Non-resonant Microwave Absorption Signals Research Articles

Non-resonant Microwave Absorption and Boundary Current Response of Ba0.34K0.64Fe2As2 Superconducting Single Crystals

Boundary current response of Ba0.34K0.64Fe2As2 (BaK122) single crystals was probed by non-resonant microwave absorption (NRMA) as a function of 100-KHz modulation field at an intermediate temperature, namely, 25 K, which is 10 K below the actual Tc. The measurements were done for both field parallel and perpendicular to the iron arsenide plane configuration. A weak anisotropic effect is noted. However, it is observed that the boundary current response depends on the modulation field value and it increases as we increase the modulation field value from 1 to 9 G. Further, this boundary current response strongly depends on the microwave power. We have shown that these modulation field and microwave power dependence of the boundary current response of NRMA signals in BaK122 iron arsenide crystal can be qualitatively understood through the Dulcic model.

Non-Resonant Microwave Absorption in Nano Nickel Added YBCO Superconducting Powders: Line Shape and Hysteresis

Nonresonant Microwave Absorption (NRMA) or the ‘Low field microwave absorption’ (LFMA) measurements on high quality Ni nanoparticle added YBCO powder (Ni-YBCO) were carried as a function of temperature. The NRMA signal has structure in its line shape. As the Ni-YBCO powder sample is warmed up from 20K towards Tc, a phase reversal of the signal occurred around 60K, indicating a cross over from ‘normal’ to ‘anomalous’ type of absorption. We believe that the phase reversal could be due to the π-junction formation because of the nano nickle particles in the weaklinks between particles. Further, we observed strong low field hysteresis in NRMA. The low field hysteresis can be deduced from the boundary current response using Dulcic model. Our studies establish that, the addition of Nano Ni , can tune the microwave absorption properties and the low field hysteresis of NRMA

Non-resonant Microwave Absorption in Nano Nickel Added YBCO Powders: Observation of Multiple Phase Reversals

We report on the non-resonant microwave absorption in the system of a nominal 2 wt % nano nickel particles added into YBCO powders (Ni–YBCO). With this dilute mixture of nano nickel particles, one is expected to have groups of normal Josephson junctions (JJs) and π JJs due to YBCO–nickel–YBCO interparticle weak links and as nickel is ferromagnetic. We experimentally show, for the first time multiple phase reversals in the non-resonant microwave absorption (NRMA) spectra from Ni–YBCO possibly due to the formation of π JJs. We also showed that these multiple phase reversals then depend on microwave power and temperature. We argue that microwave power induced coherence among some groups of JJs and breaking of some of the weaker JJs can then lead to the disappearance of multiple phase reversals at higher microwave power levels. Further, we also reported a role of pair breaking effects that shall give a linear field dependence of the derivative microwave absorption signal, essentially which is the NRMA signal. This pair breaking effect dominates at closer temperatures to T c which is expected thermodynamically.

Second and Third Peaks in the Non-resonant Microwave Absorption Spectra of Superconducting Bi2212 Crystals

Non-resonant microwave absorption (NMA) measurements at liquid nitrogen temperature with systematic microwave power variation showed a two-peak structure in the Bi-2212 textured crystals, similar to that observed in the Bi-2212 single crystals (Srinivasu et al., in J. Supercond. Incorp. Nov. Magn. 14:41, 2001). NMA signals from the aged Bi-2212 single crystals show an emergence of a ‘third peak’ as a function of microwave power. We qualitatively interpret these second and third peaks as due to the microwave power induced phase locking of several number of junctions into coherent groups and then the destruction of the phase locking by the applied DC field leading to decoupling or fluxon motion, which gives the loss in individual junctions belonging to these otherwise coherent groups.

Non-resonant microwave absorption studies of superconducting MgB2 and MgB2 + MgO

Non-resonant microwave absorption(NRMA) studies of superconducting MgB_2 at a frequency of 9.43 GHz in the field range -50 Gauss to 5000 Gauss are reported. The NRMA results indicate near absence of intergranular weak links. A linear temperature dependence of the lower critical field H_c1 is observed indicating a non s-wave superconductivity. However, the phase reversal of the NRMA signal which could suggest d-wave symmetry is also not observed.

