Magneto-elastic interaction in cubic helimagnets with B20 structure

The magneto-elastic interaction in cubic helimagnets with B20 symmetry is considered. It is shown that this interaction is responsible for negative contribution to the square of the spin-wave gap $\Delta$ which is alone has to disrupt assumed helical structure. It is suggested that competition between positive part of $\Delta^2_I$ which stems from magnon-magnon interaction and its negative magneto-elastic part leads to the quantum phase transition observed at high pressure in $Mn Si$ and $Fe Ge$. This transition has to occur when $\Delta^2=0$. For $Mn Si$ from rough estimations at ambient pressure both parts $\Delta_I$ and $|\Delta_{ME}|$ are comparable with the experimentally observed gap. The magneto-elastic interaction is responsible also for 2\m k modulation of the lattice where \m k is the helix wave-vector and contribution to the magnetic anisotropy. Experimental observation by $x$-ray and neutron scattering the lattice modulation allows determine the strength of anisotropic part of the magneto-elastic interaction responsible for above phenomena and the lattice helicity

On structure of effective action in four-dimensional quantum dilaton supergravity

A general structure of effective action in new chiral superfield model associated with $N=1$, $D=4$ supergravity is investigated. This model corresponds to finite quantum field theory and does not demand the regularization and renormalization at effective action calculation. It is shown that in local approximation the effective action is defined by two objects called general superfield effective lagrangian and chiral superfield effective lagrangian. A proper-time method is generalized for calculation of these two effective lagrangians in superfield manner. Power expansion of the effective action in supercovariant derivatives is formulated and the lower terms of such an expansion are calculated in explicit superfield form

Quantum nanostructures in strongly spin-orbit coupled two-dimensional systems

Recent progress in experimental studies of low-dimensional systems with strong spin-orbit coupling poses a question on the effect of this coupling on the energy spectrum of electrons in semiconductor nanostructures. It is shown in the paper that this effect is profound in the strong coupling limit. In circular quantum dots a soft mode develops, in strongly elongated dots electron spin becomes protected from the effects of the environment, and the lower branch of the energy spectrum of quantum wires becomes nearly flat in a wide region of the momentum space.Comment: 5 pages, 1 figur

A Four-Dimensional Theory for Quantum Gravity with Conformal and Nonconformal Explicit Solutions

The most general version of a renormalizable $d=4$ theory corresponding to a dimensionless higher-derivative scalar field model in curved spacetime is explored. The classical action of the theory contains $12$ independent functions, which are the generalized coupling constants of the theory. We calculate the one-loop beta functions and then consider the conditions for finiteness. The set of exact solutions of power type is proven to consist of precisely three conformal and three nonconformal solutions, given by remarkably simple (albeit nontrivial) functions that we obtain explicitly. The finiteness of the conformal theory indicates the absence of a conformal anomaly in the finite sector. The stability of the finite solutions is investigated and the possibility of renormalization group flows is discussed as well as several physical applications.Comment: LaTeX, 18 pages, no figure

Photodisintegration of the deuteron in the few GeV region using asymptotic amplitudes

Exclusive photodisintegration of the deuteron in the 1-4GeV range is described in terms of a simple covariant and gauge invariant approach using an effective counting rule as the hard part of the d-np vertex. At a scattering angle of theta_cm=90 degrees a prescaling behavior of the differential cross section proportional to 1/s^(n-2) with n ~ 12 is obtained; going away from 90 degrees the value of n decreases slowly, in qualitative agreement with the recent data.Comment: 10 pages, 3 figures, to appear in Physics Letters

Sensitive Dependence on Parameters of Continuous-time Nonlinear Dynamical Systems

We would like to thank the partial support of this work by the Brazilian agencies FAPESP (processes: 2011/19296-1 and 2013/26598-0, CNPq and CAPES. MSB acknowledges EPSRC Ref. EP/I032606/1.Peer reviewedPostprin

Operator ordering and causality

It is shown that causality violations [M. de Haan, Physica 132A, 375, 397 (1985)], emerging when the conventional definition of the time-normal operator ordering [P.L.Kelley and W.H.Kleiner, Phys.Rev. 136, A316 (1964)] is taken outside the rotating wave approximation, disappear when the amended definition [L.P. and S.S., Annals of Physics, 323, 1989 (2008)] of this ordering is used.Comment: References update