Electrons flow through most materials more like a gas than a fluid, meaning
they don’t interact much with one another. It was long hypothesized that
electrons could flow like a fluid, but only recent advances in materials and
measurement techniques allowed these effects to be observed in 2D materials.
In 2020, the labs of Amir Yacoby, Professor of Physics and of Applied
Physics at the Harvard John A. Paulson School of Engineering and Applied
Sciences (SEAS), Philip Kim, Professor of Physics and Professor Applied
Physics at Harvard and Ronald Walsworth, formerly of the Department of
Physics at Harvard, were among the first to image electrons flowing in
graphene like water flows through a pipe.
The findings provided a new sandbox in which to explore electron
interactions and offered a new way to control electrons — but only in
two-dimensional materials. Electron hydrodynamics in three-dimensional
materials remained much more elusive because of a fundamental behavior of
electrons in conductors known as screening. When there is a high density of
electrons in a material, as in conducting metals, electrons are less
inclined to interact with one another.
Recent research suggested that hydrodynamic electron flow in 3D conductors
was possible, but exactly how it happened or how to observe it remained
unknown. Until now.
A team of researchers from Harvard and MIT developed a theory to explain how
hydrodynamic electron flow could occur in 3D materials and observed it for
the first time using a new imaging technique.
The research is published in Nature Physics.
“This research provides a promising avenue for the search for hydrodynamic
flow and prominent electron interactions in high-carrier-density materials,”
said Prineha Narang, Assistant Professor of Computational Materials Science
at SEAS and a senior author of the study.
Hydrodynamic electron flow relies on strong interactions between electrons,
just as water and other fluids rely on strong interactions between their
particles. In order to flow efficiently, electrons in high density materials
arrange themselves in such a way that limits interactions. It’s the same
reason that group dances like the electric slide don’t involve a lot of
interaction between dancers — with that many people, it's easier for
everyone to do their own moves.
“To date, hydrodynamic effects have mostly been deduced from transport
measurements, which effectively jumbles up the spatial signatures,” said
Yacoby. “Our work has charted a different path in observing this dance and
understanding hydrodynamics in systems beyond graphene with new quantum
probes of electron correlations.”
The researchers proposed that rather than direct interactions, electrons in
high density materials could interact with one another through the quantum
vibrations of the atomic lattice, known as phonons.
“We can think of the phonon-mediated interactions between electrons by
imagining two people jumping on a trampoline, who don’t propel each other
directly but rather via the elastic force of the springs,” said Yaxian Wang,
a postdoctoral scholar in the NarangLab at SEAS and co-author of the study.
In order to observe this mechanism, the researchers developed a new
cryogenic scanning probe based on the nitrogen-vacancy defect in diamond,
which imaged the local magnetic field of a current flow in a material called
layered semimetal tungsten ditelluride.
“Our tiny quantum sensor is sensitive to small changes in the local magnetic
field, allowing us to explore the magnetic structure in a material
directly,” said Uri Vool, John Harvard distinguished science fellow and
co-lead author of the study.
Not only did the researchers find evidence of hydrodynamic flow within
three-dimensional tungsten ditelluride but they also found that the
hydrodynamic character of the current strongly depends on the temperature.
“Hydrodynamic flow occurs in a narrow regime where temperature is not too
high and not too low, and so the unique ability to scan across a wide
temperature range was crucial to see the effect,” said Assaf Hamo, a
postdoctoral scholar at the Yacoby lab and co-lead author of the study.
“The ability to image and engineer these hydrodynamic flows in
three-dimensional conductors as a function of temperature, opens up the
possibility to achieve near dissipation-less electronics in nanoscale
devices, as well as provides new insights into understanding
electron-electron interactions,” said Georgios Varnavides, a Ph.D student in
the NarangLab at SEAS and one of the lead authors of the study.” The
research also paves the way for exploring non-classical fluid behavior in
hydrodynamic electron flow, such as steady-state vortices.”
“This is an exciting and interdisciplinary field synthesizing concepts from
condensed matter and materials science to computational hydrodynamics and
statistical physics,” said Narang. In previous research, Varnavides and
Narang classified different types of hydrodynamic behaviors which could
arise in quantum materials where electrons flow collectively.
Reference:
Uri Vool, Assaf Hamo, Georgios Varnavides, Yaxian Wang, Tony X. Zhou, Nitesh
Kumar, Yuliya Dovzhenko, Ziwei Qiu, Christina A. C. Garcia, Andrew T.
Pierce, Johannes Gooth, Polina Anikeeva, Claudia Felser, Prineha Narang,
Amir Yacoby. Imaging phonon-mediated hydrodynamic flow in WTe2. Nature
Physics, 2021;
DOI: 10.1038/s41567-021-01341-w
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Physics