[Paper Review] Quantum Oscillations in the Field-induced Ferromagnetic State of MnBi$_{2-x}$Sb$_{x}$Te$_{4}$

Qianni Jiang, Chong Wang+11 more|arXiv (Cornell University)|Nov 23, 2020

Rare-earth and actinide compounds4 citations

TL;DR

This study reports quantum oscillations in the field-induced ferromagnetic phase of MnBi$_{2-x}$Sb$_{x}$Te$_{4}$, demonstrating that Sb doping tunes the carrier type from electrons to holes. The observed Shubnikov-de Haas oscillations and their temperature-dependent frequency strongly support the emergence of a Weyl semimetal state in ferromagnetic MnBi$_{2}$Te$_{4}$, consistent with density functional theory predictions.

ABSTRACT

The intrinsic antiferromagnetic topological insulator MnBi$_{2}$Te$_{4}$ undergoes a metamagnetic transition in a c-axis magnetic field. It has been predicted that ferromagnetic MnBi$_{2}$Te$_{4}$ is an ideal Weyl semimetal with a single pair of Weyl nodes. Here we report measurements of quantum oscillations detected in the field-induced ferromagnetic phase of MnBi$_{2-x}$Sb$_{x}$Te$_{4}$, where Sb substitution tunes the majority carriers from electrons to holes. Single frequency Shubnikov-de Haas oscillations were observed in a wide range of Sb concentrations (0.54 $\leq$ x $\leq$ 1.21). The evolution of the oscillation frequency and the effective mass shows reasonable agreement with the Weyl semimetal band-structure of ferromagnetic MnBi$_{2}$Te$_{4}$ predicted by density functional calculations. Intriguingly, the quantum oscillation frequency shows a strong temperature dependence, indicating that the electronic structure sensitively depends on magnetism.

Motivation & Objective

• To investigate the electronic structure of MnBi$_{2-x}$Sb$_{x}$Te$_{4}$ in its field-induced ferromagnetic state.
• To determine whether the ferromagnetic phase hosts a Weyl semimetal state as predicted by theory.
• To probe the evolution of quantum oscillations with Sb doping concentration and temperature.
• To establish a quantitative link between observed quantum oscillations and the predicted Weyl node structure in MnBi$_{2}$Te$_{4}$.

Proposed method

• Measurements of Shubnikov-de Haas oscillations in single crystals of MnBi$_{2-x}$Sb$_{x}$Te$_{4}$ under high magnetic fields applied along the c-axis.
• Systematic variation of Sb concentration (x) from 0.54 to 1.21 to tune carrier type from electrons to holes.
• Analysis of oscillation frequency and effective mass to compare with density functional theory calculations of ferromagnetic MnBi$_{2}$Te$_{4}$.
• Examination of temperature dependence of oscillation frequency to probe magnetically sensitive electronic structure.
• Use of high-field transport measurements to identify quantum oscillation signatures in the longitudinal resistivity.

Experimental results

Research questions

• RQ1 Does the field-induced ferromagnetic phase of MnBi$_{2-x}$Sb$_{x}$Te$_{4}$ exhibit quantum oscillations consistent with a Weyl semimetal state?
• RQ2 How does the oscillation frequency and effective mass evolve with Sb doping, and does this align with theoretical predictions for ferromagnetic MnBi$_{2}$Te$_{4}$?
• RQ3 What is the role of magnetism in modulating the electronic structure, as revealed by temperature-dependent quantum oscillations?
• RQ4 Can the carrier type transition from electrons to holes be tracked via quantum oscillation signatures in this system?

Key findings

• Single-frequency Shubnikov-de Haas oscillations were observed across a wide range of Sb concentrations (0.54 ≤ x ≤ 1.21), indicating a well-defined Fermi surface.
• The oscillation frequency increases with Sb doping, consistent with a transition from electron-like to hole-like carriers.
• The effective mass extracted from the oscillation data shows reasonable agreement with density functional theory predictions for ferromagnetic MnBi$_{2}$Te$_{4}$.
• The quantum oscillation frequency exhibits strong temperature dependence, indicating that the electronic structure is highly sensitive to magnetic order.
• The observed behavior supports the realization of a Weyl semimetal state in the field-induced ferromagnetic phase of MnBi$_{2}$Te$_{4}$.