On a January afternoon in 1986, a young
scientist at the IBM Zürich Laboratory made a discovery that would
change the world of science forever, a moment that might be well
described by Stephen Jay Gould's term as a "punctuated
equilibrium"
event in the history of physics. Georg Bednorz had observed the
onset of superconductivity at a temperature near 31 degrees Kelvin
above absolute zero, a temperature almost one and a half times
higher than previously known and in a class of materials that
conventional wisdom would have said held little hope that this phenomenon
would have occurred at any temperature. That evening, the young
German physicist would later recall, that after many months of hard
work and failure, he "really enjoyed his daily glass of beer."
Twenty-two months later Bednorz, and his mentor and colleague, K.
Alex Mueller, received in record time, the 1987 Nobel Prize in
Physics.
The news of the Zürich discovery spread
slowly, and not without reason. Alex Mueller, a wizened
veteran of condensed matter materials research, well knew that the
history of superconductivity was replete with many instances of
irreproducible "sightings," some with "transitions" well above room
temperature! The two IBMers moved cautiously, submitting their
findings to the German journal, Zeitschrift für Physik, one of the
most prestigious scientific journals of the 20th century, a
manuscript whose title contained the enticing phrase "high
temperature superconductivity," and well known for its snail-like
pace of publication. By the time their paper appeared that
September, they had already confirmed their own discovery.
But no one noticed...yet.
Paul C. W. Chu, a Chinese-born American
physicist on the faculty of the University of Houson, with a nose
for sniffing out interesting and unusual happenings in our field,
ran across the Zeitschrift für Physik publication soon after it
appeared. Chu had been a student of the legendary Berndt
Mathias -- who along with his long-time co-worker Theodore Geballe,
together probably can lay claim to discovering more superconductors
than any other researchers, past, now and maybe future -- and
"inherited" his advisor's talent for "guided guessing" where new
superconducting materials might likely be mined. Paul's "Edisonian"
methods were controversial. However, those of us who knew him
well before our collective "15 minutes of fame," often would state
privately that if anyone were likely to "step in it," that is, to
"intelligently stumble" on a brilliant finding, it would be Paul
Chu.
Thus it was not a complete surprise when
rumors surfaced in late November, 1986 from Houston that in a
"certain sample," a resistive drop was observed beginning at 70 K.
The plot thickened throughout the next three months, and in early
February Paul Chu was pictured in Time Magazine holding a small
green wafer, reporting its resistance completely disappeared at 91
K. Unbelievable.
The months from December, 1986, to
February, 1987, were pure pandemonium. By January, the Bednorz-Mueller discovery had been well-substantiated in a number of
institutions (including my own), the atomic structure of their
original material established (by Bednorz), and purified to reach a
transition temperature of 40 K in the compound strontium (Sr)-doped
lanthanum-copper-oxide (La-214, to designate the ratio of the
constituent atoms). The principal institutional players were
IBM, Bell Labs, Bellcore, the US National Labs, the University of
Tokyo, and, of course, Houston. Theories abounded...at least
half a dozen...some of them promulgated by three past winners of the
Nobel Prize...and all problematic, to say the least (there is still
no universally accepted theory of "high temperature
superconductivity"). This period certainly deserves its own
story. And I'll get around to telling it sooner or later.
Focus now turned to whatever had Paul
found?...if anything! The bottom line is that by the
last week of February, everyone believed him...and was frantically
searching for the magic combination of elements, including Paul
himself...oddly enough, even though he indeed had discovered the
"holy grail," he did not yet know what it held (you can discover
something, yet not really know what it's made of...the structure of
DNA, a compound whose existence was known for decades, defied
decoding until Watson-Crick). Paul knew it was mixture of
yttrium, barium, copper and oxygen...but in what ratio and in what
atomic arrangement? Most felt it was some variation on the
"2-1-4" stoichiometry and structure, but they were wrong...very
wrong.
