Effects of defects and surface roughness on the vortex penetration and vortex dynamics in superconductor-insulator-superconductor multilayer structures exposed to RF magnetic fields: numerical simulations within TDGL theory

The vortex penetration and vortex dynamics are significantly important to superconducting devices, for example the superconducting cavities, since the vortex motions would create substantial dissipation. In experiments, different kinds of defects, as well as different degrees of surface roughness were observed. By considering these in superconductor-insulator-superconductor (SIS) structures, the vortex penetration and vortex dynamics are very complex due to the interactions with defects and the influence of surface roughness, especially for radio-frequency (RF) magnetic field, which are quite different from ideal defect-free SIS multilayer structures. In this paper, within Ginzburg-Landau theory, we perform numerical simulations to study the effects of nanoscale defects, surface roughness, and cracks in the coating layer on the vortex penetration and superheating field in Nb3Sn-I-Nb multilayer structures exposed to a quasi-static magnetic field. The validations of the numerical simulations are verified by good consistency with previous theoretical results in ideal defect-free SIS multilayer and single Nb structures. Furthermore, we explore the vortex dynamics and induced voltages in SIS multilayer structures exposed to RF magnetic fields for both ideal defect-free structures and real situations including surface roughness. Our numerical simulations indicate that, unlike the quasi-static case, the advantage of SIS multilayer structures over a single Nb structure depends on the degrees of surface roughness as well as the frequency and amplitude of the RF magnetic field. The results of this paper provide deep insight to evaluate the actual performance-limiting of next-generation superconducting radio-frequency (SRF) cavities with different proposed candidate materials, which are quite susceptible to nonideal surface.

Numerical simulations for ferromagnetic resonance of nano-size island structures probed by radio-frequency scanning tunneling microscopy

We numerically calculated ferromagnetic resonance (FMR) spectra taken on a single-domain nano-size ferromagnetic island structure in the configuration of radio-frequency scanning tunneling microscopy (RF-STM), where RF electromagnetic waves are introduced into the tunneling gap through the probe tip. In this scheme, near-field in-plane azimuthal RF magnetic field induces FMR of an out-of-plane magnetized island situated below the tip under the external out-of-plane magnetic field. The amount of the magnetization of the island is effectively reduced by the resonance and the reduction can be detected from the spin-polarized tunneling conductance. From the calculated spectra we found that the FMR signal becomes larger with a smaller tip-sample distance and a sharper tip. It is also revealed that the azimuthal RF magnetic field and therefore the FMR signal are enhanced when a tip is located near the edge of the island.

Filtration of Nanoparticle Agglomerates in Aqueous Colloidal Suspensions Exposed to an External Radio-Frequency Magnetic Field

The study investigated the phenomenon of the fast aggregation of single-domain magnetic iron oxide nanoparticles in stable aqueous colloidal suspensions due to the presence of a radio-frequency (RF) magnetic field. Single-domain nanoparticles have specific magnetic properties, especially the unique property of absorbing the energy of such a field and releasing it in the form of heat. The localized heating causes the colloid to become unstable, leading to faster agglomeration of nanoparticles and, consequently, to rapid sedimentation. It has been shown that the destabilization of a stable magnetic nanoparticle colloid by the RF magnetic field can be used for the controlled filtration of larger agglomerates of the colloid solution. Two particular cases of stable colloidal suspensions were considered: a suspension of the bare nanoparticles in an alkaline solution and the silica-stabilized nanoparticles in a neutral solution. The obtained results are important primarily for biomedical applications and wastewater treatment.

Magnetically Controlled Carbonate Nanocomposite with Ciprofloxacin for Biofilm Eradication

Biofilms are the reason for a vast majority of chronic inflammation cases and most acute inflammation. The treatment of biofilms still is a complicated task due to the low efficiency of drug delivery and high resistivity of the involved bacteria to harmful factors. Here we describe a magnetically controlled nanocomposite with a stimuli-responsive release profile based on calcium carbonate and magnetite with an encapsulated antibiotic (ciprofloxacin) that can be used to solve this problem. The material magnetic properties allowed targeted delivery, accumulation, and penetration of the composite in the biofilm, as well as the rapid triggered release of the entrapped antibiotic. Under the influence of an RF magnetic field with a frequency of 210 kHz, the composite underwent a phase transition from vaterite into calcite and promoted the release of ciprofloxacin. The effectiveness of the composite was tested against formed biofilms of E. coli and S. aureus and showed a 71% reduction in E. coli biofilm biomass and an 85% reduction in S. aureus biofilms. The efficiency of the composite with entrapped ciprofloxacin was higher than for the free antibiotic in the same concentration, up to 72%. The developed composite is a promising material for the treatment of biofilm-associated inflammations.