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The interaction of electron-hole pairs with lattice vibrations exhibits a wealth of intriguing physical phenomena. The Kohn anomaly is a renowned example where electron-phonon coupling leads to non-analytic phonon dispersion at specific momentum nesting the Fermi surface. Here we report evidence of another type of phonon anomaly discovered by low temperature Raman spectroscopy in bilayer graphene where the charge density is modulated by the electric field effect. This anomaly, arising from charge-tunable modulations of particle-hole pairs that are resonantly coupled to lattice vibrations, is predicted to exhibit a logarithmic divergence in the long-wavelength optical-phonon energy. In a non-uniform bilayer of graphene, the logarithmic divergence is abated by charge density inhomogeneity leaving as a vestige an anomalous phonon softening. The observed softening marks the first confirmation of the phonon anomaly as a key signature of the resonant deformation-potential electron-phonon coupling. The high sensitivity of the phonon softening to charge density non-uniformity creates significant venues to explore the interplay between fundamental interactions and disorder in the atomic layers., Comment: 4 figures

In addition to variation in terms of single nucleotide polymorphisms (SNPs), whole regions ranging from several kilobases up to a megabase in length differ in copy number among individuals. These differences are referred to as Copy Number Variants (CNVs) and extensive mapping of these is underway. Recent studies have highlighted their great prevalence in the human genome. Segmental Duplications (SDs) are long (>1kb) stretches of duplicated DNA with high sequence identity. First, we analyzed the co-localization of SDs and find that SDs are significantly co-localized with each other, resulting in a power-law distribution, which suggests a preferential attachment mechanism, i.e. existing SDs are likely to be involved in creating new ones nearby. Second, we look at the relationship of CNVs/SDs with various types of repeats. We we find that the previously recognized association of SDs with Alu elements is significantly stronger for older SDs and is sharply decreasing for younger ones. While it might be expected that the patterns should be similar for SDs and CNVs, we find, surprisingly, no association of CNVs with Alu elements. This trend is consistent with the decreasing correlation between Alu elements and younger SDs, the activity of Alu elements has been decreasing and by now it they seem no longer active. Furthermore, we find a striking association of SDs with processed pseudogenes suggesting that they may also have mediated SD formation. Moreover, find strong association with microsatellites for both SDs and CNVs that suggests a role for satellites in the formation of both., Comment: 13 pages

We have developed the combination of an etching and deposition technique that enables the fabrication of locally gated graphene nanostructures of arbitrary design. Employing this method, we have fabricated graphene nanoconstrictions with local tunable transmission and characterized their electronic properties. An order of magnitude enhanced gate efficiency is achieved adopting the local gate geometry with thin dielectric gate oxide. A complete turn off of the device is demonstrated as a function of the local gate voltage. Such strong suppression of device conductance was found to be due to both quantum confinement and Coulomb blockade effects in the constricted graphene nanostructures., Comment: 3 pages 3 figures; separated and expanded from arXiv:0705.3044v1

Pathways are integral to systems biology. Their classical representation has proven useful but is inconsistent in the meaning assigned to each arrow (or edge) and inadvertently implies the isolation of one pathway from another. Conversely, modern high-throughput experiments give rise to standardized networks facilitating topological calculations. Combining these perspectives, we can embed classical pathways within large-scale networks and thus demonstrate the crosstalk between them. As more diverse types of high-throughput data become available, we can effectively merge both perspectives, embedding pathways simultaneously in multiple networks. However, the original problem still remains - the current edge representation is inadequate to accurately convey all the information in pathways. Therefore, we suggest that a standardized, well-defined, edge ontology is necessary and propose a prototype here, as a starting point for reaching this goal., Comment: 30 pages including 5 figures and supplemental material

We have developed a device fabrication process to pattern graphene into nanostructures of arbitrary shape and control their electronic properties using local electrostatic gates. Electronic transport measurements have been used to characterize locally gated bipolar graphene $p$-$n$-$p$ junctions. We observe a series of fractional quantum Hall conductance plateaus at high magnetic fields as the local charge density is varied in the $p$ and $n$ regions. These fractional plateaus, originating from chiral edge states equilibration at the $p$-$n$ interfaces, exhibit sensitivity to inter-edge backscattering which is found to be strong for some of the plateuas and much weaker for other plateaus. We use this effect to explore the role of backscattering and estimate disorder strength in our graphene devices., Comment: 4 pages 4 figures, to appear in Phys. Rev. Lett. Original version arXiv:0705.3044v1 was separated and expanded to this current version and arXiv:0709.1731

We present scanning tunneling microscopy (STM) images of single-layer graphene crystals examined under ultrahigh vacuum conditions. The samples, with lateral dimensions on the micron scale, were prepared on a silicon dioxide surface by direct exfoliation of single crystal graphite. The single-layer films were identified using Raman spectroscopy. Topographic images of single-layer samples display the honeycomb structure expected for the full hexagonal symmetry of an isolated graphene monolayer. The absence of observable defects in the STM images is indicative of the high quality of these films. Crystals comprised of a few layers of graphene were also examined. They exhibited dramatically different STM topography, displaying the reduced three-fold symmetry characteristic of the surface of bulk graphite., Comment: Accepted to PNAS