Non-resonant microwave absorption evidence for intrinsic Josephson coupling in Tl 2Ba 2CaCu 2O 8 single crystals

We report the results of our non-resonant microwave absorption (NRMA) studies on single crystals of Tl 2Ba 2CaCu 2O 8 (Tl 2212) which reflect the occurrence of intrinsic Josephson coupling in these crystals. We have studied the magnetic field induced microwave absorption at various temperatures from 4.2K to T c (∼104K) using a standard CW EPR spectrometer (H dc ‖ c). We observe the appearance of a characteristic feature in the NRMA signals similar to the ones observed earlier by us in Bi 2Sr 2CaCu 2O 8 (Bi 2212) starting a few degrees below T c, which on cooling passes through a maximum in intensity before disappearing at a further low temperature. This behaviour is attributed to the appearance, strengthening and disappearance of Josephson response consequent to the temperature dependence of the viscosity of the Josephson medium between the CuO 2 superconducting sheets.

Non-resonant microwave absorption studies in Bi 1.6Pb 0.4Sr 2Ca 2Cu 3O 10+ x

Non-resonant microwave absorption (NRMA) studies have been carried out on Bi 1.6Pb 0.4Sr 2Ca 2Cu 3O 10+ x pellets sintered for different durations varying from 24 to 168 h by using EPR technique. A detailed study of the temperature dependence of the NRMA signal amplitude, peak-to-peak width, hysteresis area has been carried out. Apart from showing a flux line lattice (FLL) melting temperature T m( B) (60–70 K), it is interesting to note that hysteresis loops showed the anomalous magnetic behaviour in the entire temperature regions even far below the fluctuation regime. The results are discussed in terms of several current models for the NRMA effects as well as for its anomalous magnetic hysteresis behaviour.

APPLICATION OF ESR TO HIGH-Tc SUPERCONDUCTORS AND RELATED MATERIALS IV: ALKALI K DOPING EFFECTS ON THE SUPERCONDUCTING CHARACTERISTICS OF K3C60

Several samples of K 3 C 60 have been made by the calcination of C 60 powder plus K flakes in evacuated quartz tubes at temperatures between 100°C and 700°C for 24 hours. The non-resonant microwave absorption (NRMA) measurements have been done below T c for these samples. From the linewidths and intensities of the NRMA signals, the superconducting properties (T c superconducting volume fraction and vortex size) have been studied as a function of the calcination temperature.

Anomalous magnetic behaviour in non-resonant microwave absorption study of Bi-2212 superconductors

The non-resonant microwave absorption (NRMA) studies in silver doped polycrystalline Bi/sub 2/Sr/sub 2/Ca/sub 1/Cu/sub 2/O/sub 8/ samples have been carried out. The NRMA signal has been observed not only below Tc but also above Tc which is much above the fluctuation regime when the DC magnetic field was swept from -50 G to +50 G through 0G. Anomalous magnetic hysteresis behaviour has been observed where the forward curve lies above the return curve even at lower temperatures. The observed curve has been simulated by taking into consideration the sum of reversible losses as well as irreversible losses using Dulcic and Portis models. This leads to anomalous magnetic hysteresis behaviour as has been observed experimentally.

Electron transfer through ordered metallic chains in LiNbO 3 : (Mg, Zn) : Me(3d)

In crystal and ceramic LiNbO 3 double doped with Mg or Zn above the threshold concentration ( C Mg,Zn⩾4–6 mol%) and with transition metals of the 3d group (Cr, Fe, Mn) non-resonant microwave absorption (NRMA) dependent on the magnetic field was studied. Peak-like dependencies of the NRMA signal intensity on the concentration of dopants and stoichiometry of lithium niobate were found. The EPR line of mobile electrons (Δ B=30 mT) with g=2.0023 was detected. The interaction energy between the substitutional impurities of Mg and Cr was calculated. It was concluded, that in LiNbO 3 : (Mg, Zn) : (Cr, Fe, Mn) ordered metallic chains exist with electron conductivity.

Nonresonant microwave absorption study of intrinsic Josephson coupling in Bi2Sr2CaCu2O8 single crystals.

The results of nonresonant microwave absorption (NRMA) studies in Bi2212 single crystals for the $H\Vert c$ axis and $H\Vert ab$ plane are reported. In the former orientation, over a few degrees below $T_c$ , the NRMA signal shapes evolve continuously as a function of temperature. We interpret these results as indicative of rapid strengthening of Josephson coupling between the pancake vortices in the $CuO_2$ bilayers on cooling below $T_c$ . For $H\Vert ab$ there is almost no change in the signal shape in the temperature range studied. In this case, no loss component attributable to Josephson coupling is observed.

