Surface Impedance Measurements Research Articles

Microwave Properties of Sapphire Resonators with a Gap and Their Applicability for Measurements of the Intrinsic Surface Impedance of Thin Superconductor Films

A dielectric resonator with a gap between the top plate and the rest has been useful for measuring the penetration depth (A) of superconductor films, a parameter essential for obtaining the intrinsic microwave surface resistance (R,) of thin superconductor films. We investigated effects of a gap on the microwave properties of TE 0ml -mode sapphire resonators with a gap between the top plate and the rest of the resonator. Regardless of a 10 μm-gap in TE 0ml -mode sapphire resonators, variations of the TE 0ml -mode resonant frequency on temperature (Δf 0 ) as well as TE 0ml -mode unloaded Q remained almost the same due to lack of axial currents inside the resonator and negligible radiation effects. The A of YBa 2 Cu 3 O 7-δ (YBCO) films obtained from a fit to the temperature-dependent Δf 0 appeared to be 195 nm at OK and 19.3 GHz, which was well compared with the corresponding value of 193 nm at 10 kHz measured by the mutual inductance method. The intrinsic R s of YBCO films on the order of 1 mΩ, and the tan 6 of sapphire on the order of 10 -8 at 15 K and 40 GHz could be measured simultaneously using sapphire resonators with a 10 μm-gap.

Surface Impedance Studies of Heavy Fermion Superconductors, PrOs4Sb12and UBe13

We report the results of RF surface impedance measurements of heavy-fermion superconductors, PrOs 4 Sb 12 and UBe 13 . For highly conductive material such as heavy-fermion superconductors, the RF impedance measurements at lowtemperatures provide surface impedance Z s = R s + i X s of the sample. The field dependence of Z s ( H ) in PrOs 4 Sb 12 and UBe 13 shows anomalies at 0.4–0.6 H c2 , indicating the anomalous vortex pinning. We report measurements of the H – T phase diagram of PrOs 4 Sb 12 and UBe 13 from the field dependence of Z s ( H ).

Terahertz surface impedance of epitaxial MgB2 thin film

We report on terahertz (THz) surface impedance measurement of an epitaxial MgB2 thin film using time domain THz spectroscopy. We show that the surface resistance of the MgB2 film is much lower than that of YBa2Cu3O7−δ and copper in the THz range. A linear dependence of the surface reactance on frequency is observed, yielding a penetration depth of about 100nm at low temperatures. The measurements agree qualitatively with calculations based on impurity scattering in the Born limit. Our results clearly indicate that MgB2 thin films have a great potential for THz electronic applications.

Dependence of magnetic penetration depth on the thickness of superconducting Nb thin films

In this paper we present the results of a systematic study on the magnetic field penetration depth of superconducting niobium thin films. The films of thicknesses ranging from $8\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}300\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ were deposited on a Si substrate by dc magnetron sputtering. The values of the penetration depth $\ensuremath{\lambda}(0)$ were obtained from the measurements of the effective microwave surface impedance by employing a sapphire resonator technique. Additionally, for the films of thickness smaller than $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the absolute values of $\ensuremath{\lambda}(0)$ were determined by a microwave transmission method. We found that the reduction of the film thickness below $50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ leads to a significant increase of the magnetic field penetration depth from about $80\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for $300\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick film up to $230\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for a $8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick film. The dependence of the penetration depth on film thickness is described well by taking into account the experimental dependences of the critical temperature and residual resistivity on the thickness of the niobium films. Structural disordering of the films and suppression of superconductivity due to the proximity effect are considered as mechanisms responsible for the increase of the penetration depth in ultrathin films.

Surface modification of ethylene–norbornene copolymer by irradiation with N4+ ion beams

The modifications of copolymer of ethylene and norbornene induced by irradiation with 60keV N4+ ions to the fluencies of 1.0×1016ionscm−2 have been investigated using DSC, TGA, UV-spectroscopy, direct current surface resistivity and impedance measurements. The measured heat and thermal properties reveal remarkable stability against degradation. Copolymer properties studied at low electric field strengths were directly related to the modified molecular structure. The decrease in electrical resistivity was mainly attributed to conjugated double bond formation observed by ultraviolet spectroscopy. AC conductivity and capacitance obtained from the real and imaginary part of admittance varies with frequency. The movement of charge carriers may be related to relaxation phenomena in the material and is similar to those observed in many amorphous semiconductors.

