Low-dimensional materials provide a unique platform for exploring exotic properties that are otherwise unachievable in bulk solids. ${\mathrm{C}}_3\mathrm{N}$ and ${\mathrm{C}}_3\mathrm{B}$ are two graphene-derived two-dimensional (2D) ordered alloys that have attracted increasing research attention. These materials are best known for their remarkable stability and moderate band gaps, and thus, are suitable for a range of applications. Perhaps the most interesting feature of the electronic structures of ${\mathrm{C}}_3\mathrm{N}$ and ${\mathrm{C}}_3\mathrm{B}$ is the existence of nearly parallel valence and conduction bands across a large region of the Brillouin zone. In this work, using many-body perturbation theory within the GW and Bethe–Salpeter-equation approach, we predict that the primarily $p_z$-orbital-derived nearly parallel valence and conduction bands in monolayer ${\mathrm{C}}_3\mathrm{N}$ and ${\mathrm{C}}_3\mathrm{B}$ give rise to a giant narrow-band absorption peak in their optical absorption spectra. More surprisingly, two degenerate excitonic states contribute to over 90% and 80% of the dipole absorption below 5 eV for ${\mathrm{C}}_3\mathrm{N}$ and ${\mathrm{C}}_3\mathrm{B}$, respectively. Detailed examinations of the exciton-binding energies unveil a unique shell-like distribution of the excitonic states, with each shell (series) converging to a different excitation edge. Such distinctive absorption properties are not observed in any other 2D materials. We further investigate the internal structure of the excitonic states using a multifaceted approach and reveal several important characteristics of the excitonic states in these 2D materials.

All physical systems employed for quantum information tasks must act as unbiased carriers of encoded quantum states. Ensuring such indistinguishability of information carriers is a major challenge in many quantum information applications, including advanced quantum communication protocols. For photons, the workhorses of quantum communication networks, it is difficult to obtain and maintain their indistinguishability because of environment-induced transformations and loss imparted by communication channels, especially in noisy scenarios. Conventional strategies to mitigate these transformations often require hardware or software overhead that is restrictive (e.g., adding noise), infeasible (e.g., on a satellite), or time-consuming for deployed networks. Here we propose and develop resource-efficient Bayesian optimization techniques to rapidly and adaptively calibrate the indistinguishability of individual photons for quantum networks using only information derived from their measurement. To experimentally validate our approach, we demonstrate the optimization of Hong-Ou-Mandel interference between two photons–a central task in quantum networking– finding rapid, efficient, and reliable convergence towards maximal photon indistinguishability in the presence of high loss and shot noise. We expect our resource-optimized and experimentally friendly methodology will allow fast and reliable calibration of indistinguishable quanta, a necessary task in distributed quantum computing, communications, and sensing, as well as for fundamental investigations.

The study of nanoscale chiral magnetic order in polycrystalline materials with a strong Dzyaloshinskii-Moriya interaction is interesting for the observation of magnetic phenomena at grain boundaries and interfaces. This is especially true for polycrystalline materials, which can be grown using scalable techniques, the scalability of which is promising for future device applications. One such material is sputter-deposited B20 $\mathrm{Fe}\mathrm{Ge}$ on $\mathrm{Si}$, which is actively investigated as the basis for low-power high-density magnetic memory technology in a scalable material platform. Although conventional Lorentz electron microscopy provides the requisite spatial resolution to probe chiral magnetic textures in single-crystal $\mathrm{Fe}\mathrm{Ge}$, probing the magnetism of sputtered B20 $\mathrm{Fe}\mathrm{Ge}$ is more challenging because the submicron crystal grains add confounding contrast. This is a more general problem for polycrystalline magnetic devices, where scattering from grain boundaries tends to hide comparably weaker signals from magnetism. We address the challenge of disentangling magnetic and grain contrast by applying four-dimensional Lorentz scanning transmission electron microscopy using an electron-microscope pixel-array detector. Supported by analytical and numerical models, we find that the most important parameter for imaging magnetic materials with polycrystalline grains is the ability for the detector to sustain large electron doses, where having a high-dynamic-range detector becomes extremely important. Despite the small grain size in sputtered B20 $\mathrm{Fe}\mathrm{Ge}$ on $\mathrm{Si}$, using this approach, we are still able to observe helicity switching of skyrmions and magnetic helices across two adjacent grains, as they thread through neighboring grains. We reproduce this effect using micromagnetic simulations by assuming that the grains have distinct orientation and magnetic chirality and find that magnetic helicity couples to crystal chirality. Our methodology for imaging magnetic textures is applicable to other thin-film magnets used for spintronics and memory applications, where an understanding of how magnetic order is accommodated in polycrystalline materials is important.

