Anisotropy by design in superconducting Nb thin films via ultrashort pulse laser irradiation

TL;DR

This work demonstrates that femtosecond UV laser irradiation can sculpt Nb thin films with surface LIPSS, enabling anisotropic flux pinning and tailored superconducting properties. By combining resistive, inductive, MOI, XRD, and electron microscopy measurements with TDGL simulations, the authors show that ripple-induced topography drives directional enhancements in the critical current while modestly depressing Tc and Hc2, linked to lattice compression. The results establish pulsed laser processing as a scalable, lithography-free method to engineer superconducting surfaces for flux control, with potential applications in flux lenses, diodes, and related devices. The study also provides a quantitative framework connecting microstructural changes to macroscopic electromagnetic response, bridging experiment and mesoscopic vortex dynamics.

Abstract

The ability to fabricate anisotropic superconducting layers a la carte is desired in technologies such as fluxon screening or removal in field-resilient devices, flux lensing in ultra-sensitive sensors, or in templates for imprinting magnetic structures in hybrid magnetic/superconducting multilayers. In this work, we demonstrate tailored superconductivity in polycrystalline niobium thin films exposed to femtosecond ultraviolet laser pulses. The samples exhibit significant changes in their superconducting properties, directly connected with the observed topography, crystallite geometry, and lattice parameter modifications. On the mesoscopic scale, quasi-parallel periodic ripple structures (about 260 nm of spatial period) gradually form on the film surface by progressively increasing the laser energy per pulse, Ep. This gives way to a stepwise increase of the critical current anisotropy and magnetic flux channeling effects along the ripples. As demonstrated in our resistive and inductive measurements, these superstructures determine the electromagnetic response of the sample within the regime dominated by flux-pinning. Time-dependent Ginzburg-Landau simulations corroborate the topographical origin of the customized anisotropy. Concurrently, intrinsic superconducting parameters (critical field and temperature) are moderately and isotropically depressed upon increasing Ep, as is the lattice parameter of Nb. These findings promote pulsed laser processing as a flexible, one-step, and scalable lithography-free technique for versatile surface functionalization in microelectronic superconducting technology.

Figures (15)

• Figure 1: A) Sketch of the sample geometries and laser scanning process. Square films $2b\times 2b$ were laser cut. On some of them, the Nb layer was selectively removed by laser ablation (pulse energy $E_{\rm p}=6$), defining a transport bridge geometry. Subsequently, the surface was corrugated by laser irradiation ($E_{\rm p}=3.4$). Ultrasonic wire bonding was performed on the four corners (A--D) for the square films or on top of the specific pads ($V_{\pm}, I_{\pm}$) of the bridges for resistive measurements. For visual purposes, the laser spot is not to scale.
• Figure 2: Microstructural details of square-shaped samples that show the gradual effect of increasing the laser pulse energy. A) Pristine sample (FS0); B) and C) two samples of the series FSj irradiated with two intermediate $E_{\rm p}$ values as indicated; and D) Sample FSL whose surface is fully covered by LIPSS. Upper panels: Top-view SEM (SE) images. Insets show 2D-FFT of SEM images ($\approx 70~\unit{\micro\meter}^2$ of area). Lower panels: Cross-sectional brightfield TEM images.
• Figure 3: SEM (SE) images of the surface of bridge samples: the upper panels correspond to the sample BS$_{\rm PAR}$ and show the complete circuit and a detail of the bridge between the voltage contacts. Middle and lower panels show details at higher magnification of the bridges near the center and at the edge for the three analyzed samples: BS0 (pristine), BS$_{\rm PAR}$ (parallel), and BS$_{\rm PERP}$ (perpendicular). The unwanted laser-patterning induced LIPSS of the pristine sample (BS0) occupy a lateral strip of 12.5 $\pm$ 2.5 $\mu$m (see text)
• Figure 4: Effect of changing the laser pulse energies: A) (110) diffraction peak of Nb. B) Estimated lattice parameter $a$ from the XRD experiments. C) $\chi'(T)$ scaled by $|\chi '$(2K)$|$, showing the superconducting-to-normal transition at zero DC field of square films processed with different pulse energies. D) $T_{\rm c}$ values estimated from susceptibility or resistivity measurements of several bridge and square samples, as indicated. Reference values for bulk Nb (not displayed): $a=3.3066(1)$ÅXRD, $T_{\rm c}=9.25\pm 0.01$finnemore_66. Dashed lines are guides for the eye.
• Figure 5: Fraction of the equilibrium phase diagram (upper critical field $H_{c2}$) obtained from resistive measurements. The zero field values, i.e. $T_{\rm c}$, are those displayed in Fig. \ref{['fig:figure_xrd_tc_gradual']} and were obtained with $I=1$).