Current-activated residual surface resistance in high temperature superconductors

An experimental program is being conducted to assess the applicability of high-T/sub c/ superconductors for use in high-power RF and microwave devices. The program involves the measurement of the RF surface resistance of high-T/sub c/ samples at frequencies between 0.15 and 40 GHz and RF surface magnetic fields as high as 640 G. Polycrystalline samples were found to have surface resistances which increase monotonically with RF-field amplitude, saturating at high field at a few percent of the normal-state surface resistance just above T/sub c/. The data indicate that for high-T/sub c/ materials at low-RF fields, sufficiently low surface resistances are achieved, but at high fields (e.g. >or=30 G) the surface resistances are too large by a factor typically approximately 100.

To address the issue of the pairing state in high-temperature superconductors, four different experimental investigations have been carried out on a single high-quality, untwinned single crystal of ${\mathrm{LuBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. These include measurements of the specific heat in magnetic fields, both near the transition temperature and at low temperatures, investigations of the angular dependence of the transverse magnetic moment in the nonlinear Meissner regime, and the measurement of microwave properties leading to the temperature dependences of the penetration depth and the surface resistance at low temperatures. Some of the results raise questions relating to the existence of a pairing state with line nodes in the energy gap, whereas others appear to be consistent with this picture. The results taken together suggest that studies of the pairing state be examined critically for possible alternative explanations. \textcopyright{} 1996 The American Physical Society.

In this paper a macroscopic phenomenological model for the microwave properties of superconductors is presented. The model is based on the idea that there are two kinds of current carriers, and instead of the first London's equation a new equation is derived. This model can be applied to both low- and high-temperature superconductors. Using this model, an expression for the microwave surface resistance is derived and the surface resistance versus frequency is calculated. The results show that the relation between resistance and frequency is not R/sub 3//spl sim//spl omega//sup 2/ as indicated by both BCS theory and London model, but R/sub s//spl sim//spl omega//sup a/, where a is between 1 and 2 (e.g. a=1.35) for thin film high-T/sub c/ superconductors YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//. The temperature dependence of R/sub s/ is simulated using the given model. These relations and the values of the surface resistance agree well with experimental results. A residual resistance may be interpreted from this model.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Study of the surface resistance of superconducting niobium films at 1.5 GHz

Selective spin inversion in solution by magic field cross polarization

Higher usage of the mobile phone and internet services has increased the demands on the mobile frequency spectrum resulting in the congestion of the frequency bands used for the wireless communication systems. Consequently the adjacent channel interference and hence the number of dropout calls have increased. High Temperature Superconducting (HTS) microstrip filters enable the decrease in the adjacent channel interference, improvement of the audio quality and increase of the sensitivity of wireless communication receivers. Improvements in the performance of microwave HTS devices directly depend on lowering the surface resistance of the HTS thin films. Therefore it is important to fabricate HTS thin films with the lowest possible surface resistance. In general, microwave losses of HTS films depend on several factors such as the substrate material and deposition technique. In this paper we report results on YBa/sub 2/Cu/sub 3/O/sub 7-d/ (YBCO) thin films deposited on Al/sub 2/O/sub 3/ substrates with CeO/sub 2/ sub layer. We have used a microstrip resonator for reflection parameter measurements for data processing. We have carried out the measurements of microstrip meander resonators quality factor at frequency 1.8 GHz. Resonators are prepared by the wet etching technique, from the YBCO films, with sapphire substrate and CeO/sub 2/ sub layer, which is obtained by the laser ablation method. In our investigation we have used a single-port measurement method (using reflection factor).

A theoretical analysis of high-gain Compton free electron laser dynamics when the radio frequency (rf) field evolves self-consistently with single resonant particles representing a pre-bunched electron beam is presented and the single particle phase space is studied. Using a set of universally scaled equations the optimal parameters of excitation for a free electron laser (FEL) in the single particle approximation are found. Using a new approach the relations between the initial rf field amplitude, the output field amplitude and the particle’s initial detuning from resonance are also found. A set of equations describing the self-consistent evolution of the rf field with e-bunch macro-parameters, such as bunch width and bunch mean phase, is derived from the universally scaled equations under the condition of the uniform initial distribution of electrons in the bunch. The analysis of optical field generation and amplification by compact bunches of electrons is provided. The saturation regime and the physical reason for the saturation in the high-gain Compton FEL driven by pre-bunched electrons are studied and discussed.