Materials with extremely low impedance like air can insulate efficiently waterborne sound, particularly at low frequency, however the poor resistance to load hinders further their applications. This work demonstrates that the elastic anisotropy of materials can not only achieve extremely low impedance, but also high enough stiffness for load bearing. A dimensionless quality factor is proposed to characterize integrately the performance on sound insulation and load bearing of anisotropic materials. It is found that this factor has an upper bound of unity and the corresponding conditions are derived for some particular anisotropic materials. By setting the quality factor to be unity as an objective function for microstructure optimization, a methodology based on topological optimization is proposed to design the microstructure of the corresponding materials with optimized integrated performance on waterborne sound insulation and load bearing. The obtained optimized material is validated by numerical simulation and by comparing with other similar waterborne sound insulation materials with low impedance. This study paves the way for designing load-bearing materials with extremely low impedance, and opens a route to control low-frequency underwater waves.

Write error rate (WER) in voltage-induced magnetization switching of a conically magnetized free layer with an elliptic cylinder shape is numerically calculated on the basis of the macrospin model. Simulations using the Langevin equation show that a WER lower than the order of ${10}^{-5}$, which enables practical WER with a single error-correction process, is theoretically feasible at zero-bias magnetic field and room temperature. We also show that WER at a given thermal stability factor and the volume of the free layer can be reduced by adopting the free layer with the larger in-plane shape anisotropy field and a moderate cone angle at zero-bias voltage.

Redox-based resistive random access memories (ReRAMs) are promising candidate devices for new memory and computing paradigms. However, the fundamental mechanisms that rule the conduction in these devices are still heavily debated. The present work focuses on studying one model for the conduction, the quantum point contact (QPC), and specifically a single-subband approximation (SSA) to this model. With this intent, $\mathrm{Pt}$(20 nm)/$\mathrm{Ta}$(15 nm)$\mathrm{Ta}$${}_2$$\mathrm{O}$${}_5$(5 nm)/$\mathrm{Pt}$(20 nm) resistive switching devices are fabricated and electrically characterized by measuring current-voltage ($I$-$V$) curves in both resistance states. The QPC model has been found to be hard to apply, as the starting parameters have a strong influence on the fitting results. On the other hand, the SSA has proved its ability to provide good fits to the data and to do so better than other typical conduction mechanisms considered. However, its physical basis is criticized and it is concluded that in the devices studied, multiple subbands likely contribute to the conduction, in direct opposition to the assumptions made in such an approximation. A reinterpretation of the parameters of the SSA is proposed, to reconcile the increased performance with greater physical accuracy. Beyond that, the main challenges and difficulties regarding the application of the QPC to the case of valence-change-based ReRAM are discussed.

Twisted moiré superlattices (TMSs) are emergent materials with exotic physical properties. Among their properties, higher-order topology is seldom realized and investigated in experiments. Here, we report on the experimental observation of a class of bilayer higher-order topological states in TMSs. We create the physical realization of acoustic TMSs using moiré twisting and ultrastrong interlayer couplings in bilayer honeycomb lattices of coupled acoustic cavities. Through such a design we reach an unexplored regime in TMSs that is not available in graphene TMSs and other solid-state TMS systems. We reveal that the ultrastrong interlayer couplings lead to a large acoustic band gap with unique higher-order band topology, which is characterized by unprecedented topological indices and layer-hybridized corner states. The higher-order topological edge and corner states are observed via acoustic pump-probe measurements, which show consistency with theory and simulations. Our study paves the way toward higher-order topological phenomena in TMSs.

We consider mediated interactions in an array of floating transmons, where each qubit capacitor consists of two superconducting pads galvanically isolated from ground. Each such pair contributes two quantum degrees of freedom, one of which is used as a qubit, while the other remains fixed. However, these extraneous modes can generate coupling between the qubit modes that extends beyond the nearest neighbor. We present a general formalism describing the formation of this coupling and calculate it for a one-dimensional chain of transmons. We show that the strength of coupling and its range (that is, the exponential falloff) can be tuned independently via circuit design to realize a continuum from nearest-neighbor-only interactions to interactions that extend across the length of the chain. We discuss how this coupling breaks integrability, allowing the system to locally equilibrate, and so enables exploration of quantum thermalization. We present designs with capacitance and microwave simulations showing that various interaction configurations can be achieved in realistic circuits. Such a coupling could be used in analog simulation of different quantum regimes or to increase connectivity in digital quantum systems. Thus the mechanism must also be taken into account in other types of qubits with extraneous modes.

