This work tips the balance for
the phase shift theory and will certainly channel further work. On top of all that, it was recently announced
that we are now able to approach room temperature. With some luck we may have a better theory to
work with out of all this that allows prediction in the realm of higher
temperatures.
The fact remains that we today
have a palette of working materials and working engineering protocols all
combining to bring superconducting into mainstream industrial
applications. It may not be throw away
consumer electronics yet but we will certainly take it.
This is more good news and
pertinent to the development of Magnetic Field Exclusion Vessels which will
soon be a priority engineering target..
High-temperature superconductor spills secret: A new phase of matter
An unprecedented three-pronged study has found that one type of
high-temperature superconductor may exhibit a new phase of matter. As in all
superconductors, electrons pair off (bottom) to conduct electricity with no
resistance when the material is cooled below a certain temperature. But in this
particular copper-based superconductor, many of the electrons in the material
don’t pair off; instead they form a distinct, elusive order (orange plumes)
that had not been seen before. Scientists at SLAC National Accelerator
Laboratory, Stanford University , Lawrence Berkeley National Laboratory and institutions in Japan and Thailand
(PhysOrg.com) -- Scientists from the U.S. Department of Energy's
Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of
California at Berkeley have joined with researchers at Stanford University and
the SLAC National Accelerator Laboratory to mount a three-pronged attack on one
of the most obstinate puzzles in materials sciences: what is the pseudogap?
A collaboration organized by Zhi-Xun Shen, a member of the Stanford
Institute for Materials and Energy Science (SIMES) at SLAC and a professor of
physics at Stanford University, used three complementary experimental
approaches to investigate a single material, the high-temperature
superconductor Pb-Bi2201 (lead bismuth strontium lanthanum copper-oxide). Their
results are the strongest evidence yet that the pseudogap phase, a mysterious
electronic state peculiar to high-temperature superconductors, is not a gradual
transition to superconductivity in these materials, as many have long believed.
It is in fact a distinct
phase of matter.
"This is a paradigm shift in the way we understand
high-temperature superconductivity," says Ruihua He, lead author with
Makoto Hashimoto of the paper in the March 25 issue of the
journal Science that describes the team's findings. "The
involvement of an additional phase, once fully understood, might open up new
possibilities for achieving superconductivity at even higher temperatures in
these materials." When the research was done Hashimoto and He were members
of SIMES, of Stanford's Department of Applied Physics, and of Berkeley
The pseudogap mystery
Superconductivity is the total absence of resistance to the flow of
electric current. Discovered in 1911, it was long thought to occur only in
metals and only below a critical temperature (Tc) not far above absolute zero.
"Ordinary" superconductivity commonly takes place at 30 kelvins (30
K) or less, equivalent to more than 400 degrees below zero Fahrenheit. Awkward
as reaching such low temperatures may be, ordinary superconductivity is widely
exploited in industry, health, and science.
High-Tc superconductors were discovered in 1986. "High" is a
relative term; the highest-Tc superconductors function at temperatures five
times higher than ordinary superconductors but still only about twice that of
liquid nitrogen. Many high-Tc superconductors have been found, but the record
holders for critical temperature remain the kind first discovered, the cuprates
— brittle oxides whose structure includes layers of copper and oxygen atoms
where current flows.
In this phase diagram common to many cuprate superconductors, the
insulating phase typical of undoped cuprate compounds appears at the far left
(black). Other phases appear with increased hole doping -- the dome-shaped
superconducting phase below Tc (blue), the mysterious pseudogap below T* (red),
and a “normal metallic” phase (white). New evidence from studies of Bi2201
(crystal structure inset) along the temperature range shown in greeen strongly
supports the idea that the pseudogap is in fact a distinct phase of matter that
persists into the superconducting phase. If so the T* phase transition must
terminate in a quantum critical point (Xc) at zero temperature. Credit: Ruihua
He, Lawrence Berkeley
In all known superconductors electrons join in pairs (Cooper pairs)
to move in correlated fashion through the material. It takes a certain amount
of energy to break Cooper pairs apart; in ordinary superconductors, the absence
of single-electron states below this energy constitutes a superconducting gap,
which vanishes when the temperature rises above Tc. Once in the normal state
the electrons revert to unpaired, uncorrelated behavior.
Not so for cuprate superconductors. A similar superconducting gap
exists below Tc, but when superconductivity ceases at Tc the gap doesn't close.
A "pseudogap" persists and doesn't go away until the material reaches
a higher temperature, designated T* (T-star). The existence of a pseudogap in
the normal state is itself anything but normal; its nature has been heatedly
debated ever since it was identified in cuprates more than 15 years ago.
