Advanced Physics Research最新文献

Magneto-Catalysis

Magnetic fields could “talk” with catalysis reactions through the induced electromotive force. The catalytic efficiencies for hydrogen production can be boosted depending on the relative geometric positions between the magnetic fields and electrodes. See article 2300067 by Juntao Huo, Dengsong Zhang, Guowei Li, and colleagues for more details.

The sustained interest in investigating magnetism in the 2D limit of insulating antiferromagnets is driven by the possibilities of discovering, or engineering, novel magnetic phases through layer stacking. However, due to the difficulty of directly measuring magnetic interactions in 2D antiferromagnets, it is not yet understood how intralayer magnetic interactions in insulating, strongly correlated, materials can be modified through layer proximity. Herein, the impact of reduced dimensionality in the model van der Waals antiferromagnet NiPS₃ is explored by measuring electronic excitations in exfoliated samples using Resonant Inelastic X-ray Scattering (RIXS). The resulting spectra shows systematic broadening of NiS₆ multiplet excitations with decreasing layer count from bulk down to three atomic layers (3L). It is shown that these trends originate from a decrease in transition metal-ligand and ligand–ligand hopping integrals, and by charge-transfer energy evolving from Δ = 0.83 eV in the bulk to 0.37 eV in 3L NiPS₃. Relevant intralayer magnetic exchange integrals computed from the electronic parameters exhibit a decrease in the average interaction strength with thickness. This study underscores the influence of interlayer electronic interactions on intralayer ones in insulating magnets, indicating that magnetic Hamiltonians in few-layer insulating magnets can greatly deviate from their bulk counterparts.

Memristive hardware with reconfigurable conductance levels are leading candidates for achieving artificial neural networks (ANNs). However, owing to difficulties in device character design and circuit combination, the ability to perform complicated online-learning tasks on a memristive network is not well understood. Here, tandem (T) material states are harnessed in a phase-change memory (PCM) element, i.e., the primed-amorphous state and the partial-crystallized state, by utilizing an impetus-and-consequent pair pulse through a large degree of configurational ordering, and illustrate the development of an integrated system for achieving in-memory computing and neural networks (NNs). A correct classification of 96.1% of 10,000 separate test images from the conventional Modified-National-Institute-of-Standards-and-Technology (MNIST) database in the tandem neural-network (T-NN) model is achieved, as well as image recognition for 28×28-pixel pictures. The T-NN configuration exhibits an in situ learning, with 50% of the elements stuck in the low-conductance state, and at the same time, maintains an identification accuracy of ≈90%. The structural origin of the large degree of configurational-ordering-enhanced improvement in the extent of the conductance uniformity in the T-based memristive element is revealed by theoretical studies. This work opens the door for attaining a widely relevant hardware system capable of performing artificial intelligence tasks with a large power-time efficacy.

Harnessing extra degrees of freedom at the heterostructure interface is of crucial importance to bring additional functionalities in modern spintronic devices. Here, a vertical hysteresis loop shift (vertical bias) is demonstrated in an exchange biased system of ferromagnetic thin film heterostructure of La_0.67Sr_0.33MnO₃ (10 nm)-SrRuO₃ (SRO) (20 nm), after field cooling with ±1 T below 100 K close to the Curie temperature (T_C) ≈125 K of SRO and loop sweeping under ±1 T field. Besides, a positive exchange bias (H_EB) is also observed below T_C ≈125 K showing a maximum ≈11 mT at 2 K. The vertical shift is modeled closely using micromagnetic simulations and the layers’ thickness dependency is demonstrated. The reason for the shift is attributed to the simultaneous role of the interfacial antiferromagnetic interaction and the hard anisotropy of SRO against the Zeeman energy. Finally, from the experimental and simulation results, a generalized model of controllable and tunable vertical shift is proposed applicable for other material systems possessing glassy phases, uncompensated/canted spins, absent interfacial exchange coupling, etc., and hence can be informative for the use of vertical shift in future spintronic devices.

One of the hallmark of topological insulators is having conductivity properties that are unaffected by the possible presence of defects. In this work, by going beyond backscattering immunity and topological invisibility across defects or disorder is obtained. Using a combination of chiral and mirror symmetry, the transmission coefficient is guaranteed to be unity. Importantly, but no phase shift is induced making the defect completely invisible. Many lattices possess the chiral-mirror symmetry, and the principle is chosen to be demonstrated on an hexagonal lattice model with Kekulé distortion displaying topological edge waves, and analytically and numerically is shown that the transmission across symmetry preserving defects is unity. Then this lattice in an acoustic system is realized, and the invisibility is confirmed with numerical experiments. It is foreseen that the versatility of the model will trigger new experiments to observe topological invisibility in various wave systems, such as photonics, cold atoms or elastic waves.

