Absence of Magnetic Thermal Conductivity in the Quantum Spin-Liquid Candidate ${\mathrm{YbMgGaO}}_4$

We present the ultralow-temperature specific heat and thermal conductivity measurements on single crystals of ${\mathrm{YbMgGaO}}_4$, 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 ($C_m\sim T^{0.74}$). On the contrary, we do not observe any significant contribution of thermal conductivity from magnetic excitations. In magnetic fields $H\ge 6\mathrm{T}$, 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.

Figure 1

(a) The specific heat of the ${\mathrm{YbMgGaO}}_4$ single crystal at the zero magnetic field $H=0\mathrm{T}$. (b) The specific heat of the ${\mathrm{LuMgGaO}}_4$ polycrystalline sample at $H=0\mathrm{T}$ (data from Ref. [23]). (c) The magnetic specific heat of the ${\mathrm{YbMgGaO}}_4$ single crystal at various magnetic fields up to 9 T in a log-log scale. The magnetic specific heat is extracted by subtracting the specific heat of ${\mathrm{LuMgGaO}}_4$ from that of ${\mathrm{YbMgGaO}}_4$. The solid line is the fit of the 0 T data to $C_m=c{T}^{\beta }$ between 0.10 and 0.65 K. (d) The specific heat of the ${\mathrm{YbMgGaO}}_4$ single crystal at $H=6$ and 9 T. The solid lines are the fits to $C_m=d{e}^{-\mathrm{\Delta }/k_{B}T}$, in which $\mathrm{\Delta }$ is the magnon gap in the fully polarized state.

Figure 2

The in-plane thermal conductivity of (a) the ${\mathrm{YbMgGaO}}_4$ single crystal at $H=0\mathrm{T}$ and (b) the ${\mathrm{LuMgGaO}}_4$ single crystal at $H=0$ and 9 T. The solid lines are the fits to the data below 0.3 K to $\kappa /T=a+b{T}^{\alpha -1}$. Note that the applying of a 9 T magnetic field has no effect on the thermal conductivity of nonmagnetic ${\mathrm{LuMgGaO}}_4$.

Figure 3

(a) The in-plane thermal conductivity of the ${\mathrm{YbMgGaO}}_4$ single crystal at various magnetic fields up to 9 T. (b) Field dependence of the $\kappa /T$ at 0.2 K. The $\kappa /T$ first decreases slightly for $H<2\mathrm{T}$, then increases sharply (by about 35%) for $2\mathrm{T}<H<5\mathrm{T}$, and finally saturates for $H>5\mathrm{T}$. The saturated thermal conductivity above 5 T is purely attributed to phonons, without scattering by magnetic excitations.