Microwave Surface Resistance Research Articles

Microwave spectra ofd-wave superconductors

A theory of the anomalous microwave conductivity and surface resistance of high-temperature superconductors is developed for a nested Fermi surface. The damping $\ensuremath{\Gamma}$ is derived for electron collisions across nearly parallel orbit segments. Above the superconducting temperature $\ensuremath{\Gamma}(\ensuremath{\omega},T)$ is linear in frequency $\ensuremath{\omega}$ and temperature T, in accord with the anomalous non-Drude conductivity which differentiates high-${T}_{c}$ cuprates from conventional metals. In the superconducting state a d-wave energy gap suppresses the damping---by reducing available scattering states---and thus explains why the YBCO microwave surface resistance ${R}_{s}$ drops so dramatically when the temperature is a few degrees below ${T}_{c}=90$ K. Numerical calculations of the infrared conductivity, reflectivity, microwave ${R}_{s},$ and penetration depth are presented, and comply with the YBCO experimental data if the gap has a large maximum value ${\ensuremath{\Delta}}_{\mathrm{dm}}\ensuremath{\simeq}{4T}_{c}.$ The cuprate spectral shapes differ notably from conventional BCS theory.

Study of correlation between the microstructure and phase inhomogeneities of Y-Ba-Cu-O epitaxial films and their DC and microwave properties

The influence of various kinds of structure and phase inhomogeneity on electrical properties of YBCO thin films prepared by inverted cylindrical magnetron sputtering has been investigated. A simultaneous analysis of the changes in the microstructure and electrical parameters has allowed us to define the contribution of each kind of inhomogeneity in the restriction of the film's electrical parameters. It has been found that the transition temperature depends mainly on deviations of the c-axis lattice parameter from its optimum value, the critical current density depends mainly on the out of plane misorientation of the film domains and the microwave surface resistance depends mainly on the volume of the high-angle misoriented domains. By using atomic force and scanning tunnelling microscopy regions of YBCO films with modified properties around Cu-rich second phase particles were observed. It is suggested that superconducting properties of these regions are worse than the matrix due to Cu depletion. Usually the relative area of these inhomogeneities in our films is about 30% and can occupy up to 50% and, therefore, mostly determines the electrical properties of films. It is suggested that the processes of cation disorder deforming the film lattice play an important role in formation of structure and phase inhomogeneities in YBCO thin films.

Microwave surface resistance measurements on the heavy fermion superconductor UPt 3

The relative change of the microwave surface resistance has been measured as a function of temperature in the superconducting and normal states of UPt 3 using a cavity perturbation technique. With our frequency range of ∼9–38 GHz, the microwave photon energy (ℏ ω) is ∼20–95% of the estimated zero-temperature (BCS) energy gap (2 Δ 0) of UPt 3. The surface resistance shows a change in slope as the signature of T c and has a significantly weaker temperature dependence in the superconducting state than would be predicted by the Mattis–Bardeen theory. At higher frequencies, the slope tends to change by a smaller amount. No indication of collective modes at any of the frequencies was observed down to the lowest temperatures measured (∼0.1 K).

Dependence of the microwave surface resistance on the structure and thickness of superconducting cuprate films

An investigation is made of the microwave surface resistance Rs measured at 10 GHz and 77 K, as a function of the thickness of superconducting epitaxial films of yttrium barium cuprate grown on lanthanum aluminate substrates by magnetron sputtering. Films between 200 and 1800 nm thick have low values Rs (0.3–0.5) mΩ and do not show any deterioration. The level of Rs achieved in films of comparatively large thickness is mainly attributable to the low rate of film growth (0.8 nm/min).

The influence of in-plane 0–45° grain boundary on microwave surface resistance of c-axis YBa 2Cu 3O y films on MgO substrate

The effect of in-plane 0–45° grain boundaries on surface resistance ( R s) of c-axis YBa 2Cu 3O y (YBCO) films on MgO substrates is discussed. Samples that have various densities of 45° rotated grains were prepared using a self-template technique. The R s values at 22 GHz were measured by the dielectric resonator method using a cryocooler. The R s systematically changed corresponding to the amount of 0–45° grain boundaries. Conspicuous feature of the films including 0–45° grain boundaries is a large residual surface resistance. The R s of almost perfectly in-plane aligned YBCO film scaled to 10 GHz were 0.12 mΩ at 20 K and 0.67 mΩ at 77 K on the assumption of tan δ=0, respectively.

