Investigation of the hyperfine interaction in the antiferromagnetic CsMnI3

The lower branch of the resonance spectrum of the quasi-one-dimensional triangular antiferromagnetic CsMnI₃ has been investigated experimentally. This branch possesses a gap due to the dynamic hyperfine interaction. The temperature dependence of the energy gap was studied in detail at several frequencies. A theoretical calculation of the corresponding spectrum of coupled electron-nuclear spin oscillations was performed in the “hydrodynamic approximation” with an empirical correction for thermal fluctuations of the antiferromagnetic system. The good agreement between the calculation and experimental data makes it possible to determine the zero-point spin reduction in the antiferromagnetic. Zh. éksp. Teor. Fiz. 112, 1893–1898 (November 1997)     

II. The intermediate velocity source in the 40Ca + 40Ca reaction at Elab = 35AMeV

The shape of the velocity distributions of charged particles projected on the beam direction can be explained if emissions from the hot projectile-like fragment and the target-like fragment are supplemented by an emission from an intermediate velocity source located between them. The creation of this source is predicted by a two-stage reaction model where, in the second stage, some of the nucleons identified in the first stage as participants form a group of clusters located in the region between the colliding nuclei. The cluster coalescence process is governed on the average by the maximum value of entropy, although its fluctuations are also significant. The properties of the intermediate velocity source are precisely described, including the isotopic composition of the emitted particles. Received: 12 March 2001 / Accepted: 20 June 2001     

New magnetic states in copper metaborate CuB<Subscript>2</Subscript>O<Subscript>4</Subscript>

The static and resonance properties of copper metaborate CuB₂O₄ were experimentally studied in a magnetic field applied in the crystal tetragonal plane. The field-induced second-order phase transition to a weakly ferromagnetic state was observed in the temperature range 10–20 K. The low-field state is characterized by the absence of spontaneous moment, and it represents, presumably, a long-period helicoid. At temperatures below 2 K, two sequential first-order phase transitions were observed. They were accompanied by jumps in resonance absorption with a hysteresis upon changing field-scan direction. These transitions can be caused by the transformation of the incommensurate spin structure into the helicoidal states with periods commensurate with the lattice translation period.     

Comparative study of ESR spectra in incommensurate antiferromagnets

The electron spin resonance is studied for noncollinear low-dimensional antiferromagnets RbMnBr₃ and RbFe(MoO₄)₂ in a wide range of frequencies and fields. Both compounds have incommensurate spin structures appearing due to a low-symmetry distortion of an ideal hexagonal crystal lattice. Magnetic field applied in the spin plane induces a first-order transition into the commensurate phase. The low-energy resonance branch corresponding to a uniform oscillation of the spin system in the easy plane is observed in the two compounds in both incommensurate and commensurate phases, with a dramatic change of the spectra taking place near the transition field. The resonance spectrum of a nearly commensurate spin structure with long-wave modulations is analyzed in clean and dirty limits in the framework of a hydrodynamic approach. The resonance branch with steep field dependence in the incommensurate state is attributed to the acoustic mode with the gap resulted from pinning of local domain walls (discommensurations) on defects of the crystal structure.     

Magnetic properties of defects in spin-gap magnets

The magnetic properties of defects were studied in spin-gap magnets such as spin-Peierls magnet CuGeO₃, Haldane magnet PbNi₂V₂O₈, and charge-ordered ladder magnet NaV₂O₅. Doping of these systems with nonmagnetic impurities leads to additional magnetic degrees of freedom, which manifest themselves at low temperatures, where the intrinsic magnetic susceptibility of a spin-gap system is close to zero. Magnetic susceptibility appears due to the local destruction of the singlet ground state as a result of impurity-induced breakage of spin chains. Antiferromagnetically correlated areas arise near the spin-chain breaks. The sizes of these areas and the effective spin of these specific spin clusters are estimated. The order parameter and its spatially modulated depth are determined for impurity-induced magnetically ordered phases. The magnetic properties of defects for the NaV₂O₅ ladder structure are explained in the model of electrons “hopping” near the chain break. The hopping degree of freedom effectively influences the total spin of a spin-chain fragment and magnetization of the system.     

I. The properties of hot Ca-like fragments from the 40Ca + 40Ca reaction at 35AMeV

The creation of hot Ca-like fragments was investigated in the ⁴⁰Ca + ⁴⁰Ca reaction at 35 AMeV. Using the AMPHORA 4π detector system, the primary projectile-like fragment was reconstructed and its properties were determined. Both primary and secondary distributions are compared with the predictions of a Monte Carlo code describing a heavy-ion collision as a two-step process. Some of the nucleons which are identified as participants in the first step are transferred in the second step to these final states, which correspond on the average to the maximum value of entropy (thermodynamic probability). The model allows for competition between mean-field effects and nucleon-nucleon interactions in the overlap zone of the interacting nuclei. The analysis presented here suggests a thermalized source picture of the decay of the projectile-like fragment. The validity of the reconstruction procedure for projectile-like fragments is discussed. Received: 12 March 2001 / Accepted: 20 June 2001     

The kinetics of an atom diffusing in one dimension: Hydrogen in quartz

Abstract

The time dependence of frequency recovery at 306 K of pulse-irradiated quartz is analyzed and shown to be inconsistent with the diffusion of a specie in three dimensions, but follows well diffusion in one dimension. The main feature which distinguishes these cases is a t ^?1/2 time dependence of the recovery. No extended region for a t ^?1/2 dependence is possible in three-dimensional diffusion. An extensive t ^?1/2 dependence is predicted in one-dimensional diffusion, paralleling the observation that this holds for some three decades in time and over 80% of the recovery.

Having established one-dimensional diffusion in frequency recovery, it is shown that acoustic relaxation found at 115 K and a long time component of conductivity recovery found near 300 K, as well as the frequency recovery, all appear to originate in a common mechanism—the diffusion of H⁺ in one dimension with an activation of approximately ¼ electron volt.     

ID-IE recovery in electron irradiated copper. II. Theory

A complete diffusional analysis of the low temperature, I_D-I_E post-irradiation recovery stages in copper is presented here. In applying this analysis to experiments, intrinsic defect characteristics are deduced by multi-parameter, minimum deviation fitting. The vacancy capture radius for interstitials is found to be 3.2 ± 0.3 lattice constants, for example. In addition, the shape of the initial interstitial-vacancy separation distribution function has been determined. A single distribution function describes this distribution for irradiation by electrons with energies greater than 0.6 MeV. At lower energies, the function is narrowed.