Abstract
Shubnikov de Haas resistance oscillations of highly mobile two dimensional helical electrons propagating on a conducting surface of strained HgTe 3D topological insulator are studied in magnetic fields B tilted by angle θ from the normal to the conducting layer. Strong decrease of oscillation amplitude A is observed with the tilt: A ∼ exp(-ξ/cos(θ)), where ξ is a constant. Evolution of the oscillations with temperature T shows that the parameter ξ contains two terms: ξ=ξ₁+ξ₂ T. The temperature independent term, ξ₁, describes reduction of electron mean free path in magnetic field B pointing toward suppression of the topological protection of the electron states against impurity scattering. The temperature dependent term, ξ₂ T, indicates increase of the reciprocal velocity of 2D helical electrons suggesting modification of the electron spectrum in magnetic fields.