Quantum Transport in Topological Surface States of Selectively Grown Bi₂Te₃ Nanoribbons

Abstract

Conventional 3D TIs are the van der Waals bound, binary compounds such as Bi 2 Te 3 , [10] Bi 2 Se 3 , [11,12] Sb 2 Te 3 , [13] as well as alloys of these elements. [14,15] For a quasi 1D nanowire, due to the inclusion of the Berryphase of a particle traversing the perimeter of the nanowire, the surface band structure is determined to be gapped. [16-18] By applying a magnetic flux parallel to the nanowire axis Φ = ±Φ 0 /2, where Φ 0 = h/e, this gap will be closed. The non-degenerate, topologically protected linear surface bands will re-emerge periodically with a period of a full flux quantum Φ = (n + 1/2) Φ 0 threading the wire. The observation of such periodic Aharonov-Bohm (AB) oscillations has previously been reported. [19-24] Making use of a selective area growth approach, nanowires of aforementioned materials can as well be deposited by molecular beam epitaxy (MBE). [25-28] These nanowires have a rather rectangular cross-section and are therefore referred to as nano ribbons. As a scalable bottom-up approach, these selectively grown nanoribbons are beneficial for desired Majorana surface architectures. [29] However, MBE grown 3D TI compounds usually Quasi-1D nanowires of topological insulators are candidate structures in superconductor hybrid architectures for Majorana fermion based quantum computation schemes. Here, selectively grown Bi 2 Te 3 topological insulator nanoribbons at cryogenic temperatures are investigated. The nanoribbons are defined in deep-etched Si 3 N 4 /SiO 2 nano-trenches on a silicon (111) substrate followed by a selective area growth process via molecular beam epitaxy. The selective area growth is beneficial to the device quality, as no subsequent fabrication needs to be performed to shape the nanoribbons. In the diffusive transport regime of these unintentionally n-doped Bi 2 Te 3 topological insulator nanoribbons, electron trajectories are identified by analyzing angle dependent universal conductance fluctuation spectra. When the sample is tilted from a perpendicular to a parallel magnetic field orientation, these high frequent conductance modulations merge with low frequent Aharonov-Bohm type oscillations originating from the topologically protected surface states along the nanoribbon perimeter. For 500 nm wide Hall bars low frequent Shubnikov-de Haas oscillations are identified in a perpendicular magnetic field orientation. These reveal a topological, high-mobility, 2D transport channel, partially decoupled from the bulk of the material.

Reappearance of first Shapiro step in narrow topological Josephson junctions

“…The transition from 3D topological surface states on each individual surface of the TI and quasi-1D states confined to the perimeter of a narrow TI nanoribbon is sharp, as it is solely dependent on the relation of the phase-coherence length to the nanoribbon perimeter. In previous studies, we found that for 200-nm-wide nanoribbon devices, the topological wave function traverses the perimeter phase coherently, while for devices of width W ≥ 500 nm, this is not observed ( 37 ).…”

“…We now focus on the investigation of junctions on narrow BST nanoribbons of only W = 200 nm that have been defined using a SAG approach ( 24 , 36 , 37 ). In Josephson junctions on selectively deposited nanoribbons, the geometry of the junction is defined by the width W of the TI nanoribbon and the electrode separation L of the two superconducting electrodes.…”

“…For details on the substrate preparation for the SAG of 3D TI compounds using MBE, the reader is referred to Materials and Methods. A cross-sectional analysis of SAG-grown nanoribbons is presented in ( 37 ).…”

“…A topological phase transition can be observed by applying a magnetic flux of magnitude Φ 0 /2 = h /2 e along the nanoribbon axis ( 26 , 28 , 29 ). Signatures of a periodic gap opening in magnetotransport measurements on synthesized TI nanowires ( 30 – 35 ) and just recently in MBE selectively grown nanoribbons ( 36 , 37 ) have been reported. In Josephson junctions based on confined TI nanoribbons, MBSs vanish without applied magnetic flux due to the spin degeneracy of surface states.…”

Crossed Andreev reflection in topological insulator nanowire T junctions

“…1). These or larger dimensions are experimentally realizable [45]. We use B ⊥ = 20 T such that the wires are in the quantum Hall regime (see Fig.…”