Microwave surface resistance of YBCO bulk samples prepared by addition of HgO to YBCO system

The paper reports microwave surface resistance (R/sub s/) measurement on yttrium barium copper oxide (YBCO) bulk samples, prepared by an alternate route, namely by addition of mercury oxide (HgO) to YBCO system. R/sub s/ is measured using a TE/sub 011/ mode cylindrical cavity, at 20 GHz, by the end plate substitution technique. HgO added to the YBCO system has the advantage that oxygen in HgO acts as a copious source of oxygen and therefore does not require sintering in oxygen flow. Powder X-ray diffraction studies confirm the formation of pure orthorhombic 123 phase without any indication of Hg or known Hg compounds. The specimen with 0.5 wt.% of HgO added to the YBCO system showed superior superconducting properties including microwave surface resistance (22.5 m/spl Omega/ at 77 K and 20.0014 GHz). The R/sub s/ has also been measured as a function of the area of the sample that yields consistent value confirming surface homogeneity. This is believed due to stoichiometric oxygenation and large number of well connected grains.

Microwave surface resistance of YBCO thin films on cerium oxide buffer

The effects of the crystallinity and surface roughness of CeO/sub 2/ buffer on R-plane Al/sub 2/O/sub 3/ substrate on the microwave surface resistance (R/sub s/) of YBa/sub 2/Cu/sub 3/O/sub y/(YBCO) thin films are discussed. We estimated R/sub s/ from the transmission characteristics of the microstrip line resonator at 25 K and 6.7 GHz. X-ray diffraction (XRD) of /spl theta/-2/spl theta/ and /spl phi/-scan showed that CeO/sub 2/ was completely [001]-oriented and in-plane aligned crystal. Four CeO/sub 2/ samples with different thicknesses were prepared using identical conditions except for the deposition time. The dependence of R/sub s/ on CeO/sub 2/ thickness was measured in the range from 10 nm to 200 nm. The value of R/sub s/ was minimum at CeO/sub 2/ thickness of 100 nm. The dependence of R/sub s/ vs CeO/sub 2/ thickness was similar to that of the amount of a-axis domains against the thickness. The crystallinity of thin CeO/sub 2/ was poor because the lattice was strongly strained by Al/sub 2/O/sub 3/. This affected the quality of the upper YBCO layer. In contrast, thick CeO/sub 2/ had excellent crystallinity. However, for the thickness of more than 100 nm a drastic change in surface morphology was observed by atomic force microscopy (AFM). A number of projections appeared on the CeO/sub 2/ surface. These projections act as nucleation centers for the a-axis domains.

Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ and YBa/sub 2/Cu/sub 3/O/sub 7/ films on large area MgO and sapphire substrates for high power microwave and rf applications

This paper describes recent work at DuPont to extend the high power microwave performance of YBa/sub 2/Cu/sub 3/O/sub 7/ and Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ high T/sub c/ superconducting (HTS) films on 2 inch diameter MgO and sapphire substrates. The YBa/sub 2/Cu/sub 3/O/sub 7/ films were produced by in-situ off-axis sputtering with radiant substrate heating. The Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ films were produced by off-axis sputtering followed by annealing in oxygen and Tl/sub 2/O vapor. The microwave surface resistance and power handling of these films was measured over a wide range of temperature (20 K to 85 K) using an HTS-sapphire resonator technique. At 70 K, the surface resistance of YBa/sub 2/Cu/sub 3/O/sub 7/ films on sapphire remained below 250 /spl mu//spl Omega/ (@8.1 GHz) for microwave surface current densities up to 1.7/spl times/10/sup 6/ A/cm/sup 2/. At 70 K, the surface resistance of Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ films on MgO remained below 230 /spl mu//spl Omega/ (@8.1 GHz) with microwave current densities up to 2/spl times/10/sup 6/ A/cm/sup 2/.

Large-area YBCO films on sapphire for microwave applications

Crack-free thick YBa/sub 2/Cu/sub 3/O/sub 7-x/ films are prepared on CeO/sub 2/ buffered r-cut sapphire (2 inch in diameter) with thickness up to 700 nm, smooth surfaces ('peak-to-valley' roughness 2 MA/cm/sup 2/ at 77 K and 0 T) and low microwave surface resistance (R/sub s/(77 K)/spl ap/1.4 m/spl Omega/ and R/sub s/(4.2 K)/spl ap/110 /spl mu//spl Omega/ at 19 GHz), comparable to the best values reported for YBCO films on structurally better matched substrates. The improvement of the critical thickness of YBa/sub 2/Cu/sub 3/O/sub 7-x/ on structurally poorly matched sapphire from typically d/sub c//spl ap/250-300 nm to d/sub c/>700 nm is achieved by introducing defects which do not hamper the superconducting properties of the films. The modified film growth was reflected by slightly broadened rocking curves and an altered temperature dependence of R/sub s/. The thick YBCO films were able to handle high microwave power corresponding to magnetic field amplitudes (B/sub HF/) of at least 54 and 37 mT at 4.2 and 50 K (limited by the available power), and 17 mT at 77 K (often limited by quenches). The excellent high-power performance was achieved without any degradation of the samples despite frequent thermal cycling.

