Modulation of Josephson current in out of equilibrium superconductors

Since the liquefaction of helium by Kramerlingh Onnes in 1911 and the birth of low temperature physics, superconducting systems have brought to the attention of the scientific community a number of new and interesting physical phenomena. Recently, out-of-equilibrium superconducting systems have gained much attention from both the theoretical and experimental points of view and allowed the realization of controllable Josephson junctions through injection of an electric current. The first proposal for the use of out-of-equilibrium phenomena in superconductors was put forward by Parmenter in 1961. The first demonstration of enhancement of superconductivity by quasiparticles extraction was performed by Chi and Clarke in 1979 on a three-layer stack of aluminum separated by tunnel barriers. In 1992 Blamire et al. showed superconductivity enhancement in a Nb-Al-Nb tunnel junction. In both these experiments gap-enhancement was demonstrated by observing features at finite energies in the current-voltage characteristics. More recently (1999) Manninen et al. demonstrated electron-cooling effects by extraction of quasiparticles in an array of Al-AlO-Ti junctions. The zero bias resistance of an additional junction was exploited for thermometry. It was also experimentally demonstrated that the Josephson current in superconductor-normal metal-superconductor (SNS) junctions can be controlled by injection of a current through the normal metal, even producing phase reversal of the supercurrent (pi-junction). It is in this framework that this work originates with the purpose of designing and fabricating superconducting junctions controllable by tunneling extraction of quasiparticles in order to investigate the behavior of the Josephson current in out-of- equilibrium superconductors. To do this I realized a device consisting of a superconducting titanium island of 1.5 micron x 600 nm connected to two aluminum superconducting reservoirs through tunnel junctions. These reservoirs are DC biased to extract quasiparticles from the island and in this way drive it out of equilibrium. Two additional aluminum tunnel contacts are used to measure the Josephson current as the bias voltage of the reservoirs is varied. The devices were fabricated at the Scuola Normale NEST Laboratory and measured at Helsinki University of Technology. I was able to demonstrate heating and cooling of the titanium island inducing an out-of-equilibrium distribution by varying the voltage bias of the control line. This led to the observation of supercurrent at lattice temperatures up to about twice the critical temperature of titanium, demonstrating an electron-cooling effect at bath temperatures about 350-400 mK. This phenomenology was consistently reproduced in all devices measured. About the processing phase, some useful results on the behavior of aluminum oxide for the case of interfaces with titanium deserve mention. Refinements in the shadow-evaporation technique allowed to reproducibly fabricate aluminum-titanium tunneling junctions with a good flexibility in terms of specific resistance and geometry.Protocols were optimized to consistently produce superconducting Al-AlxOx-Ti junctions with specific resistance down to 100 Ohm*micron^2, and sizes down to 150 micron x 150 micron or smaller. Experimental results on supercurrent values in out-of-equilibrium all-superconducting structures were analyzed within the present quasiequilibrium theory. Some unexpected features in the voltage-current characteristics of superconducting junctions will also be reported and discussed by extending the available quasiequilibrium model. Good qualitative agreement with the major features of the full currentvoltage characteristics and with the observed behavior of the supercurrent versus different injection voltages will be shown

A simple trick to improve the accuracy of PIV/PTV data

Particle Image Velocimetry (PIV) estimates velocities through correlations of particle images within interrogation windows, leading to a spatial modulation of the velocity field. Although in principle Particle Tracking Velocimetry (PTV) estimates locally a non-modulated particle displacement, to exploit the scattered data from PTV it is necessary to interpolate these data on a structured grid, which implies a spatial modulation effect that biases the resulting velocity field. This systematic error due to finite spatial resolution inevitably depends on the interrogation window size and on the interparticle spacing. It must be observed that all these operations (cross-correlation, direct interpolation or averaging in windows) induce modulation on both the mean and the fluctuating part. We introduce a simple trick to reduce this systematic error source of PIV/PTV measurements exploiting ensemble statistics. Ensemble Particle Tracking Velocimetry (EPTV) can be leveraged to obtain the high-resolution mean flow by merging the different instantaneous realisations. The mean flow can be estimated with EPTV, and the fluctuating part can be measured from PIV/PTV. The high-resolution mean can then be superposed to the instantaneous fluctuating part to obtain velocity fields with lower systematic error. The methodology is validated against datasets with a progressively increasing level of complexity: two virtual experiments based on direct numerical simulations (DNS) of the wake of a fluidic pinball and a channel flow and the experimental data of a turbulent boundary layer. For all the cases both PTV and PIV are analysed.This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No 949085, project: NEXTFLOW).Publicad

An end-to-end KNN-based PTV approach for high-resolution measurements and uncertainty quantification

