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We formulate the chiral decomposition rules that govern the electronic structure of a broad family of twisted N+ Mmultilayer graphene configurations that combine arbitrary stacking order and a mutual twist. We show that at the magic angle in the chiral limit the low-energy bands of such systems are composed of chiral pseudospin doublets that are energetically entangled with two flat bands per valley induced by the moiré superlattice potential. The analytic construction is supported by explicit numerical calculations based on realistic parametrization. We further show that vertical displacement fields can open energy gaps between the pseudospin doublets and the two flat bands, such that the flat bands may carry nonzero valley Chern numbers. These results provide guidelines for the rational design of topological and correlated states in generic twisted graphene multilayers.

Bi3+-activated phosphors have demonstrated great values in solid-state lighting, biomedicine, and detectors. Low quantum efficiency and unsatisfactory thermal stability are the main reasons that limit the practical application of Bi3+-activated phosphors. In this study, a high-efficiency and stable orange-light-emitting phosphor induced by D-band emission, Ba2LuGaO5:Bi3+, was successfully synthesized by selecting gallate with a highly rigid structure and appropriate cation sites. Under the excitation of near-ultraviolet light, Ba2LuGaO5:Bi3+emits strong orange light with an internal quantum efficiency of 73% and thermal stability of 80%@150 °C. Based on two possible models of metal-to-metal charge-transfer transition and Bi3+pairs/clusters, the origin of D-band emission is discussed in detail. According to the crystallographic data and spectroscopy data analysis, the luminescence of Ba2LuGaO5:Bi3+is generated by the Bi3+pair/cluster. The results indicate that Ba2LuGaO5:Bi3+is of great potential in white-light-emitting diodes.

Recently, borate compounds have received much attention in the field of rare earth doped phosphors due to their excellent luminescent performance. In this work, to explore the potential in LED and FED applications, the CsBaB3O6:Eu3+phosphor was investigated in detail by using Rietveld refinement, DFT calculations, photoluminescent and cathodoluminescent spectra. As a result, CsBaB3O6has a planar stacked three-dimensional layered structure. Under the excitation of 395 nm n-UV light and electron beam, CsBaB3O6:Eu3+phosphor exhibits a typical red emission of Eu3+. A good thermal stability and good resistance to saturation and degradation were observed in the CsBaB3O6:Eu3+phosphor. The related photoluminescent and cathodoluminescent mechanisms were studied. The results indicate that CsBaB3O6:Eu3+phosphor has potential in multifunctional applications.

We address the intrinsic polarization and screening of an external electric field in a broad range of ordered and twisted configurations of multilayer graphene, using an ab initioapproach combining density functional theory and the Wannier function formalism. We show that multilayer graphene is intrinsically polarized due to the crystal field effect, an effect that is often neglected in tight-binding models of twisted bilayer graphene and similar systems. This intrinsic polarization of the order of up to a few tens of millielectronvolts has different out-of-plane alignments in ordered and twisted graphene multilayers, while the in-plane potential modulation is found to be much stronger in twisted systems. We further investigate the dielectric permittivity ε in same multilayer graphene configurations at different electric field strengths. Our findings establish a deep insight into intrinsic and extrinsic polarization in graphene multilayers and provide parameters necessary for building accurate models of these systems.

Weyl semimetals in three-dimensional crystals provide the paradigm example of topologically protected band nodes. It is usually taken for granted that a pair of colliding Weyl points annihilate whenever they carry opposite chiral charge. In stark contrast, here we report that Weyl points in systems that are symmetric under the composition of time reversal with a π rotation are characterized by a non-Abelian topological invariant. The topological charges of the Weyl points are transformed via braid phase factors, which arise upon exchange inside symmetric planes of the reciprocal momentum space. We elucidate this process with an elementary two-dimensional tight-binding model that is implementable in cold-atom set-ups and in photonic systems. In three dimensions, interplay of the non-Abelian topology with point-group symmetry is shown to enable topological phase transitions in which pairs of Weyl points may scatter or convert into nodal-line rings. By combining our theoretical arguments with first-principles calculations, we predict that Weyl points occurring near the Fermi level of zirconium telluride carry non-trivial values of the non-Abelian charge, and that uniaxial compression strain drives a non-trivial conversion of the Weyl points into nodal lines.

