Spectral gap induced by structural corrugation in armchair graphene nanoribbons

We study the effects of the structural corrugation or rippling on the electronic properties of undoped armchair graphene nanoribbons (AGNR). First, reanalyzing the single corrugated graphene layer we find that the two inequivalent Dirac points (DP), move away one from the other. Otherwise, the Fermi velocity decrease by increasing rippling. Regarding the AGNRs, whose metallic behavior depends on their width, we analyze in particular the case of the zero gap band-structure AGNRs. By solving the Dirac equation with the adequate boundary condition we show that due to the shifting of the DP a gap opens in the spectra. This gap scale with the square of the rate between the high and the wavelength of the deformation. We confirm this prediction by exact numerical solution of the finite width rippled AGNR. Moreover, we find that the quantum conductance, calculated by the non equilibrium Green's function technique vanish when the gap open. The main conclusion of our results is that a conductance gap should appear for all undoped corrugated AGNR independent of their width.Comment: 7 pages, 5 figure

Tuning the polarized quantum phonon transmission in graphene nanoribbons

We propose systems that allow a tuning of the phonon transmission function T($\omega$) in graphene nanoribbons by using C$^{13}$ isotope barriers, antidot structures, and distinct boundary conditions. Phonon modes are obtained by an interatomic fifth-nearest neighbor force-constant model (5NNFCM) and T($\omega$) is calculated using the non-equilibrium Green's function formalism. We show that by imposing partial fixed boundary conditions it is possible to restrict contributions of the in-plane phonon modes to T($\omega$) at low energy. On the contrary, the transmission functions of out-of-plane phonon modes can be diminished by proper antidot or isotope arrangements. In particular, we show that a periodic array of them leads to sharp dips in the transmission function at certain frequencies $\omega_{\nu}$ which can be pre-defined as desired by controlling their relative distance and size. With this, we demonstrated that by adequate engineering it is possible to govern the magnitude of the ballistic transmission functions T$(\omega)$ in graphene nanoribbons. We discuss the implications of these results in the design of controlled thermal transport at the nanoscale as well as in the enhancement of thermo-electric features of graphene-based materials

Partially unzipped carbon nanotubes as magnetic field sensors

The conductance, $G(E)$, through graphene nanoribbons (GNR) connected to a partially unzipped carbon nanotube (CNT) is studied in the presence of an external magnetic field applied parallel to the long axis of the tube by means of non-equilibrium Green's function technique. We consider (z)igzag and (a)rmchair CNTs that are partially unzipped to form aGNR/zCNT/aGNR or zGNR/aCNT/zGNR junctions. We find that the inclusion of a longitudinal magnetic field affects the electronic states only in the CNT region, leading to the suppression of the conductance at low energies. Unlike previous studies, for the zGNR/aCNT/zGNR junction in zero field, we find a sharp dip in the conductance as the energy approaches the Dirac point and we attribute this non-trivial behavior to the peculiar band dispersion of the constituent subsystems. We demonstrate that both types of junctions can be used as magnetic field sensors.Comment: final version to appear in Applied Physics Letter

The role of atomic vacancies and boundary conditions on ballistic thermal transport in graphene nanoribbons

Quantum thermal transport in armchair and zig-zag graphene nanoribbons are investigated in the presence of single atomic vacancies and subject to different boundary conditions. We start with a full comparison of the phonon polarizations and energy dispersions as given by a fifth-nearest-neighbor force-constant model (5NNFCM) and by elasticity theory of continuum membranes (ETCM). For free-edges ribbons we discuss the behavior of an additional acoustic edge-localized flexural mode, known as fourth acoustic branch (4ZA), which has a small gap when it is obtained by the 5NNFCM. Then, we show that ribbons with supported-edges have a sample-size dependent energy gap in the phonon spectrum which is particularly large for in-plane modes. Irrespective to the calculation method and the boundary condition, the dependence of the energy gap for the low-energy optical phonon modes against the ribbon width W is found to be proportional to 1/W for in-plane, and 1/W$^2$ for out-of-plane phonon modes. Using the 5NNFCM, the ballistic thermal conductance and its contributions from every single phonon mode are then obtained by the non equilibrium Green's function technique. We found that, while edge and central localized single atomic vacancies do not affect the low-energy transmission function of in-plane phonon modes, they reduce considerably the contributions of the flexural modes. On the other hand, in-plane modes contributions are strongly dependent on the boundary conditions and at low temperatures can be highly reduced in supported-edges samples. These findings could open a route to engineer graphene based devices where it is possible to discriminate the relative contribution of polarized phonons and to tune the thermal transport on the nanoscale

