S. Z. Shmurak

Effect of a magnetic field on the plasticity and photo-and electroluminescence of ZnS single crystals

It is found that a ∼10 T magnetic field influences the microhardness and the photo-and electroluminescence of ZnS single crystals containing microtwins. It is established that a magnetic field irreversibly changes the properties of the crystal, leading to relaxation of the metastable states of structural defects.

Photoconductivity spectrum of C60 single crystals placed in a magnetic field

To explain mechanisms of generation of free charge carriers in C60 single crystals under the action of light or a penetrating radiation, we need to analyze both intramolecular and intermolecular electronic processes [1]. Theoretical calculations of the electronic structure [2, 3] were recently compared with experimentally observed optical transitions in molecular and solid (films and microcrystals) states of C60 [1, 4, 5]. This comparison testifies to the fact that the existence of a crystal field changes the energy of intermolecular excited states and forms a complicated electronic fine structure. In addition to the internal mechanisms, an external constant magnetic field can also noticeably affect the process of the photogeneration of charge carriers in C60 single crystals [6, 7]. The main goal of the present study is to clarify the role of intermolecular electronic processes in the photogeneration of charge carriers in fullerite single crystals and the possibility to affect these processes by a weak magnetic field (B < 1 T).

Effect of a magnetic field on the electroluminescence intensity of single-crystal ZnS

After exposure of single-crystal ZnS with microscopic twinning to magnetic fields of 1–10 T, the integral electroluminescence brightness is found to increase by several times. It is proposed that the magnetic field facilitates relaxation of a metastable state of the structural defects.

Magnetoplastic Effect: From Spin Dynamics to Dislocation Mobility

Microscopic elementary events responsible for magnetoplastic effect are studied in detail by means of photoluminescence, electron spin resonance and SQUID magnetometry in NaCl:Eu single crystals. The Eu2+ clusters being dislocation obstacles were used as a spin and luminescent labels allowed detecting simultaneous spin and atomic structure transitions in exchange-coupled few-atomic Eu2+ clusters under static magnetic field B = 5 T. Rearrangement of atomic structure of these clusters changes the lattice distortions around them and effectiveness of clusters interaction with moving dislocations during plastic flow. From the comparison of spectroscopic data and numerical calculations of aggregation pathways of small clusters it was concluded that magnetosensitive clusters contain two Eu2+ ions with parallel spins (dimers). Two different ways of creating of magnetosensitive dimers in the crystal lattice are found: (1) slow diffusion limited aggregation in freshly quenched crystals, and (2) fast aggregation stimulated by dislocations dynamical distortions of lattice provided by the plastic deformation of aged crystals.

Dislocation spectroscopy of crystals

A new method of studying the energy characteristics of dislocations is proposed, which is based on the investigation of the interaction of moving dislocations with purposefully introduced electronic and hole centers. A study has been made of KCl, NaCl, KBr, LiF, and KI alkali halide crystals containing electronic F and hole VK and Me++ (Cu++, Ag++, Tl++, In++) centers. Investigation of the temperature dependence of the dislocation interaction with the F centers permitted determination of the position of the dislocation-induced electronic band (DEB) in the band diagram of the crystal. In KCl, the DEB is separated by ≈2.2 eV from the conduction-band minimum. It is shown that dislocations transport holes from the centers lying below the dislocation-induced hole band (DHB) (X+, In++, Tl++, VK) to those above the DHB (the Cu+ and Ag+ centers). Such a process is temperature independent. The DHB position in the crystal band diagram has been determined; in KCl it is separated by ≈1.6 eV from the valence-band top. The effective radii of the dislocation interaction with the electronic F and hole X+, VK, and Tl++ centers have been found.

Magnetic field effect on the photoluminescence of an Eu impurity during its aggregation in NaCl crystals

It is found that a magnetic field up to 20 T affects photoluminescence absorption spectra in NaCl:Eu crystals in the process of impurity aggregation into complexes. The spectral changes are irreversible and are observed after quenching the crystals at the early stages of the formation of small metastable complexes containing impurity-vacancy Eu2+-V dipoles. Emporal correlations revealed between the occurrence of the magnetoplastic effect and a change in the optical properties of the crystals in a magnetic field.

Sign reversal of the magnetoplastic effect in C60 single crystals during the sc-fcc phase transition

It is found that the magnetoplastic effect in C60 single crystals in a pulsed magnetic field with induction larger than 10 T changes its sign in the vicinity of the phase transition at Tc=250–260 K: crystal strengthening is observed for TTc. This indicates a change in the crystal lattice structure in the magnetic field.

Effect of a pulsed magnetic field on the microhardness of C60 single crystals

Magnetic-field pulses with induction greater than 10 T were observed to influence the microhardness of C60 single crystals. It was established that the magnetic field reversibly alters the bulk properties of the material.

Effect of a constant magnetic field on the photoconductivity of single-crystal C60

Magnetic fields with induction B<1 T are found to affect the photocurrent in single-crystal C60. This effect does not reduce to the Hall effect. The effect of the field on the multiplicity of short-lived pairs consisting of mobile carriers and their traps is proposed as a possible explanation for the observed phenomenon.

Optical quenching of the magnetoplastic effect in NaCl crystals

Observations indicate that illuminating NaCl crystals by ultraviolet light (λ=350 nm) suppresses the magnetoplastic effect. The processes induced by illumination take place in a subsystem of point defects and are related to a change in the state of magnetically sensitive dislocation pinning sites.