  Heat flux ($J$) generally increases with temperature difference in a
material. A differential coefficient of $J$ against temperature ($T$) is called
differential thermal conductance ($k$), and an inverse of $k$ is differential
thermal resistance ($r$). Although $k$ and $r$ are generally positive, they can
be negative in a macroscopic heterojunction with positive $T$-dependent
interfacial thermal resistance (ITR). The negative differential thermal
resistance (NDTR) effect is an important effect that can realize thermal
transistor, thermal memory, and thermal logic gate. In this paper, we examine
analytical expressions of $J$, $k$, $r$, and other related quantities as a
function of parameters related to thermal conductivity ($\kappa$) and ITR in a
macroscopic heterojunction to precisely describe the NDTR effect.
