Universal lower limit on vortex creep in superconductors

Abstract

The search for a universal description of vortex matter -one that is applicable to a range of systems and regimes -is a formidable challenge, complicated by the complexity of the interactions between vortices and the environment. Vortex motion that can be induced by Magnus and Lorentz forces or thermal activation can also be counteracted by pinning forces. Because vortex cores are normal (i.e., superfluidity or superconductivity is destroyed inside them), creating a vortex costs energy, and pinning can occur when it is energetically more favorable for a vortex to appear in one location than in another. In type-II superconductors at high enough magnetic fields, vortices are formed by the penetration of magnetic flux, and material disorder that locally reduces the vortex core energy can produce pinning forces that almost completely preclude vortex motion. This results in nearly zero resistance, as long as the current density (J) does not exceed the critical current density (Jc). The caveat is that, for J<Jc, vortices are in metastable states inside pinning centers that act as finite potential wells, and thermal fluctuations may enable them to jump out (creep). Creep is seldom problematic in applications involving low-Tc superconductors (LTS), which typically exhibit slow creep (the decay over time in the persistent current, containing a uniform dispersion of BZO nanoparticles (Ba122:P+BZO) and one without (Ba122:P).Magnetization ( ∝ ! ) studies were performed in a SQUID magnetometer (see Methods). By recording successive measurements of every 15s for an hour, at different temperatures (T) and fields (H), we capture the logarithmic decay over time in J and determine both Jc(T,H) and S (T,H). We see that the nanoparticles increase Jc(5K, 200 Oe) by only ~12% and that this enhancement vanishes at high T (Fig. 1b), but increases dramatically with increasing field (400% at 2T, Fig. 1c). This field-dependent enhancement in Jc is not surprising. Similar to defects created by particle irradiation 4,14 , the efficacy of the BZO in immobilizing vortices is magnified as the number of vortices approaches the number of available pinning sites. Consistent with other studies 11,12,15,16 , Jc is also high in films without nanoparticles. Strong pinning in this material has been ascribed to spatial variations in the P dopant concentration 11,15 . Namely, the Tc and ξ in BaFe2(As1-xPx)2 are exceptionally sensitive to the stoichiometry 16 , such that these variations demarcate phase-separated regions that are effective pinning sites. These regions are spaced roughly 100 nm apart 15 , therefore, outnumber the vortices in fields less than . This is analogous to pinning in Sm1+xBa2-xCu3Oy (Sm123) films, in which Sm-rich123 precipitates of lower Tc than the Sm123 matrix are effective pinning centers 17 . Comparing S(T) in both films (Fig. 2a), we see that the addition of BZO increases S for ! = 200 Oe and 0.3 T. Hence, BZO nanoparticles counteract the strong pinning effects of the large preexisting phase-separated regions. To understan...

Glassy Dynamics in a heavy ion irradiated NbSe2 crystal

“…For example, it is unknown why the peak associated with rapid double-kink expansion in YBCO has not been observed in other materials 18–22 . Of particular interest is superconductors with low Ginzburg numbers ( Gi ), such as NbSe 2 , which can attain significantly lower creep rates 23 than superconductors with high Gi , such as YBCO ( Gi ~ 10 −2 ). This evokes the question of whether glassy states in low Gi materials manifest as a plateau at such a high S  ~ 0.02–0.04 and double-kink expansion creates a peak in S .…”

Strong pinning theory of thermal vortex creep in type-II superconductors

“…Magnetic relaxation measurements [36][37][38] represent a convenient way to study thermal vortex creep. The sample is typically cooled in zero field, then a magnetic field is applied generating a Bean critical state with a vortex density gradient.…”

Slow Vortex Creep Induced by Strong Grain Boundary Pinning in Advanced Ba122 Superconducting Tapes*

“…The plateau area shrinks with the increasing field, and disappears after 5 T. We find that the plateau value is independent of the field and remains at 0.002. This relaxation rate is one order of magnitude smaller than that of the cuprates [18], and it is comparable to that of the NbTi wire, of which the creep rate at 2.5 K is 0.00235 and 0.00432 for 0.3 T and 1 T, respectively [19]. When the temperature increases, the relaxation rate at 0.6 T undergoes a sudden jump at 10 K, gradually increases thereafter, and finally diverges at 25 K. It implies a successive transformation of vortex creep regime.…”