  A magnetic torque method is proposed that probes the warping and mass gap of
Dirac cone surface states in topological insulators like Bi2X3 (X=Se,Te). A
rotating field parallel to the surface induces a paramagnetic moment in the
helical surface states for nonzero warping. It is non-collinear with the
applied field and therefore produces torque oscillations as function of the
field angle which are a direct signature of the surface states. The torque
dependence on field strength and angle, the chemical potential and the Dirac
cone parameters like warping strength and mass gap is calculated. It is shown
that the latter leads to a symmetry reduction in the fourfold torque
oscillations.
