Thermoelectric signatures of the electron-phonon fluid in PtSn4

TL;DR

This study provides evidence that PtSn4 exhibits hydrodynamic electron-phonon transport behavior, characterized by phonon drag effects and low thermal conductivity, confirmed through detailed resistivity, thermopower, and thermal measurements.

Contribution

It demonstrates the presence of hydrodynamic electron-phonon fluid behavior in PtSn4 via comprehensive transport measurements, highlighting the role of collective quasiparticle interactions.

Findings

Observation of a phonon drag peak near 14 K

Breakdown of the Lorenz ratio below the Sommerfeld value

Hierarchy of momentum-conserving and relaxing scattering times

Abstract

In most materials, transport can be described by the motion of distinct species of quasiparticles, such as electrons and phonons. Strong interactions between quasiparticles, however, can lead to collective behaviour, including the possibility of viscous hydrodynamic flow. In the case of electrons and phonons, an electron-phonon fluid is expected to exhibit strong phonon-drag transport signatures and an anomalously low thermal conductivity. The Dirac semi-metal PtSn4 has a very low resistivity at low temperatures and shows a pronounced phonon drag peak in the low temperature thermopower; it is therefore an excellent candidate for hosting a hydrodynamic electron-phonon fluid. Here we report measurements of the temperature and magnetic field dependence of the longitudinal and Hall electrical resistivities, the thermopower and the thermal conductivity of PtSn4. We confirm a phonon drag peak in the thermopower near 14 K and observe a concurrent breakdown of the Lorenz ratio below the Sommerfeld value. Both of these facts are expected for an electron-phonon fluid with a quasi-conserved total momentum. A hierarchy between momentum-conserving and momentum-relaxing scattering timescales is corroborated through measurements of the magnetic field dependence of the electrical and Hall resistivity and of the thermal conductivity. These results show that PtSn4 exhibits key features of hydrodynamic transport.