Magnetic heat transport in $R_2\mathrm{Cu}{\mathrm{O}}_4$ ($R=\mathrm{La}$, Pr, Nd, Sm, Eu, and Gd)

We have studied the thermal conductivity $\kappa$ on single crystalline samples of the antiferromagnetic monolayer cuprates $R_2\mathrm{Cu}{\mathrm{O}}_4$ with $R=\mathrm{La}$, Pr, Nd, Sm, Eu, and Gd. For a heat current within the $\mathrm{Cu}{\mathrm{O}}_2$ planes, i.e., for ${\kappa }_{ab}$ we find high-temperature anomalies around $250\mathrm{K}$ in all samples. In contrast, the thermal conductivity ${\kappa }_c$ perpendicular to the $\mathrm{Cu}{\mathrm{O}}_2$ planes, which we measured for $R=\mathrm{La}$, Pr, and Gd, shows a conventional temperature dependence as expected for a purely phononic thermal conductivity. This qualitative anisotropy of ${\kappa }_i$ and the anomalous temperature dependence of ${\kappa }_{ab}$ give evidence for a significant magnetic contribution ${\kappa }_{\mathrm{mag}}$ to the heat transport within the $\mathrm{Cu}{\mathrm{O}}_2$ planes. Our results suggest, that a large magnetic contribution to the heat current is a common feature of single-layer cuprates. We find that ${\kappa }_{\mathrm{mag}}$ is hardly affected by structural instabilities, whereas already weak charge carrier doping causes a strong suppression of ${\kappa }_{\mathrm{mag}}$.

Figure 1

(Color online) In-plane $\left({\kappa }_{ab}\right)$ and out-of-plane $\left({\kappa }_c\right)$ thermal conductivity of ${\mathrm{Gd}}_2\mathrm{Cu}{\mathrm{O}}_4$. ${\kappa }_{ab}$ was measured on two different crystals. Solid lines are fits by the Debye model and the dashed line is ${\kappa }_c$ multiplied by a factor of 1.25 (see text).

Figure 2

(Color online) In-plane $\left({\kappa }_{ab}\right)$ and out-of-plane $\left({\kappa }_c\right)$ thermal conductivity of ${\mathrm{Pr}}_2\mathrm{Cu}{\mathrm{O}}_4$. Lines are fits by the Debye model (see text). Inset: The same data on double-logarithmic scales with ${\kappa }_c$ multiplied by a factor of 1.2. The temperature dependencies of ${\kappa }_{ab}$ and ${\kappa }_c$ around the low-temperature maxima are nearly the same. Above about $70\mathrm{K}$ ${\kappa }_c$ follows a $1∕T$ behavior (solid line), whereas an anomalous high-temperature upturn is present in ${\kappa }_{ab}$.

Figure 3

(Color online) In-plane thermal conductivity of ${\mathrm{Nd}}_2\mathrm{Cu}{\mathrm{O}}_4$, ${\mathrm{Sm}}_2\mathrm{Cu}{\mathrm{O}}_4$, and ${\mathrm{Eu}}_2\mathrm{Cu}{\mathrm{O}}_4$. For $R=\mathrm{Eu}$ the low-temperature maximum is suppressed. The inset compares ${\kappa }_{ab}\left(●\right)$ of our ${\mathrm{Nd}}_2\mathrm{Cu}{\mathrm{O}}_4$ crystal with data (엯) from Ref. 8.

Figure 4

(Color online) Magnetic contributions to the in-plane thermal conductivity, calculated via ${\kappa }_{\mathrm{mag}}={\kappa }_{ab}-{\kappa }_{ph}$, where ${\kappa }_{ph}$ is determined by a Debye fit of the low-temperature maximum. Upper panel: Values calculated from our measurements of ${\kappa }_{ab}$ of $R_2\mathrm{Cu}{\mathrm{O}}_4$ and from the data from Ref. 8. Lower panel: The same analysis for various data of ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_{4+\delta }$ taken from Refs. 11, 12, 13 and of our crystal with $T_{\mathrm{N}}=245\mathrm{K}$. Inset: The maximum of the calculated ${\kappa }_{\mathrm{mag}}$ vs. the Néel temperature for ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_{4+\delta }$ (see text).