One of the key trends in laser material processing is the usage of structured laser beams. Collimated and focused Gaussian beams are the most common tools; however, more exotic beams can be beneficial too. For instance, Bessel beams with elongated focal area and self-healing properties, or vortex beams with helical wave fronts and a dark area along the optical axis are being increasingly used. Here, we propose and experimentally demonstrate dynamical “optical drill” beams presenting nonstationary intensity distributions that resemble a spinning mechanical drill. Optical drills appear as the spatiotemporal interference of two Bessel-vortex beams of different topological charges and different carrier frequencies. By mixing a pair of high-order Bessel beams, synthesized using a liquid crystal spatial light modulator, optical drills of tuned helicities are experimentally observed, and the simplest cases of matter processing (fluorescence) with such beams are demonstrated. Optical drill beams are expected to be useful in material processing by light or in cell and particle manipulation in biomedical applications.

Brownian thermal noise associated with highly reflective coatings is a fundamental limit for several precision experiments, including gravitational-wave detectors. Research is currently ongoing to find coatings with low thermal noise that also fulfill strict optical requirements such as low absorption and scatter. We report on the optical and mechanical properties of ion-beam-sputtered magnesium fluoride thin films and we discuss the application of such coatings in current and future gravitational-wave detectors.

Superconducting boron-doped silicon is a promising material for integrated silicon quantum devices. In particular, its low electronic density and moderate disorder make it a suitable candidate for the fabrication of large inductances with low losses at microwave frequencies. We study experimentally the electrodynamics of superconducting silicon thin layers using coplanar waveguide resonators, focusing on the kinetic inductance, the internal losses, and the variation of these quantities with the resonator readout power. We report the observation in a doped semiconductor of microwave resonances with internal quality factors of a few thousand. As expected in the BCS framework, superconducting silicon presents a large sheet kinetic inductance in the 50–500-pH range, comparable to that of strongly disordered superconductors. The temperature dependence of the kinetic inductance is well described by the Mattis-Bardeen theory. However, we find an unexpectedly strong nonlinearity in the complex surface impedance that cannot be satisfactorily explained either by depairing or by quasiparticle heating.

Adiabatic quantum-control schemes are widely used in the areas of quantum-information processing and quantum sensing. The very requirement of adiabaticity in these schemes often leads to problems of low efficiency and being sensitive to decoherence. To address such issues, various methods have been developed for accelerating adiabatic processes, but they often need to be designed on an $adhoc$ basis. Here we propose and experimentally demonstrate an inverse-engineering approach, where a parameterized state of the Schrödinger equation is employed for constructing desired evolutions. Within a single and simple frame, the user-defined passages can be flexibly customized for different objectives and systems. To experimentally benchmark its performance, we implement this approach in a task of coherent state transfer in a superconducting Xmon device. We find that our method completed the transfer with the fastest speed and highest efficiency (>99.5%) among all recent experiments performed in similar configurations.

Intercalation-type batteries based on the alkali metal $\mathrm{K}$, an earth-abundant element, are attracting increasing attention as alternatives to $\mathrm{Li}$-ion batteries. However, the lack of a high-performance anode limits the success of $\mathrm{K}$-ion batteries. Here, we study the performance of monolayer ${\mathrm{C}}_5\mathrm{N}$ as a potential building block for the anode. Our theoretical data show that the metallic monolayer can accommodate 2.25 K atoms per formula unit, which corresponds to a specific capacity of 814 mA h ${\mathrm{g}}^{-1}$, and provides a low diffusion barrier of only 0.11 eV. The volume change of bulk ${\mathrm{C}}_5\mathrm{N}$ (47%) is found to be smaller than that of graphite (61%; commercial anode material). These predictions will serve the design of future anode materials for $\mathrm{K}$-ion batteries.

