Identifying open-orbit topological surface states in dual topological semimetal TaSb$_2$

TL;DR

The paper addresses identifying open-orbit topological surface states in TaSb2, a material with coexisting weak TI and TCI phases. The authors use ARPES, DFT, and magnetotransport to disentangle surface states from bulk bands on the measured plane and to characterize open surface Fermi surfaces and spin textures. They find surface-derived open Fermi surfaces parallel to high-symmetry directions, demonstrate spin-momentum locking via circular dichroism ARPES, and observe weak antilocalization in magnetotransport, consistent with spin-polarized surface transport alongside bulk carriers that are nearly electron-hole compensated. These findings position TaSb2 as a platform to study interactions between distinct topological phases and their impact on spin-polarized transport and magnetotransport phenomena.

Abstract

TaSb$_2$, a member of the transition metal dipnictide family of materials, hosts the very rare dual topological phase - weak topological insulating state and topological crystalline insulating state along different crystallographic orientations. So far, studies on the electronic structure of transition metal dipnictides have focused on their overall electronic structure and the bulk open-orbit Fermi surfaces. Using angle-resolved photoemission spectroscopy, density functional theory calculations, and transport measurements, we distinguish the intertwined bulk and surface states on the weakly topological $(20\bar{1})$ plane of TaSb$_2$. We identify multiple electron- and hole-like bulk bands, yielding a near-perfect carrier compensation. Crucially, we observe open-orbit FSs parallel to $\bar{L}$-$\bar{Y}$ direction that are entirely of surface origin. Circular-dichroism ARPES reveals $k \rightarrow -k$ spectral reversal, indicating spin-momentum locking and the topological nature of these surface states. Consistent with this, magnetotransport measurements display weak antilocalization, establishing TaSb$_2$ as a platform for spin-polarized topological transport on a weakly topological surface.

Figures (4)

• Figure 1: Crystal structure of TaSb$_2$. (a) SEM image of the TaSb$_2$ single crystal. (b) The primitive BZ of TaSb$_2$ with ($1\Bar{1}\Bar{1}$) plane. b$_1$, b$_2$ and b$_3$ are the reciprocal lattice vectors for the primitive unit cell. The $(1\bar{1}\bar{1})$ plane corresponds to the ($20\bar{1}$) plane for conventional unit cell Xu2016, which we adopt for all subsequent references. (c) The XRD data collected from ($20\Bar{1}$) plane of TaSb$_2$. (d) Schematic of the conventional unit cell of TaSb$_2$ with the ($20\Bar{1}$) plane. The blue atoms stand for Ta, and orange atoms are Sb.
• Figure 2: In-plane electronic structure of TaSb$_2$. (a) Top: FS map (left) and constant energy contours at 0.2 eV (middle) and 0.3 eV (right) below the Fermi level from ARPES ($h\nu$ = 70 eV). Bottom: corresponding bulk band calculations. (b) Top: EDMs along $\Bar{L}-\Bar{X}$ (left), $\Bar{Y}-\Bar{X1}$ (middle), and $\Bar{L}-\Bar{Y}$ (right). Bottom: corresponding bulk calculations. Blue arrows point the surface states. Green lines (top-left panel) show the momentum distribution curve (MDC) used in the out-of-plane contours in Fig. \ref{['fig:3']}(a). (c) FS map from ARPES (top) compared with surface state calculations (bottom). (d) 3D ARPES spectra showing the in-plane dispersion of the marked surface state. (e) Evolution of the band structure from $\Bar{L}-\Bar{X}$ toward $\Bar{Y}-\Bar{X1}$. Red dotted lines in (c)–(d) mark the positions of the EDM cuts.
• Figure 3: Out-of-plane electronic structure and polarization-dependent ARPES measurements. (a) Out-of-plane FS, constant energy contours acquired at 0.1 eV, and 0.6 eV below the Fermi level respectively. The blue dotted lines indicate the surface states exhibiting negligible k$_z$ warping. (b) Left panel: Out-of-plane EDM obtained at k$_x$=0$\AA^{-1}$. Right panel: Corresponding bulk calculation. (c) EDM along $\Bar{L}-\Bar{X}$ acquired by subtracting the spectral intensity obtained from right circularly polarized light (C+) and left circularly polarized light (C-) with photon energies of 70 eV and 60 eV. (d) represents the same for the EDM taken along $\Bar{Y}-\Bar{X1}$.
• Figure 4: (a) Temperature dependence of the longitudinal resistivity, $\rho_{xx}(T)$. (b) MR as a function of magnetic field at 5 K and 20 K. (c) Anisotropic MR [$\Delta R_{xx}=R_{xx}(\theta)-R_{xx}$ (0 deg)] measured at 6 T as a function of field angle $\theta$ for in-plane (top panel) and out-of-plane (bottom panel) rotations at 5 K and 300 K. The measurement geometries are demonstrated in the schematics. MR ($B,\theta$) for field rotation in a plane oriented (d) perpendicular and (e) parallel to the current direction ($b$ axis). (f) Hall resistivity, $\rho_{xy}$, versus magnetic field at three representative temperatures. (g) Longitudinal conductivity $\sigma_{xx}$ and Hall conductivity $\sigma_{xy}$ as a function of magnetic field at 5 K, with solid lines representing the two-band model fitting.