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Chladni belonged to those few scientists who have been able to arouse enthusiasm for their work not only in professional circles, but also in the lay public. During several journeys all over Europe Chladni demonstrated the sound figures he had discovered as well as the musical instruments invented by him. In this respect the invitation pronounced by Napoleon during a longer stay in Paris to show his experiments at the imperial court was undoubtedly a highlight. Chladni has given a personal report on this event. In this article is shall be tried to sketch Chladni's character, based on his own writings but also on comments of his contemporaries.

Abstract.: Chladni belonged to those few scientists who have been able to arouse enthusiasm for their work not only in professional circles, but also in the lay public. During several journeys all over Europe Chladni demonstrated the sound figures he had discovered as well as the musical instruments invented by him. In this respect the invitation pronounced by Napoleon during a longer stay in Paris to show his experiments at the imperial court was undoubtedly a highlight. Chladni has given a personal report on this event. In this article is shall be tried to sketch Chladni's character, based on his own writings but also on comments of his contemporaries.

Experimental results on spectra and wave functions of a ray-splitting microwave billiard are presented. The billiard is formed by a flat rectangular microwave cavity with a quarter-circle insert made of teflon in one of the corners. Using the Gutzwiller trace formula, the contribution of the periodic orbits of the billiard to the density of states are determined. The wave functions, many of them showing scars associated with periodic orbits, are interpreted in terms of the semiclassical Green function.

Abstract:: A set-up is described allowing the automatic registration of wavefunctions of quasi-two-dimensional microwave billiards of arbitrary shape. Tests of the apparatus with rectangular shaped billiards showed that a precision of some percent in the wavefunction amplitudes can be obtained, as far as isolated resonances are considered. For the case of overlapping resonances, however, the measurement yields wavefunctions which are close to a symmetric and an antisymmetric linear combination of the original rectangle eigenfunctions. The cause for this at first sight surprising result is discussed.

We measure the angular momentum content of modes in a flat, near-circular microwave cavity with a rough perimeter and demonstrate localization in angular momentum space. Because Schrödinger's wave mechanics and Maxwell's electrodynamics are equivalent for a 2d cavity, we compare our experimental results directly with the quantum theory of rough 2d cavities [K.M. Frahm and D.L. Shepelyansky, Phys. Rev. Lett. 78 (1997) 1440]. Introducing the concept of effective roughnesswe find good qualitative agreement.

Abstract: Spin polarized radioactive 12B, 12N, and 8Li probe nuclei are implanted in nominally undoped ZnSe at stationary concentrations ≤ 1010 cm-3. These light dopants, one donor and two acceptors, are characterized by β-radiation detected nuclear magnetic resonance (β-NMR) measurements. The donor 12B and the acceptor 12N show quite similar and rather simple implantation behaviour, the situation is markedly different for the second acceptor 8Li, however. This contrast indicates that in semi-insulating ZnSe the structure of dilute impurities is controlled by individual, element-specific properties.

The spectra of Sinai microwave billiards and rectangular billiards with statistically distributed circular scatterers have been taken as a function of the position of one wall, and of one of the scatterers, respectively. Whereas in the first case the velocity distribution and correlations obey the universal behaviour predicted by Simons and Altshuler, in the second case a completely different behaviour is observed. This is due to the fact that a shift of one wall changes the wave function globally whereas the displacement of one scatterer only leads to a local perturbance.

ß-radiation detected nuclear magnetic resonance (ß-NMR) measurements of12B occupying sites with noncubic surroundings after implantation into Si have been extended from p-type to moderately doped n-type material. The quadrupole split signals observed in both materials indicate the existence of the same interstitial related boron defect but with lower thermal stability in n-type Si.

An introduction to the principles of the ß-NMR method is presented and its potential in the field of defect research in semiconductors is discussed. This is followed by a review of experimental applications to date and a description of several examples in some detail.

