Vortex dynamics in unconventional superconductors
Zusammenfassung der Projektergebnisse
The main objective of this project was to investigate the intrinsic pinning in superconductors which violate spontaneously other symmetries beside the U(1)-gauge symmetry. One class of compounds studied are the noncentrosymmetric superconductors (i.e. CePt3Si, Li2Pt3B) and superconductors which break time reversal symmetry (i.e PrOs4Sb12). It had been proposed by Sigrist and Agterberg (1999) that in superconductors which violate the time reversal symmetry, fractionalized vortices, caring arbitrary amounts of magnetic flux can exist on domain walls separating different degenerate superconducting phases. To move outside the domain walls, the fractionalized vortices would have to recombine to form an integer flux quanta and only then they can move in the superconductor. Therefore the domain walls act like very effective barriers against flux motion. If this scenario could be experimentally verified, this would represent the first case of intrinsic pinning of vortices on domain walls produced in a superconductor without external (material) defects such as impurities or dislocations. Indeed our results obtained during this project on the noncentrosymmetric superconductors CePt3Si and Li2Pt3B reveal extreme vortex pinning, stronger than in any other known superconductor. Counter intuitively the strong pinning it is not reflected in high critical currents. This confirms that in these compounds an unconventional pinning mechanism never observed before experimentally, is at work. This very effective pinning mechanism is possibly based on the existence of fractionalized vortices on twin boundaries which can host states with broken time reversal symmetry. The results obtained for PrOs4Sb12 which beaks time reversal symmetry reveal very strong pinning though the relaxation rates are faster than in the noncentrosymmetric compounds. Our results basically open a new research chapter in the vortex dynamics in superconductors without an inversion center and this is a path to pursue in the future, both experimentally and theoretically. The results were published in two papers so far. We also studied vortex dynamics in another unconventional superconductor, the heavy fermion compound CeCoIn5. This is the only known compound where an exotic state is believed to be formed inside the superconducting phase, at low temperatures and high magnetic fields; the Fulde-Ferrell-Larkin-Ovchinnikov state were the superconducting order parameter is modulates in real space. Our results reveal an additional phase transition inside the superconducting state at low temperatures and very low magnetic fields marked by a strong enhancement in the remnant magnetization. The nature of this transition is still unknown. One possible explanation is based on the change of the vortex lattice symmetry and theoretical investigations are underway in order to clarify this problem. Pending on the theoretical results our findings will be published. In addition to the initial objectives of the project and due to the new discoveries in condensed matter physics we conducted resistivity studies under hydrostatic pressure on the pnictide superconductor EuFe2As2. The occurrence of superconductivity in FeAs based compounds discovered in 2008 has created a huge impact among physicists working in the superconductivity field, impact comparable to the one produced by the discovery of high-Tc in 1986. Our results suggest a very peculiar and interesting interaction between the superconducting state and the magnetism of the rare-earth ions in EuFe2As2 under pressure. We observed reentrant superconductivity which has not been observed in the doped RFeAsO compounds and this makes EuFe2As2 unique among the layered FeAs systems. The results have been published. Another compound investigated is the nanotube system Na2−xV3O7. Our magnetic low temperature ac susceptibility, specific heat and remnant magnetization decay measurements reveal that the previously observed transition at 76 mK is spin-glass like with no long range order. The results have been published.
Projektbezogene Publikationen (Auswahl)
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Structural and magnetic aspects of the nanotube system Na2−xV3O7, Phys. Rev. B 78, 214426 (2008) – December 18, 2008
O. Zaharko, J. L. Gavilano, Th. Strässle, C. F. Miclea, A. C. Mota, Y. Filinchuk, D. Chernyshov, P. P. Deen, B. Rahaman, T. Saha-Dasgupta, R. Valentí, Y. Matsushita, A. Dönni, and H. Kitazawa
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Evidence for a reentrant superconducting state in EuFe2As2 under pressure, Phys. Rev. B 79, 212509 (2009) – June 15, 2009
C. F. Miclea, M. Nicklas, H. S. Jeevan, D. Kasinathan, Z. Hossain, H. Rosner, P. Gegenwart, C. Geibel, and F. Steglich
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Vortex avalanches in the noncentrosymmetric superconductor Li2Pt3B, Phys. Rev. B 80, 132502 (2009) – October 6, 2009
C. F. Miclea, A. C. Mota, M. Sigrist, F. Steglich, T. A. Sayles, B. J. Taylor, C. A. McElroy, and M. B. Maple
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Extreme vortex pinning in the noncentrosymmetric superconductor CePt3Si, Phys. Rev. B 81, 014527 (2010) – January 28, 2010
C. F. Miclea, A. C. Mota, M. Nicklas, R. Cardoso, F. Steglich, M. Sigrist, A. Prokofiev, and E. Bauer
