D. V. Lopatin

Photogeneration of free charge carriers in the donor–acceptor complex TBPDA·(C60)2

The effect of a weak magnetic field with B0 < 10000 G on the photoconductivity of the layered donor–acceptor complex TBPDA·(C60)2 has been studied. The Reaction Yield Detected Magnetic Resonance (RYDMR) spectrum provides evidence of a spin nature of the mechanism of free charge carrier generation in magnetic fields. A model of triplet–triplet annihilation of charge transfer excitons has been suggested. The model interprets the effect of a weak magnetic field on photoconductivity in TBPDA·(C60)2. The photoconductivity spectrum of the complex shows two peaks at 470 and 700 nm. The comparison of this spectrum with the optical absorption one indicates that both intermolecular charge transfer between neighboring C60 molecules and direct charge transfer from the TBPDA to the C60 molecules contribute to free charge carrier generation in TBPDA·(C60)2.

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).

Reversible and irreversible magnetic-field-induced changes in the plastic properties of NaCl crystals

It is established that a weak magnetic field with induction B∼1 T gives rise to irreversible changes in NaCl crystals without freshly introduced dislocations, while a strong magnetic field with induction 16<B<35 T gives rise predominantly to reversible changes. It is inferred that there exist two different channels whereby a magnetic field influences the state of point defects.

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.

The influence of a weak magnetic field on the radiation-induced conductivity of C60 single crystals

The radiation-induced conductivity of C60 single crystals is found to increase under the influence of a weak magnetic field (B < 1 T). It is shown that the observed effect is not related to the known galvanomagnetic phenomena. The results obtained are explained in terms of the proposed mechanisms of the influence of a weak magnetic field on the spin-dependent processes involving intermediate electron-hole pairs generated under radiation excitation of the fullerite single crystals.

Influence of a Weak Magnetic Field on Photoconductivity Spectrum of C60 Single Crystal

Abstract Comparative research of the excitation photoconductivity spectra (quantum light energy 2–5 eV) of C60 single crystal in and out of magnetic field at the temperature T = 250–350 K has been carried. The spectral evolution at this temperature range is described. It is shown that the spectra changes abrupt at temperature T 1 ∼ 260 K and T 2 ∼ 315 K. An increase in the photoconductivity up to 15% was observed in the magnetic field (B = 0.4 T) within the photon energy range 2.5–4.5 eV. Local photoconductivity peaks appearances in the magnetic field have been proven that the charge transfer excitons take part in a photoconductivity.

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.

The influence of low doses of β irradiation on the conductivity of C60 single crystals

The electrical conductivity of C60 single crystals is found to increase by 55–120% under β irradiation with low doses. It is shown that this effect can be associated with multistage collision ionization of C60 molecules.

INFLUENCE OF SPIN DYNAMICS OF EXCITONIC STATES ON PHOTOCONDUCTIVITY OF FULLERITE C60

It is shown, that the photoconductivity of C60 single crystal essentially depends on a spin state of the intermediate electron-hole pairs. The distance between components of electron-hole pairs in states with uncorrelated spins and their lifetime were estimated as R 3.4 nm and ~10 s.

Local charge-carrier trapping centers in C60 single crystals

The energy spectrum of local charge-carrier trapping centers in C60 single crystals is studied in terms of the theory of space-charge-limited currents in the Gaussian approximation. The energy distribution of these centers in the energy gap is found, and the type and parameters of this distribution are determined.