V. Ferrando

Effects of neutron irradiation on polycrystalline Mg 11 B 2

We studied the influence of the disorder introduced in polycrystalline MgB2 samples by neutron irradiation. To circumvent self shielding effects due to the strong interaction between thermal neutrons and 10B we employed isotopically enriched 11B which contains 40 times less 10B than natural B. The comparison of electrical and structural properties of different series of samples irradiated in different neutron sources, also using Cd shields, allowed us to conclude that, despite the low 10B content, the main damage mechanisms are caused by thermal neutrons, whereas fast neutrons play a minor role. Irradiation leads to an improvement in both upper critical field and critical current density for an exposure level in the range 1-2x1018 cm-2. With increasing fluence the superconducting properties are depressed. An in-depth analysis of the critical field and current density behaviour has been carried out to identify what scattering and pinning mechanisms come into play. Finally the correlation between some characteristic lengths and the transition widths is analysed.

Effect of two bands on critical fields in MgB 2 thin films with various resistivity values

Upper critical fields of four MgB2 thin films were measured up to 28 Tesla at Grenoble High Magnetic Field Laboratory. The films were grown by Pulsed Laser Deposition and showed critical temperatures ranging between 29.5 and 38.8 K and resistivities at 40 K varying from 5 to 50 mWcm. The critical fields in the perpendicular direction turned out to be in the 13-24 T range while they were estimated to be in 42-57 T the range in ab-planes. In contrast to the prediction of the BCS theory, we did not observe any saturation at low temperatures: a linear temperature dependence is exhibited even at lowest temperatures at which we made the measurements. Moreover, the critical field values seemed not to depend on the normal state resistivity value. In this paper, we analyze these data considering the multiband nature of superconductivity in MgB2 We will show how the scattering mechanisms that determine critical fields and resistivity can be different.

As-grown magnesium diboride superconducting thin films deposited by pulsed laser deposition

As-grown superconducting MgB2 thin films were deposited by pulsed laser deposition on magnesium oxide and sapphire substrates. Starting from a non-stoichiometric, Mg and B mixed-powder target, we were able to grow the superconducting phase during the film deposition, without any further annealing process. So far, samples grown in the temperature range of 400-450??C, and at an argon buffer pressure of the order of 10-2?mbar turned out to be superconducting with an onset temperature of the resistive transition at about 25?K. Even if the deposition process still needs to be fully optimized, we have demonstrated that this method allows us to achieve in?situ deposition of as-grown superconducting thin films. This procedure could therefore be promising for the deposition of high-quality epitaxial MgB2 thin films.

Growth methods of c-axis oriented MgB2 thin films by pulsed laser deposition

High-quality MgB2 thin films have been obtained by pulsed laser deposition both on MgO and Al2O3 substrates using different methods. In the standard two-step procedure, an amorphous precursor layer is deposited at room temperature starting from the stoichiometric target and from the boron target. After this first step, it is annealed in a magnesium atmosphere in order to crystallize the superconducting phase. The obtained films show a strong c-axis orientation, evidenced by XRD analysis, a critical temperature up to 38 K and very high critical fields along the basal planes, up to 22 T at 15 K. Also an in situ one step technique for the realization of superconducting MgB2 thin films has been developed. In this case, the presence of an argon buffer gas during deposition is crucial, and we observe a strong dependence of the quality of the deposited film on the background gas pressure. The influence of the Ar atmosphere has been confirmed by time- and space-resolved spectroscopy measurements on the emission spectrum of the plume. The Ar pressure strongly modifies the plasma kinetics by promoting excitation and ionization of the plume species, especially of the most volatile Mg atoms, increasing their internal energy.

