Search results for "MAGNETIC FIELD"
showing 10 items of 1488 documents
Magnetic Lifshitz transition and its consequences in multi-band iron-based superconductors
2017
In this paper we address Lifshitz transition induced by applied external magnetic field in a case of iron-based superconductors, in which a difference between the Fermi level and the edges of the bands is relatively small. We introduce and investigate a two-band model with intra-band pairing in the relevant parameters regime to address a generic behaviour of a system with hole-like and electron-like bands in external magnetic field. Our results show that two Lifshitz transitions can develop in analysed systems and the first one occurs in the superconducting phase and takes place at approximately constant magnetic field. The chosen sets of the model parameters can describe characteristic ban…
Reentrant stability of superconducting films and the vanishing of dendritic flux instability
2007
We propose a mechanism responsible for the abrupt vanishing of the dendritic flux instability found in many superconducting films when an increasing magnetic field is applied. The onset of flux avalanches and the subsequent reentrance of stability in NbN films were investigated using magneto-optical imaging, and the threshold fields were measured as functions of critical current density ${j}_{c}$. The results are explained with excellent quantitative agreement by a thermomagnetic model published recently [D. V. Denisov et al., Phys. Rev. B 73, 014512 (2006)], showing that the reentrant stability is a direct consequence of a monotonously decreasing ${j}_{c}$ versus field.
Low-Frequency Imaginary Impedance at the Superconducting Transition of 2H - NbSe2
2020
The superconducting transition leads to a sharp resistance drop in a temperature interval that can be a small fraction of the critical temperature ${T}_{c}$. A superconductor exactly at ${T}_{c}$ is thus very sensitive to all kinds of thermal perturbation, including the heat dissipated by the measurement current. We show that the interaction between electrical and thermal currents leads to a sizable imaginary impedance at frequencies of the order of tens of hertz at the resistive transition of single crystals of the layered material $2H$-${\mathrm{Nb}\mathrm{Se}}_{2}$. We explain the result using models developed for transition-edge sensors. By measuring under magnetic fields and at high cu…
Microwave second-harmonic response of ceramic MgB2 samples
2005
Nonlinear microwave response of different ceramic MgB2 samples has been investigated by the technique of second-harmonic emission. The second-harmonic signal has been investigated as a function of temperature, DC magnetic field and input microwave power. The attention has mainly been devoted to the response at low magnetic fields, where nonlinear processes arising from motion of Abrikosov fluxons are ineffective. The results show that different mechanisms are responsible for the nonlinear response in the different ranges of temperature. At low temperatures, the nonlinear response is due to processes involving weak links. At temperatures close to Tc, a further contribution to the harmonic em…
Superconductivity in high‐quality (Hg 0.9 Re 0.1 )Ba 2 CaCu 2 O 6+ δ HTSC thin films
2004
High-quality epitaxial (Hg0.9Re0.1)Ba2CaCu2O6+δ (HgRe-1212) HTS thin films were successfully prepared using pulsed laser deposition (PLD) of the Re0.1Ba2CaCu2O6+δ precursor and subsequent Hg vapor annealing. The thin films exhibit a sharp superconducting transition at Tc ≈ 120 K. The resistive transitions have been investigated in magnetic fields up to 6 T parallel and perpendicular to the c-axis. We have determined the activation energy of thermally activated flux-motion for both magnetic field orientations. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Vortex-creep activation energy in YBa2Cu3O7/PrBa2Cu3O7 superlattices
2010
Abstract YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 (YBCO/PBCO) superlattices with a different ratio of the superconducting and insulating layer thicknesses were prepared by high pressure dc sputtering. The vortex-creep activation energy U 0 was determined by analyzing the in-plane resistive transition of 200 μm wide bridges with the external magnetic field B oriented along the c axis. It was found that U 0 is proportional to the thickness of the YBCO layers, and does only weakly depend on the PBCO layer thickness, when the latter exceeds two unit cells. We observed a change in the variation of U 0 with the current I in the specimen: U 0 exhibits a plateau in the low- I region, then decreases signific…
The quality factor of a superconducting rf resonator in a magnetic field.
2010
The quality factor of a superconducting NbTi resonator at 1.6 MHz in a magnetic field up to 1.2 T as well as its temperature dependence is investigated. A hysteresis effect in the superconducting surface resistance as a function of the magnetic field is observed. An unloaded Q-value of the resonator of 40,500 is achieved at 3.9 K. It is shown that this Q-value is limited by dielectric losses in the FORMVAR insulation of the coils wire. The details of the Q-value optimization are discussed. In the temperature dependence of the Q-value a steep decrease is observed above T approximately = 7.5 K. Finally, the implications of these measurements for real trap experiments are discussed in detail.
Anomalous magnetic hysteresis in the microwave surface resistance of MgB2 superconductor
2007
We report experimental results of the field-induced variations of the microwave surface resistance in samples of MgB2, produced by different methods. By sweeping the DC magnetic field at increasing and decreasing values, we have detected a magnetic hysteresis that can be ascribed to the different magnetic induction, due to the critical state of the fluxon lattice. The hysteresis observed in the bulk samples has an unusual shape, which cannot be justified in the framework of the critical-state models.
Field-induced suppression of the pi-band superconductivity and magnetic hysteresis in the microwave surface resistance of MgB_2 at temperatures near …
2009
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 supp…
Effects of Weak Links in the Nonlinear Microwave Response of MgB2 Superconductor
2006
We report experimental results of second-harmonic (SH) response at microwave frequency in several ceramic MgB2 samples, prepared by different methods. The SH signal has been investigated as a function of the temperature and DC magnetic field. The investigation has been carried out at low magnetic fields, where nonlinear processes arising from motion of Abrikosov fluxons are ineffective. We show that the low-temperature SH emission is ascribable to processes involving weak links. Comparison among the peculiarities of the SH signal radiated by the different samples shows that the presence of weak links strongly depends on the sample preparation method, as well as the purity and morphology of …