Fourth signature of the superconducting transition in cuprates
The results cap 15 years of detective work aimed at understanding how
these materials transition into a superconducting state where they can conduct electricity with no loss.

Date:
January 26, 2022
Source:
DOE/SLAC National Accelerator Laboratory
Summary:
Superconductors have four classic traits, including conducting
electric current without loss and levitating magnets. Now the
discovery of the fourth and final trait caps 15 years of detective
work.



FULL STORY ==========================================================================
When an exciting and unconventional new class of superconducting materials
was discovered 35 years ago, researchers cheered.


==========================================================================
Like other superconductors, these materials, known as copper oxides or cuprates, conducted electricity with no resistance or loss when chilled
below a certain point -- but at much higher temperatures than scientists
had thought possible. This raised hopes of getting them to work at close
to room temperature for perfectly efficient power lines and other uses.

Research quickly confirmed that they showed two more classic traits of
the transition to a superconducting state: As superconductivity developed,
the material expelled magnetic fields, so that a magnet placed on a chunk
of the material would levitate above the surface. And its heat capacity --
the amount of heat needed to raise their temperature by a given amount -- showed a distinctive anomaly at the transition.

But despite decades of effort with a variety of experimental tools,
the fourth signature, which can be seen only on a microscopic scale,
remained elusive: the way electrons pair up and condense into a sort
of electron soup as the material transitions from its normal state to
a superconducting state.

Now a research team at the Department of Energy's SLAC National
Accelerator Laboratory and Stanford University has finally revealed
that fourth signature with precise, high-resolution measurements made
with angle-resolved photoemission spectroscopy, or ARPES, which uses
light to eject electrons from the material. Measuring the energy and
momentum of those ejected electrons reveals how the electrons inside
the material behave.

In a paper published today in Nature, the team confirmed that the
cuprate material they studied, known as Bi2212, made the transition
to a superconducting state in two distinct steps and at very different temperatures.



==========================================================================
"Now we know what happens at the superconducting transition in very
fine detail, and we can think about how to make that happen at higher temperatures," said Sudi Chen, who led the study while a PhD student
at Stanford. "That's a very practical direction." Stanford Professor
Zhi-Xun Shen, an investigator with the Stanford Institute for Materials
and Energy Sciences (SIMES) at SLAC who supervised the research, said,
"This is the climax of 15 years of scientific detective work in trying
to understand the electronic structure of these materials, and it
provides the missing link for a holistic picture of unconventional superconductivity. We knew these materials should produce distinctive spectroscopic signatures as the paired electrons coalesce into a quantum condensate; the amazing thing is that it took so long to find it." Unconventional transitions In conventional superconductors, which were discovered in 1911, electrons overcome their mutual repulsion and form
what are known as Cooper pairs, which immediately condense into a sort
of electron soup that allows electrical current to travel unimpeded.

But in the unconventional cuprates, scientists have speculated that
electrons pair up at one temperature but don't condense until they're
cooled to a significantly lower temperature; only at that point does
the material become superconducting.



========================================================================== While the details of this transition had been explored with other
methods, until now it had not been confirmed with microscopic probes
like photoemission spectroscopy that study how matter absorbs light and
emits electrons. It's an important independent measure of how electrons
in the material behave.

Shen started his scientific career at Stanford just as the discovery of
the new cuprate superconductors was coming to light, and he has devoted
more than three decades to unraveling their secrets and improving
photoemission spectroscopy as a tool for doing that.

For this study, cuprate samples made by collaborators in Japan were
examined at two ARPES setups -- one in Shen's Stanford laboratory,
equipped with an ultraviolet laser, and the other at SLAC's Stanford Synchrotron Radiation Lightsource (SSRL) with the help of SLAC staff
scientists and longtime collaborators Makoto Hashimoto and Donghui Lu.

Peeling a physics onion "Recent improvements in the overall performance of those instruments were an important factor in obtaining these high-quality results," Hashimoto said.

"They allowed us to measure the energy of the ejected electrons with
more precision, stability and consistency." Lu added, "It's very
challenging to get a full understanding of the physics of high-temperature superconductivity. Experimentalists use different tools to probe different aspects of this hard problem, and this provides deeper insights."
Shen said the long-term study of these unconventional materials has
been like peeling layers from an onion to reveal the surprising and
interesting physics within.

Now, he said, confirming that the transition to superconductivity occurs
in two separate steps "gives us two knobs we can tune to get the materials
to superconduct at higher temperatures." Sudi Chen is now a postdoctoral fellow at the University of California, Berkeley. Researchers from the
National Institute of Advanced Industrial Science and Technology in Japan,
the Lorentz Institute for Theoretical Physics at Leiden University
in the Netherlands, and DOE's Lawrence Berkeley National Laboratory
also contributed to this work, which was funded by the DOE Office of
Science. SSRL is a DOE Office of Science user facility.

========================================================================== Story Source: Materials provided by
DOE/SLAC_National_Accelerator_Laboratory. Original written by Glennda
Chui. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Four_classic_signatures_of_a_superconductor ========================================================================== Journal Reference:
1. Su-Di Chen, Makoto Hashimoto, Yu He, Dongjoon Song, Jun-Feng He,
Ying-Fei
Li, Shigeyuki Ishida, Hiroshi Eisaki, Jan Zaanen,
Thomas P. Devereaux, Dung-Hai Lee, Dong-Hui Lu, Zhi-Xun
Shen. Unconventional spectral signature of Tc in a pure
d-wave superconductor. Nature, 2022; 601 (7894): 562 DOI:
10.1038/s41586-021-04251-2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220126122419.htm

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