We show perpendicular magnetic anisotropy (PMA) in substituted rare-earth iron garnet ${\mathrm{Y}}_{3-x}{\mathrm{Tm}}_x{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (x = 0–3) films epitaxially grown on (111) $({\mathrm{Gd}}_{0.63}{\mathrm{Y}}_{2.37})({\mathrm{Sc}}_2{\mathrm{Ga}}_3){\mathrm{O}}_{12}$ substrates by magnetron sputtering. With $\mathrm{Tm}$ concentrations varying from x = 0 to x = 3, the effective PMA field of the 10-nm-thick films increases monotonically from 0.3 to 2.7 kOe together with a similar coercivity increase from 2 to 65 Oe. The substituted garnet films with x ranging from 0.6 to 1.2 are structurally much more robust against epitaxial strain relaxation. Magnetotransport measurements in the ${\mathrm{Y}}_{3-x}{\mathrm{Tm}}_x{\mathrm{Fe}}_5{\mathrm{O}}_{12}$/$\mathrm{Pt}$ heterostructure show that the anomalous Hall effect and spin Seebeck effect are significant and change little with $\mathrm{Tm}$ concentration, indicating that the interface spin-exchange interaction and interface spin-current transmission mainly depend on the net moment of ${\mathrm{Fe}}^{3+}$ sublattices rather than the total moment of ${\mathrm{Y}}_{3-x}{\mathrm{Tm}}_x{\mathrm{Fe}}_5{\mathrm{O}}_{12}$. Importantly, the critical current density for the spin-orbit-torque-induced switching also changes very little with a minor increase from 1.76 × ${10}^{11}$ to 1.83 × ${10}^{11}$ A m${}^{-2}$. We tentatively attribute this weak PMA-dependent current density to modification of the domain-wall depinning by the current. Our results demonstrate that substituted rare-earth iron garnets can be a general approach to tune PMA within a large range with nearly constant switching current, and yet there is plenty of work left to be done for more efficient SOT-based garnet ferrimagnet spintronics devices.

Superconducting circuits present a promising platform with which to realize a quantum refrigerator. Motivated by this, we fabricate and perform spectroscopy of a gated Cooper-pair box, capacitively coupled to two superconducting coplanar-waveguide resonators with different frequencies. We experimentally demonstrate the strong coupling of a charge qubit to two superconducting resonators, with the ability to perform voltage driving of the qubit at gigahertz frequencies. We go on to discuss how the measured device could be modified to operate as a cyclic quantum refrigerator by terminating the resonators with normal-metal resistors acting as heat baths.

Developing a critical-element-free low-cost permanent magnet is an urgent necessity in view of rapidly developing technologies and the associated huge market demand for ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$-based magnets. Here, inspired by the abundant and low-cost nature of $\mathrm{Ce}$ and these high-performance ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ permanent magnets, we explore whether it is in fact possible to attain a useful performance in alloys based on the sister material ${\mathrm{Ce}}_2{\mathrm{Fe}}_{14}\mathrm{B}$, employing both experimental and theoretical efforts. Experimentally, we study ${\mathrm{Ce}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ with $\mathrm{Co}$, $\mathrm{La},$ and $\mathrm{Zr}$ substitutions. The $\mathrm{Zr}$ substitution is explored in view of $\mathrm{Zr}$’s frequent role in enhancing magnetic anisotropy in permanent magnets, while the $\mathrm{Co}$ and $\mathrm{La}$ substitutions serve to remedy the too-low Curie point of 433 K in the base alloy. While we find no $\mathrm{Zr}$-related anisotropy enhancement either experimentally or theoretically, the cosubstitution of $\mathrm{La}$ and $\mathrm{Co}$ indeed improves the Curie temperature as well as the magnetization, $M_s$, with a potential energy product as high as 38 MG Oe. These properties together suggest optimization of the alloy ${\mathrm{La}\mathrm{Ce}\mathrm{Fe}}_{12.7}{\mathrm{Co}}_{1.3}\mathrm{B}$ (with only 7 wt % cobalt) as a critical-element-free permanent magnet. While the substituted elements do not enhance magnetic anisotropy, from theory, we find a substantial increase, to a first anisotropy constant, $K_1$, as high as 4.24 $\mathrm{MJ}/{\mathrm{m}}^3$, associated with $\mathrm{Bi}$ substitution for $\mathrm{Ce}$. Our experimental and theoretical results demonstrate the great potential of $\mathrm{La}$, $\mathrm{Co},$ and $\mathrm{Bi}$ substitutions in developing low-cost and critical-element-free ${\mathrm{Ce}}_2{\mathrm{Fe}}_{14}\mathrm{B}$-based permanent magnets.

