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## Splitting of antiferromagnetic resonance modes in the quasi-two-dimensional collinear antiferromagnet Cu(en)(H2O)2SO4

A low-temperature magnetic resonance study of the quasi-two-dimensional antiferromagnet Cu(en)(H2O)2SO4 (en = C2H8N2) was performed down to 0.45 K. This compound orders antiferromagnetically at 0.9 K. The analysis of the resonance data within the hydrodynamic approach allowed us to identify anisotropy axes and to estimate the anisotropy parameters for the antiferromagnetic phase. Dipolar spin-spin coupling turns out to be the main contribution to the anisotropy of the antiferromagnetic phase. The splitting of the resonance modes and its nonmonotonous dependence on the applied frequency were observed below 0.6 K in all three field orientations. Several models are discussed to explain the origin of the nontrivial splitting, and the existence of inequivalent magnetic subsystems in Cu(en)(H2O)2SO4 is chosen as the most probable source.

This book is the proceedings of the IX International Conference for Professionals & Young Scientists “LOW TEMPERATURE PHYSICS” ICPYS LTP 2018 dedicated to the 100th anniversary of the National Academy of Sciences of Ukraine, and contains 150 peer-reviewed abstracts. These materials present and discuss the studies of modern aspects of experimental and theoretical physics at low and ultralow temperatures, including electronic properties of conducting and superconducting systems, magnetism and magnetic materials, optics, photonics and optical spectroscopy, quantum liquids and quantum crystals, cryocrystals, nanophysics and nanotechnologies, biophysics and physics of macromolecules, materials science, theory of condensed matter physics, technological peculiarities of the instrumentation for physical experiments, and other related fields.

The electron spin resonance doublet indicating the width of the two-spinon continuum in a spin-12 triangular lattice Heisenberg antiferromagnet Cs2CuCl4 was studied in high magnetic field. The doublet was found to collapse in a magnetic field of one-half of the saturation field. The collapse of the doublet occurs via vanishing of the high-frequency component in a qualitative agreement with the theoretical prediction for the S=12 chain. The field of the collapse is, however, much lower than expected for the S=12 chain. This is proposed to be due to the destruction of frustration of interchain exchange bonds in a magnetic field, which restores the 2D character of this spin system. In the saturated phase the mode with the Larmor frequency and a much weaker mode downshifted for 119 GHz are observed. The weak mode is of exchange origin; it demonstrates a positive frequency shift at heating corresponding to the repulsion of magnons in the saturated phase.

Despite the fact that electron transport in condensed helium has been studied for over half a century [1], observations of new intriguing effects still appear [2]. Alas, the traditional methods of injecting electrons into condensed helium (radioactive-sources, electrical discharge or field emission) lead to generation of helium ions, recombination of which is accompanied by emergence of a large number of excitations. As a result, interpretation of such experiments is not simple and sometimes may be questionable. In this respect, photoelectron emitters, which operate with energies substantially smaller than the ionization energy of helium, are preferable. However, immersion of the photocathode into condensed helium suppresses electron emission. Nevertheless, we managed to achieve electron currents (>20 fA) with the In photocathode immersed directly in liquid superfluid helium. The UV light (λ=254 nm) was guided to the photocathode through a two-meter long Al-covered quartz optical fiber.

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.

We present the results of magnetization, electron spin resonance (ESR), and nuclear magnetic resonance (NMR) measurements on single-crystal samples of the frustrated S = 1/2 chain cuprate LiCu2O2 dopedwith nonmagnetic Zn2+.As shown by the x-ray techniques, the crystals of Li(Cu1−xZnx )2O2 withx < 0.12 are single-phase,whereas for higher Zn concentrations the samples were polyphase. ESR spectra for all monophase samples (0 x < 0.12) can be explained within the model of a planar spin structure with a uniaxial type anisotropy. The NMR spectra of the highly doped single-crystal sample Li(Cu0.9Zn0.1)2O2 can be described in the frame of a planar spin-glass-like magnetic structure with short-range spiral correlations in the crystal ab planes with strongest exchange bonds. The value of magnetic moments of Cu2+ ions in this structure is close to the value obtained for undoped crystals: (0.8 ± 0.1) μB.

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.