Graded-band-gap solar cells have been shown to be a promising way of improving the power conversion efficiency of solar cells. However, due to the mixture of drift and diffusional motion of charge carriers induced by the band-gap grading in the solar cells, existing assessment techniques fail to properly characterize the electrical quality of the graded-band-gap solar cells. I develop a simple method to simultaneously characterize recombination characteristics including the minority carrier diffusion length $L_d$ and the back-surface recombination velocity $S_b$ through depth-resolved minority carrier collection length by diffusion in the graded-band-gap solar cells. The suggested method relies on wavelength-dependent lateral photocurrent in a simple custom-designed device structure, which enables to determine the recombination characteristics of the minority carrier, regardless of the band-gap grading. As a proof of concept, the suggested method is applied to a graded-band-gap $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ solar cell without even knowing the depth profile of the band-gap grading. The determined $L_d$ and $S_b$ values match well with previous reports of these values for $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ solar cells. This result suggests that this approach can be thus readily extended to other types of solar cells because this approach does not require the depth profile of the band-gap grading.

Within the paradigm of metamaterials and metasurfaces, the electromagnetic properties of composite materials can be engineered by shaping or modulating their constituents, the so-called meta-atoms. Synthesis and analysis of complex-shaped meta-atoms with general polarization properties is a challenging task. In this paper, we demonstrate that the most general dipolar response of any small object can be conceptually decomposed into a set of basic fundamental polarization phenomena, which enables immediate all-direction characterization of the electromagnetic properties of arbitrary linear materials and metamaterials. The proposed platform for modular characterization is tested on several examples of bianisotropic and nonreciprocal meta-atoms. As a demonstration of the potential of this modular analysis, we use it to design a single-layer metasurface of vanishing thickness with unitary circular dichroism. The analysis approach developed in this paper is supported by a ready-to-use computational code and can be further extended to meta-atoms engineered for other types of wave interactions, such as in acoustics and mechanics.

The study of Weyl points in electronic systems has inspired much recent research in classical systems such as photonic and acoustic lattices. Here we show how Weyl physics can also inspire the design of novel elastic structures. We construct a single-phase three-dimensional structure, an analog of the $AA$-stacked honeycomb lattice, and predict the existence of Weyl points with opposite topological charges ($\pm 1$), elastic Fermi arcs, and the associated gapless topologically protected surface states. We apply full-scale numerical simulations on the elastic three-dimensional structure and present a clear visualization of topological surface states that are directional and robust. Such designed lattices can pave the way for novel vibration control and energy harvesting on structures that are ubiquitous in many engineering applications.

In analogy to Tamm states in electronic systems, plasmonic Tamm states at the edge of terminated graphene plasmonic crystals are proposed. In contrast to plasmonic Tamm states that exist at the interface between metal-insulator-metal Bragg reflectors and metals, an edge-matching layer should be adopted to compensate the unique extra reflection phase at graphene terminals. The width of the edge-matching layer is optimized by the generalized Wiener-Hopf method according to the phase-matching condition. Moreover, graphene plasmonic crystals with merely two unit cells can compress electromagnetic fields into an extremely small mode volume with remarkable intensity enhancement by virtue of the graphene plasmonic Tamm states. Specifically, both the electric field and magnetic field intensities can be enhanced by 80 times in the ultracompact device with the length of 108.2 nm, which is more than 90 times shorter than the operation wavelength of 10 $\mu \mathrm{m}$. The robust confinement mechanism of plasmonic Tamm states enables propagating plasmons trapping and enhancing the electromagnetic field in a planar landscape. Our results open an avenue for developing deep-subwavelength integrated graphene-based trappers, resonators, and biosensors and pave the way for ultracompact on-chip applications.

Using a simple quantum-mechanical model, we explore a tunneling anisotropic magnetoresistance (TAMR) effect in ferroelectric tunnel junctions (FTJs) with a ferromagnetic electrode and a ferroelectric barrier layer, where spontaneous polarization gives rise to the Rashba and Dresselhaus spin-orbit coupling (SOC). For realistic parameters of the model, we predict sizable TAMR measurable experimentally. For asymmetric FTJs, whose electrodes have different work functions, the built-in electric field affects the SOC parameters and leads to TAMR being dependent on the ferroelectric polarization direction. The SOC change with polarization switching affects tunneling conductance, revealing an alternative mechanism of tunneling electroresistance. These results demonstrate alternative functionalities of FTJs, which can be explored experimentally and used in electronic devices.

