The Weyl semimetal has emerged as a new topologically nontrivial phase of matter, hosting low-energy excitations of massless Weyl fermions. Here, we present a comprehensive study of a type-II Weyl semimetal WP$_{2}$. Transport studies show a butterfly-like magnetoresistance at low temperature, reflecting the anisotropy of the electron Fermi surfaces. This four-lobed feature gradually evolves into a two-lobed variant with an increase in temperature, mainly due to the reduced relative contribution of electron Fermi surfaces compared to hole Fermi surfaces for magnetoresistance. Moreover, an angle-dependent Berry phase is also discovered, based on quantum oscillations, which is ascribed to the effective manipulation of extremal Fermi orbits by the magnetic field to feel nearby topological singularities in the momentum space. The revealed topological character and anisotropic Fermi surfaces of the WP$_{2}$ substantially enrich the physical properties of Weyl semimetals, and show great promises in terms of potential topological electronic and Fermitronic device applications.

Huber et al. [Phys. Rev. Lett. 118 (2017) 200502] have proved that a seven-qubit state whose three-body marginal states are all maximally mixed does not exist. Here, we propose a method to build a maximally entangled state based on orthogonal arrays to construct maximally entangled seven-qubit states. Using this method, we not only determine that a seven-qubit state whose three-body marginals are all maximally mixed does not exist, but also find the condition for maximally entangled seven-qubit states. We consider that $\pi_{\rm ME} =19/140$ is a condition for maximally entangled seven-qubit states. Furthermore, we derive three forms of maximally entangled seven-qubit states via orthogonal arrays.

We study the analytic structure for the eigenvalues of the one-dimensional Dirac oscillator, by analytically continuing its frequency on the complex plane. A twofold Riemann surface is found, connecting the two states of a pair of particle and antiparticle. One can, at least in principle, accomplish the transition from a positive energy state to its antiparticle state by moving the frequency continuously on the complex plane, without changing the Hamiltonian after transition. This result provides a visual explanation for the absence of a negative energy state with the quantum number $n=0$.

Information entropy, as a quantitative measure of complexity in nonlinear systems, has been widely researched in a variety of contexts. With the development of a nonlinear dynamic, the entropy is faced with severe challenges in dealing with those signals exhibiting extreme volatility. In order to address this problem of weighted permutation entropy, which may result in the inaccurate estimation of extreme volatility, we propose a rescaled range permutation entropy, which selects the ratio of range and standard deviation as the weight of different fragments in the time series, thereby effectively extracting the maximum volatility. By analyzing typical nonlinear systems, we investigate the sensitivities of four methods in chaotic time series where extreme volatility occurs. Compared with sample entropy, fuzzy entropy, and weighted permutation entropy, this rescaled range permutation entropy leads to a significant discernibility, which provides a new method for distinguishing the complexity of nonlinear systems with extreme volatility.

Sympathetic cooling is a method used to lower the kinetic energy of ions with complicated energy-level structures, via Coulomb interactions with laser-cooled ions in an ion trap. The ion to be sympathetically cooled is sometimes prepared outside of the trap, and it is critical to introduce this ion into the trap by temporarily lowering the potential of one endcap without allowing the coolant ion to escape. We study the time required for a laser-cooled ion to escape from a linear Paul trap when the voltage of one endcap is lowered. The escape time is on the order of a few microseconds, and varies significantly when the low-level voltage changes. A re-cooling time of a maximum of 13 s was measured, which can be reduced to approximately one hundred of milliseconds by decreasing the duration of the low-level voltage. The measurement of these critical values lays the foundation for the smooth injection and cooling of the ion to be sympathetically cooled.

We demonstrate a novel polarization control system based on a gradient descent algorithm, applied to a 450-km optical frequency transfer link. The power of the out-loop beat note is retrieved by controlling the polarization state of the transferred signal, with a recovery time of 24 ms, thereby ensuring the long-term evaluation of the fiber link. As a result, data utilization is enhanced from 70% to 99% over a continuous measurement period of $\sim$12 h. A fractional transfer instability of $7.2 \times 10^{-20}$ is achieved at an integration time of 10000 s. This work lays the foundation for the comparison of a remote optical clock system via a long-haul optical fiber link.

Novel ionic transporting phenomena emerge as nanostructures approach the molecular scale. At the sub-2 nm scale, widely used continuum equations, such as the Nernst–Planck equation, break down. Here, we extend the Nernst–Planck equation by adding a partial dehydration effect. Our model agrees with the reported ion fluxes through graphene oxide laminates with sub-2 nm interlayer spacing, outperforming previous models. We also predict that the selectivity sequences of alkali metal ions depend on the geometries of the nanostructures. Our model opens a new avenue for the investigation of the underlying mechanisms in nanofluidics at the sub-2 nm scale.

