Artifical atoms in interacting graphene quantum dots

We describe the theory of few Coulomb-correlated electrons in a magnetic quantum dot formed in graphene. While the corresponding nonrelativistic (Schr\"odinger) problem is well understood, a naive generalization to graphene's "relativistic" (Dirac-Weyl) spectrum encounters divergencies and is ill-defined. We employ Sucher's projection formalism to overcome these problems. Exact diagonalization results for the two-electron quantum dot, i.e., the artificial helium atom in graphene, are presented.Comment: 4+ pages, 2 figure

Delocalization of Wannier-Stark ladders by phonons: tunneling and stretched polarons

We study the coherent dynamics of a Holstein polaron in strong electric fields. A detailed analytical and numerical analysis shows that even for small hopping constant and weak electron-phonon interaction, polaron states can become delocalized if a resonance condition develops between the original Wannier-Stark states and the phonon modes, yielding both tunneling and `stretched' polarons. The unusual stretched polarons are characterized by a phonon cloud that {\em trails} the electron, instead of accompanying it. In general, our novel approach allows us to show that the polaron spectrum has a complex nearly-fractal structure, due to the coherent coupling between states in the Cayley tree which describes the relevant Hilbert space. The eigenstates of a finite ladder are analyzed in terms of the observable tunneling and optical properties of the system.Comment: 7 pages, 4 figure

Giant circular dichroism of a molecule in a region of strong plasmon resonances between two neighboring gold nanocrystals

We report on giant circular dichroism (CD) of a molecule inserted into a plasmonic hot spot. Naturally occurring molecules and biomolecules have typically CD signals in the UV range, whereas plasmonic nanocrystals exhibit strong plasmon resonances in the visible spectral interval. Therefore, excitations of chiral molecules and plasmon resonances are typically off-resonant. Nevertheless, we demonstrate theoretically that it is possible to create strongly-enhanced molecular CD utilizing the plasmons. This task is doubly challenging since it requires both creation and enhancement of the molecular CD in the visible region. We demonstrate this effect within the model which incorporates a chiral molecule and a plasmonic dimer. The associated mechanism of plasmonic CD comes from the Coulomb interaction which is greatly amplified in a plasmonic hot spot.Comment: Manuscript: 4+pages, 4 figures; Supplemental_Material: 10 pages, 7 figure

Semiconductor-metal nanoparticle molecules: hybrid excitons and non-linear Fano effect

Modern nanotechnology opens the possibility of combining nanocrystals of various materials with very different characteristics in one superstructure. The resultant superstructure may provide new physical properties not encountered in homogeneous systems. Here we study theoretically the optical properties of hybrid molecules composed of semiconductor and metal nanoparticles. Excitons and plasmons in such a hybrid molecule become strongly coupled and demonstrate novel properties. At low incident light intensity, the exciton peak in the absorption spectrum is broadened and shifted due to incoherent and coherent interactions between metal and semiconductor nanoparticles. At high light intensity, the absorption spectrum demonstrates a surprising, strongly asymmetric shape. This shape originates from the coherent inter-nanoparticle Coulomb interaction and can be viewed as a non-linear Fano effect which is quite different from the usual linear Fano resonance.Comment: 5 pages, 5 figures, submitted to Phys. Rev. Let