Phonon‐Dominated Energy Transport in Purely Metallic Heterostructures

Abstract

Ultrafast X‐ray diffraction is used to quantify the transport of energy in laser‐excited nanoscale gold–nickel (Au–Ni) bilayers. Electron transport and efficient electron–phonon coupling in Ni convert the laser‐deposited energy in the conduction electrons within a few picoseconds into a strong non‐equilibrium between hot Ni and cold Au phonons at the bilayer interface. Modeling of the subsequent equilibration dynamics within various two‐temperature models confirms that for ultrathin Au films, the thermal transport is dominated by phonons instead of conduction electrons because of the weak electron–phonon coupling in Au.

Dominant Energy Carrier Transitions and Thermal Anisotropy in Epitaxial Iridium Thin Films

“…Indeed an increasing contribution of the phonon‐mediated heat path within Ir layer may be due to the onset of insufficient electron–phonon coupling within a thinner Ir layer, having been recently shown experimentally in Au‐Ni bilayers. [ 26 ] As such, we note that other metals can demonstrate such enhanced thermal transport behavior given dimensions comparable to the free paths of energy carriers, exceptionally pure microstructure, and a neighboring material with compatible vibrational characteristics (see Section S15, Supporting Information). Such an interplay between carriers stresses the importance of designing entire material systems for effective thermal control.…”

“…With extreme scaling, however, dimensions approach the mean-free paths of heat carriers and a transition between their relative dominance can occur. [26] In this context, the epitaxial growth of Ir provides a foundational material system with minimal confounding microstructural effects and an opportunity to reveal the extent of electron-phonon coupling effects across a metal that have not been fully explored. [27,28] At present, similar coupling effects and their impact on thermal transport are of growing interest in materials such as doped/polar semiconductors, [29,30] bilayer graphene, [31] and transition metal oxides.…”

Concepts and use cases for picosecond ultrasonics with x-rays

“…Subsequently, the energy is transferred to phonons in Ni via electron-phonon coupling within the first 1 ps to 2 ps, which lowers the overall electron temperature and the energy density stored in electron excitations. The phonon temperatures of both layers equilibrate only on the timescale of tens of picoseconds as discussed in [62,115] and illustrated by Fig. 10(c).…”

“…with the thermal conductivity κ = κ(T (x 3 , t)) that inherits its depth-dependence from the temperature profile and differs for different materials. In addition, the description of heat transport across interfaces may require considering interface resistances that account for different dispersion relations of the involved quasi-particles [60][61][62].…”

“…In the following, we discuss the ultrafast energy transport within a metallic bilayer of 5 nm Au and 13 nm Ni in the framework of a diffusive 2 TM (Eq. ( 17)) that we previously applied to capture the energy transfer and resulting strain response in similar Au-Ni [62,115] and Au-Fe bilayers [110] across a large range of layer thicknesses. The picosecond strain response of both Au and Ni serves as a reference for the heat transfer that is dominated by electronic transport within the first hundreds of femtoseconds.…”