Hybrid superconducting InAs-nanowire based nanojunctions

Semiconductor-superconductor hybrid devices have been investigated for many years. In these devices the macroscopic quantum correlations typical of superconductors can be induced via proximity effect in a wide class of low-dimensional materials such as, topological insulator , ferromagnetic barriers ], graphene and in particular semiconducting nanowires (NWs) which o�er a unique freedom in the design and control of quantum states. Precise control of the geometry, composition and chemical properties enabled the NWs to be a potential building block in felds, like nanoelectronics, photonics, mechanical and biological systems or sensors [32]. The most intriguing consequence of such a material combination is the possibility to give raise to novel excitations and properties that none of the single device components originally possesses. Recently, intense activity was aimed at arti�cially creating topologically protected Majorana fermion states. Many proposals make use quasi-onedimensional (quasi-1D) low critical temperature superconductors (LTS) in contact with topological insulators or quasi-one-dimensional materials with strong spin-orbit interactions (InAs & InSb NWs). Efforts have been made to use unconventional barriers with LTS, which o�er a much better compatibility and therefore nominal better interfaces, thus limiting, by low critical magnetic felds and low gap energy, for instance, the range parameters necessary to observe MBSs. HTS are much harder to integrate with other systems, but present the advantage as higher critical temperature, gap energy and huge stability to the magnetic field. Device that combine semiconducting nanowires (InAs-NWs) with HTS exploit both superconducting properties available with ceramic HTS and the high-purity of InAs-NWs together with xviii Hybrid superconducting InAs-nanowire based nanojunctions xix the tunability of their transport. The NWs represent the conductive channel in a superconductor-semiconductor-superconductor �eld-e�ect transistor (SUFET) device architecture and provide an ideal mesoscopic system to study both quantum con�nement and interference e�ects at low temperature, which is a promising platform to develop novel quantum devices, and the limits of quantum suppression of superconductivity. Nanoscale ordering and phase transition in complex oxides, where the electrons self-organize in ways qualitatively di�erent from those of conventional metals and insulators, is one of the most outstanding problems in physics today, and studies of nanoscale devices may have a formidable impact on that. Mixing bottom-up and top-down nanofabrication approaches have allowed building Josephson junctions in non-suspended and suspended design. Sophisticate EBL techniques have been developed and aim to align the InAs-NWs. Design device on which the InAs-NWs are positioned in random and guided (Dielectrophoresis (DEP) technique) way are fabricated. The InAs-NWs show external native oxide shell that hampers the current ux trough the interface, reason that procedures that allow to remove the oxide and get a good transparency between InAs-NWs and superconductor has been developed. Currentvoltage measurements as function of the temperature show, for both designs, a critical current amplitude that increases when the temperature decrease. Novel insights on macroscopic superconducting coupling in extreme conditions imposed by the nanostructures have been derived. The nanotechnology platform developed for suspended LTS junctions has represented the starting point for the integration between YBCO, the most used HTS, and NWS. This is an extremely challenging, high risk but extremely rewarding activity. Many designs have been developed for this type of junction. Many nanofabrication problems due to the complex structure of YBCO unit cell have been solved. In order to minimize the YBCO damaging e�ects due to out-di�usion oxygen from unit cell the nanofabrication recipe developed includes two sessions of electron beam lithography (EBL), two e-resists (PMMA) baking steps at low temperature and for few minutes, one dry etching session performed at very low temperature and one fast wet etching Hybrid superconducting InAs-nanowire based nanojunctions xx step. Special design of devices that allows verifying electric shorts before the InAs-NWs deposition have been developed. Innovative superconducting hybrid devices InAs-NWs/YBCO have been characterized with electric transport measurements as function of temperature and show as a current injected into YBCO banks can through the InAs-NWs that works as a weak link. I have demonstrated the feasibility of the fabrication procedure of YBCO/suspended InAs-NW/YBCO junctions. The �rst achievement of my work is that junctions YBCO/ suspended InAs-NW/YBCO of length � 200nm are not insulating. Current passes through the InAs-NWS, demonstrating the feasibility of the whole fabrication process. The other encouraging result is that di�erently from InAs-NWs/Aluminium junctions resistance behavior observed for InAs- NW/YBCO devices depends on the normal length (L) of junction [43]. The work is structured in seven chapters. The �rst chapter is dedicated to a review of the general mechanisms of the electric transport in the normal state. In the second chapter an introduction to superconductivity and to the concept of Josephson coupling have been discussed. The third chapter is dedicated to the description of the properties of InAs material (bulk and nanostructutered). The fourth chapter summarizes all the nanofabrication procedures developed for the fabrication of non-suspended junction between InAsnanowire and aluminium, suspended junction between InAs-nanowire and aluminium and in the last sections,the e�orts accomplished for the fabrication of the novel type of hybrid systems between InAsnanowire/ YBCO will be shown. In the �fth chapter the cryogenic system and the measurement setup used for the characterization of the devices builded are described. In the sixth chapter the transport measures, and their elaboration, of Hybrid superconducting InAs-nanowire based nanojunctions xxi InAs-nanowires and aluminium junctions will be shown. In the seven chapter the transport measurements of InAs-nanowires/YBCO junctions and their comparative analysis performed with InAs-nanowires and aluminium junctions have been performe

