This work tips the balance forthe phase shift theory and will certainly channel further work. On top of all that, it was recently announcedthat we are now able to approach room temperature. With some luck we may have a better theory towork with out of all this that allows prediction in the realm of highertemperatures.
The fact remains that we todayhave a palette of working materials and working engineering protocols allcombining to bring superconducting into mainstream industrialapplications. It may not be throw awayconsumer electronics yet but we will certainly take it.
This is more good news andpertinent to the development of Magnetic Field Exclusion Vessels which willsoon 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 ofhigh-temperature superconductor may exhibit a new phase of matter. As in allsuperconductors, electrons pair off (bottom) to conduct electricity with noresistance when the material is cooled below a certain temperature. But in thisparticular copper-based superconductor, many of the electrons in the materialdon’t pair off; instead they form a distinct, elusive order (orange plumes)that had not been seen before. Scientists at SLAC National AcceleratorLaboratory, Stanford University , Lawrence Berkeley National Laboratory and institutions in Japan and Thailand
(PhysOrg.com) -- Scientists from the U.S. Department of Energy'sLawrence Berkeley National Laboratory (Berkeley Lab) and the University ofCalifornia at Berkeley have joined with researchers at Stanford University andthe SLAC National Accelerator Laboratory to mount a three-pronged attack on oneof the most obstinate puzzles in materials sciences: what is the pseudogap?
A collaboration organized by Zhi-Xun Shen, a member of the StanfordInstitute for Materials and Energy Science (SIMES) at SLAC and a professor ofphysics at Stanford University, used three complementary experimentalapproaches to investigate a single material, the high-temperaturesuperconductor Pb-Bi2201 (lead bismuth strontium lanthanum copper-oxide). Theirresults are the strongest evidence yet that the pseudogap phase, a mysteriouselectronic state peculiar to high-temperature superconductors, is not a gradualtransition to superconductivity in these materials, as many have long believed.
It is in fact a distinctphase of matter.
"This is a paradigm shift in the way we understandhigh-temperature superconductivity," says Ruihua He, lead author withMakoto Hashimoto of the paper in the March 25 issue of thejournal Science that describes the team's findings. "Theinvolvement of an additional phase, once fully understood, might open up newpossibilities for achieving superconductivity at even higher temperatures inthese materials." When the research was done Hashimoto and He were membersof SIMES, of Stanford's Department of Applied Physics, and of Berkeley
The pseudogap mystery
Superconductivity is the total absence of resistance to the flow ofelectric current. Discovered in 1911, it was long thought to occur only inmetals and only below a critical temperature (Tc) not far above absolute zero."Ordinary" superconductivity commonly takes place at 30 kelvins (30K) or less, equivalent to more than 400 degrees below zero Fahrenheit. Awkwardas reaching such low temperatures may be, ordinary superconductivity is widelyexploited in industry, health, and science.
High-Tc superconductors were discovered in 1986. "High" is arelative term; the highest-Tc superconductors function at temperatures fivetimes higher than ordinary superconductors but still only about twice that ofliquid nitrogen. Many high-Tc superconductors have been found, but the recordholders for critical temperature remain the kind first discovered, the cuprates— brittle oxides whose structure includes layers of copper and oxygen atomswhere current flows.
In this phase diagram common to many cuprate superconductors, theinsulating phase typical of undoped cuprate compounds appears at the far left(black). Other phases appear with increased hole doping -- the dome-shapedsuperconducting 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 stronglysupports the idea that the pseudogap is in fact a distinct phase of matter thatpersists into the superconducting phase. If so the T* phase transition mustterminate in a quantum critical point (Xc) at zero temperature. Credit: RuihuaHe, Lawrence Berkeley
In all known superconductors electrons join in pairs (Cooper pairs)to move in correlated fashion through the material. It takes a certain amountof energy to break Cooper pairs apart; in ordinary superconductors, the absenceof single-electron states below this energy constitutes a superconducting gap,which vanishes when the temperature rises above Tc. Once in the normal statethe electrons revert to unpaired, uncorrelated behavior.
Not so for cuprate superconductors. A similar superconducting gapexists below Tc, but when superconductivity ceases at Tc the gap doesn't close.A "pseudogap" persists and doesn't go away until the material reachesa higher temperature, designated T* (T-star). The existence of a pseudogap inthe normal state is itself anything but normal; its nature has been heatedlydebated ever since it was identified in cuprates more than 15 years ago.
