Daniele Di Gioacchino

Optimum inhomogeneity of local lattice distortions in La2CuO4+y

Electronic functionalities in materials from silicon to transition metal oxides are, to a large extent, controlled by defects and their relative arrangement. Outstanding examples are the oxides of copper, where defect order is correlated with their high superconducting transition temperatures. The oxygen defect order can be highly inhomogeneous, even in optimal superconducting samples, which raises the question of the nature of the sample regions where the order does not exist but which nonetheless form the “glue” binding the ordered regions together. Here we use scanning X-ray microdiffraction (with a beam 300 nm in diameter) to show that for La2CuO4+y, the glue regions contain incommensurate modulated local lattice distortions, whose spatial extent is most pronounced for the best superconducting samples. For an underdoped single crystal with mobile oxygen interstitials in the spacer La2O2+y layers intercalated between the CuO2 layers, the incommensurate modulated local lattice distortions form droplets anticorrelated with the ordered oxygen interstitials, and whose spatial extent is most pronounced for the best superconducting samples. In this simplest of high temperature superconductors, there are therefore not one, but two networks of ordered defects which can be tuned to achieve optimal superconductivity. For a given stoichiometry, the highest transition temperature is obtained when both the ordered oxygen and lattice defects form fractal patterns, as opposed to appearing in isolated spots. We speculate that the relationship between material complexity and superconducting transition temperature Tc is actually underpinned by a fundamental relation between Tc and the distribution of ordered defect networks supported by the materials.

Possibility of high temperature superconducting phases in PdH

Possible new superconducting phases with a high critical transition temperature (Tc) have been found in stable palladium–hydrogen (PdHx) samples for stoichiometric ratio x=H/Pd⩾1, in addition to the well-known low critical transition temperature (0⩽Tc⩽9) when x is in the range (0.75⩽x⩽1.00). Possible new measured superconducting phases with critical temperature in the range 51⩽Tc⩽295 K occur. This Tc varies considerably with every milli part of x when x exceeds unit. A superconducting critical current density Jc⩾6.1×104 A cm−2 has been measured at 77 K with HDC=0 T.

Frequency dependence of HTS AC harmonic susceptibility in the Kim-Anderson and collective pinning vortex glass models

By numerically solving the nonlinear magnetic flux diffusion equation, the temperature dependence of the AC magnetic susceptibility have been computed at different frequencies in the framework of the collective pinning vortex glass and of the Kim-Anderson models. In particular the frequency dependence of the amplitude of the third harmonic have been analyzed. Such dependence is determined by the dynamic curvature of the E-J characteristics, normalized to the dynamic resistivity.

Irreversibility line and magnetic field dependence of the critical current in superconducting MgB2 bulk samples

The third harmonic components of the ac susceptibility of MgB2 bulk samples have been measured as a function of applied magnetic fields, together with standard magnetization cycles. The irreversibility line (IL) of the magnetic field has been extracted from the onset of the third harmonic components. Using a (1 − t)α glass/liquid best fit where α = 1.27 IL shows a coherent length ξ divergence with exponent ν = 0.63, which indicates a 3D behaviour. Moreover, using the numerical solution of the non-linear magnetic diffusion equation, considering the creep model in a 3D vortex glass, a good description of the vortex dynamics has been obtained. The behaviour of the magnetization amplitude (~Hz) and the ac susceptibility signals (kHz), at different applied magnetic fields, 3.5 T < Hdc < 4.5 T, and at the reduced temperature 0.86 < t < 0.93 (T = 22 K), shows that the superconducting dynamic response of vortices in the MgB2 samples is not evidently dependent on the frequency.

Magnetic and transport properties of PdH: Intriguing superconductive observations

Since the discovery of superconductivity in palladium-hydrogen (PdH) and its isotopes (D,T) at low temperature, several efforts have been made to study the properties of this system. Superconductivity of PdH system has been initially claimed by resistance drop versus temperature and then confi rmed by dc magnetic susceptibility measurements. These studies have shown that the critical transition temperature is a function of the hydrogen concentration x in the PdHx system. In all these experiments, the highest concentration x of hydrogen in palladium was lower than the unit. In the last decade we defi ned a room temperature and room pressure technique to load hydrogen and its isotopes into palladium at levels higher than unit, using electrochemical set-up, followed by a stabilization process to maintain the hydrogen concentration in palladium lattice stable. In the meanwhile, several measurements of resistance versus temperature have been performed. These measurements have shown several resistive drops in the range of [18K<Tc< 273K] similar to the results presented in literature, when the superconducting phase has been discovered. Moreover, on PdH wires 6cm long the current-voltage characteristic with a current density greater than 6*104 Acm-2 has been measured at liquid nitrogen temperature. These measurements have the same behavior as superconducting I-V characteristic with sample resistivity, at 77K, of two orders of magnitude lower than copper or silver at the same temperature. The measurements of fi rst and third harmonic of ac magnetic susceptibility in PdHx system have been performed. These represent a good tool to understand the vortex dynamics, since the superconducting response is strongly non-linear. Clear ac susceptibility signals confi rming the literature data at low temperature (9K) and new signifi cant signals at high temperature (263K) have been detected. A phenomenological approach to describe the resistance behaviour of PdH versus stoichiometry x at room temperature has been developed. The value x=1.6 to achieve a macroscopic superconducting state in PdHx has been predicted.