Nuclear spins nearby nitrogen-vacancy (N-$V$) centers in diamond are excellent quantum memory for quantum computing and quantum sensing, but are difficult to initialize due to their weak interactions with the environment. Here, we propose and demonstrate a magnetic-field-independent, deterministic, and highly efficient polarization scheme by introducing chopped laser pulses into the double-resonance initialization method. With this method, we demonstrate the initialization of a single-nuclear-spin system approaching $98.1\mathrm{\%}$ and a ${}^{14}\mathrm{N}{–}^{13}\mathrm{C}$ double-nuclear-spin system approaching $96.8\mathrm{\%}$ at room temperature. The initialization is limited by a nuclear-spin depolarization effect due to chopped laser excitation. Our approach could be extended to N-$V$ systems with more nuclear spins and would be a useful tool in future applications such as nano-magnetic resonance imaging and single-cell NMR.

We design an all-carbon membrane for the filtration and desalination of water. A unique layered assembly of carbon nanostructures including graphite oxide (GO), buckypaper consisting of carbon nanotubes, and a strong carbon fabric provides high mechanical strength and thermal stability, resilience to harsh chemical cleaning agents, and electrical conductivity, thus addressing major shortcomings of commercial reverse osmosis membranes. We use ab initio density-functional-theory calculations to obtain atomic-level insight into the permeation of water molecules in between GO layers and across in-layer vacancy defects. Our calculations elucidate the reason for selective rejection of solvated ${\mathrm{Na}}^{+}$ ions in an optimized GO membrane that is structurally stabilized in a sandwich arrangement in between layers of buckypaper, which are protected on both sides by strong carbon fabric layers.

Multiple nonvolatile and well-separated capacitive states can be obtained in a two-terminal ferroelectric capacitor setup by fine tuning the polarization switching process. This approach allows for the implementation of memcomputing (same platform for storage and computing) capable ferroelectric structures. Digital and analog storage modes are exemplified in this work together with an algorithm for simple binary computation functions such as or/nor and and/nand for data processing on the same device. Results are obtained by controlling the polarization switching process in ferroelectric multilayers such as $\mathrm{Pb}({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}){\mathrm{O}}_3/{\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3/\mathrm{Pb}({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}){\mathrm{O}}_3$ and $\mathrm{Pb}({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}){\mathrm{O}}_3/{\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3/\mathrm{Pb}({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}){\mathrm{O}}_3$. Besides memcomputing, these results can be used for nondestructive capacitive reading of information in simple ferroelectric capacitors or can open the way toward applications such as neuromorphic and chaotic circuits.

We numerically study reservoir computing on a spin-torque oscillator (STO) array, describing the magnetization dynamics of the STO array by a nonlinear oscillator model. The STOs exhibit synchronized oscillation due to coupling by magnetic dipolar fields. We show that reservoir computing can be performed using the synchronized oscillation state. The performance can be improved by increasing the number of STOs. The performance becomes highest at the boundary between the synchronized and disordered states. Using an STO array, we can achieve higher performance than that of an echo-state network with a similar number of units. This result indicates that STO arrays are promising for hardware implementation of reservoir computing.

We observe a crossover from electron-phonon ($e$-ph) coupling limited energy relaxation to that governed by thermal boundary resistance (phonon-phonon coupling, ph-ph) in copper films at subkelvin temperatures. Our measurement yields a quantitative picture of heat currents, in terms of temperature dependences and magnitudes, in both $e$-ph and pp limited regimes, respectively. We show that by adding a third layer in between the copper film and the substrate, the thermal boundary resistance is increased fourfold, consistent with an assumed series connection of thermal resistances.

The information-theoretic security promised by quantum key distribution (QKD) holds as long as the assumptions in the theoretical model match the parameters in the physical implementation. The superlinear behavior of sensitive single-photon detectors represents one such mismatch and can pave the way to powerful attacks hindering the security of QKD systems, a prominent example being the after-gate attack. A long-standing tenet is that trapped carriers causing delayed detection can help mitigate this attack, but despite intensive scrutiny, it remains largely unproven. Here we approach this problem from a physical perspective and find evidence to support a detector’s secure response. We experimentally investigate two different carrier-trapping mechanisms causing delayed detection in fast-gated semiconductor avalanche photodiodes, one arising from the multiplication layer and the other arising from the heterojunction interface between absorption and charge layers. The release of trapped carriers increases the quantum bit error rate measured under the after-gate attack above the typical QKD security threshold, thus favoring the detector’s inherent security. This represents a significant step to avert quantum hacking of QKD systems.