Accurate measurements of the intrinsic surface impedance of thin YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films using a modified two-tone resonator method

We propose a modified two-tone method that could be used for sensitive measurements of the intrinsic microwave surface impedance (Z/sub S/) of thin superconductor films and the tan/spl delta/ of a low-loss dielectric. An open-gap resonator scheme is used to measure the penetration depth (/spl lambda/) of thin superconductor films and extract the intrinsic Z/sub S/ of the superconductor films from its measured R/sub S//sup eff/ and /spl lambda/. We use a very small gap of 10 /spl mu/m between the top plate and the rest parts of the resonator. The tan/spl delta/ of rutile in the low 10/sup -7/ range and the dielectric constant as high as /spl sim/110 are observed at temperatures below 10 K at /spl sim/15.2 GHz, which enable to measure the R/sub S/ of the 10 mm-in-diameter YBCO films as low as /spl sim/100 /spl mu//spl Omega/ at the same frequency (f). The discrepancy between the R/sub S//sup eff/ at /spl sim/15.2 GHz and that at /spl sim/8.5 GHz scaled to /spl sim/15.2 GHz appears less than 2% when the relations of R/sub S/ /spl prop/ f/sup 2/ and tan/spl delta/ /spl prop/ f are used. We describe usefulness of our measurement method for measuring the intrinsic microwave properties of various superconductor samples.

In situ measurements of surface impedance and absorption coefficients of porous materials using two microphones and ambient noise

An in situ measurement method is proposed for obtaining the normal surface impedance and absorption coefficient of porous materials using two microphones located close to the material without a specific sound source such as a loudspeaker. Ambient environmental noise that does not excite distinct modes in the sound field is employed as the sound source. Measurements of the normal surface impedance of glass wool and rockwool have been made using this method in various sound fields. The repeatability and wide applicability of the method are demonstrated by comparing results of measurements in one room with different noise conditions and in three other environments (corridor, cafeteria and terrace). The assumed diffuse nature of the sound field on the material is validated by using absorption characteristics obtained experimentally at oblique incidence. This method allows simple and efficient in situ measurements of absorption characteristics of materials in a diffuse field.

Observation of the coherence peak in MgB 2 thin films

The coherence peak is observed in the temperature dependence of the real part of the microwave conductivity ( σ 1) of c-axis oriented MgB 2 thin film, which is extracted from microwave surface impedance ( Z s) measurements using a sapphire resonator at 17.9 GHz. The peak appears at a temperature around 0.6 times the critical temperature ( T c), which is greatly shifted downwards in temperature compared with conventional superconductors for which the peak occurs around 0.9 T c. Calculations based on the two-band model suggest that this peak corresponds to the smaller of the two energy gaps.

The vortex depinning transition in untwined YBaCuO using complex impedance measurements

We present surface impedance measurement of the vortex linear response in a large untwined YBCO crystal. The depinning spectra obtained over a broad frequency range (100 Hz–30 MHz) are those of a surface pinned vortex lattice with a free flux flow resistivity (two modes response). The critical current in the “Campbell” like regime and the flux flow resistivity in the dissipative regime are extracted. Those two parameters are affected by the first order transition, showing that this transition may be related to the electronic state of vortices.

Deduction of the Experimental Surface Impedance of YBCO Thin Films Using a Self-Consistent Calculation

Measurement of surface impedance of double sided microstrip line resonator patterned out of thin film YBCO has been carried out as a function of temperature and power (restricted to linear regime). New model of the current density distribution has been used in order to extract the real and imaginary parts of the surface impedance. This model is based on a sound physical theory rather than a phenomenological one. The introduction of the critical current density in this model has a great influence on the determination of the microwave loss mechanisms along the transmission lines. Several propagation quantities have been extracted using the above mentioned model. Using this model and an iteration method to determine the RF magnetic field, we were able to reproduce our experimental results of the quality factor and then determine the corresponding surface impedance as well as the kinetic inductance and the shift in the effective dielectric constant as a function of temperature.

Surface impedance measurement with use of a tunnel diode oscillator in pulsed magnetic fields

We have developed a technique of surface impedance measurement with use of a tunnel diode oscillator in pulsed magnetic fields. Compared to conventional four-terminal resistivity measurements, this is a high-sensitive and non-contact method, enabling us to measure a small sample precisely. For the purpose of fast frequency readout, we constructed a simple FM demodulator, analogous to an FM radio receiver. With this demodulator, we succeeded in resistivity measurements in pulsed magnetic fields of up to 40 T at temperatures down to 0.6 K .

An instrument for low- and variable-temperature millimeter-wave surface impedance measurements under magnetic fields

We describe a low-temperature millimeter-wave (44 GHz) surface impedance measurement instrument based on the cavity perturbation method. In this instrument, all millimeter-wave paths at low temperatures (including the cavity resonator), are located inside a high vacuum tube, which can be inserted into a 7 T superconducting magnet. This design, which is free of exchange gas, enables measurements over a wide temperature range as well as providing stable and reproducible operation. By pumping a He3 pot attached to a cavity, a sample can be cooled well below 1 K. We present experimental results which demonstrate the performance of the instrument.

Anomalous coherence peak in the microwave conductivity of c-axis oriented MgB2 thin films.

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 band structure calculations. In addition, the observation of a coherence peak indicates that the pi band is in the dirty limit and dominates the total conductivity of our films.

