Physical systems with gain and loss can be described by a non-Hermitian Hamiltonian, which is degenerated at the exceptional points (EPs). Many new and unexpected features have been explored in the non-Hermitian systems with a great deal of recent interest. One of the most fascinating features is that chiral state conversion appears when one EP is encircled dynamically. Here, we propose an easy-controllable levitated microparticle system that carries a pair of EPs and realize slow evolution of the Hamiltonian along loops in the parameter plane. Utilizing the controllable rotation angle, gain and loss coefficients, we can control the structure, size and location of the loops in situ. We demonstrate that, under the joint action of topological structure of energy surfaces and nonadiabatic transitions, the chiral behavior emerges both along a loop encircling an EP and even along a straight path away from the EP. This work broadens the range of parameter space for the chiral state conversion, and proposes a useful platform to explore the interesting properties of exceptional points physics.

Traveling wave solutions have been well studied for various nonlinear systems. However, for high order nonlinear physical models, there still exist various open problems. Here, travelling wave solutions to the well-known fifth-order nonlinear physical model, the Sawada–Kotera equation, are revisited. Abundant travelling wave structures including soliton molecules, soliton lattice, kink-antikink molecules, peak-plateau soliton molecules, few-cycle-pulse solitons, double-peaked and triple-peaked solitons are unearthed.

Very recently the LHCb collaboration reported their observation of the first two fully open-flavor tetraquark states, the $X_0(2900)$ of $J^P = 0^+$ and the $X_1(2900)$ of $J^P = 1^-$. We study their possible interpretations using the method of QCD sum rules, paying special attention to an interesting feature of this experiment that the higher resonance $X_1(2900)$ has a width significantly larger than the lower one $X_0(2900)$. Our results suggest that the $X_0(2900)$ can be interpreted as the s-wave $D^{*-}K^{*+}$ molecule state of $J^P = 0^+$, and the $X_1(2900)$ can be interpreted as the p-wave $\bar c \bar s u d$ compact tetraquark state of $J^P = 1^-$. Mass predictions of their bottom partners are also given.

Motivated by the recent ATLAS results in terms of the branching ratios $ Br(t\rightarrow q\gamma)$, we consider the effects of the anomalous $tq\gamma$ couplings on the radiative $B$ meson decays $\bar{B}\rightarrow X_{D} \gamma$ and $B\rightarrow V\gamma$ with $V$ being light vector mesons $\rho,\omega,\phi$ and $K^{\ast}$. Comparing with the corresponding experimental measured data, we obtain the constraints on anomalous $tq\gamma$ couplings.

We present a general machine learning based scheme to optimize experimental control. The method utilizes the neural network to learn the relation between the control parameters and the control goal, with which the optimal control parameters can be obtained. The main challenge of this approach is that the labeled data obtained from experiments are not abundant. The central idea of our scheme is to use the active learning to overcome this difficulty. As a demonstration example, we apply our method to control evaporative cooling experiments in cold atoms. We have first tested our method with simulated data and then applied our method to real experiments. It is demonstrated that our method can successfully reach the best performance within hundreds of experimental runs. Our method does not require knowledge of the experimental system as a prior and is universal for experimental control in different systems.

We demonstrate a tunable bandpass optical filter based on a tapered optical fiber (TOF) touched by a hemispherical microfiber tip (MFT). Other than the interference and selective material absorption effects, the filter relies on the controllable and wavelength-dependent mode–mode interactions in TOF. Experimentally, a large range of tunability is realized by controlling the position of the MFT in contact with the TOF for various TOF radii, and two distinct bandpass filter mechanisms are demonstrated. The center wavelength of the bandpass filter can be tuned from 890 nm to 1000 nm, while the FWHM bandwidth can be tuned from 110 nm to 240 nm when the MFT touches the TOF in the radius range from 160 nm to 390 nm. The distinction ratio can reach $28 \pm 3$ dB experimentally. The combined TOF-MFT is an in-line tunable bandpass optical filter that has great application potential in optical networks and spectroscopy, and the principle could also be generalized to other integrated photonic devices.

The microscopic mechanism of thermal transport in liquids and amorphous solids has been an outstanding problem for a long time. There have been several approaches to explain the thermal conductivities in these systems, for example, Bridgman's formula for simple liquids, the concept of the minimum thermal conductivity for amorphous solids, and the thermal resistance network model for amorphous polymers. Here, we present a ubiquitous formula to calculate the thermal conductivities of liquids and amorphous solids in a unified way, and compare it with previous ones. The calculated thermal conductivities using this formula without fitting parameters are in excellent agreement with the experimental data. Our formula not only provides a detailed microscopic mechanism of heat transfer in these systems, but also resolves the discrepancies between existing formulae and experimental data.

