Magnetic field induced helical mode and topological transitions in a quasi-ballistic topological insulator nanoribbon with circumferentially quantized surface state sub-bands

Abstract

A topological insulator (TI) nanowire (NW), where the core is insulating and the 2D spin-helical Dirac fermion topological surface states (TSS) are circumferentially quantized into a series of 1D sub-bands, promises novel topological physics and applications. An axial magnetic flux (Φ) through the core drives periodic topological transitions in the surface sub-bands, changing from being all doubly-degenerate with a gapped Dirac point (DP) at integer (including zero) flux quanta (Φ$_0$ = h/e, with h being the Planck constant and e the electron charge), to contain a topologically-protected, non-degenerate 1D spin helical mode with restored DP at half-integer flux quanta. The resulting magnetoconductance is predicted to exhibit Aharonov-Bohm oscillations (ABO) with maxima occurring alternatively at half-integer or integer flux quanta (referred to as π-ABO or 0-ABO), depending periodically on the Fermi wavevector (k$_F$, with period 2π/C, C being the NW circumference). Here, we report a clear observation of such k$_F$-periodic alternations between 0-ABO and π-ABO in Bi$_2$Te$_3$ TI nanoribbon (NR, a rectangular cross sectional NW) field effect devices, which exhibit quasi-ballistic transport over ~2 μm (as manifested in length-independent conductance, exponential decaying ABO amplitude with increasing temperature (T), and an 1/T dependence of the extracted phase coherence length). The conductances as functions of the gate voltage at half and zero flux quanta also exhibit clear, but anti-correlated oscillations periodic in k$_F$ (with period 2π/C, equivalently when C encloses an integer multiples of Fermi wavelength 2π/k$_F$), consistent with the circumferentially quantized surface sub-bands. We also extract the minimal Fermi energy and momentum for TSS to emerge out of the bulk valence band, in agreement with the known Bi$_2$Te$_3$ bandstructure.