Quantum oscillations and nontrivial topological state in a compensated semimetal TaP2

We report systematic magnetotransport measurements and ab initio calculations on single-crystalline TaP2, a member of the transition-metal dipnictides. We observed unsaturated magnetoresistance (MR) reaching similar to 700% at a magnetic field (B) of 9 T at 2 K when B is perpendicular to the electric current (I). We also found negative longitudinal MR (n-LMR) when B slightly deviates from the direction of I along with striking Shubnikov-de Haas (SdH) oscillations. Our analysis on the SdH oscillations uncovers three fundamental magnetic oscillation frequencies of 72, 237, and 356 T, slightly different from the theoretical calculations which reveal one hole pocket and two electron pockets at the L point and one electron pocket at the Z point of the Brillouin zone. The inconsistency might be due to that one Fermi pocket is out of the detection capability by our measurements. The analysis also reveals a nonzero Berry phase, indicating nontrivial band topology. The n-LMR could be fitted with the Adler-Bell-Jackiw chiral anomaly equation but the origin remains yet ambiguous. The ab initio calculations suggest TaP2 as a weak topological insulator with the Z(2) indices of (0; 111), which exhibits topological surface states on the (001) surface.