Molecular dynamics simulations are performed to gain insights into the structural and vibrational properties of interface between porous and fused silica. The Si–O bonds formed in the interface exhibit the same lengths as the bulk material, whereas the coordination defects in the interface are at an intermediate level as compared with the dense and porous structures. Clustered bonds are identified from the interface, which are associated with the reorganization of the silica surface. The bond angle distributions show that the O–Si–O bond angles keep the average value of 109$^{\circ}$, whereas the Si–O–Si angles of the interface present in a similar manner to those in porous silica. Despite the slight structural differences, similarities in the vibrations are observed, which could further demonstrate the stability of porous silica films coated on the fused silica.

The noncentrosymmetricity of a prototypical correlated electron system Ca$_{3}$Ru$_{2}$O$_{7}$ renders extensive interest in the possible polar metallic state, along with multiple other closely competing interactions. However, the structural domain formation in this material often complicates the study of intrinsic material properties. It is crucial to fully characterize the structural domains for unrevealing underlying physics. Here, we report the domain imaging on Ca$_{3}$Ru$_{2}$O$_{7}$ crystal using the reflection of polarized light at normal incidence. The reflection anisotropy measurement utilizes the relative orientation between electric field component of the incident polarized light and the principal axis of the crystal, and gives rise to a peculiar contrast. The domain walls are found to be the interfaces between 90$^{\circ}$ rotated twin crystals by complementary magnetization measurements. A distinct contrast in reflectance is also found in the opposite cleavage surfaces, owing to the polar mode of the RuO$_{6}$ octahedra. More importantly, the analysis of the contrast between all inequivalent cleavage surfaces enables a direct determination of the crystallographic orientation of each domain. Such an approach provides an efficient yet feasible method for structural domain characterization, which can also find applications in noncentrosymmetric crystals in general.

To obtain various Ni/Mn orderings, we use a low-temperature synthesized method to modulate the Ni/Mn ordering of the ferromagnetic-ferroelastic La$_{2}$NiMnO$_{6}$ compound, and the Ni/Mn ordering is estimated by the low-temperature saturation magnetism. The microstructures, crystal structures and magnetic properties are investigated, and the Landau theory are used to describe the form and magnitude of the coupling effects between Ni/Mn ordering and magnetic order parameters. It is predicted that the Ni/Mn ordering would be a strong coupling effect with the Curie transition temperatures if the La$_{2}$NiMnO$_{6}$ sample stoichiometry is close.

Planar graphene metalens has demonstrated advantages of ultrathin thickness (200 nm), high focusing resolution (343 nm) and efficiency ($>$32%) and robust mechanical strength and flexibility. However, diffraction-limited imaging with such a graphene metalens has not been realized, which holds the key to designing practical integrated imaging systems. In this work, the imaging rule for graphene metalenses is first derived and theoretically verified by using the Rayleigh-Sommerfeld diffraction theory to simulate the imaging performance of the 200 nm ultrathin graphene metalens. The imaging rule is applicable to graphene metalenses in different immersion media, including water or oil. Based on the theoretical prediction, high-resolution imaging using the graphene metalens with diffraction-limited resolution (500 nm) is demonstrated for the first time. This work opens the possibility for graphene metalenses to be applied in particle tracking, microfluidic chips and biomedical devices.

Stacking-dependent magnetism in van der Waals materials has caught intense interests. Based on the first principle calculations, we investigate the coupling between stacking orders and interlayer magnetic orders in bilayer H-VSe$_{2}$. It is found that there are two stable stacking orders in bilayer H-VSe$_{2}$, named AB-stacking and A$^{\prime}$B-stacking. Under standard DFT framework, the A$^{\prime}$B-stacking prefers the interlayer AFM order and is semiconductive, whereas the AB-stacking prefers the FM order and is metallic. However, under the DFT+$U$ framework both the stacking orders prefer the interlayer AFM order and are semiconductive. By detailedly analyzing this difference, we find that the interlayer magnetism originates from the competition between antiferromagnetic interlayer super-superexchange and ferromagnetic interlayer double exchange, in which both the interlayer Se-4$p_{z}$ orbitals play a crucial role. In the DFT+$U$ calculations, the double exchange is suppressed due to the opened bandgap, such that the interlayer magnetic orders are decoupled with the stacking orders. Based on this competition mechanism, we propose that a moderate hole doping can significantly enhance the interlayer double exchange, and can be used to switch the interlayer magnetic orders in bilayer VSe$_{2}$. This method is also applicable to a wide range of semiconductive van der Waals magnets.

