Controlling the electron-phonon heat exchange in a metallic film by its position in a dielectric slab

Abstract

We theoretically study the heat flux between electrons and phonons in a thin metallic film embedded in a suspended dielectric slab (called a \textit{membrane}, in accordance with the established nomenclature), forming a layered structure. The thickness of the membrane is much smaller than the other two dimensions and, in the considered temperature range, is comparable to the dominant phonon wavelength. The thickness of the metallic layer is an order of magnitude smaller than the thickness of the membrane. While the dependence of the heat exchange on the thicknesses of the film and of the membrane has been studied before, it is not yet known how this depends on the position of the film inside the membrane. Here we show that the position strongly influences the heat exchange. If we denote by $T_e$ the effective temperature of the electrons in the metal and by $T_{ph}$ the effective temperature of the phonons (assumed to be uniform in the entire system), then we may write in general the heat power as $P \equiv P^{(0)}(T_e) - P^{(0)}(T_{ph})$, where $P^{(0)}(T) \equiv P_s^{(0)}(T) + P_a^{(0)}(T)$, with $P_s^{(0)}(T)$ and $P_a^{(0)}(T)$ being the contributions of the symmetric and antisymmetric Lamb modes, respectively. In the low-temperature limit, we may write $P_s^{(0)}(T) \equiv C_s T^4$ and $P_a^{(0)}(T) \equiv C_a T^{3.5}$, where $C_s$ is independent of the position of the film inside the membrane, whereas $C_a$ increases with the distance between the mid-plane of the film and the mid-plane of the membrane, being zero when the film is at the center of the membrane. Our examples show that by changing the position of the film inside the membrane one may change the electron-phonon heat power by orders of magnitude, depending on the dimensions and the temperature range.