日時 : 平成29年12月11日(月) 15 :00
所属：Materials Physics and Applications Division, CMMS, Los Alamos National Laboratory, Los Alamos, USA
題目：What is the lowest possible vortex creep in superconductors, and how can we achieve it?
Thermal and quantum fluctuations play only a minor role on the vortex properties of many conventional LTS. However, they dramatically influence vortex matter in HTS such as oxides and Fe-based, creating a proliferation of vortex liquid phases that occupy substantial portions of the phase diagram and fast dynamics of the metastable states (flux creep). This fascinating physics has been a topic of continuous interest for decades, but on the other hand is detrimental for applications. The strength of the thermal fluctuations is quantified by the Ginzburg number (Gi) that measures the ratio of the thermal energy to the condensation energy in an elemental superconducting volume. The combination of the small coherence length (x), large anisotropy (g) and high transition temperatures (Tc) in the HTS results in Gi values several orders of magnitude higher than in LTS. For instance, Gi ~ 10-9 in Nb and ~ 10-2 in YBa2Cu3O7, naturally accounting for the much faster creep rate (S) in the latter. We have found that, for strong pinning superconductors in the Anderson-Kim (A-K) creep regime, there is a universal minimum attainable S ~ Gi1/2(T/Tc). This lower limit has been achieved in a few materials including YBa2Cu3O7, MgB2 and our BaFe2(As0.67P0.33)2 films and, to our knowledge, violated by none. This hard constraint has two important, broad implications: first, the creep problem in HTS cannot be fully eliminated and there is a limit to how much it can be ameliorated, and secondly, we can confidently predict that any yet-to-be-discovered HTS will have fast creep. On the other hand, many SC exhibit S values higher, sometimes orders of magnitude higher, than the lower limit. The reason is that Gi only sets the lowest limit for S, but in order to achieve it the pinning landscape must be optimized. I will show that S can be reduced by appropriate engineering of the pinning landscape, in some cases (such as in irradiated Ba(Fe1-xCox)2A2 single crystals) dramatically so and all the way down to the lower limit imposed by Gi. Finally I will discuss some of our studies of creep outside the A-K limit and in very clean (weak pinning) samples, where collective effects are relevant and different glassy and plastic dynamic regimes can be observed and tuned by methods such as irradiation and film thinning.
紹介教員： 鹿野田 教授、為ヶ井 准教授