Microwave properties of Ba $\mathsf{_{0.6}}$ K $\mathsf{_{0.4}}$ BiO $\mathsf{_3}$ crystals

Abstract

We report on field-induced variations of the microwave surface resistance at 9.6 GHz of Ba0.6K0.4BiO3 crystals. Energy losses have been investigated as a function of the static magnetic field in the range of temperatures 4.2 K ÷ Tc. By analyzing the experimental results in the framework of the Coffey and Clem model we determine the temperature dependence of the first-penetration field, upper critical field and depinning frequency. The results show that the pinning energy of this bismuthate superconductor is weaker than those of cuprates. .Nf Response to electromagnetic fields (nuclear magnetic resonance, surface impedance, etc.) -74.25.Op Mixed state, critical fields, and surface sheaths PACS

Microwave surface resistance of pristine and neutron-irradiated MgB2 samples in magnetic field

“…Firstly, the application of a magnetic field of ≈ 1 T, which is about H c2 /15, causes a R s variation of ≈ 35% of the maximum variation. These field-induced variations of R s are much greater than those expected from the models reported in the literature [30,31,32] and detected in other superconductors [28,29]. A comparison with the results of panel (b) shows that in sample P6 a R s variation of the same order is obtained for the same value of H 0 , even though, in this case, 1 T is about H c2 /2.…”

Magnetic hysteresis in the microwave surface resistance of Nb samples in the critical state

“…Microwave losses induced by static magnetic fields have been investigated by several authors [1][2][3][4][5][6][7][8][9]32,33]. At low temperatures and for applied magnetic fields lower enough than the upper critical field, the main contribution arises from the fluxon motion; however, it has been pointed out that a noticeable contribution can arise from the presence of normal fluid, especially at temperatures near T c and for magnetic fields of the same order of H c2 ðTÞ.…”

“…From the basic point of view, it gives information on the relative magnitude of elastic and viscous forces, which rule the motion regime of the fluxon lattice [1][2][3][4][5][6]. From the technological point of view, it allows determining the magnetic-field-induced energy losses, which have important implication in a large variety of superconductor-based devices [7].…”

“…In the absence of static magnetic fields, the variation with the temperature of the condensed-fluid density determines the temperature dependence of R s . On the other hand, the field dependence of R s in superconductors in the mixed state is determined by the presence of fluxons, which bring along normal fluid in their cores, as well as the fluxon motion [1][2][3][4][5][6][8][9][10]. Measurements of the high-frequency em response allow to conveniently investigate the vortex dynamics because they probe the vortex response at very low currents, when vortices undergo reversible oscillations and are less sensitive to flux-creep processes.…”

“…Measurements of the depinning frequency have been performed in both conventional [1,3,11] and high-T c superconductors [3,4,6,12]. For temperatures lower enough than T c and applied magnetic fields smaller enough than H c2 , conventional superconductors exhibit depinning frequency of the order of MHz, while much higher values ( J 10 GHz) have been reported for cuprate high-T c superconductors.…”