Micromagnetic simulation results on dynamic hysteresis loops of clusters of iron oxide nanoparticles (NPs) with internal structure composed of nanorods are compared with the widely used macrospin approximation. Such calculations allowing for nanorod-composed NPs is facilitated by a previously developed coarse-graining method based on the renormalization group approach. With a focus on applications to magnetic hyperthermia, we show that magnetostatic interactions improve the heating performance of NPs in chains and triangles, and reduce heating performance in fcc arrangements. Hysteresis loops of triangular and fcc systems of complex NPs are not recovered within the macrospin approximation, especially at smaller interparticle distances. For triangular arrangements, the macrospin approximation predicts that magnetostatic interactions reduce loop area, in contrast to the complex NP case. An investigation of the local hysteresis loops of individual NPs and macrospins in clusters reveals the impact of the geometry of their neighbors on individual versus collective magnetic response, inhomogeneous heating within clusters, and further differences between simulating NPs with internal structure and the use of the macrospin approximation. Capturing the internal physical and magnetic structure of NPs is thus important for some applications.

A family of $M{A}_2{Z}_4$ materials has recently inspired great interest due to its exotic geometry and intriguing electronic properties. Here we investigate the electronic transport and photoelectric properties of ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{P}}_4$ monolayer (MSP ML) that has a small direct gap using first-principles calculations. We design several model nanodevices based on MSP ML, including p-n junction diodes, p-i-n junction field-effect transistors, and photoelectric transistors. We demonstrate that these MSP-ML-based nanodevices yield superb transport properties, including significant rectifying effect, high electrical anisotropy, pronounced field-effect behavior, strong photoelectric response, and large photovoltaic power. These findings reveal the multifunctional nature of ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{P}}_4$ monolayer, promising its application as a designer material in next-generation ultrathin flexible semiconductor nanodevices.

Metallic antiferromagnets are demonstrated to show an unconventional spin Hall effect (SHE) and the inverse spin Hall effect (ISHE), which are very promising for Néel-vector detection. However, the ISHE is absent in insulating antiferromagnets, as it does not conduct charge current. Here, we report our observation of the anomalous ISHE at the interface of the easy-plane insulator antiferromagnetic nickel oxide and the heavy metal platinum. We find that, at the interface of $\mathrm{Ni}\mathrm{O}/\mathrm{Pt}$, the ISHE generates an anomalous hysteretic transverse voltage in both Hall and planar Hall measurements, depending on spin-current propagation and the polarization direction. The observation of a nonzero transverse voltage in $\mathrm{Pt}$ indicates the appearance of a spin current with longitudinal polarization reflected from $\mathrm{Ni}\mathrm{O}$, implying Néel-vector reorientation from the transverse to longitudinal direction. Our simulation indicates that the Néel vector can be reoriented by a spin current injected from the heavy metal and an applied magnetic field. The observed anomalous ISHE permits the manipulation and detection of the Néel order in insulating antiferromagnets.

Rydberg atoms show significant promise as the basis for highly sensitive detectors of continuous radio-frequency (rf) electric fields (E fields). Here, we study their time-dependent response to pulse-modulated rf E fields at 19.4 GHz using a cesium vapor cell at room temperature. We use density-matrix simulations to explain the timescales that shape the transient atomic response under different laser conditions, finding them to be limited by dephasing mechanisms, including transit-time broadening, Rydberg-Rydberg collisions, and ionization. Using a matched filter, we demonstrate the detection of individual pulses with durations from 10 µs down to 50 ns and amplitudes from 15 000 µV ${\mathrm{cm}}^{-1}$ down to about 170 µV ${\mathrm{cm}}^{-1}$, corresponding to a sensitivity of about 240 nV ${\mathrm{cm}}^{-1}$ ${\mathrm{Hz}}^{-1/2}$. Finally, we highlight the potential of a Rydberg vapor cell as a receiver by detecting pulse trains from a rotating emitter on a simulated passing aircraft.

