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The role of coherence played in several phenomena in the gamma energy region has not been well recognised until recently. The Mossbauer source nuclei are noncorrelated, but the absorber response has a well-defined phase relation to the instantaneous source radiation within the relaxation time of the nuclear system. By rapidly changing this phase relation with mechanical displacement, new types of coherent transients in gamma regime have been observed. The gamma echo, replicating and amplifying the enchained decay through a thick sample, is perhaps the best known example of these phenomena. In addition, a simple gamma-ray interferometer based on rapid transit through the resonance has been demonstrated. Fast and intense gamma pulses have been obtained with stepwise phase modulation. These and the gamma-NMR double resonance experiments are reviewed in this paper.

Splitting of Mossbauer lines has been observed in conventional energy domain spectra of Permalloy samples when an external radio frequency (rf) magnetic field was tuned to be in resonance with the separation of Zeeman sublevels of either the ground or the excited state of the57Fe nucleus. Time domain measurements were performed with the frequency of the rf field close to the natural linewidth of the 14.4 keV Mossbauer state. A nonmagnetostrictive magnetically soft Permalloy sample was used in the experiments.

The rare-earth site of samples having the compositions (Fe,Cu)Sr2(Eu,Ce) n Cu2O4+2n+z (n=2 and 3) were studied by151Eu Mossbauer spectroscopy. The results obtained are compared with measurements of EuBa2Cu3O7−δ (1∶2∶3) and Bi2Sr2(Ca1−yEuy)Cu2O8+E (2∶2∶1∶2) samples. The full quadrupole Hamiltonian of the 21.5-keV γ-transition was applied in analyzing the measured spectra. Simultaneous fitting of spectra recorded from crystallites oriented in various directions was used to determine the hyperfine parameters of then= 2 and 3 samples. Two different electric field gradients, not discernible in simple random crystallite measurements, were obtained for then=3 series.

By applying stepwise phase modulation of recoilless gamma radiation in a coincidence experiment, constructive interference is produced in transmission geometry between the source and the absorber fields. The resulting regenerated decay signal is called a gamma echo. Here it is demonstrated that during the decay of the 14.4 keV state of57Fe multiple echo signals can be generated.

The enhancement of recoilless fraction (f factor) in restricted geometries is studied. The experiments were carried out using the119Sn resonance in SnCl4 molecules trapped in porous glass. A theoretical approach to explain the increase of thef factor is presented.

The influence of sintering temperatures and nominal starting compositions on the phase contents of polycrystalline Bi2Sr2Ca n-1Cu n O4+2n samples are reported. After every sintering the samples were characterized with X-ray diffraction and magnetic AC susceptibility measurements. For Mossbauer studies, in each case also a corresponding specimen containing57Fe was synthetized. The identification of various superconducting Bi2Sr2Ca n-1Cu n O4+2n phases on the basis of Mossbauer spectra consisting usually of three quadrupole doublets is discussed.

Both57Fe and151Eu Mossbauer resonances were used to study polycrystallineRBa2Cu3O7-δ samples. The dependence of hyperfine interaction parameters on the oxygen deficiency δ is studied in detail with151Eu. For57Fe spectra, a site assignment is proposed.

The influence of high-frequency magnetic modulation on Mossbauer resonance is investigated using the67Zn resonance. A phase-modulation theory theory explaining the experimental results is presented. Implications to magnetic modulation experiments with the57Fe resonance are also discussed.

Temporal aspects related to the Mossbauer resonance are reviewed, in particular time-dependent transient effects produced by high-frequency modulation of the recoilless gamma radiation. These phenomena are in many respects similar to those observed in NMR and laser spectroscopy. Transient effects can be described within a classical field model in terms of phase modulation of recoilless gamma radiation. Experimentally, use of the very narrow Mossbauer resonance of67Zn gives access to undisturbed transient phenomena without excessive difficulties due to nonideal modulation. Application of transient experiments facilitates determination of several physical parameters, of which some arc tedious to evaluate by other methods.

The use of an alternating magnetic field for phase modulation of recoilless gamma radiation is studied. The measurements were performed with the Mossbauer resonances of Fe-57 and Zn-67. Novel time-domain data are presented from experiments on the nuclear Zeeman effect at high modulation frequencies.