We present wideband microwave measurements (1-22 GHz) taken on Superconductor/Ferromagnet heterostructures with the Corbino disk technique. We apply the technique to Nb/PdNi/Nb trilayers in the vortex state and we compare the results to data taken with the same technique on a Nb thin film. We show that it is possible to directly extract the genuine flux-flow resistivity from our frequency-dependent measurements without overcoming the (depinning) critical current. The characteristic frequency for vortex relaxation can also be estimated, without resorting to a specific model. We find that the F layer determines a weakening of pinning and an enhancement of the flux-flow resistivity with respect to the Nb film.
The temperature dependences of the enhanced critical current in wide and thin Sn films exposed to the microwave field have been investigated experimentally and analyzed. It was found that the microwave field stabilizes the current state of a wide film with respect to the entry of Abrikosov vortices. The stabilizing effect of irradiation increases with frequency. Using similarity between the effects of microwave enhancement of superconductivity observed for homogeneous (narrow films) and inhomogeneous (wide films) distributions of the superconducting current over the film width, we have succeed…
Here we give a results of experimental study of enhancement of superconductivity by microwave irradiation in superconducting films. An influence of the power, frequency of microwave irradiation, as well as temperature and width of superconducting films on behavior of experimental dependencies of enhanced critical current and the current at which a vortex structure of the resistive state vanishes and the phase-slip first line appears is analyzed. The experimental studies of films with different width reveal that the effect of superconductivity enhancement by microwave field is common and occurs…
The effect of the patterning process on the nonlinearity of the microwave surface resistance $R_S$ of YBCO thin films is investigated. With the use of a sapphire dielectric resonator and a stripline resonator, the microwave $R_S$ of YBCO thin films was measured before and after the patterning process, as a function of temperature and the rf peak magnetic field in the film. The microwave loss was also modeled, assuming a $J_{rf}^2$ dependence of $Z_S(J_{rf})$ on current density $J_{rf}$. Experimental and modeled results show that the patterning has no observable effect on the microwave residual…
While the electrodynamic treatment of a superconducting thin film by Yeh et al. [Phys. Rev. B 48, 9861 (1993)] ostensibly has application for the "thin-film limit," their work is shown to employ bulk-like instead of thin-film electrodynamics, and as such obtains an ostensible violation of the first law of thermodynamics and a surface resistance independent of the resistivity. Other theoretical and experimental difficulties are briefly noted.
We report the growth of NdBa2Cu3O7-d films on (100) MgO substrate by Molecular Beam Epitaxy (MBE). Large area NdBa2Cu3O7-d films with homogeneous superconducting properties were grown by precise control of stoichiometry and the optimisation of growth parameters. The stoichiometric ratio of Nd:Ba:Cu close to 1:2:3 yields films with TC of 94 K and JC values above 3.5 MA/cm2 at 77 K on bare MgO substrate. The NdBa2Cu3O7-d films grown under optimised conditions had excellent in-plane texture and good metallicity. The most significant characteristic of our MBE grown NdBa2Cu3O7-d films is the very l…
Two issues related to the microwave surface impedance Z_s of MgB_2 thin film are discussed in this Letter, both being significant for potential microwave applications. At first, a correlation between Zs and Alpha = Xi/l was found, where Xi is the coherence length, and l is the mean free path. The surface resistance Rs decreases with Alpha at moderate and large values of Alpha and saturates when Alpha approaches one. The values of the penetration depth at zero temperature Lamda(0) for different films could be well fitted by Lamda_L (1+Alpha)^(1/2), yielding a London penetration depth Lamda_L of…
The inelastic magnetic scattering of neutrons from thin superconducting films irradiated with microwave radiations, is analyzed. This process is due to the interaction of the magnetic field generated by the neutron, with the oscillating current induced by the electromagnetic field penetrated into the film. In the Born approximation, the neutron energy can both increase and decrease by the energy quantum of the field. The scattering cross section, which increases with decreasing the field frequency, is derived, and its numerical calculations are carried out. Measurements of this cross section g…
We report measurements of the complex resistivity in $YBCO$ and $MgB_2$ thin films over a continuous frequency spectrum in the microwave range, making use of a Corbino disk geometry. The paper mainly focuses on the extraction of the resistivity from raw data, displaying data analisys procedure and its limits of validity. We obtain and show resistivity curves as a function of frequency and temperature denoting a frequency dependent widening of the superconductive transition.