Upper critical fields up to 60 T in dirty magnesium diboride thin films

Upper critical fields of several magnesium diboride thin films were measured up to 28 T at the Grenoble High Magnetic Field Laboratory (GHMFL) in Grenoble and up to 60 T at the Laboratoire National des Champs Magne/spl acute/tiques Pulse/spl acute/s (LNCMP) in Toulouse. The samples were prepared both by pulsed laser deposition (PLD) and hybrid physical chemical vapor deposition (HPCVD) technique; they have critical temperatures between 29 and 39 K and normal state resistivities between 5 and 250 /spl mu//spl Omega/ cm; one of them has been intentionally doped with carbon. The measured critical fields, /spl mu//sub 0/H/sub c2/, were exceptionally high; we obtained the record value of 52 T at 4.2 K in the parallel orientation. In contrast with the BCS predictions, no saturation in H/sub c2/ at low temperature was observed. Furthermore, films with a wide range of resistivity values showed similar critical fields, suggesting that in a two band system resistivity and H/sub c2/ are not trivially linked. The high H/sub c2/ values seem to be related with the expanded c-axis. The structure of one of the samples was carefully investigated with X-ray diffraction at European Synchrotron Radiation Facility (ESRF) in Grenoble.

Systematic study of disorder induced by neutron irradiation in MgB2 thin films

The effects of neutron irradiation on normal state and superconducting properties of epitaxial magnesium diboride thin films are studied up to fluences of 1020 cm-2. All the properties of the films change systematically upon irradiation. Critical temperature is suppressed and, at the highest fluence, no superconducting transition is observed down to 1.8 K. Residual resistivity progressively increases from 1 to 190 microohmcm; c axis expands and then saturates at the highest damage level. We discuss the mechanism of damage through the comparison with other damage procedures. The normal state magnetoresistivity of selected samples measured up to high fields (28 and 45T) allows to determine unambiguously the scattering rates in each band; the crossover between the clean and dirty limit in each sample can be monitored. This set of samples, with controlled amount of disorder, is suitable to study the puzzling problem of critical field in magnesium diboride thin films. The measured critical field values are extremely high (of the order of 50T in the parallel direction at low fluences) and turns out to be rather independent on the experimental resistivity, at least at low fluences. A simple model to explain this phenomenology is presented.The effects of neutron irradiation on normal state and superconducting properties of epitaxial magnesium diboride thin films are studied up to fluences of 1020cm−2. All the properties of the films change systematically upon irradiation. Critical temperature is suppressed and, at the highest fluence, no superconducting transition is observed down to 1.8K. Residual resistivity progressively increases from 1to190μΩcm; c axis expands and then saturates at the highest damage level. We discuss the mechanism of damage through the comparison with other damage procedures. The normal state magnetoresistivity of selected samples measured up to high fields (28 and 45T) allows to determine unambiguously the scattering rates in each band; the crossover between the clean and dirty limit in each sample can be monitored. This set of samples, with controlled amount of disorder, is suitable to study the puzzling problem of critical field in magnesium diboride thin films. The measured critical field values are extremely high...

Pulsed laser deposition of epitaxial titanium diboride thin films

Epitaxial titanium diboride thin films have been deposited on sapphire substrates by Pulsed Laser Ablation technique. Structural properties of the films have been studied during the growth by Reflection High Energy Electron Diffraction (RHEED) and ex-situ by means of X-ray diffraction techniques; both kinds of measurements indicate a good crystallographic orientation of the TiB2 film both in plane and along the c axis. A flat surface has been observed by Atomic Force Microscopy imaging. Electrical resistivity at room temperature resulted to be five times higher than the value reported for single crystals. The films resulted to be also very stable at high temperature, which is very promising for using this material as a buffer layer in the growth of magnesium diboride thin films.Abstract Epitaxial titanium diboride thin films have been deposited on sapphire substrates by pulsed laser ablation technique. Structural properties of the films have been studied during the growth by reflection high energy electron diffraction (RHEED) and ex-situ by means of X-ray diffraction techniques; both kinds of measurements indicate a good crystallographic orientation of the TiB 2 film both in plane and along the c axis. A flat surface has been observed by atomic force microscopy imaging. Electrical resistivity at room temperature resulted to be five times higher than the value reported for single crystals. The films also resulted to be very stable at high temperature, which is very promising for using this material as a buffer layer in the growth of magnesium diboride thin films.

Role of interband scattering in neutron irradiated MgB2 thin films by scanning tunneling spectroscopy measurements

A series of

Magnetoresistivity as a probe of disorder in the it and σ bands of MgB2

\mathrm{Mg}{\mathrm{B}}_{2}

Epitaxial MgB2 thin films on ZrB2 buffer layers: structural characterization by synchrotron radiation

thin films systematically disordered by neutron irradiation have been studied by scanning tunneling spectroscopy. The