arXiv subjects

C. Cirillo

Publications and source records attributed to C. Cirillo.

At least 19 recordsLinked to original sources

arXiv2026

Nitrogen-doped W0.75Re0.25 Superconducting Nanowire Single Photon Detectors

Nitrogen-doped Tungsten-Rhenium superconducting alloys were recently proposed as a promising material platform for superconducting nanowire single-photon detectors (SNSPDs), offering a favorable balance between high normal state resistivity and tunable superconducting properties. In this work, we report on the fabrication and characterization of SNSPDs based on thin W0.75Re0.25 films deposited by reactive DC magnetron sputtering in a mixed Ar/N2 atmosphere. Meander detectors with 70 nm linewidth exhibit saturated internal detection efficiency (IDE) up to 1310 nm and 85.3% IDE at 1550 nm at 2.5 K, with sub-nanosecond rise times, decay times of the order of a few nanoseconds, and timing jitter of 73.2 ps measured with room temperature amplifiers.

cond-mat.supr-con↗

arXiv2025

Tuning Superconductivity in Sputtered W0.75Re0.25 Thin Films

W0.75Re0.25, in its bulk form, has been shown to be an interesting superconducting material due to its multiple crystalline phases, each exhibiting distinct superconducting characteristics. However, little is known about how these phases manifest in thin-film form, where deposition conditions and dimensionality are critical aspects. Here, we investigate superconducting W0.75Re0.25 thin films deposited via UHV dc magnetron sputtering. In order to tune the crystalline phase of the films, we further explored the effect of incorporating N2 during the deposition. The superconducting and normal-state properties as a function of deposition conditions were investigated, revealing the role of the crystal phase on the film transport properties.

cond-mat.supr-con↗

arXiv2025

Unveiling Intrinsic Triplet Superconductivity in Noncentrosymmetric NbRe through Inverse Spin-Valve Effects

NbRe is a non-centrosymmetric superconductor that has been proposed as a candidate for intrinsic spin-triplet pairing. However, a conclusive demonstration of triplet pairing in NbRe is yet to be found. To probe the presence of equal-spin triplet Cooper pairs, we fabricated Py/NbRe/Py trilayers capped with an antiferromagnetic layer. Magnetic and electrical measurements reveal an inverse spin-valve effect, which could indicate equal-spin triplet superconductivity. The minimal sample structure and the lack of ad hoc engineered interfaces clearly associate our observation to intrinsic triplet correlations of NbRe. The availability of NbRe in thin-film form and the simplicity of the heterostructure highlight its potential as a scalable platform for superconducting spintronics.

cond-mat.supr-con↗

arXiv2025

Superconducting Nanowire Single Photon Detectors based on NbRe nitride ultrafilms

The influence of the reactive DC sputtering parameters on the superconducting properties of NbReN ultrathin films was investigated. A detailed study of the current-voltage characteristics of the plasma was performed to optimize the superconducting critical temperature, Tc. The thickness dependence of Tc for the films deposited under different conditions was analyzed down to the ultrathin limit. Optimized films were used to fabricate superconducting nanowire single photon detectors which, at T=3.5 K, show saturated internal detection efficiency (IDE) up to a wavelength of 1301 nm and 95% IDE at 1548 nm with recovery times and timing jitter of about 8 ns and 28 ps, respectively.

cond-mat.supr-con↗

arXiv2024

Single photon detection up to 2 um in pair of parallel microstrips based on NbRe ultrathin films

Superconducting Microstrip Single Photon Detectors (SMSPDs) are increasingly attracting the interest of the scientific community as a new platform for large area detectors with unprecedented advantaged in terms of fabrication. However, while their operativity at the telecommunication wavelength was achieved, working beyond 1.55 um is challenging. Here, we experimentally demonstrate single-photon operation of NbRe microstrips at wavelengths of 1.55 and 2 um. The devices are structured as pairs of parallel microstrips with widths ranging from 1.4 to 2.2 um and lengths from 5 to 10 um. This innovative design may assure large sensitive areas, without affecting the kinetic inductance, namely the time performance of the detectors. The results are discussed in the framework of the hot-spot two-temperature model.

cond-mat.supr-con↗

arXiv2023

Demonstration of high-impedance superconducting NbRe Dayem bridges

Here we demonstrate superconducting Dayem-bridge weak-links made of different stoichiometric compositions of NbRe. Our devices possess a relatively high critical temperature, normal-state resistance, and kinetic inductance. In particular, the high kinetic inductance makes this material a good alternative to more conventional niobium-based superconductors (e.g., NbN or NbTiN) for the realization of superinductors and high-quality factor resonators, whereas the high normal-state resistance yields a large output voltage in superconducting switches and logic elements realized upon this compound. Moreover, out-of-plane critical magnetic fields exceeding 2 T ensure that possible applications requiring high magnetic fields can also be envisaged. Altogether, these features make this material appealing for a number of applications in the framework of quantum technologies.

