Anomalies of the photo-response and thermal boundary resistance of a YBaCuO/YSZ structure

The photoresponse of a YBaCuO/ZrO₂ bolometric structure was measured under modulated (λ = 630 nm) and pulsed (τ ≈ 7 – 8 ns; λ = 337 nm) laser excitation. The shape of the measured photoresponse was interpreted by a thermal model; nevertheless, the pulse amplitude for vanishing YBaCuO film resistance was 5–6 times greater than predicted; the thermal boundary resistance R_Bd between YBaCuO and YSZ was evaluated ? 10^?2 K × cm²/Watt, which is considerably larger than estimated theoretically for the similar situation of YBaCuO/MgO [5].     

Surface resistance and far infrared absorptivity at T=7 K of highTc YBaCuO superconductors

A surface resistance as small as R_S=3×10^–2 Ohms has been computed at T=8 K, for v=0.6 Thz (v=20 cm^–1), for an YBaCuO film deposited on an MgO substrate. The calculations are made with the refractive index computed from the Far IR transmission spectra of a 400 Å thick film.Still lower values are obtained with another sample, 300 Å thick of exceptional quality (R_S=1.2×10^–2 Ohms).     

Performance of YBaCuO thick film rf SQUIDs at 77 K

RF SQUID behaviour has been observed at 77 K in YBaCuO thick films prepared by screen printing technique. A hole shunted with a microbridge type of geometry is patterned manually for observing rf SQUID behaviour. Flux noise spectrum is also studied and it is found to depend on the quality of the film. The spectral density of the flux noise in the white noise region is 1.7×10⁻³ Φ₀/√Hz at 77 K.     

Magnetron sputtering of YBaCuO target: Effects of discharge voltage and deposition rate variation

The time variations of the discharge voltage and YBaCuO film deposition rate in an on-axis magnetron sputterer are studied. In the case of an YBaCuO ceramic target, the discharge voltage and the rate of superconducting phase deposition decrease with time, exponentially approaching a quasi-steady regime. At a pressure of 100 Pa, a decrease in the magnetic field induction near the target from 1200 to 600 G leads to a rise in the discharge voltage by 25–30% and increases the deposition rate more than twofold. The deposited films offer high electrophysical parameters, as demonstrated with a high-frequency circuit: the intrinsic Q factor of the circuit at 64.5 MHz is found to be 2.7 × 10⁵.     

The possibility of new high-transmission, high finesse, Perot-Fabry interferometers for far ir spectroscopy

Finesse as high as 260 and peak transmission close to 98% at frequencies around 30 cm^–1 are predicted at 10 K by coating the Perot-Fabry (PF) mirrors with a suitable thickness of High Tc (HTC) superconductor YBaCuO. In fact, Finesse will be limited by the lack of parallelism and the lack of perfect flatness of the mirrors to values around 100. These predictions are based upon phenomenological models for both the YBaCuO coating and the MgO support, i.e. from direct transmission measurements of MgO and YBaCuO in the Far IR, which had to be fitted by the models.     

Adaptation of the sheet resistance of an YBaCuO layer to the substrate impedance, applications

We have studied the Far Infrared transmission spectra of an MgO plate covered with an YBaCuO film of thickness t=200 Å, at different temperatures. By cooling from room temperature to temperatures < 140 K, the Far Infrared interference fringes are displaced by half a period. Explanation in terms of different adaptations of the YBaCuO sheet resistance to the MgO substrate impedance during cooling.     

Study of highT c superconductivity in Y1Ba2Cu3O7−x by PAC

HighT _c superconductivity in the YBaCuO superconductor has been investigated through the quadrupole interaction of the probe nuclei⁹⁹Tc. The quadrupole interactions were measured by the TDPAC method from 77 to 296 K. The probe nuclei⁹⁹Tc were introduced into the YBaCuO superconducting specimen by diffusion. The derived electric quadrupole interaction parameters show that the probe nuclei are subject to a unique EFG interaction and occupy a substitutional lattice site in the YBaCuO superconductor. A strong EFG of 10¹⁹ V/cm² was observed. The temperature dependence of the EFG exhibits a linear decrease with temperature increase. Anomalies of both EFG and ν were found in the superconducting transition temperature region. The role of the oxygen vacancies in the Cu−O chains is discussed.     

