A new discovery led by Princeton University could upend our understanding of
how electrons behave under extreme conditions in quantum materials. The
finding provides experimental evidence that this familiar building block of
matter behaves as if it is made of two particles: one particle that gives
the electron its negative charge and another that supplies its magnet-like
property, known as spin.
“We think this is the first hard evidence of spin-charge separation,” said
Nai Phuan Ong, Princeton’s Eugene Higgins Professor of Physics and senior
author on the paper published this week in the journal Nature Physics.
The experimental results fulfill a prediction made decades ago to explain
one of the most mind-bending states of matter, the quantum spin liquid. In
all materials, the spin of an electron can point either up or down. In the
familiar magnet, all of the spins uniformly point in one direction
throughout the sample when the temperature drops below a critical
temperature.
However, in spin liquid materials, the spins are unable to establish a
uniform pattern even when cooled very close to absolute zero. Instead, the
spins are constantly changing in a tightly coordinated, entangled
choreography. The result is one of the most entangled quantum states ever
conceived, a state of great interest to researchers in the growing field of
quantum computing.
To describe this behavior mathematically, Nobel prize-winning Princeton
physicist Philip Anderson (1923-2020), who first predicted the existence of
spin liquids in 1973, proposed an explanation: in the quantum regime an
electron may be regarded as composed of two particles, one bearing the
electron’s negative charge and the other containing its spin. Anderson
called the spin-containing particle a spinon.
In this new study, the team searched for signs of the spinon in a spin
liquid composed of ruthenium and chlorine atoms. At temperatures a fraction
of a Kelvin above absolute zero (or roughly -452 degrees Fahrenheit) and in
the presence of a high magnetic field, ruthenium chloride crystals enter the
spin liquid state.
Graduate student Peter Czajka and Tong Gao, Ph.D. 2020, connected three
highly sensitive thermometers to the crystal sitting in a bath maintained at
temperatures close to absolute zero degrees Kelvin. They then applied the
magnetic field and a small amount of heat to one crystal edge to measure its
thermal conductivity, a quantity that expresses how well it conducts a heat
current. If spinons were present, they should appear as an oscillating
pattern in a graph of the thermal conductivity versus magnetic field.
The oscillating signal they were searching for was tiny — just a few
hundredths of a degree change — so the measurements demanded an
extraordinarily precise control of the sample temperature as well as careful
calibrations of the thermometers in the strong magnetic field.
The team used the purest crystals available, ones grown at the U.S.
Department of Energy’s Oak Ridge National Laboratory (ORNL) under the
leadership of David Mandrus, materials science professor at the University
of Tennessee-Knoxville, and Stephen Nagler, corporate research fellow in
ORNL’s Neutron Scattering Division. The ORNL team has extensively studied
the quantum spin liquid properties of ruthenium chloride.
In a series of experiments conducted over nearly three years, Czajka and Gao
detected temperature oscillations consistent with spinons with increasingly
higher resolution, providing evidence that the electron is composed of two
particles consistent with Anderson’s prediction.
“People have been searching for this signature for four decades,” Ong said,
“If this finding and the spinon interpretation are validated, it would
significantly advance the field of quantum spin liquids.”
Czajka and Gao spent last summer confirming the experiments while under
COVID restrictions that required them to wear masks and maintain social
distancing.
“From the purely experimental side,” Czajka said, “it was exciting to see
results that in effect break the rules that you learn in elementary physics
classes.”
Reference:
“Oscillations of the thermal conductivity in the spin-liquid state of
α-RuCl3” by Peter Czajka, Tong Gao, Max Hirschberger, Paula Lampen-Kelley,
Arnab Banerjee, Jiaqiang Yan, David G. Mandrus, Stephen E. Nagler and N. P.
Ong, 13 May 2021, Nature Physics.
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