PHYSIK & ASTRONOMIE_Festkörperforschung An ice-cold experiment: The samples researchers at the Max Planck Institute for Chemical Physics of Solids are investigating are cooled in this dilution fridge to below minus 273 degrees Celsius. This is done by carefully lowering the instrument into a well-insulated cryostat. Photo: Sven Döring 48 MaxPlanckResearch 2 | 11 PHYSICS & ASTRONOMY_Superconductivity Superconductivity Is Pair Work Electric cables that routinely conduct electricity without loss – physicists have been motivated by this idea ever since superconductivity was discovered 100 years ago. Researchers working with Bernhard Keimer at the Max Planck Institute for Solid State Research in Stuttgart and Frank Steglich at the Max Planck Institute for Chemical Physics of Solids in Dresden want to gain a detailed understanding of how unconventional superconductors lose their resistivity. TEXT ROLAND WENGENMAYR pril 8, 1911. At the Univer- electrical resistivity of the mercury Although superconducting technolo- sity of Leiden in the Neth- drops abruptly to zero. And there it re- gy has become indispensable in basic erlands, two men sit in a mains. The researchers’ first reaction is research, it is encountered on a routine darkened booth. Only their that it must be an error. However, sub- basis only in magnetic resonance im- instruments tell them what sequent experiments show that they agers, because the complex cooling is A is happening in the cryostat next door. have discovered a new phenomenon. expensive. The solution would be su- A man in his late fifties with a striking In 1913, Kamerlingh Onnes called this perconductivity at room temperature, walrus mustache fiddles with this effect superconductivity. but such materials have remained a enormous thermos flask: Heike Kamer- dream. Since 1993 it has not been pos- lingh Onnes has been famous ever WAITING FOR SUPER CONDUCTORS sible to raise the temperature record since he became the first person to suc- FOR EVERYDAY USE any higher. It currently stands at mi- ceed in liquefying helium in 1908. nus 138 degrees Celsius (135 Kelvin) This enabled him to reach the ex- Superconductors have excited the and is held by a high-temperature ce- tremely low temperature of 4.2 Kelvin, imagination ever since they were dis- ramic superconductor with the com- which is around 4 degrees above abso- covered. Even Kamerlingh Onnes plicated name of HgBa2Ca2Cu3O8 (Hg: lute zero, at minus 273.2 degrees Cel- dreamed of transporting electricity in mercury, Ca: calcium, Ba: barium, Cu: sius. For this achievement, Kamerlingh power grids with no losses whatsoever. copper, O: oxygen). Onnes would be awarded the Nobel The discovery of so-called high-temper- Bernhard Keimer relates that compa- Prize for Physics in 1913. ature superconductors in 1986 at the ny representatives recently asked him The cryostat now contains a small IBM research laboratory near Zurich when superconductivity at room tem- glass tube filled with mercury. The created a great deal of euphoria. The perature could be expected. “They were Leiden-based physicists want to use it two men who made the discovery, Karl disappointed when I told them that no to observe, for the first time, how the Alex Müller and Johann Georg Bed- one had an answer yet,” he says, smil- electrical resistivity of metals behaves norz, were awarded the Nobel Prize for ing. The Director at the Max Planck In- when the temperature approaches ab- Physics as early as 1987. A more sober stitute for Solid State Research in Stutt- solute zero. At 4.2 Kelvin, minus 269 atmosphere has now returned, howev- gart and this year’s recipient of the degrees Celsius, something unexpected er, as has happened so often in the his- prestigious Leibniz Prize ought to know happens: the display indicating the tory of superconductors. – after all, he has been researching high- 2 | 11 MaxPlanckResearch 49 PHYSIK & ASTRONOMIE_Festkörperforschung 12 » Superconductors have excited the imagination ever since they were discovered. Even Kamerlingh Onnes dreamed of transporting electricity in power grids with no losses whatsoever. temperature superconductors for many This is also the motivation for the re- der if it helps to save energy. And the years. The scientist wants to obtain a de- search being undertaken by Frank Steg- atoms in crystals succeed in doing just tailed understanding of how these ma- lich, a Director at the Max Planck Insti- that, as they can share certain elec- terials lose their resistivity – in order to tute for Chemical Physics of Solids in trons among each other. These elec- create the basis on which everyday su- Dresden. The honor of having been trons provide the glue, the chemical perconductors can be developed. awarded the most valuable prize in Ger- bond, between the atoms. This works man science is not all that Steglich and only with the aid of quantum phys- Keimer have in common: both re- ics: since electrons, as quantum