[Paper Review] Thermoelectric signatures of the electron-phonon fluid in PtSn4

Chenguang Fu, Thomas Scaffidi+14 more|arXiv (Cornell University)|Feb 26, 2018

Thermodynamic and Structural Properties of Metals and AlloysEngineering22 references16 citations

TL;DR

This study identifies hydrodynamic electron-phonon fluid behavior in the Dirac semimetal PtSn4 through thermoelectric and transport measurements. It demonstrates a phonon drag peak in thermopower near 14 K and a breakdown of the Lorenz ratio below the Sommerfeld value, confirming a hierarchy of momentum-conserving and momentum-relaxing scattering timescales, key signatures of viscous electron-phonon fluid dynamics.

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.

Motivation & Objective

• To investigate whether PtSn4 hosts a hydrodynamic electron-phonon fluid at low temperatures.
• To identify thermoelectric signatures of collective electron-phonon behavior, such as phonon drag and anomalous thermal transport.
• To determine the hierarchy between momentum-conserving and momentum-relaxing scattering processes in the material.
• To test the validity of hydrodynamic transport models in a Dirac semimetal with strong electron-phonon coupling.

Proposed method

• Measurement of longitudinal and Hall electrical resistivity as a function of temperature and magnetic field.
• Quantification of thermopower to detect phonon drag effects near 14 K.
• Analysis of thermal conductivity to assess momentum relaxation and deviations from the Wiedemann-Franz law.
• Evaluation of the Lorenz ratio's deviation from the Sommerfeld value to probe momentum-conserving electron-phonon scattering.
• Use of magnetic field dependence in resistivity and thermal conductivity to distinguish between momentum-conserving and momentum-relaxing scattering mechanisms.
• Comparison of experimental data with hydrodynamic transport models for electron-phonon fluids.

Experimental results

Research questions

• RQ1 Does PtSn4 exhibit phonon drag in its thermopower at low temperatures, indicating collective electron-phonon behavior?
• RQ2 Is there a breakdown of the Lorenz ratio below the Sommerfeld value, signaling momentum-conserving electron-phonon scattering?
• RQ3 What is the relative timescale hierarchy between momentum-conserving and momentum-relaxing scattering processes in PtSn4?
• RQ4 Can the magnetic field dependence of resistivity and thermal conductivity distinguish hydrodynamic transport from standard quasiparticle transport?
• RQ5 To what extent does PtSn4 exhibit hydrodynamic transport characteristics in its thermoelectric and thermal response?

Key findings

• A pronounced phonon drag peak in the thermopower is observed near 14 K, indicating strong electron-phonon coupling and collective fluid-like behavior.
• The Lorenz ratio is found to be anomalously low below the Sommerfeld value, confirming the presence of momentum-conserving electron-phonon scattering.
• The magnetic field dependence of longitudinal and Hall resistivity reveals a hierarchy between momentum-conserving and momentum-relaxing scattering timescales.
• Thermal conductivity measurements support the existence of a long-lived electron-phonon fluid with suppressed momentum relaxation.
• The combined transport and thermoelectric data provide robust evidence for hydrodynamic transport in PtSn4.
• The results establish PtSn4 as a material system hosting a hydrodynamic electron-phonon fluid at low temperatures.