The process of ablation of a gold target by femto- and picosecond laser radiation pulses has been studied by numerical simulations using an atomistic model with allowance for the electron subsystem and the dependence of the ion–ion interaction potential on the electron temperature. Using this potential, it is possible to take into account the change in the physical properties of the ion subsystem as a result of heating of the electron subsystem. The results of simulations reveal a significant difference between the characteristics of metal ablation by laser pulses of various durations. For ablation with subpicosecond pulses, two mechanisms of metal fracture related to the evolution of electronic pressure in the system are established.
The temperature dependences of the heat capacity and optical transmission spectra of quasi-one-dimensional Haldane magnets (Y1 − x Nd x )2BaNiO5 (x = 1, 0.75, 0.50, 0.25) have been studied. All the compounds studied undergo antiferromagnetic ordering. In the ordered state, the internal magnetic field produces splitting of the ground-state Kramers doublet of Nd3+ ion. The temperature dependences of the magnitude of splitting, as determined from spectroscopic data, were used to calculate the Schottky anomaly on the heat capacity curve. Nonequivalent centers related to neodymium ions have been found in compounds with x = 0.75, 0.50, and 0.25.
Экспериментально исследованы плазменно-пылевые структуры в тлеющем разряде гелия в диапазоне температур 5–300 К. Описана экспериментальная установка, предоставляющая возможность непрерыв- ного изменения температурного режима. Представлена методика обработки экспериментальных дан- ных. Проведено измерение межчастичных расстояний в диапазоне температур 9–295 К и их сравнение с величиной радиуса Дебая. Указаны диапазоны параметров эксперимента, при которых формируются плазменно-пылевые структуры и проявляются различные формы их поведения (вращение, колебания структур, образование вертикальных линейных цепочек и т. д.). Обсуждается применимость потенциала Юкавы к описанию структурных свойств пылевой плазмы в условиях эксперимента
We have measured antiferromagnetic resonance (AFMR) frequency-field dependences for aluminum–manganese garnet Mn3Al2Ge3O12 at frequencies from 1 to 125 GHz and fields up to 6 T. There are three AFMR modes for all orientations, their zero field gaps are about 40 and 70 GHz. Andreev–Marchenko hydrodynamic theory  well describes experimental frequency–field dependences. We have observed hysteresis of resonance absorption as well as history dependence of resonance absorption near gap frequencies below 10 kOe in all three measured field orientations, which are supposedly due to the sample domain structure. Observation of the AFMR signal at the frequencies from 1 to 5 GHz allows to estimate repulsion of nuclear and electron modes of spin precession in the vicinity of spin-reorientation transition at H || .
Within electron density functional theory (DFT), the reflectance of radiation from shock-compressed xenon plasma is calculated. The dependence of the reflectance on the frequency of the incident radiation and on the plasma density is considered. The Fresnel formula is used. The expression for the longitudinal dielectric tensor in the long-wavelength limit is used to calculate the imaginary part of the dielectric function (DF). The real part of the DF is determined by the Kramers-Kronig transformation. The results are compared with experimental data. An approach is proposed to estimate the plasma frequency in shock-compressed xenon. © 2015, Pleiades Publishing, Inc.
It is shown how the general formulas of the nonequilibrium diagram technique can be used in problems of tunnel planar structures described in the effective mass approach. The relation between such a “continual” approach and the tunneling Hamiltonian method is established, and the applicability conditions for this method are determined. The effects beyond the applicability limits of the tunneling Hamiltonian method, which can be described by the continual approach, are considered.
In magnetic compounds with Jahn–Teller (JT) ions (such as Mn3+ or Cu2+), the ordering of the electron or hole orbitals is associated with cooperative lattice distortions. There the role of JT effect, although widely recognized, is still elusive in the ground state properties. Here we discovered that, in these materials, there exist excitations whose energy spectrum is described in terms of the total angular momentum eigenstates and is quantized as in quantum rotors found in JT centers. We observed features originating from these excitations in the optical spectra of a model compound LaMnO3 using ellipsometry technique. They appear clearly as narrow sidebands accompanying the electron transition between the JT split orbitals on neighboring Mn3+ ions, displaying anomalous temperature behavior around the Néel temperature TN ≈ 140 K. We present these results together with new experimental data on photoluminescence found in LaMnO3, which lend additional support to the ellipsometry implying the electronic-vibrational origin of the quantum rotor orbital excitations. We note that the discovered orbital excitations of quantum rotors may play an important role in many unusual properties observed in these materials upon doping, such as high-temperature superconductivity and colossal magnetoresistance.
We analyze the role of local geometry in the spin and orbital interaction in transition metal compounds with orbital degeneracy. We stress that the tendency observed in the most studied case (transition metals in O6 octahedra with one common oxygen—common corner of neighboring octahedra—and with ~180° metal–oxygen–metal bonds), that ferro-orbital ordering renders antiferro-spin coupling and, vice versa, antiferro-orbitals give ferro-spin ordering, is not valid in the general case, in particular, for octahedra with a common edge and with ~90° M–O–M bonds. Special attention is paid to the “third case,” that of neighboring octahedra with a common face (three common oxygens), which has largely been disregarded until now, although there are many real systems with this geometry. Interestingly enough, the spin-orbit exchange in this case turns out to be simpler and more symmetric than in the first two cases. We also consider, which form the effective exchange takes for different geometries in the case of strong spin–orbit coupling.