Low-Temperature Heat Transport in the Low-Dimensional Quantum Magnet ${\mathrm{NiCl}}_2\mathrm{\text{−}}4\mathrm{SC}({\mathrm{NH}}_2{)}_2$

We report a study of the low-temperature thermal conductivity of ${\mathrm{NiCl}}_2\mathrm{\text{−}}4\mathrm{SC}({\mathrm{NH}}_2{)}_2$, which is a spin-1 chain system exhibiting the magnon Bose-Einstein condensation (BEC) in a magnetic field. It is found that the low-$T$ thermal conductivity along the spin-chain direction shows strong anomalies at the lower and upper critical fields of the magnon BEC state. In this state, magnons act mainly as phonon scatterers at relatively high temperature, but change their role to heat carriers upon temperature approaching zero. The result demonstrates a large thermal conductivity in the magnon BEC state and points to a direct analog between the magnon BEC and the conventional one.

Figure 1

Temperature-field phase diagram of DTN ($H\parallel c$) obtained from magnetization, specific heat, and magnetocaloric effect measurements [17, 18, 19, 20]. PM and SP represent the low-field quantum paramagnetic state and high-field spin-polarized state, respectively. Magnon BEC is a magnetic-field-induced AF ordered state, with the lower and upper critical fields $H_{c1}(\sim 2\mathrm{T})$ and $H_{c2}(\sim 12\mathrm{T})$. The maximum of field-dependent critical temperature is about 1.2 K, where $H_{c1}$ and $H_{c2}$ merge. Inset: Unit cell of tetragonal structure showing Ni and Cl atoms. The other atoms are omitted for clarity.

Figure 2

Temperature dependences of thermal conductivities of DTN single crystals for ${\kappa }_c$ and ${\kappa }_{ab}$ in both the zero field and several different magnetic fields up to 14 T, which applied along the $c$ axis or the $ab$ plane. The dashed lines in panels (b) and (d) indicate that the zero-field thermal conductivities at sub-Kelvin temperatures show a $T^{2.7}$ temperature dependence.

Figure 3

Magnetic-field dependences of thermal conductivities of DTN single crystals at low temperatures.

Figure 4

Comparison of ${\kappa }_c(H)$ and ${\kappa }_{ab}(H)$ isotherms for $H\parallel c$ at sub-Kelvin temperatures. (a) $T=0.97\mathrm{K}$; (b) $T=0.7\mathrm{K}$; (c) $T=0.55\mathrm{K}$; (d) $T=0.38\mathrm{K}$.