Seminar: Helicity protected ultrahigh mobility Weyl fermions in NbP

Helicity protected ultrahigh mobility Weyl fermions in NbP

Speaker: Yi Zheng  

Department of Physics, Zhejiang University 

Time: 2:00pm, Monday, October 26, 2015

Place: Room 423, teaching building number 12, Yuquan Campus 

Abstract:
    Non-centrosymmetric transition metal monopnictides, including TaAs, TaP, NbAs and NbP, have been very recently proposed to be Weyl semimetals (WSMs), in which the low-energy quasiparticle excitations are the condensed-matter-physics realization of the long-sought-out Weyl fermions, which previously only appear in theoretical high-energy physics. The first fundamental question in this emergent field is that which compound is the ideal platform for exploring various fascinating quantum phenomena and novel device concepts. Although chiral WSM states have been observed in TaAs and NbAs, there is no conclusive evidence on the existence of Weyl fermions in NbP. Here, we use angle-dependent quantum oscillations to reveal that NbP has four pairs of unusually large Weyl fermion pockets in the kz=0 plane near the high symmetry points Σ, dominating over the coexisting massive hole pockets and the previous reported WSM pockets in the kz=1.18π/c plane. Such dominant WSM pockets are highly anisotropic in k-space and approaching the parabolic band top along the internode direction. The corresponding Fermi surface is consisting of helical Weyl fermions with unprecedented mobility of 1×107 cm2V-1s-1 at 1.5 K, well protected from defect backscattering by real spin conservation associated to the chiral Weyl nodes. Inter-pocket pumping of Weyl fermions with opposite helicity becomes feasible when the magnetic field and electric field are applied in parallel, manifested as robust chiral anomaly induced negative MR. Surprisingly, we have observed a strong offset to the π Berry’s phase, which may be correlated to the non-negligible mass of Weyl fermions. 
[1] H. M. Wen, et al, Phys. Rev. X 5, 011029 (2015) 
[2] Z. Wang, Y. Zheng, Z. Shen, Y. Zhou, X. Yang, Y. Li, C. Feng, and Z. A. Xu, arXiv:1506.00924.
  In collaboration with  Zhu-An Xu, Zhen Wang, and Zhixuan Shen