Viscous fingering (VF) is a well-known interfacial instability occurring when a viscous fluid in a porous medium is displaced by a lower viscosity fluid. In this work, we show that electrokinetic control of the instability is possible, even in the case of a perfect dielectric viscous defending fluid. Using Hele-Shaw cell experiments in which a nonconducting viscous mineral oil is displaced by a conducting less viscous brine solution, we observe that we are able to effectively control the stability of the interface. We evaluate the interfacial stability at different field strengths, both positive and negative with respect to the pressure-driven flow and find that an electric field in the same direction as the pressure-driven flow significantly stabilizes the interface. In contrast, an electric field opposing the pressure-driven flow further destabilizes the displacement. We attribute this to the interplay between the different $\zeta$ potentials (fluid-fluid and fluid-wall) and preferential oil wettability of the system, resulting in the formation of an oil-film layer throughout the displacement. The nonconducting mineral oil in our system acts then as a dead-end pore for the brine, which under the influence of an electric field develops an electro-omostic flow velocity. We qualitatively and quantitatively show that for all positive electric field strengths VF is reduced, while for negative electric fields VF is increased and that the electric field strength has limited influence on the degree of (de)stabilization. The results from this work are of particular relevance in applications where VF occurs in a system containing one nonconducting fluid, which are common in many applications such as enhanced oil recovery.

Dispersion-suppressed mode depletion in a resonant cavity is usually challenging due to the entanglement of amplitude and phase response dictated by the Kramers-Kronig relationship. In this study, we examine the dispersion-suppressed mode depletion induced by parity–time- (PT-) symmetric dual resonators by taking advantage of the exotic property at the exceptional point (EP). We show that this technique can modify the linear spectrum of an optical resonator for relaxed phase-matching conditions in nonlinear optics, thus profoundly altering the nonlinear behavior of integrated Kerr combs. Indeed, dissipation-induced modulation instability (MI) is predicted and numerically verified in normal group-velocity dispersion (GVD) microresonators, and the single-soliton-generation probability is greatly enhanced in the anomalous GVD regime. The merging of two vibrant disciplines, i.e., non-Hermitian optics and nonlinear optics, especially Kerr microcombs, can deepen our understanding of non-Hermitian optical frequency combs and opens broad applications ranging from fiber-optic communications, microwave photonics, and ultrafast optics to quantum state manipulations.

We analyze a high-fidelity two-qubit gate using fast flux pulses on superconducting fluxonium qubits. The gate is realized by temporarily detuning the magnetic flux through the fluxonium loop away from the half flux quantum sweet spot. We simulate dynamics of two capacitively coupled fluxoniums during the flux pulses and optimize the pulse parameters to obtain a highly accurate $\sqrt{i\mathrm{SWAP}}$-like entangling gate. We also evaluate the effect of the flux noise and qubit relaxation on the gate fidelity. Our results demonstrate that the gate error remains below ${10}^{-4}$ for a currently achievable magnitude of the flux noise and qubit relaxation time.

It is well known that a particle put into an ultrasonic standing wave tends to move towards an equilibrium position, where the acoustic pressure-induced force on its surface compensates the particle weight. We demonstrate, by means of a full three-dimensional numerical analysis and a thorough experimental study, that the acoustic force, and thus the particle’s behavior, critically depends on its size. While particles within certain size ranges, including those smaller than half the wavelength, are trapped on axis around the pressure nodes, particles in other size ranges are trapped off axis nearby the pressure antinodes. This behavior, related with sign inversions of the radiation force, implies that the magnitude of the force, and thus the trapping stiffness, can be maximum or null for some specific sizes. As a case of study, we analyze expanded polystyrene particles levitated in air with an ultrasonic frequency of 40 kHz, a relevant system due to recent applications for the development of volumetric displays. Yet, our results illustrate a general behavior of radiation-based traps with structured wave fields.

Effective low-energy theories represent powerful theoretical tools to reduce the complexity in modeling interacting quantum many-particle systems. However, common theoretical methods rely on perturbation theory, which limits their applicability to weak interactions. Here we introduce the Variational Adiabatic Gauge Transformation (VAGT), a nonperturbative hybrid quantum algorithm that can use nowadays quantum computers to learn the variational parameters of the unitary circuit that brings the Hamiltonian to either its block-diagonal or full-diagonal form. If a Hamiltonian can be diagonalized via a shallow quantum circuit, then VAGT can learn the optimal parameters using a polynomial number of runs. The accuracy of VAGT is tested through numerical simulations, as well as simulations on Rigetti and IonQ quantum computers.