Radio-frequency (rf) superconducting devices are verysensitive to corrosion because of the small penetration depthsλ<300 nm of the rfmagnetic fields, Hrf. Qualification of high temperature superconductor (HTS) filters for launchinginto outer space requires resistance against corrosion for three weeks at40 °C and 93% relative humidity. A protective layer is essential to shieldthe HTS against these severe atmospheric conditions and to avoiddegradation of HTS films. Such degradation has been reduced by a thinBaF2/Y F3 protective layerproduced by soaking the YBa2Cu3O7−δ (YBCO) films in HF solution. Results on the surface resistanceRs(77 K,Hrf) of YBCO films of different quality before and after HF soaking, as well as after exposure toa highly humid atmosphere, are presented. They are explained by weak link corrosion bydeoxygenation, easing the permeation of Josephson fluxons. These nonlinear effects, edgeenhanced by strips and holes, ultimately limit the applications of strip line orTE011 modes, being quantitatively related to Josephson fluxon hysteresis losses causing localheating. These results on YBCO films shed new light on HTS material science andcorrosion, into thermal runaway of HTS and nonlinear surface impedances of HTS films,and their dependences on the rf mode.

The effect of 57Fe hyperfine interaction radiofrequency (rf) modulation by external rotating magnetic field was studied in thin Permalloy foil by means of Mossbauer spectroscopy. The rf effect was investigated as a function of intensity for several rf field frequencies. The experiments show that the external rotating rf field causes considerable changes in the hyperfine pattern. The obtained spectra are in disagreement with those obtained by Perlow [Phys. Rev. 172 (1968) 319]. They also are inconsistent with magnetostriction hypothesis. Proceeding from the Mossbauer spectrum analysis one may conclude that the magnetization of investigated foil changes its direction in a complex manner. However, the undertaken experiments show that the essential number of Mossbauer nuclei experience the rotating magnetic field influence.

In this paper we report the results of an experimental study of the coherent dynamics (CD) of the (2J+1), with J=1, 2⋅⋅⋅6, low-field, equally spaced Zeeman components of the oxygen molecule O322 (O16–O16) and O342 (O16–O18) driven by a radio frequency (rf) field in a supersonic molecular beam. A supersonic molecular beam magnetic resonance spectrometer has been employed in the ’’flop out’’ mode, where two intense inhomogeneous magnetic fields are antiparallel, so that the resonating Zeeman levels are defocused at the detector, diminishing the beam intensity. The experimental signal interrogates the total number of molecules in a given total angular momentum state J which changed their initial MJ component. The rf amplitude spectroscopy has been applied to study the dependence of the signal on the amplitude of the rf field for various Zeeman manifolds on-resonance, and to investigate the dependence of the signal on the rf frequency. An analytical solution for the coherent dynamics of a spin J system driven by an rf field, which is valid both for on-resonance and for off-resonance situations, was utilized for the analysis of the experimental data. This confrontation between experiment and theory elucidated some of the universal features of the CD of multilevel systems. Inhomogeneous dephasing of the CD effects, which is due to the velocity distribution in the beam, was quantitatively analyzed, providing a novel method for the determination of translational temperatures in supersonic beams.

The surface resistanceR s of Tl2Ba2CaCu2O8 films fabricated on LaAlO3 wafers up to 3 inches (7.6 cm) in diameter through a post-deposition anneal process was measured over the frequency range 5.55–94.1 GHz by the following techniques: 5.55 and 27.5 GHz high-temperature superconductor (HTS)-sapphire resonators, 10 GHz parallel plate resonator, and 94.1 GHz scanning confocal resonator.R s was found to exhibit a quadratic dependence on frequencyf at 77 K:R s ∝f 2.0±0.1. The highest-quality films yieldR s =145±15μΩ at 10 GHz and 77 K. Scanning confocal resonator mapping ofR s across a 2-inch (5.1 cm) diameter wafer yielded a base value forR s of 16±1 mΩ at 77 K and 94.1 GHz (equivalent to 180±10μΩ at 10 GHz) and good uniformity inR s across the wafer. HTS-sapphire resonator measurements ofR s for fifteen 1.2 cm square parts cut from a 3-inch diameter wafer yieldedR s values scaled to 10 GHz of 196±10μΩ at 80 K. Similar values were measured for Tl2Ba2CaCu2O8 films prepared on both sides of a 2-inch diameter wafer.Rs values at 10 GHz and 80 K of 147−214μΩ were maintained over the course of 40 independent and successive deposition runs and corresponding anneals under nominally identical film fabrication conditions. Surface resistance at 5.55 GHz remained below 80μΩ for maximum rf magnetic fields up to 85 Oe at 4.2 K and 7 Oe at 80 K, respectively. Results are compared with predictions of the two-fluid model. The relative advantages and disadvantages of the different techniques for measuring surface resistance are discussed.