parti- searchers work in the field of “uncon- cles, are also spatially extended waves, ventional superconductors,” although they can bind neighboring atoms to Steglich’s team in Dresden works on so- each other. called heavy fermion systems, which In metals and semiconductors, how- become superconducting at very low ever, not all electrons act only as glue for temperatures. the crystal. Some of them escape from “In order to understand what un- the atoms and move with almost com- conventional superconductivity means, plete freedom through the lattice, but classical, conventional superconductiv- they require a certain energy to do this ity first needs to be explained,” Steglich in semiconductors. These free conduc- emphasizes. It was 1957 before the tion electrons carry the electric current. three Americans John Bardeen, Leon This loss leaves the “atomic cores” with Cooper and Robert Schrieffer managed a positive electric charge. to crack this tough nut. In 1972 they received the Nobel Prize for Physics for GENTLE VIBRATIONS BIND this work. The BCS theory – the name TWO ELECTRONS derives from their initials – can be un- derstood only by immersing oneself in “The conduction electrons can even Layers with zero resistivity: In the the physics of solids. move through a perfect crystal without superconductor, which goes by the name Like most solids, superconducting meeting any resistance,” explains Frank of YBCO and consists of yttrium (gray), barium (green), copper (red) and oxygen materials consist of crystals in which Steglich. This is due to the uniform ar- (blue), the Cooper pairs move along the the atoms organize themselves into a rangement of the atoms: if the distanc- copper oxide planes (pyramids). regular spatial lattice. Nature loves or- es between the peaks and troughs of the Photo: David Ausserhofer for the German Research Foundation (top, 2), graphic: MPI Solid State 50 MaxPlanckResearch 2 | 11 PHYSICS & ASTRONOMY_Superconductivity 1 To gain a better understanding of high-temperature superconductivity, Bernhard Keimer and his colleagues use the apparatus in the far right of the picture to produce thin crystalline layers with customized superconducting properties. 2 Daniel Pröpper and Bernhard Keimer observe how the cryostat in the foreground cools the sample of a high- temperature superconductor that loses its resistivity only at temperatures far below freezing point. electron wave match the distances be- binds it with the first electron, al- The properties of the Cooper pairs are tween the atoms, the electron can leap though the two particles violently re- radically different from those of the unhindered through the crystal. pel each other because they have the electrons. Pursuant to the rules of quan- But perfect crystals do not exist. Ev- same, negative charge. Only the large tum mechanics, electrons possess a ery crystal lattice has its flaws. They do temporal and thus also spatial separa- half-integer spin. They thus belong to not match the rhythm of the electron tion of up to one micrometer – a thou- the group of particles known as fermi- waves, which is why they literally rico- sandth of a millimeter – between the ons. These are quite acquisitive, each chet off them. And even if crystals with two allows the marriage in the soft lat- requiring a quantum state for it alone. perfectly ordered atoms did exist, the tice bed. For comparison: the distance In the Cooper pair, the two half-integer temperature would prevent the current between the atoms in the crystal lat- spins of the two electrons either sub- from flowing with zero resistivity, be- tice is around one thousand times tract to a total spin of zero or, more cause the higher it becomes, the stron- smaller, around one tenth of a nano- rarely, they also add up to one. ger and faster is the vibration of the meter – one nanometer is a millionth Quantum particles with integer crystal’s atomic lattice – like a mattress of a millimeter. spin, however, belong to the group of being jumped on with wild abandon. particles known as bosons. Bosons are This also interferes with the crystalline POWERFUL VIBRATIONS so sociable that they all like to con- perfection and hinders the wave of con- DESTROY THE COOPER PAIRS dense together into one quantum state duction electrons. if the heat energy in the system be- In conventional superconductors, This is how two free electrons form one comes low enough for this to happen. the vibrations of the atomic mattress be- Cooper pair, which is named after Leon For Cooper pairs, this is the supercon- come slow and gentle at low tempera- Cooper, one of the three fathers of BCS ducting state in which many Cooper tures – as happens in essentially all ma- theory. His idea provided the break- pairs permeate each other to form one terials. When a conduction electron through for the explanation of conven- “macroscopic” quantum state. This races through the lattice on its path, tional superconductivity in 1957.