448 research outputs found
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
A mechanical model of the smartphone's accelerometer
To increase the attention of students, several physics experiments can be
performed at school, as well at home, by using the smartphone as laboratory
tools. In the paper we describe a mechanical model of the smartphone's
accelerometer, which can be used in classroom to allow students to better
understand the principle of the accelerometer even by students at the beginning
of the study in physics.Comment: 4 pages, 1 embedded figur
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.
Surface-Barrier Effects in the Microwave Second-Harmonic Response of Superconductors in the Mixed State
We report on transient effects in the microwave second-order response of
different type of superconductors in the mixed state. The samples have
contemporarily been exposed to a dc magnetic field, varying with a constant
rate of 60 Oe/s, and a pulsed microwave magnetic field. The time evolution of
the signal radiated at the second-harmonic frequency of the driving field has
been measured for about 500 s from the instant in which the dc-field sweep has
been stopped, with sampling time of about 0.3 s. We show that the
second-harmonic signal exhibits two relaxation regimes; an initial exponential
decay, which endures roughly 10 s, and a logarithmic decay in the time scale of
minutes. Evidence is given that the decay in the time scale of minutes is ruled
by magnetic relaxation over the surface barrier.Comment: 6 pages, 6 embedded figure
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
Nonlinear Microwave Spectrometer for Investigating High-Tc Superconductors
We describe a nonlinear microwave spectrometer, which can be conveniently used for investigating the harmonic generation at microwave frequencies in high-Tc superconductors. The use of this technique allows highlighting mechanisms responsible for the nonlinear microwave response of high-Tc superconductors as well as measuring specific characteristics of the investigated samples. We report a brief review of the main results obtained
Depinning frequency in a heavily neutron-irradiated MgB2 sample
The magnetic-field-induced variations of the microwave surface resistance
have been investigated in a heavily neutron-irradiated MgB2 sample, in which
the irradiation has caused the merging of the two gaps into a single value. The
experimental results have been analyzed in the framework of the Coffey and Clem
model. By fitting the experimental data, we have determined the field
dependence of the depinning frequency, omega_0, at different values of the
temperature. Although the pinning is not particularly effective, the value of
omega_0 obtained at low temperatures is considerably higher than that observed
in conventional low-temperature superconductors.Comment: 15 pages, 6 figures; to be published in Physica
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.
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
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
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