Peterborough, N.H. -- A joint project between IBM's T.J. Watson Research Center and a group at the University of Twente (Enschede, Netherlands) has made observations of high-critical-temperature superconductors that confirm the exotic nature of the electron behavior producing the effect. The new data makes it clear that the electron-pairing mechanism in the high-Tc material yttrium-barium-copper-oxide is essentially different than that of low-temperature superconductors such as niobium or lead.
While not offering the vital clue that will lead physicists out of the wilderness of proposed theories, the new results will at least eliminate some candidates, and may lead to even more definitive experiments, said Chang Tsuei, who led the IBM project.
"People say that there are as many models of high-Tc superconductors as there are theorists. The situation is quite unsettled," said Tsuei, who was assisted in the work by John Kirtley. "However, we do know something about high-temperature superconduction. We know that it results from a condensate of Cooper pairs--spin-up and spin-down pairing--and a very important thing is to understand what is the symmetry of such pairs."
Tsuei and his colleagues at IBM Research have spent the last 10 years performing experiments designed to distinguish between the two possible types of symmetry, known as d-wave and s-wave pairing. "We ... have done a series of experiments that settles this decade-long s-wave vs. d-wave debate in favor of predominately d-wave symmetry," Tsuei said.
"One of the questions that people asked right after the first discoveries was: Are these exotic materials? Is the physics exotic--is the explanation going to be something that is completely unexpected?" said Tom Theis, director of physical sciences at IBM Research. "Now it is generally acknowledged that yes, these are very exotic materials, and we just don't understand them."
It has now been 20 years since Alex Muller and Georg Bednorz at IBM's Zurich research center reported superconduction in a rare-earth ceramic at the unprecedented temperature of 35 Kelvin, but no satisfactory explanation of why electrical resistance disappears at such relatively high temperatures has emerged. In the meantime, experimental physicists have pushed the temperature at which materials can conduct electricity at zero resistance to 150 K. There seems to be little progress in finding a model that explains the phenomenon.
However, it took more than 40 years from the time that Kamerlingh Onnes at the University of Leiden discovered the existence of superconductivity in lead wires in 1911 to the publication in 1955 of the definitive explanation, the Bardeen-Cooper-Schieffer model, named after the three physicists who proposed it. One reason for the long delay was the need to invent and develop quantum mechanics as a context for understanding the phenomenon.
The experiment at IBM, which was published recently in the journal Nature Physics, eliminates any doubt about how electrons pair up in high-temperature superconductors, which should help to winnow out competing models.
The researchers defined superconducting rings where one segment of the ring is YBCO and the remaining segment is niobium. The niobium overlaps the YBCO at the two boundaries, forming two tunnel junctions. If the two superconductors have the same s-wave pairing symmetry, then theory predicts that the magnetic flux through the ring will be quantized in integer increments. That would also be the case if the ring were formed from a single s-wave superconductor.
If YBCO has a predominantly d-wave pairing, then the ring system will experience what physicists call "frustration"--although both pairing states are always present, one state or the other always predominates. "In this case, what nature does is very clever; it generates a very small supercurrent to compensate. That current produces a half-integer magnetic flux," Tsuei explained.
That was the original insight that led to a series of experiments establishing the d-wave symmetry of electron pairs in YBCO. All that was needed was to measure the magnetic flux of the composite superconducting ring and verify that it was quantized in half-integer increments. Experi- ments performed at IBM Research in 1994 by Tsuei and his collaborators found the predicted half-integer quantum flux in specially designed YBCO/niobium rings.
"This was the fundamental principle pioneered by Chang that allowed his group to be the first to perform fundamental experiments that definitively showed the existence of d-wave symmetry," said Theis.
Asymmetry in lattice
There is a complicating factor with high-Tc materials, however. Low-temperature superconductors are formed from a single element. Lead atoms bind with each other in a simple repetitive pattern that has a high degree of symmetry. But a compound like YBCO is composed of four elements and has a complex, layered structure where copper and oxygen atoms bond together to form planes that are separated by yttrium and barium atoms. Consequently, superconducting properties are highly dependent on the direction of the lattice.
Because of that asymmetry in the crystal lattice, measurements at one fixed orientation do not give a complete picture of the pairing symmetry. The recent experiments were designed to test the previous results at a large number of lattice orientations, in order to eliminate suspicion that the specialized d-wave results might simply be a result of a particular orientation.
A ring of YBCO can be cut at different angles, measured from the center of the ring, to expose a face at different lattice orientations. In a series of experiments, 72 YBCO/niobium rings were fabricated where one of the interfaces between the materials was cut at angles changing in 5° increments. A second series used angles at 0.5° increments. The University of Twente group, led by Hans Hilgenkamp, was instrumental in carrying out the fabrication of these rings, which Tsuei said was a very difficult feat.
"What Chang and his colleagues found was that while the magnitude of the pairing did change with the crystal orientation of the interface between the YBCO and niobium, the change in magnitude corresponded closely to what would be predicted by d-wave pairing," said Theis. "That is expected to eliminate any lingering doubts about the nature of electron pairing in YBCO.
"I'm always fascinated by the connections between basic research, where these samples were developed to understand a fundamental new physical phenomenon, and the fact that people are now publishing papers where these kinds of topologically frustrated junctions are being thought about as a potential element in a quantum computer," Theis said.
Tsuei added that d-wave symmetry in quantum-computing devices would greatly simplify the engineering, since no magnetic bias would have to be applied to maintain coherence. Recent papers on the kind of Josephson s that the IBM group has been studying have revealed macroscopic quantum tunneling, which would lead to an inherently stable device for representing quantum bits of information.
"Room-temperature superconductors are a dream of everyone working in this field. What we need is a viable theory to guide the search," said Tsuei, who noted that with the ability to fabricate structures at the nanoscale, it might be possible to create artificial materials that support resistanceless conduction at room temperature, even if they don't exist in nature. If a complete theory of high-critical-temperature superconductors existed at this point, materials scientists could go to work discovering the crystal structures that support them or experimenting with promising nanoscale structures. "You can take your cue from these high-Tc superconductors, they are all layers--copper/oxygen layers," he said.
IBM looked at the possibility of replacing interconnect in computing systems with high-Tc superconductors but concluded that the advantages would not be great enough to justify developing the concept at the time. But much has changed since then, said Theis. "Now we are being limited by two things that would make a huge difference: not only the wiring, but also the power dissipation in the entire system," he said. "There would great potential for new devices that would have greatly reduced power consumption, if we could get room-temperature superconductors."