Wave packets in graphene under external fields: Vortices formation

In this work we analyzed the time propagation of wave packets on a sheet of graphene under the action of external magnetic and electric fields in the Hall configuration. The treatment given in this work to the problem of particle propagation in graphene is based on the tight-binding model, not requiring to consider the linear approximation of the band structure around point K in the Brillouin zone. So, our calculation is able to describe the behavior of the particle in more general cases, not only the case of low lying excited states, the so-called massless Dirac electrons. Evaluating the time evolution of the wave function we assume as an initial state a Gaussian with a given velocity. We have considered the symmetric gauge for the vector potential. For specific cases one is able to show a very interesting effect such as the apparition of vortices, i.e., the initial wave is split into components each one of these forming vortices that remain stationaries as time goes. Moreover, for a packet with a wave vector near point K in the Brillouin zone, one is able to show the presence of the effect of zitterbewegung, that is, a trembling motion of the centroid of the wave packet. The inclusion of a dc electric field in the plane of the graphene lattice displaces the vortices in a direction perpendicular to the field.     

Polarization of waves in a warm magnetoplasma which propagate nearly perpendicular to the magnetic field

We compute the polarization of the waves which propagate with a wave vectork nearly perpendicular to the confining magnetic field. We show a qualitative alteration with respect to the perpendicular modes.     

Excitation of an upper hybrid wave by two intense laser beams and particle acceleration

This Letter presents an investigation of the excitation of an upper hybrid wave (UHW) by cross focusing of two intense laser beams in a collisionless hot magnetoplasma, when relativistic and ponderomotive nonlinearities are operative. The electric vectors of the two beams are polarized along uniform static magnetic field and the beams propagate perpendicular to the static magnetic field. Analytical expressions for the beam width of the laser beams, electric vector and power of the excited UHW and energy gain have been obtained. The UHW generation at the difference frequency and particle acceleration has also been studied. The nonlinear coupling between intense laser beams and UHW is so strong that UHW gets excited and a large fraction of the laser beam energy gets transferred to UHW and this UHW accelerates electrons. It has been shown that the presence of a magnetic field affects significantly the power of the UHW and energy gain by the electron in the presence of the UHW.     

Dirac particle with anomalous magnetic moment in a plane-polarized wave and stationary transverse magnetic and electric fields

A determination is made of the wave function of an uncharged Dirac particle with an anomalous magnetic moment in a plane-polarized wave and stationary magnetic and electric fields. The stationary magnetic field is parallel to the magnetic field of the wave and perpendicular to the stationary electric field. Institute of Chemical Physics and Combustion. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 66–69, May, 1998.     

Deformation of nematic layers in crossed electric and magnetic fields

A parallel nematic layer will show a Fréedericksz-transition in an electric field when the voltage exceeds a threshold value U_c. If a magnetic field is applied parallel to the layer but perpendicular to the molecular orientation the threshold voltage U_c increases with increasing magnetic field field H above a threshold field H_c2. From a plot of U_c vs H all three elastic constants can be obtained. Theoretical results are compared with data of HBAB.     

Horizontal rolls in convective flow above a partially heated surface

Considering the nonlinearity arising from the interaction between electrons and lattice vibrations, an effective electronic model with a self-interaction cubic term is employed to study the interplay between electron-electron and electron-phonon interactions. Based on numerical solutions of the time-dependent nonlinear Schroedinger equation for an initially localized two-electron singlet state, we show that the magnitude of the electron-phonon coupling χ necessary to promote the self-trapping of the electronic wave packet decreases as a function of the electron-electron interaction U. We show that such dependence is directly linked to the narrowing of the band of bounded two-electron states as U increases. We obtain the transition line in the χ × U parameter space separating the phases of self-trapped and delocalized electronic wave packets. The present results indicates that nonlinear contributions plays a relevant role in the electronic wave packet dynamics, particularly in the regime of strongly correlated electrons.     

Formation of convective cells by modulational instability of drift Alfvén waves

A model equation describing drift Alfvén wave with E × B nonlinearity is derived. For a special ordering a nonlinear Schrödinger equation is derived, which governs modulational instability of the drift Alfvén wave. Translational invariance is assumed along the magnetic field. The relation between the characteristic scale lengths parallel and perpendicular to the drift flow for the onset of cell formation has been found. The influence of perpendicular ion viscosity is also discussed.     

