[Paper Review] Surface chiral metal in a bulk half-integer quantum Hall insulator

J. Y. Liu, Jinling Yu+23 more|arXiv (Cornell University)|Jul 15, 2019

Topological Materials and Phenomena4 citations

TL;DR

This study reports the first observation of a bulk half-integer quantum Hall effect in the topological semimetal BaMnSb2, where the extreme quantum limit stabilizes a 2D chiral metal at the surface, representing a novel topological quantum liquid state not previously seen in bulk single crystals.

ABSTRACT

The study of the quantum Hall effect (QHE) in two-dimensional (2D) systems such as 2D electron gases and graphene has led to important breakthroughs in the development of many new concepts in modern physics. Although the QHE is not generally expected for bulk materials due to the band dispersion along the magnetic field direction, a bulk QHE has been observed in several materials. Here, we report on the observation of a unique bulk half-integer QHE in a topological semimetal BaMnSb2. In the extreme quantum limit, its quantum Hall state is accompanied by a 2D chiral metal at the surface, which represents a novel topological quantum liquid, not previously observed in bulk single crystal materials.

Motivation & Objective

• To investigate the emergence of quantum Hall effects in bulk topological semimetals beyond conventional 2D systems.
• To explore whether topological surface states can host chiral metals under extreme quantum conditions.
• To determine if bulk materials can sustain half-integer quantum Hall states despite band dispersion along the magnetic field direction.
• To identify and characterize a novel topological quantum liquid phase in a bulk single crystal.

Proposed method

• Application of high magnetic fields to achieve the extreme quantum limit in single-crystalline BaMnSb2.
• Measurement of longitudinal and transverse resistivity to detect quantum Hall plateaus.
• Use of angle-resolved transport measurements to probe surface state contributions.
• Analysis of Landau level quantization and chiral edge states in the bulk quantum Hall state.
• Identification of chiral surface states via transport anisotropy and quantized Hall conductance.
• Comparison of observed Hall conductance with theoretical predictions for half-integer QHE in topological semimetals.

Experimental results

Research questions

• RQ1 Can a bulk topological semimetal exhibit a half-integer quantum Hall effect despite 3D band dispersion?
• RQ2 What is the role of surface states in stabilizing chiral metals under extreme quantum conditions?
• RQ3 Does the observed quantum Hall state host a novel topological quantum liquid not previously realized in bulk materials?
• RQ4 How does the chiral surface metal emerge in the context of a bulk half-integer QHE?
• RQ5 What is the topological nature of the surface state and its relation to the bulk quantum Hall response?

Key findings

• A bulk half-integer quantum Hall effect was observed in single-crystalline BaMnSb2 under high magnetic fields.
• The quantum Hall state is accompanied by a 2D chiral metal at the surface, indicating a novel topological quantum liquid.
• The chiral surface metal persists even when the bulk is insulating, suggesting topological protection.
• The half-integer quantization of the Hall conductance indicates a nontrivial topological invariant in the bulk.
• The surface chiral metal is robust and distinct from conventional edge states in 2D systems.
• The system realizes a new class of topological quantum matter where bulk and surface states coexist in a quantized, chiral fashion.