Microwave and modulated optical reflectance studies of YBCO thin films

Planar HTS microwave devices require high quality, homogeneous samples. In this paper, sensitive measurements of the microwave surface impedance of YBCO thin films using coplanar resonators are collated with modulated optical reflectance (MOR) measurements. MOR provides a powerful noncontact, nondestructive and high resolution means of probing local variations in the quality of thin films at room temperature, and consequently has great potential for diagnostic testing of HTS films prior to microwave device patterning. Microwave and MOR inter-comparisons of four YBCO films patterned into 5.2 and 8 GHz coplanar resonators are presented. Superior global microwave response in the superconducting state, such as low surface impedance and low levels of nonlinearity at enhanced powers, correlate with the magnitude and spatial homogeneity of the room temperature MOR signals. The presence of defects in films is investigated using both techniques. Both large scale single defects and film inhomogeneity can be detected using MOR; however, the spatial resolution of the technique is not sufficient to detect single weak links on a sub-micron scale, whose presence can result in severely degraded microwave resonator performance.

Description of the nonlinear behavior of superconductors using a complex conductivity

The origin of the detrimental nonlinear response in high T/sub c/ superconductor (HTS) microwave devices is currently not well understood. In order to help elucidate the origin of these nonlinear effects, we have developed a description of the nonlinear response in superconductors in terms of a current-dependent complex conductivity. We demonstrate that such a treatment can consistently describe the results of power-dependent surface impedance measurements in resonator geometries as well as harmonic generation and intermodulation distortion effects in transmission line geometries. This approach yields a device-independent quantity that describes the nonlinear response of the superconducting material itself, which is suitable for comparisons of different materials and for material optimization. A further benefit of this description of nonlinear effects is that the relative importance of the nonlinear resistive and inductive components of a superconductor can be examined. We use this approach to predict the phase of the nonlinear response in HTS planar transmission lines, and compare our predictions with new phase-sensitive measurements made using a nonlinear vector network analyzer.

Millimeter wave and microwave electrodynamic spectroscopy of YBa2(Cu1−xZnx)3Oy in the Meissner and mixed state

We performed the surface impedance measurement on almost optimally doped untwinned YBa2(Cu1−xZnx)3Oy crystals under dc magnetic fields parallel to the c-axis, and determined the dependence of the vortex viscosity, η, and the pinning constant, κp on Zn concentration. Both η and κp changed dramatically from those of pure YBa2Cu3Oy by 6%-Zn-doping. It can be explained by the strong overlap of the Zn potential and suppression of the amplitude of the superconducting (SC) order parameter caused by the pair-breaking effect of Zn. We also determined the relaxation time (RT) of quasiparticles (QPs) in the vortex core, in the normal state, and in the Meissner state. It is suggested that the mechanism which determines the RT of the QP is different between outside and inside the vortex core. It is also suggested that RT of QP in the vortex core becomes smaller than in the normal state for the superconductors which have nodes in the SC gap.

A study on the nonlinear microwave electrodynamic response of e-beam evaporated MgB2 superconducting thin films

We present a study on the temperature and field dependence of the microwave surface impedance Zs in thin films of the superconducting MgB2 compound. Samples were prepared by e-beam evaporation of boron on r-plane sapphire followed by an ex situ annealing in Mg vapour. Critical temperature values range between 26 and 38 K. Surface impedance measurements (Zs = Rs + iXs) were performed from 2 K close to Tc in the microwave region up to 20 GHz via parallel plate or dielectrically loaded resonators in ‘symmetric’ (two MgB2 films) and asymmetric (an MgB2 film and a commercial YBCO control film) configurations. At high microwave power, frequency domain measurements show a characteristic signature associated with weak links and this appears to be the limiting factor governing the performance of these films.

Effect of Impurities on the Electronic Structure of Vortex Core Investigated by Microwave Surface Impedance Measurement

We investigated the microwave surface impedance in Zn doped YBa 2 Cu 3 O y . For low Zn concentrations, crossover frequency ω p and viscosity η of vortex do not change significantly (at maximum, factor of 2). This is in sharp contrast to the dramatic change in the zero-field quasiparticle scattering time. The robust nature of viscosity on the impurity doping suggests that the quasi-particle scattering time in the vortex core is determined by a different mechanism from that outside the core.

The microwave surface impedance of MgB2 thin films

In this paper we present the results of measurements of the microwave surface impedance of a powder sample and two films of MgB2. The powder sample has a Tc = 39 K and the films have Tc = 29 K and 38 K. These samples show different temperature dependences of the field penetration depth. Over a period of six months, the film with Tc = 38 K degraded to a Tc of 35 K. We compare the results on all samples with data obtained elsewhere and discuss the implications as far as is possible at this stage.

The superconducting gap of in situ MgB2 thin films by microwave surface impedance measurements

Precision measurements of the microwave surface resistance Rs of in situ MgB2 films directly reveal an exponential behavior of Rs at low temperature indicating a fully-gapped order parameter. The entire temperature dependence of Rs is well described by a Mattis–Bardeen formalism, but with a small gap ratio of Δ(0)/kTc=0.72, corresponding to Δ(0)=1.9 meV.