Magnetoelectric barriers in monolayer graphene: Red and blue shifts of the low energy conductance peaks and its relation to the spectrum of bound states
We investigate the possible formation of a molecular condensate, which might be, for instance, the analogue of the alpha condensate of nuclear physics, in the context of multicomponent cold atoms fermionic systems. A simple paradigmatic model of N-component fermions with contact interactions loaded into a one-dimensional optical lattice is studied by means of low-energy and numerical approaches. For attractive interaction, a quasi-long-range molecular superfluid phase, formed from bound-states made of N fermions, emerges at low density. We show that trionic and quartetting phases, respectively for N = 3,4, extend in a large domain of the phase diagram and are robust against small symmetry-breaking perturbations.
The microscopic mechanism of charge instabilities and the formation of inhomogeneous states in systems with strong electron correlations is investigated. We demonstrate that within a strong coupling expansion the single-band Hubbard model shows an instability towards phase separation and extend the approach also for an analysis of phase separation in the Hubbard-Kanamori hamiltonian as a prototypical multiband model. We study the pairing fluctuations on top of an inhomogeneous stripe state where superconducting correlations in the extended s-wave and d-wave channels correspond to (anti)bound states in the two-particle spectra. Whereas extended s-wave fluctuations are relevant on the scale of the local interaction parameter U, we find that d-wave fluctuations are pronounced in the energy range of the active subband which crosses the Fermi level. As a result, low energy spin and charge fluctuations can transfer the d-wave correlations from the bound states to the low energy quasiparticle bands. Our investigations therefore help to understand the coexistence of stripe correlations and d-wave superconductivity in cuprates.
A heteroepitaxial system of h-BN/monolayer graphene on Ni(111) has been investigated by means of vibrational spectroscopy, accompanied by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The system was prepared in an epitaxial manner on a Ni(111) surface by chemical vapor deposition (CVD). We found that phonon peaks in observed spectra showed typical features of Fuchs–Kliewer (FK) phonons. The vibrational spectra of h-BN films ranging in thickness from 0 to 8.7 monolayers have been compared with theoretical spectra based on a dielectric theory. Detailed analysis has revealed new types of phonons, of which the vibrational amplitudes are localized at edges of h-BN nanocrystals. In addition, we have observed subsidence phenomena of the graphene and h-BN layers into Ni substrate at elevated temperatures.
We define a family of non local and chirally symmetric low energy lagrangians motivated by theoretical studies on Quantum Chromodynamics. These models lead to quark propagators with non trivial momentum dependencies. We define the formalism for two body bound states and apply it to the pion. We study the coupling of the photon and W bosons with special attention to the implementation of local gauge invariance. We calculate the pion decay constant recovering the Goldberger-Treiman and the Gell-Mann-Oakes-Renner relations. We recover a form of the axial current consistent with PCAC. Finally we study the pion form factor and we construct the operators involved in its parton distribution.
Using the massless Dirac–Weyl model for monolayer graphene sheet, we study the low-lying spectra of a single Dirac electron system bound to an on-center positively charged Coulomb impurity, under both electrostatic potential and magnetic field. Numerical results obtained from diagonalization show that, the increase of the electrostatic potential causes the low-lying states to evolve from one Landau-type plateau to the higher ones, and the whole spectra exhibit similar features with slight shifts in eigenenergies when the on-center impurity is considered. Electrostatic-potential dependent optical spectra with their corresponding absorption coefficients as functions of incident photon energies for transitions between low-lying states are presented.
Robust low-energy Andreev bound states in semiconductor-superconductor structures: Importance of partial separation of component Majorana bound states