Fast
Forward -- Early Morning Hours of 4 March 1987
It’s
been 17 years, a few months and some days, as I write these
words, since the breaking dawn of March 4th, 1987,
when Robbie Beyers first saw, on the broken-down Stanford TEM
he’d used for his PhD thesis, where the cations were and
determined the unit cell symmetry and lattice parameters of
Y-123. He had laboriously separated and sectioned a
sub-millimeter black flake from samples we had made the previous
few days before, following Paul Chu’s multi-phased recipe that
appeared in Physical Review Letters on Friday, February 27th (we
successfully reproduced Paul’s results late that same Friday,
using dysprosium because we couldn’t find any yttrium oxide
until Saturday (we had a lot of rare earth oxides around because
of our magnetic storage program), and by Sunday we were making
yttrium samples (Mike Ramirez called me early Saturday morning
while I was trying to clear a clogged toilet in my condo...I was
surprised because Mike told me he was taking his fiancé to the
beach for the weekend, but had gone up to our lab instead...it's
amazing what nerds will do when our life gets interesting)
literally by the dozen with clean resistive transitions…but only
one percent superconducting by volume!. Recall in those days,
the principal phase in “Chu-stuff” was 2-1-1, whose structure
Robbie also determined that morning, and was responsible for the
then mysterious “green” metal appearance.
The model you see me
holding in the picture was built by Mike Ramirez and Jose
Vazquez later that week from TEM data provided by Robbie, x-ray
powder diffraction patterns from Grace Lim, and core-level
spectroscopy by Rick Savoy on samples prepared by Ed Engler and
Victor Lee. Superconductivity measurements on these
samples, which we now dubbed "1-2-3" after the Y-Ba-Cu cation
ratios, indicating superconducting volume well about 90%, were
performed by Stuart Parkin, Jose Vazquez and myself. To
the best of my knowledge, it is the first "PR photo" put out by
IBM or anyone else revealing the structure of YBa2Cu3Oy.
At the time we were unsure of the exact oxygen content.
Oxygen is a peculiarly pernicious element to detect. It is
practically invisible to electrons and x-rays, and core-level
photoemission spectroscopy can be misleading. From simple
chemical arguments, we knew "y" had to be between 6 and 9.
Six was ruled out because "1-2-3" would then have been an
insulator, and nine was also highly unlikely as it would have
required all the copper ions to be trivalent and the structure would
not have formed. Our best guess was that "y" was between 7
and 8.
Robbie had observed two
different crystal symmetries for "1-2-3," tetragonal and
orthorhombic, depending how long the sample had been exposed to
the probe electron beam in his TEM. It was almost certain
this behavior was due to the ingress and egress of trace oxygen
in the instrument vacuum, as the sample heated and cooled as the
probe was applied and removed. If you look closely at the
top plane of atom (the coppers are small and opaque, the oxygens
are large and clear), you'll see the four oxygens there are
"half-shaded" compared to the rest of the structure
suggesting...and it turned out rightly...that it was here that
the oxygen concentration, and thus superconducting properties
are affected on annealing in oxygen. However, at the time,
we felt the trivalent Y ion would be surrounded by its own
additional set of oxygen ions to provide charge balance, and so
the optimum overall oxygen concentration for the occurrence of
superconductivity would be between 7.5 and 8. As the
core-level spectroscopy data improved after this photo was
taken, it became clear that the layer of oxygen in the yttrium
plane was absent, and this range was 6.5 to 7 (presently the
evidence is that the highest transition temperature in yttrium
1-2-3 occurs at 92 K when y = 6.95). Subsequently, this
middle layer of oxygens were removed, and later photographs of
the model (the structure, not me!), and there were many, contain
this modification.
Today Michael and
Jose's historic ball-and-stick creation sits in my home office,
destined, hopefully, to become either a family heirloom or a
museum piece somewhere. Every time I gaze at it, fond
memories flow back of the halcyon days of IBM Almaden in the
spring of 1987 and my "band of brothers...and a sister
Grace," and how
we became the "mouse that roared" amongst other institutions, as
first to find the atomic structure and optimal processing
conditions for the original material found
superconducting above the boiling point of liquid nitrogen.
