Dynamical Dzyaloshinsky-Moriya interaction in KCuF3: Raman evidence for an antiferrodistortive lattice instability

In the orbitally ordered, quasi-one dimensional Heisenberg antiferromagnet KCuF3 the low-energy Eg and B1g phonon modes show an anomalous softening (25% and 13%) between room temperature and the characteristic temperature T_S = 50 K. In this temperature range a freezing-in of F ion dynamic displacements is proposed to occur. In addition, the Eg mode at about 260 cm-1 clearly splits below T_S. The width of the phonon lines above T_S follows an activated behavior with an activation energy of about 50 K. Our observations clearly evidence a reduction of the structural symmetry below T_S and indicate a strong coupling of lattice and spin fluctuations for T>T_S.Comment: 7 pages, 9 figure

Low temperature mixed spin state of Co3+ in LaCoO3 evidenced from Jahn-Teller lattice distortions

One- and multi-phonon excitations of the single crystalline LaCoO3 were studied using Raman spectroscopy in the temperature region of 5 K - 300 K. First-order Raman spectra show a larger number of phonon modes than allowed for the rhombohedral structure. Additional phonon modes are interpreted in terms of activated modes due to lattice distortions, arising from the Jahn-Teller (JT) activity of the intermediate-spin (IS) state of Co3+ ions. In particular, the 608-cm-1 stretching-type mode shows anomalous behavior in peak energy and scattering intensity as a function of temperature. The anomalous temperature dependence of the second-order phonon excitations spectra is in accordance with the Franck-Condon mechanism that is characteristic for a JT orbital order.Comment: 11 pages, 9 figures, to be published in J. Low. Temp. Physic

Spin-flop transition in uniaxial antiferromagnets: magnetic phases, reorientation effects, multidomain states

The classical spin-flop is the field-driven first-order reorientation transition in easy-axis antiferromagnets. A comprehensive phenomenological theory of easy-axis antiferromagnets displaying spin-flops is developed. It is shown how the hierarchy of magnetic coupling strengths in these antiferromagnets causes a strongly pronounced two-scale character in their magnetic phase structure. In contrast to the major part of the magnetic phase diagram, these antiferromagnets near the spin-flop region are described by an effective model akin to uniaxial ferromagnets. For a consistent theoretical description both higher-order anisotropy contributions and dipolar stray-fields have to be taken into account near the spin-flop. In particular, thermodynamically stable multidomain states exist in the spin-flop region, owing to the phase coexistence at this first-order transition. For this region, equilibrium spin-configurations and parameters of the multidomain states are derived as functions of the external magnetic field. The components of the magnetic susceptibility tensor are calculated for homogeneous and multidomain states in the vicinity of the spin-flop. The remarkable anomalies in these measurable quantities provide an efficient method to investigate magnetic states and to determine materials parameters in bulk and confined antiferromagnets, as well as in nanoscale synthetic antiferromagnets. The method is demonstrated for experimental data on the magnetic properties near the spin-flop region in the orthorhombic layered antiferromagnet (C_2H_5NH_3)_2CuCl_4.Comment: (15 pages, 12 figures; 2nd version: improved notation and figures, correction of various typos

Bound, virtual and resonance SS-matrix poles from the Schr\"odinger equation

A general method, which we call the potential $S$-matrix pole method, is developed for obtaining the $S$-matrix pole parameters for bound, virtual and resonant states based on numerical solutions of the Schr\"odinger equation. This method is well-known for bound states. In this work we generalize it for resonant and virtual states, although the corresponding solutions increase exponentially when $r\to\infty$. Concrete calculations are performed for the $1^+$ ground and the $0^+$ first excited states of $^{14}\rm{N}$, the resonance $^{15}\rm{F}$ states ($1/2^+$, $5/2^+$), low-lying states of $^{11}\rm{Be}$ and $^{11}\rm{N}$, and the subthreshold resonances in the proton-proton system. We also demonstrate that in the case the broad resonances their energy and width can be found from the fitting of the experimental phase shifts using the analytical expression for the elastic scattering $S$-matrix. We compare the $S$-matrix pole and the $R$-matrix for broad $s_{1/2}$ resonance in ${}^{15}{\rm F}$Comment: 14 pages, 5 figures (figures 3 and 4 consist of two figures each) and 4 table

