Few-electron eigenstates of concentric double quantum rings

Few-electron eigenstates confined in coupled concentric double quantum rings are studied by the exact diagonalization technique. We show that the magnetic field suppresses the tunnel coupling between the rings localizing the single-electron states in the internal ring, and the few-electron states in the external ring. The magnetic fields inducing the ground-state angular momentum transitions are determined by the distribution of the electron charge between the rings. The charge redistribution is translated into modifications of the fractional Aharonov-Bohm period. We demonstrate that the electron distribution can be deduced from the cusp pattern of the chemical potentials governing the single-electron charging properties of the system. The evolution of the electron-electron correlations to the high field limit of a classical Wigner molecule is discussed.Comment: to appear in Physical Review

Paramagnetic adsorbates on graphene: a charge transfer analysis

We introduce a modified version of the Hirshfeld charge analysis method and demonstrate its accurateness by calculating the charge transfer between the paramagnetic molecule NO2 and graphene. The charge transfer between paramagnetic molecules and a graphene layer as calculated with ab initio methods can crucially depend on the size of the supercell used in the calculation. This has important consequences for adsorption studies involving paramagnetic molecules such as NO2 physisorbed on graphene or on carbon nanotubes.Comment: 4 pages, 4 figures, submitted to Applied Physics Letter

Double quantum dots defined in bilayer graphene

Artificial molecular states of double quantum dots defined in bilayer graphene are studied with the atomistic tight-binding and its low-energy continuum approximation. We indicate that the extended electron wave functions have opposite parities on each of the sublattices at both graphene layers and that the ground-state wave function components change from bonding to antibonding with the interdot distance. In the weak coupling limit -- the most relevant for the quantum dots defined electrostatically -- the signatures of the interdot coupling include -- for the two-electron ground state -- formation of states with symmetric or antisymmetric spatial wave functions split by the exchange energy. In the high energy part of the spectrum the states with both electrons in the same dot are found with the splitting of energy levels corresponding to simultaneous tunneling of the electron pair from one dot to the other

Saddle point states and energy barriers for vortex entrance and exit in superconducting disks and rings

The transitions between the different vortex states of thin mesoscopic superconducting disks and rings are studied using the non-linear Ginzburg-Landau functional. They are saddle points of the free energy representing the energy barrier which has to be overcome for transition between the different vortex states. In small superconducting disks and rings the saddle point state between two giant vortex states, and in larger systems the saddle point state between a multivortex state and a giant vortex state and between two multivortex states is obtained. The shape and the height of the nucleation barrier is investigated for different disk and ring configurations.Comment: 10 pages, 18 figure

Plasmons and their interaction with electrons in trilayer graphene

The interaction between electrons and plasmons in trilayer graphene is investigated within the Overhauser approach resulting in the 'plasmaron' quasi-particle. This interaction is cast into a field theoretical problem, nd its effect on the energy spectrum is calculated using improved Wigner-Brillouin perturbation theory. The plasmaron spectrum is shifted with respect to the bare electron spectrum by $\Delta E(\mathbf{k})\sim 50\div200\,{\rm meV}$ for ABC stacked trilayer graphene and for ABA trilayer graphene by $\Delta E(\mathbf{k})\sim 30\div150\,{\rm meV}$ ($\Delta E(\mathbf{k})\sim 1\div5\,{\rm meV}$) for the hyperbolic linear) part of the spectrum. The shift in general increases with the electron concentration $n_{e}$ and electron momentum. The dispersion of plasmarons is more pronounced in \textit{ABC} stacked than in ABA tacked trilayer graphene, because of the different energy band structure and their different plasmon dispersion.Comment: arXiv admin note: substantial text overlap with arXiv:1310.623

Wigner crystallization in transition metal dichalcogenides: A new approach to correlation energy

We introduce a new approach for the correlation energy of one- and two-valley two-dimensional electron gas (2DEG) systems. Our approach is based on a random phase approximation at high densities and a classical approach at low densities, with interpolation between the two limits. This approach gives excellent agreement with available Quantum Monte Carlo (QMC) calculations. We employ the two-valley 2DEG model to describe the electron correlations in monolayer transition metal dichalcogenides (TMDs). The zero-temperature transition from a Fermi liquid to a quantum Wigner crystal phase in monolayer TMDs is obtained using density-functional theory within the local-density approximation. Consistent with QMC, we find that electrons crystallize at $r_s=30.5$ in one-valley 2DEG. For two-valleys, we predict Wigner crystallization at $r_s= 29.5$, indicating that valley degeneracy has little effect on the critical $r_s$, in contrast to an earlier claim.Comment: 5 pages, 3 figure

Klein paradox for a pn junction in multilayer graphene

Charge carriers in single and multilayered graphene systems behave as chiral particles due to the particular lattice symmetry of the crystal. We show that the interplay between the meta-material properties of graphene multilayers and the pseudospinorial properties of the charge carriers result in the occurrence of Klein and anti-Klein tunneling for rhombohedral stacked multilayers. We derive an algebraic formula predicting the angles at which these phenomena occur and support this with numerical calculations for systems up to four layers. We present a decomposition of an arbitrarily stacked multilayer into pseudospin doublets that have the same properties as rhombohedral systems with a lower number of layers.Comment: 5 pages, 4 figure