50 Scientific American, October 2019trons, which create positively charged ions) form a crystal lat-
tice—a structure with regular spacing—plus a sea of free elec-
trons that, when a voltage is applied, flow through the lattice to
form an electric current. Typically lattice imperfections and
vibrations resulting from heat impede this flow and create resis-
tance. According to BCS theory, however, electrons can foil this
friction with a feat of quantum aikido that turns lattice motions
to their advantage. First, as an electron moves through the lattice
it bends the lattice’s atoms in its direction of travel (because of
the attraction between its negative charge and the lattice’s posi-
tive charge). This bending bunches positive charges together, and
the resulting concentration of positive charge pulls a second elec-
tron into the first’s wake, bonding the two into a so-called Cooper
pair. Second, those pairs, acting more like waves than particles
now, overlap, synchronize and coalesce into one big wave called a
Bose-Einstein condensate that is too large to be impeded by the
lattice and so flows through it without any resistance at all.
BCS theory has led to many successful predictions, including
the so-called critical temperatures above which superconductors
lose their superpowers. Nevertheless, it has generally been of lit-
tle help in the search for new superconductors with higher criti-
cal temperatures. In fact, the most successful superconductor
hunter in history was an experimentalist named Bernd Matthias
who deemed BCS irrelevant to his pursuit. Matthias discovered
hundreds of superconductors (many of which were metal alloys)
between the 1950s and the 1970s by testing countless materials in
his lab, guided largely by five empirical rules relating to material
properties (for example, “high symmetry is good”) and one over-
arching principle: “Stay away from theorists.”
But despite Matthias’s many conquests, the highest critical
temperature seen in a superconductor rose only slightly, from 17
to 23 kelvins, between 1955 and 1973. And there it stayed until
1986, when Georg Bednorz and Alex Müller, two IBM scientists
in Zurich, discovered superconductivity in a class of complex
layered ceramics called cuprates. These materials still hold the
record for high temperature at ambient pressure that they set
in 1993: 135 kelvins. Unlike Matthias, Bednorz and Müller “had
a very robust theoretical view about what they were looking for,”
says physicist Peter Littlewood of the University of Chicago.
“Now those ideas are probably wrong.”
Wrong because they were based on BCS theory and the way
it invokes atomic lattice vibrations, or phonons, to create Coo-
per pairs. Although such pairs, and the Bose-Einstein conden-
sate they form, are believed to underlie the cuprates’ supercon-
ductivity, many experts today believe the Cooper bonds in
cuprates depend on some form of direct electromagnetic inter-
action between the electrons instead of, or at least in addition
to, phonons. Alas, those direct interactions are so difficult to
model mathematically that more than three decades of inten-
sive research have failed to yield an equivalent to BCS theory for
the cuprates or even to create a consensus on the details of the
electrons’ pairing mechanism. Scientists lump cuprates into a
catchall category with several other classes of superconductors
whose success seems to depend on various types of direct elec-
tron-to-electron interactions. These materials are called uncon-
ventional superconductors to distinguish them from the con-
ventional, phonon-driven kind described by BCS.
So Bednorz and Müller found what they were looking for,
but it did not work the way they thought it would. Yet that is
superconductivity’s serendipitous way. For example, in 2006
scientists stumbled on iron-based superconductors—another
unconventional class that lacks a theory to describe or predict
it—while doing research to improve flat-panel displays. “Almost
invariably, some new weird material is discovered,” Littlewood
says, “and that then teaches us about a new mechanism [for
electron pairing] that we hadn’t thought about.”THE TEMPERATURE BARRIER
Superconductivity favorS a chill, says Michael Norman, a materi-
als scientist at Argonne, because “temperature is just bad” for sus-
taining wavelike quantum behavior at useful, macroscopic scales.
The energy of heat tends to break up the bonds in Cooper pairs and
disrupt the coordinated quantum state of a wavelike condensate.
The number of pairs in a condensate and the strength of the
bonds holding them together provide a barrier to thermal dis-
ruption. A superconductor’s critical temperature represents the
height of this barrier—above this point it cannot withstand the
heat. (The high barriers of the cuprates, for example, are
thought to result from the way their direct electron-to-electron
interactions create stronger Cooper pair bonds than those that
come from the indirect mechanism of phonons.)
And yet “I don’t think anybody now doubts that there is a pos-
sibility for a room-temperature superconductor at ambient pres-
sure,” says Norman, partly because of the way new superconduc-
tors and pairing mechanisms keep cropping up. Even for conven-
tional superconductors, there is “no fundamental limit” to
critical temperature, says Igor Mazin, a physicist at the Naval
Research Laboratory in Washington, D.C. Instead, he says, thereMADDURY SOMAYAZULU has spent decades trying to create
superconductors that can operate at warm temperatures.