Magnetic skyrmions are chiral spin textures with attractive features, such as ultrasmall size, solitonic nature, and easy mobility with small electrical currents that make them promising as information-carrying bits in low-power high-density memory and in logic applications. However, it is essential to guarantee the positional stability of skyrmions for reliable information extraction. Using micromagnetic simulations for the minimum energy path, we compute the energy barriers associated with stabilizing notches along a racetrack. We vary the material parameters, specifically, the strength of the chiral Dzyaloshinskii-Moriya interactions, the notch geometry, and the thickness of the racetrack, to obtain the optimal barrier height. We find that the reduction of skyrmion size as it squeezes past the notch gives rise to the energy barrier. We find a range of energy barriers up to approximately $45k_{B}T$ for a racetrack of 5-nm thickness that can provide a positional lifetime of years for skyrmions for long-term memory applications, while requiring a moderate amount of current (approximately ${10}^{10}{\text{A/m}}^2$) to move the skyrmions. Furthermore, we derive quasianalytical equations to estimate the energy barrier. We also explore other pinning mechanisms, such as local variation of the material parameters in a region, and find that notched geometry provides the highest energy barrier. Our results open up possibilities to design practical skyrmion-based racetrack geometries for spintronics applications.

Continuous-variable systems realized in high-coherence microwave cavities are a promising platform for quantum information processing. While strong dynamic nonlinear interactions are desired to implement fast and high-fidelity quantum operations, static cavity nonlinearities typically limit the performance of bosonic quantum error-correcting codes. Here we study theoretical models of nonlinear oscillators describing superconducting quantum circuits with asymmetric Josephson-junction loops. Treating the nonlinearity as a perturbation, we derive effective Hamiltonians using the Schrieffer-Wolff transformation. We support our analytical results by numerical experiments and show that the effective Kerr-type couplings can be canceled by an interplay of higher-order nonlinearities. This can be better understood in a simplified model supporting only cubic and quartic nonlinearities. Our results show that a cubic interaction allows an increase in the effective rates of both linear and nonlinear operations without an increase in the undesired anharmonicity of an oscillator which is crucial for many bosonic encodings.

We apply the magnetoconductivity characteristic and angle-resolved photoemission spectroscopy (ARPES) to visualize the evolution of surface-selected electronic features of quasi-one-dimensional material ${\mathrm{Bi}}_4{\mathrm{Br}}_4$. The transport measurements indicate the quantum-interference correction to conductivity possesses a symbolic spin-rotational characteristic correlated to the value of Berry phase with the effects of weak localization and weak antilocalization for (001) and (100) surfaces, respectively. The ARPES spectra provide the experimental evidence for a quasi-one-dimensional surface state at the side (100) surface and an anisotropic massive Dirac surface state at the top (001) surface, respectively, which is highly coincided with the angle-dependent scaling behavior of magnetoconductivity. Our results reveal the facet-dependent electronic quantum diffusion in quasi-one-dimensional ${\mathrm{Bi}}_4{\mathrm{Br}}_4$, stimulating further investigations of this topological class and applications of the feasible technologies of topological spintronics.

Significant efforts have been devoted to further increasing the photoconversion efficiency of two-dimensional (2D) organic-inorganic halide perovskites, which are promising photovoltaic materials with superior stability but lower efficiency compared to their three-dimensional counterparts. One of the main factors that limit their photoconversion efficiency is the nonradiative recombination of photoexcited carriers. A widely used strategy for tuning the photoconversion efficiency of 2D perovskites is exchanging various organic cations. However, due to mixed effects contributed to by extrinsic factors, such as defect concentration and sample morphology, experiments alone are insufficient to explain the role of organic cations in phonon-assisted carrier recombination. With time-domain simulations based on first-principles nonadiabatic molecular dynamics, we investigate six prototypical 2D Ruddlesden-Popper perovskites to reveal the impact of organic cations on tuning the nonradiative recombination time through displacing inorganic ions and affecting electron-phonon coupling. The phonon-assisted band-to-band nonradiative recombination time is on the order of a few hundred nanoseconds and can be tuned up to 5 times by modifying organic cations. A distinct correlation between the pure-dephasing time and inorganic atom distortions induced by the motion of organic parts is revealed. Compared to the pure-dephasing time, nonadiabatic coupling (NAC) associated with the strength of electron-phonon coupling plays a more important role in determining the nonradiative carrier recombination time. Notably, fluorinated organic spacers increase the NAC between frontier electronic states and speed up the nonradiative recombination process.

