  The behavior of charge density wave (CDW) in an external magnetic field is
dictated by both orbital and Pauli (Zeeman) effects. A quasi-one-dimensional
(Q1D) system features Q1D Fermi surfaces that allow these effects to be
distinguished, which in turn can provide sensitive probe to the underlying
electronic states. Here we studied the field dependence of an incommensurate
CDW in a transition-metal chalcogenide Ta2NiSe7 with a Q1D chain structure. The
angle-dependent magnetoresistance (MR) is found to be very sensitive to the
relative orientation between the magnetic field and the chain direction. With
an applied current fixed along the b axis (the chain direction), the
angle-dependent MR shows a striking change of the symmetry below T_CDW only for
a rotating magnetic field in the ac plane. In contrast, the symmetry axis
remains unchanged for other configurations (H in ab and bc plane). The orbital
effect conforms to the lattice symmetry, while Pauli effect in the form of
{\mu}B B / v_F can be responsible for such symmetry change, provided that the
Fermi velocity v_F is significantly anisotropic and the nesting vector changes
in a magnetic field, which is corroborated by our first-principles
calculations. Our results show that the angle-dependent MR is a sensitive
transport probe of CDW and can be useful for the study of low-dimensional
systems in general.
