Effects of Strain on Electronic Properties of Graphene

We present first-principles calculations of electronic properties of graphene under uniaxial and isotropic strains, respectively. The semi-metallic nature is shown to persist up to a very large uniaxial strain of 30% except a very narrow strain range where a tiny energy gap opens. As the uniaxial strain increases along a certain direction, the Fermi velocity parallel to it decreases quickly and vanishes eventually, whereas the Fermi velocity perpendicular to it increases by as much as 25%. Thus, the low energy properties with small uniaxial strains can be described by the generalized Weyl's equation while massless and massive electrons coexist with large ones. The work function is also predicted to increase substantially as both the uniaxial and isotropic strain increases. Hence, the homogeneous strain in graphene can be regarded as the effective electronic scalar potential.Comment: 4 pages, 6 figures; Published versio

The stability of graphene band structures against an external periodic perturbation; Na on Graphene

We report that the $\pi$ band of graphene sensitively changes as a function of an external potential induced by Na especially when the potential becomes periodic at low temperature. We have measured the band structures from the graphene layers formed on the 6H-SiC(0001) substrate using angle-resolved photoemission spectroscopy with synchrotron photons. With increasing Na dose, the $\pi$ band appears to be quickly diffused into background at 85 K whereas it becomes significantly enhanced its spectral intensity at room temperature (RT). A new parabolic band centered at $k\sim$1.15 \AA$^{-1}$ also forms near Fermi energy with Na at 85 K while no such a band observed at RT. Such changes in the band structure are found to be reversible with temperature. Analysis based on our first principles calculations suggests that the changes of the $\pi$ band of graphene be mainly driven by the Na-induced potential especially at low temperature where the potential becomes periodic due to the crystallized Na overlayer. The new parabolic band turns to be the $\pi$ band of the underlying buffer layer partially filled by the charge transfer from Na adatoms. The five orders of magnitude increased hopping rate of Na adatoms at RT preventing such a charge transfer explains the absence of the new band at RT.Comment: 6 pages and 6 figure

Electronic topological transition in sliding bilayer graphene

We demonstrate theoretically that the topology of energy bands and Fermi surface in bilayer graphene undergoes a very sensitive transition when extremely tiny lateral interlayer shift occurs in arbitrary directions. The phenomenon originates from a generation of effective non-Abelian vector potential in Dirac Hamiltonian by the sliding motions. The characteristics of the transition such as pair annihilations of massless Dirac fermions are dictated by the sliding direction owing to a unique interplay between the effective non-Abelian gauge fields and Berry's phases associated with massless electrons. The transition manifests itself in various measurable quantities such as anomalous density of states, minimal conductivity, and distinct Landau level spectrum.Comment: title changed, an extended version for regular article format, 10 pages, 5 figure

Biscrolled Carbon Nanotube Yarn Structured Silver-Zinc Battery

Flexible yarn- or fiber-based energy storing devices are attractive because of their small dimension, light weight, and suitability for integration into woven or textile application. Some Li-ion based yarn or fiber batteries were developed due to their performance advantages, realizing highly performing and practically safe wearable battery still remains a challenge. Here, high performance and safe yarn-based battery is demonstrated by embedding active materials into inner structure of yarn and using water based electrolyte. Thanks to biscrolling method, loading level of silver and zinc in yarn electrodes increased up to 99 wt%. Our high loaded Silver and Zinc yarn electrodes enables high linear capacity in liquid electrolyte (0.285 mAh/cm) and solid electrolyte (0.276 mAh/cm), which are significantly higher than previously reported fiber batteries. In additions, due to PVA-KOH based aqueous electrolyte, our yarn battery system is inflammable, non-explosive and safe. Consequently, these high-capacities enable our Silver-Zinc aqueous yarn battery to be applicable to the energy source of portable and wearable electronics like an electric watch. © 2018, The Author(s).1