Marcello Gombos - Academia.edu (original) (raw)

Papers by Marcello Gombos

Superconductor Science and Technology, 2017

IEEE Transactions on Applied Superconductivity, 2022

Gd1212 (GdSr2RuCu2O8-y) is a superconductor, isostructural to Y123 (YBa2Cu3O7-x), whose high inte... more Gd1212 (GdSr2RuCu2O8-y) is a superconductor, isostructural to Y123 (YBa2Cu3O7-x), whose high interest is due to the coexistence of superconducting and magnetic ordered phases in the same unit cell. Up to now the best superconducting properties for Gd1212 have been achieved by TSG (Top-Seeded Melt-Textured growth), a well-known technique used to grow high quality samples from High Temperature Superconductors characterized by a peritectic melting reaction. Due to the relevant production of liquid during the melting of Y123-like materials, the standard TSG procedure involves the addition of secondary phases in the starting powders mixture, generally the peritectic solid phase or one of its precursors, to enhance the structural stability of pellets during the process. This practice also provides additional pinning centers in the final samples, improving their Superconducting features. Unfortunately, Gd1212 peritectic reaction produces an insufficient amount of liquid phase, so preventing from obtaining wide crystallographic domains, and the addition of the peritectic solid phase Gd1210 (GdSr2RuO6), although beneficial on the superconducting properties, even worsen this problem. In this work we tried to grow larger Gd1212 domains by inverting the usual approach: i.e., by adding in the precursor powders some peritectic liquid phase (CuO) instead of Gd1210. Anyway, previous analyses suggested us that the chemistry of Gd1212 melting may change for excess CuO mixtures. We have characterized the obtained samples by means of Raman spectroscopy and mapping, data were compared with those of Gd1210-doped and pure Gd1212 samples, results confirm our hypothesis of different melting reaction for CuO rich samples.

Scientific Reports, 2016

Vortices are topological defects accounting for many important effects in superconductivity, supe... more Vortices are topological defects accounting for many important effects in superconductivity, superfluidity, and magnetism. Here we address the stability of a small number of such excitations driven by strong external forces. We focus on Abrikosov-Josephson vortex that appears in lateral superconducting S/S'/S weak links with suppressed superconductivity in S'. In such a system the vortex is nucleated and confined in the narrow S' region by means of a small magnetic field and moves under the effect of a force proportional to an applied electrical current with a velocity proportional to the measured voltage. Our numerical simulations show that when a slow moving Abrikosov-Josephson vortex is driven by a strong constant current it becomes unstable with respect to a faster moving excitation: the Josephon-like vortex. Such a current-driven transition explains the structured dissipative branches that we observe in the voltage-current curve of the weak link. When vortex matter is strongly confined phenomena as magnetoresistance oscillations and reentrance of superconductivity can possibly occur. We experimentally observe these phenomena in our weak links.

$Research paper thumbnail of Low frequency transport measurements in $ \mathsf {GdSr_2RuCu_2O_8}$$

The European Physical Journal B - Condensed Matter, 2003

Low frequency transport measurements are performed on GdSr2RuCu2O8 pellets. The observed current-... more Low frequency transport measurements are performed on GdSr2RuCu2O8 pellets. The observed current-voltage curves are qualitatively explained in the framework of a simple phenomenological model accounting for coexistence of ferromagnetism and superconductivity in the sample. A Curie temperature TcM =133 K and a superconducting critical temperature TcS=18 K, with an onset temperature TcO=33 K, are extracted from the analysis of the current-voltage curves.

cm diameter single domain NdBaCuO pellets, ob- tained from powders of Nd Ba Cu O (Nd123) with Nd ... more cm diameter single domain NdBaCuO pellets, ob- tained from powders of Nd Ba Cu O (Nd123) with Nd BaO (Nd210) addition, have been successfully prepared by Top Seeding Melt Textured Growth technique. Distribution of Nd Ba Cu O (Nd422) precipitates in the Nd123 matrix has been analyzed by means of Polarized Light Optical Microscopy and Scanning Elec- tron Microscopy. Relevant distribution inhomogeneity in the pe- ripheral region of pellets with a high Nd210 addition have been re- vealed, as well as in regions of dendritic growth and around pores. Increasing difficulties in obtaining a complete growth of the single domain samples, in dependence of increasing Nd210 addition has been observed too. A qualitative growth model for high Nd concen- tration samples has been proposed to explain both these behaviors.

Superconductor Science and Technology

We report an experimental and numerical study of magneto-transport properties of mesoscopic later... more We report an experimental and numerical study of magneto-transport properties of mesoscopic lateral S/N/S superconducting weak links where the N region is made of the same material as the S banks, though with strongly reduced critical temperature. Magnetoresistance oscillations and clear dc and ac Josephson effects are observed. Experimental results are analyzed in the framework of the time-dependent Ginzburg–Landau model for mesoscopic type II superconductors with an inhomogeneous critical temperature. The analysis suggests that dissipative branches of the current–voltage curve of the weak link in the presence of a magnetic field are accounted for by moving 'Josephson-like' vortices. These relatively fast excitations are anisotropic as per the ordinary Josephson vortex in tunnel junctions, but have a normal core like the ordinary Abrikosov vortex in plain superconducting strips. Moreover, unlike the vortex in tunneling junctions, in the lateral S/N/S weak link, the extension of the moving vortex is larger than the extension of the static one. Further, we report in some detail on the lateral proximity effect, and the deviations from the ideality of the current–phase relation of this kind of lateral weak link in the Josephson regime.

