Том 106, № 12 (2017)

We highlight the importance of quantum fluctuations in organizing a dissipative quantum phase transition for the driven Jaynes–Cummings interaction with variable qubit-cavity detuning. The system response presents a substantial difference from the predictions of the semiclassical theory, the extent of which is revealed in the properties of quantum bistability, and visualized with the help of quasi-distribution functions for the cavity field, subject to an appropriate scale parameter. States anticipated by the neoclassical theory of radiation coexist in the quantum picture, following the occurrence of spontaneous dressed-state polarization and phase bistability at resonance.

The nuclear recoil effect on the g-factor of H- and Li-like heavy ions is evaluated to all orders in αZ. The calculations include an approximate treatment of the nuclear size and the electron–electron interaction corrections to the recoil effect. As the result, the second largest contribution to the theoretical uncertainty of the g-factor values of ²⁰⁸Pb⁷⁹⁺ and ²³⁸U⁸⁹⁺ is strongly reduced. Special attention is paid to tests of the QED recoil effect on the g-factor in experiments with heavy ions. It is found that, while the QED recoil effect on the gfactor value is masked by the uncertainties of the nuclear size and nuclear polarization contributions, it can be probed on a few-percent level in the specific difference of the g-factors of H- and Li-like heavy ions. This provides a unique opportunity to test QED in a new region of the strong-coupling regime beyond the Furry picture.

It has been shown that the electric current of massive fermions can be excited along an external magnetic field when particles have anomalous magnetic moments and undergo the electroweak interaction with background matter. This current has been calculated on the basis of the exact solution of the Dirac equation in external fields. It has been shown that the magnetic field becomes unstable if this current is included in Maxwell’s equations. It has been found that the seed magnetic field can be significantly enhanced in the particular case of a degenerate electron gas existing in a neutron star. The astrophysical applications of the results have been discussed.

The main result of the paper contains the conclusion that the magnetic phase transition in MnSi always remains first order at any temperature and magnetic field. In these aims, a model of coupling of an order parameter with other degrees of freedom is used. The coupling of magnetic order parameters with long-wave acoustic phonons, in the presence of the nonsingular parts of the bulk and shear moduli, a first-order transition occurs, participle near the transition the heat capacity and the compressibility remain finite, if the heat capacity becomes infinite in the system disregarding the acoustic phonons. The role of the Frenkel heterophase fluctuations is discussed. The impurity effect shows that, for some phases, the heat capacity of the system remains continuous and finite at the transition point. It is supposed that the transition is progressively smoothed by these fluctuations at the application of the magnetic field.

The temperature dependence of the conditions of formation of limiting (steady) structural states at severe plastic deformation of amorphous alloys has been theoretically analyzed. Within an approach based on principles of nonequilibrium thermodynamics, it has been shown that, depending on the temperature of severe plastic deformation, the free energy of the steady state can be both above and below the energy of the initial state.

A sequence of magnetocaloric anomalies occurring with the change in a magnetic field H is predicted for an open nanowire with the Rashba spin–orbit coupling and the induced superconducting pairing potential. The nature of such anomalies is due to the cascade of quantum transitions related to the successive changes in the fermion parity of the nanowire ground state with the growth of the magnetic field. It is shown that the critical H_c values fall within the parameter range corresponding to the nontrivial values of the Z₂ topological invariant of the corresponding 1D band Hamiltonian characteristic of the D symmetry class. It is demonstrated that such features in the behavior of the open nanowire are retained even in the presence of Coulomb interactions.

A graphene-based superlattice formed owing to the periodic modulation of the Fermi velocity is considered. Such a modulation is possible in graphene deposited on a strip substrate of materials with significantly different static dielectric constants. The dispersion relation for plasmons has been derived for this system in the case where the Fermi level lies in the low miniband. The problem of absorption of modulated external electromagnetic radiation because of the excitation of plasmons has been discussed.

The reduction of the area of the cross section of a spin-valve-like structure to a nanoscale is an important problem of modern spin electronics. However, the transverse quantization of electronic states in the spin valve, which forms a magnetic nanobridge at this scale, additionally affects not only the magnetoresistance but also the spin-transfer torques. In this work, features of the quantization of the magnetoresistance and spin-angular momentum associated with the spin transfer in a Co/Au/Co metallic nanobridge with metallic contacts have been theoretically analyzed. It has been shown that these features are manifested in oscillations of the microwave sensitivity of a spin-torque diode based on the spin-valve structure mentioned above.