cond-mat.supr-con↗

arXiv2019

Emergence of a metallic meta-stable phase induced by electrical current in Ca_2RuO_4

A comprehensive study of the behavior of the Mott insulator Ca_2RuO_4 under electrical current drive is performed by combining two experimental probes: the macroscopic electrical transport and the microscopic X-Ray diffraction. The resistivity, rho, vs electric current density, J, and temperature, T , rho(J,T), resistivity map is drawn. In particular, the meta-stable state, induced between the insulating and the metallic thermodynamic states by current biasing Ca_2RuO_4 single crystals, is investigated. Such an analysis, combined with the study of the resulting RuO_6 octahedra energy levels, reveals that a metallic crystal phase emerges in the meta-stable regime. The peculiar properties of such a phase, coexisting with the well-established orthorhombic insulating and tetragonal metallic phases, allow to explain some of the unconventional and puzzling behaviors observed in the experiments, as a negative differential resistivity.

cond-mat.str-el↗

arXiv2016

Emergence of the stripe-domain phase in patterned Permalloy films

The occurrence of stripe domains in ferromagnetic Permalloy (Py=Fe$_{20}$Ni$_{80}$) is a well known phenomenon which has been extensively observed and characterized. This peculiar magnetic configuration appears only in films with a thickness above a critical value ($d_{cr}$), which is strongly determined by the sputtering conditions (i.e. deposition rate, temperature, magnetic field). So far, $d_{cr}$ has usually been presented as the boundary between the homogeneous (H) and stripe-domains (SD) regime, respectively below and above $d_{cr}$. In this work we study the transition from the H to the SD regime in thin films and microstructured bridges of Py with different thicknesses. We find there is an intermediate regime, over a quite significant thickness range below d$_{cr}$, which is signaled in confined structures by a quickly changing domain-wall configuration and by a broadening of the magnetoresistance dip at the coercive field. We call this the emerging stripe-domains (ESD) regime. The transition from the ESD to the SD regime is accompanied by a sharp increase of the magnetoresistance ratio at the thickness where stripes appear in MFM.

cond-mat.mes-hall↗

arXiv2015

Robustness of the $0 -π$ transition against compositional and structural ageing in S/F/S heterostructures

We have studied the temperature induced $0 -π$ thermodynamic transition in Nb/PdNi/Nb Superconductor/Ferromagnetic/Superconductor (SFS) heterostructures by microwave measurements of the superfluid density. We have observed a shift in the transition temperature with the ageing of the heterostructures, suggesting that structural and/or chemical changes took place. Motivated by the electrodynamics findings, we have extensively studied the local structural properties of the samples by means of X-ray Absorption Spectroscopy (XAS) technique, and the compositional profile by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). We found that the samples have indeed changed their properties, in particular for what concerns the interfaces and the composition of the ferromagnetic alloy layer. The structural and compositional data are consistent with the shift of the $0-π$ transition toward the behaviour of heterostructures with different F layers. An important emerging indication to the physics of SFS is the weak relevance of the ideality of the interfaces: even in aged samples, with less-than-ideal interfaces, the temperature-induced $0-π$ transition is still detectable albeit at a different critical F thickness.

cond-mat.supr-con↗

arXiv2014

Superconducting nanowire quantum interference device based on Nb ultrathin films deposited on self-assembled porous Si templates

Magnetoresistance oscillations were observed on networks of superconducting ultrathin Nb nanowires presenting evidences of either thermal or quantum activated phase slips. The magnetic transport data, discussed in the framework of different scenarios, reveal that the system behaves coherently in the temperature range where the contribution of the fluctuations is important.

cond-mat.supr-con↗

arXiv2014

Thermodynamic nature of the 0-PI quantum transition in superconductor-ferromagnet-superconductor trilayers

In structures made up of alternating superconducting and ferromagnet layers (S/F/S heterostructures), it is known that the macroscopic quantum wavefunction of the ground state changes its phase difference across the F--layer from 0 to PI under certain temperature and geometrical conditions, whence the name "0-PI" for this crossover. We present here a joint experimental and theoretical demonstration that the "0-PI" is a true thermodynamic phase transition: microwave measurements of the temperature dependence of the London penetration depth in Nb/Pd_0.84Ni_0.16/Nb trilayers reveal a sudden, unusual decrease of the density of the superconducting condensate (square modulus of the macroscopic quantum wavefunction) with decreasing temperature, which is predicted by the theory here developed as a transition from the 0- to the PI-state. Our result for the jump of the amplitude of the order parameter is the first thermodynamic manifestation of such temperature-driven quantum transition.