We introduce a novel end-to-end approach to improving the resolution of PIV measurements. The method blends information from different snapshots without the need for time-resolved measurements on grounds of similarity of flow regions in different snapshots. The main hypothesis is that, with a sufficiently large ensemble of statistically-independent snapshots, the identification of flow structures that are morphologically similar but occurring at different time instants is feasible. Measured individual vectors from different snapshots with similar flow organisation can thus be merged, resulting in an artificially increased particle concentration. This allows to refine the interrogation region and, consequently, increase the spatial resolution. The measurement domain is split in subdomains. The similarity is enforced only on a local scale, i.e. morphologically-similar regions are sought only among subdomains corresponding to the same flow region. The identification of locally-similar snapshots is based on unsupervised K-nearest neighbours search in a space of significant flow features. Such features are defined in terms of a Proper Orthogonal Decomposition, performed in subdomains on the original low-resolution data, obtained either with standard cross-correlation or with binning of Particle Tracking Velocimetry data with a relatively large bin size. A refined bin size is then selected according to the number of sufficiently close snapshots identified. The statistical dispersion of the velocity vectors within the bin is then used to estimate the uncertainty and to select the optimal K which minimises it. The method is tested and validated against datasets with a progressively increasing level of complexity: two virtual experiments based on direct simulations of the wake of a fluidic pinball and a channel flow and the experimental data collected in a turbulent boundary layer.This project has received funding from the European Research Council (ERC) under the European Union Horizon 2020 research and innovation program (grant agreement No 949085). Funding for APC: Universidad Carlos III de Madrid (Read & Publish Agreement CRUE-CSIC 2022)

Experimental comparison between traditional and cryogenic cooling conditions in rough turning of Ti-6Al-4V

Titanium alloys, mainly because of their poor thermal conductivity, need to be cut at relatively low cutting speeds to avoid a severe diffusion wear, with obvious negative consequences on the profitability of machining. An important amount of research activities has been done in order to increase productivity in titanium machining operations and one of the most promising solutions is represented by the use of liquid nitrogen as a coolant during the machining operation. The aim of this paper is to compare traditional and cryogenic turning of Ti6Al4V in a region of cutting parameters particularly relevant to the aerospace industry where no previous data are available. The cutting parameters are those typical of titanium alloys rough machining which is considered, cost-wise, the most important operation because, for aerospace components, the so-called Buy-To-Fly ratio can reach values up to 20:1. The experiments have been performed using a full factorial design in order to statistically evaluate, using ANOVA and regression analyses, the significance of the input factors on the process most interesting outputs. The considered input factors are: type of cooling method, cutting speed and feed rate. The main analysed responses are: tool wear, surface roughness, cutting forces, coefficient of friction and chip morphology. The results show the significance of the cooling method on the tool life and that cryogenic machining is able to increase the tool life with respect to wet cutting. On the other hand, the beneficial effect of the liquid nitrogen cooling is reduced at high cutting speed and feed rate. Besides, the results showed that a small but significant reduction can be achieved for both the repulsion force and the coefficient of friction at the tool-workpiece interface

On the mechanics of chip formation in Ti-6Al-4V turning with spindle speed variation

"In order to enhance material removal rate (MRR), a strategy that relies on higher depths of cut could be chosen if vibrational issues due to regenerative chatter did not occur. A lot of research was done to suppress regenerative chatter without detrimental effects on productivity. One of the most interesting chatter suppression methods, mainly due to its flexibility and relative ease of implementation, is spindle speed variation (SSV), which consists in a continuous modulation of the nominal cutting speed. Sinusoidal spindle speed variation (SSSV) is a specific technique that exploits a sinusoidal law to modulate the cutting speed. The vast scientific literature on SSV was mainly focused on cutting process stability issues fully neglecting the study of the mechanics of chip formation in SSV machining. The aim of this work is to fill this gap: thus, finite element method (FEM) models of Ti-6Al-4V turning were setup to simulate both SSSV and constant speed machining (CSM). The models consider both the micro-geometry of the insert and the coating. Numerical results were experimentally validated on dry turning tests of titanium tubes exploiting the experimental assessment of cutting forces, cutting temperatures and chip morphology. Tool-chip contact pressure, tool engagement mechanism and the thermal distribution in the insert are some of the analysed numerical outputs because they cannot be easily assessed by experimental procedures. These quantities were useful to compare thermo-mechanical loads of the insert both in CSM and SSSV machining: it was observed that the loads significantly differ. Compared to CSM, the modulation of the cutting speed involves a higher tool-chip contact pressure peak, a higher maximum temperature and higher temperature gradients that could foster the main tool wear mechanisms. (C) 2013 Elsevier Ltd. All rights reserved.