2D materials with long-range ferromagnetic order hold promises for the development of compact spintronic devices with unprecedented multifunctionality and tunability. Among various 2D magnets, self-intercalated transition metal chalcogenides Cr_1+δTe₂ exhibit unique features, especially excellent ambient stability and intrinsic ferromagnetic ordering above room temperature, which are critical requirements for real-life device applications. Despite the many investigations of the magnetic properties of the Cr_1+δTe₂ family on the averaging macroscopic level, the domain evolution on the microscale, which is vital to nanoscale spintronics, is yet to be fully understood. Here, the evolution of magnetic behaviors of Cr_0.92Te crystals is presented on both macro- and micro-scales under magnetic field and thermal excitation. The crystal exhibits a high Curie temperature (T_c ≈ 343 K) among the Cr_1+δTe₂ family with weak magnetic anisotropy and in-plane magnetic easy axis. Utilizing magnetic force microscopy, a pristine multidomain state and typical domain-switching behavior are observed. Moreover, the evolution of domain texture under thermal excitation shows statistical power-law scaling as approaching T_c. The results provide microscopic insight into the ferromagnetic behavior of a room-temperature quasi-2D crystal, which can be useful for further engineering of domain texture in low-dimensional magnetic materials.

Hybrid organic–inorganic perovskites (HOIPs) are promising candidates for next-generation photovoltaic materials. However, there is a debate regarding the impact of interactions between the organic center and the surrounding inorganic cage on the solar cell's high diffusion lengths. It remains unclear whether the diffusion mechanism is consistent across various halide perovskite families and how light illumination affects carrier lifetimes. The focus is on ion kinetics of (CH₃NH₃)PbX₃ (X = Br, Cl) perovskite halide single crystals. Muon spectroscopy (μ⁺SR)is employed to investigate the fluctuations and diffusion of ions via the relaxation of muon spins in local nuclear field environments. Within a temperature range of 30–340 K, ion kinetics are studied with and without white-light illumination. The results show a temperature shift of the tetragonal-orthorhombic phase transition on the illuminated samples, as an effect of increased organic molecule fluctuations. This relation is supported by density functional theory (DFT) calculations along the reduction of the nuclear field distribution width between the phase transitions. The analysis shows that, depending on the halide ion, the motional narrowing from H and N nuclear moments represents the molecular fluctuations. The results demonstrate the importance of the halide ion and the effect of illumination on the compound's structural stability and electronic properties.

Bismuth(Bi)-doped optical fibers are widely used in various aspects such as fiber lasers and amplifiers. Despite extensive research efforts to explore high-gain Bi-doped fibers, the pursuit of Bi-doped fibers with emission over an extremely wide wavelength range remains unfulfilled. Here, the strategy of cluster control via regulating glass structure is proposed. It is applied to construct Bi-doped photonic glass with ultra-broadband near-infrared (NIR) emission and a full width at half maximum (FWHM) of 343 nm. The strategy is further combined with the modified chemical vapor deposition (MCVD) technique with solution doping. The fiber exhibits ultra-broadband amplified spontaneous emission (ASE) covering the entire fiber communication band (1260–1625 nm). Finally, the fiber exhibits higher optical gain and bandwidth compared with the fiber without cluster control. This device can be utilized to potentially enhance the capacity of telecommunication systems.

Ultralow SET and RESET voltage are essential for high-density, low-power, and small heat dissipation nonvolatile random-access memory (NVRAM) elements. A nanoscale polycrystalline Hf_0.75Zr_0.25O₂ (HZO) thin films on Pt/Si substrate are fabricated and investigated for suitability for bipolar resistive switching. The device illustrates monoclinic and tetragonal/orthorhombic phases with weak ferroelectricity and robust resistive switching. Small remanent polarization (≈0.1 μC cm⁻²) may assist in the height reduction of barrier height and ease the electron for transport. Remarkably, the Al/HZO/Pt/Si device, consisting of thin films with 10 and 5 nm thicknesses, exhibits a switching voltage below −30 mV from a low-resistance state (LRS) to a high-resistance state (HRS). It shows a significant R_OFF/R_ON ratio of 10⁶, making it suitable for low power consumption and minimal heat dissipation devices. Moreover, the utilization of an ultrathin film (5 nm) results in an improved reduction (< 0.7 V) of the operating window at the positive voltage.  Direct tunneling and the Fowler–Nordheim tunneling model are performed in current–voltage (I–V) data to study the charge transportation behavior over a trapezoidal and triangular potential barrier. These results of the HZO candidate may stimulate the futuristic nonvolatile resistive random-access memory (ReRAM) in the optoelectronic industry.

Rotational symmetry is ubiquitous in nature. However, self-assemblies of soft condensed matter such as liquid crystals (LCs) are entropy-driven, making the tailoring of rotational symmetry challenging. Here, an approach is proposed to control the rotational symmetry of LC domain lattices based on the anchoring condition predesign and the orientational-order inheritance during the nematic-smectic phase transition. By this means, periodic and quasiperiodic LC textures with C_i symmetry (i = 2–6) determined by the preset alignment lattices are realized, and verified by symmetries of diffraction patterns. Topological analysis is carried out to disclose distinct evolutions of orders between disclination line textures and defect wall textures of different rotational symmetries. The influences of anchoring conditions, phase transitions, and mechanical stress on the self-assembly of LCs, as well as the underlying mechanisms and dynamics, are investigated. This work realizes controllable rotational symmetry for large-area self-organized LCs and brings new insights to soft condensed matter.