the evolution of the 2D Fermi-Hubbard model—again on a 2D qubit array—with $O(N)$ gates and $O(\sqrt{N})$ circuit depth. Finally, we discuss how these algorithms can be used to determine the ground-state properties and phase diagrams of strongly correlated quantum systems using the Hubbard model as an example.

Applications of magnetostrictive materials commonly involve the use of the dynamic deformation, i.e., the piezomagnetic effect. Usually, this effect is described by the strain derivative $\partial \lambda /\partial H$, which is deduced from the quasistatic magnetostrictive curve. However, the strain derivative might not be accurate to describe dynamic deformation in semihard materials as cobalt ferrite (CFO). To highlight this issue, dynamic magnetostriction measurements of cobalt ferrite are performed and compared with the strain derivative. The experiment shows that measured piezomagnetic coefficients are much lower than the strain derivative. To point out the direct application of this effect, low-frequency magnetoelectric (ME) measurements are also conducted on bilayers $\mathrm{CFO}/\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\text{O}}_3$. The experimental data are compared with calculated magnetoelectric coefficients which include a measured dynamic coefficient and result in very low relative error ($<5\%$), highlighting the relevance of using a piezomagnetic coefficient derived from dynamic magnetostriction instead of a strain derivative coefficient to model ME composites. The magnetoelectric effect is then measured for several amplitudes of the alternating field $H_{\mathrm{ac}}$, and a nonlinear response is revealed. Based on these results, a trilayer CFO/$\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\text{O}}_3$/CFO is made exhibiting a high magnetoelectric coefficient of $578\mathrm{mV}/\mathrm{A}$ (approximately $460\mathrm{mV}/\mathrm{cm}\mathrm{Oe}$) in an ac field of $38.2\mathrm{kA}/\mathrm{m}$ (about 48 mT) at low frequency, which is 3 times higher than the measured value at $0.8\mathrm{kA}/\mathrm{m}$ (approximately 1 mT). We discuss the viability of using semihard materials like cobalt ferrite for dynamic magnetostrictive applications such as the magnetoelectric effect.

The magnetic phase-transition tunnel-magnetoresistance (MPT-TMR) effect with a single magnetic electrode is investigated by first-principles calculations. The calculations show that the MPT-TMR of an ${\alpha }^{\prime }\text{−}\mathrm{FeRh}/\mathrm{MgO}/\mathrm{Cu}$ tunnel junction can be as high as hundreds of percent when the magnetic structure of ${\alpha }^{\prime }\text{−}\mathrm{FeRh}$ changes from $G$-type antiferromagnetic ($G\text{−}\mathrm{AFM}$) to ferromagnetic order. This type of MPT-TMR may be superior to the tunnel anisotropic magnetoresistance because of its huge magnetoresistance effect and similar structural simplicity. The main mechanism for the giant MPT-TMR can be attributed to the formation of interface resonant states at the $G\text{−}\mathrm{AFM}$ $\mathrm{FeRh}/\mathrm{MgO}$ interface. A direct $\mathrm{FeRh}/\mathrm{MgO}$ interface is found to be necessary for achieving a high MPT-TMR experimentally. Moreover, we find the ${\alpha }^{\prime }\text{−}\mathrm{FeRh}/\mathrm{MgO}$ interface with FeRh in the ferromagnetic phase has nearly full spin polarization due to the negligible majority transmission and significantly different Fermi surface of two spin channels. Thus, it may act as a highly efficient and tunable spin injector. In addition, the electric-field-driven MPT of FeRh-based heteromagnetic nanostructures can be utilized to design various energy-efficient tunnel-junction structures and the corresponding lower-power-consumption devices. We also discuss the consequence of various junction defects on MPT-TMR. The interface oxygen layer is found to be detrimental to MPT-TMR. The sign of MPT-TMR is reversed with Rh termination due to the lack of contribution from the interface resonance states. However, the MPT-TMR may be robust against the oxygen vacancy inside of MgO and the shift of the Fermi energy. Our results will stimulate further experimental investigations of MPT-TMR and other fascinating phenomenon of FeRh-based tunnel junctions that may be promising in antiferromagnetic spintronics.