The microwave surface resistance Rs of MgB2 films with the zero-resistance temperature of - 39 K was measured at 8.0 - 8.5 GHz. The MgB2 films were prepared by deposition of boron films on c-cut sapphire, followed by annealing in a vaporized magnesium environment. The Rs appeared significantly reduced by ion milling of the as-grown MgB2 film surface, with the observed Rs of ~ 0.8 mohm at 24 K for an ion-milled MgB2 film as small as 1/15 of the value for the corresponding as-grown MgB2 film. The reduced Rs of the ion-milled MgB2 films is attributed to the effects of the Mg-rich metallic layer e…
Precision measurements of the microwave surface resistance $R_{s}$ of in situ $MgB_{2}$ films directly reveal an exponential behavior of $R_{s}$ at low temperature indicating a fully-gapped order parameter. The entire temperature dependence of $R_{s}$ is well described by a Mattis-Bardeen formalism but with a small gap ratio of $Δ(0)/kT_{c}=0.72$, corresponding to $Δ(0)=1.9meV$.
We develop an explicitly gauge-invariant semiclassical approach to investigate nonlinear response of superconductors to monochromatic THz radiation. We demonstrate that in clean superconductors charge conservation forbids non-linear response to a uniform field. We apply our approach to quasi-two-dimensional films and obtain an explicit expression for the photoinduced current. We find that the photoinduced current exhibits a strong dependence on polarization and incidence angle of the radiation. Our predictions may be directly verified in experiments with quasi-two-dimensional superconducting f…
Critical fluctuations above $T_c$ are studied in the microwave complex conductivity of BSCCO-2212, BSCCO-2223, and YBCO thin films. The analysis of the experimental data yields the temperature dependences of the reduced coherence length $ξ(T)/ξ_0$. Two critical regimes are observed, having the static critical exponent $ν=1$ close to $T_c$, and a crosover to $ν=2/3$ at higher temperatures. In more anisotropic superconductors the reduced coherence lengths are larger, and the critical states extend to higher temperatures.
A localized and strong RF magnetic field, created by a magnetic write head, is used to examine the linear electrodynamic properties of a Nb superconducting film. The complex reflection coefficient of the write head held in close proximity to the films is measured as a function of sample temperature. A model combining a magnetic circuit (magnetic write head inductively coupled to the sample) and transmission line (microwave circuit) is given to interpret the linear response measurement. Additionally, this reflection linear response measurement can be used to determine the temperature dependence…
We have grown superconducting TiN films by atomic layer deposition with thicknesses ranging from 6 to 89 nm. This deposition method allows us to tune the resistivity and critical temperature by controlling the film thickness. The microwave properties are measured, using a coplanar-waveguide resonator, and we find internal quality factors above a million, high sheet inductances (5.2-620 pH), and pulse response times up to 100 μs. The high normal state resistivity of the films (> 100 μΩcm) affects the superconducting state and thereby the electrodynamic response. The microwave response is modele…
When a thin-film ring with supercurrent is irradiated by coherent microwave field, the field induces coherent oscillations of all the Cooper pairs involved in the supercurrent. The oscillating charged condensate will emit photons with energies determined by the difference of quantized energy levels of the superconducting ring. This effect can be observed only in the rings, the thickness of which is less than both the field skin-depth and the London penetration depth. The probability of the microwave field-induced single-photon and two-photon emission is calculated. The energy - angle distribut…
We report on the presence of weak links in the intermetallic superconductor MgB2 from the non-resonant microwave absorption (NRMA) studies on the MgB2 thin film. The strong low field signal, which is the characteristics of weak links and the presence of hysteresis, which arises due to flux trapping, are observed. The studies are carried out as a function of temperature, microwave power, magnitude of modulation field and the range of field scan. Presence of weak links is discussed in terms of increase in the oxygen content of grain boundaries in the thin film and because of disorder effects ari…
We investigated the microwave conductivity spectrum of FeSe$_{0.5}$Te$_{0.5}$ epitaxial films on CaF$_2$ in the vicinity of the superconducting transition. We observed the critical behavior of the superconducting fluctuations in these films with a dimensional crossover from two-dimensional to three-dimensional as the film thickness increased. From the temperature dependence of the scaling parameters we conclude that the universality class of the superconducting transition in FeSe$_{0.5}$Te$_{0.5}$ is that of the 3D-XY model. The lower limit of the onset temperature of the superconducting fluct…
