R. P. Huebener

Application of low‐temperature scanning electron microscopy to superconductors

Scanning electron microscopy applied to specimens in close thermal contact with a liquid‐helium bath can be used for a two‐dimensional display of various sample responses arising from the localized excitation of the sample by the electron beam. As shown in recent experiments, the method becomes particularly interesting when applied to superconductors. A general response theory is outlined, and a detailed treatment is given for the special case of the thermal response associated with a strongly temperature‐dependent electrical resistance of the sample.

New family of superconducting copper oxides: Bi cuprates of type 1212

Abstract Bi cuprates of type 1212 have been synthesized above 970°C from the proper starting materials. The oxides of idealized composition (Bi 0.5 Cu 0.5 )Sr 2 YCu 2 O 7− z crystalline in the tetragonal space group P4/mmm ( a=3.815(5), c=11.73(1) A ) and show solid solution behaviour. The as-prepared materials are non-superconducting above 5 K. However, post-treatment in flowing oxygen at moderate temperature (≈500°C) introduces superconductivity. The highest superconducting transition temperature is situated at 68 K for a sample of nominal composition (Bi 0.5 Cu 0.5 )Sr 2 Y 0.8 Cu 2.2 O 6.95 .

Spatially resolved observation of current filament dynamics in semiconductors

Abstract Two-dimensional imaging of the nucleation and the dynamics of current filaments generated in homogeneous p -doped germanium at 4.2 K during impurity impact ionization induced avalanche breakdown has been performed. The images were obtained by scanning the specimen surface with an electron beam and by recording the beam-induced current change in the voltage-biased samples. This new method is expected to identify in particular the filament configurations showing chaotic temporal resistance behavior.

Spontaneous oscillations and chaos in p-germanium

Abstract We have observed spontaneous oscillations and the transition to chaos in the post-breakdown regime of p-Ge at temperatures between 1.7 and 4.2 K. At least three different routes to chaos were found.

Nonlinearity in the flux-flow behavior of thin-film superconductors

The nonlinear flux-flow behavior due to the nonequilibrium distribution of quasiparticles predicted by Larkin and Ovchinnikov has been observed in thin films of Al, In, and Sn. From the voltage-current characteristics the inelastic scattering time τE of the electrons was obtained. For Al and In the values of τE are in excellent agreement with the previous results. For Sn the value τE=5×10−10 sec is slightly larger than the previous results.

Imaging of self-generated multifilamentary current patterns in GaAs

Two-dimensional imaging of current filament patterns generated in homogeneousn-type GaAs during avalanche breakdown at low temperatures is reported. The self-generated formation and subsequent growth behavior of distinct single- and multifilament configurations could globally be visualized by means of a scanning electron microscope equipped with a liquid-helium stage. From local conductivity measurements in the smallest possible filaments (typical diameter of about 10 μm) carrier mobilities as high as about 4·106 cm2/Vs at 4.2 K were estimated. Such high-mobility filament channels may become interesting for applications in ultrafast electronic circuits.

A p-Ge semiconductor experiment showing chaos and hyperchaos

Abstract A p-Ge semiconductor experiment is investigated by the help of both probabilistic and dynamical characterization methods. Dimensions, Lyapunov exponents, and the corresponding scaling functions are calculated. Two exemplary files of data from the p-Ge semiconductor experiment exhibiting spontaneous (i.e., undriven) resistance oscillations in the low-temperature avalanche breakdown are shown to be chaotic and hyperchaotic, respectively. For the first file, we obtained a fractal dimension between two and three and one positive Lyapunov exponent, whereas for the second file we found a fractal dimension between three and four and two positive Lyapunov exponents. Adopting the terminology introduced by Rossler, the behavior corresponding to the latter file is called hyperchaotic. Furthermore, using the language of the thermodynamical formalism, the probabilistic scaling function evaluated for the hyperchaotic state indicates a phase-transition-like-behavior.

Dissipative flux motion in epitaxial YBa2Cu3O7−δ and Bi2Sr2CaCu2O8+x films

Abstract We have performed a comparative study of the dissipative flux motion in the anisotropic and the very anisotropic high temperature superconductors YBa 2 Cu 3 O 7−δ and Bi 2 Sr 2 CaCu 2 O 8+x , respectively, by AC susceptibility measurements in an applied DC magnetic field between 0.01 and 7 T. The limit of small current densities (10 −8 J cm 2 ) was invesvestigated by using small amplitudes of the driving AC field. For the very anisotropic Bi 2 Sr 2 CaCu 2 O 8+ x superconductor the thermally activated motion of two-dimensional vortex lattice defects, which we refer to as plastic flux creep, is found to prevail over the motion of elastically correlated flux bundles, or elastic creep. The magnetic field dependence of the activation barrier and the irreversibility lines are found in good agreement with plastic flux creep theory. For the less anisotropic superconductor YBa 2 Cu 3 O 7−δ the flux motion is dominated by elastic flux creep. For this material the detailed measurement of the frequency and amplitude dependence of the AC susceptibility transition gives clear evidence for a finite temperature phase transition to a vortex glass state. The relevant critical exponents and the magnetic field dependent glass temperature are derived. Our results demonstrate the influence of the anisotropy of the superconducting material on the dissipative flux motion.

Resonance imaging of dynamical filamentary current structures in a semiconductor

Abstract The spatially resolved observation of the nonlinear dynamical behavior of spontaneous current oscillations obtained during low-temperature avalanche breakdown of homogeneously p-doped germanium is reported. Stationary current filaments developing in the breakdown regime through impurity impact ionization were observed two-dimensionally by means of a scanning electron microscope equipped with a liquid-helium cryostage. Further, spontaneous current oscillations showing typical transitions to chaos were localized two-dimensionally by means of a novel resonance imaging technique, which provides spatially resolved analysis of the nonlinear dynamical behavior. From these measurements different oscillation frequencies were clearly identified as spatially separated oscillation centers localized along the stationary current filaments. The electron beam was demonstrated to act as an exemplary control parameter, which can be manipulated both spatially and temporally.

Analysis of the critical current density in grain boundary Josephson junctions on a nanometer scale

The critical current Ic of YBa2Cu3O7−δ bicrystal grain boundary Josephson junctions (GBJs) have been measured in high magnetic fields up to B=12 T. Even at the highest applied magnetic fields, Ic varied periodically with varying magnetic field. By comparing the envelope of the measured Ic(B) dependencies to model calculations we obtain information on the spatial distribution of the supercurrent density jc on a length scale down to below 1 nm. Our analysis gives clear evidence that jc has spatial inhomogeneities on all length scales down to 1 nm with a probability distribution p(a) for the characteristic length scale a of the inhomogeneities.