Self-trapping of interacting electrons in crystalline nonlinear chains

Considering the nonlinearity arising from the interaction between electrons and latticevibrations, an effective electronic model with a self-interaction cubic term is employedto study the interplay between electron-electron and electron-phonon interactions. Basedon numerical solutions of the time-dependent nonlinear Schroedinger equation for aninitially localized two-electron singlet state, we show that the magnitude of theelectron-phonon coupling χ necessary to promote the self-trapping of theelectronic wave packet decreases as a function of the electron-electron interactionU. We show that such dependence is directly linked to the narrowing ofthe band of bounded two-electron states as U increases. We obtain thetransition line in the χ × U parameter space separatingthe phases of self-trapped and delocalized electronic wave packets. The present resultsindicates that nonlinear contributions plays a relevant role in the electronic wave packetdynamics, particularly in the regime of strongly correlated electrons.     

Semiclassical dynamics of vortices in 2D easy-plane ferromagnets

Semiclassical dynamics of magnetic vortices in 2D lattice models of easy-plane ferromagnets is investigated. It is shown that the low-energy part of the spectrum of vortices treated as quantum excitations of the system exhibits a nontrivial structure. The simplest spectrum is observed for standard magnetic vortices, in which magnetization at long distances from the center of a vortex is parallel to the basal plane. In this case, the spectrum has a band structure consisting of several nonintersecting bands, whose number is determined only by the value of atomic spin S and lattice symmetry. For purely 2D magnets with a single spin per unit cell, the number of bands is S or 2S for integral and half-integral values of spin S, respectively. For a lattice with the basis with an even number 2n of spins per unit cell, the number of bands is 2nS for any spins. The situation radically changes for vortices in the cone state as compared to standard vortices, for which the magnetization at a long distance from the center of a vortex rotates in the easy plane of the magnet. Vortices in the cone state are formed under the action of a constant external field perpendicular to the easy plane of the magnet. As a rule, the spectrum for such vortices is not a standard band spectrum and forms a set such that a forbidden energy value can be found in any small neighborhood of an allowed value, and vice versa. The possibility of an oscillatory motion of a vortex under the action of a constant external force is indicated (analog of Bloch oscillations of electrons in crystals). Possible realization of these effects in other ordered media with vortices is considered.     

Thickness-shear modes and magnetoelastic waves in a longitudinally magnetized ferromagnetic plate

Magnetoelastic (ME) waves and thickness-shear modes in the ferromagnetic plate are studied. Coupled vibrations of magnetization and shear elastic deformations excited simultaneously by a variable magnetic field propagate in two mutually perpendicular directions: parallel and normal to a surface. For parameters characteristic of isotropic ferromagnet with the sample magnetization and Zeeman field parallel to the surface, resonant frequencies of shear modes are computed and their dispersion law is examined. It is shown that the dependence of dimensional resonances frequencies on wave number k_z of ME wave propagating along saturating field direction occurs. The possibility of excitation of ME waves with different k_z explains multimode character of thickness ME resonances.     

A microscopic semiclassical confining field equation forU(1) lattice gauge theory in 2+1 dimensions

We present a semiclassical nonlinear field equation for the confining field in 2+1-dimensionalU(1) lattice gauge theory (compact QED). The equation is derived directly from the underlying microscopic quantum Hamiltonian by means of truncation. Its nonlinearities express the dynamic creation of magnetic monopole currents leading to the confinement of the electric field between two static electric charges. We solve the equation numerically and show that it can be interpreted as a London relation in a dual superconductor.     

Enhancement of the intrinsic pinning by a magnetic field in a single crystal of high-temperature superconductor TmBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

This paper presents a comparative study of the complex permeability μ* of single crystals of hightemperature superconductors RBa₂Cu₃Oy (R = Y, Tm) as a function of the magnetic field applied along the crystallographic ab-plane. Contributions to μ* from the oscillatory motion of vortices perpendicular to layers of the crystal lattice, μ_v, and realization of the critical state along the layers are obtained. It is found that, as the temperature approaches T _c, the behavior of the field dependences of μ_v and the critical current is substantially different for the given samples. This effect is related to the manifestation of an additional unusual mechanism of intrinsic pinning, which arises when Y₃₊ is replaced with the Tm³⁺ magnetic ion. The revealed specificities of the interaction of vortices with the magnetic ion layer suggest that, most probably, they have a magnetic nature and are not related to the variation in the condensation energy in the core of the vortex.     

Vortex deconfinement in the XY model with a magnetic field

We study vortex unbinding for the classical two-dimensional XY model in a magnetic field on square and triangular lattices. A renormalization group analysis combined with duality in the model shows that at high temperature and high field, the vortices unbind as the magnetic field is lowered in a two-step process: strings of overturned spins first proliferate and then vortices unbind. The transitions are highly continuous but are not of the Kosterlitz-Thouless type. The unbound vortex fixed point is shown to inherit properties of the underlying lattice, in particular containing a set of nodal lines that reflect the lattice symmetry.     

