The discovery in the early 1990s that C60 crystals could superconduct at temperatures of several tens of kelvins raised hopes that such “buckyballs” might rival the copper-oxide superconductors, whose T c’s go as high as 150 K under pressure. Those hopes have now been fulfilled.
The first C60 superconductors were made by incorporating alkali atoms into the spaces between the closely packed C60 spheres. These atoms acted as electron donors because their outer electrons were quickly stripped away by the buckyballs, which have a high affinity for electrons. Unfortunately, T c seemed to reach a maximum of 40 K, obtained by doping C60 with rubidium atoms. Last year, while working at Bell Labs, Lucent Technologies, Jan Hendrik Schön (also of the University of Konstanz, Germany), Christian Kloc, and Bertram Batlogg (now at ETH Zürich) took T c up to 52 K by finding a way to inject holes instead of electrons (see January 2001, page 15). Nice, but still far from the T c region reached by the cuprates.
The achievement of 52 K was only a taste of things to come, however. The same Bell Labs–ETH team noted at the time that the T c for electron-doped C60 increases with the size of the dopants: Larger dopants spread the C60 molecules farther apart and lead to higher values of T c, by as much as a factor of 3 for a small percentage change in lattice spacing: When the C60 molecules are farther apart, the energy bands grow narrower, so that the density of states—and concomitantly the T c—gets higher. Perhaps, the trio reasoned, the same trend would also govern hole-doped crystals.
There was certainly room to expand the C60 crystal. Unlike in electron doping, in which electrons are brought in by the intercalated alkali atoms, the holes are added to the crystal electronically. The T c of 52 K had been measured on a C60 crystal with excess holes but no intercalated atoms. For the new study, the researchers added neutral molecules to expand the crystal, hoping to realize the promised gain in T c.
They have now succeeded in their quest. 1 By intercalating molecules of tribromomethane (CHBr3) into the C60 lattice, the experimenters changed the cubic lattice constant from 14.16 Å to 14.43 Å and thereby raised T c to 117 K. As shown in the top panel of the figure, the higher the hole concentration, the higher the T c, up to around 3–3.5 holes per C60 molecule. At this doping level, the critical temperature increases from 52 K to about 80 K and 117 K with insertion of trichloromethane (CHCl3) and CHBr3, respectively, as seen in the bottom panel of the figure.
Electronic doping
The finding of T c = 117 K for hole-doped C60 is only the latest in a continuing series of feats that Schön, Kloc, and Batlogg have performed in the past two years. Thanks to a unique capability they have developed for adding charges to organic crystals by means of a field-effect transistor and for producing exceptionally high-quality crystals, Batlogg and his colleagues have seen both superconductivity and lasing in pentacene crystals, and have also demonstrated the quantum Hall effect there (see May 2000, page 23, and September 2000, page 17). Their capability, which several groups are reportedly trying to emulate, is yielding much new physics. Zachary Fisk of Florida State University says, “Their program is fantastic. They are creating electronic states you couldn’t otherwise produce.”
The method for injecting charges is shown in the inset in the figure. When a voltage is applied to the gate, charges of opposite sign accumulate on the interface between the insulating aluminum oxide layer and the single buckyball crystal. In this way, the experimenters can dope the crystal with either electrons or holes, depending on the sign of the gate voltage. The resistivity is measured by monitoring the current that flows from the source to the drain electrode. The experimenters must have crystals of exceptional purity; otherwise, the charge carriers can be trapped locally.
The magnitude of the gate voltage controls the degree of doping, and the doping determines the onset of superconductivity, as seen in the figure. There is considerable evidence that the charge carriers are all confined to a single layer of molecules at the surface of the crystal. So what’s seen is superconductivity in two dimensions.
Picking a winner
In searching for a compound that would expand the C60 crystal by just the right amount, the Bell Labs–ETH team drew on numerous studies of intercalated compounds, including previous measurements 2 of the lattice parameters of C60 intercalated with CHBr3 and CHCl3. Kloc had had experience slipping those molecules from solution into a C60 lattice. 3 Unlike the case of alkali metal atoms, in which the transfer of electrons results in ionic bonding between the intercalated atoms and the C60 lattice, the CHCl3 and CHBr3 molecules remain neutral; with them in place, the crystal barely holds together. Still, the experimenters hope to find another molecule that will expand the lattice spacing by another percent because just that small amount can increase T c to 150 K. If they do succeed, the results will be of interest for a limited range of applications: “You can’t make wires out of this stuff,” says Batlogg, “but the equivalent of thin-film electronics will be feasible.”
“The implications of this work are gigantic,” said Robert Cava of Princeton University. “They show us that the only thing that’s stood in the way of finding more 100-K superconductors is thermodynamics.” According to a prevailing view, he explained, the same interactions that give rise to conventional superconductivity can also break crystal structures apart or cause structural distortions that change the electronic states at the Fermi energy and thus kill the potential for superconductivity. Normally, compounds are made by heating the components together at high temperatures; that process gives the system the energy needed to overcome the activation barriers and reach its thermodynamic ground state, where distortions set in. But Batlogg and company have foiled thermodynamics by forming their compound, with the correct size and electron count, at low temperature. The same trick can be tried on other materials, Cava suggested.
Schön, Kloc, and Batlogg are now looking for superconductivity in both electron- and hole-doped C70. If an electron–phonon mechanism is responsible for superconductivity in these materials, one would anticipate a lower Tc for C70 because of the weaker electron–phonon coupling expected in those molecules. The next step would then be to turn to smaller molecules such as C36, where the coupling, and hence the T c, is expected to be even higher than in C60.
The new results on C60 will no doubt stimulate a renewed look at superconductivity in the fullerenes. Although an electron–phonon mechanism has been a leading candidate to explain electron pairing in these materials, the question has never been fully settled. What’s the role, for example, of the very high-frequency intramolecular vibrations of the buckyballs, or of the electron–electron interactions? For superconductivity governed strictly by electron–phonon interactions, some theorists predicted in the 1970s that T c would be limited to about 30 K. That assertion is clearly challenged by the 117-K result.
Onset of superconductivity in C60 crystals varies with the degree of hole doping and with the size of the intercalated molecules. As seen in the top panel, when tribromomethane (CHBr3) molecules sit between the C60 spheres, the critical temperature T c (where resistivity drops) gets higher for higher levels of hole doping. As seen in the bottom panel, T c = 52 K for a pure C60 crystal (green curve) and rises to 80 K and 117 K, respectively, when CHCl3 (red) and CHBr3 (blue) molecules slip into the lattice. The inset shows the field-effect transistor setup that electronically “dopes” charges into the single crystal.
Onset of superconductivity in C60 crystals varies with the degree of hole doping and with the size of the intercalated molecules. As seen in the top panel, when tribromomethane (CHBr3) molecules sit between the C60 spheres, the critical temperature T c (where resistivity drops) gets higher for higher levels of hole doping. As seen in the bottom panel, T c = 52 K for a pure C60 crystal (green curve) and rises to 80 K and 117 K, respectively, when CHCl3 (red) and CHBr3 (blue) molecules slip into the lattice. The inset shows the field-effect transistor setup that electronically “dopes” charges into the single crystal.
