K. M. Masum Habib

Electron optics with p-n junctions in ballistic graphene

Shaowen Chen,1,2∗ Zheng Han,1,7∗ Mirza M. Elahi, K. M. Masum Habib,3† Lei Wang, Bo Wen, Yuanda Gao, Takashi Taniguchi, Kenji Watanabe, James Hone, Avik W. Ghosh, and Cory R. Dean1‡ Department of Physics, Columbia University, New York, NY 10027, USA Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, VA 22904, USA Department of Physics, Cornell University, Ithaca , NY 14853, USA Department of Mechanical Engineering, Columbia University, New York, NY 10027, USA National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0047, Japan and Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P. R. China (Dated: February 29, 2016)Electrons transmitted across a ballistic semiconductor junction are expected to undergo refraction, analogous to light rays across an optical boundary. In graphene, the linear dispersion and zero-gap band structure admit highly transparent p-n junctions by simple electrostatic gating. Here, we employ transverse magnetic focusing to probe the propagation of carriers across an electrostatically defined graphene junction. We find agreement with the predicted Snell’s law for electrons, including the observation of both positive and negative refraction. Resonant transmission across the p-n junction provides a direct measurement of the angle-dependent transmission coefficient. Comparing experimental data with simulations reveals the crucial role played by the effective junction width, providing guidance for future device design. Our results pave the way for realizing electron optics based on graphene p-n junctions.

Permanent Electric Dipole Moments of Carboxyamides in Condensed Media: What Are the Limitations of Theory and Experiment?

Electrostatic properties of proteins are crucial for their functionality. Carboxyamides are small polar groups that, as peptide bonds, are principal structural components of proteins that govern their electrostatic properties. We investigated the medium dependence of the molar polarization and of the permanent dipole moments of amides with different state of alkylation. The experimentally measured and theoretically calculated dipole moments manifested a solvent dependence that increased with the increase in the media polarity. We ascribed the observed enhancement of the amide polarization to the reaction fields in the solvated cavities. Chloroform, for example, caused about a 25% increase in the amide dipole moments determined for vacuum, as the experimental and theoretical results demonstrated. Another chlorinated solvent, 1,1,2,2-tetrachloroethane, however, caused an abnormal increase in the experimentally measured amide dipoles, which the theoretical approaches we used could not readily quantify. We showed and discussed alternatives for addressing such discrepancies between theory and experiment.

The coherent interlayer resistance of a single, rotated interface between two stacks of AB graphite

The coherent, interlayer resistance of a misoriented, rotated interface between two stacks of AB graphite is determined for a variety of misorientation angles. The quantum-resistance of the ideal AB stack is on the order of 1 to 10u2009mΩu2009μm2. For small rotation angles, the coherent interlayer resistance exponentially approaches the ideal quantum resistance at energies away from the charge neutrality point. Over a range of intermediate angles, the resistance increases exponentially with cell size for minimum size unit cells. Larger cell sizes, of similar angles, may not follow this trend. The energy dependence of the interlayer transmission is described.

Effect of Random, Discrete Source Dopant Distributions on Nanowire Tunnel FETs

The finite number, random placement, and discrete nature of the dopants in the source of an InAs nanowire tunnel field-effect transistor affect the drive current and the inverse subthreshold slope. The impact of source scattering is negligible, since the current is limited by the interband tunneling. The most significant effect of the discrete dopants is to create variations of the electric fields in the tunnel barrier, which cause variations in the current. The relative variation in the ON current decreases as the average doping density and/or nanowire diameter increases. Results from full self-consistent nonequilibrium Greens function calculations and semiclassical calculations are compared.

Multi-state current switching by voltage controlled coupling of crossed graphene nanoribbons

The interlayer transport between two semi-infinite crossed graphene nanoribbons (GNRs) is governed by the quantum interference between the standing waves of the individual GNRs. An external bias applied between the GNRs controls the wavelength and hence the relative phase of these standing waves. Sweeping the applied bias results in multiple constructive and destructive interference conditions. The oscillatory nature of the voltage controlled interference gives rise to an oscillatory current-voltage response with multiple negative differential resistance regions. The period of oscillation is inversely proportional to the length of the finite ends of the GNRs. Quantum interference is explicitly shown to be the physical mechanism controlling the interlayer current by direct evaluation of the interlayer matrix element using analytical expressions for the wavefunctions.

Interlayer magnetoconductance of misoriented bilayer graphene ribbons

The coherent, interlayer conductance of misoriented bilayer graphene ribbons is a strong function of the Fermi energy and magnetic field. Edge states can result in a large peak in the interlayer transmission at the charge neutrality point that is several orders of magnitude larger than the surrounding low-energy transmission. The coherent interlayer conductance is consistently asymmetric around the charge neutrality point for all structures with the value differing by up to 3 orders of magnitude at Efu2009=u2009±0.05u2009eV. The low-energy states exhibit a high magnetoconductance ratio, and the magnetoconductance ratio tends to increase as the width of the ribbons decrease. The maximum value for the 35u2009nm wide bilayer ribbons at 10u2009T is 15u2009000%. Non-equilibrium Greens function calculations of the interlayer transport properties are also supported by semi-analytical calculations based on Fermis Golden Rule.

Modified Dirac Hamiltonian for efficient quantum mechanical simulations of micron sized devices

Representing massless Dirac fermions on a spatial lattice poses a potential challenge known as the Fermion Doubling problem. Addition of a quadratic term to the Dirac Hamiltonian provides a possible way to circumvent this problem. We show that the modified Hamiltonian with the additional term results in a very small Hamiltonian matrix when discretized on a real space square lattice. The resulting Hamiltonian matrix is considerably more efficient for numerical simulations without sacrificing on accuracy and is several orders of magnitude faster than the atomistic tight binding model. Using this Hamiltonian and the non-equilibrium Greens function formalism, we show several transport phenomena in graphene, such as magnetic focusing, chiral tunneling in the ballistic limit, and conductivity in the diffusive limit in micron sized graphene devices. The modified Hamiltonian can be used for any system with massless Dirac fermions such as Topological Insulators, opening up a simulation domain that is not readily ...

Quantum transport at the Dirac point: Mapping out the minimum conductivity from pristine to disordered graphene

The phase space for graphenes minimum conductivity

Graphene Klein tunnel transistors for high speed analog RF applications

sigma_mathrm{min}

Chiral tunneling of topological states: towards the efficient generation of spin current using spin-momentum locking.

is mapped out using Landauer theory modified for scattering using Fermis Golden Rule, as well as the Non-Equilibrium Greens Function (NEGF) simulation with a Monte Carlo sampling over impurity distributions. The resulting `fan diagram spans the range from ballistic to diffusive over varying aspect ratios (