Late-time evolution of massive Dirac fields in the Kerr background

The late-time tail of massive Dirac fields in Kerr spacetime is investigated by using the black hole Green function. It is shown that in the intermediate late times there are two kinds of new properties. The one is that the asymptotic behaviour of the massive Dirac fields is dominated by a decaying tail without any oscillation, which is different from the oscillatory decaying tails of the massive scalar field; the other is that the dumping exponent for the massive Dirac field depends not only on the multiple number of the wave mode and the mass of the Dirac particle but also on the rotating parameter of the black hole.

Quasilocal Energy for Static Charged Black Holes in String Theory

The Brown-York quasilocal energies of some static charged dilaton black holes are calculated, and then the validity of Martinez's conjecture is explored in string theory. It is shown that the energy is positive and monotonically decreases to the ADM mass at spatial infinity, and the conjecture that the Brown-York quasilocal energy at the outer horizon of black hole reduces to twice of its irreducible mass is still applicable for the static charged black holes in string theory. The result is different from Bose-Naing's one.

Thermodynamics of a Schwarzschild Black Hole With a Cosmic String or a Global Monopole

It is well known that thermodynamics of the black hole whose space-time isasymptotically flat obeys the four laws proposed by Bardeen, Carter and Hawking.But for the Schwarzschild type black hole whose space-time is no longerasymptotically flat, whether the Bardeen-Carter-Hawking laws are valid is worthstudying. We all know that certain gauge theories allow possibility of topological de-

Displacement Current in Domain Walls of Bismuth Ferrite

In 1861,Maxwell conceived the idea of the displacement current,which then made laws of electrodynamics more complete and also resulted in the realization of devices exploiting such displacement current.Interestingly,it is presently unknown if such displacement current can result in large intrinsic ac current in ferroic systems possessing domains,despite the flurry of recent activities that have been devoted to domains and their corresponding conductivity in these compounds.Here,we report firstprinciples-based atomistic simulations that predict that the transverse(polarization-related)displacement currents of 71°and 109°domains in the prototypical BiFeO_(3) multiferroic material are significant at the walls of such domains and in the GHz regime,and,in fact,result in currents that are at least of the same order of magnitude than previously reported dc currents(that are likely extrinsic in nature and due to electrons).Such large,localized and intrinsic ac currents are found to originate from low-frequency vibrations at the domain walls,and may open the door to the design of novel devices functioning in the GHz or THz range and in which currents would be confined within the domain wall.

Dynamics of antipolar distortions

Materials possessing antipolar cation motions are currently receiving a lot of attention because they are fundamentally intriguing while being technologically promising.Most studies devoted to these complex materials have focused on their static properties or on their zone-center phonons.As a result,some important dynamics of antipolar cation distortions,such as the temperature behavior of their phonon frequencies,have been much less investigated,despite the possibility to exhibit unusual features.Here,we report the results and analysis of atomistic simulations revealing and explaining such dynamics for BiFeO3 bulks being subject to hydrostatic pressure.It is first predicted that cooling such material yields the following phase transition sequence:the cubic paraelectric Pm3m state at high temperature,followed by an intermediate phase possessing long-range-ordered in-phase oxygen octahedral tiltings,and then the Pnma state that is known to possess antipolar cation motions in addition to in-phase and antiphase oxygen octahedral tiltings.Antipolar cation modes are found to all have high phonon frequencies that are independent of temperature in the paraelectric phase.On the other hand and in addition to antipolar cation modes increasing in number,some phonons possessing antipolar cation character are rather soft in the intermediate and Pnma states.Analysis of our data combined with the development of a simple model reveals that such features originate from a dynamical mixing between pure antipolar cation phonons and fluctuations of oxygen octahedral tiltings,as a result of a specific trilinear energetic coupling.The developed model can also be easily applied to predict dynamics of antipolar cation motions for other possible structural paths bringing Pm3m to Pnma states.

Strain control of layer-resolved negative capacitance in superlattices

Negative capacitance in BaTiO_(3)/SrTiO_(3) superlattices is investigated by Monte Carlo simulations in an atomistic effective Hamiltonian model,using fluctuation formulas for responses to the local macroscopic field that incorporates depolarizing fields.We show that epitaxial strain can tune the negative capacitance of the BaTiO_(3) ferroelectric layer and the overall capacitance of the system.In addition,we predict and explain an original switching of the negative capacitance from the BaTiO3 layer to the SrTiO_(3) layer at low temperatures for intermediate strains.

Strain-induced resonances in the dynamical quadratic magnetoelectric response of multiferroics

For the last few years,the research interest in magnetoelectric(ME)effect,which is the cross-coupling between ferroelectric and magnetic ordering in multiferroic materials,has experienced a significant revival.The extensive recent studies are not only conducted towards the design of sensors,actuators,transducers,and memory devices by taking advantage of the cross-control of polarization(or magnetization)by magnetic(or electric)fields,but also aim to create a clearer picture in understanding the sources of ME responses and the novel effects associated with them.Here we derive analytical models allowing to understand the striking and novel dynamics of ME effects in multiferroics and further confirm it with atomistic simulations.Specifically,the role of strain is revealed to lead to the existence of electroacoustic magnons,a new quasiparticle that mixes acoustic and optical phonons with magnons,which results in resonances and thus a dramatic enhancement of magnetoelectric responses.Moreover,a unique aspect of the dynamical quadratic ME response under a magnetic field with varying frequencies,which is the second harmonic generation(SHG),has not been discussed prior to the present work.These SHGs put emphasis on the fact that nonlinearities should be considered while dealing with such systems.