Striking features have been found in the nonlinear microwave (8 GHz) surface impedance $Z_s=R_s + jX_s$ of high-quality YBaCuO thin films with comparable low power characteristics [$R_{res}\sim 35--60 μΩ$ and $λ_L(15 K)\sim 130--260 nm$]. The surface resistance $R_s$ is found to increase, decrease, or remain independent of the microwave field $H_{rf}$ (up to 60 mT) at different temperatures and for different samples. However, the surface reactance $X_s$ always follows the same functional form. Mechanisms which may be responsible for the observed variations in $R_s$ and $X_s$ are briefly discus…
We have measured the power dependent microwave properties of a weak link in a YBa2Cu3O7 thin film formed by writing a line of damage using a focused ion beam. The measurement was made using a parallel plate resonator at 5.5 GHz with the weak link written across the width of one of the plates. The ion induced damage was characterized using a TRIM computer simulation and the dc properties of similar weak links was measured. Using a 200 eV Si ion dose of 2e13 cm-2, the Tc of the damaged region was reduced by 5.5 K and the normal resistivity was doubled. Surprisingly, the microwave measurements di…
Vortices in superconductors driven at microwave frequencies exhibit a response related to the interplay between the vortex viscosity, pinning strength, and flux creep effects. At the same time, the trapping of vortices in superconducting microwave resonant circuits contributes excess loss and can result in substantial reductions in the quality factor. Thus, understanding the microwave vortex response in superconducting thin films is important for the design of such circuits, including superconducting qubits and photon detectors, which are typically operated in small, but non-zero, magnetic fie…
The morphological, transport and terahertz optical properties of DC magnetron sputtered granular molybdenum thin-films with nano-grains embedded in an amorphous matrix have been studied in the normal and superconducting states. The superconducting transition temperatures of these films are much higher than that of bulk molybdenum. The optical properties of these thin-films have been studied using terahertz time-domain spectroscopy. Their properties have been compared with the existing materials used for the development of radiation detectors. The resistivity of the films lies in >100 micro-Ohm…
We report the observation of a peak in the microwave (9.55 GHz) surface resistance in an epitaxial DyBa2Cu3O7-y superconducting film in magnetic fields (parallel to the c axis) ranging between 0.2 to 0.9 Tesla. Such a peak is absent in the measurements done in zero-field. The temperature and field dependence of the peak suggests that this peak could be associated with the peak effect phenomenon reflecting the order-disorder transformation in the flux-line lattice. A strong dependence of this peak effect at frequencies close to the depinning frequency of the flux line lattice is observed.
The anomalous decrease in microwave surface resistance, R_{s}, of superconducting YBa_{2}Cu_{3}O_{7-d} (YBCO) thin films in the presence of a low dc magnetic field is studied using a microstrip resonator technique. We have done a dc field hysteresis measurement of R_{s} to study the effects of vortex penetration on the anomalous effect. It is shown that the anomaly happens at a field level far below the low critical field, H_{c1,strip}, of the superconducting microstrip and vortex (Abrikosov) penetration would eliminate the anomalous effect observed at low field. This implies that the anomalou…
Results of X-band microwave surface impedance measurements of FeSe1-xTex very thin film are reported. The effective surface resistance shows appearance of peak at T less and near Tc when plotted as function of temperature. The authors suggests that the most well-reasoned explanation can be based on the idea of the changing orientation of the microwave magnetic field at a SN phase transition near the surface of a very thin film. The magnetic penetration depth exhibits a power-law behavior of delta lambda proportional to T with an exponent n = 2.4 at low temperatures, which is noticeably higher …
Surface impedance measurements of LuNi2B2C superconducting thin films as a function of temperature have been performed down to 1.5 K and at 20 GHz using a dielectric resonator technique. The magnetic penetration depth closely reproduces the standard B.C.S. result, but with a reduced value of the energy gap at low temperature. These data provide evidence for an anisotropic s-wave character of the order parameter symmetry in LuNi2B2C. From the evaluation of the real part of complex conductivity, we have observed constructive (type II) coherence effects in the electromagnetic absorption below Tc.