Magnetic tunnel junctions (MTJs) have attracted strong research interest within the last decades due to their potential use as nonvolatile memory such as magnetoresistive random access memory as well as for magnetic logic applications. Half-metallic magnets (HMMs) have been suggested as ideal electrode materials for MTJs to achieve an extremely large tunnel-magnetoresistance (TMR) effect. Despite their high TMR ratios, MTJs based on HMMs do not exhibit current rectification, i.e., a diode effect, which was achieved in a magnetic tunnel junction concept [E. Şaşıoğlu et al., ACS Appl. Electron. Mater. 1, 1552–9 (2019)] based on HMMs and type-II spin-gapless semiconductors (SGSs). The proposed concept has recently been experimentally demonstrated using Heusler compounds. In the present work, we investigate from first-principles MTJs based on type-II SGS and HMM quaternary Heusler compounds $\mathrm{Fe}\mathrm{V}\mathrm{Ta}\mathrm{Al}$, $\mathrm{Fe}\mathrm{V}\mathrm{Ti}\mathrm{Si}$, $\mathrm{Mn}\mathrm{V}\mathrm{Ti}\mathrm{Al}$, and $\mathrm{Co}\mathrm{V}\mathrm{Ti}\mathrm{Sb}$. Our ab initio quantum transport calculations based on a nonequilibrium Green’s function method have demonstrated that the MTJs under consideration exhibit current rectification with relatively high on:off ratios. We show that, in contrast to conventional semiconductor diodes, the rectification bias voltage window (or breakdown voltage) of the MTJs is limited by the spin gap of the HMM and SGS Heusler compounds. A unique feature of the present MTJs is that the diode effect can be configured dynamically, i.e., depending on the relative orientation of the magnetization of the electrodes, the MTJ allows the electrical current to pass either in one or the other direction, which leads to an inverse TMR effect. The combination of nonvolatility, reconfigurable diode functionality, tunable rectification voltage window, and high Curie temperature of the electrode materials makes the proposed MTJs very promising for room-temperature spintronic applications and opens ways to magnetic memory and logic concepts as well as logic-in-memory computing.

We demonstrate an algorithm-dependent computational ghost imaging (GI) by proposing a pattern multiplexing method. Based on the characteristics of three typical GI algorithms (i.e., traditional second-order GI, differential GI, total variational regularization GI), we design a set of pseudorandom patterns to achieve different reconstructed images with different GI algorithms. With our designed patterns, total variational regularization GI can be used for imaging unknown scenes or prehidden images, while traditional second-order GI and differential GI are used for encrypt other different images. Not only being critical for imaging quality in GI, reconstruction algorithms in our simulations and experiments demonstrate their potential role in multiple-image reconstruction, providing an idea for the application of GI in image encryption.

We show that resonant plasmonic detection dramatically increases the sensitivity of the terahertz detectors based on a gated graphene $p$-$i$-$n$ (GPIN) field-effect transistor (FET) structure. In the proposed device, the gated $p$ and $n$ regions serve as the hole and electron reservoirs and the terahertz resonant plasma cavities. The current-voltage ($I$-$V$) characteristics are strongly nonlinear due to the Zener-Klein interband tunneling in the reverse-biased $i$ region between the gates. The terahertz signal rectification by this region enables the terahertz detection. The resonant excitation of the hole and electron plasmonic oscillations results in a substantial increase in the terahertz detector responsivity at the signal frequency close to the plasma frequency and its harmonics. Because of the transit-time effects, the GPIN-FET response at the higher plasmonic modes could be stronger than for the fundamental mode. Our estimates predict the detector responsivity up to a few ${10}^5$ V/W at room temperature, much larger than for other electronic terahertz detectors, such as Schottky diodes, $p$-$n$ junctions, Si CMOS, and III-V and III-N HEMTs.

The advantages of using quantum states of light for object detection are often highlighted in schemes that use simultaneous and optimal measurements. Here, we describe a theoretical but experimentally realizable quantum illumination scheme based on nonsimultaneous and nonoptimal measurements, which can maintain this advantage. In particular, we examine the multiclick-heralded two-mode-squeezed vacuum state as a probe signal in a quantum illumination process. The increase in conditioned signal intensity associated with multiclick heralding is greater than that from a single detector-heralded signal. Our results show, for lossy external conditions, the presence of the target object can be revealed earlier using multiclick measurements. We demonstrate this through sequential shot measurements based on Monte Carlo simulation.