Ag-doped double-sided PLD-YBCO thin films for passive microwave devices in future communication systems

Pulsed laser deposited (PLD) Ag-doped YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films on both sides of 3-inch diameter sapphire wafers are used routinely for development of microwave filters for future communication systems. The reproducibly deposited YBCO:Ag films of about 250 nm thickness show critical current densities of 4 MA/cm/sup 2/ at 77 K and laterally homogeneous maps of microwave surface resistance R/sub s/ of about 45 m/spl Omega/ at 145 GHz and 77 K measured by an open resonator technique. The R/sub s/ at 8.4 GHz and 77 K determined in the center position of the YBCO:Ag films with a sapphire resonator technique remains constant at about 380 /spl mu//spl Omega/ up to a microwave surface magnetic field of 7-10 mT. Correlations of transport and microwave properties to the film microstructure are shown in terms of in-plane epitaxy, size of particulates on the films, and composition ratios Cu/O and Y/O, and growth defects like stacking faults as shown by Raman spectroscopy, SEM, and SNMS depth profiling, and TEM cross sections, respectively. The optimum Ag-content of the PLD-YBCO target was determined to be about 4 weight -%. The results demonstrate that Ag-doping supports the PLD process for YBCO in terms of reliability and cost effectiveness.

Role of oxygen pressure during deposition on the microwave properties of YBCO films

We have studied the effect of oxygen pressure (PO/sub 2/) during pulsed laser deposition on the properties of YBCO films, with particular attention power microwave surface resistance R/sub s/. Above a threshold oxygen pressure the properties of the films are nearly independent of PO/sub 2/ during deposition and are typical of high quality YBCO films. The films made below this threshold pressure have increased disorder which produces a reduced T/sub c/ and an expanded c-axis lattice parameter. However, these films also have significantly reduced low temperature R/sub s/, which is likely a direct result of the increased scattering in these films. Preliminary Raman measurements show no increase in the Y-Ba cation disorder in these low PO/sub 2/ films, so that different disorder mechanism must be present.

Microstructure and microwave surface resistance of YBaCuO thin films

Epitaxial YBaCuO thin films on 73 mm diam. sapphire and 50 mm LaAlO/sub 3/ wafers prepared for microwave applications were characterized by optical and electron microscopy as well as by X-ray diffraction in order to guide optimizing the film properties. The surface resistance R/sub s/, measured at 8.5, 19 or 145 GHz, partially as a function of microwave field amplitude B/sub s/, was taken as the key parameter. Typical results scaled quadratically in frequency to 0.25-0.75 m/spl Omega/ at 10 GHz, 77 K, and low field levels. In case of sapphire substrates, a challenge for applications is microcracking of the films. However, cracks are less deleterious if "dispersed" by other heterogeneities such as a-oriented grains. Narrow microcracks should allow for tunnel currents and flux pinning. In case of LaAlO/sub 3/, films sensitive to high microwave power exhibited some in-plane rotational misorientation and a-oriented grains as well as inhomogeneous layering of these grains and of additional Cu-O planes within the film thickness, with possible influence on R/sub s/.

Anomalous Peak in Microwave Surface Resistance Studies on Silver Doped Bi1.84Pb0.4Sr2Ca2.2Cu3O10 Superconductor

We report surface resistance studies on silver (0 to 25 wt%) doped Bi1.84Pb0.4Sr2Ca2.2Cu3O10 samples prepared by spray drying technique. The XRD studies show the predominance of 2223 phase in the samples. The 10 wt% silver doped sample shows optimum superconducting characteristics viz, improved microstructure characteristics and the reduced resistive grain boundaries. Temperature dependent microwave surface resistance behaviour shows the apparence of two anomalous peaks (high temperature at Tc and low temperature near TCI2) in samples when the Ag dopant level was increased beyond 10 Wt%.

Microwave Surface Resistance of Superconducting YBa2Cu3O7-δ Thin Films

One of the serious material problems with high Tc superconductors is their granularity which greatly affects their microwave transmission (as well as absorption) properties. High quality near-epitaxial thin films of these oxide superconductors have been found to be advantageous for microwave devices due to their low conduction loss. Pulsed laser ablation deposition (PLAD) technique has been extensively used for the growth of high quality films of oxide superconductors. Work carried out at TIFR during the last several years has convincingly shown that significant improvement in microstructure, and consequently, in microwave transmission properties of YBa2Cu3O7-δ (YBCO) thin films can be realised by incorporating a small quantity (about 5%) of Ag in the YBCO target used for PLAD. Microwave surface resistance of YBCO films has been measured using microstrip resonator configuration at various temperatures in the range 15-88K and in the frequency range 1.5 to 12 GHz. Due to the much improved microstructure, Ag-YBCO films have not only shown a low surface resistance but also a good power handling capability. Au-YBCO films too have shown improved properties with the added advantage of much improved environmental stability. This paper reviews significant advances in the growth and microwave measurements of Ag-YBCO and Au-YBCO thin films and show why they would be of great advantage for microwave passive devices operating at 77K.

