Frontiers of Physics最新文献

Quantum secure direct communication (QSDC) can transmit secret messages without keys, making it an important branch of quantum communication. We present a hybrid entanglement-based quantum secure direct communication (HE-QSDC) protocol with simple linear optical elements, combining the benefits of both continuous variables (CV) and discrete variables (DV) encoding. We analyze the security and find that the QSDC protocol has a positive security capacity when the bit error rate is less than 0.073. Compared with previous DV QSDC protocols, our protocol has higher communication efficiency due to performing nearly deterministic Bell-state measurement. On the other hand, compared with CV QSDC protocol, this protocol has higher fidelity with large α. Based on these advantages, our protocol may provide an alternative approach to realize secure communication.

We studied the quantum correlations of a three-body Unruh–DeWitt detector system using genuine tripartite entanglement (GTE) and geometric quantum discord (GQD). We considered two representative three-body initial entangled states, namely the GHZ state and the W state. We demonstrated that the quantum correlations of the tripartite system are completely destroyed at the limit of infinite acceleration. In particular, it is found that the GQD of the two initial states exhibits “sudden change” behavior with increasing acceleration. It is shown that the quantum correlations of the W state are more sensitive than those of the GHZ state under the effect of Unruh thermal noise. The GQD is a more robust quantum resource than the GTE, and we can achieve robustness in discord-type quantum correlations by selecting the smaller energy gap in the detector. These findings provide guidance for selecting appropriate quantum states and resources for quantum information processing tasks in a relativistic setting.

High-quality antiferromagnetic Mott insulator thin films of LaTiO₃ (LTO) were epitaxially grown onto SrTiO₃ (STO) (110) substrates using the pulsed laser deposition. The LTO/STO heterostructures are not only highly conducting and ferromagnetic, but also show Kondo effect, Shubnikov-de Haas (SdH) oscillations with a nonzero Berry phase of π, and low-field hysteretic negative magnetoresistance (MR). Angle-dependent SdH oscillations and a calculation of the thickness of the interfacial conducting layer indicate the formation of a 4-nm high mobility two-dimensional electron gas (2DEG) layer at the interface. Moreover, an amazingly large low-field negative MR of ∼61.8% is observed at 1.8 K and 200 Oe, which is approximately one to two orders of magnitude larger than those observed in other spin-polarized 2DEG oxide systems. All these results demonstrate that the 2DEG is spin-polarized and the 4-nm interfacial layer is ferromagnetic, which are attributed to the presence of magnetic Ti³⁺ ions due to interfacial oxygen vacancies and the diffusion of La³⁺ ions into the STO substrate. The localized Ti³⁺ magnetic moments couple to high mobility itinerant electrons under magnetic fields, giving rise to the observed low-field MR. Our work demonstrates the great potential of antiferromagnetic titanate oxide interface for designing spin-polarized 2DEG and spintronic devices.

Spin–rotation coupling (SRC) is a fundamental interaction that connects electronic spins with the rotational motion of a medium. We elucidate the Einstein–de Haas (EdH) effect and its inverse with SRC as the microscopic mechanism using the dynamic spin–lattice equations derived by elasticity theory and Lagrangian formalism. By applying the coupling equations to an iron disk in a magnetic field, we exhibit the transfer of angular momentum and energy between spins and lattice, with or without damping. The timescale of the angular momentum transfer from spins to the entire lattice is estimated by our theory to be on the order of 0.01 ns, for the disk with a radius of 100 nm. Moreover, we discover a linear relationship between the magnetic field strength and the rotation frequency, which is also enhanced by a higher ratio of Young’s modulus to Poisson’s coefficient. In the presence of damping, we notice that the spin-lattice relaxation time is nearly inversely proportional to the magnetic field. Our explorations will contribute to a better understanding of the EdH effect and provide valuable insights for magneto-mechanical manufacturing.

We investigated 1-µm multimode fiber laser based on carbon nanotubes, where multiple typical pulse states were observed, including Q-switched, Q-switched mode-locked, and spatiotemporal mode-locked pulses. Particularly, stable spatiotemporal mode-locking was realized with a low threshold, where the pulse duration was 37 ps and the wavelength was centred at 1060.5 nm. Moreover, both the high signal to noise and long-term operation stability proved the reliability of the mode-locked laser. Furthermore, the evolution of the spatiotemporal mode-locked pulses in the cavity was also simulated and discussed. This work exhibits the flexible outputs of spatiotemporal phenomena in multimode lasers based on nanomaterials, providing more possibilities for the development of high-dimensional nonlinear dynamics.

