A group of scientists from Boston College have developed a novel metallic specimen whereby the motion of electrons flows in the same way water flows in a pipe, thereby changing from particle-like to fluid-like dynamics.
The team’s findings, which have been published in the journal Nature Communications, confirm theoretical predictions that certain metallic specimens could support an electron-phonon liquid phase.
Boston College Assistant Professor of Physics Fazel Tafti, alongside colleagues from the University of Texas at Dallas and Florida State University, has discovered that in the metal superconductor – a synthesis of Niobium and Germanium (NbGe2) – a strong interaction between electrons and phonons changes the transportation of electrons from the diffusive, or particle-like, to hydrodynamic, or fluid-like, movement.
Tafti explained that these novel findings mark the first detection of an electron-phonon liquid inside NbGe2.
“We wanted to test a recent prediction of the ‘electron-phonon fluid’,” Tafti commented, stating that phonons are the vibrations of a crystal structure. “Typically, electrons are scattered by phonons which leads to the usual diffusive motion of electrons in metals. A new theory shows that when electrons strongly interact with phonons, they will form a united electron-phonon liquid. This novel liquid will flow inside the metal exactly in the same way as water flows in a pipe.”
By confirming the hypothesis of theoreticians, the research findings of the experimental physicist Tafti, together with his Boston College colleague Professor of Physics Kenneth Burch, Luis Balicas of FSU, and Julia Chan of UT-Dallas, will facilitate further investigation of the material and its possible applications.
Tafti remarked that our day-to-day lives are dependent on the flow of water in pipes and electrons in wires. While these phenomena may sound similar, they are fundamentally different processes. Water molecules flow as a fluid continuum, not as individual molecules, complying with the laws of hydrodynamics. Meanwhile, electrons flow as individual particles and disperse inside metals while they are scattered by lattice vibrations.
The researchers’ novel inquiry, with significant contributions from graduate student researcher Hung-Yu Yang, concentrated on the conduction of electricity in the new metal, NbGe2.
The team employed three experimental techniques: electrical resistivity measurements exhibited a higher-than-expected mass for electrons; Raman scattering displayed a change of behaviour in the vibration of the NbGe2 crystal as a result of the special flow of electrons; and X-ray diffraction exposed the crystal structure of the material.
By utilising a specific method called ‘quantum oscillations’ to assess the mass of electrons in the material, the group discovered that the mass of electrons in all trajectories was triple the expected value, explained Tafti.
“This was truly surprising because we did not expect such ‘heavy electrons’ in a seemingly simple metal,” Tafti added. “Eventually, we understood that the strong electron-phonon interaction was responsible for the heavy electron behaviour. Because electrons interact with lattice vibrations, or phonons, strongly, they are ‘dragged’ by the lattice and it appears as if they have gained mass and become heavy.”
Going forward, the team’s mission is to find other materials in this hydrodynamic regime by leveraging the electron-phonon interactions. The group will also focus on controlling the hydrodynamic fluid of electrons in such materials and engineering new electronic devices.