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discovered 02_2012

discovered 02.12 FOCUS WWW.Hzdr.DE want a female“, describes the theoretician. If only a few men are looking for a female, very little happens because the well- connected pairs don’t allow themselves to be influenced by the few troublemakers. If the surplus of males increases too dramatically however, then the system rapidly breaks down and the pairs separate from each other. Similarly, a weak magnetic field produces a smaller surplus of electrons with a certain spin in a superconductor that does not yet disturb the Cooper pairs. However, if the critical Zeeman energy is exceeded, then the surplus will grow too rapidly and the superconductivity breaks down. “Even under this scenario however one can reduce the influence of the troublemakers for a while by forcing them into a corner“, the Dresden physicist continues. From this corner they are less able to disturb the pairs – irrespectively of whether we are dealing with humans or electrons. Since the pairs try to keep away from the corner with the isolated ones they tend to be in certain places, as can sometimes be observed very well on a dance floor. With a superconductor it is the same concept: the density of the Cooper pairs varies from place to place. Sometimes, theoreticians need to have a lot of patience This “electron motion“ does not however take place on the dance floor but under the laws of quantum mechanics. Also the “corner“ is not in the “real space“, but in the “momentum space“, both of which are difficult to imagine for physicists let alone for non-physicists. “That doesn’t matter, the basic principle is still the same“, Peter Fulde reassures us. Even with this very vivid explanation, the FFLO effect under the influence of strong magnetic fields was still only a theory. Numerous attempts had been made to support the theory with laboratory experiments. “For this, however, very clean superconductors are essential that preferentially are in the form of very thin layers“ explains Joachim Wosnitza – and naturally an outstanding magnetic field technology such as the one at the HZDR. None of the earlier experiments were able to meet all of these prerequisites and Peter Fulde had to wait for almost half a century until Joachim Wosnitza (the scientist with whom he had often discussed this theory) from the Helmholtz center in Dresden came up with the crucial evidence that the FFLO effect that had been named after Fulde (among others) did actually exist in practice. Strong magnetic fields and thin layers “We tested the theory using organic superconductors, the layers of which at 1.8 nanometers are only a little thicker than a millionth of a millimeter“, reports Joachim Wosnitza. By comparison a human hair is twenty thousand times thicker. These organic superconductors consist of many thousands of organic layers approx. one nanometer in thickness and similarly thin layers of insulating molecules. When HZDR researchers placed these superconductors absolutely in parallel with a magnetic field with the strength of approx. eleven tesla, the Zeeman energy had clearly been exceeded. Nevertheless the material was still showing superconductivity. Did this mean that the long sought after FFLO effect had finally been proven in practice? Did this mean that the magnetic field had actually relegated the troublemakers for the Cooper pairs to certain corners? When pairs are no longer in sync If the answer is yes, then the effect should disappear when the researchers slightly tilt the superconductor towards the magnetic field. In this case the magnetic field is able to penetrate through the superconducting layers and interrupt the swinging Cooper pairs that now risk losing their rhythm and the troublemakers resist being pushed into a corner. If the researchers swivel the superconductor by 0.1 degrees with respect to the magnetic field, the superconductivity is maintained at first. Even after rotating it by 0.2 or 0.4 degrees not much would happen. By contrast, if the sample was turned by half a degree, the superconductivity would disappear completely. The troublemakers would have the upper hand, just as FFLO had predicted. After almost half a century the theory could finally be proven in practice. For fundamental physics however this experiment has tremendous meaning because the FFLO effect cannot only be observed for human pairs or electrons. Similar pairs are also formed with other elementary particles such as quarks, but also with neutrons and protons, which atomic nuclei are made up of, and also with certain ultra-cold atomic gases. In the meantime the FFLO superconducting state has also been found with ultra-cold lithium atoms. “As a component in superconducting electronics this kind of state has a very practical significance“, Peter Fulde suggests. Joachim Wosnitza and Peter Fulde, the two Dresden physicists, have started a new chapter in the history of physics, which is waiting to be completed by other researchers. LITERATURE R. Beyer, B. Bergk et al.: “Angle-dependent evolution of the Fulde-Ferrell-Larkin-Ovchinnikov state in an organic superconductor”, in Physical Review Letters, vol. 109 (2012), p. 027003 (DOI: 10.1103/PhysRevLett.109.027003) CONTACT _Dresden High Magnetic Field Laboratory at HZDR Prof. Joachim Wosnitza _Max Planck Institute for the Physics of Complex Systems Prof. Peter Fulde Contact: Anett Pacholik