Magnetic and transport properties of PdH: intriguing superconductive observations

Abstract

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 P dHx 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 * 10 4 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 P dHx 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 P dHx has been predicted.

Electrical resistivity and hydrogen solubility of PdHc thin films

“…Hydrogen-induced resistivity changes in bulk palladium have been widely investigated during the past decades, concerning the exact determination of the resistivity increment Dq H =at%H [1,2], hydrogen-induced changes in the electronic band structure of PdH c and related scattering mechanisms [3], temperature effects [4] as well as superconducting properties of highly concentrated alloys [5]. Based on this, resistivity data are a commonly applied operator for the determination of pressure-concentration isotherms, phase boundaries and enthalpies of hydride formation of PdH c [6,7].…”

Effect of spacer layer on flux-pinning properties of iron-based superconductors

“…To perform analysis on flux pinning dominant processes in single crystals present in the sample we use the analysis of the higher harmonic (III Harmonic) behaviour of the multi-harmonic ac susceptibility in function of temperature and magnetic field. In fact, the high harmonic components ac detect only the non-linear components in the signal, separate and amplify the signals due to different superconducting phases present in the sample [41][42][43][44][48][49][50][51], even when there are small amount of superconducting phases or evanescent superconducting phases are present [52,53]. The latter are connected to changes of the effective flux-diffusivity tuned by the flux-pinning interaction [39,40].…”

Answer to the remarks on superconductivity in PdH

“…Hence the measured T c rarely corresponds to the measured x value during the H loading process. We have bestowed on the H stability for high x value in PdH samples much attention and many experimental experiences based on electrochemical approach explained in our paper [5]. Our PdH samples are stable and only if the mean x value in PdH sample is well stable, it is possible to have a "true" correspondence between T c and x parameter.…”

“…Following this issue, other three further important papers on magnetic measurements [5][6][7] have been published. The paper [5] is a review on the PdH samples preparation, temperature coefficient resistivity measurements, resistance measurements, a phenomenological model and some ac susceptibility magnetic data.…”

“…The paper [5] is a review on the PdH samples preparation, temperature coefficient resistivity measurements, resistance measurements, a phenomenological model and some ac susceptibility magnetic data. In these last three papers [5][6][7] new and significant tests of the HTS phases in our PdH samples have been shown. In particular the following items:…”