This work aims to study the $N$-coupled Hirota equations in an optical fiber under the zero boundary condition at infinity. By analyzing the spectral problem, a matrix Riemann–Hilbert problem on the real axis is strictly established. Then, by solving the presented matrix Riemann–Hilbert problem under the constraint of no reflection, the bright multi-soliton solutions to the $N$-coupled Hirota equations are explicitly gained.

Direct ZnO x-ray detectors with tunable sensitivity are realized by delicately controlling the oxygen flux during the sputtering deposition process. The photocurrents induced by x-rays from a 40 kV x-ray tube with a Cu anode increase apparently as the oxygen flux decreases, which is attributed to the introduction of $V_{\rm o}$ detects. By introducing $V_{\rm o}$ defects, the annihilation rate of the photo-generated electron-hole pairs will be greatly slowed down, leading to a remarkable photoconductive gain. This finding informs a novel way to design the x-ray detectors based on abundant oxide materials.

New experimental cross-section data for the $^{180}$W(n,2n)$^{179{\rm m}}$W, $^{186}$W(n,2n)$^{185{\rm m}}$W and $^{186}$W(n,p)$^{186}$Ta reactions at the neutron energies of 13.5 and 14.4 MeV are obtained by the activation technique. The neutron beams are produced by means of the $^{3}$H(d,n)$^{4}$He reaction. The gamma activities of the product nuclei are measured by a high-resolution gamma-ray spectrometer with a coaxial high-purity germanium detector. The neutron fluence is determined using the monitor reaction $^{93}$Nb(n,2n)$^{92{\rm m}}$Nb. The results in the current work are discussed and compared with the measurement results found in the literature. It is shown that these higher accuracy experimental cross-section data around the neutron energy of 14 MeV agree with some previous experimental values from the literature within experimental uncertainties.

The field shift and mass shift parameters of the 2$s2p\,{}^{3,1}\!P_{1}\to 2s^{2}\,{}^{1}\!S_0$ transitions in Be-like ions ($70 \le Z \le 92$) are calculated using the multi-configuration Dirac–Hartree–Fock and the relativistic configuration interaction methods with the inclusion of the Breit interaction and the leading QED corrections. We find that the mass shift parameters of these two transitions do not change monotonously along the isoelectronic sequence in the high-$Z$ range due to the relativistic nuclear recoil effects. A minimum value exists for the specific mass shift parameters around $Z=80$, especially for the 2$s2p\,{}^{3}\!P_{1}\to 2s^{2}\,{}^{1}\!S_0$ transition. In addition, the field shifts and mass shifts of these two transitions are estimated using an empirical formula, and their contributions are compared along the isoelectronic sequence.

FUNDAMENTAL AREAS OF PHENOMENOLOGY(INCLUDING APPLICATIONS)

We demonstrate laser ultrasonic generation in polyetheretherketone (PEEK). A middle infrared ZnGeP$_{2}$ optical parametric oscillator (ZGP-OPO) pumped by a Q-switched Ho:YAG laser is employed as the ultrasonic excitation source. The ZGP-OPO has a spectral range of 3.2–3.4 μm. At an output wavelength of 3.4 μm, the maximum average output power of ZGP-OPO is 3.05 W with a pulse width of 24.3 ns, corresponding to a peak power of approximately 127.5 kW. The ultrasound is generated by the laser converted from 3.2 to 3.4 μm in the PEEK composite. The maximum ultrasonic signal amplitude in PEEK is 33 mV under the condition of thermoelastic excitation at 3.4 μm. Ablation occurs in the CPRF sample when the energy fluence is over 122.45 mJ/cm$^{2}$. PEEK has a stronger absorption at 3.4 μm and laser-ultrasound generation is influenced by the wavelength of the laser.

We develop a two-stage end-pumped Nd:YVO$_{4}$ amplifier seeded by a passively Q-switched microchip laser. An average output power of 13.5 W with repetition rate up to 7 kHz and pulse duration of $\sim$1.24 ns is obtained, corresponding to a pump extraction efficiency of 16.1% (19.5% for the second stage) and peak power of $\sim $1.5 MW. The beam quality factors at maximum output power are measured to be $M_{x}^{2}=1.56$ and $M_{y}^{2}=1.48$. We introduce an analytical model to estimate gain and beam quality after amplification. This model focuses on the influence of ratio of seed spot radius to pump spot radius when designing an amplifier. Moreover, our experiments reveal that the re-imaging system in the double-pass configuration can be used to enhance the beam quality.

