Nonfrustrated magnetoelectric with incommensurate magnetic order in magnetic field

We discuss a model nonfrustrated magnetoelectric in which strong enough magnetoelectric coupling produces incommensurate magnetic order leading to ferroelectricity. Properties of the magnetoelectric in magnetic field directed perpendicular to wave vector describing the spin helix are considered in detail. Analysis of classical energy shows that in contrast to naive expectation the onset of ferroelectricity takes place at a field $H_{c1}$ that is lower than the saturation field $H_{c2}$. One has $H_{c1}=H_{c2}$ at strong enough magnetoelectric coupling. We show that at H=0 the ferroelectricity appears at $T=T_{FE}<T_N$. Qualitative discussion of phase diagram in $H-T$ plane is presented within mean field approach.Comment: 12 pages, 3 figures, accepted in JET

Influence of strain and oxygen vacancies on the magnetoelectric properties of multiferroic bismuth ferrite

The dependencies on strain and oxygen vacancies of the ferroelectric polarization and the weak ferromagnetic magnetization in the multiferroic material bismuth ferrite, BiFeO_3, are investigated using first principles density functional theory calculations. The electric polarization is found to be rather independent of strain, in striking contrast to most conventional perovskite ferroelectrics. It is also not significantly affected by oxygen vacancies, or by the combined presence of strain and oxygen vacancies. The magnetization is also unaffected by strain, however the incorporation of oxygen vacancies can alter the magnetization slightly, and also leads to the formation of Fe^{2+}. These results are discussed in light of recent experiments on epitaxial films of BiFeO_3 which reported a strong thickness dependence of both magnetization and polarization.Comment: 9 pages, 3 figure

The Hyperfine Molecular Hubbard Hamiltonian

An ultracold gas of heteronuclear alkali dimer molecules with hyperfine structure loaded into a one-dimensional optical lattice is investigated. The \emph{Hyperfine Molecular Hubbard Hamiltonian} (HMHH), an effective low-energy lattice Hamiltonian, is derived from first principles. The large permanent electric dipole moment of these molecules gives rise to long range dipole-dipole forces in a DC electric field and allows for transitions between rotational states in an AC microwave field. Additionally, a strong magnetic field can be used to control the hyperfine degrees of freedom independently of the rotational degrees of freedom. By tuning the angle between the DC electric and magnetic fields and the strength of the AC field it is possible to control the number of internal states involved in the dynamics as well as the degree of correlation between the spatial and internal degrees of freedom. The HMHH's unique features have direct experimental consequences such as quantum dephasing, tunable complexity, and the dependence of the phase diagram on the molecular state

Image translation and rotation on hexagonal structure

Image translation and rotation are becoming essential operations in many application areas such as image processing, computer graphics and pattern recognition. Conventional translation moves image from pixels to pixels and conventional rotation usually comprises of computation-intensive CORDIC operations. Traditionally, images are represented on a square pixel structure. In this paper, we perform reversible and fast image translation and rotation based on a hexagonal structure. An image represented on the hexagonal structure is a collection of hexagonal pixels of equal size. The hexagonal structure provides a more flexible and efficient way to perform image translation and rotation without losing image information. As there is not yet any available hardware for capturing image and for displaying image on a hexagonal structure, we apply a newly developed virtual hexagonal structure. The virtual hexagonal structure retains image resolution during the process of image transformations, and almost does not introduce distortion. Furthermore, images can be smoothly and easily transferred between the traditional square structure and the hexagonal structure. © 2006 IEEE

Quasi-particle scattering and protected nature of topological states in a parent topological insulator Bi2_2Se3_3

We report on angle resolved photoemission spectroscopic studies on a parent topological insulator (TI), Bi$_2$Se$_3$. The line width of the spectral function (inverse of the quasi-particle lifetime) of the topological metallic (TM) states shows an anomalous behavior. This behavior can be reasonably accounted for by assuming decay of the quasi-particles predominantly into bulk electronic states through electron-electron interaction and defect scattering. Studies on aged surfaces reveal that topological metallic states are very much unaffected by the potentials created by adsorbed atoms or molecules on the surface, indicating that topological states could be indeed protected against weak perturbations.Comment: accepted for publication in Phys. Rev. B(R

Energy exchange during stimulated Raman scattering of a relativistic laser in a plasma

Energy exchange between pump and daughter waves during the stimulated Raman scattering process in a plasma is investigated, including the effect of a damping coefficient of electron-ion collision at different initial three-wave phases. To obey the energy and momentum conservations, the resonance conditions are satisfied at an optimal initial phase difference between the interacting waves. The amplitudes of the interacting waves exhibit behaviors such as a parametric oscillator. The variations in initial three-wave phase difference generate a phase mismatch, which enhances the rate of the amplitude variations of the interacting waves. The relativistic mass effect modifies the dispersion relations of the interacting waves, and consequently the energy exchange during the stimulated Raman scattering is affected. The collisional damping in the plasma is shown to have an important effect on the evolution of the interacting waves.open91

Basic transformations on virtual hexagonal structure

Hexagonal structure is different from the traditional square structure for image representation. The geometrical arrangement of pixels on hexagonal structure can be described in terms of a hexagonal grid. Hexagonal structure provides an easy way for image translation and rotation transformations. However, all the existing hardware for capturing image and for displaying image are produced based on square architecture. It has become a serious problem affecting the advanced research based on hexagonal structure. In this paper, we introduce a new virtual hexagonal structure. Based on this virtual structure, a more flexible and powerful image translation and rotation are performed. The virtual hexagonal structure retains image resolution during the process of image transformations, and does not introduce distortion. Furthermore, images can be smoothly and easily transferred between the traditional square structure and the hexagonal structure. © 2006 IEEE

Giant Magnetoelectric Effect in a Multiferroic Material with a High Ferroelectric Transition Temperature

We present a unique example of giant magnetoelectric effect in a conventional multiferroic HoMnO3, where polarization is very large (~56 mC/m2) and the ferroelectric transition temperature is higher than the magnetic ordering temperature by an order. We attribute the uniqueness of the giant magnetoelectric effect to the ferroelectricity induced entirely by the off-center displacement of rare earth ions with large magnetic moments. This finding suggests a new avenue to design multiferroics with large polarization and higher ferroelectric transition temperature as well as large magnetoelectric effects

Effective thermodynamics of strongly coupled qubits

Interactions between a quantum system and its environment at low temperatures can lead to violations of thermal laws for the system. The source of these violations is the entanglement between system and environment, which prevents the system from entering into a thermal state. On the other hand, for two-state systems, we show that one can define an effective temperature, placing the system into a `pseudo-thermal' state where effective thermal laws are upheld. We then numerically explore these assertions for an n-state system inspired by the spin-boson environment.Comment: 9 pages, 3 figure