Lifetime of Gapped Excitations in a Collinear Quantum Antiferromagnet

We demonstrate that local modulations of magnetic couplings have a profound effect on the temperature dependence of the relaxation rate of optical magnons in a wide class of antiferromagnets in which gapped excitations coexist with acoustic spin waves. In a two-dimensional collinear antiferromagnet with an easy-plane anisotropy, the disorder-induced relaxation rate of the gapped mode, Gamma_imp=Gamma_0+A(TlnT)^2, greatly exceeds the magnon-magnon damping, Gamma_m-m=BT^5, negligible at low temperatures. We measure the lifetime of gapped magnons in a prototype XY antiferromagnet BaNi2(PO4)2 using a high-resolution neutron-resonance spin-echo technique and find experimental data in close accord with the theoretical prediction. Similarly strong effects of disorder in the three-dimensional case and in noncollinear antiferromagnets are discussed.Comment: 4.5 pages + 2.5 pages supplementary material, published versio

Crystal structure of mixed fluorites Ca(1-x)Sr(x)F(2) and Sr(1-x)Ba(x)F(2) and luminescence of Eu(2+) in the crystals

Within the framework of the virtual crystal method implemented in the shell model and pair potential approximation the crystal structure of mixed fluorites Ca(1-x)Sr(x)F(2) and Sr(1-x)Ba(x)F(2) has been calculated. The impurity center Eu(2+) and the distance Eu(2+)-F in this crystals have been also calculated. The low level position of excited 4f65d configuration of the Eu(2+) ion has been expressed using phenomenological dependence on distance E(2+)-F. The dependences of Stokes shift and Huang-Rhys factor on concentration x have been received for yellow luminescence in Sr(1-x)Ba(x)F(2):Eu(2+). The value x, for which the eg -level of Eu(2+) ion will be in conduction band in Sr(1-x)Ba(x)F(2):Eu(2+) has been calculated.Comment: 8 pages, 3 figures. The manuscript is sent to journal 'Physics of the solid state'. The results will be submitted on inernational conference SCINTMAT'2002 in oral session (june,20-22,2002,Ekaterinburg,Russia). Corresponding author e-mail: [email protected]

Instability of antiferromagnetic magnons in strong fields

We predict that spin-waves in an ordered quantum antiferromagnet (AFM) in a strong magnetic field become unstable with respect to spontaneous two-magnon decays. At zero temperature, the instability occurs between the threshold field $H^*$ and the saturation field $H_c$. As an example, we investigate the high-field dynamics of a Heisenberg antiferromagnet on a square lattice and show that the single-magnon branch of the spectrum disappears in the most part of the Brillouin zone.Comment: RevTeX, 4 pages, 3 figures, accepted to PR

Holes in the t-J_z model: a thorough study

The t-J_z model is the strongly anisotropic limit of the t-J model which captures some general properties of the doped antiferromagnets (AF). The absence of spin fluctuations simplifies the analytical treatment of hole motion in an AF background and allows us to calculate the single- and two-hole spectra with high accuracy using regular diagram technique combined with real-space approach. At the same time, numerical studies of this model via exact diagonalization (ED) on small clusters show negligible finite size effects for a number of quantities, thus allowing a direct comparison between analytical and numerical results. Both approaches demonstrate that the holes have tendency to pair in the p- and d-wave channels at realistic values of t/J. The interactions leading to pairing and effects selecting p and d waves are thoroughly investigated. The role of transverse spin fluctuations is considered using perturbation theory. Based on the results of the present study, we discuss the pairing problem in the realistic t-J-like model. Possible implications for preformed pairs formation and phase separation are drawn.Comment: 21 pages, 15 figure

Rapid Mass Spectrometric Study of a Supercritical CO2-extract from Woody Liana Schisandra chinensis by HPLC-SPD-ESI-MS/MS

Woody liana Schisandra chinensis contains valuable lignans, which are phenylpropanoids with valuable biological activity. Among green and selective extraction methods, supercritical carbon dioxide (SC-CO2) was shown to be the method of choice for the recovery of these naturally occurring compounds. Carbon dioxide (CO2) was the solvent with the flow rate (10−25 g/min) with 2% ethanol as co-solvent. In this piece of work operative parameters and working conditions were optimized by experimenting with different pressures (200–400 bars) and temperatures (40–60 °C). The extraction time varied from 60 to 120 min. HPLC-SPD-ESI -MS/MS techniques were applied to detect target analytes. Twenty-six different lignans were identified in the S. chinensis SC-CO2 extracts