Wavefront shaping technology has mainly been applied to microscopic fluorescence imaging through turbid media, with the advantages of high resolution and imaging depth beyond the ballistic regime. However, fluorescence needs to be introduced extrinsically and the field of view is limited by memory effects. Here we propose a new method for microscopic imaging light transmission through turbid media, which has the advantages of label-free and discretional field of view size, based on transmission-matrix-based wavefront shaping and the random matrix theory. We also verify that a target of absorber behind the strong scattering media can be imaged with high resolution in the experiment. Our method opens a new avenue for the research and application of wavefront shaping.

A target in layered medium can be located by the ridge-like distribution time reversal and reverse time migration (TR-RTM) mixed method. However, this method cannot distinguish between acoustic field distributions of the interface and target for the wider acoustic pulse signals, which may result in inaccurate location of the target. A snapshot TR-RTM mixed method is proposed to solve this problem. The principle of snapshot TR-RTM mixed method is first given. Experiments are then carried out, and a mountain-like acoustic field distribution is obtained by processing experimental data. The results show that the location of the peak is that of the target, and the ratio of the scattered signal and interface reflection signal (signal-to-interference ratio) is improved by about four times after processing. Furthermore, this method can effectively suppress the interface reflection signal and enhance the target scattering signal. Therefore, it can achieve effective detection and location of a target in a layered medium.

We propose an innovative method to generate acoustic vortex waves based on a disc piezoelectric transducer that is coated with multi-arm coiled electrodes. Finite element simulation results for single-arm to four-arm coiled electrodes indicate that the method could modulate amplitude and phase spatial distribution of the acoustic waves near the acoustic axis by acoustic field synthesis principle, making the waves rotate spirally in space and form stable focused vortex beams. Compared with the traditional method that requires electronic control of an array consisting of a large number of transducers, this method provides a more effective and compact solution.

CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES

Based on density functional theory calculations, it is found that for substitutional N in diamond the $C_{3v}$ symmetry structure is more stable, while $C_{3v}$ and $D_{2d}$ symmetry patterns for the substitutional P in diamond have comparable energies. Moreover, the substitutional N is a deep donor for diamond, while P is a shallow substitutional n-type dopant. This is attributed to the different doping positions of dopant (the N atom is seriously deviated from the substitutional position, while the P atom nearly locates in the substitutional site), which are determined by the atomic radius.

Molecular dynamics simulations are performed to investigate the effects of low-energy recoils on the microscopic structure of porous silica. Exhibiting a logistic growth with the recoil energy, the displacement probability of Si is shown to be smaller than that of O at the same primary knock-on level. Computations of pair distribution functions and bond angle distributions reveal that this material upon irradiation with energies around the displacement thresholds mainly undergoes structural changes in the medium-range order. In the porous network, while the formation of nonbridging oxygen defects tends to induce shorter Si–O bonds than those formed by bridging oxygen atoms, a remarkable increase of inter-tetrahedral bond angles created by multiple recoils can be observed and associated with the rearrangement of ring statistics.

Developing accurate self-assembly is the key for constructing functional materials from a bottom-up approach. At present, it is mainly hindered by building blocks and driving modes. We design a new self-assembly method based on the magnetic coupling between spin-polarized electrons. First-principles calculations show that spin-polarized electrons from different endohedral metallofullerene (EMF) superatoms can pair each other to ensure a one-dimensional extending morphology. Furthermore, without ligand passivation, the EMF superatoms maintain their electronic structures robustly in self-assembly owing to the core-shell structure and the atomic-like electron arrangement rule. Therefore, it should noted that the magnetic coupling of monomeric electron spin polarization can be an important driving mechanism for high-precision self-assembly. These results represent a new paradigm for self-assembly and offer fresh opportunities for functional material construction at the atomic level.

The key to fully understanding water-solid interfaces relies on the microscopic nature of hydrogen bond networks, including their atomic structures, interfacial interactions, and dynamic behaviors. Here, we report the observation of two types of simplest water chains on Au(111) surface which is expected unstable according to the rules of hydrogen network on noble metal surfaces. A common feature at the end of chain structures is revealed in high resolution scanning tunneling microscopy images. To explain the stability in observed hydrogen bond networks, we propose a structure model of the water chains terminated with a hydroxyl group. The model is consistent with detailed image analysis and molecular manipulation. The observation of simplest water chains suggests a new platform for exploring fundamental physics in hydrogen bond networks on surfaces.

