arXiv subjects

Weichang Hao

Publications and source records attributed to Weichang Hao.

22 records · Page 2Linked to original sources

arXiv2020

Knot theory for two-band model of two-dimensional square lattice with high topological numbers

A knot theory for two-dimensional square lattice is proposed, which sheds light on design of new two-dimensional material with high topological numbers. We consider a two-band model, focusing on the Hall conductance σxy = e^2/hbar*P, where P is a topological number, the so-called Pontrjagin index. By re-interpreting the periodic momentum components kx and ky as the string parameters of two entangled knots, we discover that P becomes the Gauss linking number between the knots. This leads to a successful re-derivation of the typical P-evaluations in literature: P = 0;{\pm}1. Furthermore, with the aid of this explicit knot theoretical picture we modify the two-band model to achieve higher topological numbers, P = 0;{\pm}1;{\pm}2.

cond-mat.str-el↗

arXiv2019

High third-order Kerr optical nonlinearity in BiOBr 2D films measured by the Z-scan method

We investigate the nonlinear optical properties of BiOBr nanoflakes, a novel two-dimensional (2D) layered material from the Bismuth oxyhalide family. We measure the nonlinear absorption and Kerr nonlinearity of BiOBr nanoflakes at both 800 nm and 1550 nm via the Z scan technique. We observe a large nonlinear absorption coefficient beta = 10^-7 m/W as well as a large Kerr coefficient n2 = 10^-14 m^2/W. We also observe strong dispersion in n2, with it reversing sign from negative to positive as the wavelength varies from 800 nm to 1550 nm. In addition, we characterize the thickness-dependence of the nonlinear optical properties of BiOBr nanoflakes, observing that both the magnitudes of beta and n2 increase for very thin flakes. Finally, we integrate BiOBr nanoflakes onto silicon integrated waveguides and characterize the linear optical properties of the resulting hybrid integrated devices, with the measurements agreeing with calculated parameters using independent ellipsometry measurements. These results verify the strong potential of BiOBr as an advanced nonlinear optical material for high-performance hybrid integrated photonic devices.

physics.optics↗

arXiv2015

Metal-Silicene Interaction Studied by Scanning Tunneling Microscopy

Ag atoms have been deposited on 3x3 silicene and R3xR3 silicene films by molecular beam epitaxy method in ultrahigh vacuum. Using scanning tunneling microscopy and Raman spectroscopy, we found that Ag atoms do not form chemical bonds with both 3x3 silicene and R3xR3 silicene films,which is due to chemically inert surface of silicene. On 3x3 silicene films, Ag atoms mostly form into stable flat top Ag islands. In contrast, Ag atoms form nanoclusters and glide on silicene films, suggesting more inert nature. Raman spectroscopy suggests that there is more sp2 hybridization in R3xR3 than in R7xR7/3x3silicene films

cond-mat.mtrl-sci↗

arXiv2015

Investigation of Electron-Phonon Coupling in Epitaxial Silicene by In-situ Raman Spectroscopy

In this letter, we report that the special coupling between Dirac fermion and lattice vibrations, in other words, electron-phonon coupling (EPC), in silicene layers on Ag(111) surface was probed by an in-situ Raman spectroscopy. We find the EPC is significantly modulated due to tensile strain, which results from the lattice mismatch between silicene and the substrate, and the charge doping from the substrate. The special phonon modes corresponding to two-dimensional electron gas scattering at edge sites in the silicene were identified. Detecting relationship between EPC and Dirac fermion through the Raman scattering will provide a direct route to investigate the exotic property in buckled two-dimensional honeycomb materials.

cond-mat.mtrl-sci↗