Nucleus cross-polarization technique in a rotating frame of reference is analyzed as applied to NMR experiments with sample magic-angle spinning. The concept of simultaneous phase and amplitude modulation is suggested. According to this suggestion, the form of the Hamiltonian of recoupled dipolar interaction remains unchanged if phase inversion is accompanied by inversion of the difference of radio-frequency (RF) field amplitudes. A theoretical treatment is given in terms of the average Hamiltonian theory. The concept is demonstrated experimentally and by numerical analysis for several particular cases. Periodic phase inversion in cross-polarization experiments was shown to have the practically important advantage of suppressing chemical shift interactions and the effect of inaccurate tuning of RF field parameters.
The dynamics of magnetization in synthetic antiferromagnetic systems with the magnetic dipole coupling in a rapidly oscillating field has been examined. It has been revealed that the system can behave similar to the Kapitza pendulum. It has been shown that an alternating magnetic field can be efficiently used to control the magnetic state of a cell of a synthetic antiferromagnet. Analytical relations have been obtained between the parameters of such an antiferromagnet and an external magnetic field at which certain quasistationary states are implemented.
The theory of the dynamic remagnetization of a synthetic antiferromagnetic system and magnetic points located on a magnetic substrate in an external magnetic field has been considered. The energies of the equilibrium states of the system have been calculated. The conditions of switching between equilibrium states have been described. The conditions of applicability of this theory have been formulated. It has been shown that the process of remagnetization can be implemented in an inertialess regime, escaping the long-term relaxation of the system to a new equilibrium position with the use of a special shape of the field signal. The possibility of the reduction of the switching field amplitude by varying the pulse duration has been demonstrated.
Recently a new effect in the Raman scattering of x-ray radiation has been predicted theoretically and discovered in experiments, the effect of restoration of the selection rules for the scattering tensor under strong electron-vibrational interaction. We propose a fairly simple model for describing this effect, a model that allows for an exact solution and takes into account the real vibrational structure of the molecule and electron-vibrational interaction.
The formation of a short-range order in soft magnetic Fe-Si alloys depending on the annealing temperature has been investigated theoretically and experimentally. The B2-type short-range order has been observed in samples quenched from temperatures T > T(C) (where T(C) is the Curie temperature) with the content c(Si) close to the boundary of the two-phase region. Annealing at temperatures T < T(C) for the content c(Si) >= 0.08 leads to an increase in the fraction of regions with the D0(3)-type short-range order. The mechanism of the formation of the short-range order in Fe-Si solid solutions has been analyzed by the Monte Carlo simulation with the ab initio calculated interatomic interaction parameters. It has been shown that the energy of the effective Si-Si interaction in bcc iron strongly depends on the magnetic state of the matrix. As a result, the B2-type short-range order is formed at T > T(C) and is fixed at quenching, whereas the D0(3)-type short-range order is equilibrium in the ferromagnetic state. The results reveal the decisive role of magnetism in the formation of the short-range order in Fe-Si alloys and allow the explanation of the observed structural features of the alloys depending on the composition and temperature.
Thermal radiation from hot combustion products has virtually no effect on the flame propagation in a gas medium. We consider a different situation when even a small concentration of microparticles suspended in a gas absorbs the thermal radiation and heats the gas mixture ahead of the combustion wave front by transferring it to the gas. The mixture heating ahead of the flame front can lead either to a moderate increase in the combustion wave velocity for a fast flame or to its significant increase for a slow flame, depending on the gas mixture reactivity and the normal laminar flame velocity. For a slow flame, the heat transfer by radiation from the combustion products can become the dominant mechanism compared to the ordinary molecular thermal conduction that determines the combustion wave structure and velocity. The radiative heating for a spatially nonuniform distribution of particles ahead of the flame front is shown to give rise to a temperature gradient that, in turn, can lead to the ignition of different combustion regimes, depending on the radiation absorption length. In accordance with the Zeldovich gradient mechanism, both deflagration and detonation regimes can be formed in this case. A hydrogen-oxygen flame is used as an example to illustrate the ignition of different combustion wave propagation regimes, depending on the radiation absorption length.
The north-south asymmetry for cosmic-ray particles was measured with one instrument of the PAMELA satellite-borne experiment in the period June 2006-May 2009. The analysis has been performed by two independent methods: by comparing the count rates in regions with identical geomagnetic conditions and by comparing the experimental distribution of particle directions with the simulated distribution that would be in the case of an isotropic particle flux. The dependences of the asymmetry on energy release in the PAMELA calorimeter and on time have been constructed. The asymmetry (N (n) - N (s) )/(N (n) + N (s) ) is 0.06 +/- 0.004 at the threshold energy release in the calorimeter and gradually decreases with increasing energy release. The observed effect is shown to be produced by electrons in the energy range 10-100 GeV.
New measurements of the energy spectra of cosmic-ray protons and helium nuclei with significantly increased statistics owing to an improvement of the event selection technique and the involvement of all data over the period 2006-2013 in the analysis have been made at energies above 0.8 TeV/nucleon with a position-sensitive calorimeter based on data from the PAMELA satellite-borne experiment.
The pasta phases predicted to occur near the inner boundary of the crust of a neutron star resemble liquid crystals, a smectic A in the case of sheet-like nuclei (lasagna) and the columnar phase in the case of rod-like nuclei (spaghetti). An important difference compared with usual liquid crystals is that the nucleons are superfluid. We develop the hydrodynamic equations for this system and use them to study collective oscillations. Nucleon superfluidity leads to important qualitative differences in the spectra of these oscillations and also increases their frequencies compared with ordinary liquid crystals. We discuss a number of directions for future work.
The dynamics of the motion of the magnetic moment averaged over an ensemble of nonequilibrium spin-injected electrons in a ferromagnetic junction is considered with allowance for the exchange interaction, as well as the interaction with an external electromagnetic field and a thermostat. The solution of this problem is important for the experimental development of compact terahertz-band radiation sources. The rate of quantum transitions of electrons with opposite spins, which determine the spin relaxation under the interaction with a thermostat, is calculated within the density matrix formalism. It is shown that two spin-relaxation modes can be implemented that correspond to low- and high-Q precession of spin-nonequilibrium injected electrons. The effect of the characteristic features of spin-flip transitions under the relaxation of the magnetic moment on the emission and absorption of photons with-energy corresponding to the energy of effective exchange splitting of spin subbands is discussed.