Surface barrier effects in non-resonant microwave absorption by thin superconducting films of YBa 2Cu 3O 7−δ

We report an unusual temperature dependence of hysteresis in the non-resonant microwave absorption signals from superconducting thin films of YBa 2Cu 3O 7−δ. We observe that the hysteresis increases with increasing temperature up to T c which we interpret as evidence for the presence of surface barriers in the single crystalline films.

MICROWAVE ABSORPTION STUDIES ON HIGH-TcSUPER CONDUCTORS AND RELATED MATERIALS IX—NON-RESONANT MICROWAVE ABSORPTION IN AN ULTRA-THIN (~100 Å THICK) Bi-Sr-Ca-Cu-O FILM

Non-resonant microwave absorption (NRMA) studies have been done for an ultra-thin (~100 Å thick) Bi-Sr-Ca-Cu-O film fabricated on MgO(100) by MOCVD. The measurements have been done in two cases; either (a) the rear surface of the substrate or (b) the film surface was covered with an aluminum foil to shield microwave radiation. Three types of NRMA signals were observed for the former case, whereas only one type of signals were detected in the latter. The intensities, linewidths and their temperature dependences are qualitatively and quantitatively different in these two cases, possibly indicating a surface effect rather than a volume effect for the NRMA phenomenon.

MICROWAVE ABSORPTION STUDIES ON HIGH-Tc SUPERCONDUCTORS AND RELATED MATERIALS VI—NON-RESONANT MICROWAVE ABSORPTION IN ULTRA-THIN Bi-Sr-Ca-Cu-O FILMS—

Non-resonant microwave absorption (NRMA) studies have been done for Bi-Sr-Ca-Cu-O (BSCCO) films with thicknesses of 30 Å and 15 Å fabricated on MgO substrate by MOCVD. For the latter two kinds of samples have been fabricated: BSCCO/MgO and BiOx/BSCCO/MgO . NRMA signals have been detected up to about 70–75 K for all the samples. The lower critical magnetic field H c1 * of the 30 Å film was severely increased by the application of a magnetic field of several kGauss, whereas it is almost unaffected in the case of 15 Å films. H c1 * increases but the signal intensity decreases with increasing temperature for all the 30 Å and 15 Å samples. The temperature dependence of the intensity is quite different from that of thicker films (100 Å and 350 Å), in which the intensity vs. temperature relation has a peak (or peaks) at particular temperature(s).

MICROWAVE ABSORPTION STUDIES ON HIGH-Tc SUPERCONDUCTORS AND RELATED MATERIALS V-ANOMALOUS BEHAVIOR OF LOW MAGNETIC FIELD MICROWAVE ABSORPTION IN AN ULTRA-THIN Bi-Sr-Ca-Cu-O FILM

Non-resonant microwave absorption (NRMA) studies have been done on a 100 Å thick Bi-Sr-Ca-Cu-O film fabricated on MgO substrate by MOCVD. Temperature dependence of NRMA intensity reveals that the intensity becomes maximum at around 30 K, 40 K and 55 K, and minimum at around 35 K and 50 K. The sign (or phase) of the signal changes by passing through these intensity minimum temperatures. Furthermore, the linewidths of the NRMA signals anomalously increase in the vicinity of these temperatures.

Magnetoresistance origin for nonresonant microwave absorption in antiferromagnetically coupled epitaxial Fe/Cr/Fe(001) sandwiches

Nonresonant microwave-absorption signals have been observed in a series of molecular-beam-epitaxy-grown Fe/Cr/Fe (001) sandwiches which were previously shown to exhibit an antiferromagnetic coupling of the two Fe layers. Using field-modulation detection, one observes signals which display an unusual magnetic-field dependence containing sharp features in the range 0–2 kOe. The magnetic-field locations of these features are independent of the microwave frequency, but their intensity depends strongly on the field-modulation amplitude. The features also have a strong in-plane angular dependence. Comparison with earlier magnetization and magnetoresistance data on the same samples shows that a sharp feature occurs when the magnetization orientations in the Fe films change abruptly, and that one can explain the microwave-absorption signals in terms of magnetoresistance effects in the sandwich. Thus, microwave absorption provides a contactless way to investigate the magnetoresistance in such samples.