Janusz Konopka

Random telegraph signals and low-frequency voltage noise in Y-Ba-Cu-O thin films

Excess low‐frequency noise extending to MHz frequencies was observed in dc current biased granular high‐Tc thin films. At particular bias conditions random telegraph signal produced by a single, fast two‐level fluctuator dominated the noise properties of the sample. Lifetimes of the low‐ and high‐voltage states of the fluctuating system were found to be exponentially distributed. Power spectra of the excess noise signal could be well fitted with a single Lorentzian contribution. Duty cycle dependence of the random telegraph signal on bias conditions was used to get an insight into physical mechanism causing the fluctuations. Charge trapping events in the intergranular intrinsic Josephson junctions and trapped flux hopping were identified as possible alternative sources of the observed noise.

Microwave detection and mixing in Y-Ba-Cu-O thin films at liquid-nitrogen temperatures

Microwave properties of an Y‐Ba‐Cu‐O microstrip detector have been studied at 24 GHz frequency band at the liquid‐nitrogen temperature. The amplitude of the video detection signal was found to be proportional to the incident microwave power. Direct mixing action was also demonstrated, setting the upper limit for the film response time as short as 40 ps. Occurrence of microwave mixing in the absence of dc current bias was observed, suggesting the presence of the inverse Josephson effect. Physical properties of the detector response were very similar to the enhanced mode of detection, characteristic for granular superconducting films.

Microwave noise emission from high Tc thin films

A pronounced X‐band microwave noise emission from Y‐Ba‐Cu‐O thin films has been observed. Emission depends strongly on temperture, bias current, and magnetic field. Two types of emission lines can be distinguished in the power versus dc bias spectra. The lines which remain at stable current positions against temperature we tentatively ascribe to the interaction of depinned flux flow with a two‐dimensional Josephson junction array. The lines that appear at fixed voltage positions may be attributed to the Josephson radiation of a distributed network of junctions. No significant emission was detected from Bi‐Ca‐Sr‐Cu‐O films.

Dielectric properties of nanoparticulate Ba2EuZrO5.5 perovskite at microwave frequencies

Dielectric properties of a complex perovskite material, Ba2EuZrO5.5, which was proposed as a suitable substrate for high TC superconducting films [R. Jose et al., J. Mater. Res., 15, 2125 (2000)], were measured in the frequency range from 0.47 to 26 GHz and in the temperature range from 77 to 350 K. In the present research, sintered bulk Ba2EuZrO5.5 material composed of nanocrystallites was used. The dielectric constant: e′=32,25±0.3% was measured at 300 K, and the material was found to be rather lossy with tan δ close to 10−2. The temperature measurements show unusual behavior of the dielectric properties. In the range from 100 to 250 K, a considerable increase of the dielectric constant and losses were observed. This effect might be due to the tilting of B(B′)O6 octahedra commonly found in complex perovskite materials. Due to such behavior, the applications of Ba2EuZrO5.5 as a substrate for YBCO and BISCCO superconductors at high and microwave frequencies should be rather ruled out.

Current enhanced two-level fluctuator noise in high Tc thin films

Excess low frequency noise was observed in granular high Tc films. Random telegraph signal produced by a single, fast two level fluctuator dominated the noise properties of the sample at some particular bias conditions. Exponentially distributed switching rates fall in the MHz range. Apart from simple switching also more complicated three stage processes were observed. Duty cycle of the two level telegraph signal strongly depends on bias conditions. Power spectra of the excess noise signal can be well fitted with a single Lorentzian.

Spectral properties of rf emission from high Tc films

Spectral properties of rf radiation from intrinsic Josephson junctions in high {Tc} Y-Ba-Cu-O thin film have been measured in the frequency range up to 1.5 GHz. Narrow emission lines with the 3 dB bandwidth of the order of 20 MHz were detected indicating that Josephson clusters radiate coherently. Synchronization conditions are determined by dc current and external magnetic field bias. Frequency locking of radiation to external resonant circuit was also observed. Spectral line narrowing due to resonant lock was distinguished from the coherence-induced narrowing by different tuning properties of the emission line. Noncoherent Josephson radiation manifests itself as a broadband background noise increase. A pronounced 1/{ital f}-like tail sensitive to dc bias and magnetic field was observed in a low frequency part of the spectrum.

Deposition and properties of high critical temperature superconducting ceramic thin films

We report our studies on the fabrication and superconducting properties of thin films in the YBaCuO and BiSrCaCuO systems. The films were manufactured on ZrO2:Y, ZrO2:Gd, MgO, and LaAlO3 substrates, by chemical deposition of properly mixed nitrate precursors, as well as by d.c. magnetron sputtering from a single oxide target. In all cases, as-deposited films required a high temperature heat treatment to become superconducting. After annealing, our films were granular with the thickness ranging from 0.5 to 5 μm, and exhibited sharp superconducting transitions with the zero resistivity state reached at 89 K and 77 K for YBaCuO and BiSrCaCuO films respectively. Patterning techniques, most suitable for our deposition methods, were also developed and tested.

Microwave absorption in YBa2Cu3O7−y thin films

Abstract We report our measurements of high frequency properties of the YBa2Cu3O7−y thin films prepared on heated sapphire substrates using a standard dc magnetron deposition process. The film was exposed to a uniform illumination of the millimeter-wavelength microwave radiation. We observed an abnormal microwave absorption, which manifested itself in a substantial reduction of the film supercurrent. Our results are explained assuming a granular nature of the film.

On the origin of low frequency noise in HTCS thin films

Abstract Pronounced low frequency voltage noise extending into MHz frequencies was observed in current biased granular Y-Ba-Cu-O films. At some particular bias conditions it was possible to isolate the random telegraph voltage noise produced by a single elementary two-level fluctuator. The telegraph signal and the power spectrum of the noise depended strongly on the dc bias current and on the applied magnetic field. On the basis of experimental observations two alternative mechanisms; charge carrier trapping in the barriers of intrinsic Josephson junctions and magnetic flux jumping, auto-detected by intrinsic dc SQUIDS, are proposed as possible sources of the 1/f noise in superconducting high- T c films.

Detection of Millimetre Wave Radiation by Y-Ba-Cu-O Thin Films at Liquid Nitrogen Temperatures

Detecting properties of Y-Ba-Cu-O granular thin film structures obtained by two differnt technological methods and using three different substrate materials have been studied at millimetre waves. The response to 25 and 100 GHz radiation is presented as the function of dc bias current for several temperatures around 77 K. For power levels below saturation the amplitudes of the recovered video signals were found to be proportional to the incident power. Sensitiviy obtained at 110 GHz is comparable to that of crystal point-contact detector. Preliminary results on mixing of two Ku-band sources for IF frequencies up to 5 GHz are presented, indicating that the response time of Y-Ba-Cu-O films is shorter than 40 ps. Due to pronounced quantum effects, which manifest themselves especially in narrow superconducting strips below 60 K, at present state of the technology the usefulness of investigated detectors is limited to liquid nitrogen temperatures.