Direct observation of large electron–phonon interaction effect on phonon heat transport

Abstract

As a foundational concept in many-body physics, electron–phonon interaction is essential to understanding and manipulating charge and energy flow in various electronic, photonic, and energy conversion devices. While much progress has been made in uncovering how phonons affect electron dynamics, it remains a challenge to directly observe the impact of electrons on phonon transport, especially at environmental temperatures. Here, we probe the effect of charge carriers on phonon heat transport at room temperature, using a modified transient thermal grating technique. By optically exciting electron-hole pairs in a crystalline silicon membrane, we single out the effect of the phonon–carrier interaction. The enhanced phonon scattering by photoexcited free carriers results in a substantial reduction in thermal conductivity on a nanosecond timescale. Our study provides direct experimental evidence of the elusive role of electron–phonon interaction in phonon heat transport, which is important for understanding heat conduction in doped semiconductors. We also highlight the possibility of using light to dynamically control thermal transport via electron–phonon coupling.

Reduced Thermal Conductivity in Ultrafast Laser Heated Silicon Measured by Time-Resolved X-ray Diffraction

“…Figure 5 shows the notable reduction in the thermal conductivity as compared to the bulk value k bulk for increasing the carrier concentration. In particular, the values that were obtained in this work with the carrier concentration within the surface region in the range of 3.5∼8.2 × 10 19 cm −3 estimated from the absorption length (10 µm) and photon energy (1.55 eV) show a consistent trend with the past work that were performed with different techniques [9,[51][52][53]. Nevertheless, we note that the reduced k values from 50% to 30% are noticeably smaller than that previously reported.…”

“…The carrier density in this work was estimated from mean surface carrier density within the X-ray extinction depth at ∆t = 0. The results from previous research using different measurement techniques (e.g., the transient thermal grating measurement [9] and doped Si [51][52][53]) are also displayed.…”

“…Similar difficulties may also arise under an extreme level of photoexcitation in solids, which often induces a dramatic local temperature gradients within a very short distance that may be further enhanced by nonlinear, non-radiative recombination processes. However, surprisingly little is known regarding how such non-equilibrium processes may affect the eventual outcome of macroscopic thermophysical properties, such as thermal diffusion [9]. Such scientific and practical needs combined with arrivals of femtosecond laser technologies have instigated extensive research towards understanding physical mechanisms of heat generation (electron-phonon interactions), relaxation, and propagation at microscopic scales by using all optical pump-probe experiments [10][11][12].…”

Observation of second sound in a rapidly varying temperature field in Ge

“…The resulting R at the energy corresponding to the probe wavelength is provided by the first-order expansion R = (∂R/∂T)T + (∂R/∂n)n (16). Interestingly enough, the contribution provided by the variation of the carrier concentration is expected to dominate the way the reflectivity is affected in experiments, like the present ones, involving pulsed laser sources, i.e., for high electronic excitation densities (17,18). However, in our excitation conditions, the electronic contribution to the optical reflectivity can be neglected; hence, the optical reflectivity is dominated by the temperature of the lattice (T) for all excitation frequencies.…”

Thermal Management Systems and Waste Heat Recycling by Thermoelectric Generators—An Overview

“…However, thermal conductivities can be modified via microstructural manipulation to increase the heat carriers scattering and reduce thermal conductivities. Significant efforts by the TE community over the last few decades have been concentrated on various techniques to improve thermoelectric properties, based on modifying S, σ, and κ [53][54][55][56][57], such as nanostructuring [58][59][60][61]. For instance, Figure 8 illustrates the impact of microstructure through changes in particle morphology on the thermoelectric properties, including electrical conductivity, Seebeck coefficients, lattice thermal conductivity, and power factor.…”