A Rice University-led study is forcing physicists to rethink
superconductivity in uranium ditelluride, an A-list material in the
worldwide race to create fault-tolerant quantum computers.
Uranium ditelluride crystals are believed to host a rare "spin-triplet" form
of superconductivity, but puzzling experimental results published this week
in Nature have upended the leading explanation of how the state of matter
could arise in the material. Neutron-scattering experiments by physicists
from Rice, Oak Ridge National Laboratory, the University of California, San
Diego and the National High Magnetic Field Laboratory at Florida State
University revealed telltale signs of antiferromagnetic spin fluctuations
that were coupled to superconductivity in uranium ditelluride.
Spin-triplet superconductivity has not been observed in a solid-state
material, but physicists have long suspected it arises from an ordered state
that is ferromagnetic. The race to find spin-triplet materials has heated up
in recent years due to their potential for hosting elusive quasiparticles
called Majorana fermions that could be used to make error-free quantum
computers.
"People have spent billions of dollars trying to search for them," Rice
study co-author Pengcheng Dai said of Majorana fermions, hypothetical
quasiparticles that could be used to make topological quantum bits free from
the problematic decoherence that plagues qubits in today's quantum
computers.
"The promise is that if you have a spin-triplet superconductor, it can
potentially be used to make topological qubits," said Dai, a professor of
physics and astronomy and member of the Rice Quantum Initiative. "You can't
do that with spin-singlet superconductors. So, that's why people are
extremely interested in this."
Superconductivity happens when electrons form pairs and move as one, like
couples spinning across a dance floor. Electrons naturally loathe one
another, but their tendency to avoid other electrons can be overcome by
their inherent desire for a low-energy existence. If pairing allows
electrons to achieve a more sloth-like state than they could achieve on
their own—something that's only possible at extremely cold temperatures—they
can be coaxed into pairs.
The coaxing comes in the form of fluctuations in their physical environment.
In normal superconductors, like lead, the fluctuations are vibrations in the
atomic lattice of lead atoms inside the superconducting wire. Physicists
have yet to identify the fluctuations that bring about unconventional
superconductivity in materials like uranium ditelluride. But decades of
study have found phase changes—watershed moments where electrons
spontaneously rearrange themselves—at the critical points where pairing
begins.
In the equations of quantum mechanics, these spontaneous ordered
arrangements are represented by terms known as order parameters. The name
spin triplet refers to the spontaneous breakdown of three symmetries in
these ordered arrangements. For example, electrons spin constantly, like
tiny bar magnets. One order parameter relates to their spin axis (think
north pole), which points up or down. Ferromagnetic order is when all spins
point the same direction, and antiferromagnetic order is when they alternate
in an up-down, up-down arrangement. In the only confirmed spin-triplet,
superfluid helium-3, the order parameter has no fewer than 18 components.
"All other superconductivity is spin singlet," said Dai, who's also a member
of Rice's Center for Quantum Materials (RCQM). "In a spin singlet, you have
one spin up and one spin down, and if you put a magnetic field on, it can
easily destroy superconductivity."
That's because the magnetic field pushes spins to align in the same
direction. The stronger the field, the stronger the push.
"The problem with uranium ditelluride is the field required to destroy
superconductivity is 40 Tesla," Dai said. "That's huge. For 40 years, people
thought the only possibility for that to occur is that when you put a field
on, the spins are already aligned in one direction, meaning it's a
ferromagnet."
In the study, Dai and Rice postdoctoral research associate Chunruo Duan, the
study's lead author, worked with Florida State co-author Ryan Baumbach,
whose lab grew the single crystal samples of uranium ditelluride used in the
experiment, and UC San Diego co-author Brian Maple, whose lab tested and
prepared the samples for neutron-scattering experiments at Oak Ridge's
Spallation Neutron Source.
"What the neutron does is come in with a particular energy and momentum, and
it can flip the Cooper pair spins from an up-up state to an up-down state,"
Dai said. "It tells you how the pairs are formed. From this neutron spin
resonance, one can basically determine the electron pairing energy" and
other telltale properties of the quantum mechanical wave function that
describes the pair, he said.
Dai said there are two possible explanations for the result: either uranium
ditelluride is not a spin-triplet superconductor, or spin-triplet
superconductivity arises from antiferromagnetic spin fluctuations in a way
that physicists haven't previously imagined. Dai said decades of
experimental evidence points to the latter, but this appears to violate
conventional wisdom about superconductivity. So Dai teamed up with Rice
colleague Qimiao Si, a theoretical physicist who specializes in emergent
quantum phenomena like unconventional superconductivity.
Si, a study co-author, has spent much of the past five years showing a
theory of multiorbital pairing he co-developed with former Ph.D. student
Emilian Nica explains contradictory experimental findings in several kinds
of unconventional superconductors, including heavy fermions, the class that
includes uranium ditelluride.
In multiorbital pairing, electrons in some atomic shells are more likely to
form pairs than others. Si recalled thinking that uranium had the potential
to contribute paired electrons from any of seven orbitals with 14 possible
states.
"Multiorbitals was the first thing that came to mind," he said. "It wouldn't
be possible if you only had one band or one orbital, but orbitals bring a
new dimension to possible unconventional superconductor pairings. They're
like a palette of colors. The colors are the internal quantum numbers, and
the f electrons in the uranium-based, heavy-fermion materials are naturally
set up to have these colors. They lead to new possibilities that go beyond
the 'periodic table of pairing states.' One of these new possibilities turns
out to be spin-triplet pairing."
Si and Nica, who's now at Arizona State University, showed antiferromagnetic
correlations could give rise to plausible, low-energy, spin-triplet pairing
states.
"Spin-triplet pairing states are highly improbable in the vast majority of
cases because pairs will form as spin-singlets in order to lower their
energy," Si said. "In uranium ditelluride, spin-orbit coupling can change
the energy landscape in a way that makes spin-triplet pairing states more
competitive with their spin-singlet counterparts."
Si is the Harry C. and Olga K. Wiess Professor in Rice's Department of
Physics and Astronomy and director of RCQM. Additional co-authors include
Andrey Podlesnyak of Oak Ridge and Yuhang Deng, Camilla Moir and Alexander
Breindel of UC San Diego.
Reference:
Pengcheng Dai, Resonance from antiferromagnetic spin fluctuations for
superconductivity in UTe2, Nature (2021).
DOI: 10.1038/s41586-021-04151-5
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