Метод построения решающих правил неразрушающего контроля композитов на основе гистограммного анализа

Досліджено можливість застосування гістограмного аналізу для побудови вирішальних правил у безеталонній дефектоскопії композиційних матеріалів. Показано, що гістограм ний метод дозволяє надійно класифікувати експериментальні дані навіть у випадку, коли характеристики положення і розсіювання законів розподілу еталонних класів інформативних ознак повністю збігаються.Possibility of histogram analysis method using for decision rules creation in etalon-free nondestructive testing of composite material was investigated. Proved, that histogram method allows reliable experimental data classification in case when position and dispersion characteristics of distribution low is absolutely equal.Исследована возможность использования гистограммного анализа для построения решающих правил в безэталонной дефектоскопии композиционных материалов. Показано, что гистограммный метод позволяет надежно классифицировать экспериментальные данные даже в случае, когда характеристики положения и рассеивания законов распределения эталонных классов полностью совпадают

Phononic and magnetic excitations in the quasi-one-dimensional Heisenberg antiferromagnet KCuF₃

The Raman-active phonons and magnetic excitations in the orbitally ordered, quasi-one-dimensional Heisenberg antiferromagnet KCuF₃ are studied as a function of temperature in the range between 3 and 290 K in various scattering configurations. The low-energy Eg and B1g phonon modes show an anomalous softening (~25% and ~13%) between room temperature and the characteristic temperature TS = 50 K. In this temperature range a freezing-in of F– ion dynamic displacements is proposed to occur. In addition, the Eg mode at about 260 cm⁻¹ clearly splits below TS. The widths of the phonon lines above TS follow an activated behavior with an activation energy of about 50 K. Our observations clearly evidence a reduction of the lattice symmetry below TS and indicate a strong coupling of lattice, spin, and orbital fluctuations for T > TS. A strongly polarization dependent quasielastic scattering is observed at temperatures above TN = 39 K due to magnetic-energy fluctuations. For temperatures below TN a rich spectrum of additional modes is observed, with different lineshape, polarization and temperatures dependence. We attribute them to longitudinal and transverse magnetic modes as well as to a continuum of magnetic excitations

Метод оценивания расширенной неопределенности результатов измерений с помощью преобразования Джонсона

Запропоновано та досліджено метод оцінювання розширеної невизначеності результату вимірювання. Метод базується на процедурі нормалізації даних за допомогою сімейства розподілів Джонсона. Досліджено точність визначення розширеної невизначеності запропонованим методом у порівнянні із методом заміни результуючого закону розподілу невизначеності гауссівським розподілом. Показана перевага методу оцінювання розширеної невизначеності за допомогою перетворення Джонсона.Method of expanded uncertainty estimation was introduced and investigated. The methods is based on procedure of data normalization due to Jonson transformation. The expanded uncertainty evaluation precision of investigated method versus method of substitution to Gauss distribution was explore. The advantage of method of measuring result’s expanded uncertainty estimation based on Jonson transformation was shown.Предложен и исследован метод оценивания расширенно неопределенности результата измерения. Метод базируется на процедуре нормализации данных с помощью семейства распределений Джонсона. Исследована точность определения расширенной неопределенности предложенным методом в сравнении с методом замены результирующего закона распределения неопределенности гауссовским распределением. Показано преимущество метода оценивания расширенной неопределенности при помощи преобразования Джонсона

Phonons and magnons in stripe-ordered nickelates. A Raman scattering study

Electronic correlation effects in La₂-xSrxNiO₄ (x = 1/3 and 0.225) lead to spontaneous phase separation into microscopic spin/charge stripes with commensurate and incommensurate order, respectively. Raman scattering experiments on such single crystalline materials show a rich phenomenology of phonon and magnon anomalies due to the new, self-organized periodicities. These effects are observable as function of temperature but can also be induced by cooling in a seemingly small magnetic field leading to a reorganization of stripe structure