Enhancing the critical current of a superconducting film in a wide range of magnetic fields with a conformal array of nanoscale holes

The maximum current (critical current) a type-II superconductor can transmit without energy loss is limited by the motion of the quantized magnetic flux penetrating into a superconductor. Introducing nanoscale holes into a superconducting film has been long pursued as a promising way to increase the critical current. So far the critical current enhancement was found to be mostly limited to low magnetic fields. Here we experimentally investigate the critical currents of superconducting films with a conformal array of nanoscale holes that have nonuniform density while preserving the local ordering.We find that the conformal array of nanoscale holes provides a more significant critical current enhancement at high magnetic fields. The better performance can be attributed to its arching effect that not only gives rise to the gradient in hole density for pinning vortices with a wide range of densities but also prevents vortex channeling occurring in samples with a regular lattice of holes

Anisotropy of the critical temperature of a superconducting niobium thin film with an array of nanoscale holes in an external magnetic field

We report on magnetotransport experiments investigating the effect of a regular array of nanoscale holes on the anisotropic response in the transition temperature of a superconducting niobium thin film. We find that the angle dependence of the critical temperature exhibits two strong anisotropic effects: Little-Parks oscillations whose period varies with field direction and a smooth background arising from one-dimensional confinement by the finite lateral space between neighboring holes. The two components of the anisotropy are intrinsically linked and appear in concert with one superimposed on top of the other

Catalyst-Free Growth of Millimeter-Long Topological Insulator Bi₂Se₃ Nanoribbons and the Observation of the π‑Berry Phase

We report the growth of single-crystalline Bi₂Se₃ nanoribbons with lengths up to several millimeters via a catalyst-free physical vapor deposition method. Scanning transmission electron microscopy analysis reveals that the nanoribbons grow along the (112̅0) direction. We obtain a detailed characterization of the electronic structure of the Bi₂Se₃ nanoribbons from measurements of Shubnikov–de Haas (SdH) quantum oscillations. Angular dependent magneto-transport measurements reveal a dominant two-dimensional contribution originating from surface states. The catalyst-free synthesis yields high-purity nanocrystals enabling the observation of a large number of SdH oscillation periods and allowing for an accurate determination of the π-Berry phase, one of the key features of Dirac fermions in topological insulators. The long-length nanoribbons open the possibility for fabricating multiple nanoelectronic devices on a single nanoribbon

Origin of the turn-on temperature behavior in WTe2

A hallmark of materials with extremely large magnetoresistance (XMR) is the transformative turn-on temperature behavior: when the applied magnetic field H is above certain value, the resistivity versus temperature ρ(T) curve shows a minimum at a field dependent temperature T∗, which has been interpreted as a magnetic-field-driven metal-insulator transition or attributed to an electronic structure change. Here, we demonstrate that ρ(T) curves with turn-on behavior in the newly discovered XMR material WTe2 can be scaled as MR∼(H/ρ0)m with m≈2 and ρ0 being the resistivity at zero field. We obtained experimentally and also derived from the observed scaling the magnetic field dependence of the turn-on temperature T∗∼(H-Hc)ν with ν≈1/2, which was earlier used as evidence for a predicted metal-insulator transition. The scaling also leads to a simple quantitative expression for the resistivity ρ∗≈2ρ0 at the onset of the XMR behavior, which fits the data remarkably well. These results exclude the possible existence of a magnetic-field-driven metal-insulator transition or significant contribution of an electronic structure change to the low-temperature XMR in WTe2. This work resolves the origin of the turn-on behavior observed in several XMR materials and also provides a general route for a quantitative understanding of the temperature dependence of MR in both XMR and non-XMR materials