Effect of piezoelectric substrate on phonon-drag thermopower in monolayer graphene

TL;DR

This study investigates how a piezoelectric substrate influences phonon-drag thermopower in monolayer graphene, revealing significant effects at low temperatures and proposing new relations involving mobility and electron concentration.

Contribution

It provides the first detailed analysis of piezoelectric substrate effects on phonon-drag thermopower in graphene, including temperature and electron density dependencies and new theoretical relations.

Findings

S_g can reach ~20 μV at 10 K in large graphene samples

Crossover between piezoelectric and deformation potential phonon contributions occurs around 5 K

New relation S_g μ_p ~ n_s^0 predicted, highlighting screening effects

Abstract

The phonon-drag thermopower is studied in monolayer graphene on a piezoelectric substrate. The phonon-drag contribution S^g_PA from the extrinsic potential of piezoelectric surface acoustic (PA) phonons of a piezoelectric substrate (GaAs) is calculated as a function of temperature T and electron concentration n_s. At very low temperature, S^g_PA is found to be much greater than S^g_DA of the intrinsic deformation potential of acoustic (DA) phonons of the graphene. There is a crossover of S^g_PA and S^g_DA at around ~5 K. In graphene samples of about >10 um size, we predict S_g ~20 uV at 10 K, which is much greater than the diffusion component of the thermopower and can be experimentally observed. In the Bloch-Gruneisen (BG) regime T and n_s dependence are, respectively, given by the power laws S^g_PA (S^g_DA) ~ T^2(T^3) and S^g_PA, S^g_DA ~ n_s^(-1/2). The T (n_s) dependence is the manifestation of the two-dimensional phonons (Dirac phase of the electrons). The effect of the screening is discussed. Analogous to Herring's law (S_g mu_p ~T^-1), we predict a new relation S_g mu_p ~n_s^0, where mu_p is the phonon-limited mobility. We suggest that n_s dependent measurements will play a more significant role in identifying the Dirac phase and the effect of screening.