Suppression of the "notch effect” in microwave surface resistance in YBa2Cu3O7 films on MgO (100) substrate by deposition of ultra-thin SrTiO3 seed layers

RHEED controlled ultra-thin buffer layers of SrTiO 3 have been deposited on (100) MgO by pulsed laser deposition to growth YBaCuO films for microwave applications. A buffer layer with a thickness between 2 and 15 nm, i.e. 5 to 40 unit cells of SrTiO 3 , is sufficient to expand to more than 60 °C the range of deposition temperatures leading to low microwave surface resistance. The Rs values are as good as those obtained on LaAlO 3 substrates with a slighly lower magnetic field dependency. The SrTiO 3 seed layer induces an oriented epitaxial growth with YBCO [100] // MgO [100] over this range of deposition temperatures.

Microstructure and microwave surface resistance oftypical YBaCuO thin films on sapphire andLaAlO3

Epitaxial c-oriented YBCO films laser deposited onto 3 in diameter CeO2-buffered sapphire wafers and LaAlO3 cylinders as well as sputter deposited onto 2 in diameter LaAlO3 wafers were characterized by integral and spatially resolved measurements of the critical current density jc and the microwave surface resistance Rs, by microstructure investigations using optical and electron microscopy and by x-ray diffraction. Epitaxial misorientations of in-plane-rotated as well as of a-axis-oriented grains were found in amounts up to 10%. The in-plane rotation seriously degraded Rs while the a orientation mainly lowered jc. Moreover, a degradation of Rs and of the microwave power handling could be clearly correlated with the density of microcracks occasionally found in YBCO films on sapphire. Inhomogeneities like a-axis-oriented grains were observed to `disperse' microcracks, probably in favour of the electrical properties. The impact of further microstructure imperfections on Rs, in particular of the typical twin lamellae and their domains, is discussed in view of findings from transmission electron microscopy.

Suppression of the "notch effect” in microwave surface resistance in YBa2Cu3O7 films on MgO (100) substrate by deposition of ultra-thin SrTiO3 seed layers

Suppression of the "notch effect” in microwave surface resistance in YBa2Cu3O7 films on MgO (100) substrate by deposition of ultra-thin SrTiO3 seed layers

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Using a dielectric resonator method at 10.7GHz in the TE011 mode, we investigated the microwave surface resistance (Rs) of screen-printed Bi2Sr2CaCu2Ox (Bi2212) thick films on Ag substrate subjected to a partial-melt solidification process. Heat treatment conditions on the partial-melt solidification process influence the Rs value of thick films through changes in microstructures therein. We obtained a low Rs value of 1.6mΩ at 20K by heating the film up to a maximum temperature of 880°C, subsequent slow cooling at 4°C/h from 880 to 860°C, isothermal annealing for 10h at 860°C and final cooling to room temperature. To our knowledge, this Rs value is the lowest record for Bi2212 thick films subjected to the partial-melt solidification process. The factor dominating the Rs value of Bi2212 thick films was investigated from a microstructural point of view. The film with low Rs value shows a large grain size, small amount of impurity phases and also a high degree of grain alignment. A significant correlation between microstructures and Rs values of Bi2212 thick films was suggested.

Sensitivity in the measurement of microwave surface resistance of high- superconducting samples of different sizes by the cavity method

Cavity resonators are conventional devices for measuring the temperature and frequency dependence of microwave surface resistance, , of superconducting samples. The temperature dependence can provide information about the losses due to those conduction electrons that are not condensed into the superconducting state. Since surface resistance characterizes the response to the electromagnetic field near the surface, it qualifies the suitability of the superconducting material for microwave device applications. The measurement of by the cavity technique is performed by replacing the end plate of mode cavity with the superconducting sample. The sensitivity of measurement is restricted for the samples whose dimension is less than that of the end plate. In this paper, we report the measurement of surface resistance of bulk samples of varying dimensions, using a lamina with a circular opening in the end plate. Sensitivity and the error in the measurement of is discussed with respect to the uncertainties in the Q-value and the -value of the background copper cavity.

Peculiarity of surface microwave resistance in overdoped YBCO films

The temperature dependence of the microwave surface resistance for overdoped YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films has been investigated experimentally. The observed unusual behavior of the surface resistance is described by the schemes based on an idea of the spin-flip scattering of carriers, resulting in the pair-breaking, and the subsequent recovery of superconductivity caused by the ordering of the magnetic impurities at low temperatures.