Coherent transport in extremely underdoped Nd1.2Ba1.8Cu3Oz nanostructures

Proximity-effect and resistance magneto-fluctuations measurements in submicron Nd1.2Ba1.8Cu3Oz (NBCO) nano-loops are reported to investigate coherent charge transport in the non-superconducting state. We find an unexpected inhibition of cooper pair transport, and a destruction of the induced superconductivity, by lowering the temperature from 6K to 250mK. This effect is accompanied by a significant change in the conductance-voltage characteristics and in the zero bias conductance response to the magnetic field pointing to the activation of a strong pair breaking mechanism at lower temperature. The data are discussed in the framework of mesoscopic effects specific to superconducting nanostructures, proximity effect and high temperature superconductivity.Comment: to appear on new journal of Physic

Enhanced Josephson coupling in hybrid nanojunctions

We have fabricated NbN/Au nanogaps and bridged them with an Al superconductor using Ti as an interlayer. The nanodevices show a critical current density at 300 mK as high as 3 7106A/cm2, which is 30% higher than that of Al nanowires with the same lateral dimensions as the NbN-based devices. The response of the critical current as a function of the external magnetic field clearly showed a Fraunhofer-like behavior, indicating a Josephson coupling between the NbN electrodes through the Al barrier. The superconducting transport evolves into different transport regimes as a function of the temperature. These findings demonstrate the importance of using superconducting barriers in hybrid nanodevices to achieve very high Josephson current in nanodevices of great relevance in superconducting circuits requiring high integration density

Little-Parks effect in single YBaCuO sub-micron rings

The properties of single submicron high-temperature superconductor (HTS) rings are investigated. The Little-Parks effect is observed and is accompanied by an anomalous behavior of the magnetic dependence of the resistance, which we ascribe to non-uniform vorticity (superfluid angular momentum) within the ring arms. This effect is linked to the peculiar HTS-relationship between the values of the coherence length and the London penetration depth.Comment: 14 pages, 3 figure

Work stress and burnout among physicians and nurses in Internal and Emergency Departments

Burnout has been defined as loss of enthusiasm for work, feelings of cynicism, and a low sense of personal accomplishment. Work environment and working conditions exposes the individual to numerous factors of stress. Stress-related diseases are defined as burnout. The increased workload, the repeated reorganizations in the hospital with iterative downsizing suggestions and budget cuts, without any perspective of career progression, with a social culture of bureaucracy and blame, resulting both in subtracting direct care time with patients and in the fear by healthcare professionals from the burden of their responsibility, are the backgrounds on which more and more frequent cases of burnout may develop. We need to establish homogenous standards all over the national territory on workload and about the procedures that have to be implemented for the prevention of burnout in our wards

Fast Tunable High-Q-Factor Superconducting Microwave Resonators

We present fast tunable superconducting microwave resonators fabricated from planar NbN on a sapphire substrate. The 3 lambda/4 wavelength resonators are tuning fork shaped and tuned by passing a dc current that controls the kinetic inductance of the tuning fork prongs. The lambda/4 section from the open end operates as an integrated impedance converter that creates a nearly perfect short for microwave currents at the dc terminal coupling points, thus preventing microwave energy leakage through the dc lines. We measure an internal quality factor Q(int) > 10(5) over the entire tuning range. We demonstrate a tuning range of greater than 3% and tuning response times as short as 20 ns for the maximum achievable detuning. Because of the quasifractal design, the resonators are resilient to magnetic fields of up to 0.5 T

A FEM Free Vibration Analysis of Variable Stiffness Composite Plates through Hierarchical Modeling