The role of Y2BaCuO5 in PAC-experiments on YBa2Cu3O7

The “green phase” of the superconductor YBaCuO was studied by means of the PAC-method.¹¹¹In was implanted into sintered bulk samples of Y₂BaCuO₃ at 400 keV. One of the three observed fractions shows combined interaction below the Néel temperature T_N=28 K. Therefore, these¹¹¹In-probes occupy the Cu-site, the oxygen coordination of which is nearly identical with that of Cu₂-site in YBaCuO. The other fractions are often observed in PAC-experiments on YBaCuO after diffusion, which points to a disintegration of these samples. Supported by the BMFT under the contract number FK2 13N 54930     

The EFG of111Cd in Y2Cu2O5

The perturbed angular correlation (PAC) method was applied to study the “blue-phase” of the high T_c-superconductor YBaCuO.¹¹¹In was implanted at 400 keV into pressed powder of Y₂Cu₂O₅. PAC spectra were taken at measureing temperatures between 23 K and 843 K. Four fractions were observed which show a strong motional narrowing with increasing temperature. Two of these fractions have PAC parameters like Y₂O₃ and the other two are similar to the “Cul-site” in YBaCuO. This work was supported by the BMFT, contract: FKZ 13 N 54930     

Characterization of superconducting YBaCuO-films by low temperature scanning electron microscopy

Low-temperature scanning electron microscopy has been performed for imaging the spatial distribution of the critical current densityj _c(x,y) and of the critical temperatureT _c(x,y) in polycrystalline superconducting YBaCuO films. Strongly inhomogeneous behavior has been observed, and the spatial resolution limit has been found to be 1–2 m. The local temperature increment in the specimen film caused by the electron beam scanning has been demonstrated experimentally as the underlying mechanism of the imaging principle, and the beam-induced thermal perturbation of the high-T _c film/substrate configuration is discussed in detail. The radiation hardness of the sample films against the electron beam irradiation in our imaging experiments has been evaluated. No radiation damage could be detected up to the maximum applied dose of well above 10²⁰ electrons/cm² for a typical beam energy of 26 keV.     

Structure and transport properties of ultrathin YBa2Cu3O7−x films

The crystal structure and transport properties of epitaxial c-oriented YBa₂Cu₃O_7−x films are investigated for high-T _c layer thicknesses from 5 to 300 nm. The films were prepared by laser deposition. Films less than 30 nm thick become predominantly single-domain in the direction of the c axis. As the thickness decreases, the orthorhombicity parameter of the YBaCuO lattice decreases, which correlates with the critical temperature degradation observed in films less than 9 nm thick. The obtained thickness dependence of the effective microwave surface resistance of a YBaCuO film agrees well with the computational result obtained in the framework of local electrodynamics for samples with a constant microwave conductance. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 608–613 (25 April 1996)     

Superconducting and structural properties of YBaCuO thin films deposited by inverted cylindrical magnetron sputtering

Thin films of YBaCuO have been deposited onto Al₂O₃, MgO, SrTiO₃ and ZrO₂ substrates by inverted cylindrical magnetron sputtering. The main advantage of this preparation technique is the high reproducibility of the results allowing systematic studies of the film properties as a function of deposition parameters. Optimum preparation conditions were a low discharge voltage (100 V), a high oxygen partial pressure (2×10^–1 Torr) in an oxygen argon mixture, and substrate temperatures around 800°C. The films grow highly textured on all substrates yielding single crystalline growth on SrTiO₃ and MgO as proved by X-ray diffraction and channeling. The zero resistance values of the sharp transtions usually are around 90 K. highest critical current densities were determined in films grown on MgO and SrTiO₃ with values up to 5.5×10⁶ A/cm² at 77 K.     

An experimental study of Josephson effect in bulk YBaCUO

We have observed Josephson effect in bulk YBaCuO. The size of bulk is 7.5×2.2×0.3 mm and the microwave frequency is 9.82 GHz in our experiment. Several microwave induced steps can easily be observed. When an external magnetic field is applied to the bulk, the critical supercurrent at zero voltage is suppressed significantly. It has been demonstrated that YBaCuO bulk can be seen as a network of Josephson junctions. An experimental study of Josephson effect in bulk YBaCuO at millimeter wave frquency is in progress.     

