The basic transition parameters and excess noise of Mo/Au-based transition-edge sensor (TES) x-ray detectors without banks and with Au absorber are analyzed as a function of TES dimensions and absorber stems position. We show the effect of the absorber stems and TES dimensions on these parameters. We observe that the logarithmic derivatives of R(T) are reduced and develop non-monotonous behavior vs bias as stems are placed on the Mo/Au sensor area, progressively disturbing the current flow inside the TES. TESs with outer tangent stems display higher excess noise but also higher figures of merit because of their higher alpha and beta values. We find that both longer and narrower TESs have higher excess noise so that a trade-off is required for TES performance optimization. We discuss some hints for that. The possible origins of the experimental excess noise of these devices are also explored and discussed.

Oxygen-depleted YBa2Cu3O7-delta exhibits a substantial drop in the normal-state resistivity and an increase in the superconducting critical temperature when illuminated with visible light. The photo-induced states are metastable, slowly decaying at high temperatures and essentially persistent at low temperatures. In this work, this effect is exploited to modify the response of half-wavelength YBa2Cu3O7-delta resonators and simultaneously the high-sensitivity of the resonant circuit is used to investigate the persistent photodoping of this material. Under illumination, the bolometric effect and photodoping are clearly distinguished by the different time scales associated with each mechanism. Using a 60 mu m-wide laser spot, the properties of the resonator are locally and reversibly modified, and the position-dependent sensitivity of the device is demonstrated. This enables the direct imaging of standing waves at both the fundamental resonance and the second harmonic.

Oxygen‐depleted YBa 2 Cu 3 O 7−δ exhibits a substantial drop in the normal‐state resistivity and an increase in the superconducting critical temperature when illuminated with visible light. The photo‐induced states are metastable, slowly decaying at high temperatures and essentially persistent at low temperatures. In this work, this effect is exploited to modify the response of half‐wavelength YBa 2 Cu 3 O 7−δ resonators and simultaneously the high‐sensitivity of the resonant circuit is used to investigate the persistent photodoping of this material. Under illumination, the bolometric effect and photodoping are clearly distinguished by the different time scales associated with each mechanism. Using a 60 µm‐wide laser spot, the properties of the resonator are locally and reversibly modified, and the position‐dependent sensitivity of the device is demonstrated. This enables the direct imaging of standing waves at both the fundamental resonance and the second harmonic.

Transition-Edge Sensors (TESs) constitute highly sensitive particle and radiation detectors, widely used in many applications. Each of these requires optimization of TES performances and designs, including sizes and geometries. These may have implications on the superconducting transition mechanisms and, therefore, on TESs performances and stability, through the specific shape of the resistance vs temperature and current R(T,I) and the nature of noise. In this study, we investigate the dependence of the superconducting transition, characterized by R(T,I), on TES size and bias current density. Through analyses of R(T,I) in bare Mo/Au TESs with T-c tuned for this study, we observe how the weak link behavior induced by the superconducting leads weakens and disappears as TES length or driving current increase, being substituted by another dominant transition mechanism, which might be related to a Berezinskii-Kosterlitz-Thouless transition. We also observe a significant broadening of the transition's upper part, attributed to the longitudinal proximity effect induced by the pads; for the shorter devices, this effect is observed for R > 70% R-n and results in TES resistances considerably lower than R-n up to temperatures well above the TES transition: R < R-n up to 3 K for a 8 mu m-long device.

The search for low-mass dark matter (DM) goes in parallel with the identification of new detection channels and the development of suitable detectors. Detection of the resulting small energy depositions is challenging: it requires extremely high sensitivity, only achievable by cryogenic thermal detectors, which might be put to the limit. Understanding the processes which can limit performances of these detectors can be thus crucial for evaluating the feasibility of the proposed new detection schemes and to design the detectors and tune their performance. In this paper we focus on a promising detection scheme, the excitation of single optical phonons in polar materials, to evaluate one of the possible limiting factors of cryogenic thermal detectors, i.e. the phonon dynamics in the target/absorber. We present a detailed theoretical analysis, within an entirely ab initio scheme, of the downconversion and propagation processes undergone by optical phonons, created by the interaction of a low-mass DM particle in an Al2O3 target, until they reach the interface with a phonon Al collector. After a preliminary methodological survey that reveals the limitations of any Relaxation Time Approximation based method, we developed a 3D beyond-RTA phonon Monte Carlo that allowed us to introduce the spatial dimension of the device and address questions about impact of target size and scattering position. We analyse also the effect of the phonon energy and wavevector and show that isotopes can, perhaps counterintuitively, result in a larger heat flux by providing transport channels of higher velocities, thus favoring detection. Our results suggest that, though challenging, the direct detection of light DM via athermal phonon generation appears feasible, and that the phonon downconversion followed by quasi-ballistic propagation does not appear to be a major bottleneck in terms of reducing the signal.

