A. Kittel

Near-field heat transfer in a scanning thermal microscope

We present measurements of the near-field heat transfer between the tip of a thermal profiler and planar material surfaces under ultrahigh vacuum conditions. For tip-sample distances below 10(-8) m, our results differ markedly from the prediction of fluctuating electrodynamics. We argue that these differences are due to the existence of a material-dependent small length scale below which the macroscopic description of the dielectric properties fails, and discuss a heuristic model which yields fair agreement with the available data. These results are of importance for the quantitative interpretation of signals obtained by scanning thermal microscopes capable of detecting local temperature variations on surfaces.

Delayed feedback control of chaos by self-adapted delay time☆

Abstract We present a novel technique for stabilizing unstable periodic orbits of chaotic systems. It uses a continuous feedback loop in the form of the difference between an actual and a delayed output signal of the system with a variable delay time. This time is chosen to be equal to the interval between the last and the k th previous maximum of the output signal and is changed at every k th maximum. During the procedure, the delay time asymptotically tends to the period of the period- k unstable periodic orbits and the control signal vanishes. The method is illustrated with the help of the Rossler ordinary differential equations and the Mackey-Glass delay differential equations.

Persistent photoconductivity in highly porous ZnO films

ZnO and ZnO-dye hybrid films prepared by electrochemical deposition are highly porous if fabricated in the presence of structure directing agents and they can easily be sensitized by various molecules. If the material is sensitized with the appropriate molecules, it becomes interesting for various sensor applications, i.e., gas sensors and biosensors, or as an electrode material for solar energy conversion in dye sensitized solar cells. In the present work, the focus is on dye sensitized ZnO as a model system. The long term photoconductivity transients have been investigated in such kind of material. Upon excitation with different wavelengths, the conductivity increases already under sub-band-gap illumination due to widely distributed trap states within the band gap. The slow photoconductivity transients follow a stretched exponential law if the illumination is rapidly changing in a dry atmosphere. The underlying mechanism of persistent photoconductivity can be attributed to a lattice relaxation process of surface states, immediately after electrons have been photoexcited into distributed surface states located inside the band gap of the ZnO thin film.

On a quantitative method to analyze dynamical and measurement noise

This letter reports on a new method of analysing experimentally gained time series with respect to different types of noise involved, namely, we show that it is possible to differentiate between dynamical and measurement noise. This method does not depend on previous knowledge of model equations. For the complicated case of a chaotic dynamics spoiled at the same time by dynamical and measurement noise, we even show how to extract from data the magnitude of both types of noise. As a further result, we present a new criterion to verify the correct embedding for chaotic dynamics with dynamical noise.

Generalized synchronization of chaos in electronic circuit experiments

Abstract Two examples of one-way coupled electronic circuits displaying generalized synchronization of chaos are considered. In one of them, the dynamics of both the response and the driving systems represent a double-scroll chaos oscillator. In another example, the double-scroll oscillator is driven by an electronic analog of the Mackey-Glass system. To detect the generalized synchronization, an auxiliary response system that is a replica of the original one is used. In these systems, we have discovered two types of generalized synchronization, namely, a strong and a weak synchronization, which correspond to the existence of a smooth and a nonsmooth map from the trajectories of the driving attractor to the trajectories of the response system, respectively.

Magnetism in SQUIDs at Millikelvin Temperatures

We have characterized the temperature dependence of the flux threading dc SQUIDs cooled to millikelvin temperatures. The flux increases as 1/T as temperature is lowered; moreover, the flux change is proportional to the density of trapped vortices. The data are compatible with the thermal polarization of surface spins in the trapped fields of the vortices. In the absence of trapped flux, we observe evidence of spin-glass freezing at low temperature. These results suggest an explanation for the universal 1/f flux noise in SQUIDs and superconducting qubits.

Tool to recover scalar time-delay systems from experimental time series.

We propose a method that is able to analyze chaotic time series, gained from exp erimental data. The method allows to identify scalar time-delay systems. If the dynamics of the system under investigation is governed by a scalar time-delay differential equation of the form

Near-field thermal imaging of nanostructured surfaces

dy(t)/dt = h(y(t),y(t-\tau_0))

HEAT TRANSFER IN ULTRAHIGH VACUUM SCANNING THERMAL MICROSCOPY

, the delay time

The near-field scanning thermal microscope.

\tau_0