It is widely believed that the skyrmion Hall effect, often disruptive for device applications, vanishes for overall topologically neutral structures such as (synthetic) antiferromagnetic skyrmions and skyrmioniums due to a compensation of Magnus forces. In this paper, however, we report that in contrast to the case of spin-transfer-torque-driven skyrmion motion, this notion is generally false for spin-orbit-torque-driven objects. We show that the skyrmion Hall angle is directly related to their helicity and imposes an unexpected roadblock for developing faster and lower input racetrack memories based on spin-orbit torques.

Modeling and simulations of entanglement-based quantum-networking protocols commonly assume perfect entangled states. Some investigations have been performed, which show how imperfections cause the efficiency of the protocols to rapidly deteriorate. For polarization-entangled states created by spontaneous-parametric down-conversion (SPDC), the fundamentals of phase matching lead to a trade-off problem for the optimal properties of the state. We present a SPDC method, which circumvents the trade-off problem and allows one to obtain degeneracy, indistinguishability, and heralded-single-photon spectral purity greater than $99\mathrm{\%}$ for any target SPDC wavelength in the visible and near-infrared spectrum. Therefore, our method can, in principle, generate polarization-entangled states that are optimal for polarization-entanglement-based quantum-networking protocols.

In functional magnonic devices, spin-wave couplers serve a key role in controlling the spin-wave transmission across different magnonic units. Here we study a reconfigurable spin-wave coupler based on domain-wall channels. The design relies on the dipolar interaction between spin waves propagating in neighboring domain walls. The dipolar-coupled spin-wave modes and their propagation in domain walls are analyzed using micromagnetic simulations and analytical calculations. The distance between domain walls and thus the spin-wave coupling strength can be tuned dynamically. Such a feature results in a dynamically manipulatable spin-wave coupler that enables full control over the propagation properties of coupled spin waves. The spin-wave coupler based on domain-wall channels can be achieved in a single magnetic film, avoiding complex nanofabrication, and enabling possibilities in reconfigurable magnonic functional devices.

Herein, we propose a technique for megaelectronvolt ultrafast electron diffraction that can achieve a few-femtosecond time resolution. The technique modulates a relativistic electron beam using two radially polarized laser pulses of varying frequencies, which enable the condensation of the electrons into an ultrashort pulse. Moreover, our study shows that this technique can suppress the arrival-time jitter of the electron beam at the sample location. Numerical simulations show that using realistic laser parameters corresponding to the current gigawatt-power-level laser systems, a 3.3-fs (rms over 45% charge) electron beam can be obtained and the arrival-time jitter can be 1.35 fs (rms). This technique may have a considerable impact on ultrafast electron diffraction, microscopy, and other ultrafast research facilities that require few-femtosecond electron beams with a small time jitter.

Two-dimensional (2D) semiconductors offer a promising prospect for high-performance and energy-efficient devices especially in the sub-10-nm regime. Inspired by the successful fabrication of 2D β-${\mathrm{Te}\mathrm{O}}_2$ and the high on:off ratio and high air stability of fabricated FETs [Zavabeti et al., Nat. Electron. 4, 277 (2021)], we provide a comprehensive investigation of the electronic structure of monolayer β-${\mathrm{Te}\mathrm{O}}_2$ and the device performance of sub-10-nm MOSFETs based on this material. The anisotropic electronic structure of monolayer β-${\mathrm{Te}\mathrm{O}}_2$ plays a critical role in the anisotropy of transport properties for MOSFETs. We show that the 5.2-nm gate-length n-type MOSFET holds an ultrahigh on-state current exceeding 3700 µA/µm according to International Roadmap for Devices and Systems (IRDS) 2020 goals for high-performance devices, which is benefited by the highly anisotropic electron effective mass. Moreover, monolayer β-${\mathrm{Te}\mathrm{O}}_2$ MOSFETs can fulfill the IRDS 2020 goals for both high-performance and low-power devices in terms of on-state current, subthreshold swing, delay time, and power-delay product. This study unveils monolayer β-${\mathrm{Te}\mathrm{O}}_2$ as a promising candidate for ultrascaled devices in future nanoelectronics.

With continuing improvements on the quality of fabricated quantum devices, it becomes increasingly crucial to analyze noisy quantum process in greater details such as characterizing the non-Markovianity in a quantitative manner. In this work, we propose an experimental protocol, termed spectral transfer tensor maps (STTMs), to accurately predict the Rivas-Huelga-Plenio non-Markovian measure of any Pauli channels without state preparation and measurement errors. In fact, for Pauli channels, STTM even allows the reconstruction of a highly precise noise power spectrum for qubits. At last, we also discuss how STTM can be useful to approximately characterize non-Markovianity of non-Pauli channels via Pauli twirling in an optimal basis.