In cavity quantum electrodynamics, the multiple reflections of a photon between two mirrors defining a cavity is exploited to enhance the light-coupling of an intra-cavity atom. We show that this paradigm for enhancing the interaction of a flying particle with a localized object can be generalized to spintronics based on van der Waals 2D magnets. Upon tunneling through a magnetic bilayer, we find that the spin transfer torques per electron incidence can become orders of magnitude larger than $\hbar /2$, made possible by electron's multi-reflection path through the ferromagnetic monolayers as an intermediate of their angular momentum transfer. Over a broad energy range around the tunneling resonances, the damping-like spin transfer torque per electron tunneling features a universal value of $(\hbar/2)\tan (\theta /2)$, depending only on the angle $\theta$ between the magnetizations. These findings expand the scope of magnetization manipulations for high-performance and high-density storage based on van der Waals magnets.

Fundamental understanding of interfacial charge behaviors is of great significance for the optoelectronic and photovoltaic applications. However, the crucial roles of perovskite terminations in charge transport processes have not been completely clear. We investigate the charge transfer behaviors of the CsPbI$_{3}$/black phosphorus (BP) van der Waals heterostructure by using the density functional theory calculations with a self-energy correction. The calculations at the atomic level demonstrate the type-II band alignments of the CsPbI$_{3}$/BP heterostructure, which make electrons transfer from the perovskite side to monolayer BP. Moreover, the stronger interaction and narrower physical separation of the interfaces can lead to higher charge tunneling probabilities in the CsPbI$_{3}$/BP heterostructure. Due to different electron affinities, the PbI$_{2}$-terminated perovskite slab tends to collect electrons from the adjacent materials, whereas the CsI-termination prefers to inject electrons into transport materials. In addition, the interface coupling effect enhances the visible-light-region absorption of the CsPbI$_{3}$/BP heterostructure. This study highlights the importance of the perovskite termination in the charge transport processes and provides theoretical guidelines to develop high-performance photovoltaic and optoelectronic devices.

Recently, the theoretically predicted lanthanum superhydride, LaH$_{10 \pm \delta}$, with a clathrate-like structure was successfully synthesized and found to exhibit a record high superconducting transition temperature $T_{\rm c} \approx 250$ K at $\sim $170 GPa, opening a new route for room-temperature superconductivity. However, since in situ experiments at megabar pressures are very challenging, few groups have reported the $\sim $250 K superconducting transition in LaH$_{10 \pm \delta}$. Here, we establish a simpler sample-loading procedure that allows a relatively large sample size for synthesis and a standard four-probe configuration for resistance measurements. Following this procedure, we successfully synthesized LaH$_{10 \pm \delta}$ with dimensions up to $10 \times 20$ μm$^{2}$ by laser heating a thin La flake and ammonia borane at $\sim $1700 K in a symmetric diamond anvil cell under the pressure of 165 GPa. The superconducting transition at $T_{\rm c} \approx 250$ K was confirmed through resistance measurements under various magnetic fields. Our method will facilitate explorations of near-room-temperature superconductors among metal superhydrides.

Metal–insulator transition (MIT) is one of the most conspicuous phenomena in correlated electron systems. However such a transition has rarely been induced by an external magnetic field as the field scale is normally too small compared with the charge gap. We present the observation of a magnetic-field-driven MIT in a magnetic semiconductor $\beta $-EuP$_3$. Concomitantly, we find a colossal magnetoresistance in an extreme way: the resistance drops billionfold at 2 K in a magnetic field less than 3 T. We ascribe this striking MIT as a field-driven transition from an antiferromagnetic and paramagnetic insulator to a spin-polarized topological semimetal, in which the spin configuration of Eu$^{2+}$ cations and spin-orbital coupling play a crucial role. As a phosphorene-bearing compound whose electrical properties can be controlled by the application of field, $\beta $-EuP$_3$ may serve as a tantalizing material in the basic research and even future electronics.

We investigate the magnetic damping parameter of Fe$_{1-x}$Cr$_x$ thin films using the time-resolved magneto-optical Kerr effect technique. It is demonstrated that the overall effective damping parameter is enhanced with the increasing Cr concentration. The effective damping at high field $\alpha_{0}$ is found to be significantly enhanced when increasing the Cr concentration with the $\alpha_{0}=0.159$ in the Fe$_{45}$Cr$_{55}$ enhanced by 562% compared with that of $\alpha_{0}=0.024$ in the pure Fe film. This study provides a new approach of controlling the effective damping parameter with a desired magnitude via varying Cr composition.

We systematically investigate the magnetic properties of Cu$_{4-x}$Zn$_x$(OH)$_6$FBr using the neutron diffraction and muon spin rotation and relaxation (μSR) techniques. Neutron-diffraction measurements suggest that the long-range magnetic order and the orthorhombic nuclear structure in the $x = 0$ sample can persist up to $x = 0.23$ and 0.43, respectively. The temperature dependence of the zero-field μSR spectra provides two characteristic temperatures, $T_{A_0}$ and $T_{\lambda}$, which are associated with the initial drop close to zero time and the long-time exponential decay of the muon relaxation, respectively. Comparison between $T_{A_0}$ and $T_{\rm M}$ from previously reported magnetic-susceptibility measurements suggest that the former comes from the short-range interlayer-spin clusters that persist up to $x = 0.82$. On the other hand, the doping level where $T_{\lambda}$ becomes zero is about 0.66, which is much higher than threshold of the long-range order, i.e., $\sim$0.4. Our results suggest that the change in the nuclear structure may alter the spin dynamics of the kagome layers and a gapped quantum-spin-liquid state may exist above $x = 0.66$ with the perfect kagome planes.

