arXiv subjects

Jiacheng Gao

Publications and source records attributed to Jiacheng Gao.

21 records · Page 2Linked to original sources

arXiv2020

Layer Construction of Topological Crystalline Insulator LaSbTe

Topological crystalline insulator (TCI) is one of the symmetry-protected topological states. Any TCI can be deformed into a simple product state of several decoupled two-dimensional (2D) topologically nontrivial layers in its lattice respecting its crystalline symmetries called the layer construction (LC) limit. In this work, based on first-principles calculations we have revealed that both tetragonal LaSbTe (t-LaSbTe) and orthorhombic LaSbTe (o-LaSbTe) can be interpreted as stacking of 2D topological insulators in each lattice space. The structural phase transition from t-LaSbTe to o-LaSbTe due to soft phonon modes demonstrates how the real space change can lead to the modification of topological states. Their symmetry-based indicators and topological invariants have been analyzed based on LC. We propose that LaSbTe is an ideal example demonstrating the LC paradigm, which bridges the crystal structures in real space to the band topology in momentum space.

cond-mat.mtrl-sci↗

arXiv2020

Electronic structures and topological properties in nickelates $Ln_{n+1}$Ni$_n$O$_{2n+2}$

After the significant discovery of the hole-doped nickelate compound Nd$_{0.8}$Sr$_{0.2}$NiO$_2$, an analysis of the electronic structure, orbital components, Fermi surfaces and band topology could be helpful to understand the mechanism of its superconductivity. Based on the first-principles calculations, we find that Ni $3d_{x^2-y^2}$ states contribute the largest Fermi surface. $Ln~5d_{3z^2-r^2}$ states form an electron pocket at $Γ$, while $5d_{xy}$ states form a relatively bigger electron pocket at A. These Fermi surfaces and symmetry characteristics can be reproduced by our two-band model, which consists of two elementary band representations: $B_{1g}@1a~\oplus~A_{1g}@1b$. We find that there is a band inversion near A, giving rise to a pair of Dirac points along A--M below the Fermi level once including spin-orbit coupling. Furthermore, we have performed the LDA+Gutzwiller calculations to treat the strong correlation effect of Ni 3d orbitals. In particular, the bandwidth of $3d_{x^2-y^2}$ has been renormalized largely. After the renormalization of the correlated bands, the Ni $3d_{xy}$ states and the Dirac points become very close to the Fermi level. Thus, a hole pocket at A could be introduced by hole doping, which may be related to the observed sign change of Hall coefficient. By introducing an additional Ni $3d_{xy}$ orbital, the hole-pocket band and the band inversion can be captured in our modified model. Besides, the nontrivial band topology in the ferromagnetic two-layer compound La$_3$Ni$_2$O$_6$ is discussed and the band inversion is associated with Ni $3d_{x^2-y^2}$ and La $5d_{xy}$ orbitals.

cond-mat.mtrl-sci↗

arXiv2019

Weyl Semimetals with $S_4$ symmetry

In the time-reversal-breaking centrosymmetric systems, the appearance of Weyl points can be guaranteed by an odd number of all the even/odd parity occupied bands at eight inversion-symmetry-invariant momenta. Here, based on symmetry analysis and first-principles calculations, we demonstrate that for the time-reversal-invariant systems with $S_4$ symmetry, the Weyl semimetal phase can be characterized by the inequality between a well-defined invariant $η$ and an $S_4$ indicator $z_2$. By applying this criterion, we find that some candidates, previously predicted to be topological insulators, are actually Weyl semimetals in the noncentrosymmetric space group with $S_4$ symmetry. Our first-principles calculations show that four pairs of Weyl points are located in the $k_{x,y}$ = 0 planes, with each plane containing four same-chirality Weyl points. An effective model has been built and captures the nontrivial topology in these materials. Our strategy to find the Weyl points by using symmetry indicators and invariants opens a new route to search for Weyl semimetals in the time-reversal-invariant systems.

cond-mat.mtrl-sci↗