Current-induced spin-orbit torque (SOT) is regarded as a promising mechanism for driving neuromorphic behavior in spin-orbitronic devices. In principle, the strong SOT in a heavy-metal-based magnetic heterostructure is attributed to the spin-orbit coupling (SOC)-induced spin Hall effect and/or the spin Rashba-Edelstein effect. Recently, SOC-free mechanisms such as the orbital-angular-momentum-induced orbital Hall effect and/or the orbital Rashba-Edelstein effect have been proposed to generate sizable torques comparable to those from the conventional spin Hall mechanism. In this work, we show that the orbital current can be effectively generated by the nitrided light metal $\mathrm{Cu}$. The overall dampinglike SOT efficiency, which consists of both the spin and the orbital current contributions, can be tailored from ${\xi }_{\mathrm{DL}}\approx 0.06\text{ to }0.4$ in a $\mathrm{Pt}/\mathrm{Co}/{\mathrm{Cu}\mathrm{N}}_x$ magnetic heterostructure by tuning the nitrogen doping concentration. Current-induced magnetization switching further verifies the efficacy of such orbital current with a critical switching current density as low as $J_c$ ∼ 5 × ${10}^{10}{\mathrm{A}/\mathrm{m}}^2$. Most importantly, orbital-current-mediated memristive switching behavior can be observed in such heterostructures, which reveals that gigantic SOT and efficient magnetization switching are the trade-offs for the applicable window of memristive switching. Our work provides insights into the role that orbital current might play in SOT neuromorphic devices for making energy-efficient spin-orbitronic devices.

Although metal-organic frameworks attract much attention in electronic devices, the discontinuity at and weak interfacial contact with the substrate are still challenging. In this paper, an $\mathrm{Ag}$/(polyethylene glycol + zeolitic imidazole framework-8) @ (polyvinyl alcohol + 2-methylimidazole)/fluorine-doped tin oxide [$\mathrm{Ag}$/(PEG + ZIF8) @ (PVA + 2MIM)/FTO] structured resistive memory is fabricated. The interfacial contact and active site are enhanced by modifying FTO with (PVA + 2MIM). The continuity of ZIF-8 film is enhanced by compositing PEG. The resistive memory exhibits a lower set voltage of 1.3 V, a higher endurance of more than 140 cycles, and a longer retention time of over 10 000 s, in comparison with control devices of $\mathrm{Ag}$/ZIF-8/FTO and $\mathrm{Ag}/$ZIF-8 @ (PVA + 2MIM)/FTO. The mechanism of the resistive switching behavior is analyzed by conduction models, and explained by redox reactions of the top active electrode to form and rupture conductive filaments under the electric field. This study suggests a facial and general technology for synthesizing a continuous film with strong contact with the substrate, benefiting electronic devices.

Studies of micromechanical and acoustic modes in the quantum regime have shed light on quantum properties of massive objects. Integrating these systems into superconducting circuits shows great promise for applications as quantum memory elements, bosonic codes, or in frequency conversion. To this end, investigation of nonclassical properties of acoustic degrees of freedom is critical also for applications. Here, we investigate a strongly driven system consisting of a transmon qubit interacting with a high-overtone bulk acoustic resonator. We observe multiphonon transitions, which enable mapping the energy landscape in the coupled system. At a high driving amplitude comparable to the qubit-oscillator coupling, we observe a shift of the multiphonon spectral lines, reminiscent of Stark shift, which is explained using a simple model. Our work thus also provides insight in multiquanta transitions in other qubit-oscillator systems, not limited to acoustics or circuit quantum electrodynamics.

Synthesized three-dimensional space provides us with a platform to explore the intriguing acoustic valley Hall insulator. In this paper, a rotational degree of freedom is introduced to construct a synthetic three-dimensional space. Based on simulations, our results show that Weyl points and Fermi arcs and topologically nontrivial edge modes appear in a synthetic space. Not only rotational operation but also boundary truncation can modulate the frequencies of nontrivial edge states, which result in two completely different edge states at the upper and lower boundaries. Using frequencies of edge states protected by Weyl points, we devise two different acoustic topological rainbow devices. The two devices are able to trap states with different frequency components at different spatial positions: one device is capable of capturing low-frequency components at close range, while the other has the ability to confine low-frequency components at long distances. This work may boost further the development of topological rainbow devices in a synthetic space related to the acoustic valley Hall insulator.