Basing on the phenomenological model of the microwave response of thin superconducting films with trapped magnetic flux elaborated in Luzhbin et al (2005) Supercond. Sci. Technol. Vol.18, pp.1112-7, detailed analysis of the ac linear response of a superconducting strip-line resonator cooled down in FC and ZFC regimes is presented. Special emphasis on a nonuniform distribution of the trapped vortices and on the magnetic field and temperature dependence of the phenomenological model parameters such as critical current density, vortex pinning and viscous motion coefficients is made. For a ZFC cas…
It is now well known that the properties of the disordered s wave superconductors can deviate significantly from the prediction of standard Bardeen-Cooper-Schrieffer (BCS) theory. By measuring the temperature dependence of the resonance frequency, $f_0$, of microwave resonators made from disordered NbTiN and TiN thin films, at low temperatures, below half of the superconducting critical temperature, $T_c$, Driessen et al. \cite{driessen2012PRL} demonstrated that $f_0$ vanishes faster than predicted from the BCS theory. Here, we report on the temperature dependence of $f_0$ of microwave resonat…
The magneto-optical imaging technique is used to visualize the penetration of the magnetic induction in YBa$_{2}$Cu$_{3}$O$_{7-δ}$ thin films during surface resistance measurements. The in-situ surface resistance measurements were performed at 7 GHz using the dielectric resonator method. When only the microwave magnetic field $H_{rf}$ is applied to the superconductor, no $H_{rf}$-induced vortex penetration is observed, even at high rf power. In contrast, in the presence of a constant magnetic field superimposed on $H_{rf}$ we observe a progression of the flux front as $H_{rf}$ is increased. A …
The temperature dependence of the real part of the microwave complex conductivity at 17.9 GHz obtained from surface impedance measurements of two c-axis oriented MgB2 thin films reveals a pronounced maximum at a temperature around 0.6 times the critical temperature. Calculations in the frame of a two-band model based on Bardeen-Cooper-Schrieffer (BCS) theory suggest that this maximum corresponds to an anomalous coherence peak resembling the two-gap nature of MgB2. Our model assumes there is no interband impurity scattering and a weak interband pairing interaction, as suggested by bandstructure…
We have carried out non-linear microwave (8 GHz) surface impedance measurements of three YBaCuO thin films in dc magnetic fields $H_{dc}$ (parallel to c axis) up to 12 mT using a coplanar resonator technique. In zero dc field the three films, deposited by the same method, show a spread of low-power residual surface resistance, $R_{res}$ and penetration depth, $λ$ (T=15 K) within a factor of 1.9. However, they exhibit dramatically different microwave field, $H_{rf}$ dependences of the surface resistance, $R_s$, but universal $X_s(H_{rf})$ dependence. Application of a dc field was found to affec…
We have measured the frequency and temperature dependences of complex ac conductivity, σ(ω)=σ_1(ω)-iσ_2(ω), of NbN films in zero magnetic field between 0.1 to 10 GHz using a microwave broadband technique. In the vicinity of superconducting critical temperature, Tc, both σ_1(ω) and σ_2(ω) showed a rapid increase in the low frequency limit owing to the fluctuation effect of superconductivity. For the films thinner than 300 nm, frequency and temperature dependences of fluctuation conductivity, σ(ω,T), were successfully scaled onto one scaling function, which was consistent with the Aslamazov and …
The two tone intermodulation distortion arising in MgB2 thin films synthesized by hybrid physical-chemical vapour deposition is studied in order to probe the influence of the two bands on the nonlinear response of this superconductor. The measurements are carried out by using a dielectrically loaded copper cavity operating at 7 GHz. Microwave data on samples having critical temperatures above 41 K, very low resistivity values, and residual resistivity ratio larger than 10, are shown. The dependence of the nonlinear surface losses and of the third order intermodulation products on the power fee…
