From Andrea Bianchi

Site: UnconventionalSuperconductivity

The origin of superconductivity in the high-Tc cuprates and in heavy-fermion materials is one of today’s most intriguing phenomena in condensed matter physics. Superconducting phases in these materials are thought to be close to quantum critical points arising from competing quantum phases. However, the relationship between emergent superconductivity and strong fluctuations associated with the critical point is not clear. In the heavy fermions magnetic fluctuations lead to a renormalization of the mass of the conduction electrons at low temperatures instead of the magnetic order as could be guessed from the magnetic behaviour at high temperatures. At even lower temperatures, these now “heavy” electrons form superconducting pairs suggesting that the binding of the Cooper pairs is magnetically mediated. Of all these systems, CeCoIn5 has proven to be the cleanest and one of the most interesting ever studied.

First order superconducting phase transition

While studying the superconducting phase transition in the heavy fermion superconductor CeCoIn5 with magnetic fields H close to Hc2, by exploring the (H,T)-phase diagram using specific heat and magnetocaloric measurements, I discovered that the transition from the normal to the superconducting state becomes first order at high fields (1). This has been predicted in the 1960’s for a Pauli limited clean type-II superconductor. However, this is the first time that such a behavior has been observed in a bulk sample. In addition, at even higher fields

(H-T)-phase diagram of CeCoIn5

In addition, at even higher fields and lower temperatures, I found a second anomaly inside the superconducting phase diagram due to the formation of a Fulde-Ferrell-Larkin-Ovchinnikov state (2,3). This spatially inhomogeneous superconducting state is of fundamental interest to the condensed matter community and is currently actively studied in quantum gases. The FFLO state is also of great interest to the high energy physics community, as the same mechanism could explain the phase slips in the rotational period of quasars. This study would not have been possible without the high quality samples that were grown out of an excess indium flux.

Field dependence of the form factor

Studying the vortex lattice in CeCoIn5 by small angle neutron scattering (4), an effort undertaken in collaboration with colleagues from PSI, Notre Dame, and Birmingham, we found that the magnetic contrast seen by the neutrons gets stronger with increasing field, almost all the way up to the upper critical field of Hc2. This behaviour is unprecedented, as the neutron scattering form factor should decrease roughly exponentially with increasing field, as the magnetic contrast is determined by the two characteristic lengths of a type II superconductor: The magnetic penetration depth and the superconducting coherence length.

  1. A. Bianchi, R. Movshovich, N. Oeschler, P. Gegenwart, F. Steglich, J. D. Thompson, P. G. Pagliuso, and J. L. Sarrao, “First order superconducting phase transition in CeCoIn5”, Phys. Rev. Lett. 89, 137002 (2002).
  2. R. Movshovich, A. Bianchi, C. Capan, M. Jaime, and R. G. Goodrich, “Electron-spin domains - Magnetic enhancement of superconductivity”, Nature 427, 802 (2004).
  3. A. Bianchi, R. Movshovich, C. Capan, P.G. Pagliuso, and J. L. Sarrao, “Possible Fulde - Ferrell-Larkin-Ovchinnikov superconducting state in CeCoIn5”, Phys. Rev. Lett. 91, 187004 (2003).
  4. A. D. Bianchi, M. Kenzelmann, L. DeBeer-Schmitt, Jo. S. White, E. M. Forgan, J. Mesot, M. Zolliker, J. Kohlbrecher, R. Movshovich, E. D. Bauer, J. L. Sarrao, Z. Fisk, C. Petrovic, and M. Ring Eskildsen, "Superconducting Vortices in CeCoIn5: Toward the Pauli-Limiting Field", Science 319, 177 (2008).
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