Heat transport of the quasi-one-dimensional alternating spin chain material (CH${}_3$)${}_2$NH${}_2$CuCl${}_3$

We report a study of the low-temperature heat transport in the quasi-one-dimensional $S=1/2$ alternating antiferromagnetic-ferromagnetic chain compound (CH${}_3$)${}_2$NH${}_2$CuCl${}_3$. Both the temperature and magnetic-field dependencies of thermal conductivity are very complicated, pointing to the important role of spin excitations. It is found that magnetic excitations act mainly as the phonon scatterers in a broad temperature region from 0.3 to 30 K. In magnetic fields, the thermal conductivity shows drastic changes, particularly at the field-induced transitions from the low-field Néel state to the spin-gapped state, the field-induced magnetic ordered state, and the spin polarized state. In high fields, the phonon conductivity is significantly enhanced because of the weakening of spin fluctuations.

Figure 1

(a) A schematic plot of the spin structure of MCCL. There are two types of dimers along the $a$ axis: (i) the AFM dimers with the intradimer interaction $J_1$ and the neighboring AFM interaction $J_A$; (ii) the FM dimers with the intradimer interaction $J_2$ and the neighboring AFM interaction $J_B$. Spin chains (along the $c$ axis) are contacted by alternating AFM dimers and FM dimers through AFM interaction $J_3$ between two types of dimers. (b) The $H-T$ phase diagram of MCCL obtained from the former experiments.[19, 20, 21, 24, 25] The magenta area and the cyan one represent the low-field spontaneous order state and the high-field induced magnetic order state, respectively, with the field-induced gapped state between them.

Figure 2

Specific heat of MCCL as a function of temperature for the magnetic field applied perpendicular to the $bc$ plane.

Figure 3

Temperature dependencies of the thermal conductivity of MCCL single crystal in the zero field and several different magnetic fields up to 14 T, which are applied perpendicular to the $bc$ plane. The dashed line indicates a $T^{2.7}$ dependence of $\kappa$ at sub-K temperatures. The inset shows the temperature dependence of the phonon mean free path $l$ divided by the averaged sample width $W$ in 14-T magnetic field.

Figure 4

Magnetic-field dependencies of thermal conductivity of MCCL crystal at low temperatures. The magnetic fields are applied perpendicular to the $bc$ plane. (a) $T=$ 0.38–0.7 K. (b) $T=$ 0.97–3 K. (c) Zoom in of the low-field plots for $T=$ 0.38–0.7 K.