Spontaneous Magnon Decays

A theoretical overview of the phenomenon of spontaneous magnon decays in quantum antiferromagnets is presented. The intrinsic zero-temperature damping of magnons in quantum spin systems is a fascinating many-body effect, which has recently attracted significant attention in view of its possible observation in neutron-scattering experiments. An introduction to the theory of magnon interactions and a discussion of necessary symmetry and kinematic conditions for spontaneous decays are provided. Various parallels with the decays of anharmonic phonons and excitations in superfluid 4He are extensively used. Three principal cases of spontaneous magnon decays are considered: field-induced decays in Heisenberg antiferromagnets, zero-field decays in spiral antiferromagnets, and triplon decays in quantum-disordered magnets. Analytical results are compared with available numerical data and prospective materials for experimental observation of the decay-related effects are briefly discussed.Comment: v3.0, asymptotically close to the published versio

Thermal drag revisited: Boltzmann versus Kubo

The effect of mutual drag between phonons and spin excitations on the thermal conductivity of a quantum spin system is discussed. We derive general expression for the drag component of the thermal current using both Boltzmann equation approach and Kubo linear-response formalism to leading order in the spin-phonon coupling. We demonstrate that aside from higher-order corrections which appear in the Kubo formalism both approaches yield identical results for the drag thermal conductivity. We discuss the range of applicability of our result and provide a generalization of our consideration to the cases of fermionic excitations and to anomalous forms of boson-phonon coupling. Several asymptotic regimes of our findings relevant to realistic situations are highlighted.Comment: 14 pages, 3 figures, published version, extended discussio

Использование отечественных кремнийорганических жидкостей в качестве неподвижных фаз в газожидкостной хроматографии

The thermal stability of organosilicon stationary phases for gas-liquid chromatography has been tested under conditions approximating the real operation of chromatographic column.Проведены испытания термостойкости отечественных кремнийорганических неподвижных фаз в условиях, приближенных к обычным условиям работы хроматографических колонн. Показано, что значения верхнего температурного предела большинства кремнийорганических неподвижных фаз были завышены. Определены значения верхнего температурного предела этих неподвижных фаз, соответствующие современным требования

Выделение некоторых высококипящих элементоорганических соединений методом препаративной газовой хроматографии

Preparative gas chromatography is proposed to isolate some high-boiling organometallic compounds. Isolation of high-boiling substances should be conducted at a column temperature significantly below the boiling point, because most isolated compounds are thermally unstable at such temperatures. Stationary phases for preparative gas chromatography have a temperature limit of 350°C. The reduction of the column temperature is based on simultaneous changing the parameters of the chromatographic experiment (column length, impregnation degree, flow rate of the carrier gas). The influence of reducing the column temperature on the shape of the chromatographic peak is shown. The peak has an asymmetric shape, and its width increases. Therefore, the possibility of high-boiling substances preparative isolation depends on temperature decrease as the column separation efficiency is maintained.Метод препаративной газовой хроматографии предложен для выделения некоторых высококипящих элементоорганических соединений. Выделение высококипящих веществ необходимо проводить при температуре, значительно ниже температуры кипения, так как большинство выделяемых соединений термически неустойчивы при таких температурах. Неподвижные фазы для препаративной газовой хроматографии имеют температурный предел 350ºС. Рассмотрены особенности выделения высококипящих веществ при снижении температуры колонки относительно температуры кипения вещества. Понижение температуры колонки достигнуто путем одновременного изменения параметров хроматографического опыта (длины колонки, степени пропитки, скорости газа-носителя). Однако при увеличении отношения температуры кипения к температуре колонки наблюдается изменение формы пика, возрастание его ширины. Поэтому возможность препаративного выделения высококипящих веществ зависит от снижения температуры колонки при сохранении эффективности разделения

Выделение некоторых органохлорсиланов методом препаративной газовой хроматографии

The preparative gas chromatography method has been used for isolation and analysis of the organochlorsilanes isomers. The reaction of the organochlorsilanes with vapour of the water in the air has been studied. The method for isolation of the organochlorsilanes is proposed. It is based on using chromatographic column with efficiency up to 10000 theoretical plates. A high degree of purity (up to 99%) of the isolated compounds is achieved by an increase of separation selectivity, which is turned results from a temperature decrease. The effect of the parameters of chromatographic experiment (the column length, impregnation degree, the rate of gas-carrier) on the isolation of the compounds boiling up to 400o C was investigated.Метод газовой хроматографии, основанный на использовании хроматографических колонок с эффективностью до 10000 теоретических тарелок, предложен для выделения и анализа некоторых изомеров органохлорсиланов, которые активно реагируют с влагой воздуха. Высокая чистота (до 99%) выделенных соединений достигается за счет увеличения селективности путем понижения температуры. Исследованы условия выделения некоторых изомеров органо- хлорсиланов c температурами кипения до 400о С методом препаративной газовой хроматографии. Показаны пути преодоления методических трудностей, возникающих при выделении высококипящих изомеров. Приведены данные по выделению изомеров хлорсиланов, содержащих различные радикалы (метил-, винил-, фенил-, адамантил-