Single-mode lasing based on PT-breaking of two-dimensional photonic higher-order topological insulator

Topological lasers are a new class of lasers that seek to exploit the special properties of topological states of light. A typical limiting factor in their performance is the existence of non-topological states with quality factors comparable to the desired topological states. We show theoretically that by distributing uniform gain and loss on two sublattices of a two-dimensional higher-order topological insulator (HOTI) lattice, single-mode lasing based on topological corner states can be sustained over a wide range of pump strengths. This behavior stems from the parity/time-reversal breaking of the topological corner states, which supplies them with more effective gain than the edge and bulk states, rather than through localized pumping of the domain corners. These results point to opportunities for exploiting non-Hermitian phenomena and designing compact high performance topological lasers

Lanthanum distribution and dielectric properties of intergrowth Bi₅ˍₓLaₓTiNbWO₁₅ ferroelectrics

Bi₅ˍₓLaₓTiNbWO₁₅ (x=0–1.50)ceramics prepared by conventional solid-state reaction were studied using x-ray diffraction(XRD),dielectric spectroscopy and Raman scattering techniques. The XRD analysis implied that single-phase intergrowth bismuth layered perovskite structure was obtained for all the samples and when x=0.75, the Bi³⁺ in (Bi₂O₂)²⁺ layer begins to be substituted by La³⁺. The dielectric spectra showed that, when Bi³⁺ in (Bi₂O₂)²⁺ is substituted, the Curie temperature becomes diffusive and the dielectricpermittivity at room temperature is increased in a wide frequency range. Especially when x=1.50, the dielectricpermittivity reaches its maximum of 270, nearly two times larger than that of the La3+ undoped sample. The Raman scattering experiments evidenced further that Bi³⁺ in (Bi₂O₂)²⁺ is substituted when x⩾0.75 and revealed the orthorhombic distortion of the octahedra is responsible for the increase of the dielectricpermittivity at x⩾1.25.This work was supported by the Ministry of Science and Technology of China through 973-Project under Grant No. 2002CB613307

Orbitally relieved magnetic frustration in NaVO2

The magnetic properties of NaVO2 are investigated using full-potential linearized augmented plane wave method. We perform calculations for three structures. For the rhombohedral structure at 100 K, the t2g orbitals of V ions are split into upper a1g and lower e'g orbitals by a trigonal distortion of compression. For the monoclinic structure at 91.5 K, the system behaves like a frustrated spin lattice with spatially anisotropic exchange interactions. For another monoclinic structure at 20 K, the magnetic frustration is relieved by a lattice distortion which is driven by a certain orbital ordering, and the long-range magnetic ordering is thus formed. Moreover, the small magnetic moment originates from the compensation oforbital moment for the spin moment.Comment: Accepted by PR

FOCUSING MAGNETIC FIELD DESIGN FOR A FEL LINAC

Abstract A linac-based Free Electron Laser is planned to be built in Huazhong University of Science and Technology (HUST). As an important component of the linac, the focusing magnetic field is carefully designed. Spacecharge force is calculated at first to give a rough estimation of the focusing field. Start-to-end simulation shows that the magnetic field has only significant effect on spot size and phase space. With the final designed field, 3-ps-length (FWHM) pulse containing 200pC electrons can be obtained and the corresponding RMS normalized emittance and RMS radius are 7 mm·mrad and 0.25 mm, respectively. Finally, a new double-peak scheme is discussed and excitation current is proposed as the evaluation index