111Cd hyperfine probes in YBaCuO

Different techniques like implantation or in-diffusion have been applied to place the hyperfine probe¹¹¹Cd at YBaCuO lattice sites. In this paper different experiments are reviewed which show that the standard PAC-probe can be placed at any regular site of YBaCuO. The PAC-parameters of these sites are collected and discussed. This work was supported by the BMFT, contract: FKZ 13 N 54930     

Effect of cation composition on the superconducting properties and on the microstructure of YBaCuO thin films

The surface morphology and superconducting properties of YBaCuO epitaxial films prepared through magnetron sputtering from targets of different cation composition were systematically studied. It was shown that small changes in the growth conditions and relatively small variations in the cation composition of the condensate noticeably affect the surface morphology of the films and their structural and superconducting properties, thus offering an efficient way of controlling the YBCO film parameters. It was found that the 90° off-axis configuration of the magnetron sputtering system permits realization of growth conditions in which the grown films do not contain CuO precipitates and exhibit good superconducting properties (T_c≥88 K, j_c(77 K)≥4×10⁶ A/cm²).     

Dispersion of magnetostatic surface waves in ferrite-htsc structures near the phase transition

We present a detailed study of the dispersion properties of surface magnetostatic waves in layered structures of ferrite and high temperature superconductor (HTSC) layers. We solve the propagation problem for the surface magnetostatic waves under a quasistatic approximation, and obtain the dispersion relation. We present an analysis of this dispersion near the phase transition temperature in the HTSC layer. The calculations show that when the HTSC material transforms to the superconducting state the dispersion properties of the magnetostatic waves changes suddenly: the damping decreases by 4–10 dB and the phase velocity increases by a factor of 2–4. These results support experiments which have been performed, in which surface magnetostatic waves have been studied in a YIG film with gallium impurities and a YBaCuO film on a lithium niobate substrate.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 50–55, March, 1995.     

Optical properties of YBaCuO thin films at Far-Infrared monochromatic light

By using monochromatic sources of radiation we measured the reflectivity and transmission vs temperature behavior of YBaCuO thin film in the Far Infrared region in the range 300-20 K. A smooth change of optical properties is observed from critical temperature down to 40 K with an evident reduction of the losses for all the frequencies we examined. To analyze the use of YBaCuO films as semireflecting elements in submillimeter devices we made a preliminary study of the optical properties of the substrate in the same conditions. The permanence of the sample in a superconducting phase can be deduced from its room-temperature transmittance, as it changes drastically if some degeneracy occurs in the film.Partially supported by the National Research Council of Italy, CNR, under the Progetto Finalizzato Superconductive and Cryogenics Technologies     

57Fe ion implantation studies on superconducting oxides

Ion implantation of⁵⁷Fe in YBaCuO pellets with different oxygen contents was tested for three parameters: total fluence, energy of the Fe ions and target temperature. The CEMS spectra obtained at room temperature are complex and we present here the analysis obtained for distinct symmetric doublets. The results are discussed on the basis of iron species present in sintered Fe: YBaCuO samples and spurious phases like Y₂BaCuO₅.     

高温超导YBaCuO薄膜的不稳定性实验

一、引言 高温氧化物超导材料一出现,科学家们就注意将它制成薄膜材料、以此迈向实用化。利用高温氧化物超导材料制造超导量子干涉探测器,高速微芯片,红外探测器、高温超导晶体管等是各国科学家应用开发的重要领域,其他如超导混频器、超导天线等可用于通信、军事等领域具有广阔的前途。已经发展了许多方法制备高温超导薄膜,诸如DC或RF磁控溅射、     

Measurement of the impedance of very thin superconducting YBaCuO films at 10 GHz with a cavity perturbation method between 4 and 300 K

We have measured the complex film impedance 1/σd (σ conductivity, d film thickness) of three YBaCuO thin films with d = 44, 115, and 168 nm on MgO substrates at 10.2 GHz in the temperature range between 300 and 4 K. Below T_c, the experimental results are discussed in terms of the two-fluid model and the BCS theory. The residual resistance decreases with the film thickness. The thinnest film has a residual surface resistance of 3 · 10^?4 Ω. For this film, the complex microwave conductivity is calculated and compared with the models. Apart from the residual resistance, the measured conductivity is in agreement with the peak caused by the energy gap of the BCS theory. All measurements were performed with a cavity perturbation method which we have to our knowledge applied for the first time to superconducting thin films. The method allows to determine the complex impedance of films with arbitrary thickness. In particular, films with thicknesses small compared to the skin depth δ or the London penetration depth λ can be measured. Therefore, we are able to measure the impedance both in the normal and superconducting state.