Ulrich Poppe

Growth and etching phenomena on pulsed laser deposited YBa2Cu3O7-d films.

Abstract The surface morphology of pulsed-laser deposited YBa 2 Cu 3 O 7−δ films is investigated by STM AFM. Instead of spiral growth, a 2D nucleation and growth behaviour is observed. As we find these 2D nuclei also on high-oxygen pressure DC sputtered films grown at a much lower growth rate, we conclude that the supersaturation is not a decisive parameter for the predominance of either growth mode. Instead, we attribute the absence of growth spirals to the non-steady state growth conditions inherent to the pulsed nature of the laser-ablation process. Growth spirals only develop, if a non-vanishing diffusional flow of adatoms towards the step edge is maintained. The number of growth spirals observed on a films is therefore not necessarily a measure for the number of screw dislocations. After wet-etching the films in Br-ethanol, we observe that etch pits are formed consisting of concentric steps. We conclude that these pits are due to repetitive nucleation around linear defects. The etchpit density identified in this way is of the order of 1 per μm 2 .

High-

Among various discussed ways of explosive detection, the techniques using electromagnetic radiation are considered as having great potential and research activities are recommended in this field. To identify new threats, like liquid explosives, with low rate of false alarms, fast spectral measurements are required in a broad frequency range from microwave to terahertz. We attract attention to a great potential of high-Tc Josephson technology in security applications and present our results in developing high-Tc Josephson junctions for Hilbert spectroscopy and detector arrays.

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Hilbert spectroscopy is based on the frequency-selective detection of weak electromagnetic radiation by Josephson junctions described by the resistively shunted junction (RSJ) model. The YBa2Cu3O7-x grain-boundary Josephson junctions fabricated on twin-free NdGaO3 bicrystals were found to be close to the RSJ model. Laboratory prototypes of general-purpose Hilbert spectrometers were developed and characterized. The spectral bandwidth of Hilbert spectroscopy for any junction temperature between 30 and 85 K is as large as one frequency decade. The middle frequency of this bandwidth scaled with the characteristic frequency f c = (2e/h)I c R n of the Josephson junction and a total bandwidth of almost two orders, from 50 GHz to 4 THz, was covered by one Josephson junction at two temperatures. A spectral resolution δf/f of the order of 10-3 was realized in the terahertz range. The dynamic range of intensities of radiation which can be measured by Hilbert spectroscopy is close to five orders. With the help of Hilbert spectroscopy we measured the following emission spectra: Lorentz spectra of Josephson oscillations, spectra of high-harmonic content in commercial millimeter-wave oscillators, spectra of terahertz radiation from optically-pumped gas lasers and spectra of transition radiation from relativistic electron bunches at DESY (Hamburg).

Josephson Square-Law Detectors and Hilbert Spectroscopy for Security Applications

The detector response of YBa2Cu3O7−x Josephson grain-boundary junctions to monochromatic radiation with the frequency f in the range from 60 GHz to 4 THz has been studied. Frequency-selective odd-symmetric resonances in the responses ΔI(V) of these junctions to radiation with different frequencies f have been observed near the voltages V=hf/2e in almost a decade of spectral range for any operating temperature in the range from 30 to 85 K. The spectral range of the selective detection has scaled with the IcRn product of the Josephson junction, reaching the range of 0.16–3.1 THz for a IcRn product of 1.5 mV. A resolving power δf/f of around 10−3 has been demonstrated in the selective detection by Josephson junctions. The high-frequency falldown of the amplitude of the selective response has been found to be proportional to exp[−P/P0], where P=(hf/2e)2/Rn is the power dissipated in the junction at the resonance and P0 is a characteristic power level. The values of P0 for our junctions were around 20 μW at 34...