Attempts to explain what's going on in the pseudogap have coalesced
around two main schools of thought. Traditional thinking holds that the
pseudogap represents a foreshadowing of the superconducting phase. As the temperature
is lowered, first reaching T*, a few electron pairs start to form, but they are
sparse and lack the long-range coherence necessary for superconductivity — they
can't "talk" to one another. As the temperature continues to fall,
more such pairs are formed until, upon reaching Tc, virtually all conducting
electrons are paired and act in correlation; they're all talking. In this
scheme, there's only a single phase transition, which occurs at Tc.
Another school of thought argues that the appearance of the pseudogap
at T* is also a true phase transition. The pseudogap does not represent a
smooth shift to the superconducting state but is itself a state distinct from
both superconductivity and normal "metallicity" (the usual state of
delocalized, uncorrelated electrons). This new phase implies the existence of a
"quantum critical point" — a point along a line at zero temperature
where competing phases meet. In theory, with competing phases wildly
fluctuating in the neighborhood of a quantum critical point, there may be
entirely new routes to superconductivity.
"Promising as the 'quantum critical' paradigm is for explaining a
wide range of exotic materials, high-Tc superconductivity in cuprates has
stubbornly refused to fit the mold," says Joseph Orenstein of Berkeley
Lab's Materials Sciences Division, a professor in physics at UC Berkeley, whose
group conducted one of the research team's three experiments. "For 20
years, the cuprates managed to conceal any evidence of a phase-transition line
where the quantum critical point is supposed to be found."
In recent years, however, hints have emerged. "New ultrasensitive
probes have found fingerprints of phase transitions in high-Tc materials,"
Orenstein says, "although there's been no smoking gun. The burning question
is whether we can discover the nature of the new phase or phases."
A multipronged attack on the pseudogap
In the Stanford-Berkeley study, three groups of researchers joined
forces to probe the pseudogap phase on the same sample.
"Pb-Bi2201 was chosen because, first, it is structurally simple,
and second, it has a relatively wide temperature range between Tc and T*,"
says Ruihua He. "This permits a clean separation of any remnant effect of
superconductivity from genuine pseudogap physics."
Groups led by Z.-X. Shen at beamline 5‑4 of the Stanford Synchrotron
Radiation Lightsource (SSRL) at SLAC and by Zahid Hussain, ALS Division Deputy
for Scientific Support, at beamline 10.0.1 of Berkeley Lab's ALS, studied the
sample with angle-resolved photoemission spectroscopy (ARPES). In ARPES, a beam
of x-rays directed at the sample surface excites the emission of valence
electrons. By monitoring the kinetic energy and momentum of the emitted
electrons over a wide temperature range the researchers map out the material's
low-energy electronic band structure, which determines much of its electrical
and magnetic properties.
At Stanford, researchers led by Aharon Kapitulnik of SIMES, a professor
in applied physics at Stanford
University
Finally, Orenstein's group at Berkeley
All these experimental techniques had previously pointed to the
possibility of a phase transition in the neighborhood of T* in different
cuprate materials. But no single result was strong enough to stand alone.
ARPES experiments performed in 2010 by the same group of experimenters
as in the present study revealed the abrupt opening of the pseudogap at T* in
Pb-Bi2201. Variations in T* in different materials and even different samples,
as well as in the surface conditions to which ARPES is sensitive, had left room
for uncertainty, however.
In 2008, the Kerr effect was measured in another cuprate, also by the
same group as in the present study, and showed a change in magnetization from
zero to finite across T*. This was long-sought thermodynamic evidence for the
existence of a phase transition at T*. But compared to the pronounced spectral
change seen by ARPES, the extreme weakness of the Kerr-effect signal left doubt
that the two results were connected.
Finally, since the late 1990s various experiments with time-resolved
reflectivity in different cuprates have reported signals setting in near T* and
increasing in strength as the temperature drops, until interrupted by the onset
of a separate signal below Tc. The probe is complex and there was a lack of
corroborating evidence for the same cuprates; the results did not receive wide
attention.
Now the three experimental approaches have all been applied to the same
material. All yielded consistent results and all point to the same conclusion: there
is a phase transition at the pseudogap phase boundary – the three techniques
put it precisely at T*. The electronic states dominating the pseudogap phase do
not include Cooper pairs, but nevertheless intrude into the lower-lying
superconducting phase and directly influence the motion of Cooper pairs in a
way previously overlooked.
"Instead of pairing up, the electrons in the pseudogap phase
organize themselves in some very different way," says He. "We
currently don't know what exactly it is, and we don't know whether it helps
superconductivity or hurts it. But we know the direction to take to move
forward."
Says Orenstein, "Coming to grips with a new picture is a little
like trying to steer the Titanic, but the fact that all three of these
techniques point in the same direction adds to the mounting evidence for the
phase change."
Hussain says the critical factor was bringing the Stanford and Berkeley
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