The two tone intermodulation arising in MgB2 thin films deposited in-situ by planar magnetron sputtering on sapphire substrates is studied. Samples are characterised using an open-ended dielectric puck resonator operating at 8.8 GHz. The experimental results show that the third order products increase with the two-tone input power with a slope ranging between 1.5 and 2.3. The behaviour can be understood introducing a mechanism of vortex penetration in grain boundaries as the most plausible source of non linearities in these films. This assumption is confirmed by the analysis of the field depen…
We report a systematic study of the dynamic microwave conductivity near $T_c$ for La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) thin films with $x$=0.07 to 0.16. The strong frequency dependence of the phase stiffness together with scaling analysis of the ac fluctuating conductivity of superconductivity provide direct evidence for the 2D-XY behavior of nearly decoupled CuO$_2$ planes in underdoped LSCO ($x$=0.07 and 0.12). On the other hand, the critical exponents for slightly overdoped LSCO ($x$=0.16) were found to agree with those for the relaxational 3D-XY model, indicating that the universality class in L…
The microwave response of high quality niobium films in a perpendicular static magnetic field has been investigated. The complex frequency shift was measured up to the upper critical fields. The data have been analyzed by the effective conductivity model for the type-II superconductors in the mixed state. This model is found to yield consistent results for the coherence lengths in high-kappa superconducting samples, and can be used with HTSC even at temperatures much below T_c. It is shown that for samples with high values of depinning frequency, one should measure both components of the compl…
The carrier concentration of Tl2Ba2CaCu2O8 films was modified by annealing in N2 gas. X-ray analysis of the structure and the oxygen content revealed a correspondence between carrier concentration and oxygen depletion. The TC and nonlinear surface impedance was measured using a dielectric resonator and the nonlinearity slope parameter r=dXS/dRS was found to converge to unity at the critical temperature, indicating a dominance of Josephson fluxon hysteresis on the nonlinearity. Highly inductive nonlinearity was observed in a small range of doping levels between 0.180<p<0.195 holes/Cu, which doe…
We report a set of experimental data on local third-harmonic generation at microwave frequencies (0.5 GHz) in $YBa_2Cu_3O_7$ and Nb films. For local investigations of the nonlinear response a probe with inductive coupling was elaborated. The map of the nonlinear microwave response of a $YBa_2Cu_3O_7$ thin film is plotted below $T_c$ with high resolution. The third-harmonic power is measured as a function of temperature, input power and dc magnetic field at some areas of the film. The correlation between the depinning current density ${J_p}$ and the nonlinear microwave response is also demonstr…
The microwave properties of polycrystalline MgB2 thin films prepared by the so-called in-situ method are investigated. The characterization of the films at microwave frequencies was obtained by a coplanar resonator technique. The analysis of the experimental data results in the determination of penetration depth, surface impedance and complex conductivity. The aim of this work is to set the experimental results in a consistent framework, involving the two-band model in the presence of impurity scattering. The energy gaps are calculated and the contribution of intra- and inter-band scattering i…
The vortex dynamics in YBa2Cu3O7-d thin films have been studied at microwave frequencies. A pronounced peak in the surface resistance, Rs, is observed in these films at frequencies of 4.88 and 9.55 GHz for magnetic fields varying from 0.2 to 0.8 T. The peak is associated with an order-disorder transformation of the flux line lattice as the temperature or field is increased. The occurrence of the peak in Rs is crucially dependent on the depinning frequency, wp and on the nature and concentration of growth defects present in these films. Introduction of artificial defects by swift heavy ion irra…