In this paper we study the coherent transient property of a &Lambda;-three-level system (&Omega;_<i>d</i> = 0) and a quasi- &Lambda; -four-level system (&Omega;_<i>d</i>&gt;0). Optical switching of the probe field can be achieved by applying a pulsed coupling field or rf field. In &Lambda; -shaped three-level system, when the coupling field was switched on, there is a almost total transparency of the probe field and the time required for the absorption changing from 90% to 10% of the maximum absorption is 2.9_&Gamma;0 (_&Gamma;0 is spontaneous emission lifetime). When the coupling field was switched off, there is an initial increase of the probe field absorption and then gradually evolves to the maximum of absorption of the two-level absorption, the time required for the absorption of the system changing from 10% to 90% is 4.2_&Gamma;0. In four-level system, where rf driving field is used as switching field, to achieve the same depth of the optical switching, the time of the optical switching is 2.5_&Gamma;0 and 6.1_&Gamma;0, respectively. The results show that with the same depth of the optical switching, the switch-on time of the four-level system is shorter than that of the three-level system, while the switch-off time of the four-level system is longer. The depth of the optical switching of the four-level system was much larger than that of the three-level system, where the depth of the optical switching of the latter is merely 14.8% of that of the former. The speed of optical switching of the two systems can be increased by the increase of Rabi frequency of coupling field or rf field.

Films nominally 80 μm thick of Bi-Sr-Ca-Cu-O on silver substrates with surface areas from 1.25 to 182 cm2 were fabricated using two different processing procedures. Their rf surface resistances were measured as functions of temperature in the frequency range 2.65–29.2 GHz at low rf field amplitudes using cylindrical resonant cavities. The critical temperatures of the films were in the range 81–83 K. At X-band frequencies, the surface resistances matched that of room-temperature copper. The results were comparable to recent data on small pellets of bulk Bi-Sr-Ca-Cu-O.

The knowledge of the surface resistance Rs of superconducting thin film at microwave and terahertz (THz) regions is significant to design, make and assess superconducting microwave and THz electronic devices. In this paper we reported the Rs of MgB2 films at microwave and THz measured with sapphire resonator technique and the time-domain THz spectroscopy, respectively. Some interesting results are revealed in the following: (1) A clear correlation is found between Rs and normal-state resistivity right above Tc, ρ0, i.e., Rs decreases almost linearly with the decrease of ρ0. (2) A low residual Rs ,l ess than 50µ� at 18 GHz is achieved by different deposition techniques. In addition, between 10 and 14 K, MgB2 has the lowest Rs compared with two other superconductors Nb3Sn and the high-temperature superconductor YBa2Cu3O7−δ(YBCO). (3) From THz measurement it is found that the Rs of MgB2 up to around 1 THz is lower than that of copper and YBCO at the temperature below 25 K. (4) The frequency dependence of Rs follows ω n ,w hereω is angular frequency, and n is power index. However, n changes from 1.9 at microwave to 1.5 at THz. The above results clearly give the evidences that MgB2 thin film, compared with other superconductors, is of advantage to make superconducting circuits working in the microwave and THz regions.

This paper summarizes recent measurements of the penetration depth and surface resistance of YBCO, with particular emphasis on the asymptotic behaviour at low temperatures and its dependence on hole doping and cation substitution. A complete set of measurements of the penetration depth tensor indicates that λ(T) has a predominantly linear temperature dependence in both the a and b directions at low temperatures, but the c-axis penetration depth is much flatter, without a clear linear term. Ni and Ca substitution are found to have little effect on the linear behaviour of λ(T), but Zn impurities alter the behaviour; from T linear to T2 and then to a non-monotonic temperature dependence at a Zn concentration of 1%. The surface resistance in the ab-plane also seems to vary linearly with temperature at low temperatures, except when the sample is doped with Zn. Substitution of Zn for Cu leads to upturns in the surface resistance below 4 K that are seen at 3.8 GHz, but not at 35 GHz, suggesting impurity effects at a very low energy scale.