The energetic-particle-induced geodesic acoustic mode (EGAM) is studied using gyrokinetic particle simulations in tokamak plasmas. In our simulations, exponentially growing EGAMs are excited by energetic particles with a slowing-down distribution. The frequencies of EGAMs are always below the frequencies of GAMs, which is due to the non-perturbative contribution of energetic particles (EPs). The mode structures of EGAMs are similar to the corresponding mode structures of GAMs. Our gyrokinetic simulations show that a high EP density can enhance the EGAM growth rate, due to high EP free energy, and that EPs' temperature and the pitch angle of the distribution modify the EGAM frequency/growth rate by means of the resonance condition. Kinetic effects of the thermal electrons barely change the EGAM frequency, and have a weak damping effect on the EGAM. Benchmarks between the gyrokinetic particle simulations and a local EGAM dispersion relation exhibit good agreement in terms of EGAM frequency and growth rate.

We investigate the surface structure and electronic properties of $\beta$-Sn islands deposited on a graphitized 6H-SiC (0001) substrate via low temperature scanning tunneling microscopy and spectroscopy. Owing to the confinement of the island geometry, quantum well states (QWSs) are formed, manifesting as equidistant peaks in the tunneling spectra. Furthermore, a distinct strip feature appears on the surfaces of odd-layer Sn islands, ranging from 15–19 layers, which is not present on the surfaces of even-layer Sn islands. The spatial distribution of strips can be modified by applying a bias pulse, using an STM tip. Furthermore, the strip-like structure shows significant impacts on the QWS. An energy splitting of the lowest unoccupied QWSs is observed in strip regions; this may be ascribed to caused the phase shift of the wave functions of the QWSs on the top surface, due to surface distortions created by the aforementioned strips.

The CdS/CdTe heterojunction plays an important role in determining the energy conversion efficiency of CdTe solar cells. However, the interface structure remains unknown, due to the large lattice mismatch between CdS and CdTe, posing great challenges to achieving an understanding of its interfacial effects. By combining a neural-network-based machine-learning method and the stochastic surface walking-based global optimization method, we first train a neural network potential for CdSTe systems with demonstrated robustness and reliability. Based on the above potential, we then use simulated annealing to obtain the optimal structure of the CdS/CdTe interface. We find that the most stable structure has the features of both bulks and disorders. Using the obtained structure, we directly calculate the band offset between CdS and CdTe by aligning the core levels in the heterostructure with those in the bulks, using one-shot first-principles calculations. Our calculated band offset is 0.55 eV, in comparison with 0.70 eV, obtained using other indirect methods. The obtained interface structure should prove useful for further study of the properties of CdTe/CdS heterostructures. Our work also presents an example which is applicable to other complex interfaces.

Two-dimensional (2D) blue phosphorene with a honeycomb structure is the phosphorus analog of graphene, and is regarded as a promising 2D material with a large tunable band gap and high charge-carrier mobility. Here, using the molecular beam epitaxy method, we synthesize monolayer blue phosphorene on the Ag(111) surface. Combined with first-principles calculations, scanning tunneling microscopy measurements reveal that the blue phosphorene on the Ag(111) surface consists of 2D clusters with a buckling $1\times 1$ lattice, arranged regularly on the Ag(111). The formation of these phosphorus clusters stems from the strain modulation induced by the lattice mismatch between blue phosphorene and the Ag(111) substrate. Moreover, x-ray photoelectron spectroscopy measurements are performed to study the instability of the blue phosphorene clusters in air. The realization of regular nanoclusters of blue phosphorene with unique sizes and morphology provides an ideal platform for the exploration of the quantum physical properties and applications of blue phosphorene.

This paper proposes a method of repairing interface defects by supercritical nitridation technology, in order to suppress the threshold voltage shift of AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs). We find that supercritical NH$_{3}$ fluid has the characteristics of both liquid NH$_{3}$ and gaseous NH$_{3}$ simultaneously, i.e., high penetration and high solubility, which penetrate the packaging of MIS-HEMTs. In addition, NH$_{2}^{-}$ produced via the auto coupling ionization of NH$_{3}$ has strong nucleophilic ability, and is able to fill nitrogen vacancies near the GaN surface created by high temperature processes. After supercritical fluid treatment, the threshold voltage shift is reduced from 1 V to 0 V, and the interface trap density is reduced by two orders of magnitude. The results show that the threshold voltage shift of MIS-HEMTs can be effectively suppressed by means of supercritical nitridation technology.