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

The topological edge states of two-dimensional topological insulators with large energy gaps furnish ideal conduction channels for dissipationless current transport. Transition metal tellurides $X$Te$_{5}$ ($X$=Zr, Hf) are theoretically predicted to be large-gap two-dimensional topological insulators, and the experimental observations of their bulk insulating gap and in-gap edge states have been reported, but the topological nature of these edge states still remains to be further elucidated. Here, we report our low-temperature scanning tunneling microscopy/spectroscopy study on single crystals of HfTe$_{5}$. We demonstrate a full energy gap of $\sim$80 meV near the Fermi level on the surface monolayer of HfTe$_{5}$ and that such an insulating energy gap gets filled with finite energy states when measured at the monolayer step edges. Remarkably, such states are absent at the edges of a narrow monolayer strip of one-unit-cell in width but persist at both step edges of a unit-cell wide monolayer groove. These experimental observations strongly indicate that the edge states of HfTe$_{5}$ monolayers are not trivially caused by translational symmetry breaking, instead they are topological in nature protected by the 2D nontrivial bulk properties.

Systematic theoretical calculations are performed to investigate the dopant effect of Fe on stability, electronic and magnetic properties of the newly synthesized all-boron fullerene B$_{40}$. The results reveal that as a typical ferromagnetic element, Fe atoms can either be chemically externally adsorbed on, or internally encapsulated in the cage of B$_{40}$, with the binding energies ranging from 3.07 to 5.31 eV/atom. By introducing the dopant states from the doped Fe atom, the energy gaps of the Fe-doped B$_{40}$-based metallofullerenes are decreased. Our spin-polarized calculations indicate that Fe-doped metallofullerenes have attractive magnetic properties: with alternative binary magnetic moments between 4.00$\mu_{_{\rm B}}$ and 2.00$\mu_{_{\rm B}}$, depending on the resident sites of the doped Fe atom. The findings of the tunable electronic properties and binary magnetic moments of the Fe-doped B$_{40}$-based metallofullerenes imply that this type of metallofullerene may be applied in single molecular devices.

Nodal line semimetal (NLS) is a new quantum state hosting one-dimensional closed loops formed by the crossing of two bands. The so-called type-II NLS means that these two crossing bands have the same sign in their slopes along the radial direction of the loop, which requires that the crossing bands are either right-tilted or left-tilted at the same time. According to the theoretical prediction, Mg$_{3}$Bi$_{2}$ is an ideal candidate for studying the type-II NLS by tuning its spin-orbit coupling (SOC). High-quality Mg$_{3}$Bi$_{2}$ films are grown by molecular beam epitaxy (MBE). By in-situ angle resolved photoemission spectroscopy (ARPES), a pair of surface resonance bands around the $\bar{{{\it \Gamma}}}$ point are clearly seen. This shows that Mg$_{3}$Bi$_{2}$ films grown by MBE are Mg(1)-terminated by comparing the ARPES spectra with the first principles calculations results. Moreover, the temperature dependent weak anti-localization effect in Mg$_{3}$Bi$_{2}$ films is observed under magneto-transport measurements, which shows clear two-dimensional (2D) $e$–$e$ scattering characteristics by fitting with the Hikami–Larkin–Nagaoka model. Therefore, by combining with ARPES, magneto-transport measurements and the first principles calculations, this work proves that Mg$_{3}$Bi$_{2}$ is a semimetal with topological surface states. This paves the way for Mg$_{3}$Bi$_{2}$ to be used as an ideal material platform to study the exotic features of type-II nodal line semimetals and the topological phase transition by tuning its SOC.

High-quality epitaxial LaRhO$_{3}$ (LRO) thin films on SrTiO$_{3}$ (110) single-crystalline substrates are fabricated by pulsed laser deposition and their photoconductivity properties are studied. The transient photoconductivity (TPC) effect is found in this semiconductor LRO film at room temperature. The magnitude of TPC increases almost linearly with the laser power intensities and the photon energies in visible light range. Moreover, the difference in the TPC results under two airflow conditions confirms that both intrinsic photoinduced carrier accumulation and extrinsic photoinduced heating effects contribute to the magnitude of TPC effect.