Scattering polarization in the presence of magnetic and electric fields

The polarization of radiation by scattering on an atom embedded in combined external quadrupole electric and uniform magnetic fields is studied theoretically. Limiting cases of scattering under Zeeman effect, and Hanle effect in weak magnetic fields are discussed. The theory is general enough to handle scattering in intermediate magnetic fields (Hanle-Zeeman effect) and for arbitrary orientation of magnetic field. The quadrupolar electric field produces asymmetric line shifts, and causes interesting level-crossing phenomena either in the absence of an ambient magnetic field, or in its presence. It is shown that the quadrupolar electric field produces an additional depolarization in the Q/I profiles and rotation of the plane of polarization in the U/I profile over and above that arising from magnetic field itself. This characteristic may have a diagnostic potential to detect steady-state and time-varying electric fields that surround radiating atoms in solar atmospheric layers.     

Cyclotron resonance study of conduction electrons in n-type indium antimonide under a strong magnetic field—II: Hot electrons

A system of hot electrons in the n-InSb under the application of a strong magnetic field has been studied by far IR cyclotron resonance. A three band model and an energy independent scattering time were assumed in analyzing the line shape variation with electric field applied either parallel or perpendicular to the magnetic field. Two kinds of electron temperature, inter- and intra-subband, were introduced to describe the electron distribution in energy space. The electron distribution function was found to deviate from an essentially Maxwellian form in the manner predicted by Yamada and Kurosawa. A remarkable difference exists between the two geometries: E∥H and E⊥U. A brief survey of cyclotron emission, and the reverse process of hot electron cyclotron absorption, is summarized at the end as an addendum.     

Flux flow in current driven mesoscopic superconductors: size effects

Flux-flow phenomena in a superconducting mesoscopic stripe submitted to an appliedcurrent and external magnetic field is studied. The time-dependent Ginzburg-Landauequations are solved numerically to obtain the electric and magnetic response of thesystem. It is shown that the I-V curves, for the wider strips, present a universalbehaviour. The dependence of the flux-flow resistivity on the magnetic field and widthallow us to propose a criterion characterizing, both, the macroscopic and mesoscopicregimes. The power spectrum of the average voltage permits identifying the effect ofsurface currents in vortices movement. Based on the maximum value of the power spectrumfirst harmonic we propose a geometric condition for matching between the sample dimensionsand the vortex lattice parameter.     

Hyperfine interactions and electronic structures of BaFe 2 As 2 : a first-principles LDA+U study

The hyperfine parameters of hyperfine fields, electric field gradients and isomer shifts at the Fe site are investigated based on the first-principles calculations of the electronic structures using LDA (GGA)+U method in the low-temperature orthorhombic antiferromagnetic phase of undoped BaFe₂As₂. It is fond that the electric field gradient of Fe nucleus is highly related with the electronic structures close to the Fermi level. Though the addition of negative on-site Coulomb interaction to Fe-3d states improves the calculated magnetic moment of Fe atom and the hyperfine parameters of Fe nucleus when U = ?0.1 Ry (?0.08 Ry) for GGA+U (LDA+U) method, a negative U correction does not capture the right physics of this system. The calculations prove the strong coupling between the magnetic, structural and electronic properties in antiferromagnetic BaFe₂As₂ parent.     

Semiclassical dynamics of electron wave packet states with phase vortices

We consider semiclassical higher-order wave packet solutions of the Schr?dinger equation with phase vortices. The vortex line is aligned with the propagation direction, and the wave packet carries a well-defined orbital angular momentum (OAM) variant Planck's over 2pil (l is the vortex strength) along its main linear momentum. The probability current coils around the momentum in such OAM states of electrons. In an electric field, these states evolve like massless particles with spin l. The magnetic-monopole Berry curvature appears in momentum space, which results in a spin-orbit-type interaction and a Berry/Magnus transverse force acting on the wave packet. This brings about the OAM Hall effect. In a magnetic field, there is a Zeeman interaction, which, can lead to more complicated dynamics.     

Vector boson in the constant electromagnetic field

The propagator and the complete sets of in-and out-solutions of the wave equation, together with the Bogoliubov coefficients relating these solutions are obtained for the vector W-boson (with the gyromagnetic ratio g=2) in a constant electromagnetic field. When only the electric field is present, the Bogoliubov coefficients are independent of the boson polarization and are the same as for the scalar boson. For the collinear electric and magnetic fields, the Bogoliubov coefficients for states with the boson spin perpendicular to the field are again the same as in the scalar case. For the W ^? spin parallel (antiparallel) to the magnetic field, the Bogoliubov coefficients and the one-loop contributions to the imaginary part of the Lagrange function are obtained from the corresponding expressions for the scalar case by the substitution m ² → m ²+2eH (m ² → m ²-2eH). For the gyromagnetic ratio g=2, the vector boson interaction with the constant electromagnetic field is described by the functions that can be expected by comparing the scalar and Dirac particle wave functions in the constant electromagnetic field.