Transparent conducting materials (TCMs) have been widely used in optoelectronic applications such as touchscreens, flat panel displays and thin film solar cells. These applications of TCMs are currently dominated by n-type doped oxides. High-performance p-type TCMs are still lacking due to their low hole mobility or p-type doping bottleneck, which impedes efficient device design and novel applications such as transparent electronics. Here, based on first-principles calculations, we propose chalcogenide perovskite YScS$_{3}$ as a promising p-type TCM. According to our calculations, its optical absorption onset is above 3 eV, which allows transparency to visible light. Its hole conductivity effective mass is 0.48$m_{0}$, which is among the smallest in p-type TCMs, suggesting enhanced hole mobility. It could be doped to p-type by group-II elements on cation sites, all of which yield shallow acceptors. Combining these properties, YScS$_{3}$ holds great promise to enhancing the performance of p-type TCMs toward their n-type counterparts.

We examine quantum anomalous Hall (QAH) insulators with intrinsic magnetism displaying quantized Hall conductance at zero magnetic fields. The spin-momentum locking of the topological edge stats promises QAH insulators with great potential in device applications in the field of spintronics. Here, we generalize Haldane's model on the honeycomb lattice to a more realistic two-orbital case without the artificial real-space complex hopping. Instead, we introduce an intraorbital coupling, stemming directly from the local spin-orbit coupling (SOC). Our $d_{xy}/d_{x^{2}-y^{2}}$ model may be viewed as a generalization of the bismuthene $p_{x}/p_{y}$-model for correlated $d$-orbitals. It promises a large SOC gap, featuring a high operating temperature. This two-orbital model nicely explains the low-energy excitation and the topology of two-dimensional ferromagnetic iron-halogenides. Furthermore, we find that electronic correlations can drive the QAH states to a $c=0$ phase, in which every band carries a nonzero Chern number. Our work not only provides a realistic QAH model, but also generalizes the nontrivial band topology to correlated orbitals, which demonstrates an exciting topological phase transition driven by Coulomb repulsions. Both the model and the material candidates provide excellent platforms for future study of the interplay between electronic correlations and nontrivial band topology.

We report superconductivity in a new ternary molybdenum pnictide Rb$_{2}$Mo$_{3}$As$_{3}$, synthesized via the solid state reaction method. Powder x-ray diffraction analysis reveals a hexagonal crystal structure with space group $P\bar{6}m2$ (No. 187), and the refined lattice parameters are $a = 10.431(5)$ Å, $c = 4.460(4)$ Å. SEM images show rod-like grains with good ductility, confirming a quasi-one-dimensional (Q1D) structure. Electrical resistivity and dc magnetic susceptibility characterizations exhibit superconductivity with an onset of $T_{\rm c}=10.5$ K. The upper critical field of Rb$_{2}$Mo$_{3}$As$_{3}$ is estimated to be 28.2 T at zero temperature, providing an evidence of possible unconventional superconductivity. Our recent discovery of MoAs-based superconductors above 10 K provides a unique platform for the study of exotic superconductivity in $4d$ electron systems with Q1D crystal structures.

Using the newly-developed solid ionic gating technique, we measure the electrical transport property of a thin-flake NbSe$_{2}$ superconductor ($T_{\rm c} = 6.67$ K) under continuous Li intercalation and electron doping. It is found that the charge-density-wave transition is suppressed, while at the same time a carrier density, decreasing from $7\times 10^{14}$ cm$^{-2}$ to $2\times 10^{14}$ cm$^{-2}$ also occurs. This tunable capability in relation to carrier density is 70%, which is 5 times larger than that found using the liquid ionic gating method [Phys. Rev. Lett. 117 (2016) 106801]. Meanwhile, we find that the scattering type of conduction electrons transits to the $s$–$d$ process, which may be caused by the change of the occupied states of 4$d$-electrons in Nb under the condition of Li intercalation. Simultaneously, we observe a certain decrement of electron-phonon coupling (EPC), based on the electron-phonon scattering model, in the high temperature range. Based on data gathered from in situ measurements, we construct a full phase diagram of carrier density, EPC and $T_{\rm c}$ in the intercalated NbSe$_{2}$ sample, and qualitatively explain the variation of $T_{\rm c}$ within the BCS framework. It is our opinion that the in situ solid ionic gating method provides a direct route to describing the relationship between carrier density and superconductivity, which is helpful in promoting a clearer understanding of electronic phase competition in transition metal dichalcogenides.

