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.

Waveguide quantum electrodynamics studies interactions of matter with photons traveling via a transmission guide and how these can be exploited to control quantum emitters and to establish quantum correlations between them. Here, we explore the competition between such light-mediated interactions with intrinsic matter-matter interactions. For this, we couple a superconducting line to a magnetic material made of organic free radical molecules. We find that molecules belonging to one of the two crystal sublattices form one-dimensional spin chains. Temperature then controls spin correlations along these chains in a continuous and monotonic way. In the paramagnetic region (T > 0.7 K), the microwave transmission evidences a collective coupling of quasi-identical spins to the propagating photons, with coupling strengths that reach values close to the dissipation rates. As T decreases, the growth of spin correlations, combined with the anisotropy in the spin-spin exchange constants, tend to suppress the collective spin-photon coupling. In this regime, the spin visibility in transmission also reflects a gradual change in the nature of the dominant spin excitations, from single-spin flips to bosonic magnons. Waveguide quantum electrodynamics offers a platform to study the interplay between light-mediated and intrinsic matter-matter interactions. Here, the authors couple a superconducting transmission line to a model 1D magnetic material, revealing photon-induced transitions to collective excitations that evolve as temperature decreases from super radiant spin modes to magnons, with implications for quantum information processing and spintronic devices.

The spin frustration and other magnetic properties of the "cartwheel" heptanuclear cluster [FeIII 7O3(O2C t Bu)9(Me-dea)3(H2O)3] (Me-deaH2 = N-methyldiethanolamine) have been previously investigated; we present here a Mossbauer spectroscopic study and sub-Kelvin magnetization and ac susceptibility measurements which enable a complete magnetic picture of this frustrated cluster. 57Fe Mossbauer spectra above 150 K showed three doublets in a 1:3:3 ratio, which could be assigned by their respective quadrupole splittings to the central Fe(1) and the peripheral Fe(2) and Fe(3). The field dependence of the corresponding magnetic sextets at 3 K showed that the spins on the central Fe(1) and the three peripheral Fe(2) sites with O5N coordination are oriented mutually coparallel, while these are antiparallel to the spins on the peripheral Fe(3) sites with O6 coordination, resulting in an overall S = 5/2 ground state. This provides experimental confirmation of the previously proposed spin ground state structure. Upon cooling to sub-Kelvin temperatures, a crossover to spin blocking with T B approximate to 0.21 K could be observed. This single-molecule magnet behavior had been expected but had not been observable with a conventional SQUID. The anisotropy barrier, of 3-fold symmetry, can be described in terms of the parameter D/k B = -0.47 K and a fourth-order perturbation; the latter enables thermally activated quantum tunneling through the excited sublevel m z = +/- 3/2, with an activation barrier of U/k B = 1.9 K.

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.

Transition edge sensors (TESs) are extremely sensitive thermometers made of superconducting materials operating at their transition temperature, where small variations in temperature give rise to a measurable increase in electrical resistance. Coupled to suitable absorbers, they are used as radiation detectors with very good energy resolution in several experiments. Particularly interesting are the applications that TESs may bring to single photon detection in the visible and infrared regimes. In this work, we propose a method to enhance absorption efficiency at these wavelengths. The operation principle exploits the generation of highly absorbing plasmons on the metallic surface. Following this approach, we report nanostructures featuring theoretical values of absorption reaching 98%, at the telecom design frequency (λ = 1550 nm). The optimization process takes into account the TES requirements in terms of heat capacity, critical temperature and energy resolution leading to a promising design for an operating device. Neural networks were first trained and then used as solvers of the optical properties of the nanostructures. The neural network topology takes the geometrical parameters, the properties of materials and the wavelength of light as input, predicting the absorption spectrum at single wavelength as output. The incorporation of the material properties and the dependence with frequency was crucial to reduce the number of required spectra for training. The results are almost indistinguishable from those calculated with a commonly used numerical method in computational electromagnetism, the finite-difference time-domain algorithm, but up to 106 times faster than the numerical simulation.

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.