The elementary excitation characteristic of magnetoelectric (ME) multiferroics is a magnon endowed with electric activity, which is referred to as an electromagnon. The electromagnon resonance mediated by the bilinear exchange coupling potentially exhibits strong terahertz light-matter interaction with optical properties different from the conventional magnon excitation. Here we report the robust electromagnon resonance on helimagnetic $Y$-type hexaferrites in a wide temperature range including room temperature. Furthermore, the efficient magnetic field controls of the electromagnon are demonstrated on the flexible spin structure of these compounds, leading to the generation or annihilation of the resonance as well as the large resonance energy shift. These terahertz characteristics of the electromagnon exemplify the versatile magneto-optical functionality driven by the ME coupling in multiferroics, paving a way for possible terahertz applications as well as terahertz control of a magnetic state of matter.

We construct a class of two-dimensional (2D) phosphorus allotropes by assembling a previously proposed ultrathin metastable phosphorus nanotube into planar structures in different stacking orientations. Based on first-principles methods, the structures, stabilities, and fundamental electronic properties of these allotropes are systematically investigated. Our results show that these 2D van der Waals phosphorene allotropes possess remarkable stabilities due to the strong intertube van der Waals interactions, which cause an energy release of about $30–70\mathrm{meV}/\mathrm{atom}$, depending on their stacking details. Most of them are confirmed to be energetically more favorable than the experimentally viable $\alpha \text{−}\mathrm{P}$ and $\beta \text{−}\mathrm{P}$. Three of them, showing a relatively higher probability of being synthesized in the future, are further confirmed to be dynamically stable semiconductors with strain-tunable band gaps and intrinsic piezoelectricity, which may have potential applications in nanosized sensors, piezotronics, and energy harvesting in portable electronic nanodevices.

We develop a theoretical framework that lays out the fundamental rules under which a periodic (Floquet) driving scheme can induce nonreciprocal transport. Our approach utilizes an extended Hilbert space where a Floquet network with an extra (frequency) dimension naturally arises. The properties of this network (its on-site potential and the intersite couplings) are in one-to-one correspondence with the initial driving scheme. Its proper design allows for a control of the multipath scattering processes and the associated interferences. We harness this degree of freedom to realize driving schemes with narrow-band or broadband nonreciprocal transport.

We propose a general topology-optimization framework for metasurface inverse design that can automatically discover highly complex multilayered metastructures with increased functionalities. In particular, we present topology-optimized multilayered geometries exhibiting angular phase control, including a single-piece nanophotonic metalens with angular aberration correction, as well as an angle-convergent metalens that focuses light onto the same focal spot regardless of the angle of incidence.

Inspired by two-dimensional material with their unique physical properties and innovative device applications, here we report a design framework on monolayer GaSe, an important member of the two-dimensional material family, in an effort to tune the electronic, optical, and magnetic properties through a vertical electric field. A transition from indirect to direct band gap in monolayer GaSe is found with an electric field of $0.09\mathrm{V}/\AA$. The giant Stark effect results in a reduction of the band gap with a Stark coefficient of 3.54 Å. Optical and dielectric properties of monolayer GaSe are dependent on the vertical electric field. A large regulation range for polarization $\mathbf{E}\parallel \widehat{\mathbf{c}}$ is found for the static dielectric constant. The optical anisotropy with the dipole transition from $\mathbf{E}\parallel \widehat{\mathbf{c}}$ to $\mathbf{E}\perp \widehat{\mathbf{c}}$ is achieved. Induced by the spin-orbit coupling, spin-splitting energy at the valence band maximum increases linearly with the electric field. The effective mass of holes is highly susceptible to the vertical electric field. Switchable spin-polarization features in spin texture of monolayer GaSe are predicted. The tunable electronic, optical, and magnetic properties of monolayer GaSe hold great promise for applications in both the optoelectronic and spintronic devices.