The recent demonstration of neuromorphic computing with spin-torque nano-oscillators has opened a path to energy efficient data processing. The success of this demonstration hinged on the intrinsic short-term memory of the oscillators. We extend the memory of the spin-torque nano-oscillators through time-delayed feedback. We leverage this extrinsic memory to increase the efficiency of solving pattern recognition tasks that require memory to discriminate different inputs. The large tunability of these nonlinear oscillators allows us to control and optimize the delayed-feedback memory using different operating conditions of applied current and magnetic field.

Quantum key distribution (QKD) based on coherent states is well known for its implementation simplicity, but it suffers from loss-dependent attacks based on optimal unambiguous state discrimination. Crucially, previous research has suggested that coherent-state QKD is limited to short distances, typically below 100 km, assuming standard optical fiber loss and system parameters. In this work, we propose a six-coherent-state phase-encoding QKD protocol that is able to tolerate the total loss of up to 38 dB, assuming realistic system parameters, and up to 56 dB loss, assuming zero noise. The security of the protocol is calculated using a recently developed security proof technique based on semidefinite programming, which assumes only the inner-product information of the encoded coherent states, the expected statistics, and that the measurement is basis independent. Our results thus suggest that coherent-state QKD could be a promising candidate for high-speed provably secure QKD.

Chiral magnetism that manifests in the existence of skyrmions or chiral domain walls offers an alternative way for creating anisotropies in magnetic materials that might have large potential for application in future spintronic devices. Here we show experimental evidence for an alternative type of in-plane exchange-bias effect present at room temperature that is created from a chiral ${90}^{\circ }$ domain wall at the interface of a ferrimagnetic-ferromagnetic $\mathrm{Dy}$-$\mathrm{Co}$/$\mathrm{Ni}$-$\mathrm{Fe}$ bilayer system. The chiral interfacial domain wall forms due to the exchange coupling of $\mathrm{Ni}$-$\mathrm{Fe}$ and $\mathrm{Dy}$-$\mathrm{Co}$ at the interface and the presence of Dzyaloshinskii-Moriya interaction in the $\mathrm{Dy}$-$\mathrm{Co}$ layer. As a consequence of the preferred chirality of the interfacial domain wall, the sign of the exchange-bias effect can be reversed by changing the perpendicular orientation of the $\mathrm{Dy}$-$\mathrm{Co}$ magnetization. The chirality-created tunable exchange bias in $\mathrm{Dy}$-$\mathrm{Co}$/$\mathrm{Ni}$-$\mathrm{Fe}$ is very robust against high in-plane magnetic fields (${\mu }_{0}H\le 6\mathrm{T}$) and does not show any aging effects. Therefore, it overcomes the limitations of conventional exchange-bias systems.

Magnetfree circulators are crucial devices to enable full-duplex communications with the goal of enhancing the spectral efficiency of wireless radios compared to currently deployed half-duplex systems. Nevertheless, implementations proposed to date at radio- and millimeter-wave frequencies have been based on spatiotemporally modulating a discrete number of nonlinear elements, such as varactors or switched capacitors, which, despite achieving strong nonreciprocity, suffer from spurious emission in adjacent channels and limited power handling. These problems jeopardize the practical use of these components in many applications, including wireless communications and quantum computing, and make them less attractive compared to harmonicfree magnetic devices. We address these problems by introducing N-way circulators, which consist of N spatiotemporally modulated angular-momentum (STM AM) biased units connected either in series or in parallel and modulated through a rotating phase pattern. We show that these circuits, for a sufficiently large N, effectively impart an overall linear and continuous angular-momentum bias, which significantly reduces spurious emission and increases the overall power handling. This, in turn, strengthens the argument of magnetless circulators compared to their magnetic counterparts, and opens exciting venues to address the realistic needs of practical systems.