We develop an experimental tool to investigate the order parameter of superconductors by combining point-contact spectroscopy measurement with high-pressure technique. It is demonstrated for the first time that planar point-contact spectroscopy measurement on noncentrosymmetric superconducting PbTaSe$_2$ single crystals is systematically subjected to hydrostatic pressures up to 12.1 kbar. Under such a high pressure, the normal-state contact resistance is sensitive to the applied pressure, reflecting the underlying variation of contact transparency upon pressures. In a superconducting state, the pressure dependence of the energy gap $\varDelta_0$ and the critical temperature $T_{\rm c}$ for gap opening/closing are extracted based on a generalized Blond–Tinkham–Klapwijk model. The gap ratio $2\varDelta_0/k_{_{\rm B}}T_{\rm c}$ indicates a crossover from weak coupling to strong coupling in electron pairing strength upon pressure for PbTaSe$_2$. Our experimental results show the accessibility and validity of high-pressure point-contact spectroscopy, offering rich information about high-pressure superconductivity.

The Mott transition is one of the fundamental issues in condensed matter physics, especially in the system with antiferromagnetic long-range order. However, such a transition is rare in quantum spin liquid (QSL) systems without long-range order. Here we report the experimental pressure-induced insulator to metal transition followed by the emergence of superconductivity in the QSL candidate NaYbSe$_{2}$ with a triangular lattice of 4$f$ Yb$^{3+}$ ions. Detail analysis of transport properties in metallic state shows an evolution from non-Fermi liquid to Fermi liquid behavior when approaching the vicinity of superconductivity. An irreversible structure phase transition occurs around 11 GPa, which is revealed by the x-ray diffraction. These results shed light on the Mott transition in the QSL systems.

In this work, we present a new mechanism for designing phase-gradient metasurfaces (PGMs) to control an electromagnetic wavefront with high efficiency. Specifically, we design a transmission-type PGM, formed by a periodic subwavelength metallic slit array filled with identical dielectrics of different heights. It is found that when Fabry–Pérot (FP) resonances occur locally inside the dielectric regions, in addition to the common phenomenon of complete transmission, the transmitted phase differences between two adjacent slits are exactly the same, being a nonzero constant. These local FP resonances ensure total phase shift across a supercell, fully covering a range of 0 to $2\pi$, satisfying the design requirements of PGMs. Further research reveals that, due to local FP resonances, there is a one-to-one correspondence between the phase difference and the permittivity of the filled dielectric. A similar approach can be extended to the reflection-type case and other wavefront transformations, creating new opportunities for wave manipulation.

Distinctive superconducting behaviors between bulk and monolayer FeSe make it challenging to obtain a unified picture of all FeSe-based superconductors. We investigate the ultrafast quasiparticle (QP) dynamics of an intercalated superconductor (Li$_{1-x}$Fe$_{x}$)OHFe$_{1-y}$Se, which is a bulk crystal but shares a similar electronic structure with single-layer FeSe on SrTiO$_{3}$. We obtain the electron-phonon coupling (EPC) constant $\lambda_{{A}_{\rm 1g}}$ ($0.22 \pm 0.04$), which well bridges that of bulk FeSe crystal and single-layer FeSe on SrTiO$_{3}$. Significantly, we find that such a positive correlation between $\lambda_{{A}_{\rm 1g}}$ and superconducting $T_{\rm c}$ holds among all known FeSe-based superconductors, even in line with reported FeAs-based superconductors. Our observation indicates possible universal role of EPC in the superconductivity of all known categories of iron-based superconductors, which is a critical step towards achieving a unified superconducting mechanism for all iron-based superconductors.

We examine an amorphous oxide semiconductor (AOS) of ZnRhCuO. The $a$-ZnRhCuO films are deposited at room temperature, having a high amorphous quality with smooth surface, uniform thickness and evenly distributed elements, as well as a high visible transmittance above 87% with a wide bandgap of 3.12 eV. Using $a$-ZnRhCuO films as active layers, thin-film transistors (TFTs) and gas sensors are fabricated. The TFT behaviors demonstrate the p-type nature of $a$-ZnRhCuO channel, with an on-to-off current ratio of $\sim$$1\times 10^{3}$ and field-effect mobility of 0.079 cm$^{2}$V$^{-1}$s$^{-1}$. The behaviors of gas sensors also prove that the $a$-ZnRhCuO films are of p-type conductivity. Our achievements relating to p-type $a$-ZnRhCuO films at room temperature with TFT devices may pave the way to practical applications of AOSs in transparent flexible electronics.