Electron–electron interactions play an important role in graphene and related systems and can induce exotic quantum states, especially in a stacked bilayer with a small twist angle1–7. For bilayer graphene where the two layers are twisted by the ‘magic angle’, flat band and strong many-body effects lead to correlated insulating states and superconductivity4–7. In contrast to monolayer graphene, the band structure of untwisted bilayer graphene can be further tuned by a displacement field8–10, providing an extra degree of freedom to control the flat band that should appear when two bilayers are stacked on top of each other. Here, we report the discovery and characterization of displacement field-tunable electronic phases in twisted double bilayer graphene. We observe insulating states at a half-filled conduction band in an intermediate range of displacement fields. Furthermore, the resistance gap in the correlated insulator increases with respect to the in-plane magnetic fields and we find that the gfactor, according to the spin Zeeman effect, is ~2, indicating spin polarization at half-filling. These results establish twisted double bilayer graphene as an easily tunable platform for exploring quantum many-body states.

We investigate twisted double bilayer graphene (TDBG), a four-layer system composed of two AB-stacked graphene bilayers rotated with respect to each other by a small angle. Our ab initio band structure calculations reveal a considerable energy gap at the charge-neutrality point that we assign to the intrinsic symmetric polarization (ISP). We then introduce the ISP effect into the tight-binding parametrization and perform calculations on TDBG models that include lattice relaxation effects down to very small twist angles. We identify a narrow region around the magic angle characterized by a manifold of remarkably flat bands gapped out from other states even without external electric fields. To understand the fundamental origin of the magic angle in TDBG, we construct a continuum model that points to a hidden mathematical link to the twisted bilayer graphene model, thus indicating that the band flattening is a fundamental feature of TDBG and is not a result of external fields.

We present an open-source software package WannierTools , a tool for investigation of novel topological materials. This code works in the tight-binding framework, which can be generated by another software package Wannier90 (Mostofi et al., 2008). It can help to classify the topological phase of a given material by calculating the Wilson loop, and can get the surface state spectrum, which is detected by angle resolved photoemission (ARPES) and in scanning tunneling microscopy (STM) experiments. It also identifies positions of Weyl/Dirac points and nodal line structures, calculates the Berry phase around a closed momentum loop and Berry curvature in a part of the Brillouin zone (BZ). Program summary Program title: WannierTools Program Files doi: http://dx.doi.org/10.17632/ygsmh4hyh6.1 Licensing provisions: GNU General Public Licence 3.0 Programming language: Fortran 90 External routines/libraries used: • BLAS ( http://www/netlib.org/blas ) • LAPACK ( http://www.netlib.org/lapack ) Nature of problem: Identifying topological classifications of crystalline systems including insulators, semimetals, metals, and studying the electronic properties of the related slab and ribbon systems. Solution method: Tight-binding method is a good approximation for solid systems. Based on that, Wilson loop is used for topological phase classification. The iterative Green’s function is used for obtaining the surface state spectrum. [ABSTRACT FROM AUTHOR]

For solving the problem that Eu3+-activated phosphors cannot be well excited by the near UV-LED chips, the charge transfer band (CTB) of Eu3+-N3–in α-M3B2N4(M = Ca, Sr) has been investigated in this work. Compared with the sharp excitation peaks due to the f–f transitions of Eu3+, the charge transfer band is broad and the excitation band of α-M3B2N4(M = Ca, Sr):Eu3+centered at 370 or 350 nm lies in near-UV, implying a promising excitation. The electron and crystal structures of α-M3B2N4(M = Ca, Sr) has been analyzed in detailed, indicating that they all crystallize in cubic phase with the space group (Im3̅m) and have similar band-gap structures. The double bond of [N = BN]3–combining with the face-shared (Ca, Sr)N6octahedron forms the stable and compact crystal structure, and the band gap with the value about 4 eV is suitable for Eu3+to form a luminescent center. Under near-UV excitation, the sharp red light that peaked at 613 and 593 nm resulting from the f–f transition of Eu3+ions has been obtained. The optimal concentration of Eu3+in the α-M3B2N4(M = Ca, Sr) has been measured, and the decay curves of samples prove that the energy transfer happens between Eu3+ions through the dipole–dipole interaction. The thermal stability of samples has been measured to access their application in the WLED.

A series of Ca2YZr2Al3O12:Eu3+(CYZA) phosphors were successfully synthesized through conventional solid-state method. The electronic structure and their photoluminescence or cathodoluminescence properties were investigated in detail. Under n-UV excitation, the CYZA:Eu3+exhibits more intense red emission than the commercial Y2O3:Eu3+phosphor. A WLED lamp with good color render index was obtained by fabricating the phosphor with BAM:Eu2+and LuAG:Ce3+phosphors. The phosphor also exhibits red emission with high current saturation and high resistance under low voltage electron bombardment. The degradation resistance can be compared to the commercial Y2O3:Eu3+phosphor. All the results indicate that the CYZA:Eu3+has potential applications in both white LEDs and FEDs.