Nuclear magnetic resonance measurements were carried out on neutron activated 20F(T1/2=11s) nuclei in a single crystal of KZnF3. The quadrupolar splitted NMR spectrum, detected via the 20F β-radiation asymmetry, could be observed using a radio frequency modulation technique. The quadrupole coupling constant was determined to e2q Q/h= + (12.0 ± 1.5) MHz at room temperature. The sign of e2q Q was obtained from a simultaneous γ-ray anisotropy measurement on the succeeding 20Ne transition. Utilising a calculated field gradient of the fluorine atom, an fQ= 4.6% is determined. This value is compared with literature data of similar compounds.

Spin-polarized β-active 23Ne (T1/2= 37.2s) nuclei were created by capture of polarized cold neutrons in solid and liquid neon. The angular asymmetry of the β-radiation was used to monitor the nuclear polarization. From nuclear magnetic resonance curves the value for the dipole moment |μ(23Ne)| = 1.079(1) n.m. (uncorrected) was obtained which is more precise than the literature value. Comparing the measured spin-lattice relaxation rates with those of stable 21Ne the nuclear quadru­pole moment of 23N e was determined. It agrees with a shell model calculation. The feasibility of 23Ne-β-NMR for investigations in condensed rare gases in demonstrated.

In the glass Li2O·3B2O3the nuclear spin-lattice relaxation behaviour of 8Li (T1/2=0.84 s) and 12B (T1/2=20 ms) was studied by use of the method of β-radiation detected nuclear magnetic resonance/relaxation. The transient behaviour of the polarization of both nuclear species is characterized by and exp (−l/T1inh) law, which can be explained by inhomogeneous polarization averaging. The investigation of the temperature and magnetic-field dependence of the relaxation rate Ti inh−1showed a lithium motion induced increase of the relaxation rate above room temperature. The value obtained for the apparent activation energy indicates partly localized lithium diffusion.

Polarised ß emitting12B nuclei were recoil implanted into single crystalline vanadium atT=305 K. ß radiation detected nuclear magnetic resonance measurements showed that two implanation sites occur simultaneously which were identified as the substitutional site and an interstitial site with axial symmetry. Extensive cross-relaxation measurements were performed on12B in both lattice positions. By this means the interstitial site could be identified as the octahedral one. The impurity induced electric field gradients at nearest and at next nearest51V neighbours about12B could be determined for both implantation sites.

Multiple quantum nuclear magnetic resonance (NMR) transitions were observed on polarized8Li nuclei, which were produced by capture of polarized neutrons in a single crystal of LiTaO3. The asymmetric8Liß-radiation distribution was used for the detection of NMR. A quadrupole moment ratio ¦Q(8Li)/Q(7Li)¦=0.78(1) was determined. Saturation of multiple quantum transitions in nuclear radiation detected NMR may lead to a reduction in measuring time of up to two orders of magnitude, as compared to single quantum detection methods. The measured spectra agree well with an exact lineshape calculation. The same measurements were also performed on a LiTaO3 powder sample. This was done to test a method to obtain quadrupole coupling constants from high field NMR multiple quantum powder patterns, which are easily detectable, also for higher nuclear spins. This latter method may be applicable also to conventional NMR detection techniques.

Polarized20F nuclei (T1/2=11s,I=2) have been produced in a tetragonal MgF2 single crystal by capture of polarized neutrons. Nuclear magnetic resonance transitions have been induced and observed via the asymmetric? decay to20Ne, yielding the quadrupole coupling constant ¦e2q Q/h¦=5.77(2)MHz, the quadrupole moment ¦Q(20F)¦=0.064(12) b and the asymmetry parameter of the electric field gradient?=0.317(2). In order to study possible lattice defects produced by the recoil displacement due to the capture?-rays NMR signals were registered both at room temperature and at 12 K. Width and depth of the resonance curves indicated that at room temperature nearly all20F ions occupy normal lattice sites with an undisturbed environment. At 12 K, however, where no annealing occurs, this is only true for about half of the20F ions which are stopped directly at defect free sites. The other half of the20F ions at 12 K shows resonance frequencies shifted by defect interactions up to the order of some 100 kHz. Furthermore two simple methods are reported which allow a quick estimate of the strength of the quadrupolar interaction: (1) observation of the? decay asymmetry versus the magnetic field strength (decoupling curve), (2) broad-band modulation of the inducedrf field and observation of the? asymmetry versus the depth of the modulation.

