[Paper Review] Absence of magnetic thermal conductivity in the quantum spin liquid candidate YbMgGaO4

Yang Xu, Jun Zhang+5 more|PubMed|Nov 23, 2016

Advanced Condensed Matter Physics11 citations

TL;DR

This study reports ultra-low-temperature specific heat and thermal conductivity measurements on YbMgGaO4 single crystals, revealing a power-law magnetic specific heat (Cₘ ∼ T⁰.⁷⁴) indicating gapless excitations, yet no detectable magnetic thermal conductivity at zero field. The absence of magnetic thermal conductivity despite large magnetic specific heat strongly constrains theoretical models of its quantum spin liquid ground state.

ABSTRACT

We present the ultralow-temperature specific heat and thermal conductivity measurements on single crystals of YbMgGaO_{4}, which was recently argued to be a promising candidate for a quantum spin liquid (QSL). In a zero magnetic field, a large magnetic contribution of specific heat is observed, and exhibits a power-law temperature dependence (C_{m}∼T^{0.74}). On the contrary, we do not observe any significant contribution of thermal conductivity from magnetic excitations. In magnetic fields H≥6 T, the exponential T dependence of C_{m} and the enhanced thermal conductivity indicate a magnon gap of the fully polarized state. The absence of magnetic thermal conductivity at the zero field in this QSL candidate puts a strong constraint on the theories of its ground state.

Motivation & Objective

• To investigate the low-energy magnetic excitations in YbMgGaO4, a proposed quantum spin liquid (QSL) candidate, using ultra-low-temperature measurements.
• To resolve the apparent contradiction between gapless magnetic specific heat and the absence of magnetic thermal conductivity in YbMgGaO4.
• To determine whether the ground state is a gapless U(1) QSL with a spinon Fermi surface or a gapped state with localized excitations.
• To constrain theoretical models of the QSL ground state by measuring the thermal response of high-quality single crystals.
• To assess the role of disorder and scattering mechanisms in suppressing spinon thermal transport despite high magnetic specific heat.

Proposed method

• Performed ultra-low-temperature specific heat measurements (0.05–3 K) using a physical property measurement system with a dilution refrigerator.
• Measured thermal conductivity via the standard four-wire steady-state method with in-situ calibrated RuO2 chip thermometers on single crystals cut along the ab-plane.
• Applied magnetic fields up to 9 T along the c-axis to probe field-induced transitions to the fully polarized state.
• Used LuMgGaO4 as a non-magnetic reference to isolate phononic contributions to thermal conductivity.
• Analyzed data using the kinetic formula for thermal conductivity: κₘ = (1/3)Cₘv_Fl, relating magnetic specific heat, Fermi velocity, and mean free path of spinons.
• Compared experimental results with theoretical predictions for U(1) QSLs and spinon Fermi surface models to assess consistency.

Experimental results

Research questions

• RQ1 Does YbMgGaO4 exhibit magnetic thermal conductivity consistent with gapless spinon excitations in a quantum spin liquid?
• RQ2 What is the origin of the large magnetic specific heat (Cₘ ∼ T⁰.⁷⁴) at zero field, and why is it not reflected in thermal conductivity?
• RQ3 How do the magnetic excitations in YbMgGaO4 behave under magnetic fields, and what does this imply about the nature of the ground state?
• RQ4 Could disorder or scattering mechanisms localize spinons despite their gapless nature, explaining the absence of magnetic thermal conductivity?
• RQ5 To what extent do the observed thermal transport properties constrain the validity of proposed U(1) QSL and spinon Fermi surface models?

Key findings

• A large magnetic specific heat with power-law temperature dependence (Cₘ ∼ T⁰.⁷⁴) was observed down to 0.1 K, indicating the presence of gapless magnetic excitations.
• No significant contribution to thermal conductivity from magnetic excitations was detected at zero magnetic field, despite the large magnetic specific heat.
• Thermal conductivity at zero field was dominated by phonons, with additional scattering by magnetic excitations reducing its magnitude.
• In magnetic fields H ≥ 6 T, the specific heat exhibited exponential T-dependence, consistent with a fully polarized state and a magnon gap.
• The estimated spinon mean free path was only 8.6 Å, about 2.5 times the inter-spin distance, indicating strong localization despite gapless character.
• The combination of large Cₘ and negligible κₘ strongly constrains theoretical models, suggesting that spinons in YbMgGaO4 are highly localized, possibly due to Mg²⁺-Ga³⁺ site disorder in the triangular bipyramids.