Dielectric engineering plays a crucial role in the process of device miniaturization. Herein we investigate the electrical properties of bilayer GaSe metal-oxide-semiconductor field-effect transistors (MOSFETs), considering hetero-gate-dielectric construction, dielectric materials and GaSe stacking pattern. The results show that device performance strongly depends on the dielectric constants and locations of insulators. When high-k dielectric is placed close to the drain, it behaves with a larger on-state current (I_on) of 5052 µA/µm when the channel is 5 nm. Additionally, when the channel is 5 nm and insulator is HfO₂, the largest I_on is 5134 µA/µm for devices with AC stacking GaSe channel. In particular, when the gate length is 2 nm, it still meets the HP requirements of ITRS 2028 for the device with AA stacking when high-k dielectric is used. Hence, the work provides guidance to regulate the performance of the two-dimensional nanodevices by dielectric engineering.

Non-Hermitian systems with parity–time (PT)-symmetry have been extensively studied and rapidly developed in resonance wireless power transfer (WPT). The WPT system that satisfies PT-symmetry always has real eigenvalues, which promote efficient energy transfer. However, meeting the condition of PT-symmetry is one of the most puzzling issues. Stable power transfer under different transmission conditions is also a great challenge. Bound state in the continuum (BIC) supporting extreme quality-factor mode provides an opportunity for efficient WPT. Here, we propose theoretically and demonstrate experimentally that BIC widely exists in resonance-coupled systems without PT-symmetry, and it can even realize more stable and efficient power transfer than PT-symmetric systems. Importantly, BIC for efficient WPT is universal and suitable in standard second-order and even high-order WPT systems. Our results not only extend non-Hermitian physics beyond PT-symmetry, but also bridge the gap between BIC and practical application engineering, such as highperformance WPT, wireless sensing and communications.

Two-dimensional materials with high-temperature ferromagnetism and half-metallicity have the latest applications in spintronic devices. Based on first-principles calculations, we have investigated a novel two-dimensional CrS₂ phase with an orthorhombic lattice. Our results suggest that it is stable in dynamics, thermodynamics, and mechanics. The ground state of monolayer orthorhombic CrS₂ is both ferromagnetic and half-metallic, with a high Curie temperature of 895 K and a large spin-flipping gap on values of 0.804 eV. This room-temperature ferromagnetism and half-metallicity can maintain stability against a strong biaxial strain ranging from −5% to 5%. Meanwhile, increasing strain can significantly maintain the out-of-plane magnetic anisotropy. A density of states analysis, together with the orbital-resolved magnetic anisotropy energy, has revealed that the strain-enhanced MAE is highly related to the 3d-orbital splitting of Cr atoms. Our results suggest the monolayer orthorhombic CrS₂ is an ideal candidate for future spintronics.

We theoretically studied the exciton geometric structure in layered semiconducting transition metal dichalcogenides. Based on a three-orbital tight-binding model for Bloch electrons which incorporates their geometric structures, an effective exciton Hamiltonian is constructed and solved perturbatively to reveal the relation between the exciton and its electron/hole constituent. We show that the electron—hole Coulomb interaction gives rise to a non-trivial inheritance of the exciton geometric structure from Bloch electrons, which manifests as a valley-dependent center-of-mass anomalous Hall velocity of the exciton when two external fields are applied on the electron and hole constituents, respectively. The obtained center-of-mass anomalous velocity is found to exhibit a non-trivial dependence on the fields, as well as the wave function and valley index of the exciton. These findings can serve as a general guide for the field-control of the valley-dependent exciton transport, enabling the design of novel quantum optoelectronic and valleytronic devices.

Developing advanced hydrogen storage materials with high capacity and efficient reversibility is a crucial aspect for utilizing hydrogen source as a promising alternate to fossil fuels. In this paper, we have systematically investigated the hydrogen storage properties of neutral and negatively charged C₉N₄ monolayer based on density functional theory (DFT). Our foundings indicate that injecting additional electrons into the adsorbent significantly boosts the adsorption capacity of C₉N₄ monolayer to H₂ molecules. The gravimetric density of negatively charged C₉N₄ monolayer can reach up to 10.80 wt% when fully covered with hydrogen. Unlike other hydrogen storage methods, the storage and release processes happen automatically upon introducing or removing extra electrons. Moreover, these operations can be easily adjusted through activating or deactivating the charging voltage. As a result, the method is easily reversible and has tunable kinetics without requiring particular activators. Significantly, C₉N₄ is proved to be a suitable candidate for efficient electron injection/release due to its well electrical conductivity. Our work can serve as a valuable guide in the quest for a novel category of materials for hydrogen storage with high capacity.