The spin-lattice relaxation of12B in single-crystal aluminum was measured by ß NMR in the temperature range 290 to 700 K. The total relaxation rate was separated into two well understood contributions caused by the interaction with conduction electrons and by12B diffusion, and into a third unexplained part at high temperatures. From the comparison of the diffusional relaxation rate with theoretical calculations it was concluded that the nearest neighbours of12B, trapped in the octahedral interstitial site, are displaced away by 8.8 (30)%.

The dipole-dipole interaction of extremely diluted probe nuclei with their surrounding host nuclei gives rise to mutual flip-flop processes of the nuclear spins. As energy has to be conserved in this so-called cross-relaxation (CR) process a transfer of polarization from the probes to the host nuclei occurs only at certain values of the inductionB leading to resonance-like dips in the probe's polarization. For the system12B in single-crystal aluminium CR spectra were measured forB=7...400 mT andT=310...410 K at various crystal orientations relative toB. All the spectra could be well explained by a theoretical calculation assuming the octahedral interstitial site for12B.

Emission spectra of Ag atoms isolated in Ar, Kr, and Xe matrices have been studied during X-ray irradiation. The spectra consist of broad lines being red shifted by about 1 eV relative to the absorption lines. The spectra are similar to luminescence spectra observed after optical excitation of the inner-shell states (4d95s5p) of Ag but differ from the emission spectra observed after resonance excitation of the state (4d105p). During isothermal annealing of the sample after the end of the X-ray irradiation thermoluminescence is observed on all Ag emission lines. X-ray irradiation leads primarily to the production of photoelectrons which upon thermalization generate several hundred excitons. It is concluded that the excitons are partly trapped at the Ag impurities and give rise to the observed emission spectra. Another part of the excitons leads to the production of trapped charge carriers which are released upon annealing. The temperature dependence of the thermoluminescence shows that different trapping configurations with a broad distribution of activation enthalpies are present.

Absorption and magnetic circular dichroism line shapes have been calculated for a2S-2P transition of an impurity center in a cubic environment. Linear Jahn-Teller coupling to all symmetry allowed lattice vibration modes as well as spin-orbit interaction have been considered. In the calculations a modified semiclassical Franck-Condon approximation was applied. Whereas in the ordinary Franck-Condon approximation line moments are calculated correctly up to the second one, the modified version yields the third moment correctly, too. At low temperatures the modification leads to lineshapes with characteristic wings on the high frequency side as observed in several experiments on impurity centers in solids.

Polarized116In (I=1,T1/2=14.1 s) nuclei were produced by capture of polarized thermal neutrons in undoped InP and InSb and in InSb∶Te single crystals. As a consequence of the nuclear reaction charged and paramagnetic point defects were produced with a rate of about 0.8 per absorbed neutron. Spin-lattice relaxation rates of the116In probe nuclei were measured at temperatures between 1.3 K and 90 K, as well as NMR line profiles and magnetic field dependences of the nuclear polarization up to 0.63 T. Electric field gradients, produced by the defects, could be determined, and defect induced additional relaxation processes were observed. Further, characteristic phonon frequencies and spin-phonon coupling constants for the undisturbed crystal lattices could be derived.