arXiv: Superconductivity, 2001

Low frequency transport measurements are performed on GdSr$_2$RuCu$_2$O$_8$ pellets. The observed... more Low frequency transport measurements are performed on GdSr$_2$RuCu$_2$O$_8$ pellets. The observed current-voltage curves are qualitatively explained in the framework of a simple phenomenological model accounting for coexistence of ferromagnetism and superconductivity in the sample. A Curie temperature TcMT_{cM}TcM=133 K and a superconducting critical temperature TcST_{cS}TcS=18 K, with an onset temperature TcOT_{cO}TcO=33 K, are extracted from the analysis of the current-voltage curves.

Strongly anisotropic magnetic and superconducting properties have been recently observed to coexi... more Strongly anisotropic magnetic and superconducting properties have been recently observed to coexist in Gd1212 . Magnetic ordering appears to be due to Ru planes while Cu planes are correlated to the superconducting properties. The unavailability of single crystals makes relevant the fabrication of oriented samples of macroscopic size by melt texturing techniques. Then we have grown MTG samples starting from 1 cm diameter pellets. Gd1210 is added, analogously to Y211 in YBCO MTG processes. Preliminary studies have been performed on Gd1212 powders to explore the range of temperatures and the reactions involved. XRD analysis shows that a certain degree of orientation is present in our sam- ples. By Polarized Light Optical Microscopy we have observed large oriented domains of almost millimeter size. Larger domains are expected by optimization of growth parameters. Images show even a homogeneous distribution of Gd1210 precipitates with a mean size of a few micrometers. SEM and EDS analys...

IEEE Transactions on Applied Superconductivity

IEEE Transactions on Applied Superconductivity, 2016

Physica C Superconductivity

GdSr 2 RuCu 2 O 8Àd (Gd1212) rutheno-cuprate compound is widely studied because it shows the coex... more GdSr 2 RuCu 2 O 8Àd (Gd1212) rutheno-cuprate compound is widely studied because it shows the coexistence of superconductivity and highly anisotropic magnetic ordering in the same cell. Anyway the nature of the magnetic ordering is still debated and the fabrication of macroscopic samples oriented along a crystallographic axis, that could help to clarify its details, appears to be quite difficult. For this reason, accurate investigation on Gd1212 incongruent melting reaction, producing liquid copper oxide and the solid GdSr 2 RuO 6 (Gd1210), appears to be necessary to improve the fabrication of crystallographically oriented bulk superconducting samples. Thermo-gravimetric and differential thermal measurements were performed to analyse the thermal behaviour up to 1200 °C and the melt-ing temperatures of different mixtures of Gd1212 with Gd1210 or CuO, in view to perform a scan of the whole CuO–Gd1212–Gd1210 coexistence line of the Gd–Sr–Ru–Cu–O phase diagram. Dependence on the atmosphe...

International Journal of Modern Physics B, 2003

Topographic and spectroscopic information on GdSr2RuCu2O8 sintered pellets have been obtained by ... more Topographic and spectroscopic information on GdSr2RuCu2O8 sintered pellets have been obtained by a home built low temperature Scanning Tunneling Microscope (STM) operating at 4.2 K. The topographic image of the surface showed non homogeneous samples with grains of typical size of about 100 nm. In many locations studied, the Tunneling Spectroscopy reveals the presence of charging effects in the current-voltage characteristics over a voltage range up to 100 mV. Two types of charging effects are clearly distinguished: one corresponds to the reduction of the tunneling conductance around zero bias and is attributed to the Coulomb blockade, and another onw, a stepwise increasing of the current as a function of the bias voltage is identified as Coulomb staircase regime. Besides these spurious charging effects, the current-voltage characteristics often show a pronounced non-linearity around 4.0 mV. This non-linearity, disappearing above the critical temperature of the materials, is connecte...

Physica C: Superconductivity, 2002

Physica C: Superconductivity, 2007

Journal of Materials Research, 2003

In this work, we report on the observation of Nd1Ba6Cu3O10,5 (Nd163) phase of the NdBaCuO system ... more In this work, we report on the observation of Nd1Ba6Cu3O10,5 (Nd163) phase of the NdBaCuO system in melt-textured Nd123 bulk samples grown from a mixture of Nd123 and Nd210 phase powders. The observation was performed with polarized light optical microscopy and scanning electron microscopy–energy dispersive x-ray analyses. Images of the identified phase crystals show an aspect quite different from Nd422 crystals. Unexpectedly, Nd163 was individuated, even in “pure” Nd123 samples. Moreover, after long exposure to air, Nd163 disappeared completely in samples synthesized from powders containing Nd210. Thermogravimetry analyses of powders show that the stability of this phase in air is limited to temperatures higher than 900 °C, so Nd163 is unstable and highly reactive at room temperature. Moreover, an explanation of the observation of Nd163 in Nd210 free samples, based on the spontaneous formation of Nd163 phase in a Nd123 melt, is proposed.

Journal of Magnetism and Magnetic Materials, 2004