http://www.surfacetreatments.it/thinfilms Probing the Nonlinear Meissner Effect and the Enhancement of the Field Onset of Penetration of Vortices in Dirty Nb Films Under RF and DC Fields (Alex Gurevich - 30') Speaker: Alex Gurevich - National High Magnetic Field Laboratory | Duration: 30 min. Abstract Alex Gurevich National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32303, USA The first observation of the nonlinear Meissner effect in dirty Nb films revealed by the measurements of the frequency shift of the Nb thin film 20GHz resonator strip line as a function of the parallel dc magnetic field is reported [1]. The use of films thinner than the London penetration depth has enabled us to circumvent the main obstacle in the observation of the nonlinear Meissner effect due to penetration of vortices. A significant enhancement of the vortex penetration field up to  1T for a 65 nm dirty Nb film is demonstrated. We developed a theory based on the Usadel equations, which describe the experimental data very well. The nonlinear Meissner effect caused by impurities at the surface of Nb cavities can contribute to the medium and high-field Q slope. The thin film strip line resonator method can be very useful for measurements of the surface resistance and the onset of vortex penetration for emerging SRF materials used in thin film coating structures for Nb cavities. [1]. N. Groll, A. Gurevich, and I. Chiorescu, Phys. Rev. B 81, 020504(R) (2010).

1. Nonlinear Meissner Effect and Enhancement of the Field Onset of Penetration of Vortices in Dirty Nb Films Under RF and DC Fields. Alex Gurevich National High Magnetic Field Laboratory, Tallahassee, FL 32310 6-th Thin Film SRF Workshop, Padova, Italy, Oct. 4-6, 2010 N. Groll, AG and I. Chiorescu, Phys. Rev. B81, 020504(R) (2010)

10. Effect of dc filed y Field B inclined by angle  with respect to the x-axis induces vector potentials: A y = -zBcos  and A x = zBcos  +  A  : only A x is coupled linearly with weak rf field The NLME correction to is quadratic in B and has 2 – cos2  angular dependence Total rf current defines  (H) d w z x H   A 