Experimental probe of Berry phase in Dirac nodal line systems

In topological semimetals, a non-zero Berry phase originates from topologically non-trivial band crossings, such as Dirac nodes. A non-trivial Berry phase in the fundamental oscillations of the quantum oscillations (QOs) has been treated as a smoking gun for Dirac or Weyl nodal lines. A recent study suggested that this criterion did not uniquely identify non-trivial Berry curvature, and rather analysis of higher order harmonics in magnetization (de Haas can Alphen [dHvA]) was needed [1]. This motivates the search for high quality topological semimetals where higher harmonic QOs are observed, which, in turn, would allow the unambiguous determination of non-trivial Berry curvatures. We take this approach in a series of isostructural AlB2-type compounds, where strong dHvA oscillations with higher harmonics (up to 4^th order) are observed. We utilize the LK formula [1] including the spin damping term and higher harmonic contributions to fit the temperature-dependent QOs and determine the Berry phase. Together with the angle-dependent quantum oscillations, density functional theory (DFT) calculations and angle resolved photoemission spectroscopy (ARPES), we discuss on the fermiology and band topology of different Fermi pockets in these materials.

[1] Phys. Rev. X 8, 011027 (2018)