Gate-induced decoupling of surface and bulk state properties in selectively-deposited Bi$_2$Te$_3$ nanoribbons

Abstract

Three-dimensional topological insulators (TIs) host helical Dirac surface states at the interface with a trivial insulator. In quasi-one-dimensional TI nanoribbon structures the wave function of surface charges extends phase-coherently along the perimeter of the nanoribbon, resulting in a quantization of transverse surface modes. Furthermore, as the inherent spin-momentum locking results in a Berry phase offset of π of self-interfering charge carriers an energy gap within the surface state dispersion appears and all states become spin-degenerate. We investigate and compare the magnetic field dependent surface state dispersion in selectively deposited Bi2Te3 TI micro-and nanoribbon structures by analysing the gate voltage dependent magnetoconductance at cryogenic temperatures. Hall measurements on microribbon field effect devices show a high bulk charge carrier concentration and electrostatic simulations show an inhomogeneous gate potential profile on the perimeter of the TI ribbon. In nanoribbon devices we identify a magnetic field dependency of the surface state dispersion as it changes the occupation of transverse subbands close to the Fermi energy. We quantify the energetic spacing in between these subbands by measuring the conductance as a function of the applied gate potential and use an electrostatic model that treats the inhomogeneous gate profile and the initial charge carrier densities on the top and bottom surface. In the gate voltage dependent transconductance we find oscillations that change their relative phase by π at half-integer values of the magnetic flux quantum applied coaxial to the nanoribbon providing evidence for a magnetic flux dependent topological phase transition in narrow, selectively deposited TI nanoribbon devices.

Fabry–Pérot interferometry with gate-tunable 3D topological insulator nanowires

“…Based on reasonable assumptions regarding the experimental feasibility, we consider a gated 3D TI nanoribbon with rectangular cross section and with the following device and surface-state characteristics: E F = 50 meV, v F = 3.5 × 10 5 m s −1 , W = 100 nm, H = 10 nm, P = 220 nm, C eff = 2 × 10 −3 F m −2 , with W, H, P respectively the width, height, and perimeter (P = 2W + 2H) of the nanoribbon cross section, and C eff the effective capacitance of the gated section per unit area [12,14,[41][42][43][44]. The surface-state energy spectrum in the central section is shifted as a function of the gate voltage V g .…”

“…This makes it difficult to resolve the transport properties of 3D TI surface states, with the majority of the charge carriers originating from the bulk. To get into a regime where the 3D TI surface-state (magneto) transport signatures are more pronounced, 3D TI nanosamples with electrostatic gating are commonly considered [6][7][8][9][10][11][12][13][14]. Usually, gated devices are considered in which the carrier density and Fermi level are shifted across the whole device, including the contact regions.…”

Metallization and proximity superconductivity in topological insulator nanowires

“…In the past few years there has been substantial progress in the growth of thin TI nanowire devices [9,[13][14][15][16][17][18][19][20][21], including the growth of bulk insulating devices in which the chemical potential can be tuned close to the Dirac point [22,23]. In TI nanowires several transport signatures of the quantum confinement of surface states have been reported, previously these consisted of conductivity oscillations [24,25] as a function of magnetic field or gate voltage [13][14][15][16][17][18][19][20][21][22].…”

“…In the past few years there has been substantial progress in the growth of thin TI nanowire devices [9,[13][14][15][16][17][18][19][20][21], including the growth of bulk insulating devices in which the chemical potential can be tuned close to the Dirac point [22,23]. In TI nanowires several transport signatures of the quantum confinement of surface states have been reported, previously these consisted of conductivity oscillations [24,25] as a function of magnetic field or gate voltage [13][14][15][16][17][18][19][20][21][22]. Recently, a non-reciprocal transport effect in bulk insulating nanowires provided strong evidence not only for quantum confinement of the surface states but also for the splitting of initially spindegenerate subbands that is predicted due to the breaking of inversion symmetry by a non-uniform potential through the cross-section of the nanowire [23].…”

Disorder effects in topological insulator nanowires

“…Recently, substantial experimental progresses has been made in thin TI nanowires and nanoribbons, where all mobile electrons (holes) are located at the wire sufrace [17][18][19][20][21][22][23][24][25][26][27]. They attract attention because the Dirac spectrum of surface electrons (holes) splits in equidistant surface subbands sepa-rated by the substantial gap ∆ due to the small wire crosssection perimeter.…”