We present an experimental investigation on the scaling of resistance in individual single walled carbon nanotube devices with channel lengths that vary four orders of magnitude on the same sample. The electron mean free path is obtained from the linear scaling of resistance with length at various temperatures. The low temperature mean free path is determined by impurity scattering, while at high temperature the mean free path decreases with increasing temperature, indicating that it is limited by electron-phonon scattering. An unusually long mean free path at room temperature has been experimentally confirmed. Exponentially increasing resistance with length at extremely long length scales suggests anomalous localization effects., Comment: 4 pages 4 figures, to appear in Phys. Rev. Lett

We investigate electronic transport in lithographically patterned graphene ribbon structures where the lateral confinement of charge carriers creates an energy gap near the charge neutrality point. Individual graphene layers are contacted with metal electrodes and patterned into ribbons of varying widths and different crystallographic orientations. The temperature dependent conductance measurements show larger energy gaps opening for narrower ribbons. The sizes of these energy gaps are investigated by measuring the conductance in the non-linear response regime at low temperatures. We find that the energy gap scales inversely with the ribbon width, thus demonstrating the ability to engineer the band gap of graphene nanostructures by lithographic processes., Comment: 7 pages including 4 figures

Gate-modulated low-temperature Raman spectra reveal that the electric field effect (EFE), pervasive in contemporary electronics, has marked impacts on long wavelength optical phonons of graphene. The EFE in this two dimensional honeycomb lattice of carbon atoms creates large density modulations of carriers with linear dispersion (known as Dirac fermions). Our EFE Raman spectra display the interactions of lattice vibrations with these unusual carriers. The changes of phonon frequency and line-width demonstrate optically the particle-hole symmetry about the charge-neutral Dirac-point. The linear dependence of the phonon frequency on the EFE-modulated Fermi energy is explained as the electron-phonon coupling of mass-less Dirac fermions., Comment: 4 pages, 4 figures

We have measured electron transport in small bundles of identical conducting Molybdenum Selenide nanowires where the number of weakly interacting one-dimensional chains ranges from 1-300. The linear conductance and current in these nanowires exhibit a power-law dependence on temperature and bias voltage respectively. The exponents governing these power laws decrease as the number of conducting channels increase. These exponents can be related to the electron-electron interaction parameter for transport in multi-channel 1-D systems with a few defects., Comment: 4 pages, 4 figures, accepted for publication in Physical Review Letters

When electrons are confined in two-dimensional (2D) materials, quantum mechanically enhanced transport phenomena, as exemplified by the quantum Hall effects (QHE), can be observed. Graphene, an isolated single atomic layer of graphite, is an ideal realization of such a 2D system. Here, we report an experimental investigation of magneto transport in a high mobility single layer of graphene. Adjusting the chemical potential using the electric field effect, we observe an unusual half integer QHE for both electron and hole carriers in graphene. Vanishing effective carrier masses is observed at Dirac point in the temperature dependent Shubnikov de Haas oscillations, which probe the 'relativistic' Dirac particle-like dispersion. The relevance of Berry's phase to these experiments is confirmed by the phase shift of magneto-oscillations, related to the exceptional topology of the graphene band structure.

We have developed a unique micromechanical method to extract extremely thin graphite samples. Graphite crystallites with thicknesses ranging from 10 - 100 nm and lateral size $\sim$ 2 $\mu$m are extracted from bulk. Mesoscopic graphite devices are fabricated from these samples for electric field dependent conductance measurements. Strong conductance modulation as a function of gate voltage is observed in the thinner crystallite devices. The temperature dependent resistivity measurements show more boundary scattering contribution in the thinner graphite samples., Comment: 3 pages, 3 figures included, submitted to Appl. Phys. Lett

Electric field effect devices based on mesoscopic graphite are fabricated for galvanomagnetic measurements. Strong modulation of magneto-resistance and Hall resistance as a function of gate voltage is observed as sample thickness approaches the screening length. Electric field dependent Landau level formation is detected from Shubnikov de Haas oscillations in magneto-resistance. The effective mass of electron and hole carriers has been measured from the temperature dependant behavior of these oscillations., Comment: 4 pages, 4 figures included, submitted to Phys. Rev. Lett

Thermoelectric power (TEP) of individual single walled carbon nanotubes (SWNTs) has been measured at mesoscopic scales using a microfabricated heater and thermometers. Gate electric field dependent TEP-modulation has been observed. The measured TEP of SWNTs is well correlated to the electrical conductance across the SWNT according to the Mott formula. At low temperatures, strong modulations of TEP were observed in the single electron conduction limit. In addition, semiconducting SWNTs exhibit large values of TEP due to the Schottky barriers at SWNT-metal junctions., Comment: to be published in Phys. Rev. Lett

Dislocation-free decoration images containing up to 80,000 vortices have been obtained on high quality Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ superconducting single crystals. The observed flux line lattices are in the random manifold regime with a roughening exponent of 0.44 for length scales up to 80-100 lattice constants. At larger length scales, the data exhibit nonequilibrium features that persist for different cooling rates and field histories., Comment: 4 pages, 3 gif images, to appear in PRB rapid communication

The electronic density of states of atomically resolved single-walled carbon nanotubes have been investigated using scanning tunneling microscopy. Peaks in the density of states due to the one-dimensional nanotube band structure have been characterized and compared with the results of tight-binding calculations. In addition, tunneling spectroscopy measurements recorded along the axis of an atomically-resolved nanotube exhibit new, low-energy peaks in the density of states near the tube end. Calculations suggest that these features arise from the specific arrangement of carbon atoms that close the nanotube end., Comment: 4 pages including 4 eps figures, to apear in Phys. Rev. Lett