### Article

## Formation of gapless triplets in the bond-doped spin-gap antiferromagnet (C4H12N2)(Cu2Cl6)

We report results of an electron spin resonance (ESR) study of a spin-gap antiferromagnet (C4H12N2)(Cu2Cl6) (nicknamed PHCC) with chlorine ions partially substituted by bromine. We found that up to 10% of nominal doping the contribution of the random defects to the absorption spectra remains at about 0.1% per copper ion, which is almost the same as in the pure system. Instead, a particular kind of ESR absorption corresponding to gapless S = 1 triplets is observed at low temperatures in samples with high nominal bromine content x 5%. Increase of bromine concentration also leads to the systematic broadening of ESR absorption line, indicating reduction of the quasi-particles lifetime.

The electron spin resonance spectrum of a quasi-1D S=1/2 antiferromagnet K2CuSO4Br2 was found to demonstrate an energy gap and a doublet of resonance lines in a wide temperature range between the Curie-Weiss and Neèl temperatures. This type of magnetic resonance absorption corresponds well to the two-spinon continuum of excitations in S=1/2 antiferromagnetic spin chain with a uniform Dzyaloshinskii-Moriya interaction between the magnetic ions. A resonance mode of paramagnetic defects demonstrating strongly anisotropic behavior due to interaction with spinon excitations in the main matrix is also observed.

We present a detailed analysis of the phase diagram of antiferromagnets with competing exchange-driven and field-induced order parameters. By using the quasi-1D antiferromagnet BaCu2Si2O7 as a test case, we demonstrate that a model based on a Landau type of approach provides an adequate description of both the magnetization process and of the phase diagram. The developed model not only accounts correctly for the observed spin-reorientation transitions, but it predicts also their unusual angular dependence.

We report the study of spin relaxation in the Eu1−xGdxB6 (0 ≤ x ≤ 0.039) single crystals with the help of 60 GHz electron spin resonance (ESR) technique. A drastic change in the linear slopes of the temperature dependences of the ESR linewidth is discovered in the paramagnetic phase of Eu1−xGdxB6. The corresponding crossover temperature T0 is shown to decrease from T0(x = 0) ∼ 60 K down to T0(x = 0.039) ∼ 15 K under rising of Gd content. A non-bottlenecked Korringa relaxation is discussed as the main factor that governs spin dynamics in the unordered state of Eu1−xGdxB6 below T0. Using of the band parameters extracted from static magnetic and transport data allows to estimate on-site exchange constant between localized spins and itinerant electrons, which is effectively tuned from 110 meV for x = 0 down to 43 meV for x = 0.039 under gradual filling of the Eu1−xGdxB6 conduction band.

The dynamics of a two-component Davydov-Scott (DS) soliton with a small mismatch of the initial location or velocity of the high-frequency (HF) component was investigated within the framework of the Zakharov-type system of two coupled equations for the HF and low-frequency (LF) fields. In this system, the HF field is described by the linear Schrödinger equation with the potential generated by the LF component varying in time and space. The LF component in this system is described by the Korteweg-de Vries equation with a term of quadratic influence of the HF field on the LF field. The frequency of the DS soliton`s component oscillation was found analytically using the balance equation. The perturbed DS soliton was shown to be stable. The analytical results were confirmed by numerical simulations.

Radiation conditions are described for various space regions, radiation-induced effects in spacecraft materials and equipment components are considered and information on theoretical, computational, and experimental methods for studying radiation effects are presented. The peculiarities of radiation effects on nanostructures and some problems related to modeling and radiation testing of such structures are considered.