We study the logarithmic transition sensitivities to temperature and current, α and β, of bare Mo/Au TES sensors without any normal metal structure, with and without relevant weak link effects. Both parameters display a smooth dependence with bias. We analyze them as a function of bias, bath temperature and TES size. We observe relevant differences in the behavior of α and β as the aspect ratio increases.

Transition-Edge Sensors (TES) are radiation detectors working at cryogenic temperatures [1,2] (~ 100 mK) having capability for sensing very small amounts of energy coming from X-rays (~ few keV), for example, with superior sensitivity (~ 1 eV). TES can even detect single photons and measure their energy with high accuracy. TES are used in Astrophysics and Cosmology applications, as well as in Nanotechnology and Quantum Technologies. TES have already been implemented on telescopes and in a future (2032) the detector of the high spectral resolution instrument of the next telescope of X-rays from the European Spatial Agency (ESA), Athena, will be constituted by TES [3]. TES are microcalorimeters (electrothermal devices) made of a superconducting (SC) thin film (or bilayer). They take advantage of the steep resistive transition of the SC material between the normal and superconducting states (typically a few mK). This is what makes TES very sensitive to incoming radiation. TES performances (their spectral energy resolution and response time) depend on TES parameters, which are extracted from fits to the measured complex impedance Z(w); these fits require an electrothermal model of the TES, that is, knowing the number of relevant thermal blocks and their configuration. Therefore, in order to optimize TES performances and improve them, electrothermal modeling plays a key role. Usually, TES parameters are extracted by using the simplest electrothermal model, that is, considering the TES constituted by a single thermal block (1 TB). This is, though, an approximation even when no absorber is present. In this work we develop fits to Z(w) of bare TES considering different configurations with 2 thermal blocks (2 TBs) and analyze when the second TB becomes relevant, and what is its impact on basic TES parameters. We report on the results obtained so far, including a critical analysis of fits reliability and the TES size effects on the TES thermal parameters, which in the end should help us to identify the present TBs.

Transition-edge sensors (TESs) are used as very sensitive thermometers in microcalorimeters aimed at detection of different wavelengths. In particular, for soft X-ray astrophysics, science goals require very high-resolution microcalorimeters which can be achieved with TESs coupled to suitable absorbers. For many applications, there is also need for a high number of pixels which typically requires multiplexing in the readout stage. Frequency-domain multiplexing (FDM) is a common scheme and is the baseline proposed for the ATHENA mission. FDM requires biasing the TES in AC at MHz frequencies. Recently, there has been reported degradation in performances under AC with respect to DC bias. In order to assess the performances of TESs to be used with FDM, it is thus of great interest to compare the performances of the same device both under AC bias and DC bias. This requires two different measurement set-ups with different processes for making the characterization. We report in this work the preliminary results of a single-pixel characterization performed on a TiAu TES under AC and afterwards under DC bias in different facilities. Extraction of dynamical parameters and noise performances are compared in both cases as a first stage for further AC/DC comparison of these devices.

Resumen del trabajo presentado a la XXXVII Reunion Bienal de la Real Sociedad Espanola de Fisica celebrada en Zaragoza, del 15 al 21 de julio de 2019.

Poster presentado al International Workshop on Low Temperature Detectors (LTD), celebrado en Milan (Italia) del 22 al 26 de julio de 2019.

We report on the sensitivity of superconducting transition temperature (T-c) to the individual layers' thickness in Mo/Au proximity bilayers to be used in transition-edge sensors (TESs). The achieved good reproducibility and quality of the bilayers allow a clear determination of the superconducting critical temperature T-c as a function of the Mo and Au thicknesses. One objective of this work is to analyse the quality of the Mo/Au interface and to assess the possible effects of the double Au layer we use to fabricate these bilayers and TESs based on them. Experimental data are analysed on the basis of Usadel equations using the model developed by Martinis and co-workers, in which the proximity effect in the bilayer is mainly governed by the interface transparency between the superconductor and the normal metal. We find that this model describes quite well the behavior of T-c, even for quite thick Au layers, and that the double Au layer does not play any relevant role on the proximity effect.

Resumen del poster presentado a la XXXVII Reunion Bienal de la Real Sociedad Espanola de Fisica celebrada en Zaragoza, del 15 al 21 de julio de 2019.

The R(T, I) shape of the superconducting transition in transition edge sensors (TESs) is of crucial importance to determine their ultimate performance. This paper reports a study of the temperature and current dependences of the transition of Mo/Au TESs, focused on the low resistance region, where these devices preferentially operate. A large broadening of the transition is observed when increasing the applied current. An empirical analytic expression for R(T, I) is found, which describes the transition of devices with different critical temperatures, from R = 0 up to at least 30% R-n (in some cases nearly 80% R-n). Several mechanisms for this behaviour are considered; results show that a current assisted vortex pair unbinding mechanism (Berezinskii-Kosterlitz-Thouless transition) could be the possible origin for this behaviour. Finally, the consequences of the current-induced transition broadening for TES properties and operation are outlined.

