Magneto-mechanical interplay in spin-polarized point contacts

We investigate the interplay between magnetic and structural dynamics in ferromagnetic atomic point contacts. In particular, we look at the effect of the atomic relaxation on the energy barrier for magnetic domain wall migration and, reversely, at the effect of the magnetic state on the mechanical forces and structural relaxation. We observe changes of the barrier height due to the atomic relaxation up to 200%, suggesting a very strong coupling between the structural and the magnetic degrees of freedom. The reverse interplay is weak, i.e. the magnetic state has little effect on the structural relaxation at equilibrium or under non-equilibrium, current-carrying conditions.Comment: 5 pages, 4 figure

On possible wormhole solutions supported by non-commutative geometry within f(R,Lm)f(R, L_m) gravity

Non-commutativity is a key feature of spacetime geometry. The current article explores the traversable wormhole solutions in the framework of $f(R,L_m)$ gravity within non-commutative geometry. By using the Gaussian and Lorentzian distributions, we construct tideless wormholes for the nonlinear $f(R,L_m)$ model $f(R,L_m)=\dfrac{R}{2}+L_m^\alpha$. For both cases, we derive shape functions and discuss the required different properties with satisfying behavior. For the required wormhole properties, we develop some new constraints. The influence of the involved model parameter on energy conditions is analyzed graphically which provides a discussion about the nature of exotic matter. Further, we check the physical behavior regarding the stability of wormhole solutions through the TOV equation. An interesting feature regarding the stability of the obtained solutions via the speed of sound parameters within the scope of average pressure is discussed. Finally, we conclude our results.Comment: AOP accepted versio

Fluctuation theorems and atypical trajectories

In this work, we have studied simple models that can be solved analytically to illustrate various fluctuation theorems. These fluctuation theorems provide symmetries individually to the distributions of physical quantities like the classical work ($W_c$), thermodynamic work ($W$), total entropy ($\Delta s_{tot}$) and dissipated heat ($Q$), when the system is driven arbitrarily out of equilibrium. All these quantities can be defined for individual trajectories. We have studied the number of trajectories which exhibit behaviour unexpected at the macroscopic level. As the time of observation increases, the fraction of such atypical trajectories decreases, as expected at macroscale. Nature of distributions for the thermodynamic work and the entropy production in nonlinear models may exhibit peak (most probable value) in the atypical regime without violating the expected average behaviour. However, dissipated heat and classical work exhibit peak in the regime of typical behaviour only.Comment: 14 pages, 7 figure

Mixing quantum and classical mechanics and uniqueness of Planck's constant

Observables of quantum or classical mechanics form algebras called quantum or classical Hamilton algebras respectively (Grgin E and Petersen A (1974) {\it J Math Phys} {\bf 15} 764\cite{grginpetersen}, Sahoo D (1977) {\it Pramana} {\bf 8} 545\cite{sahoo}). We show that the tensor-product of two quantum Hamilton algebras, each characterized by a different Planck's constant is an algebra of the same type characterized by yet another Planck's constant. The algebraic structure of mixed quantum and classical systems is then analyzed by taking the limit of vanishing Planck's constant in one of the component algebras. This approach provides new insight into failures of various formalisms dealing with mixed quantum-classical systems. It shows that in the interacting mixed quantum-classical description, there can be no back-reaction of the quantum system on the classical. A natural algebraic requirement involving restriction of the tensor product of two quantum Hamilton algebras to their components proves that Planck's constant is unique.Comment: revised version accepted for publication in J.Phys.A:Math.Phy

Interplay of structure and spin-orbit strength in magnetism of metal-benzene sandwiches: from single molecules to infinite wires

Based on first-principles density functional theory calculations we explore electronic and magnetic properties of experimentally producible sandwiches and infinite wires made of repeating benzene molecules and transition-metal atoms of V, Nb, and Ta. We describe the bonding mechanism in the molecules and in particular concentrate on the origin of magnetism in these structures. We find that all the considered systems have sizable magnetic moments and ferromagnetic spin-ordering, with the single exception of the V3-Bz4 molecule. By including the spin-orbit coupling into our calculations we determine the easy and hard axes of the magnetic moment, the strength of the uniaxial magnetic anisotropy energy (MAE), relevant for the thermal stability of magnetic orientation, and the change of the electronic structure with respect to the direction of the magnetic moment, important for spin-transport properties. While for the V-based compounds the values of the MAE are only of the order of 0.05-0.5 meV per metal atom, increasing the spin-orbit strength by substituting V with heavier Nb and Ta allows to achieve an increase in anisotropy values by one to two orders of magnitude. The rigid stability of magnetism in these compounds together with the strong ferromagnetic ordering makes them attractive candidates for spin-polarized transport applications. For a Nb-benzene infinite wire the occurrence of ballistic anisotropic magnetoresistance is demonstrated.Comment: 23 pages, 8 figure

Static traversable wormhole solutions in f(R,Lm)f(R,L_m) gravity

In this study, we explore the new wormhole solutions in the framework of new modified $f(R,L_m)$ gravity. To obtain a characteristic wormhole solution, we use anisotropic matter distribution and a specific form of energy density. As second adopt the isotropic case with a linear EoS relation as a general technique for the system and discuss several physical attributes of the system under the wormhole geometry. Detailed analytical and graphical discussion about the matter contents via energy conditions is discussed. In both cases, the shape function of wormhole geometry satisfies the required conditions. Several interesting points have evolved from the entire investigation along with the features of the exotic matter within the wormhole geometry. Finally, we have concluding remarks.Comment: Chinese Journal of Physics accepted versio

Large Charge Four-Dimensional Extremal N=2 Black Holes with R^2-Terms

We consider N=2 supergravity in four dimensions with small R^2 curvature corrections. We construct large charge extremal supersymmetric and non-supersymmetric black hole solutions in all space, and analyze their thermodynamic properties.Comment: 18 pages. v2,3: minor fixe

Large Charge Four-Dimensional Non-Extremal N=2 Black Holes with R^2-Terms

We consider N=2 supergravity in four dimensions with small R^2 curvature corrections. We construct large charge non-extremal black hole solutions in all space, with either a supersymmetric or a non-supersymmetric extremal limit, and analyze their thermodynamic properties. This generalizes some of the extremal solutions presented in [arXiv:0902.0831]. The indexed entropy of the non-extremal extension of the supersymmetric black hole, has the form of the extremal entropy, with the charges replaced by a function of the charges, the moduli at infinity and the non-extremality parameter. This is the same behavior as in the case without R^2-terms.Comment: 13 pages. v2: stripped down to letter format, based on the background given in [arXiv:0902.0831]. v3: up to date with CQG versio