Growth and microwave properties of FeSe thin films and comparison with Fe(Se,Te)

TL;DR

The paper demonstrates PLD growth of FeSe thin films on CaF$_2$ substrates and reports their microwave response in a high-field environment, benchmarked against Fe(Se,Te). Using a dielectric-loaded resonator at ~8 GHz, the authors extract surface resistance changes under $\mu_0H=12\,\mathrm{T}$ and map the superconducting transition with temperature, revealing that FeSe exhibits a larger field-induced Tc shift but weaker pinning compared to Fe(Se,Te). The results indicate substantial potential for FeSe in microwave haloscopes, provided vortex-pinning can be optimized. Overall, the work highlights the interplay between substrate-induced strain, thin-film microwaves, and vortex dynamics in determining the suitability of IBS films for cryogenic, high-field detector applications.

Abstract

In this work, we have grown $\sim$100 nm thick pristine FeSe films by pulsed laser deposition. The films were structurally characterized with X-ray diffraction and their surface morphology checked through atomic force microscopy. Microwave measurements, performed with a dielectric loaded resonator tuned at the frequency of 8 GHz, allowed the characterization of the samples surface resistance, in view of potential applications in microwave haloscopes for dark matter search. Here, we report the comparison of the microwave properties of FeSe with Fe(Se,Te) thin films, as the temperature is swept from 4 K to 20 K. By applying a constant static magnetic field of 12 T, it was also possible to discern the magnetic field resilience of the two samples. FeSe showed a larger critical temperature drift as the field is applied, while the Fe(Se,Te) response broadens remarkably less. A preliminary analysis of vortex pinning shows margins for optimizing pinning in FeSe.

Figures (5)

• Figure 1: $\theta$-$2\theta$ XRD pattern of the FeSe film on CaF$_2$ substrate. In the inset the rocking curve of the (001) reflection is shown.
• Figure 2: AFM image of the FeSe film surface over a $2\,\unit{\micro\meter}\times2\,\unit{\micro\meter}$ area.
• Figure 3: Normalized electrical resistivity for the FeSe film as a function of temperature, with respect to the room temperature value. The inset highlights the superconducting transition region.
• Figure 4: Upper panel: $\Delta R_s(T)$ at the fixed fields $\mu_0H =\qtylist{0;12}{\tesla}$ in FeSe and Fe(Se,Te), normalized with respect to their maximum values $\Delta R_s(T_{max})$. Lower panel: temperature derivative of $R_s$ normalized at the maximum. Symbols are reported in the legend. FeSe has a lower magnetic field resilience with respect to Fe(Se,Te). Data for $dR_s/dT$ have been averaged to reduce noise. Only 5% of the data are shown to reduce clutter.
• Figure 5: Magnetic-field-induced change in the surface resistance ${\delta r_s=(\Delta R_s(\qty{12}{\tesla})-\Delta R_s(0)})/\Delta R_s(T_{max})$ in FeSe and Fe(Se,Te). Symbols are reported in the legend. Data in FeSe point to a much higher residual level of $\delta r_s$ below the superconducting transition with respect to Fe(Se,Te), an indication of much lower depinning frequency. Only 5% of the data are shown to reduce clutter.