Critical fluctuations have been studied in the microwave conductivity of Bi_2Sr_2CaCu_2O_(8+δ), Bi_2Sr_2Ca_2Cu_3O_(10+delta), and YBa_2Cu_3O_(7-delta) thin films above T_c. It is found that a consistent analysis of the real and imaginary parts of the fluctuation conductivity can be achieved only if an appropriate wavevector or energy cutoff in the fluctuation spectrum is taken into account. In all of the three underdoped superconducting films one observes strong fluctuations extending far above T_c. The coherence length inferred from the imaginary part of the conductivity exhibits the static c…
We present measurements of the magnetic field-induced microwave complex resistivity changes at 47 GHz in Tl$_2$Ba$_2$CaCu$_2$O$_{8+δ}$ (TBCCO) thin films, in the ranges 58 K$<T<T_{c}$ and 0$<μ_{0}H<$0.8 T. The large imaginary part $Δρ_{2}(H)$ points to strong elastic response, but the data are not easily reconciled with a rigid vortex model. We find that, over a wide range of temperatures, all the pairs of curves $Δρ_{1}(H)$ and $Δρ_{2}(H)$ can be collapsed on a pair of scaling curves $Δρ_{1}[H/H^{*}(T)]$, $Δρ_{2}[H/H^{*}(T)]$, with the same scaling field $H^{*}(T)$. We argue that $H^{*}(T)$ i…
The surface resistance (Rs) and the real part (sigma_1) of the microwave complex conductivity of a ~380 nm-thick polycrystalline MgB2 film with the critical temperature (Tc) of 39.3 K were investigated at ~8.5 GHz as a function of temperature. Two coherence peaks were observed in the sigma_1 versus temperature curve at temperatures of ~0.5 Tc and ~0.9 Tc, respectively, providing a direct evidence for the two-gap nature of MgB2. The film appeared to have a pi-band gap energy of 1.8 meV. For the MgB2 film ion-milled down to the thickness of ~320 nm, two coherence peaks were still observed with t…
Temperature dependencies of microwave surface impedance were measured for perfect c-oriented YBCO thin films deposited on CeO2-buffered sapphire substrates. The measurements revealed a distinct two-peak structure of Rs(T) and Xs(T) dependencies with peaks at 28-30 K and 50 K. The peaks become smeared at higher frequencies as well as in applied dc magnetic field about 1 kOe, while the peak positions remain almost unchanged. For less perfect, e.g., PLD films, Rs(T) and Xs(T) dependencies are monotonous (power law). The two-peak Zs(T)dependencies for YBCO films differ from those for high quality …
We present measurements of the field induced changes in the 47 GHz complex resistivity, $Δ\tilde ρ(H,T)$, in Tl$_{2}$Ba$_{2}$CaCu$_{2}$O$_{8+x}$ (TBCCO) thin films with $T_{c}\simeq$ 105 K, prepared on CeO$_{2}$ buffered sapphire substrates. At low fields ($μ_{0}H<$10 mT) a very small irreversible feature is present, suggesting a little role of intergranular phenomena. Above that level $Δ\tilde ρ(H,T)$ exhibits a superlinear dependence with the field, as opposed to the expected (at high frequencies) quasilinear behaviour. We observe a crossover between predominantly imaginary to predominantly …
We present a high-resolution microwave spectrometer to measure the frequency-dependent complex conductivity of a superconducting thin film near the critical temperature. The instrument is based on a broadband measurement of the complex reflection coefficient, $S_{\rm 11}$, of a coaxial transmission line, which is terminated to a thin film sample with the electrodes in a Corbino disk shape. In the vicinity of the critical temperature, the standard calibration technique using three known standards fails to extract the strong frequency dependence of the complex conductivity induced by the superco…
Two high-quality Y-Ba-Cu-O thin films on MgO substrates have been investigated using the coplanar resonator technique at 8 and 16 GHz. Both films exhibit an anomalous decrease in their surface impedance, Zs as a function of microwave field, Hrf. In zero dc field, Hrf-dependences of Rs and Xs for both the samples are uncorrelated, and only one of the quantities, Rs or Xs, displays anomalous behavior. Here, application of relatively weak (~5 mT) dc magnetic fields, Hdc can produce a correlated decrease of Rs(Hrf) and Xs(Hrf). The dependences of Zs on Hdc in both low and high microwave power regi…