Thermomechanical properties of a single hexagonal boron nitride sheet

Using atomistic simulations we investigate the thermodynamical properties of a single atomic layer of hexagonal boron nitride (h-BN). The thermal induced ripples, heat capacity, and thermal lattice expansion of large scale h-BN sheets are determined and compared to those found for graphene (GE) for temperatures up to 1000 K. By analyzing the mean square height fluctuations $< h^2>$ and the height-height correlation function $H(q)$ we found that the h-BN sheet is a less stiff material as compared to graphene. The bending rigidity of h-BN: i) is about 16% smaller than the one of GE at room temperature (300 K), and ii) increases with temperature as in GE. The difference in stiffness between h-BN and GE results in unequal responses to external uniaxial and shear stress and different buckling transitions. In contrast to a GE sheet, the buckling transition of a h-BN sheet depends strongly on the direction of the applied compression. The molar heat capacity, thermal expansion coefficient and the Gruneisen parameter are estimated to be 25.2 J\,mol$^{-1}$\,K$^{-1}$, 7.2$\times10^{-6}$K$^{-1}$ and 0.89, respectively

Anderson impurity in the one-dimensional Hubbard model on finite size systems

An Anderson impurity in a Hubbard model on chains with finite length is studied using the density-matrix renormalization group (DMRG) technique. In the first place, we analyzed how the reduction of electron density from half-filling to quarter-filling affects the Kondo resonance in the limit of Hubbard repulsion U=0. In general, a weak dependence with the electron density was found for the local density of states (LDOS) at the impurity except when the impurity, at half-filling, is close to a mixed valence regime. Next, in the central part of this paper, we studied the effects of finite Hubbard interaction on the chain at quarter-filling. Our main result is that this interaction drives the impurity into a more defined Kondo regime although accompanied in most cases by a reduction of the spectral weight of the impurity LDOS. Again, for the impurity in the mixed valence regime, we observed an interesting nonmonotonic behavior. We also concluded that the conductance, computed for a small finite bias applied to the leads, follows the behavior of the impurity LDOS, as in the case of non-interacting chains. Finally, we analyzed how the Hubbard interaction and the finite chain length affect the spin compensation cloud both at zero and at finite temperature, in this case using quantum Monte Carlo techniques.Comment: 9 pages, 9 figures, final version to be published in Phys. Rev.

Thermal properties of fluorinated graphene

Large scale atomistic simulations using the reactive force field approach (ReaxFF) are implemented to investigate the thermomechanical properties of fluorinated graphene (FG). A new set of parameters for the reactive force field potential (ReaxFF) optimized to reproduce key quantum mechanical properties of relevant carbon-fluor cluster systems are presented. Molecular dynamics (MD) simulations are used to investigate the thermal rippling behavior of FG and its mechanical properties and compare them with graphene (GE), graphane (GA) and a sheet of BN. The mean square value of the height fluctuations $$ and the height-height correlation function $H(q)$ for different system sizes and temperatures show that FG is an un-rippled system in contrast to the thermal rippling behavior of graphene (GE). The effective Young's modulus of a flake of fluorinated graphene is obtained to be 273 N/m and 250 N/m for a flake of FG under uniaxial strain along arm-chair and zig-zag direction, respectively.Comment: To appear in Phys. Rev.