Flux Dynamics in Iron-Based Superconductors

The newly discovered iron-based superconductors share a common layered structure. After the iron pnictide (1111) family, other iron-based superconductors such as the iron chalcogenides FeTe1 - xSex (11) family have been synthesized. The latter system, characterized by a simple crystal structure, represents an interesting reference system. Indeed, all iron-based superconductors have a stacked structure composed of a layer of iron atoms linked by tetrahedrally coordinated pnictogens (P, As) or a chalcogen (Se, Te) anion: the active layer. The latter is either simply stacked together, as in the FeSe (11), or separated by spacer layers with alkali (e.g., Li), alkaline earth (e.g., Ba), or rare earth oxides/fluorides. In this contribution, we present an ac multiharmonic susceptibility study of the flux dynamic in representative 1111 and 11 iron-based superconductors. Data analysis has been performed in the glass-weak pinning scenario. In particular, the comparison of the third harmonic components vs. temperature and magnetic field returned information on pinning strength and dimensionality. Although in the presence of large thermal fluctuations, because of the high Tc, susceptibility data points out a three-dimensional flux dynamic and, unexpectedly, an increase of the pinning amplitude in the Fe-based superconductor systems where the spacer layer is present.

Deuterium overloading of palladium wires by means of high power μs pulsed electrolysis and electromigration: suggestions of a “phase transition” and related excess heat

Abstract We describe room-temperature hydrogen and deuterium loading of palladium wires by means of pulsed electrolysis and the electromigration effect. The D Pd atomic ratio has been measured by means of the dependence of the resistivity upon the D Pd ratio. Values of the D Pd ratio up to 0.95 or even higher have been reached in short times. A correlation between an anomalous temperature rise and a resistivity “transition” of the overloaded palladium clearly appears.

A REVIEW OF HIGH TEMPERATURE SUPERCONDUCTING PROPERTY OF PDH SYSTEM

The discovery of superconductivity in palladium-hydrogen (PdH) and its isotopes (D,T) at low temperature, brought about extensive study of this system. These studies have shown that the critical transition temperature is a function of the H concentration x in the PdHx system with Tc=9K for x=1. In the last decade we defined a room temperature and room pressure technique to load H and maintain stable the stoichiometry in Pd lattice at levels higher than unit. Several magnetic and electric transport measurements have been performed showing transition temperature in the range of [18K< Tc < 273K]. Moreover in a typical critical current measurement configuration, current density greater than 6*104Acm-2⊐ has been measured at liquid nitrogen temperature. The 263.5K superconducting transition after a week of sample storage at room pressure and temperature, decreased down to 261.5K and after 2 years it became 160.5K, demonstrating a fairly good stability of the sample. Evidences of the flux exclusion (ZFC measurements) and the flux expulsion (FC measurements) have been found at very high transition temperature (Tc=235K) for the PdH system.

Flux Dynamic Changes by Neutron Irradiation in BISCCO: High Harmonics AC Susceptibility Analysis

High harmonics ac susceptibility measurements joint with neutron irradiations on quasi-bi-dimensional high temperature superconductor (quasi-2D-HTSC) are good tools to study the flux dynamics and its interaction with pinning processes in these superconductors. Flux neutron intensity of shows a deep change in the flux pinning dynamics in Bi-Sr-Ca-Cu-O (BSCCO) system. Third harmonic susceptibility signal increases in amplitude after the neutron irradiation, followed by a rise of the pinning and critical current. Moreover, after the irradiation, the measurements underline the demise of the anomalous peak effect (PE) associated with a three-dimensional/two-dimensional (3D/2D) flux lattice transition.

Glass-collective pinning and flux creep dynamics regimes in MgB/sub 2/ bulk

Superconducting dynamic response in zero field cooling (ZFC) of MgB/sub 2/ bulk has been analyzed using high harmonic ac susceptibility measurements (/spl chi//sub n/). The driving external ac magnetic field is in the frequency range [107Hz<f<2070Hz], the driving external dc magnetic field is in the range [0T<H/sub dc/<5T] and the temperature has been varied in the range [15K<T<38K]. These responses are characterized from the critical states that decay in glass states via creep processes. In this analysis different frequency values at fixed H/sub dc/ investigate different current levels. This way, different pinning regimes as single vortex, small bundle, large bundle are studied. Using this approach some parts of the MgB/sub 2/ three-dimensional phase diagram H-T-J can be described.