The findings confirm theoretical predictions that certain metallic specimens
could support an electron-phonon liquid phase.
A team of researchers from Boston College has created a new metallic
specimen where the motion of electrons flows in the same way water flows in
a pipe — fundamentally changing from particle-like to fluid-like dynamics,
the team reports in Nature Communications.
Working with colleagues from the University of Texas at Dallas and Florida
State University, Boston College Assistant Professor of Physics Fazel Tafti
found in the metal superconductor, a synthesis of Niobium and Germanium
(NbGe2), that a strong interaction between electrons and phonons alters the
transport of electrons from the diffusive, or particle-like, to
hydrodynamic, or fluid-like, regime.
The findings mark the first discovery of an electron-phonon liquid inside
NbGe2, Tafti said.
“We wanted to test a recent prediction of the ‘electron-phonon fluid’,”
Tafti said, noting 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 predictions of theoreticians, the experimental physicist
Tafti — working with his Boston College colleague Professor of Physics
Kenneth Burch, Luis Balicas of FSU, and Julia Chan of UT-Dallas — says the
discovery will spur further exploration of the material and its potential
applications.
Tafti noted that our daily lives depend on the flow of water in pipes and
electrons in wires. As similar as they may sound, the two phenomena are
fundamentally different. Water molecules flow as a fluid continuum, not as
individual molecules, obeying the laws of hydrodynamics. Electrons, however,
flow as individual particles and diffuse inside metals as they get scattered
by lattice vibrations.
The team’s investigation, with significant contributions from graduate
student researcher Hung-Yu Yang, who earned his doctorate from BC in 2021,
focused on the conduction of electricity in the new metal, NbGe2, Tafti
said.
They applied three experimental methods: electrical resistivity measurements
showed a higher-than-expected mass for electrons; Raman scattering showed a
change of behavior in the vibration of the NbGe2 crystal due to the special
flow of electrons; and X-ray diffraction revealed the crystal structure of
the material.
By using a specific technique known as the “quantum oscillations” to
evaluate the mass of electrons in the material, the researchers found that
the mass of electrons in all trajectories was three times larger than the
expected value, said Tafti, whose work is supported by the National Science
Foundation.
“This was truly surprising because we did not expect such ‘heavy electrons’
in a seemingly simple metal,” Tafti said. “Eventually, we understood that
the strong electron-phonon interaction was responsible for the heavy
electron behavior. 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.”
Tafti said the next step is to find other materials in this hydrodynamic
regime by leveraging the electron-phonon interactions. His team will also
focus on controlling the hydrodynamic fluid of electrons in such materials
and engineering new electronic devices.
Reference:
Evidence of a coupled electron-phonon liquid in NbGe2 6 September 2021,
Nature Communications.
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