Frontiers of Physics最新文献

Rare-earth orthoferrite REFeO₃ (where RE is a rare-earth ion) is gaining interest. We created a high-entropy orthoferrite (Tm_0.2Nd_0.2Dy_0.2Y_0.2Yb_0.2) FeO₃ (HEOR) by doping five RE ions in equimolar ratios and grew the single crystal by optical floating zone method. It strongly tends to form a single-phase structure stabilized by high configurational entropy. In the low-temperature region (11.6–14.4 K), the spin reorientation transition (SRT) of Γ₂ (F_x, C_y, G_z)–Γ₂₄–Γ₄ (G_x, A_y, F_z) occurs. The weak ferromagnetic (FM) moment, which comes from the Fe sublattices distortion, rotates from the a- to c-axis. The two-step dynamic processes (Γ₂–Γ₂₄–r₄) are identified by AC susceptibility measurements. SRT in HEOR can be tuned in the range of 50–60000 Oe, which is an order of magnitude larger than that of orthoferrites in the peer system, making it a candidate for high-field spin sensing. Typical spin-switching (SSW) and continuous spin-switching (CSSW) effects occur under low magnetic fields due to the strong interactions between RE–Fe sublattices. The CSSW effect is tunable between 20–50 Oe, and hence, HEOR potentially can be applied to spin modulation devices. Furthermore, because of the strong anisotropy of magnetic entropy change (−ΔS_m) and refrigeration capacity (RC) based on its high configurational entropy, HEOR is expected to provide a novel approach for refrigeration by altering the orientations of the crystallographic axes (anisotropic configurational entropy).

We examine the electronic and transport properties of a new phase PdSe monolayer with a puckered structure calculated by first-principles and Boltzmann transport equation. The spin–orbit coupling is found to play a negligible effect on the electronic properties of PdSe monolayer. The lattice thermal conductivity of PdSe monolayer exhibits remarkable anisotropic characteristic due to anisotropic phonon group velocity along different directions and its intrinsic structure anisotropy. The compromised electronic mobility despite a relatively low thermal conduction results in a moderate ZT value but significantly anisotropic thermoelectric performance in single-layer PdSe. The present work suggests that the remarkable thermal transport anisotropy of PdSe monolayer can be used for thermal management, and enhance the scope of possibilities for heat flow manipulation in PdSe based devices. The sizeable puckered cages and wiggling lattice implies it an ideal platform for ionic and molecular engineering for thermoelectronic applications.

With the emergence of the Internet of Things (IoT) and the rapid growth of big data generated by edge devices, there has been a growing need for electronic devices that are capable of processing and transmitting data at low power and high speeds. Traditional Complementary Metal-Oxide-Semiconductor (CMOS) devices are nonvolatile and often limited by their ability for certain IoT applications due to their unnecessary power consumption for data movement in von Neuman architecture-based systems. This has led to a surge in research and development efforts aimed at creating innovative electronic components and systems that can overcome these shortcomings and meet the evolving needs of the information era, which share features such as improved energy efficiency, higher processing speeds, and increased functionality. Memristors are a novel type of electronic device that has the potential to break down the barrier between storage and computing. By storing data and processing information within the same device, memristors can minimize the need for data movement, which allows for faster processing speeds and reduced energy consumption. To further improve the energy efficiency and reliability of memristors, there has been a growing trend toward diversifying the selection of dielectric materials used in memristors. Halide perovskites (HPs) have unique electrical and optical properties, including ion migration, charge trapping effect caused by intrinsic defects, excellent optical absorption efficiency, and high charge mobility, which makes them highly promising in applications of memristors. In this paper, we provide a comprehensive overview of the recent development in resistive switching behaviors of HPs and the underlying mechanisms. Furthermore, we summarize the diverse range of HPs, their respective performance metrics, as well as their applications in various fields. Finally, we critically evaluate the current bottlenecks and possible opportunities in the future research of HP memristors.

Progress in the studies of chirality in atomic nuclei at iThemba LABS is reviewed. New regions of chirality, around mass 80 and 190 have been discovered using the AFRODITE array, specifically in the nuclei ⁷⁴As, ^78,80,82Br, ⁸¹Kr, and ^193,194,198Tl. Many phenomena have been observed, including multiple chiral bands in the same nucleus, the coexistence of octupole correlations and nuclear chirality, and the coexistence of pseudo spin and nuclear chirality. The best example of chiral degeneracy to date was found in ¹⁹⁴Tl. The level scheme of ¹⁰⁶Ag has been revisited and interpreted in terms of two- and four-quasiparticle bands. Investigations using the particle-rotor model have shown that the fingerprints of chirality in the two-quasiparticle system only can occur in an idealised model description. For systems with a higher number of quasiparticles, the calculations showed that nuclear chirality can persist.

Two-dimensional transition metal dichalcogenides (TMDs) exhibit promising application prospects in the domains of electronic devices, optoelectronic devices and spintronic devices due to their distinctive energy band structures and spin-orbit coupling properties. Cr-based chalcogenides with narrow or even zero bandgap, covering from semiconductors to metallic materials, have considerable potential for wide-band photodetection and two-dimensional magnetism. Currently, the preparation of 2D CrX_n (X = S, Se, Te) nanosheets primarily relies on chemical vapor deposition (CVD) and molecule beam epitaxy (MBE), which enable the production of high-quality large-area materials. This review article focuses on recent progress of 2D Cr-based chalcogenides, including unique crystal structure of the CrX_n system, phase-controlled synthesis, and heterojunction construction. Furthermore, a detailed introduction of room-temperature ferromagnetism and electrical/optoelectronic properties of 2D CrX_n is presented. Ultimately, this paper summarizes the challenges associated with utilizing 2D Cr-based chalcogenides in preparation strategies, optoelectronics devices, and spintronic devices while providing further insights.