The contactless selective manipulation of individual objects at the microscale is powerfully enabled by acoustical tweezers based on acoustical vortices [M. Baudoin et al., Sci. Adv. 5, eaav1967 (2019)]. Nevertheless, the ability to assemble multiple objects with these tweezers has not been demonstrated yet and is critical for many applications, such as tissue engineering or microrobotics. To achieve this goal, it is necessary to overcome a major difficulty: the high-intensity ring which ensures the trapping of inner particles at the core of the vortex beam is repulsive for particles located outside the trap. This prevents the assembly of multiple objects. In this paper, we show (in the Rayleigh limit and in two dimensions) that this problem can be overcome by trapping the target objects at the core of two synchronized vortices. Indeed, in this case, the destructive interference between neighboring vortices enables us to create an attractive path between the captured objects. The present work may pioneer the precise assembly and patterning of particles with multitweezers.

Driven by the technological importance of the recently discovered ferroelectric ${\mathrm{Hf}\mathrm{O}}_2$, we explore a magnetoelectric effect at the ${\mathrm{Hf}\mathrm{O}}_2$-based ferroelectric-ferromagnetic interface. Using density-functional-theory calculations of the $\mathrm{Ni}/{\mathrm{Hf}\mathrm{O}}_2/\mathrm{Ni}$ (001) heterostructure as a model system, we predict a stable and sizable ferroelectric polarization in a few-nm-thick ${\mathrm{Hf}\mathrm{O}}_2$ layer. For the $\mathrm{Ni}/{\mathrm{Hf}\mathrm{O}}_2$ interface with opposite polarization directions (pointing to or away from the interface), we find a sizable difference in the interfacial $\mathrm{Ni}$—$\mathrm{O}$ bonding, resulting in dissimilar degrees of depletion of the electron density around the interface. The latter affects the relative population of the exchange-split majority and minority spin bands at the interface and thus the interfacial magnetic moments. The sizable change in the interface magnetization with ferroelectric polarization reversal of ${\mathrm{Hf}\mathrm{O}}_2$ manifests a significant ferroelectrically induced magnetoelectric effect at the $\mathrm{Ni}/{\mathrm{Hf}\mathrm{O}}_2$ interface. Our results reveal promising prospects of ferroelectric-ferromagnetic composite multiferroics based on ${\mathrm{Hf}\mathrm{O}}_2$-based ferroelectric materials.

We report a gradient metasurface design at microwave bands as an elegant approach to realize the goal of “rainbow trapping” for the storage of waves involving wave localization and absorption phenomena. A longitudinally placed coplanar waveguide is loaded with gradient metasurfaces on both sides, where split-ring resonators (SRRs) are the basic cell. The same SRRs are arranged along the transverse direction to establish magnetoinductive channels. Waves of different frequencies are coupled to corresponding SRRs at different positions in metasurfaces. Resonant trapping with a long oscillation life time enhances the absorption caused by inherent losses of the materials, thereby suppressing reflections. Both simulations and measurements verify the existence of “rainbow trapping.” The proposed strategy enhances the interaction between waves and matter, opening an avenue for further component designs, including absorptive filters, multiplexers, and buffers.

Resonant magnetoelastic coupling (MEC) is demonstrated in an $\mathrm{Fe}$ thin film epitaxially grown on a piezoelectric $\mathrm{Ga}\mathrm{As}$ substrate with application of subgigahertz surface acoustic waves (SAWs). The frequency at which resonant MEC is achieved is reduced far below 1 GHz by the application of a small in-plane magnetic field. Moreover, the resonance, observable by attenuation and velocity changes of the SAW, can be switched on and off by a small (${0.1}^{\circ }$) angular rotation of this in-plane field. This angular sensitivity makes SAW-ferromagnet devices attractive for sensing applications, such as wireless, battery-free, and interrogable magnetic-field monitors. Using a simple magnetization dynamics model that takes into account the $\mathrm{Fe}$ magnetic anisotropy and the softening of the magnetic precession modes, we are able to describe the observed salient features.

To construct global quantum communication networks, reliable free-space quantum key distribution is essential, but there has appeared a bottleneck severely hindering practical implementation, i.e., background noise. Rather than questing for filters closer to the ultimate performance, we seek a preferable solution from continuous-variable techniques employing coherent detection. In this regard, we investigate the dependence of the performance of continuous-variable quantum key distribution on background noise; the background noise is quantified and experimentally verified. The results indicate effective resistance against background noise. This characteristic demonstrates the feasibility of all-day free-space quantum communication using coherent detection, thus offering an alternative candidate for building networks.