Depairing critical current density and the vortex-free state in FeSe nanobridges

TL;DR

The paper addresses the challenge of reaching the depairing critical current density $J_{ m{c}}$ and achieving a vortex-free state by geometric confinement in FeSe. By fabricating nanobridges at a fixed crystal location using the FIB 'pickup' method, they vary the width $W$ relative to the Pearl length $\Lambda$ to obtain a homogeneous current distribution and approach the depairing limit. They find that for $W < \Lambda$, $J_{ m{c}}$ exceeds the depinning value by more than an order of magnitude, reaching about $2\times 10^{5}$ A cm$^{-2}$ at 4 K, and remains robust against fields up to roughly $1$ kOe, signaling a vortex-free state aided by increased $\mu_0 H_{ m{c1}}$. GL scaling near $T_c$ describes the temperature dependence via $J_{ m{c}}(t) = J_{ m{c}}(0)(1 - t)^{3/2}$, while simple Abrikosov-based estimates of $H_{c1}$ underestimate the field behavior due to demagnetization and surface pinning effects. Overall, the work provides a practical route to high-$J_{ m{c}}$, low-noise superconducting devices in iron-based superconductors and motivates direct imaging of vortex expulsion in future studies.

Abstract

The depairing limit and the vortex-free state in a superconductor is crucial for both the study of supercurrent related physics and the application eliminating noise linked to vortex motion. In this work, we report the evidence of depairing limit and the vortex-free state achieved by geometric constraint in FeSe superconductors. A series of narrow bridges with varying widths at the same location of a single crystal were prepared by the \textquotedblleft pickup\textquotedblright method using successive focused ion beam millings. By simply reducing the width of bridge, the magnitude of critical current density ($J_{\rm{c}}$) is enhanced more than one order, evidence the achievement of depairing limit. Moreover, in the bridge with a width smaller than the penetration depth ($λ$), $J_{\rm{c}}$ is found to be robust against magnetic field up to 1 kOe. The field-robust $J_{\rm{c}}$ is a strong piece of evidence for vortex-free state, which is created by the enhancement of lower critical fields due to geometric constraint.

Figures (5)

• Figure 1: (a)-(d)Schematics of the "pickup" method by using FIB to fabricate FeSe nanobridge. The scanning ion microscopy images of the fabricated FeSe nanobridges with (e) $W$ = 1250 nm, and (f) $W$ = 390 nm. The nanobridge in (f) was fabricated by reducing the width of the bridge in (e). (g) Temperature dependence of resistance for the two devices. The $I-V$ curves measured at different temperatures at zero field for the devices with (h) $W$ = 1250 nm, and (i) $W$ = 390 nm.
• Figure 2: Temperature dependence of $J_{\rm{c}}$ under zero field for FeSe nanobridges with $W \gg \varLambda$, $W \sim 2\varLambda$, and $W \textless$$\varLambda$. Inset shows the the reduced temperature ($t$ = $T$/$T_{\rm{c}}$) dependence of current density density, normalized to the extrapolated value $J_{\rm{c}}$(0) for the device with $W \textless$$\varLambda$. The solid and the dashed lines represent the results from the GL theory and KL theory, respectively
• Figure 3: Magnetic field dependence of $J_{\rm{c}}$ at different temperatures obtained from the $I-V$ measurements in the FeSe nanobridge with $W \textless$$\varLambda$ for (a) $H \parallel ab$, and (b) $H \parallel c$.
• Figure 4: Comparison of the field dependence of $J_{\rm{c}}$ at 4 K for FeSe with $W \textless$$\varLambda$ and $W \gg \varLambda$ for both $H \parallel ab$, and $H \parallel c$.
• Figure 5: Bridge width dependence of $\mu_0H_{\rm{c1}}$ in FeSe calculated in the range of 0.1 $\leq$$W/\lambda$$\leq$ 10 based on Abrikosov's formula.