Reperti lapidei di et\ue0 romana rinvenuti a Fornovo Taro: provenienza e circolazione

Nelle collezioni del Museo Archeologico Nazionale di Parma tra i reperti litici di et\ue0 romana figurano anche tre oggetti recuperati a Fornovo Taro o nelle sue adiacenze diversi per contesto di ritrovamento e destinazione d\u2019uso, di cui si \ue8 analizzata la pietra ai fini di determinarne la provenienza e le possibili rotte commerciali. In particolare sono stati presi in esame:- una epigrafe frammentaria (n.inv. L 49), databile al II secolo d.C., gi\ue0 murata in uno dei pilastri della Pieve ed entrata a far parte delle collezioni museali nel 18671; - una piccola scultura raffigurante Afrodite accovacciata secondo il modello dello scultore ellenistico Doidalsas (n.inv. MANPr 31963), recuperata negli anni Ottanta del secolo scorso al margine occidentale di Piazza IV Novembre, databile ai primi decenni sempre del II secolo d.C.2; - una base di colonna (mancante di n.inv. e abbreviata con la sigla BCG), recuperata nel 1980 a Roncolungo di Sivizzano tra i materiali accatastati ai margini della strada statale dal proprietario del terreno in attesa di disfarsene

Accuracy of Microwave Transistor fT and fMAX Extractions

We present a complete methodology to evaluate the accuracy of microwave transistor figures-of-merit fT (current gain cut-off frequency) and fMAX (maximum oscillation frequency). These figures-of-merit are usually extracted from calibrated S-parameter measurements affected by residual calibration and measurement uncertainties. Thus, the uncertainties associated to fT and fMAX can be evaluated only after an accurate computation of the S-parameters uncertainties, including the contribution from de-embedding. This was done with the aid of two recently released software tools. We also present an analysis on how different interpolation/extrapolation methodologies affect uncertainty. Finally, an overview of the possible causes of errors and suggestions on how to avoid them are given. With the continued rise of reported fT /fMAX values, this study has become necessary in order to add confidence intervals to these figures-of-meri

Millimeter-wave GaN-based HEMT development at ETH-Zürich

We review the AlGaN/GaN high electron mobility transistor (HEMT) activities in the Millimeter-Wave Electronics Group at ETH-Zürich. Our group's main thrust in the AlGaN/GaN arena is the extension of device bandwidth to higher frequency bands. We demonstrated surprising performances for AlGaN/GaN HEMTs grown on high-resistivity (HR) silicon (111) substrates, and extended cutoff frequencies of 100nm gate devices well into the millimeter (mm)-wave domain. Our results narrow the performance gap between GaN-on-SiC (or sapphire) and GaN-on-silicon and establish GaN-on-Si as a viable technology for low-cost mm-wave electronics. We here contrast the difference in behaviors observed in our laboratory between nominally identical devices built on high-resistivity silicon (HR-Si) and on sapphire substrates; we show high-speed devices with high-cutoff frequencies and breakdown voltages which combine fT,MAX×BV products as high as 5-10 THz V, and show AlGaN/GaN HEMTs with fT values exceeding 100GHz on HR-Si. Although the bulk of our activities have so far focused on AlGaN/GaN HEMTs on HR-Si, our process produces excellent device performances when applied to GaN HEMTs on SiC as well: 100nm gate transistors with fT>125GHz have been realized at ETH-Züric

Triple-Cut Computer-Aided Design-Computer-Aided Modeling: More Oncologic Safety Added to Precise Mandible Modeling

PURPOSE: Computer-aided design-computer-aided modeling (CAD-CAM) has become standard in mandibular reconstruction because it offers better outcomes. Occasionally, the reconstructive plans need to be changed intraoperatively and the custom-made prefabricated devices may become inadequate. We present an efficient adjunct to the standard CAD-CAM technique that resolves this problem. MATERIALS AND METHODS: Customized surgical devices with our "triple-cut" concept were used in 5 patients for mandibular reconstruction with free fibula flap (4 after mandibular resection for squamous cell carcinoma and 1 after mandibular osteoradionecrosis). In all patients the mandibular and fibular cutting guides were provided with 3 different cutting levels per side. RESULTS: Three different cutting levels on the mandible permitted an accurate resection based on the intraoperative needs. The corresponding 3 "cutting levels" on the fibula created perfectly matching segments of vascularized bone. Good contact of bony segments was obtained in all patients. CONCLUSIONS: The prefabricated triple-cut cutting guides make changing the dimensions of bony resection, while still using the prefabricated CAD-CAM reconstructive plate, possible