cond-mat.supr-con↗

arXiv2013

Nonlinear current-voltage characteristics due to quantum tunneling of phase slips in superconducting Nb nanowire networks

We report on the transport properties of an array of N about 30 interconnected Nb nanowires, grown by sputtering on robust porous Si substrates. The analyzed system exhibits a broad resistive transition in zero magnetic field, H, and highly nonlinear V(I) characteristics as a function of H which can be both consistently described by quantum tunneling of phase slips.

cond-mat.supr-con↗

arXiv2012

Superconducting and structural properties of Nb/PdNi/Nb trilayers

The superconducting and structural properties of S/F/S (Superconductor/Ferromagnet/Superconductor) heterostructures have been studied by means of microwave measurements (1-20 GHz) and x-ray absorption fine structure (XAFS) spectroscopy. Nb/PdNi/Nb trilayers have been studied as a function of F layer thicness. With respect to pure Nb, XAFS analysis shows that the heterostructures exhibit larger structural disorder in the S layers. Microwave measurements show evidence for a progressively weaker vortex pinning with increasing F thickness. However, no clear correlation is found with the local disorder in Nb: the weakest pinning is not in the most disordered trilayer. Therefore the structural disorder in the superconducting material cannot explain on its own the changes in vortex pinning. We argue that the F layer acts on the superconducting state itself. We propose possible explanations for the observed behavior.

cond-mat.supr-con↗

arXiv2012

Microwave properties of Nb/PdNi/Nb trilayers. Observation of flux flow in excess of Bardeen-Stephen theory

We combine wideband (1-20 GHz) Corbino disk and dielectric resonator (8.2 GHz) techniques to study the microwave properties in Nb/PdNi/Nb trilayers, grown by UHV dc magnetron sputtering, composed by Nb layers of nominal thickness $d_S$=15 nm, and a ferromagnetic PdNi layer of thickness $d_F$= 1, 2, 8 and 9 nm. We focus on the vortex state. Magnetic fields up to $H_{c2}$ were applied. The microwave resistivity at fixed $H/H_{c2}$ increases with $d_F$, eventually exceeding the Bardeen Stephen flux flow value.

cond-mat.supr-con↗

arXiv2012

Quantum phase slips in superconducting Nb nanowire networks deposited on self-assembled Si templates

Robust porous silicon substrates were employed for generating interconnected networks of superconducting ultrathin Nb nanowires. Scanning electron microscopy analysis was performed to investigate the morphology of the samples, which constitute of polycrystalline single wires with grain size of about 10 nm. The samples exhibit nonzero resistance over a broad temperature range below the critical temperature, fingerprint of phase slippage processes. The transport data are satisfactory reproduced by models describing both thermal and quantum fluctuations of the superconducting order parameter in thin homogeneous superconducting wires.

cond-mat.supr-con↗

arXiv2012

Non-monotonic behaviour of the superconducting order parameter in Nb/PdNi bilayers observed through point contact spectroscopy

Point contact spectroscopy measurements have been performed on Nb/PdNi bilayers in which the thickness of the Nb layer, dNb, was kept constant to 40 nm while the thickness of PdNi, dPdNi, was changed from 2 nm to 9 nm. Features related to the superconducting gap induced in the ferromagnet have been observed in the dV/dI versus V curves. These structures show a non-monotonic behaviour as a function of dPdNi as a consequence of the damped oscillatory behaviour of the superconducting order parameter in the ferromagnetic layer.

cond-mat.supr-con↗

arXiv2011

Vortex motion in Nb/PdNi/Nb trilayers: new aspects in the flux flow state

We study the dynamics of vortex lines in Supercondutor/Ferromagnet/Superconductor (SFS) heterostructures at microwave frequencies. We have employed swept-frequency, Corbino-disk and resonant, dielectric-resonator techniques to obtain the field and temperature dependence of the vortex-state parameters. We concentrate here on the genuine flux-flow resistivity $ρ_{ff}$, that we access at subcritical currents using a sufficiently high driving frequency. We find that $ρ_{ff}$ does not follow the well-known Bardeen-Stephen model. Instead, it is well described by a full time-dependent Ginzburg-Landau expression at very thin F layer thickness, but changes to a previously unreported field-dependence when the F layer exceeds a few nm.

cond-mat.supr-con↗

arXiv2009

Evidence of fractional matching states in nanoperforated Nb thin film grown on porous silicon

Resistive transitions have been measured on a perforated Nb thin film with a lattice of holes with the period of the order of ten nanometers. Bumps in the dR/dH versus H curves have been observed at the first matching field and its fractional values, 1/4, 1/9 and 1/16. This effect has been related to different vortex lattice configurations made available by the underlying lattice of holes.

cond-mat.supr-con↗