Defect luminescence has been considered to be one of the most efficient ways to resolve the problem of the recent limited supply and increased demand for rare-earth elements. So, in this work, a novel self-activated long-persistent green-yellow and yellow emitting phosphors of α-Ca3B2N4and β-Ca3B2N4has been successfully synthesized by sample-pressuring sintering. The α-Ca3B2N4and β-Ca3B2N4all have a broad excitation band peaked at 300 and 350 nm and emit yellow-green light peaked at 550 nm and yellow light peaked at 575 nm respectively with the long afterglow properties. The crystal structures and electron structures of α-Ca3B2N4and β-Ca3B2N4have been investigated to show that orthorhombic β-Ca3B2N4corresponds to an ordered superstructure of cubic α-Ca3B2N4and the phase transformation between α-Ca3B2N4and β-Ca3B2N4would happen during the preparation process, which produces many intrinsic defects forming the luminescent center. The thermoluminescence (TL) curve shows that there are traps in the host lattice with the TL peak located at 310 K. The results indicate that α-Ca3B2N4and β-Ca3B2N4with the defect-related PL and LPL properties have potential applications in the LED and LPL used phosphors.

Interest in ZrTe5has been reinvigorated in recent years owing to its potential for hosting versatile topological electronic states and intriguing experimental discoveries. However, the mechanism of many of its unusual transport behaviors remains controversial: for example, the characteristic peak in the temperature-dependent resistivity and the anomalous Hall effect. Here, through employing a clean dry-transfer fabrication method in an inert environment, we successfully obtain high-quality ZrTe5thin devices that exhibit clear dual-gate tunability and ambipolar field effects. Such devices allow us to systematically study the resistance peak as well as the Hall effect at various doping densities and temperatures, revealing the contribution from electron–hole asymmetry and multiple-carrier transport. By comparing with theoretical calculations, we suggest a simplified semiclassical two-band model to explain the experimental observations. Our work helps to resolve the longstanding puzzles on ZrTe5and could potentially pave the way for realizing novel topological states in the two-dimensional limit.

The band theory of solids is arguably the most successful theory of condensed-matter physics, providing a description of the electronic energy levels in various materials. Electronic wavefunctions obtained from the band theory enable a topological characterization of metals for which the electronic spectrum may host robust, topologically protected, fermionic quasiparticles. Many of these quasiparticles are analogues of the elementary particles of the Standard Model, but others do not have a counterpart in relativistic high-energy theories. A complete list of possible quasiparticles in solids is lacking, even in the non-interacting case. Here we describe the possible existence of a hitherto unrecognized type of fermionic excitation in metals. This excitation forms a nodal chain—a chain of connected loops in momentum space—along which conduction and valence bands touch. We prove that the nodal chain is topologically distinct from previously reported excitations. We discuss the symmetry requirements for the appearance of this excitation and predict that it is realized in an existing material, iridium tetrafluoride (IrF4), as well as in other compounds of this class of materials. Using IrF4as an example, we provide a discussion of the topological surface states associated with the nodal chain. We argue that the presence of the nodal-chain fermions will result in anomalous magnetotransport properties, distinct from those of materials exhibiting previously known excitations.

Powder samples of Sr8La2[(PO4)4.5(SiO4)2(BO4)0.5](BO2) (SLP) doped with Mn2+ and Ce3+ were prepared by solid-state reaction method and their photoluminescence properties were studied for the first time. The crystalline structure of the samples was monitored by the X-ray powder diffraction (XRD) technique. The emission spectra of the phosphors show a blue broad band at about 420 nm of Ce3+ and an orange band at around 620 nm of Mn2+. Energy transfer mechanism from Ce3+ to Mn2+ in SLP host matrix was studied based on the decay lifetime data. In this work, the tunable emission colors from blue to orange have been achieved by tuning the relative ratio of the Ce3+ to Mn2+ based on the principle of energy transfer, and the developed phosphor could be promising as a single-component white-light-emitting phosphor for white light-emitting diodes (W-LEDs).

