An analysis of electron transport in graphene in the presence of various arrangements of delta-function like magnetic barriers is presented. The motion through one such barrier gives an unusual non-specular refraction leading to asymmetric transmission. The symmetry is restored by putting two such barriers in opposite directions side by side. Periodic arrangements of such barriers can be used as Bragg reflectors whose reflectivity has been calculated using a transfer matrix formalism. Such Bragg reflectors can be used to make resonant cavities. We also analyze the associated band structure for the case of infinite periodic structures.
Transport of massless Dirac fermions in graphene monolayers is analysed in the presence of a combination of singular magnetic barriers and applied electrostatic potential. Extending a recently proposed (Ghosh and Sharma 2009 J. Phys.: Condens. Matter 21 292204) analogy between the transmission of light through a medium with modulated refractive index and electron transmission in graphene through singular magnetic barriers to the present case, we find the addition of a scalar potential profoundly changes the transmission. We calculate the quantum version of the Goos-Hänchen shift that the electron wave suffers upon being totally reflected by such barriers. The combined electric and magnetic barriers substantially modify the band structure near the Dirac point. This affects transport near the Dirac point significantly and has important consequences for graphene-based electronics.
Magnetic dipole-dipole interaction dominated Bose-Einstein condensates are discussed under spinful situations. We treat the spin degrees of freedom as a classical spin vector, approaching from large spin limit to obtain an effective minimal Hamiltonian; a version extended from a non-linear sigma model. By solving the Gross-Pitaevskii equation we find several novel spin textures where the mass density and spin density are strongly coupled, depending upon trap geometries due to the long-range and anisotropic natures of the dipole-dipole interaction.PACS numbers: 03.75. Mn, 03.75.Hh, 67.57.Fg Bose-Einstein condensates (BEC) with internal degrees of freedom, the so-called spinor BEC have attract much attention experimentally and theoretically in recent years [1]. Spinor BEC opens up a new paradigm where the order parameter of condensates is described by a multi-component vector [2,3]. This can be possible by optically trapping cold atoms where all hyperfine states are liberated, while magnetic trapping freezes its freedom. So far 23 Na (the hyperfine state F = 1), and 87 Rb (F = 2) are extensively investigated.Griesmaier et al. [4] have recently succeeded in achieving BEC of 52 Cr atom gases whose magnetic moment per atom is 3 µ B (Bohr magneton). There has been already emerging [5] several novel aspects associated with larger magnetic moment in 52 Cr atom even in this magnetic trapping, where all spin moments are polarized along an external magnetic field. Namely the magnetic dipoledipole (d-d) interaction, which is proportional to F 2 is expected to play an important role in a larger spin atom.It is natural to expect realization of BEC with still larger spin atomic species under the spinful situations by optical trapping or control the d-d interaction via the Feshbach resonance relative to other interaction channels. There has already been existing a large amount of theoretical studies for dipolar BEC [6]. Most of them treat the polarized case where the dipolar moments are aligned along an external field. The intrinsic anisotropic or tensorial nature of the d-d interaction relative to the polarization axis manifests itself in various properties. The head-to-tail moment arrangement due to the d-d interaction is susceptible to a shape instability by concentrating atoms in the central region. We have seen already that tensorial and long-ranged d-d interaction is responsible for this kind of shape dependent phenomenon where the mass density is constrained by the polarization axis.In contrast the theoretical studies of the spinor dipolar BEC are scarce, and just started with several impressive works [7,8,9,10]. They consider either the F = 1 spinor BEC by taking into account the d-d interaction or F = 3 for 52 Cr atom gases in a realistic situation. Here one must handle a 7-component spinor with 5 different interaction channels g 0 , g 2 , g 4 , g 6 , and g d . The parameter space to hunt is large and difficult enough to find a stable configuration. The situation becomes further hard towards a larger F where the d-d...
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