We report the magnetoresistance (MR), de Haas-van Alphen (dHvA) oscillations and the electronic structures of single-crystal PtGa. The large unsaturated MR is observed with the magnetic field $B \parallel [111]$. Evident dHvA oscillations with the $B \parallel [001]$ configuration are observed, from which twelve fundamental frequencies are extracted and the spin-orbit coupling (SOC) induced band splitting is revealed. The light cyclotron effective masses are extracted from the fitting by the thermal damping term of the Lifshitz–Kosevich formula. Combining with the calculated frequencies from the first-principles calculations, the dHvA frequencies $F_1/F_3$ and $F_{11}/F_{12}$ are confirmed to originate from the electron pockets at $\mit\Gamma$ and $R$, respectively. The first-principles calculations also reveal the existence of spin-3/2 Rarita–Schwinger–Weyl fermions and time-reversal doubling of the spin-1 excitation at $\mit\Gamma$ and $R$ with large Chern numbers of $\pm4$ when SOC is included.

We theoretically investigate physical properties of two-dimensional (2D) Fe$_{2}$Ga$_{2}$S$_{5}$ by employing first-principles calculations. It is found that it is an antiferromagnet with zigzag magnetic configuration orienting in the in-plane direction, with Néel temperatures around 160 K. The band structure of the ground state shows that it is a semiconductor with the indirect band gap of about 0.9 eV, which could be effectively tuned by the lattice strain. We predict that the carrier transport is highly anisotropic, with the electron mobility up to the order of $\sim$$10^3$ cm$^2$/(V$\cdot$s) much higher than the hole. These fantastic electronic properties make 2D Fe$_{2}$Ga$_{2}$S$_{5}$ a promising candidate for the future spintronics.

Two-dimensional (2D) ferromagnets with high Curie temperature have long been the pursuit for electronic and spintronic applications. CrI$_{3}$ is a rising star of intrinsic 2D ferromagnets, however, it suffers from weak exchange coupling. Here we propose a general strategy of self-intercalation to achieve enhanced ferromagnetism in bilayer CrI$_{3}$. We show that filling either Cr or I atoms into the van der Waals gap of stacked and twisted CrI$_{3}$ bilayers can induce the double exchange effect and significantly strengthen the interlayer ferromagnetic coupling. According to our first-principles calculations, the intercalated native atoms act as covalent bridge between two CrI$_{3}$ layers and lead to discrepant oxidation states for the Cr atoms. These theoretical results offer a facile route to achieve high-Curie-temperature 2D magnets for device implementation.

We report an efficient and economical way for mass production of large-scale graphene films with high quality and uniformity. By using the designed scrolled copper-graphite structure, a continuous graphene film with typical area of $200 \times 39$ cm$^{2}$ could be obtained in 15 min, and the production rate of the graphene film and space utilization rate of the CVD reactor can reach 520 cm$^{2}$$\cdot$min$^{-1}$ and 0.38 cm$^{-1}$$\cdot$min$^{-1}$, respectively. Our method provides a guidance for the industrial production of graphene films, and may also accelerate its large-scale applications.

We fabricated Sb$_{2}$Se$_{3}$ thin film solar cells using tris(8-hydroxy-quinolinato) aluminum (Alq$_{3}$) as an electron transport layer by vacuum thermal evaporation. Another small organic molecule of N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)benzidine (NPB) was used as a hole transport layer. We took ITO/NPB/Sb$_{2}$Se$_{3}$/Alq$_{3}$/Al as the device architecture. An open circuit voltage ($V_{\rm oc}$) of 0.37 V, a short circuit current density ($J_{\rm sc}$) of 21.2 mA/cm$^{2}$, and a power conversion efficiency (PCE) of 3.79% were obtained on an optimized device. A maximum external quantum efficiency of 73% was achieved at 600 nm. The $J_{\rm sc}$, $V_{\rm oc}$, and PCE were dramatically enhanced after introducing an electron transport layer of Alq$_{3}$. The results suggest that the interface state density at Sb$_{2}$Se$_{3}$/Al interface is decreased by inserting an Alq$_{3}$ layer, and the charge recombination loss in the device is suppressed. This work provides a new electron transport material for Sb$_{2}$Se$_{3}$ thin film solar cells.