We envision the use of an indium tin oxide (ITO) thin film as part of a bilayered silicon-photonics subwavelength device to boost nonlinearity-assisted all-passive and ultrafast nonreciprocal behavior. Asymmetric $p$-polarized oblique excitation of a mode near the epsilon-near-zero frequency, with a highly confined and enhanced normal electric field component and large absorption, allows one to harness the strong subpicosecond nonlinear response of ITO for the generation of notable nonreciprocal performance in an ultracompact two-port element. Although the results are limited by loss, we find the optimal operational point and the maximum attainable nonreciprocal transmittance ratio of the metasurface versus light intensity—including an apparent upper bound slightly over 2—and we perform exhaustive numerical simulations considering nonlinear processes of both anharmonic and thermal nature that validate our predictions, including steady-state and pulsed-laser excitation.

Ghost imaging in terahertz range has proved that amplitude and phase holographic imaging could be realized by detecting time-resolved electromagnetic waves, which shows application potential in several areas, such as nondestructive evaluation, biological sample imaging, etc. Compared with terahertz electromagnetic waves, ultrasound is easier to emit, modulate, and detect, and its shorter wavelength can produce higher phase and spatial-imaging resolutions. Here, we investigate ultrasonic holographic ghost imaging (UHGI). A linear-array ultrasonic transducer is used to emit ultrasonic waves, which travel through the sample closely attached to the transducer, then propagate for a certain distance in water, and finally are received by a hydrophone. By encoding the ultrasonic waves emitted by the linear-array transducer and correlating the ultrasonic waves received by the hydrophone and prearranged sampling patterns, holographic imaging of transmittance and phase shift can be realized. Combined with near-field imaging, the spatial resolution (determined by the element size of the transducer) can reach 0.3 mm. The UHGI experiment is easy to carry out and can add more capabilities to ultrasonic medical imaging, nondestructive evaluations, and the imaging of acoustic metasurfaces.

The purpose of this study is to develop a method for directly measuring the sheet resistance of a junction partner for a chalcogenide light absorber in a thin-film solar cell. The developed method is applied to $\mathrm{Cd}\mathrm{S}$ in a $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ (CIGS) thin-film solar cell as a proof of concept. The suggested method utilizes a combination of the lateral photocurrent flowing along $\mathrm{Cd}\mathrm{S}$ in a custom-designed cell and the current density–voltage curve of a lab-scale CIGS solar cell. By combining an equivalent circuit analysis and a derived approximate equation, a semiempirical formula is proposed for directly measuring the sheet resistance of $\mathrm{Cd}\mathrm{S}$ in the CIGS thin-film solar cell. The semiempirical formula is a function of the basic characteristics (fill factor, short-circuit current density, open-circuit voltage) of the CIGS solar cell, and the linear lateral photocurrent density measured in the custom-designed cell. The proposed semiempirical formula is validated by measuring the sheet resistance of a $\mathrm{Zn}\mathrm{O}:\mathrm{Al}$ thin film in a custom-modified cell. The sheet resistance value agrees well with the sheet resistance value measured by a four-point probe. This method can be easily extended to ${\mathrm{Cu}}_2{\mathrm{Zn}\mathrm{Sn}\mathrm{S}}_4$, $\mathrm{Sn}\mathrm{S}$, and ${\mathrm{Sb}}_2{\mathrm{Se}}_3$ thin-film solar cells, which have very similar device structures to a CIGS thin-film solar cell.

Acoustic and elastic metamaterials with time- and space-dependent material properties have received great attention recently as a means to break reciprocity for propagating mechanical waves, achieving greater directional control. One nonreciprocal device that has been demonstrated in the fields of acoustics and electromagnetism is the circulator, which achieves unirotational transmission through a network of ports. This work investigates an elastic wave circulator composed of a thin elastic ring with three semi-infinite elastic waveguides attached, creating a three-port network. Nonreciprocity is achieved for both longitudinal and transverse waves by modulating the elastic modulus of the ring in a rotating fashion. Two numerical models are derived and implemented to compute the elastic wave field in the circulator. The first is an approximate model based on coupled-mode theory, which makes use of the known mode shapes of the unmodulated system. The second model is a finite-element approach that includes a Fourier expansion in the harmonics of the modulation frequency and radiation boundary conditions at the ports. The coupled-mode model shows excellent agreement with the finite-element model and the conditions on the modulation parameters that enable a high degree of nonreciprocity are presented. This work demonstrates that it is possible to create an elastic wave circulator that enables nonreciprocal mode-splitting of an incident longitudinal wave into outgoing longitudinal and transverse waves.