a b c d

1 m

m

Nonlocal effect and microscopic parameters in superconductors

Vladimir Kozhevnikov, Tulsa Community College, DMR 0904157

Magneto-optical images of the In sample in the intermediate state. (T, K; HII, H Oea, (2.9, 0, -7); b, (1.7, 150, 33); c, (1.7, 180, 30); d, (1.7, 196, 0.5)

The most long-standing problem of superconductivity is a problem of the intermediate superconducting state (IS) in type-I superconductors. The IS takes place in all type-I samples other than long rods or films in a parallel magnetic field. Contrary to the Meissner state where the entire sample is in the superconducting state, samples in the IS are split for macroscopic superconducting and normal domains, which shape, size and spacing between are determined by microscopic parameters and depend on the sample shape/size and orientation of the magnetic field. Strikingly enough, in spite of the huge contribution into understanding of superconductivity made by studies of the IS over their nearly eighty years history, an adequate model of the IS was still missing. Unsolved fundamental problems also included the magnetic filed in the normal (N) domains and the critical field for the IS-normal transition. The IS provides access to one of the most fundamental parameters, namely the Pippard coherence length 0 (the size of Cooper pairs). However, a verified recipe to extract 0 from the IS parameters was also missing. Having for the first time directly measured values of Pippard coherence length (see highlights 2010-2012) in In and Sn, we received an unique opportunity for reviewing the status of the IS and to address the aforementioned fundamental problems. Targeting to that we performed a magneto-optical/electrical-transport study of the IS with high purity indium film samples in magnetic fields with independently controlled in- and out-of-plane components. Examples of the obtained IS images are shown in the right upper corner. The least expected observation was that the critical field in the IS (green line in the figure on the right) can be less than half of the thermodynamic critical field (blue line). This indicates that the magnetic field in the normal domains can be significantly smaller than the thermodynamic critical field, contrarily to the long established paradigm stating that the normal phase is unstable in such fields. Our experiments resulted in development of a novel and for the first time comprehensive theoretical model consistently describing this and all other properties of the IS. Moreover, our model allows for quantitative determination of the microscopic parameters and sheds new light on properties of superconductors. This outlines the main results achieved in our project this year.

Students of TCC Special topics in physics class (left) Emily Main and Sarah Smith in the Canadian Neutron Beam Center, Chalk River Laboratories; (right) Sasha Townsend near her poster at Oklahoma Research Day conference, University of Central Oklahoma, 2013.

Tulsa Community College is a two-years college of primary high education. TCC students involved in the project take a class “Selected topics in physics”. The class meets once a week; each meeting students make presentations on a foremost topic of physics staying beyond the standard curriculum of college physics classes. Students completed the class have their tuition reimbursed from the grant. The most active students participate in the neutron’s experiments in Chalk River Laboratories, Canada, and in the muon’s experiments in Paul Scherrer Institute, Switzerland. The students take part at the APS March Meetings, and at the Oklahoma Research Day conferences. This year the latter was held in the University of Central Oklahoma, Edmond. As of today (August/2013) the class has been offered for six semesters; total 20 students graduated the class. One former student is currently a graduate student in the physics department of the University of Tulsa; one is employed in the Science and Math division at TCC, Metro; others continue their education in STEM specialties at universities of Oklahoma and elsewhere.

Nonlocal effect and microscopic parameters in superconductors

Vladimir Kozhevnikov, Tulsa Community College, DMR 0904157