Terahertz Hilbert Spectroscopy by High-Tc Josephson Junctions

The authors investigated low-frequency noise in RF and DC superconducting quantum interference devices (SQUIDs) with step-edge Josephson junctions (SEJs). The noise properties have been measured as a function of frequency, temperature, bias current, and magnetic field. The low-frequency noise has a telegraph-like character in certain conditions and depends quasi-periodically on flux and temperature. This telegraph noise was also observed in single step-edge junctions. The experimental results were explained by a model of such a junction in which an array of parallel and series connected weak links acts like a two-level fluctuator. This fluctuator arises from the quantum states of a multijunction SQUID with spatially inhomogeneous SEJs resembling a multijunction array.<<ETX>>

Frequency-selective incoherent detection of terahertz radiation by high-Tc Josephson junctions

A demonstrator of a terahertz (THz) Hilbert-transform spectrum analyzer with subsecond scanning times and accuracy of the order of 10-3 has been developed. The spectrum analyzer is based on a high-Tc Josephson junction in a compact Stirling cryocooler. Operation of the spectrum analyzer has been critically considered in the THz range and sources of errors have been studied. The voltage and frequency scales of the analyzer have been calibrated with accuracy of 7 ·10-4 by Shapiro steps, induced on the I(V) curve of the junction by intensive monochromatic radiation. Dynamic errors of the analyzer have been minimized with scanning rates up to 4 THz/s. An instrumental function of the spectrum analyzer is of a Lorentz type with a spectral width of 1.5 GHz and free from any harmonic and subharmonic contributions with accuracy of around 10-3. Rapid and detailed characterization of the THz sources, based on frequency multiplication of microwave radiation, has been demonstrated with the developed spectrum analyzer. The developed spectrum analyzer will be effective in new demanding THz applications, where a combination of high speed and accuracy is required.

Noise properties of SQUIDs with step-edge Josephson junctions

We have investigated thin sputtered films of the highTc material YBa2Cu3O7 by means of Raman spectroscopy at different stages of the preparation process. We find that the films are amorphous after sputtering. The Raman spectra indicate that random polycrystalline layers, as well as crystalline layers with preferred orientation, are obtained by an additional thermal treatment.

THz Hilbert-Transform Spectrum Analyzer Based on High-

A scanning SQUID microscope based on high-temperature superconductor (HTS) dc-SQUIDs was developed. An extremely soft magnetic amorphous foil was used to guide the flux from room temperature samples to the liquid-nitrogen-cooled SQUID sensor and back. The flux guide passes through the pick-up loop of the HTS SQUID, providing an improved coupling of magnetic flux of the object to the SQUID. The device measures the z component (direction perpendicular to the sample surface) of the stray field of the sample, which is rastered with submicron precision in the x–y direction by a motorized computer-controlled scanning stage. A lateral resolution better than 10 µm, with a field resolution of about 0.6 nT Hz−1/2 was achieved for the determination of the position of the current carrying thin wires. The presence of the soft magnetic foil did not significantly increase the flux noise of the SQUID.

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Thin films of high‐temperature superconducting YBaCuO deposited by dc‐sputtering on SrTiO3 substrates are structured by electron beam lithography on a submicron scale. Details of the technology processes involved are presented. The sudden transition of structures below 1 μm into the semiconducting state is discussed.

Josephson Junction in Stirling Cryocooler

We have studied electrical transport and low-frequency noise properties of [100]-tilt bicrystal YBa2Cu3O7-x junctions, which show a lower degree of structural disorder and higher characteristic voltages IcRnmiddot in comparison with conventional [001]-tilt grain-boundary junctions. The oxygen content of the junction barrier was varied by an annealing in atomic oxygen and by aging in molecular oxygen. The modification of the I-V curves and low-frequency noise were monitored. The terahertz losses in the junctions were derived and were shown to decrease with the increase of the oxygen content. It was found that the resistance and critical current fluctuations are completely antiphase correlated and intensities of normalized resistance and critical current low-frequency fluctuations are equal in these junctions. Consequently, quasiparticles and Cooper pairs in the [100]-tilt junctions tunnel directly through the same parts of the barrier, and a band-bending model with the charge fluctuations at the barrier can applied to explain the modification of the [100]-tilt grain-boundary junctions after oxygen loading and aging.