Quantized conductance in a CVD-grown nanoribbon with hidden Rashba effect

Quantized conductance in quasi-one-dimensional systems not only provides a hallmark of ballistic transport, but also serves as a gateway for exploring quantum phenomena. Recently, a unique hidden Rashba effect attracts tremendous attention, which arises from the compensation of opposite spin polarizations of a Rashba bilayer in inversion symmetric crystals with dipole fields, such as bismuth oxyselenide ($\mathrm{Bi}{2}\mathrm{O}{2}\mathrm{Se}$). However, investigating this effect utilizing conductance quantization is still challenging. Here we report the conductance quantization observed in a chemical vapor deposition (CVD)-grown high-mobility $\mathrm{Bi}{2}\mathrm{O}{2}\mathrm{Se}$ nanoribbon, where quantized conductance plateaus up to $44\cdot 2e^{2}/{h}$ ($e$ is the elementary charge, $h$ is the Planck constant, and the factor $2$ results from spin degeneracy) are achieved at zero magnetic field. Due to the hidden Rashba effect, the quantized conductance remains in multiples of $2e^{2}/{h}$ without Zeeman splitting even under magnetic field up to $12$ T. Moreover, within a specific range of magnetic field, the plateau sequence exhibits the Pascal triangle series, namely $(1,3,6,10,15\dots )\cdot 2e^{2}/{h}$, reflecting the interplay of size quantization in two transverse directions. These observations are well captured by an effective hidden Rashba bilayer model. Our results demonstrate $\mathrm{Bi}{2}\mathrm{O}{2}\mathrm{Se}$ as a compelling platform for spintronics and the investigation of emergent phenomena.

💡 Research Summary

In this work the authors demonstrate ballistic transport and quantized conductance in a chemically vapor‑deposited (CVD) Bi₂O₂Se nanoribbon, exploiting the material’s hidden Rashba effect. The nanoribbon, 50 nm thick and 550 nm long, exhibits an exceptionally high field‑effect mobility (~2 × 10⁴ cm² V⁻¹ s⁻¹) that enables clear conductance plateaus at low temperature (1.5 K). By sweeping a back‑gate voltage, they observe conductance steps in units of 2e²/h up to 44 × 2e²/h, the highest index reported for an individual one‑dimensional system.

Bi₂O₂Se possesses an inversion‑symmetric crystal structure with intrinsic dipole fields, leading to opposite Rashba spin splittings in alternating Bi‑O layers. The opposite spin textures cancel in the total band structure, producing a “hidden Rashba” state: each subband remains spin‑degenerate while the effective g‑factor is strongly suppressed. Consequently, even under perpendicular magnetic fields up to 12 T the conductance plateaus stay at integer multiples of 2e²/h; half‑integer plateaus that would signal Zeeman splitting are absent.

Within a specific magnetic‑field window (≈7–9 T) the plateau indices follow the Pascal‑triangle sequence (1, 3, 6, 10, 15…)·2e²/h. This pattern arises from simultaneous size quantization in the two transverse directions of the ribbon. The confinement in both directions can be modeled as harmonic oscillators with equal spacing, so subbands are labeled by quantum numbers (nₓ, n_y). Degeneracies of the form (nₓ + n_y + 1)(nₓ + n_y)/2 generate the triangular numbers observed experimentally.

To rationalize the data the authors construct an effective Hamiltonian for a hidden‑Rashba bilayer. The model includes opposite Rashba terms for the two layers, a harmonic confinement potential in x and y, magnetic‑field‑induced cyclotron frequency, and interlayer coupling. Parameters such as the effective mass and g‑factor are obtained from ab‑initio calculations on a 40‑unit‑cell thick ribbon. Gaussian broadening (0.7 meV) accounts for finite temperature and measurement noise. Numerical simulations of the Landau‑level spectrum reproduce the full set of observations: high‑order integer plateaus at zero field, suppression of Zeeman splitting, and the Pascal‑triangle series at intermediate fields.

The study establishes Bi₂O₂Se nanoribbons as a compelling platform where high mobility, strong spin‑orbit coupling, and hidden Rashba physics coexist. The ability to maintain spin degeneracy under large magnetic fields while accessing high conductance indices opens avenues for spin‑layer‑locked spintronic devices, gate‑tunable quantum point contacts, and exploration of emergent topological phenomena in low‑dimensional systems.