337 research outputs found
Microwave response of bulk MgB2 samples of different granularity
The microwave response of three high-density bulk MgB2 samples has been
investigated in the linear and nonlinear regimes. The three samples,
characterized by different mean size of grains, have been obtained by reactive
infiltration of liquid Mg in powdered B preforms. The linear response has been
studied by measuring the microwave surface impedance; the nonlinear response by
detecting the power radiated by the sample at the second-harmonic frequency of
the driving field. Our results suggest that bulk MgB2 prepared by the liquid Mg
infiltration technique is particularly promising for manufacturing resonant
cavities operating at microwave frequencies.Comment: 4 pages, 2 embedded figures; Proceedings of 7th EUCAS Conference
(11-15 September 2005, Vienna - Austria
Fluxon dynamics by microwave surface resistance measurements in MgB2
Field-induced variations of the microwave surface resistance, Rs(H), have
been investigated in high-density ceramic MgB2. At low temperatures, several
peculiarities of the Rs(H) curves cannot be justified in the framework of
models reported in the literature. We suggest that they are ascribable to the
unconventional vortex structure in MgB2, related to the presence of two gaps.
On the contrary, the results near Tc can be accounted for by the Coffey and
Clem model, with fluxons moving in the flux-flow regime, provided that the
anisotropy of the upper critical field is taken into due account.Comment: 6 pages, 4 figure
Microwave Harmonic Emission in MgB2 Superconductor: Comparison with YBaCuO
We report results of microwave second-harmonic generation in ceramic samples
of MgB2, prepared by different methods. The SH signal has been investigated as
a function of the temperature and the static magnetic field. The results are
discussed in the framework of models reported in the literature. We show that
the peculiarities of the SH signal are related to the specific properties of
the sample. A comparison with the results obtained in ceramic and crystalline
YBa(2)Cu(3)O(7) shows that the second-harmonic emission in MgB2 is weaker than
that observed in ceramic YBa(2)Cu(3)O(7).Comment: 13 pages, 6 figures; Proceedings of Third Workshop on Metamaterials
and Special Materials for Electromagnetic Applications and TLC (Rome, 30-31
March, 2006
Josephson effect in MgB_2 break junctions
We present the first observation of the DC and AC Josephson effect in MgB_2
break junctions. The junctions, obtained at 4.2 K in high-quality, high-density
polycrystalline metallic MgB_2 samples, show a non-hysteretic DC Josephson
effect. By irradiating the junctions with microwaves we observe clear Shapiro
steps spaced by the ideal value. The temperature dependence of the
DC Josephson current and the dependence of the height of the steps on the
microwave power are obtained. These results are a direct prove for the
existence of pairs with charge 2e in this new metallic superconductor and give
evidence of the superconductor-normal metal-superconductor weak link character
of these junctions.Comment: 4 RevTEX pages, 4 eps figure
Near-Tc second-harmonic emission in high-density bulk MgB2 at microwave frequency
We discuss the microwave second-harmonic generation in high-density bulk
MgB2, prepared by the reactive liquid Mg infiltration technology. The intensity
of the harmonic signal has been investigated as a function of temperature and
amplitudes of the DC and microwave magnetic fields. The results are discussed
in the framework of a phenomenological theory, based on the two-fluid model,
which assumes that both the microwave and static magnetic fields, penetrating
in the surface layers of the sample, weakly perturb the partial concentrations
of the normal and superconducting fluids. We show that, in order to account for
the experimental results, it is essential to suppose that in MgB2 the densities
of the normal and condensed fluids linearly depend on the temperature.Comment: 6 pages, 4 embedded figures, submitted to Eur. Phys. J.
Detection of the Vortex Dynamic Regimes in MgB2 by Third Harmonic AC Susceptibility Measurements
In a type-II superconductor the generation of higher harmonics in the
magnetic response to an alternating magnetic field is a consequence of the
non-linearity in the I-V relationship. The shape of the current-voltage (I-V)
curve is determined by the current dependence of the thermal activation energy
U(J) and is thus related to the dynamical regimes governing the vortex motion.
In order to investigate the vortex dynamics in MgB2 bulk superconductors we
have studied the fundamental (chi1) and third (chi3) harmonics of the ac
magnetic susceptibility. Measurements have been performed as a function of the
temperature and the dc magnetic field, up to 9 T, for various frequencies and
amplitudes of the ac field. We show that the analysis of the behaviour in
frequency of chi3(T) and chi3(B) curves can provide clear information about the
non-linearity in different regions of the I-V characteristic. By comparing the
experimental curves with numerical simulations of the non-linear diffusion
equation for the magnetic field we are able to resolve the crossover between a
dissipative regime governed by flux creep and one dominated by flux flow
phenomena.Comment: to be published in "Horizons in Superconductivity Research" (Nova
Science Publishers, Inc., NY, 2003
Carrier Transport in Magnesium Diboride: Role of Nano-inclusions
Anisotropic-gap and two-band effects smear out the superconducting transition
(Tc) in literature reported thermal conductivity of MgB2, where large
electronic contributions also suppress anomaly-manifestation in their
negligible phononic-parts. Present thermal transport results on scarcely
explored specimens featuring nano-inclusions exhibit a small but clear
Tc-signature, traced to relatively appreciable phononic conduction, and its
dominant electronic-scattering. The self-formed MgO as extended defects
strongly scatter the charge carriers and minutely the phonons with their
longer-mean-free-path near Tc. Conversely, near room temperature, the
shorter-dominant-wavelength phonon's transport is hugely affected by these
nanoparticles, undergoing ballistic to diffusive crossover and eventually
entering the Ioffe-Regel mobility threshold regime.Comment: 14 pages, 4 figures, 28 reference
A new interpretation of the 1982-84 unrest episode at Campi Flegrei caldera (Italy) by numerical inversion
The 1982-84 unrest episode at Campi Flegrei was characterized by huge deformation (about 1.8 m uplift) located inside the caldera and significant gravity variations correlated with the elevation changes (about -213 Gal/m). Due to the bell shape of the uplift, the source is usually interpreted to have a fixed spherical shape.
In the present study, we combine simple point source mechanisms (dipoles and double couples) to represent arbitrary sources such as sphere, ellipsoid or sill.
The models are realized by Finite Element and the medium may be characterized by elastic heterogeneities.
We study the deformation detected by leveling and EDM techniques by coupling the FE forward models with an inversion procedure.
The potential point sources are contained in a volume of 888 km located beneath Pozzuoli, the site of maximum displacement.
We calculate the displacement field at each data point for each basic mechanism and we compare the result with the observed value.
From the inversion of geodetic data we retrieve the best-fitting source parameters, without fixing the shape a priori.
The best-fitting source is located beneath Pozzuoli at about 4.8 km b.s.l. and undergoes to horizontal compression and vertical dilatation
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