1,957 research outputs found
Resonant Tunneling through S- and U-shaped Graphene Nanoribbons
We theoretically investigate resonant tunneling through S- and U-shaped
nanostructured graphene nanoribbons. A rich structure of resonant tunneling
peaks are found eminating from different quasi-bound states in the middle
region. The tunneling current can be turned on and off by varying the Fermi
energy. Tunability of resonant tunneling is realized by changing the width of
the left and/or right leads and without the use of any external gates.Comment: 6 pages, 7 figure
Helical edge states in silicene and germanene nanorings in perpendicular magnetic field
Due to nonzero intrinsic spin-orbit interaction in buckled honeycomb crystal
structures, silicene and germanene exhibit interesting topological properties,
and are therefore candidates for the realization of the quantum spin Hall
effect. We employ the Kane-Mele model to investigate the electron states in
hexagonal silicene and germanene nanorings having either zigzag or armchair
edges in the presence of a perpendicular magnetic field. We present results for
the energy spectra as function of magnetic field, the electron density of the
spin-up and spin-down states in the ring plane, and the calculation of the
probability current density. The quantum spin Hall phase is found at the edges
between the nontrivial topological phase in silicene and germanene and vacuum.
We demonstrate that the helical edge states in zigzag silicene and germanene
nanorings can be qualitatively well understood by means of classical magnetic
moments. However, this is not the case for comparable-sized armchair nanorings,
where the eigenfunctions spread throughout the ring. Finally, we note that the
energy spectra of silicene and germanene nanorings are similar and that the
differences between the two are mainly related to the difference in magnitude
of the spin-orbit coupling.Comment: 17 pages, 10 figure
Theoretical investigation of electron-hole complexes in anisotropic two-dimensional materials
Trions and biexcitons in anisotropic two-dimensional materials are
investigated within an effective mass theory. Explicit results are obtained for
phosphorene and arsenene, materials that share features such as a direct
quasi-particle gap and anisotropic conduction and valence bands. Trions are
predicted to have remarkably high binding energies and an elongated
electron-hole structure with a preference for alignment along the armchair
direction, where the effective masses are lower. We find that biexciton binding
energies are also notably large, especially for monolayer phosphorene, where
they are found to be twice as large as those for typical monolayer transition
metal dichalcogenides.Comment: 3 figures, 5 pages + Supplementary Material, accepted for publication
in Phys. Rev.
Enhanced stability of hydrogen atoms at the graphene/graphane interface of nanoribbons
The thermal stability of graphene/graphane nanoribbons (GGNRs) is
investigated using density functional theory. It is found that the energy
barriers for the diffusion of hydrogen atoms on the zigzag and armchair
interfaces of GGNRs are 2.86 and 3.17 eV, respectively, while the diffusion
barrier of an isolated H atom on pristine graphene was only ~0.3 eV. These
results unambiguously demonstrate that the thermal stability of GGNRs can be
enhanced significantly by increasing the hydrogen diffusion barriers through
graphene/graphane interface engineering. This may provide new insights for
viable applications of GGNRs.Comment: 13 pages, 1 figure, 2 tables to appear in Appl. Phys. Let
An efficient finite-difference scheme for computation of electron states in free-standing and core-shell quantum wires
The electron states in axially symmetric quantum wires are computed by means
of the effective-mass Schroedinger equation, which is written in cylindrical
coordinates phi, rho, and z. We show that a direct discretization of the
Schroedinger equation by central finite differences leads to a non-symmetric
Hamiltonian matrix. Because diagonalization of such matrices is more complex it
is advantageous to transform it in a symmetric form. This can be done by the
Liouville-like transformation proposed by Rizea et al. (Comp. Phys. Comm. 179
(2008) 466-478), which replaces the wave function psi(rho) with the function
F(rho)=psi(rho)sqrt(rho) and transforms the Hamiltonian accordingly. Even
though a symmetric Hamiltonian matrix is produced by this procedure, the
computed wave functions are found to be inaccurate near the origin, and the
accuracy of the energy levels is not very high. In order to improve on this, we
devised a finite-difference scheme which discretizes the Schroedinger equation