Highly sensitive magnetometry reveals paramagnetism in dendrimer-coated gold nanoparticles. Different types of such nanoparticles, as a result of (i) functionalizing with two distinct Percec-type dendrons, linked to gold via dodecanethiol groups, and (ii) postsynthesis annealing in a solvent-free environment that further promotes their growth have been prepared. Ultimately, for each of the two functionalization configurations, we obtain highly monodisperse and stable nanoparticles of two different sizes, with spherical shape. These characteristics allow singling out the source of the measured paramagnetic signals as exclusively arising from the undercoordinated gold atoms on the surfaces of the nanoparticles. Bulk gold and the functional groups of the ligands contribute only diamagnetically.

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.

We explore how to encode more than a qubit in vanadyl porphyrin molecules hosting a S = 1/2 electronic spin coupled to a I = 7/2 nuclear spin. The spin Hamiltonian and its parameters, as well as the spin dynamics, have been determined via a combination of electron paramagnetic resonance, heat capacity, magnetization and on-chip magnetic spectroscopy experiments performed on single crystals. We find low temperature spin coherence times of micro-seconds and spin relaxation times longer than a second. For sufficiently strong magnetic fields (B > 0.1 T, corresponding to resonance frequencies of 9-10 GHz) these properties make vanadyl porphyrin molecules suitable qubit realizations. The presence of multiple equispaced nuclear spin levels then merely provides 8 alternatives to define the '1' and '0' basis states. For lower magnetic fields (B < 0.1 T), and lower frequencies (<2 GHz), we find spectroscopic signatures of a sizeable electronuclear entanglement. This effect generates a larger set of allowed transitions between different electronuclear spin states and removes their degeneracies. Under these conditions, we show that each molecule fulfills the conditions to act as a universal 4-qubit processor or, equivalently, as a d = 16 qudit. These findings widen the catalogue of chemically designed systems able to implement non-trivial quantum functionalities, such as quantum simulations and, especially, quantum error correction at the molecular level.

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.

Trabajo presentado al XIV.0 Scientific Meeting (virtual) of the Spanish Astronomical Society (SEA), celebrado del13 al 15 de julio de 2020.

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.

The electronic structure of AgCuO2, and more specifically the possible charge delocalization and its implications for the transport properties, has been the object of debate. Here the problem is faced by means of first-principles density functional theory calculations of the electron and phonon band structures as well as molecular dynamics simulations for different temperatures. It is found that both Cu and Ag exhibit noninteger oxidation states, in agreement with previous spectroscopic studies. The robust CuO2 chains impose a relatively short contact distance to the silver atoms, which are forced to partially use their d z2 orbitals to build a band. This band is partially emptied through overlap with a band of the CuO2 chain, which should be empty if copper were in a Cu3+ oxidation state. In that way, although structural correlations could roughly be consistent with an Ag+Cu3+O2 formulation, the appropriate oxidation states for the silver and copper atoms become Ag(1+δ)+ and Cu(3-δ)+, and as a consequence, the stoichiometric material should be metallic. The study of the electronic structure suggests that Ag atoms form relatively stable chains that can easily slide despite the linear coordination with oxygen atoms of the CuO2 chains. Phonon dispersion calculations and molecular dynamics simulations confirm the stability of the structure although pointing out that sliding of the silver chains is an easy motion that does not lead to substantial modifications of the electronic structure around the Fermi level and, thus, should not alter the good conductivity of the system. However, this sliding of the silver atoms from the equilibrium position explains the observed large thermal factors.

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.

Trabajo presentado a la Applied Superconductivity Conference (ASC), celebrada en Washington State Convention Center (US) del 28 de octubre al 2 de noviembre de 2018.

Resumen del trabajo presentado al 14th International Workshop on Magnetism & Superconductivity at the Nanoscale, celebrado en Coma-Ruga, El Vendrell (Espana) del 1 al 6 de julio de 2018.

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.

Trabajo presentado a la Applied Superconductivity Conference (ASC), celebrada en Washington State Convention Center (US) del 28 de octubre al 2 de noviembre de 2018.