Ch. Renner

A vertical piezoelectric inertial slider

We have developed a linear translation device using piezoelectric‐induced slip‐stick motion. Reproducible single steps of about 30 A, as well as continuous stepping with an overall translation speed of 0.25 mm/s, are routinely realized. The notable feature of this device is that this performance is achieved in the vertical orientation with the translator moving against gravity. This remarkable result is made possible using cycloidal functions instead of sawtooth signals to activate the motion. We have realized a very simple translator which can be used in any orientation with a displacement onset voltage of 15 V. The instrument was successfully tested in the temperature range from 1.6 to 300 K. Since no mechanical connections are required, this design is well suited for many applications, including scanning tunneling microscopy.

On the statistics of wind gusts

Wind measurementsnear the North Seaborder of Northern Germany andvelocity measurements under localisotropic conditions of a turbulent wakebehind a cylinder are compared. It is shownthat wind gusts, measured by means ofvelocity increments, do show similar statisticsto the laboratory data if they are conditionedon an averaged wind speed value.Clear differences between the laboratory dataand the atmospheric measurements arefound for the waiting time statistics betweensuccessive gusts above acertain threshold.

Comment on “Indispensable Finite Time Corrections for Fokker-Planck Equations from Time Series Data”

A Comment on the Letter by Mario Ragwitz and Holger Kantz, Phys. Rev. Lett. 87, 254501 (2001). The authors of the Letter offer a Reply.

Universality of small scale turbulence.

The proposed universality of small scale turbulence is investigated for a set of measurements in a cryogenic free jet with a variation of the Reynolds number (Re) from 8500 to 10(6) (max(R(lambda) approximately 1200). The traditional analysis of the statistics of velocity increments by means of structure functions or probability density functions is replaced by a new method which is based on the theory of Markov processes. It gives access to a more complete characterization by means of joint probabilities of finding velocity increments at several scales. Based on this more comprehensive method, our results are very far from a possible universal state, even for R(lambda) above 1000.

A versatile low‐temperature scanning tunneling microscope

We present a new scanning tunneling microscope (STM) developed specifically for use at low temperatures within a standard helium cryostat. This compact STM (diameter 25 mm and length 61 mm) has been designed to be compatible with a superconducting magnet. It incorporates a novel approach system which operates in any orientation including the vertical one, without any mechanical connections to the STM. It is able to translate a sample by 30 A steps in any orientation. The coarse and the fine approach are driven by a single piezoelectric tube via inertial slip‐stick motion of the sample holder. During the coarse approach, the translator can be moved over 15 mm at 0.25 mm/s against gravity. We have operated this STM down to 1.6 K and we have obtained images with atomic resolution on highly oriented pyrolitic graphite (HOPG) and NbSe2 from 300 K down to 90 K. This partially thermal compensated instrument is able to monitor temperature dependent surface changes on atomic scale in real time.

A 3He refrigerated scanning tunneling microscope in high magnetic fields and ultrahigh vacuum

We present a scanning tunneling microscope (STM) designed to operate between 275 mK and room temperature, in magnetic fields up to 14 T and in ultrahigh vacuum (UHV). The system features a compact STM connected to an UHV compatible 3He refrigerator fitting into a bottom loading cryostat with a superconducting magnet. In this configuration, the cryostat is sitting on top of the UHV chamber, resulting in a very short distance between the STM access and the experimental position. It further enables proper thermal anchoring of the entire STM setup, allowing millikelvin temperatures to be reached in true UHV conditions. We achieve a hold time of about 40 h at 275 mK and a turnaround time of 10 h between room and base temperature. We demonstrate atomic resolution and present tunneling spectra obtained at 275 mK on the high-Tc superconductors Bi2Sr2CaCu2O8+δ and YBa2Cu3O7−δ.

Gap distribution of the tunneling spectra in Bi2Sr2CaCu2Ox and some other superconductors

Abstract We discuss the conductance data of superconducting single crystal Bi2Sr2CaCu2Ox obtained with a scanning tunneling microscope in the vacuum tunneling mode. For this purpose, we introduce and apply a method involving filtering techniques to study the structure of the order parameter. We find that the order parameter distribution has a rich structure spread on a large energy scale in addition to a strong peak around 30 meV.

Oxygen doping and temperature dependence of the tunneling spectroscopy on Bi2Sr2CaCu2O8+δ

We present local probe tunneling spectroscopy of Bi2Sr2CaCu2O8+δ single crystals for different oxygen concentrations, from optimally doped (Tc=92.2 K) to highly overdoped (Tc=56.0 K) phases. With increasing oxygen overdoping, the superconducting gap (Δp) is reduced and the dip structure beyond Δp at negative sample bias1 shifts toward the Fermi energy. Apart from the shift in energy of these features, the generic shape of the tunneling spectra remains unchanged. The gap roughly scales with Tc, and 2Δp/kBTc stays large even in the highly overdoped phase. We also present preliminary results on the temperature dependence of the tunneling spectra. They are consistent with a gap that is largely independent on temperature up to the vicinity of Tc

Study of field‐emitting microstructures using a scanning tunneling microscope

Field emission from broad metal cathodes is known to be strongly enhanced at a small number of emitting sites per cm2 compared to the expected Fowler–Nordheim emission from ideal, flat surfaces. We have operated a scanning tunneling microscope (STM) in the field emission regime (typical tip voltage: +80 V) and measured the local field emission strengths and variations on niobium samples. With a modulation technique, which is an adaptation of the standard work function measurement, maps of the field enhancement factor β have been obtained. An example of an emission site is presented where STM topograph and β map are compared with a secondary electron microscope image and with field emission data obtained in a standard way using high‐voltage anodes. This demonstrates the capability of a scanning tunneling microscope to localize enhanced field emission sites (with typical β values of 50 in the present work) with high spatial resolution and to study surfaces down to the limit β=1.

Markov properties of high frequency exchange rate data

We present a stochastic analysis of a data set consisiting of 10^6 quotes of the US Doller - German Mark exchange rate. Evidence is given that the price changes x(tau) upon different delay times tau can be described as a Markov process evolving in tau. Thus, the tau-dependence of the probability density function (pdf) p(x) on the delay time tau can be described by a Fokker-Planck equation, a gerneralized diffusion equation for p(x,tau). This equation is completely determined by two coefficients D_{1}(x,tau) and D_{2}(x,tau) (drift- and diffusion coefficient, respectively). We demonstrate how these coefficients can be estimated directly from the data without using any assumptions or models for the underlying stochastic process. Furthermore, it is shown that the solutions of the resulting Fokker-Planck equation describe the empirical pdfs correctly, including the pronounced tails.