In this paper, we report on the results of features in the discovered effect of a strong change in the microwave losses (quenching) in HTS-based coplanar waveguide (CPW) at certain values of the input power $P_{in}$ and direct current (DC) $I_{dc}$. Two waveguides were studied: CPW-150 and CPW-75, fabricated of epitaxial 150 nm and 75 nm YBCO films. The insertion loss of CPW, $IL = 10 lg (P_{out} / P_{in})$, was measured versus both the operating temperature and the bias DC current $I_{dc}$ when a fixed level of the input X-band pulsed MW signal $P_{in}$ was applied. Here $P_{out}$ is the powe…
Boron-doped diamond granular thin films are known to exhibit superconductivity with an optimal critical temperature of Tc = 7.2K. Here we report the measured complex surface impedance of Boron-doped diamond films in the microwave frequency range using a resonant technique. Experimentally measured inductance values are in good agreement with estimates obtained from the normal state sheet resistance of the material. The magnetic penetration depth temperature dependence is consistent with that of a fully-gapped s-wave superconductor. Boron-doped diamond films should find application where high ki…
We have measured the temperature dependence of the microwave surface impedance Zs = Rs + iΩμ_0 \Lamda of two c-axis oriented MgB2 films at a frequency Ω/(2π) of 17.9 GHz employing a dielectric (sapphire) resonator technique. The temperature dependence of the magnetic field penetration depth \Lamda can be well fitted from 5 K close to Tc by the standard BCS integral expression assuming the reduced energy gap Delta(0)/kTc to be as low as 1.13 and 1.03 for the two samples. For the penetration depth at zero temperatures, values of 110 nm and 115 nm were determined from the fit. Our results clearly…
In the mixed state of type II superconductors penetrated by an external magnetic field in the form of a lattice of Abrikosov vortices the dc resistance is known to increase with increasing velocity of the vortex lattice. Accordingly, vortex pinning sites impeding the vortex motion are widely used to preserve the low-dissipative response of the system. Here, while subjecting superconducting Nb films with nanogrooves to a combination of dc and ac current stimuli and tuning the number of mobile and pinned vortices by varying the magnetic field around the so-called matching values, we observe a co…
The new superconductor MgB2 may prove useful for microwave applications at intermediate temperatures. MgB2 films with the thickness of 300 - 400 nm have surface resistance Rs less than 0.01 mOhms at a frequency (f) of 8.5 GHz and 7 K,and ~ 1.5 mOhms at 87 GHz and 4.2 K. The critical temperature (Tc) of these films is ~ 39 K when they are prepared under optimum conditions. The Rs appears to scale as f^2 up to 30 K. After surface ion-milling, a reduction of the Tc and an enhanced resistivity rho(Tc) are observed consistently at 8.5 GHz and 87 GHz along with a reduced Rs at low temperatures. The …
We have measured the temperature dependence of the microwave surface impedance Z_s = R_s + iΩμ_0 of a MgB2 film at a frequency Ω/2Πof 18 GHz employing a dielectric resonator technique. We found that the temperature dependence of the magnetic field penetration depth \Lamda can be fitted by \Lamda(T)= \Lamda(0) [1-(T/Tc)^2] ^(-1/2) with \Lamda(0)=(260 +/- 20)nm. The absolute value of \Lamda(0) was confirmed by direct measurements employing submillimeter wave transmission spectroscopy at 430 GHz. The analysis of the \Lamda(T) data below Tc/2 revealed significant deviations from the quadratic temp…
In this work, we report the preparation of a superconductor thin film of the BSCCO system using a good quality powder with nominal composition Bi_{1.8}Pb_{0.4}Sr_2CaCu_2O_x which was thermally treated using a domestic microwave oven (2.45 GHz, 800 W). This film was grew on a single crystal of LaAlO_3(100) substrate and exhibited a crystalline structure with the c-axis perpendicular to the plane of the substrate. An onset superconducting transition temperature was measured at 80 K.
