Fabrication and characterization of AlMn alloy superconducting films for 0vbb experiments

TL;DR

This work addresses the need for fast, high-resolution TES calorimeters in neutrinoless double-beta decay searches by enabling $T_c$ tuning of AlMn films to the $10$--$20$ mK range. It uses DC magnetron sputtering to fabricate AlMn films with Mn doping around $1800$--$2000$ ppm, followed by systematic annealing and four-terminal $R$–$T$ measurements to map how $T_c$, $igtriangleup T_c$, and $I_c$ depend on Mn content, film thickness, and annealing temperature, with TOF-SIMS used to study Mn-depth distribution. Key findings include achieving $T_c$ in the 10--20 mK window (e.g., 150 nm, 1800 ppm at $T_{annealing} oughly 182^ ing{C}$) and identifying an annealing window (120–200°C) for minimizing $igtriangleup T_c$, alongside magnetic-field sensitivity where out-of-plane fields shift $T_c$ and the GL-consistent $I_c(T_c)$ relation. The Mn depth profiling shows annealing drives more uniform Mn distribution, suggesting a mechanism for Tc tuning. Overall, the results define fabrication parameters and magnetic-field considerations for AlMn TES detectors in CUPID-like 0νββ experiments and have implications for broader low-temperature calorimetry.

Abstract

Neutrinoless double-beta decay (0vbb) experiments constitute a pivotal probe for elucidating the characteristics of neutrinos and further discovering new physics. Compared to the neutron transmutation doped germanium thermistors (NTD-Ge) used in 0vbb experiments such as CUORE, transition edge sensors (TES) theoretically have a relatively faster response time and higher energy resolution. These make TES detectors good choice for next generation 0vbb experiments. In this paper, AlMn alloy superconducting films, the main components of TES, were prepared and studied. The relationship between critical temperature (Tc) and annealing temperature was established, and the impact of magnetic field on Tc was tested. The experimental results demonstrate that the Tc of AlMn film can be tuned in the required range of 10 - 20 mK by using the above methods, which is a key step for the application of AlMn TES in 0vbb experiment. In the test range, the Tc of AlMn film is sensitive to out-of-plane magnetic field but not to the in-plane magnetic field. Furthermore, we find that a higher annealing temperature results in a more uniform distribution of Mn ions in depth, which opens a new avenue for elucidating the underlying mechanism for tuning Tc.

Figures (5)

• Figure 1: Influence of sputtering power and Ar pressure on sputtering rate. (a) The red data points indicate the relationship between sputtering rate and sputtering power. (b) The blue data points indicate the relationship between sputtering rate and sputtering pressure.
• Figure 2: (a) R-T of films are characterized by the four-terminal method in a dilution refrigerator. [id=PS]Upper-right inset: schematic of the thin-film sample and sample holder used for the measurements. (b) Example of R versus T measured by the four-terminal method with a DC current source around film superconducting transition. [id=PS]The horizontal axis represents the measurement temperature, and the vertical axis shows the film resistance measured by the four-terminal method; the five R–T curves correspond to different annealing temperatures.
• Figure 3: (a) The change of $T_c$ of 2000 ppm AlMn alloy film with film thickness and annealing temperature. (b) The change of $\Delta T_c$ of AlMn alloy film with film thickness and annealing temperature. (c) The enlarged view of the data corresponding to the annealing temperature of 182 to $205\degreeCelsius$ in Fig3.(a), with linear fitting of the data. (d) The $T_c$ of 1800 ppm AlMn alloy films baked above $150\degreeCelsius$ presents a linear relationship with the annealing temperature, and the slope decreases with the decrease of Mn doping concentration.
• Figure 4: Relationship between $T_c$ of 2000 ppm AlMn alloy films and external magnetic field, as well as magnetization of strong magnetic field. (a) and (b) The variation of $T_c$ of 150 nm AlMn alloy film with external magnetic field. (c) The $T_c$ of magnetized 2000 ppm AlMn alloy films exposed to different magnetic fields is compared with that of unmagnetized films. (d) The $T_c$ and critical current $I_c$ of AlMn alloy films satisfy the function $I_c(T_c)=I_{c0}(1-T_c/T_{c0})^3{^{{^/2}}}$ with $I_{c0}=200 \mu{A}$ and $T_{c0}$=37.3 mK, which is consistent with the Ginzburg-Landau (GL) theory.
• Figure 5: The measured distribution of $Al^+$ and $Mn^+$ content in depth for [id=PS]annealed and one as-depositedbaked and unbaked AlMn alloy films. (a) The absolute numbers of ions at different depths. (b) The ratio of the number of $Al^+$ to $Mn^+$.