Phonon Softening and Weak Temperature-dependent Lorenz Number for Bio-supported Ultra-thin Ir Film

TL;DR

This study investigates the temperature-dependent Lorenz number of bio-supported ultra-thin Ir film, revealing phonon softening, high residual resistivity, and a Lorenz number close to the Sommerfeld value with weak temperature dependence.

Contribution

It provides the first detailed analysis of the Lorenz number behavior in bio-supported ultra-thin Ir films, highlighting phonon softening and the applicability of the Wiedemann-Franz Law at low temperatures.

Findings

Large residual resistivity due to electron scattering

Significant reduction in thermal conductivity

Lorenz number remains close to Sommerfeld value

Abstract

This work reports on the first-time study of the temperature-dependent behavior of the Lorenz number of bio-supported average 3.2 nm-thin Ir film down to 10 K. Due to the strong imperfection-electron scattering, a very large residual resistivity is observed for the film that dominates the overall electron transport. The Debye temperature (221 K)of the film is found much smaller than that of bulk (308 K). This phonon softening strongly confirms the extensive surface and grain boundary electron scatterings. More than one order of magnitude reduction is observed for the thermal conductivity of the film. We find the Wiedemann-Franz Law still applies to our film even at low temperatures. The overall Lorenz number and that of imperfection structure are close to the Sommerfeld value and shows little temperature dependence. This is contrast to other studied low dimensional metallic structures that have a much larger Lorenz number. Electron tunneling and hopping in the biomaterial substrate are speculated responsible for the observed Lorenz number.