Nearly massless Dirac fermions hosted by Sb square net in BaMnSb2

Abstract

Layered compounds AMnBi2 (A = Ca, Sr, Ba, or rare earth element) have been established as Dirac materials. Dirac electrons generated by the two-dimensional (2D) Bi square net in these materials are normally massive due to the presence of a spin-orbital coupling (SOC) induced gap at Dirac nodes. Here we report that the Sb square net in an isostructural compound BaMnSb2 can host nearly massless Dirac fermions. We observed strong Shubnikov-de Haas (SdH) oscillations in this material. From the analyses of the SdH oscillations, we find key signatures of Dirac fermions, including light effective mass (~0.052m0; m0, mass of free electron), high quantum mobility (1280 cm(2)V(-1)S(-1)) and a π Berry phase accumulated along cyclotron orbit. Compared with AMnBi2, BaMnSb2 also exhibits much more significant quasi two-dimensional (2D) electronic structure, with the out-of-plane transport showing nonmetallic conduction below 120 K and the ratio of the out-of-plane and in-plane resistivity reaching ~670. Additionally, BaMnSb2 also exhibits a G-type antiferromagnetic order below 283 K. The combination of nearly massless Dirac fermions on quasi-2D planes with a magnetic order makes BaMnSb2 an intriguing platform for seeking novel exotic phenomena of massless Dirac electrons.

Figure 1. Crystal strucutre, magnetic and transport properties of BaMnSb₂.

(a) Crystal and magnetic structure of BaMnSb₂. (b) Susceptibility as a function of temperature measured with a 2T magnetic field applied along the c axis (χ_c) and along the ab plane (χ_ab) under zero field cooling (ZFC) and field cooling (FC) histories. Red: χ_c of FC; dark red: χ_c of ZFC; purple: χ_ab of FC; blue: χ_ab of ZFC. Inset in (b), an optical image of a typical BaMnSb₂ single crystal. (c) Isothermal magnetization along the c axis (red) and along the ab plane (blue). (d) Temperature dependence of the Bragg peak intensity at (101) indicates the magnetic order below 283 K. Inset in (d), the Bragg peak (101) scanned at the selected temperatures. (e) In-plane resistivity (ρ_in) and out-of-plane resistivity (ρ_out) as a function of temperature under zero magnetic field. (f) ρ_in and ρ_out plotted on logarithmic scale.

Figure 2. Quantum transport properties of BaMnSb₂.

(a) The in-plane resistivity, ρ_in, as a function of field up to 31T at different temperatures. (b) The oscillatory component of ρ_invs. 1/B at different temperatures. (c) The temperature dependence of the normalized FFT amplitude. The dashed line curve is the fit to the Lifshitz-Kosevich (LK) formula from 2 to 40 K. Inset: FFT spectra of Δρ_in(B) at different temperatures (the FFT was done in the field range of 3T–31T). (d) The out-of-plane resistivity, ρ_out. as a function of field at different temperatures. (e) The oscillatory component of ρ_out vs. 1/B. (f) The temperature dependence of the normalized FFT amplitude. The dashed line is the fit to the LK formula. Inset: FFT spectra of Δρ_out(B) at different temperatures (the FFT was done in the field range of 5T–31T).

Figure 3. Berry phase, quantum mobility and Hall resistivity of BaMnSb₂.

(a) The oscillatory component of in-plane resistivity, ρ_in, vs. 1/B at 2 K. As the longitudinal resistivity (ρ_in = ρ_xx) is much larger than transverse resistivity (ρ_xy), integer Landau level (LL) indices are assigned to the maximum of resistivity (see text). (b) LL fan diagram. The blue dashed line represents the linear fit. c, Dingle plot for the in-plane quantum oscillations Δρ_in at 2 K. (d) Hall resistivitivity as a function of field at various temperatures (T = 2, 5, 10, 20, 50, 100, 150, 200, 250 and 300 K).

Figure 4. Angular dependences of SdH oscillations and the oscillation frequencies for BaMnSb₂.

(a) The oscillatory component of in-plane resistivity, Δρ_in, vs. 1/B measured under different field orientations. The data has been shifted for clarity. (b) The FFT spectra of Δρ_in(B) at different field orientations. Inset: the diagram of the measurement setup; θ is defined as the angle between the magnetic field and the out-of-plane direction. (c) The angle dependence of SdH oscillation frequency determined from the FFT of Δρ_in(B). The dashed curve is the fit to F(θ) = F(0)/cosθ. (d) The oscillatory component of out-of-plane resistivity, Δρ_out. vs. 1/B measured under different field orientations. The data has been shifted for clarity. (e) The FFT spectra of Δρ_out(B) at different field orientations. Inset: the diagram of the measurement setup. (f) The angular dependence of SdH oscillation frequency determined from the FFT of Δρ_out(B). The dashed curves are the fits to F(θ) = F(0)/cosθ.