103 research outputs found
Field-induced suppression of the pi-band superconductivity and magnetic hysteresis in the microwave surface resistance of MgB_2 at temperatures near T_c
We report on the magnetic-field-induced variations of the microwave surface
resistance, R_s, in a polycrystalline MgB_2 sample, at different values of
temperature. We have detected a magnetic hysteresis in R_s, which exhibits an
unexpected plateau on decreasing the DC magnetic field below a certain value.
In particular, at temperatures near T_c the hysteresis manifests itself only
through the presence of the plateau. Although we do not quantitatively justify
the anomalous shape of the magnetic hysteresis, we show that the results
obtained in the reversible region of the R_s(H) curve can be quite well
accounted for by supposing that, in this range of magnetic field, the pi-gap is
almost suppressed by the applied field and, consequently, all the pi-band
charge carriers are quasiparticles. On this hypothesis, we have calculated
R_s(H) supposing that fluxons assume a conventional (single core) structure and
the flux dynamics can be described in the framework of conventional models.
From the fitting of the experimental results, we determine the values of
H_{c2}^pi(T) at temperatures near T_c. In our opinion, the most important
result of our investigation is that, at least at temperatures near T_c, the
value of the applied field that separates the reversible and irreversible
regions of the R_s(H) curves is just H_{c2}^pi(T); a qualitative discussion of
the possible reason of this finding is given.Comment: 20 pages, 8 embedded figures, 2 Appendices, accepted for publication
in Supercond. Sci. Techno
Critical-state effects on microwave losses in type-II superconductors
We discuss the microwave energy losses in superconductors in the critical
state. The field-induced variations of the surface resistance are determined,
in the framework of the Coffey and Clem model, by taking into account the
distribution of the vortex magnetic field inside the sample. It is shown that
the effects of the critical state cannot generally be disregarded to account
for the experimental data. Results obtained in bulk niobium at low temperatures
are quantitatively justified.Comment: 4 pages, 4 embedded figures, to be published on Eur. Phys. J.
Time relaxation of microwave second order response of superconductors in the critical state
Relaxation of the microwave second order response of YBa2Cu3O7 and
Ba0.6K0.4BiO3 crystals in the critical state is studied. The samples are
exposed to static and pulsed microwave magnetic fields. The second harmonic
signals decay during the time in which the microwave pulse endures. The decay
times depend on the supercon-ductor investigated and on the way the value of
the static field has been reached.Comment: 2 pages, 1 figur
Fluxon dynamics in Li-Al codoped MgB2 by microwave surface resistance measurements
The magnetic-field-induced variations of the microwave surface resistance,
R_s, have been investigated in ceramic Mg_{1-x}(LiAl)_xB_2, with x in the range
0.1 - 0.4. The measurements have been performed on increasing and decreasing
the DC magnetic field, H_0, at fixed temperatures. At low temperatures, we have
observed a magnetic hysteresis in the R_s(H_0) curves in all the investigated
samples. On increasing the temperature, the range of H_0 in which the
hysteretic behavior is visible shrinks; however, in the sample with x = 0.1 it
is present up to temperatures close to T_c. We show that the field dependence
of R_s can be quantitatively justified taking into account the critical-state
effects on the fluxon lattice only in the sample with x = 0.4. On the contrary,
in the samples with x < 0.4 the hysteresis exhibits an unusual shape, similar
to that observed in others two-gap MgB_2 samples, which cannot be justified in
the framework of the critical-state models.Comment: 8 pages, 3 embedded figures; proceedings of the Sixth International
Conference on Vortex Matter in Nanostructured Superconductors (VORTEX VI)
17-24 September 2009, Rhodes, Greec
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
Preliminary Assessment of Parmigiano Reggiano Authenticity by Handheld Raman Spectroscopy
Raman spectroscopy, and handheld spectrometers in particular, are gaining increasing attention in food quality control as a fast, portable, non-destructive technique. Furthermore, this technology also allows for measuring the intact sample through the packaging and, with respect to near infrared spectroscopy, it is not affected by the water content of the samples. In this work, we evaluate the potential of the methodology to model, by multivariate data analysis, the authenticity of Parmigiano Reggiano cheese, which is one of the most well-known and appreciated hard cheeses worldwide, with protected denomination of origin (PDO). On the other hand, it is also highly subject to counterfeiting. In particular, it is critical to assess the authenticity of grated cheese, to which, under strictly specified conditions, the PDO is extended. To this aim, it would be highly valuable to develop an authenticity model based on a fast, non-destructive technique. In this work, we present preliminary results obtained by a handheld Raman spectrometer and class-modeling (Soft Independent Modeling of Class Analogy, SIMCA), which are extremely promising, showing sensitivity and specificity of 100% for the test set. Moreover, another salient issue, namely the percentage of rind in grated cheese, was addressed by developing a multivariate calibration model based on Raman spectra. It was possible to obtain a prediction error around 5%, with 18% being the maximum content allowed by the production protocol
Time evolution of the microwave second-order response of YBaCuO powder
Transient effects in the microwave second-order response of YBaCuO powder are
investigated. The time evolution of the second harmonic signal has been
measured for about 300 s after the sample had been exposed to variations of the
DC magnetic field. We show that in different time scales the transient response
has different origin. In the time scale of milliseconds the transient response
of samples in the critical state is ascribable to processes of flux
redistribution induced by the switching on/off of the microwave field. At
longer times, the time evolution of the second harmonic signal can be ascribed
to motion of fluxons induced by the variation of the DC magnetic field. In
particular, diffusive motion of fluxons determines the response in the first 10
seconds after the stop of the magnetic field variation; magnetic relaxation
over the surface barrier determines the response in the time scale of minutes.Comment: 16 pages, 7 figures, submited to Physica
Superconducting Microwave Cavity Made of Bulk MgB2
We report the successful manufacture and characterization of a microwave
resonant cylindrical cavity made of bulk MgB2 superconductor (Tc = 38.5 K),
which has been produced by the Reactive Liquid Mg Infiltration technique. The
quality factor of the cavity for the TE011 mode, resonating at 9.79 GHz, has
been measured as a function of the temperature. At T = 4.2 K, the unloaded
quality factor is 2.2x10^5; it remains of the order of 10^5 up to T ~ 30 K. We
discuss the potential performance improvements of microwave cavities built from
bulk MgB2 materials produced by reactive liquid Mg infiltration.Comment: 7 pages, 2 embedded figures, accepted for publication in Supercond.
Sci. Techno
Electromagnetic response of LaO_0.94F_0.06FeAs: AC susceptibility and microwave surface resistance
We discuss on the electromagnetic response of a polycrystalline sample of
LaO_0.94F_0.06FeAs exposed to DC magnetic fields up to 10 kOe. The low- and
high-frequency responses have been investigated by measuring the AC
susceptibility at 100 kHz and the microwave surface resistance at 9.6 GHz. At
low as well as high DC magnetic fields, the susceptibility strongly depends on
the amplitude of the AC driving field, highlighting enhanced nonlinear effects.
The field dependence of the AC susceptibility exhibits a magnetic hysteresis
that can be justified considering the intragrain-field-penetration effects on
the intergrain critical current density. The microwave surface resistance
exhibits a clockwise magnetic hysteresis, which cannot be justified in the
framework of the critical-state models of the Abrikosov-fluxon lattice; it may
have the same origin as that detected in the susceptibility.Comment: 8 pages, 4 embedded eps figures; Proceedings of the 9th EUCAS
Conference (Dresden, Germany, September 13-17, 2009
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