Multiple Topological States in LaAgAs2, a Failed Square-Net Semimetal

Abstract

The rational design of new materials emerges as an important direction to explore new topological materials, which is based on the understanding of the correlation between crystal and electronic structures. In this paper, we perform a comprehensive study on the crystal and electronic structures in LaAgAs2 through a combination of single-crystal x-ray diffraction (XRD), quantum oscillation, and angle-resolved photoemission spectroscopy (ARPES) experimental measurements, and density functional theory (DFT) calculations. Single-crystal XRD measurements reveal that LaAgAs2 crystallizes into a HfCuSi2-derived structure with the square net distorted into cis-trans chains. Quantum oscillation measurements reveal two frequencies with small effective masses and quasi-two-dimensional (2D) characters. ARPES measurements reveal an electronic structure strikingly different from the square-net-based semimetals, such as LaAgAs2. The Fermi surface is quasi-two-dimensional (2D), with Dirac-like hole pockets at the zone center and a quasi-1D elliptical electron pocket at the zone boundary. Based on the DFT calculations, the measured electronic structure can be well understood regarding the cis-trans distortion, which transforms the two-dimensional square net-derived Dirac bands into quasi-1D trivial bands. Intriguingly, multiple topological states can be identified around the zone center, including a nontrivial Z2 topological surface state and a bulk Dirac state. Our study clarifies the impact of cis-trans distortion and identifies LaAgAs2 as a topological material with multiple topological states near the Fermi level, providing a guideline for intentionally designing new topological materials.

Figures (7)

• Figure 1: Crystal structure and basic physical properties of LaAgAs2. (a) Crystal structure of LaAgAs2. To avoid confusion, the crystallographic direction for $a$, $b$, and $c$ were denoted as $x$, $z$, and $y$, respectively. Top view of [AgAs] layer (b) and As planar layer (c), where panels (b) and (c) use the same coordinate axis as shown in panel (c). Black lines in panels (a-c) indicate the unit cell of LaAgAs2. (d) XRD pattern for (0 $k$ 0) surface of a flat LaAgAs2 crystal. The insert shows the enlarged view of the (0 8 0) reflection. (e) Core-level electronic structure of LaAgAs2, measured using a photon energy of 150 eV. The inset shows an x-ray Laue pattern of the (H, 0, L) reciprocal plane. (f) Temperature dependence of in-plane resistivity $\rho_{xx}$ ($j \parallel ac$) and out-of-plane resistivity $\rho_{zz}$ ($j \parallel b$) measured with $B$ = 0 T for LaAgAs$_2$, where $j$ is the electric current applied for the resistivity measurements. (g) Typical Hall resistivity $\rho_{xy}(B)$ curves measured at different temperatures with $B \parallel b$. The insert shows the temperature dependence of carrier concentration obtained from the single-band fitting of the high-field part of $\rho_{xy} (B)$ curves.
• Figure 2: Quantum oscillations in LaAgAs2. (a)(e) Magnetic field dependence of magnetization [$M(B)$] and magnetoresistance [MR($B$)], respectively. Each subsequent MR curve is shifted upward by 4% for clarity. (b)(f) Inverse field dependence of oscillatory components of magnetization (${\Delta}M$, dHvA oscillations) and magnetoresistance (${\Delta}{\rho}$, SdH oscillations), respectively. ${\Delta}M$/${\Delta}{\rho}$ are obtained by subtracting the polynomial background from the data in (a)/(e), respectively. (c)(g) FFT spectra of dHvA/SdH oscillations at various temperatures. The inset of (g) shows the angular dependence of FFT peaks and the geometry of the measurements, where $\theta$ is defined as the angle between the magnetic field $B$ and the crystallographic $b$-axis. Further details for the angular dependence of SdH oscillation can be found in Supplementary Fig. S2 SM. (d)(h) Temperature dependence of the FFT amplitude for $F_\alpha$ and $F_\beta$, which are the FFT peaks inferred from (c) and (g), respectively. Solid lines represent the fits with the Lifshitz-Kosevich formula.
• Figure 3: Electronic structure of LaAgAs$_2$ from ARPES. (a) The Fermi surface is taken at $h\nu=70$ eV. The black, blue, and red dotted squares represent the $1^{st}$, $2^{nd}$, and $3^{rd}$ surface BZs, respectively, with the symmetry points indicated. (b) The $E(k)$ dispersion along the $\bar{\Gamma}-\bar{\mathrm{M}}$ line of the surface BZ (solid horizontal line in (a)). (c) The $k_z$ dependence of the states at the Fermi level ($E=0$) along the same momentum line. (d) The same as in (b), but at $E=-0.6$ eV marked by the black solid line in (b). The maps in (c, d) are obtained by using the photon energies in the range from 55 to 90 eV and the free-electron approximation for the final electron state, $k_z=1/\hbar\sqrt{2m_e(E_kcos^2(\theta)+V_0)}$, where $E_k$ is the kinetic energy of a photoelectron and $V_0\sim10$ eV is the inner potential. The dashed and dotted horizontal lines mark the $\Gamma$ and Z planes in the 16$^{th}$ 3D BZ, respectively. All the spectra were taken at $T$ = 15 K using the horizontally linearly polarized light.
• Figure 4: In-plane electronic structure of LaAgAs$_2$. (a) The Fermi surface from the spot with 2 orthogonal domains. (b) The Fermi surface from the spot with a single domain dominating. (c) Band structure along the $k_y=0$ (marked as 1) line from (a). (d) The same as (b) (marked as 1'). (e) Band structure along the $k_x=0$ (marked as 2') line from (b). (f) Band structure along the $k_y=\pi/c$ (marked as 3') line from (b). The red arrow points to the suppressed intensity of the $2^{nd}$ zone states. The red square in (a,b) represents the $1^{st}$ BZ. All the spectra were taken at $T=15$ K using the horizontally linearly polarized light at 100 eV.
• Figure 5: High-resolution electronic structure of the Fermi pockets. (a) The Fermi surface and (b) the intensity at $E=-0.2$ eV taken at $h\nu=100$ eV (c) Band structure along the $k_y=0$ line from (a). The dotted lines at $k_x>0$ indicate the three resolved hole bands. The black arrows point to the splitting of the outer hole doublet. (d) The Fermi surface taken at $h\nu=70$ eV (e) Band structure along the $k_y=\pi/c$ line from (d). The red arrows indicate the small electron pocket inside the main one. The red dashed curve represents the parabolic fit to the MDC-derived dispersion.