β-active 110Ag nuclei have been produced in AgF, AgCl, AgBr by capture of polarized thermal neutrons in the reaction 109Ag(npol,y) 110Agpol. The capture process produces in the vicinity of the 110Ag nuclei many point defects which have been studied by β-NMR on 110Ag. A detailed analysis of the NMR line shape showed that 30-40% of the 110Ag nuclei see an undisturbed lattice whereas 60-70% see a lot of Ag+ vacancies and interstitials. In the case of AgCl and AgBr defect annealing has been observed in the temperature range 10-25 K.

Electron spin resonance (ESR) and optical absorption spectra of K atoms embedded in Ar matrices have been measured simultaneously. ESR and optical spectra could be correlated comparing the effects of annealing and light induced site modifications. Electron spin relaxation times were estimated by saturation measurements. A large variety of ESR spectra was found which could be arranged into four groups according to their A and g factor matrix shifts. Two of these groups could be correlated with optical absorptions namely with the so-called red and blue triplet bands. The other two groups belong to K-(H2O)n complexes. Part of the absorption disappeared irreversibly upon annealing at about 12 K. There is considerable experimental evidence that this annealing process indicates the transition from an amorphous to a microcrystalline structure at about 12 K.

Neutron activated20F nuclei (T1/2=11 s) in CaF2 serve for two purposes: first they produce well defined point defects by their (n, γ)-production mechanism, second, they act as probes to detect the properties of these defects in aβ- andγ-ray detected nuclear magnetic resonance experiment. Below about 70 K an interstitial defect is observed due to a quadrupolar splitting of the20F-NMR line. The defect induced electric field gradient is oriented along the 〈111〉-axis of the crystal and causes a quadrupole coupling constante2qQ/h=−2.0(1) MHz at the20F site. The defect anneals above 70 K with an Arrhenius like temperature dependence corresponding to an activation energyEM=0.17(4) eV.

Cross-processes of neutron activated, polarized nucleiI with surrounding stable host nucleiS are observed in diverse solids, using an in-beam NMR technique where the polarizedI nuclei are produced by capture of polarized thermal neutrons. The polarization is determined by measuring the 0°–180° asymmetry of theß-decay radiation of these nuclei. The following cross-processes are studied:8Li-6Li innatLi metal;8Li-7Li in7Li metal;8Li-6Li innatLiF (single crystal);8Li-93Nb in7LiNbO3 (single crystal);20F-19F in MgF2 (single crystal). Two applications are presented: a) Determination of the temperature dependence of the electrical quadrupole coupling constant of93Nb in LiNbO3; b) Observation of radiation induced point defects in MgF2 and their thermal annealing. All measurements are performed without any radiofrequency irradiation.

An experimental method is presented which allows the determination of the sign of nuclear electric quadrupole interactions in solids. Activated target nuclei with a purely dipolar spin polarization are produced by capture of polarized thermal neutrons. The quadrupole coupling of the nuclei to the electric field gradient tensor in the target crystal converts this (dipolar) polarization partly into a (quadrupolar) alignment, which can be measured by the anisotropy of theγ-ray emission in a succeeding nuclear transition. The sign of the alignment created in this reorientation process depends on the sign of the electric quadrupole interaction. The reorientation effect can be enhanced by selective induction of nuclear magnetic resonance transitions. The method has been applied to measure the sign of the crystal electric field gradient (efg) in tetragonal MgF2. Further, the sign of an efg in cubic CaF2 originating from a19F interstitial adjacent to the activated20F probe nucleus has been determined. The method is in principle applicable to a considerable number of nuclides.

Using the equivalence between the quantum-mechanical probability density in a quantum billiard and the Poynting vector in the corresponding microwave system, current distributions were studied in a quantum dot like cavity, as well as in a Robnik billiard with λ=0.4, and an introduced ferrite cylinder. Spatial auto correlation functions for currents and vorticity were studied and compared with predictions from the random-superposition-of-plane-waves hypothesis. In addition different types of vortex neighbour spacing distributions were determined and compared with theory.