Recently universal dynamic scaling is observed in several systems, which exhibit a spatiotemporal self-similar scaling behavior, analogous to the spatial scaling near phase transition. The latter one arises from the emergent continuous scaling symmetry. Motivated by this, we investigate the possible relation between the scaling dynamics and the continuous scaling symmetry in this paper. We derive a theorem that the scaling invariance of the quenched Hamiltonian and the initial density matrix can lead to the universal dynamic scaling. It is further demonstrated both in a two-body system analytically and in a many-body system numerically. For the latter one, we calculate the dynamics of quantum gases quenched from the zero interaction to a finite interaction via the non-equilibrium high-temperature virial expansion. A dynamic scaling of the momentum distribution appears in certain momentum-time windows at unitarity as well as in the weak interacting limit. Remarkably, this universal scaling dynamics persists approximately with smaller scaling exponents even if the scaling symmetry is fairly broken. Our findings may offer a new perspective to interpret the related experiments. We also study the Contact dynamics in the BEC–BCS crossover. Surprisingly, the half-way time displays a maximum near unitarity while some damping oscillations occur on the BEC side due to the dimer state, which can be used to detect possible two-body bound states in experiments.

Decreasing of layer thickness causes the decrease of polarization until it disappears due to the existence of depolarization field. Therefore, the search for strong piezoelectric materials is highly desirable for multifunctional ultra-thin piezoelectric devices. Herein, we propose a common strategy for achieving strong piezoelectric materials through the electronic asymmetry induced by the intrinsically asymmetric atomic character of different chalcogen atoms. Accordingly, in the tetrahedral lattice structures, for example, M₄X₃Y₃ (M = Pd/Ni, X/Y = S, Se or Te, X ≠ Y) monolayers are proved to display excellent out-of-plane piezoelectricity. Ni₄Se₃Te₃ possesses the largest piezoelectric coefficient d₃₃ of 61.57 pm/V, which is much larger than that of most 2D materials. Enhancing the electronic asymmetry further increases the out-of-plane piezoelectricity of Janus M₄X₃Y₃ materials. Correspondingly, the out-of-plane piezoelectricity is positively correlated with the ratio of electronegativity difference (R_ed) and the electric dipole moment (P). This work provides alternative materials for energy harvesting nano-devices or self-energized wearable devices, and supplies a valuable guideline for predicting 2D materials with strong out-of-plane piezoelectricity.

We concentrate on the skin effects and topological properties in the multilayer non-Hermitian Su–Schrieffer–Heeger (SSH) structure, by taking into account the nonreciprocal couplings between the different sublattices in the unit cells. Following the detailed demonstration of the theoretical method, we find that in this system, the skin effects and topological phase transitions induced by nonreciprocal couplings display the apparent parity effect, following the increase of the layer number of this SSH structure. On the one hand, the skin effect is determined by the parity of the layer number of this SSH system, as well as the parity of the band index of the bulk states. On the other hand, for the topological edge modes, such an interesting parity effect can also be observed clearly. Next, when the parameter disorders are taken into account, the zero-energy edge modes in the odd-layer structures tend to be more robust, whereas the other edge modes are easy to be destroyed. In view of these results, it can be ascertained that the findings in this work promote to understand the influences of nonreciprocal couplings on the skin effects and topological properties in the multilayer SSH lattices.

We propose a scheme to realize antibunched multiple-photon bundles based on phonon blockade in a quadratically coupled optomechanical system. Through adjusting the detunings to match the conditions of phonon blockade in the photon sidebands, we establish super-Rabi oscillation between zero-photon state and multiple-photon states with adjustable super-Rabi frequencies under appropriate single-phonon resonant conditions. Taking the system dissipation into account, we numerically calculate the standard and generalized second-order functions of the cavity mode as well as the quantum trajectories of the state populations with Monte Carlo simulation to confirm that the emitted photons form antibunched multiple-photon bundles. Interestingly, the desirable n-photon states are reconstructed after a direct phonon emission based on phonon blockade, and thus the single-phonon emission heralds the cascade emission of n-photon bundles. Our proposal shows that the optomechanical system can simultaneously behave as antibunched multiple-photon emitter and single-phonon gun. Such a nonclassical source could have potential applications in quantum information science.

Non-orthogonality in non-Hermitian quantum systems gives rise to tremendous exotic quantum phenomena, which can be fundamentally traced back to non-unitarity. In this paper, we introduce an interesting quantity (denoted as η) as a new variant of the Petermann factor to directly and efficiently measure non-unitarity and the associated non-Hermitian physics. By tuning the model parameters of underlying non-Hermitian systems, we find that the discontinuity of both η and its first-order derivative (denoted as ∂η) pronouncedly captures rich physics that is fundamentally caused by non-unitarity. More concretely, in the 1D non-Hermitian topological systems, two mutually orthogonal edge states that are respectively localized on two boundaries become non-orthogonal in the vicinity of discontinuity of η as a function of the model parameter, which is dubbed “edge state transition”. Through theoretical analysis, we identify that the appearance of edge state transition indicates the existence of exceptional points (EPs) in topological edge states. Regarding the discontinuity of ∂η, we investigate a two-level non-Hermitian model and establish a connection between the points of discontinuity of ∂η and EPs of bulk states. By studying this connection in more general lattice models, we find that some models have discontinuity of ∂η, implying the existence of EPs in bulk states.