in the first step, and then applies the Liouville-like transformation to the
difference equation. Such a procedure gives a symmetric Hamiltonian matrix,
resulting in an accuracy comparable to the one obtained with the finite element
method. The superior efficiency of the new finite-difference (FDM) scheme is
demonstrated for a few rho-dependent one-dimensional potentials which are
usually employed to model the electron states in free-standing and core-shell
quantum wires. The new scheme is compared with the other FDM schemes for
solving the effective-mass Schroedinger equation, and is found to deliver
energy levels with much smaller numerical error for all the analyzed
potentials. Moreover, the PT symmetry is invoked to explain similarities and
differences between the considered FDM schemes
Tuning of energy levels and optical properties of graphene quantum dots
We investigate theoretically the magnetic levels and optical properties of
zigzag- and armchair-edged hexagonal graphene quantum dots (GQDs) utilizing the
tight-binding method. A new bound edge state at zero energy appears for the
zigzag GQDs in the absence of a magnetic field. The magnetic levels of GQDs
exhibit a Hofstadter-butterfly spectrum and approach the Landau levels of
two-dimensional graphene as the magnetic field increases. The optical
properties are tuned by the size, the type of the edge, and the external
magnetic field.Comment: 5 pages, 7 figures. to appear in Phys. Rev.
Realization of Artificial Ice Systems for Magnetic Vortices in a Superconducting MoGe Thin-film with Patterned Nanostructures
We report an anomalous matching effect in MoGe thin films containing pairs of
circular holes arranged in such a way that four of those pairs meet at each
vertex point of a square lattice. A remarkably pronounced fractional matching
was observed in the magnetic field dependences of both the resistance and the
critical current. At the half matching field the critical current can be even
higher than that at zero field. This has never been observed before for
vortices in superconductors with pinning arrays. Numerical simulations within
the nonlinear Ginzburg-Landau theory reveal a square vortex ice configuration
in the ground state at the half matching field and demonstrate similar
characteristic features in the field dependence of the critical current,
confirming the experimental realization of an artificial ice system for
vortices for the first time.Comment: To appear in Phys. Rev. Let
Changing guards: time to move beyond Body Mass Index for population monitoring of excess adiposity
With the obesity epidemic, and the effects of aging populations, human phenotypes have changed over two generations, possibly more dramatically than in other species previously. As obesity is an important and growing hazard for population health, we recommend a systematic evaluation of the optimal measure(s) for population-level excess body fat. Ideal measure(s) for monitoring body composition and obesity should be simple, as accurate and sensitive as possible, and provide good categorisation of related health risks. Combinations of anthropometric markers or predictive equations may facilitate better use of anthropometric data than single measures to estimate body composition for populations. Here we provide new evidence that increasing proportions of aging populations are at high health-risk according to waist circumference, but not body mass index (BMI), so continued use of BMI as the principal population-level measure substantially underestimates the health-burden from excess adiposity
Electronic stability of silicon front-end hybrids
No description supplie
Type II and heterotic one loop string effective actions in four dimensions
We analyze the reduction to four dimensions of the R^4 terms which are part
of the ten-dimensional string effective actions, both at tree level and one
loop. We show that there are two independent combinations of R^4 present, at
one loop, in the type IIA four dimensional effective action, which means they
both have their origin in M-theory. The d=4 heterotic effective action also has
such terms. This contradicts the common belief thathere is only one R^4 term in
four-dimensional supergravity theories, given by the square of the Bel-Robinson
tensor. In pure N=1 supergravity this new R^4 combination cannot be directly
supersymmetrized, but we show that, when coupled to a scalar chiral multiplet
(violating the U(1) -symmetry), it emerges in the action after elimination
of the auxiliary fields.Comment: v2: 22 pages. Discussion on the new R^4 term and extended
supergravity has been abridged and improved. Published versio
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