$\mathrm{Mn}\mathrm{N}$ is a centrosymmetric collinear antiferromagnet belonging to the transition-metal-nitride family with a high Néel temperature ($660\mathrm{K}$), a low anisotropy field, and a large magnetic moment ($3.3{\mu }_B$ per $\mathrm{Mn}$ atom). Despite several recent experimental and theoretical studies, the spin symmetry (magnetic point group) and magnetic domain structure of the material remain unknown. In this work, we use optical second-harmonic generation (SHG) to study the magnetic structure of thin epitaxially grown single-crystal (001) $\mathrm{Mn}\mathrm{N}$ films. Our work shows that spin moments in $\mathrm{Mn}\mathrm{N}$ are tilted away from the [001] direction and the components of the spin moments in the (001) plane are aligned along one of the two possible in-plane symmetry axes ([100] or [110]), resulting in a magnetic-point-group symmetry of 2/m1’. Our work rules out magnetic-point-group symmetries 4/mmm1’ and mmm1’ that have been previously discussed in the literature. Four different spin domains consistent with the 2/m1’ magnetic-point-group symmetry are possible in $\mathrm{Mn}\mathrm{N}$. A statistical model based on the observed variations in the polarization-dependent intensity of the second-harmonic signal collected over large sample areas puts an upper bound of $0.65\mu \mathrm{m}$ on the mean domain size. Our results show that SHG can be used to probe the magnetic order in metallic antiferromagnets. This work is expected to contribute to the recent efforts in using antiferromagnets for spintronic applications.

The current-induced type-x spin-orbit-torque (SOT) switching configuration describes the orthogonal relationship between the magnetic easy axis (EA) of a ferromagnetic (FM) layer and the injected spin polarization, $\sigma$, from a heavy-metal (HM) layer, which has the potential to eclipse the conventional type-y scenario (EA parallel to $\sigma$) at the sub-nanosecond pulse regime. Here, we show that, in HM/FM bilayer heterostructures, current-driven differential planar Hall signals can serve as efficient means to probe type-x SOT switching in a simple measurement fashion. Through this approach, we demonstrate field-free type-x SOT switching in all devices with a canted EA, which are engineered via field-annealing processes. By analyzing the switching-phase diagrams, we further verify that such field-free switching stems from a z-direction effective field contributed to by both the dampinglike torque and the canted EA. Our work indicates that the canted EA, as engineered by a field-annealing process, can give rise to a robust field-free SOT switching and further reduce the critical switching current density.

Single rare-earth ions in solids show great potential for quantum applications, including single-photon emission, quantum computing, and high-precision sensing. However, the linewidths of single rare-earth ions are often broadened due to perturbations associated with the detection methods. Identifying the dominant broadening sources is key to reduce the linewidths for practical applications. We report a spectral broadening study on a single ${\mathrm{Er}}^{3+}$ ion in a $\mathrm{Si}$ nanotransistor. The single ion spectra display a Lorentzian lineshape at all light intensities considered. The linewidth remains nearly constant at 32±2 MHz in the low-intensity regime, and shows a monotonic increase with the intensity in the high-intensity regime. The power broadening does not persist over the microsecond time scales considered after resonant excitation. Nor does it depend on the resonant excitation intensity or the Zeeman shift. These observations and temperature-dependent measurements suggest that charge fluctuations are likely to be a dominant broadening source. Laser heating may also contribute to the power broadening. Charge suppression in the ${\mathrm{Er}}^{3+}$-doped region and coupling ${\mathrm{Er}}^{3+}$ ions to a confined optical mode could be implemented to reduce the spectral linewidth and to enhance the sensing precision of single ${\mathrm{Er}}^{3+}$ ions in $\mathrm{Si}$.

Charge injection from electrodes into doped organic films is a widespread technology used in the majority of state-of-the-art organic semiconductor devices. Although such interfaces are commonly considered to form Ohmic contacts via strong band bending, an experiment that directly measures the contact resistance has not yet been demonstrated. In this study, we use a simple metal/doped-organic-semiconductor/metal stack and study its voltage-dependent resistance. A transport-layer thickness variation proves that the experiment presented gains direct access to the contact resistance of the device. We can quantify that for an operating current density of $10{\text{mA/cm}}^2$, the investigated material system exhibits a voltage drop over the metal-organic interface of about $200\mathrm{mV}$, which can be reduced by more than one order of magnitude when employing an additional injection layer. The experiment presented proposes a simple strategy to measure the contact resistance between any metal and doped organic film without applying numerical tools or elaborate techniques. Furthermore, the simplistic device architecture allows for very high, homogeneous, and tunable electric fields within the organic layer, which enables a clear investigation of the Poole-Frenkel effect.

We present a classical algorithm to find approximate solutions to instances of quadratic unconstrained binary optimization. The algorithm can be seen as an analog of quantum annealing under the restriction of a product-state space, where the dynamical evolution in quantum annealing is replaced with a gradient-descent-based method. This formulation is able to quickly find high-quality solutions to large-scale problem instances and can naturally be accelerated by dedicated hardware such as graphics processing units. We benchmark our approach for large-scale problem instances with tunable hardness and planted solutions. We find that our algorithm offers a similar performance to current state-of-the-art approaches within a comparably simple gradient-based and nonstochastic setting.