As new light sources for next-generation illumination, white light-emitting diodes (WLEDs) have been extensively developed and are commercially available due to their excellent advantages, such as high efficiency, energy-saving, compactness, long operational lifetime and environmental friendliness. Currently, WLEDs with high color rendering are mainly based on wavelength conversion by one or more phosphor materials. In this review, the recent developments of phosphors for WLEDs were introduced combined with the relative work of our group. The common methods for generating white light for blue/ultraviolet (UV) WLEDs were summarized, including: (1) optimizing the commercially used phosphors; (2) developing some new phosphors based on UV LEDs chips; (3) realizing white light emission based on single host. Moreover, some typical new developed phosphors and their luminescence properties were introduced.

Ba3-xSi6O9N4:xEu2+ (0.03 [?] x [?] 0.10) phosphors were successfully synthesized by a solid state reaction method. Under electron beam excitation, the cathodoluminescence intensity reached a maximum at x = 0.07. The corresponding emission spectrum peaked at [?]493 nm (full-width at half maximum [?]46 nm) with CIE coordinate of (0.101, 0.418). The results show that Ba2.93Si6O9N4: 0.07Eu2+ phosphor exhibits high brightness, high color purity, and a higher thermal stability compared to the commercial green Topstar, LMS520B phosphor, which indicates that it could be a candidate for field-emission displays (FEDs).

Several coumarins were studied using cyclic voltammetry, differential pulse voltammetry, and chronocoulometry. The results show that umbelliferone (7‐hydroxycoumarin) and several structurally similar coumarins undergo a pH‐dependent, irreversible oxidation with product adsorption.

Distributed multimedia applications would be a useful tool in the manufacturing industry. They are typically time-based applications which impose several time related constraints on communication systems. Quality of Service (QoS) parameters are necessary to describe the requirements to transfer multimedia information in communication systems. The international standard Manufacturing Message Specification (MMS) is an application layer protocol which specifies services for the distributed manufacturing applications. It enables communications between heterogeneous devices, but is not suitable for multimedia applications because it does not present QoS parameters to its objects and services. This paper proposes an extension of MMS for integrating multimedia functions. The goal is to define an MMS multimedia companion standard. Like the VMD which models manufacturing devices for MMS communication, a Virtual Communication Support (VCS) is proposed to virtualize a communication system in an MMS environment. Potential uses of VCS are also discussed.

We present an open-source program irvsp , to compute irreducible representations of electronic states for all 230 space groups with an interface to the Vienna ab-initio Simulation Package. This code is fed with plane-wave-based wavefunctions (e.g. WAVECAR) and space group operators (listed in OUTCAR), which are generated by the VASP package. This program computes the traces of matrix presentations and determines the corresponding irreducible representations for all energy bands and all the k -points in the three-dimensional Brillouin zone. It also works with spin–orbit coupling (SOC), i.e. , for double groups. It is in particular useful to analyze energy bands, their connectivities, and band topology, after the establishment of the theory of topological quantum chemistry. Accordingly, the associated library – irrep_bcs.a – is developed, which can be easily linked to by other ab-initio packages. In addition, the program has been extended to orthogonal tight-binding (TB) Hamiltonians, e.g. electronic or phononic TB Hamiltonians. A sister program ir2tb is presented as well. Program title: irvsp CPC Library link to program files: https://doi.org/10.17632/y9ds5nnm2f.1 Licensing provisions: GNU Lesser General Public License Programming language: Fortran 90/77 Nature of problem: Determining irreducible representations for all energy bands and all the k -points in 230 space groups. It is in particular useful to analyze energy bands, their connectivities, and band topology. Solution method: By computing the traces of matrix presentations of space group operators for the eigen-wavefunctions at a certain k -point in a given space group, one can determine irreducible representations for them. [ABSTRACT FROM AUTHOR]

Powder samples of Sr8La2[(PO4)4.5(SiO4)2(BO4)0.5](BO2) (SLP) doped with Mn2+and Ce3+were prepared by solid-state reaction method and their photoluminescence properties were studied for the first time. The crystalline structure of the samples was monitored by the X-ray powder diffraction (XRD) technique. The emission spectra of the phosphors show a blue broad band at about 420 nm of Ce3+and an orange band at around 620 nm of Mn2+. Energy transfer mechanism from Ce3+to Mn2+in SLP host matrix was studied based on the decay lifetime data. In this work, the tunable emission colors from blue to orange have been achieved by tuning the relative ratio of the Ce3+to Mn2+based on the principle of energy transfer, and the developed phosphor could be promising as a single-component white-light-emitting phosphor for white light-emitting diodes (W-LEDs).

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