Attempts to explain what's going on in the pseudogap have coalescedaround two main schools of thought. Traditional thinking holds that thepseudogap represents a foreshadowing of the superconducting phase. As the temperatureis lowered, first reaching T*, a few electron pairs start to form, but they aresparse and lack the long-range coherence necessary for superconductivity — theycan't "talk" to one another. As the temperature continues to fall,more such pairs are formed until, upon reaching Tc, virtually all conductingelectrons are paired and act in correlation; they're all talking. In thisscheme, there's only a single phase transition, which occurs at Tc.
Another school of thought argues that the appearance of the pseudogapat T* is also a true phase transition. The pseudogap does not represent asmooth shift to the superconducting state but is itself a state distinct fromboth superconductivity and normal "metallicity" (the usual state ofdelocalized, uncorrelated electrons). This new phase implies the existence of a"quantum critical point" — a point along a line at zero temperaturewhere competing phases meet. In theory, with competing phases wildlyfluctuating in the neighborhood of a quantum critical point, there may beentirely new routes to superconductivity.
"Promising as the 'quantum critical' paradigm is for explaining awide range of exotic materials, high-Tc superconductivity in cuprates hasstubbornly refused to fit the mold," says Joseph Orenstein of BerkeleyLab's Materials Sciences Division, a professor in physics at UC Berkeley, whosegroup conducted one of the research team's three experiments. "For 20years, the cuprates managed to conceal any evidence of a phase-transition linewhere the quantum critical point is supposed to be found."
In recent years, however, hints have emerged. "New ultrasensitiveprobes have found fingerprints of phase transitions in high-Tc materials,"Orenstein says, "although there's been no smoking gun. The burning questionis 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 joinedforces 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 ofsuperconductivity from genuine pseudogap physics."
Groups led by Z.-X. Shen at beamline 5‑4 of the Stanford SynchrotronRadiation Lightsource (SSRL) at SLAC and by Zahid Hussain, ALS Division Deputyfor Scientific Support, at beamline 10.0.1 of Berkeley Lab's ALS, studied thesample with angle-resolved photoemission spectroscopy (ARPES). In ARPES, a beamof x-rays directed at the sample surface excites the emission of valenceelectrons. By monitoring the kinetic energy and momentum of the emittedelectrons over a wide temperature range the researchers map out the material'slow-energy electronic band structure, which determines much of its electricaland magnetic properties.
At Stanford, researchers led by Aharon Kapitulnik of SIMES, a professorin applied physics at Stanford University
Finally, Orenstein's group at Berkeley
All these experimental techniques had previously pointed to thepossibility of a phase transition in the neighborhood of T* in differentcuprate materials. But no single result was strong enough to stand alone.
ARPES experiments performed in 2010 by the same group of experimentersas in the present study revealed the abrupt opening of the pseudogap at T* inPb-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 roomfor uncertainty, however.
In 2008, the Kerr effect was measured in another cuprate, also by thesame group as in the present study, and showed a change in magnetization fromzero to finite across T*. This was long-sought thermodynamic evidence for theexistence of a phase transition at T*. But compared to the pronounced spectralchange seen by ARPES, the extreme weakness of the Kerr-effect signal left doubtthat the two results were connected.
Finally, since the late 1990s various experiments with time-resolvedreflectivity in different cuprates have reported signals setting in near T* andincreasing in strength as the temperature drops, until interrupted by the onsetof a separate signal below Tc. The probe is complex and there was a lack ofcorroborating evidence for the same cuprates; the results did not receive wideattention.
Now the three experimental approaches have all been applied to the samematerial. All yielded consistent results and all point to the same conclusion: thereis a phase transition at the pseudogap phase boundary – the three techniquesput it precisely at T*. The electronic states dominating the pseudogap phase donot include Cooper pairs, but nevertheless intrude into the lower-lyingsuperconducting phase and directly influence the motion of Cooper pairs in away previously overlooked.
"Instead of pairing up, the electrons in the pseudogap phaseorganize themselves in some very different way," says He. "Wecurrently don't know what exactly it is, and we don't know whether it helpssuperconductivity or hurts it. But we know the direction to take to moveforward."
Says Orenstein, "Coming to grips with a new picture is a littlelike trying to steer the Titanic, but the fact that all three of thesetechniques point in the same direction adds to the mounting evidence for thephase change."
Hussain says the critical factor was bringing the Stanford and Berkeley
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