W. Kreische

Improvement of K 2 -entropy calculations by means of dimension scaled distances

Abstract The K2-entropy is evaluated for experimental time series as well as for numerically simulated ones. The conventional calculation procedure leads to results that can be extrapolated to obtain a K2-value for the time series, but in the case of experimental systems the outcome is hardly significant for a classification of the states. The reason for this is a distortion of distances due to the phase space embedding procedure by time-delay coordinates. Therefore, a K2-calculation based on dimension scaled distances (DSD) is suggested. A comparison to the often suggested use of the maximum norm is discussed. By the application of DSD the convergence problems are avoided and clear results were obtained in all investigated situations.

Electric hyperfine interaction in solid CF4 at the fluorine site

Using the time differential perturbed angular distribution method (TDPAD), the quadrupole coupling constant for CF4 has been measured to be νQ = 59.7(3) MHz. This value is discussed in connection with measurements of the other carbon tetrahalides and can be well understood in the framework of the Townes and Dailey theory.

A simple explanation of unexpected efg's in19F-TDPAD measurements

In our19F — TDPAD experiments often efgs are appearing, which are due to bonds of the probe nucleus that do not exist in the target substance. A simple mechanism is proposed to explain these unexpected coupling constants in molecular crystals.

Temperature dependence of the electric field gradient in CdSe and HfO2

The time differential perturbed angular correlation technique was applied to study the electric field gradient in the semiconductor CdSe and the insulator HfO2 at different temperatures. Whereas the semiconductor CdSe shows an increasing quadrupole coupling constant with increasing temperature, the insulator HfO2 exhibits no temperature dependence of the electric field gradient over the whole temperature range investigated.

Charge carrier governed temperature dependence of the electric field gradient for111Cd in tellurium

Using the TDPAC-method with the proble nucleus111Cd the electric quadrupole interaction (QI) in the trigonal semiconductor Te has been investigated in the temperature range 77 K≦T≦655 K. In contrast to most metals the quadrupole frequencyvQ increases with increasing temperature. This is a consequence of the fact that in metals the charge carrier density is temperature independent, whereas in semiconductors it strongly increases with temperature. A comparison between the temperature variation of the quadrupole coupling constantvQ and that of the charge carrier density leads to the conclusion that the QI in Te is governed by changes in the free electron density.

The electric hyperfine interaction in solid fluorine

Abstract The quadrupole coupling constant of F 2 in the solid phase has been investigated at a temperature of 17.5 K by means of the time differential perturbed angular distribution technique. For the coupling constant a value of ν Q = 127.2(1.0) MHz results. The data are discussed in comparison with NQR-studies in the molecules I 2 , Br 2 and Cl 2 , and in terms of the Townes and Dailey theory. A small fraction of the 19 F nuclei was exposed to an electric field gradient of 85(5) MHz. Possible explanations are proposed.

Temperature dependence of the electric field gradient for111Cd in antimony

For the temperature dependence of the electric field gradient of111Cd in the semimetal antimony the well knownT3/2 relationship is valid over a wide temperature range. The slope parameterB differs from that for121, 123Sb in antimony by a factor of about 2.3.

QUADRUPOLE INTERACTION OF 111CD IN THE SEMICONDUCTOR CDS

The time differential perturbed angular correlation technique was applied to study the electric quadrupole coupling of 111Cd in the II‐VI semiconductor CdS (20 K≤T≤1000 K). Two kinds of sample preparation were used: implantation and diffusion. Obtained results for the regular lattice sites were independent of the preparation. The measured electric field gradient (EFG) was between 6.7±0.4 MHz and 9.1±0.2 MHz. Data were confirmed by point‐charge calculations. Implantation however caused a second EFG (≊78 MHz) that was assigned to a defect due to the preceding irradiation.

Nuclear quadrupole interaction in solid HF and trifluoroaminoboranes

Abstract Using the time differential perturbed angular distribution method (TDPAD) solid HF and trifluoroaminoboranes have been investigated. In the case of solid HF some new aspects appear in the interpretation of former experiments. The new measurements either reproduce the older data that yielded one frequency and a distinct ν, or show two frequencies and no distinct ν. This discrepancy can be explained with different target preparations. While the one-frequency experiments were done with untempered targets the two-frequency targets were tempered. Trifluoroaminoboranes (F 3 BNH 3 ) are known as charge-transfer complexes. Since ammonia acts preferentially as an electron donor and 19 F is the most electronegative element, a lowering of the observed coupling constant should appear. Experiment, however, does not support this; it gives an unchanged coupling constant, but the asymmetry parameter is changed by a factor of more than 2. This behaviour can be understood by calculating the electron distribution in both molecules within the framework of the Townes and Dailey theory. It turns out that charge transfer does take place, but the transferred charge does not lead to a more isotopicc electronic environment for fluorine, but rather reduces the asymmetric population of its p x and p ŷ orbitals.

Electric quadrupole interaction of111Cd in the system Sb1−xMx (M=Ag,Cd,In,Sn), Sb2Te3, Bi2Te3 and In2Te3

We have performed TDPAC-measurements to investigate the static quadrupole interaction of111Cd in the classic semimetal Sb. The coupling constant depends on the concentration of small amounts of metal admixtures. The only exception is the system Sb1−xAgx. The temperature dependence of the efg in the narrow gap semiconductors Sb2Te3 and Bi2Te3 is similar to that one in Te. In contrast to these results the efg in the III–VI-semiconductor In2Te3 is temperature independent.