Probing the Shape of a Graphene Nanobubble

Gas molecules trapped between graphene and various substrates in the form of bubbles are observed experimentally. The study of these bubbles is useful in determining the elastic and mechanical properties of graphene, adhesion energy between graphene and substrate, and manipulating the electronic properties via strain engineering. In our numerical simulations, we use a simple description of elastic potential and adhesion energy to show that for small gas bubbles ($\sim 10$ nm) the van der Waals pressure is in the order of 1 GPa. These bubbles show universal shape behavior irrespective of their size, as observed in recent experiments. With our results the shape and volume of the trapped gas can be determined via the vibrational density of states (VDOS) using experimental techniques such as inelastic tunneling and inelastic neutron scattering. The elastic energy distribution in the graphene layer which traps the nanobubble is homogeneous apart from its edge, but the strain depends on the bubble size thus variation in bubble size allows control of the electronic and optical properties.Comment: 5 Figures (Supplementary: 1 Figure), Accepted for publication in PCC

Covariant Symmetry Classifications for Observables of Cosmological Birefringence

Polarizations of electromagnetic waves from distant galaxies are known to be correlated with the source orientations. These quantities have been used to search for signals of cosmological birefringence. We review and classify transformation properties of the polarization and source orientation observables. The classifications give a firm foundation to certain practices which have sprung up informally in the literature. Transformations under parity play a central role, showing that parity violation in emission or in the subsequent propagation is an observable phenomenon. We also discuss statistical measures, correlations and distributions which transform properly and which can be used for systematic data analysis.Comment: 8 pages, revtex, 1 postscript figur

Density dependence of valley polarization energy for composite fermions

In two-dimensional electron systems confined to wide AlAs quantum wells, composite fermions around the filling factor $\nu$ = 3/2 are fully spin polarized but possess a valley degree of freedom. Here we measure the energy needed to completely valley polarize these composite fermions as a function of electron density. Comparing our results to the existing theory, we find overall good quantitative agreement, but there is an unexpected trend: The measured composite fermion valley polarization energy, normalized to the Coulomb energy, decreases with decreasing density

Structural characterization of carbon nanotubes via the vibrational density of states

The electrical and chemical properties of carbon nanotubes vary significantly with different chirality and diameter, making the experimental determination of these structural properties important. Here, we show that the vibrational density of states (VDOS) contains information on the structure of carbon nanotubes, particularly at low frequencies. We show that the diameter and chirality of the nanotubes can be determined from the characteristic low frequency $L$ and $L'$ modes in the VDOS. For zigzag nanotubes, the $L$ peak splits into two peaks giving rise to another low energy $L"$ peak. The significant changes in the frequencies and relative intensities of these peaks open up a route to distinguish among structurally different nanotubes. A close study of different orientations of Stone-Wales defects with varying defect density reveals that different structural defects also leave distinct fingerprints in the VDOS, particularly in the $L$ and $L'$ modes. With our results, more structural information can be obtained from experiments which can directly measure the VDOS, such as inelastic electron and inelastic neutron spectroscopy.Comment: 5 Figures, Accepted for publication in Carbo

Composite Fermions in Quantum Dots

We demonstrate the formation of composite fermions in two-dimensional quantum dots under high magnetic fields. The composite fermion interpretation provides a simple way to understand several qualitative and quantitative features of the numerical results obtained earlier in exact diagonalization studies. In particular, the ground states are recognized as compactly filled quasi-Landau levels of composite fermions.Comment: Revtex. Postscript files of figures are appended the tex

Search for Narrow-Width ttbar Resonances in ppbar Collisions at center of mass energy = 1.8 TeV

We present a preliminary result on a search for narrow-width resonances that decay into ttbar pairs using 130 pb^{-1} of lepton plus jets data in ppbar collisions at center of mass energy = 1.8 TeV. No significant deviation from Standard Model prediction is observed. 95% C.L. upper limits on the production cross section of the narrow-width resonance times its branching fraction to ttbar are presented for different resonance masses, M_X. We also exclude the existence of a leptophobic topcolor particle, X, with M_X < 560 GeV/c^2 for a width \Gamma_X = 0.012 M_X.Comment: 3 pages, 1 figure; Submitted for proceedings of 5th International Conference on Quark Confinement and Hadron spectrum, held in Italy, from 11-14 Sep., 200

Activation gaps for the fractional quantum Hall effect: realistic treatment of transverse thickness

The activation gaps for fractional quantum Hall states at filling fractions $\nu=n/(2n+1)$ are computed for heterojunction, square quantum well, as well as parabolic quantum well geometries, using an interaction potential calculated from a self-consistent electronic structure calculation in the local density approximation. The finite thickness is estimated to make $\sim$30% correction to the gap in the heterojunction geometry for typical parameters, which accounts for roughly half of the discrepancy between the experiment and theoretical gaps computed for a pure two dimensional system. Certain model interactions are also considered. It is found that the activation energies behave qualitatively differently depending on whether the interaction is of longer or shorter range than the Coulomb interaction; there are indications that fractional Hall states close to the Fermi sea are destabilized for the latter.Comment: 32 pages, 13 figure

Tunneling Spectroscopy of Disordered Two-Dimensional Electron Gas in the Quantum Hall Regime

Recently, Dial et al. presented measurements of the tunneling density of states into the bulk of a two dimensional electron gas under strong magnetic fields. Several high energy features appear in the measured spectrum showing a distinct dependence on filling factor and a unique response to temperature. We present a quantitative account of the observed structure, and argue it results from the repulsive Coulomb interactions between the tunneling electron and states localized at disorder potential wells. The quenching of the kinetic energy by the applied magnetic field leads to an electron addition spectrum that is primarily determined by the external magnetic field and is nearly independent of the disorder potential. Using a Hartree-Fock model we reproduce the salient features of the observed structure