Magnetic materials that possess large bulk perpendicular magnetic anisotropy (PMA) are essential for the development of magnetic tunnel junctions (MTJs) used in future spintronic memory and logic devices. The addition of an antiferromagnetic layer to these MTJs was recently predicted to facilitate ultrafast magnetization switching. Here, we report a demonstration of a bulk perpendicular synthetic antiferromagnetic (PSAFM) structure comprised of a (001) textured $\mathrm{Fe}\text{−}\mathrm{Pd}/\mathrm{Ru}/\mathrm{Fe}\text{−}\mathrm{Pd}$ trilayer with a face-centered-cubic (fcc) phase Ru spacer. The ${L1}_0\mathrm{Fe}\text{−}\mathrm{Pd}$ PSAFM structure shows a large bulk PMA ($K_u\sim 10.2\mathrm{Merg}/{\mathrm{cm}}^3$) and strong antiferromagnetic coupling ($-J_{\mathrm{IEC}}\sim 2.60\mathrm{erg}/{\mathrm{cm}}^2$). Full perpendicular magnetic tunnel junctions (PMTJs) with a ${L1}_0\mathrm{Fe}\text{−}\mathrm{Pd}$ PSAFM layer are then fabricated. Tunneling magnetoresistance ratios of up to approximately 25% (approximately 60%) are observed at room temperature (5 K) after postannealing at $350°\mathrm{C}$. Exhibiting high thermal stabilities and large $K_u$, the bulk PMTJs with an ${L1}_0\mathrm{Fe}\text{−}\mathrm{Pd}$ PSAFM layer could pave a way for next-generation ultrahigh-density and ultralow-energy spintronic applications.

Fast-gated avalanche photodiodes (APDs) are the most commonly used single photon detectors for high-bit-rate quantum key distribution (QKD). Their robustness against external attacks is crucial to the overall security of a QKD system, or even an entire QKD network. We investigate the behavior of a gigahertz-gated, self-differencing (In,Ga)As APD under strong illumination, a tactic Eve often uses to bring detectors under her control. Our experiment and modeling reveal that the negative feedback by the photocurrent safeguards the detector from being blinded through reducing its avalanche probability and/or strengthening the capacitive response. Based on this finding, we propose a set of best-practice criteria for designing and operating fast-gated APD detectors to ensure their practical security in QKD.

We propose a graphene-dielectric-graphene corrugated structure to achieve a multiwavelength optical switch. The transmission and reflection properties of the structure are discussed, and multiultranarrow resonant peaks in the transmission and reflection spectra are found. By adjusting the Fermi energy of graphene, the resonant peaks will shift obviously. Based on this shifting property we present an active multiwavelength optical switch and achieve the on-off of four different wavelengths simultaneously. We also discuss the modulation depths of transmission and reflection. For the transmission of all four wavelengths we can get a very high modulation depth close to 100%.

The geometry-dependent energy transfer rate from an electrically pumped inorganic semiconductor quantum well into an organic molecular layer is studied theoretically. We focus on Förster-type nonradiative excitation transfer between the organic and inorganic layers and include quasimomentum conservation and intermolecular coupling between the molecules in the organic film. (Transition) partial charges calculated from density-functional theory are used to calculate the coupling elements. The partial charges describe the spatial charge distribution and go beyond the common dipole-dipole interaction. We find that the transfer rates are highly sensitive to variations in the geometry of the hybrid inorganic-organic system. For instance, the transfer efficiency is improved by up to 2 orders of magnitude by tuning the spatial arrangement of the molecules on the surface: Parameters of importance are the molecular packing density along the effective molecular dipole axis and the distance between the molecules and the surface. We also observe that the device performance strongly depends on the orientation of the molecular dipole moments relative to the substrate dipole moments determined by the inorganic crystal structure. Moreover, the operating regime is identified where inscattering dominates over unwanted backscattering from the molecular layer into the substrate.