Variable Angle Tow (VAT) laminates offer a promising alternative to classical straight-fiber composites in terms of design and performance. However, analyzing these structures can be more complex due to the introduction of new design variables. Carrera’s unified formulation (CUF) has been successful in previous works for buckling, vibrational, and stress analysis of VAT plates. Typically, one-dimensional (1D) and two-dimensional (2D) CUF models are used, with a linear law describing the fiber orientation variation in the main plane of the structure. The objective of this article is to expand the CUF 2D plate finite elements family to perform free vibration analysis of composite laminated plate structures with curvilinear fibers. The primary contribution is the application of Reissner’s mixed variational theorem (RMVT) to a CUF finite element model. The principle of virtual displacements (PVD) and RMVT are both used as variational statements for the study of monolayer and multilayer VAT plate dynamic behavior. The proposed approach is compared to Abaqus three-dimensional (3D) reference solutions, classical theories and literature results to investigate the effectiveness of the developed models. The results demonstrate that mixed theories provide the best approximation of the reference solution in all cases

Effetti della riduzione degli input di coltivazione sul girasole (<i>Helianthus annuus</i> L.) coltivato in diversi ambienti mediterranei

A two year field experiment was carried out on six different environmental sites of Middle and Southern Italy with the aim of evaluating the effects of some low input cultural techniques on yield of sunflower (Helianthus annuus L.). In each sites sunflower commerciai hybrids were sown at different dates between January to April. Normal and minimum soil tillage were included in the experimental design, and irrigation treatments were also tested in two sites. Yield response of sunflower to minimum tillage treatments did not show in most cases a significant variation in comparison with ordinary tillage methods. Early sowing enhanced or decreased sunflower performance according to the agrometeorological features of the sites. Irrigation determined a significant yield effect but in some cases water efficiency was lower in early sowing treatments. The results indicate the possibility of reducing energy input in sunflower without remarkable yield depression, by adapting cultural techniques and hybrid choice to the environmental characteristics. Due anni di sperimentazione in campo sono stati condotti in sei differenti ambienti dell'Italia Centro-Meridionale al fine di valutare la possibilità di una semplificazione della tecnica colturale in girasole. Sono stati saggiati ibridi di diversa precocità seminati in differenti epoche e sottoposti ad un diverso grado di intensificazione delle lavorazioni. In due delle unità operative (UUOO) sono stati sperimentati anche diversi trattamenti irrigui. Nella maggior parte dei casi la lavorazione ridotta non ha determinato differenze significative di resa rispetto alla tecniche tradizionali di preparazione del terreno, mettendo in evidenza la capacità del girasole di sfruttare l'effetto residuo delle lavorazioni. L'anticipo delle semine ha avuto un effetto sulle rese variabile a seconda del sito di sperimentazione, indicando che in alcuni ambienti questo accorgimento consente di utilizzare al meglio le eventuali piogge invernali e primaverili. L'eliminazione o la riduzione degli apporti irrigui ha invece determinato significative riduzioni di produzione, anche se in alcune situazioni questo effetto è risultato meno marcato con le epoche di semina anticipate. Il primo biennio di sperimentazione ha messo in evidenza che la riduzione degli input nella coltivazione del girasole sia proponibile anche dal punto di vista economico purché si adegui la scelta delle tecniche colturali e dei genotipi alle caratteristiche ambientali

Current-phase relation of a short multi-mode Bi2Se3 topological insulator nanoribbon Josephson junction with ballistic transport modes

We used the asymmetric superconducting quantum interference device (SQUID) technique to extract the current phase relation (CPR) of a Josephson junction with a 3D-topological insulator (3D-TI) Bi2Se3 nanobelt as the barrier. The obtained CPR shows deviations from the standard sinusoidal CPR with a pronounced forward skewness. At temperatures below 200 mK, the junction skewness values are above the zero temperature limit for short diffusive junctions. Fitting of the extracted CPR shows that most of the supercurrent is carried by ballistic topological surface states (TSSs), with a small contribution of diffusive channels primarily due to the bulk. These findings are instrumental in engineering devices that can fully exploit the properties of the topologically protected surface states of 3D TIs

Hall-effect in the MnBi_2Te_4 crystal using silicon nitride nanomembrane via contacts

Utilizing an interplay between band topology and intrinsic magnetism, the two-dimensional van der Waals (vdW) system MnBi_2Te_4 provides an ideal platform for realizing exotic quantum phenomena and offers great opportunities in the emerging field of antiferromagnetic spintronic technology. Yet, the fabrication of MnBi_2Te_4-based nanodevices is hindered by the high sensitivity of this material, which quickly degrades when exposed to air or to elevated temperatures. Here, we demonstrate an alternative route of fabricating vdW-MnBi_2Te_4-based electronic devices using the cryogenic dry transfer of a printable circuit embedded in an inorganic silicon nitride membrane. The electrical connections between the thin crystal and the top surface of the membrane are established through via contacts. Our magnetotransport study reveals that this innovative via contact approach enables exploring the MnBi_2Te_4-like sensitive 2D materials and engineering synthetic heterostructures as well as complex circuits based on the two-dimensional vdW systems