Magnetic field-induced helical mode and topological transitions in a topological insulator nanoribbon

The spin-helical Dirac fermion topological surface states in a topological insulator nanowire or nanoribbon promise novel topological devices and exotic physics such as Majorana fermions. Here, we report local and non-local transport measurements in Bi 2 Te 3 topological insulator nanoribbons that exhibit quasi-ballistic transport over ∼2 μm. The conductance versus axial magnetic flux Φ exhibits Aharonov–Bohm oscillations with maxima occurring alternately at half-integer or integer flux quanta ( Φ 0 = h / e , where h is Planck's constant and e is the electron charge) depending periodically on the gate-tuned Fermi wavevector ( k F ) with period 2π/ C (where C is the nanoribbon circumference). The conductance versus gate voltage also exhibits k F -periodic oscillations, anti-correlated between Φ = 0 and Φ 0 /2. These oscillations enable us to probe the Bi 2 Te 3 band structure, and are consistent with the circumferentially quantized topological surface states forming a series of one-dimensional subbands, which undergo periodic magnetic field-induced topological transitions with the disappearance/appearance of the gapless Dirac point with a one-dimensional spin helical mode. Conductance oscillations periodic in Fermi energy and in magnetic flux measured in topological insulator nanoribbons reveal characteristic topological transport with quantized topological surface state subbands.