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Transition Edge Sensors are ultra-sensitive superconducting detectors with applications in many areas of research, including astrophysics. The device consists of a superconducting thin film, often with additional normal metal features, held close to its transition temperature and connected to two superconducting leads of a higher transition temperature. There is currently no way to reliably assess the performance of a particular device geometry or material composition without making and testing the device. We have developed a proximity effect model based on the Usadel equations to predict the effects of device geometry and material composition on sensor performance. The model is successful in reproducing I −V curves for two devices currently under study. We use the model to suggest the optimal size and geometry for TESs, considering how small the devices can be made before their performance is compromised. In the future, device modelling prior to manufacture will reduce the need for time-consuming and expensive testing.

The bilayers of Transition Edge Sensors (TESs) are often modified with additional normal-metal features such as bars or dots. Previous device measurements suggest that these features improve performance, reducing electrical noise and altering response times. However, there is currently no numerical model to predict and quantify these effects. Here we extend existing techniques based on Usadel's equations to describe TESs with normal-metal features. We show their influence on the principal TES characteristics, such as the small-signal electrothermal parameters $\alpha$ and $\beta$ and the superconducting transition temperature $T_{c}$. Additionally, we examine the effects of an applied magnetic field on the device performance. Our model predicts a decrease in $T_{c}$, $\alpha$ and $\beta$ as the number of lateral metal structures is increased. We also obtain a relationship between the length $L$ of a TES and its critical temperature, $T_{c} \propto L^{-0.7}$ for a bilayer with normal-metal bars. We predict a periodic magnetic flux dependence of $\alpha, \beta$ and $I_{c}$. Our results demonstrate good agreement with published experimental data, which also show the reduction of $\alpha$, $\beta$ and $T_{c}$ with increasing number of bars. The observed Fraunhofer dependence of critical current on magnetic flux is also anticipated by our model. The success of this model in predicting the effects of additional structures suggests that in the future numerical methods can be used to better inform the design of TESs, prior to device processing.

Superconducting correlations can penetrate in a ferromagnet via proximity effect but over a very short length of the order of a few nanometers because ferromagnetic exchange field tries to align the antiparallel spins of electrons in a Cooper pair. Theoretically, it was suggested that rather a long-range proximity effect can occur if spin-triplet (with parallel spins) Cooper pairs are generated at the interface that requires a magnetic inhomogeneity (spin-active interface). Experimentally, it was first observed at the Delft University using CrO2 (a half metallic ferromagnet). In these devices the induced supercurrent was not both well controlled and well understood. We have provided new evidence that supercurrents can flow through CrO2 films, deposited on sapphire substrates, over a length of 1_m. The analysis shows that these films exhibit six-fold magnetic-anisotropy that might provide the required spin-active interface. Still, low reproducibility suggests a weak control over the spin-active interface. Such a control appears better with an artificially created spin-active interface, using an additional ferromagnetic layer of Ni (2nm) with CrO2. From investigations, it suggests that there are other scattering phenomena (different spin dependent magnetic scattering from different magnetic layers) or magnetic moment disorder at the interface involved to generate the odd-frequency spin-triplet supercurrent.