[Paper Review] Unconventional Magnetic Oscillations in Kagome Mott Insulators

Guoxin Zheng, Yuan Zhu+13 more|arXiv (Cornell University)|Oct 12, 2023

Cold Atom Physics and Bose-Einstein Condensates4 citations

TL;DR

This study reports unconventional magnetic oscillations in the kagome Mott insulator YCu₃(OH)₆Br₂, where a 1/9 magnetization plateau and strong quantum-like oscillations in magnetic torque emerge in a robust Mott insulator. These oscillations, periodic in magnetic field H rather than 1/H, arise from Dirac spinons coupled to an emergent gauge field, providing direct evidence for fractionalized excitations in a quantum spin liquid state.

ABSTRACT

In metals, electrons in a magnetic field undergo cyclotron motion, leading to oscillations in physical properties called quantum oscillations. This phenomenon has never been seen in a robust insulator because there are no mobile electrons. We report the first exception to this rule. We study a Mott insulator on a kagome lattice which does not order magnetically down to milli-Kelvin temperatures despite antiferromagnetic interactions. We observe a plateau at magnetization equal to 1/9 Bohr magneton per magnetic ion, accompanied by oscillations in the magnetic torque, reminiscent of quantum oscillations in metals. The temperature dependence obeys Fermi distribution. These phenomena are consistent with a quantum spin liquid state whose excitations are fermionic spinons with a Dirac-like spectrum coupled to an emergent gauge field.

Motivation & Objective

• To investigate the magnetic response of the kagome Mott insulator YCu₃(OH)₆Br₂ under strong magnetic fields, which exhibits no long-range magnetic order down to mK temperatures.
• To determine whether the observed 1/9 magnetization plateau and associated oscillations can be explained by a quantum spin liquid state with fractionalized spinon excitations.
• To probe the origin of unconventional oscillations in a wide-gap insulator, which defy conventional quantum oscillation behavior seen in metals.
• To establish the presence of emergent gauge fields via the field and angular dependence of oscillation periods, testing predictions of Dirac spinon physics.

Proposed method

• Conducted pulsed magnetic field measurements (up to 73 T) on single-crystal YCu₃(OH)₆Br₂ using compensated-coil extraction magnetometry to measure magnetization and differential susceptibility (dM/dH).
• Performed cantilever magnetometry in DC and hybrid magnets (up to 42 T) on separate single crystals to confirm magnetization and oscillation features across field orientations.
• Analyzed the angular dependence of oscillation periods by rotating the magnetic field relative to the crystal c-axis, revealing orbital character of the oscillations.
• Modeled the system using a parton mean-field theory with spinons obeying a Dirac spectrum, coupled to an emergent gauge field, and derived oscillation frequency from Landau-level quantization.
• Used the LK formula for quantum oscillations, with modified cyclotron frequency ω_c = eαB_c/m_h, to fit oscillatory behavior and extract effective mass and Fermi velocity.
• Estimated the Dirac fermion velocity v_D ≈ 5140 m/s using magnetic susceptibility data near the Dirac point, consistent with oscillation period analysis.

Experimental results

Research questions

• RQ1 Can unconventional quantum oscillations be observed in a Mott insulator without electronic band dispersion, and what is their origin?
• RQ2 Do the observed 1/9 magnetization plateau and oscillations in magnetic torque indicate the presence of a quantum spin liquid with Dirac spinons?
• RQ3 Why do the oscillations exhibit periodicity in magnetic field H rather than 1/H, and what does this imply about the underlying Fermi surface?
• RQ4 How does the angular dependence of the oscillation period reveal the orbital nature of the oscillations and support an emergent gauge field mechanism?
• RQ5 What is the effective Fermi velocity of the spinons, and how does it compare across independent experimental and theoretical estimates?

Key findings

• A 1/9 magnetization plateau was observed at high magnetic fields (H ∥ c-axis), with the plateau's onset shifting to higher fields as the field was rotated toward the ab-plane.
• Strong oscillations in magnetic torque were detected near the 1/9 plateau, with a period that is approximately linear in magnetic field H rather than 1/H, indicating non-conventional quantum oscillations.
• The oscillation period showed strong angular dependence, consistent with an orbital origin and supporting the presence of an emergent gauge field coupling to spinons.
• The effective Fermi velocity of the spinons was estimated at v_D ≈ 5140 m/s using both the oscillation period and magnetic susceptibility data, showing consistency across independent methods.
• The magnetic susceptibility near the Dirac point exhibited a linear field dependence (dM/dH ∝ |B - B₀|), confirming that the 1/9 plateau corresponds to the chemical potential crossing the Dirac node.
• The ratio α of the emergent gauge field to the applied magnetic field was estimated to be close to unity, suggesting strong coupling between spinons and the emergent gauge field in this system.