[Paper Review] Magneto-transport Effects in Topological Insulator Bi$_2$Se$_3$ Nanoribbons

Hao Tang, Liang Dong+2 more|arXiv (Cornell University)|Mar 31, 2010

Topological Materials and Phenomena8 references3 citations

TL;DR

This study investigates magneto-transport in Bi₂Se₃ nanoribbons, revealing a robust, temperature-independent linear magnetoresistance (MR) in perpendicular magnetic fields up to room temperature, attributed to quantum linear MR from topological surface states with linear dispersion. The observation of suppressed out-of-plane Fermi momentum via boundary scattering further supports dominant 2D surface-state transport over bulk contributions.

ABSTRACT

Magneto-resistance (MR) of Bi$_2$Se$_3$ nanoribbons is studied over a broad range of temperature ($T$=300K-2K) and under various magnetic field ($B$) orientations. The MR is strongly anisotropic with the perpendicular MR much larger than the longitudinal and transverse MRs. The perpendicular MR exhibits quadratic $B$-dependence in low fields and becomes linear at high $B$. However, when $T$ increases, the perpendicular MR becomes linear over the whole magnetic field range (0-9T) up to room temperature. This unusual linear MR is discussed in the context of the linear quantum MR of the topological surface-states. We also observe the boundary-scattering effect in MR at low temperatures, which indicates that the out-of-plane Fermi momentum is much smaller the in-plane Fermi momentum, excluding the simple three-dimensional Fermi surface picture.

Motivation & Objective

• To investigate the anisotropic magneto-transport response in Bi₂Se₃ nanoribbons under varied magnetic field orientations.
• To determine the origin of the observed linear magnetoresistance (MR) at high temperatures and its evolution with temperature and field strength.
• To probe the Fermi surface topology by analyzing boundary scattering effects in low magnetic fields.
• To distinguish the contributions of topological surface states from bulk electronic states in transport properties.
• To assess the validity of quantum linear MR (QLMR) theory in high-quality, single-crystalline Bi₂Se₃ nanoribbons.

Proposed method

• Synthesized pure, stoichiometric Bi₂Se₃ nanoribbons via vapor-liquid-solid growth using Au catalysts, confirmed by X-ray and TEM analysis.
• Fabricated four-terminal devices on Si/SiO₂ substrates with Pd/Ti electrodes via e-beam evaporation and lift-off for ohmic contact.
• Performed low-frequency lock-in four-terminal transport measurements in a Quantum Design PPMS from 2K to 300K under magnetic fields up to 9T.
• Measured magnetoresistance (MR) in three field orientations: perpendicular (B⊥), longitudinal (B∥ current), and transverse (B∥ surface, ⊥ current).
• Extracted Fermi momentum (kF) from critical field Bc of boundary scattering using r_c = ℏkF/eBc = d/2, where d is the relevant sample dimension.
• Analyzed MR field and temperature dependence to distinguish between classical MR mechanisms and quantum linear MR (QLMR) from linear dispersion.

Experimental results

Research questions

• RQ1 What causes the linear magnetoresistance observed in Bi₂Se₃ nanoribbons at temperatures up to 300K, particularly in the perpendicular field configuration?
• RQ2 How does the anisotropy in magnetoresistance between perpendicular, longitudinal, and transverse field orientations reflect the Fermi surface topology?
• RQ3 To what extent do boundary scattering effects in low magnetic fields reveal the anisotropy of Fermi momentum in the nanoribbon?
• RQ4 Can the observed linear MR be explained by quantum linear MR (QLMR) from Dirac electrons, or is it due to classical inhomogeneity effects?
• RQ5 What evidence supports the dominance of topological surface states over bulk states in the transport response?

Key findings

• The perpendicular-field magnetoresistance exhibits a linear B-dependence up to 9T and persists linearly across the entire field range at temperatures above 90K, extending to room temperature.
• At low temperatures (T < 90K), the perpendicular MR transitions from quadratic B-dependence at low fields (<3T) to linear behavior at high fields, indicating a crossover from classical to quantum regime.
• Boundary scattering effects are clearly resolved in low magnetic fields (B < 1T), with critical fields Bc ≈ 0.5T (perpendicular) and Bc ≈ 0.75T (transverse), indicating a significant anisotropy in Fermi momentum.
• Fermi momentum estimates from boundary scattering yield kF ≈ 3.6 × 10⁸ m⁻¹ (perpendicular) and kF ≈ 1.0 × 10⁸ m⁻¹ (in-plane), showing that out-of-plane kF is much smaller than in-plane kF.
• The observed kF anisotropy contradicts the 3D Fermi surface model of bulk Bi₂Se₃ but is consistent with quasi-2D surface states dominating transport.
• The robust, temperature-independent linear MR is most consistent with quantum linear MR (QLMR) from Dirac electrons with linear dispersion, supporting the presence of topological surface states.