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Register a new userFrequency dependence of the intergranular magnetic flux penetration in ceramic $YBa_2Cu_3O_x$ uperconductor
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By means of small magnetic field ac susceptibility measurement at 10 kHz we found that the real and imaginary parts of ceramic $YBa_2Cu_3O_ x$ susceptibility in presence of the external low frequency field close to 0.1 Hz exhibit frequency dependence. The wide maximum of hysteresis losses and {color{black} the exponential dependence} of the {color{black} effectiveness of flux penetration} with increasing of external field frequency were obtained. We observed a nonlinear dynamic magnetic response in presence dc field and we suggest that this behavior is due to the dynamic and static Josephson vortex-vortex interaction.
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We report transverse field and zero field muon spin rotation studies of the superconducting rhenium oxide pyrochlore, Cd2Re2O7. Transverse field measurements (H=0.007 T) show line broadening below Tc, which is characteristic of a vortex state, demonstrating conclusively the type-II nature of this superconductor. The penetration depth is seen to level off below about 400 mK (T/Tc~0.4), with a rather large value of lambda (T=0)~7500A. The temperature independent behavior below ~ 400 mK is consistent with a nodeless superconducting energy gap. Zero-field measurements indicate no static magnetic fields developing below the transition temperature.
One of the features of the unconventional $s_pm$ state in iron-based superconductors is possibility to transform to the $s_{++}$ state with the increase of the nonmagnetic disorder. Detection of such a transition would prove the existence of the $s_pm$ state. Here we study the temperature dependence of the London magnetic penetration depth within the two-band model for the $s_pm$ and $s_{++}$ superconductors. By solving Eliashberg equations accounting for the spin-fluctuation mediated pairing and nonmagnetic impurities in the $T$-matrix approximation, we have derived a set of specific signatures of the $s_pm to s_{++}$ transition: (1) sharp change in the behavior of the penetration depth $lambda_{L}$ as a function of the impurity scattering rate at low temperatures; (2) before the transition, the slope of $Delta lambda_{L}(T) = lambda_{L}(T)-lambda_{L}(0)$ increases as a function of temperature, and after the transition this value decreases; (3) the sharp jump in the inverse square of the penetration depth as a function of the impurity scattering rate, $lambda_{L}^{-2}(Gamma_a)$, at the transition; (4) change from the single-gap behavior in the vicinity of the transition to the two-gap behavior upon increase of the impurity scattering rate in the superfluid density $rho_{s}(T)$.
Muon-spin rotation spectroscopy has been used to measure the internal magnetic field distribution in NbSe2 for Hc1 << H < 0.25 Hc2. The deduced profiles of the supercurrent density indicate that the vortex-core radius in the bulk decreases sharply with increasing magnetic field. This effect, which is attributed to increased vortex-vortex interactions, does not agree with the dirty-limit microscopic theory. A simple phenomenological equation in which the core radius depends on the intervortex spacing is used to model this behaviour. In addition, we find for the first time that the in-plane magnetic penetration depth increases linearly with H in the vortex state of a conventional superconductor.
The flux penetration near a semicircular indentation at the edge of a thin superconducting strip placed in a transverse magnetic field is investigated. The flux front distortion due to the indentation is calculated numerically by solving the Maxwell equations with a highly nonlinear $E(j)$ law. We find that the excess penetration, $Delta$, can be significantly ($sim$ 50%) larger than the indentation radius $r_0$, in contrast to a bulk supercondutor in the critical state where $Delta=r_0$. It is also shown that the flux creep tends to smoothen the flux front, i.e. reduce $Delta$. The results are in very good agreement with magneto-optical studies of flux penetration into an YBa$_2$Cu$_3$O$_x$ film having an edge defect.
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