We report on the fabrication and characterization of Mo/Au-based transition-edge sensors (TES), intended to be used in X-ray detectors. We have performed complete dark characterization using I–V curves, complex impedance and noise measurements at different bath temperatures and biases. Devices with two designs, different sizes and different membranes have been characterized, some of them with a central bismuth absorber. This has allowed extraction of the relevant parameters of the TES, analyses of their standard behavior and evaluation of their prospects.

The semimetal character of bismuth and its large photon absorbing power make of this element the most suitable absorber material for X-ray low temperature detectors. This application requires coatings of Bi with thicknesses and properties that only electrodeposition methods may achieve. Although there are studies on electrodeposition of bismuth for these detectors and other devices, the process is not straightforward and has not been sufficiently studied in terms of the desired final properties, neither the effect of different parameters is well known or easily reproduced. This work reports the influence of two different electrolytes, of the deposition method, and of heating and stirring on the structure, microstructure and transport properties of bismuth films. Typically, rhombohedral Bi is obtained upon electrodeposition with very good crystallinity, and some crystal preferential orientation, while significant empirical correlations are found among electrochemical parameters, microstructure, and resistivity. Such correlation allows the identification of the deposition parameters for coatings that yield the optimal functional properties.

We report on the development of Mo/Au based transition edge sensors (TESs) aimed at soft X-ray detection. TESs of different sizes with T c ~ 100 mK and very narrow transitions have been fabricated. Dark characterization based on I-V, complex impedance, and noise measurements has allowed us to obtain their basic functional parameters at different bath temperatures and operating points. Electrodeposited Bi films, to be used as X-ray absorbers, have been developed and characterized.

Raman spectroscopy studies of BaZrxTi(1-x)O(3) (BZT) epitaxial thin films are presented over the entire compositional range (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1). The analysis of the results has allowed the elaboration of the phase diagram of the BZT thin films in which three types of ferroelectric behaviour were distinguished based on the polar order extent: long range conventional ferroelectricity for x <= 0.2, relaxor behaviour for 0.3 <= x <= 0.7 and weak polar interaction for 0.8 <= x <= 0.9. In order to verify the validity of this phase diagram, Raman spectra of the films were also performed over a wide temperature range (from 97 K to 497 K). The results reveal that the presence or absence of a temperature dependent structural transition correlates with the polar order extent and, thus, the ferroelectric behaviour of BZT. (C) 2017 Elsevier B.V. All rights reserved.

The isovalent susbstitution of Ti4+ by Zr4+ in BaZrxT1-xO3 modifies the dielectric character of ferroelectric BaTiO3 yielding different behaviours such as relaxor, polar cluster, etc. The dynamic coupling between BaTiO3 polar nanoregions and BaZrO3 nonpolar ones as well as microstrain between them are thought to be behind such a rich phase diagram. However, these short-range compositonal variations are elusive to detect and this topic is thus rarely addressed. We have grown epitaxial thin films of BaZrxT1-xO3 on (001)-oriented SrTiO3 substrates by pulsed laser deposition sweeping the entire composition range between BaTiO3 and BaZrO3 in increments of 0.1 in x. Several characterization techniques (AFM, TEM, XRD, Raman spectroscopy) were used for this research in order to understand the morphological and structural properties of the deposited films. Ellipsometric measurements allowed the calculation of the band gap energy of the films. This work demonstrates the existence of a heterogeneous distribution in the substitution of titanium by zirconium yielding relaxor and polar cluster nanoregions. (C) 2015 Elsevier B.V. All rights reserved.

The X-ray spectroscopy telescope Athena has been designed to implement the science theme "the hot and energetic universe", selected by the European Space Agency as the second large mission of its Cosmic Vision program. X-IFU, one of the two interchangeable focal plane instruments of Athena, is a high resolution X-ray spectrometer made of a large array of Transition Edge Sensors. Two options are under consideration for the X-IFU microcalorimeters: Ti/Au bilayers or Mo/Au bilayers. Here we report on our efforts to develop Mo/Au-based TES. The TES are made of high quality superconducting Mo/Au bilayers fabricated at room temperature on low stress Si3N4 membranes; Mo is deposited by RF magnetron sputtering and in-situ covered by a thin (15nm) Au layer deposited by DC sputtering; in a second step, the Au layer thickness is increased ex-situ by e-beam deposition, to obtain suitable resistance Rn and operation temperature values. Very sharp transitions (~few mK transition width) are obtained, with typically Rn~25mΩ and Tc~ 100-120mK for 65/215 bilayers. First simple TES designs are being tested. Also, Bi films several μm thick, intended to constitute the X-ray absorber, are fabricated by electrochemical deposition.