arXiv subjects

Vincent H. Crespi

Publications and source records attributed to Vincent H. Crespi.

60 records · Page 4Linked to original sources

arXiv2006

Fractional Quantum Hall Effect in Graphene

Unlike regular electron spin, the pseudospin degeneracy of Fermi points in graphene does not couple directly to magnetic field. Therefore, graphene provides a natural vehicle to observe the integral and fractional quantum Hall physics in an elusive limit analogous to zero Zeeman splitting in GaAs systems. This limit can exhibit new integral plateaus arising from interactions, large pseudoskyrmions, fractional sequences, even/odd numerator effects, composite-fermion pseudoskyrmions, and a pseudospin-singlet composite-fermion Fermi sea. The Dirac nature of the B=0 spectrum, which induces qualitative changes in the overall spectrum, has no bearing on the fractional quantum Hall effect in the $n=0$ Landau level of graphene. The second Landau level of graphene is predicted to show more robust fractional quantum Hall effect than the second Landau level of GaAs.

cond-mat.str-el↗

arXiv2004

Discrete transverse superconducting modes in nano-cylinders

Spatial variation in the superconducting order parameter becomes significant when the system is confined at dimensions well below the typical superconducting coherence length. Motivated by recent experimental success in growing single-crystal metallic nanorods, we study quantum confinement effects on superconductivity in a cylindrical nanowire in the clean limit. For large diameters, where the transverse level spacing is smaller than superconducting order parameter, the usual approximations of Ginzburg-Landau theory are recovered. However, under external magnetic field the order parameter develops a spatial variation much stronger than that predicted by Ginzburg-Landau theory, and gapless superconductivity is obtained above a certain field strength. At small diameters, the discrete nature of the transverse modes produces significant spatial variations in the order parameter with increased average magnitude and multiple shoulders in the magnetic response.

cond-mat.supr-con↗

arXiv2004

Doping effects on the electronic and structural properties of CoO2: An LSDA+U study

A systematic LSDA+U study of doping effects on the electronic and structural properties of single layer CoO2 is presented. Undoped CoO2 is a charge transfer insulator within LSDA+U and a metal with a high density of states (DOS) at the Fermi level within LSDA. (CoO2)$^{1.0-}$, on the other hand, is a band insulator with a gap of 2.2 eV. Systems with fractional doping are metals if no charge orderings are present. Due to the strong interaction between the doped electron and other correlated Co d electrons, the calculated electronic structure of (CoO2)$^{x-}$ depends sensitively on the doping level x. Zone center optical phonon energies are calculated under the frozen phonon approximation and are in good agreement with measured values. Softening of the $E_g$ phonon at doping x ~0.25 seems to indicate a strong electron-phonon coupling in this system. Possible intemediate spin states of Co ions, Na ordering, as well as magnetic and charge orderings in this system are also discussed.

cond-mat.supr-con↗

arXiv2001

Stochastic Heterostructures in B/N-Doped Carbon Nanotubes

Carbon nanotubes are one-dimensional and very narrow. These obvious facts imply that under doping with boron and nitrogen, microscopic doping inhomogeneity is much more important than for bulk semiconductors. We consider the possibility of exploiting such fluctuations to create interesting devices. Using self-consistent tight-binding (SCTB), we study heavily doped highly compensated nanotubes, revealing the spontaneous formation of structures resembling chains of random quantum dots, or nano-scale diode-like elements in series. We also consider truly isolated impurities, revealing simple scaling properties of bound state sizes and energies.

cond-mat.mtrl-sci↗

arXiv2000

Tuning Fermi-surface properties through quantum confinement in metallic meta-lattices: New metals from old atoms

We describe a new class of nanoscale structured metals wherein the effects of quantum confinement are combined with dispersive metallic electronic states to induce modifications to the fundamental low-energy microscopic properties of a three-dimensional metal: the density of states, the distribution of Fermi velocities, and the collective electronic response.

cond-mat.mtrl-sci↗

arXiv1999

Helium mixtures in nanotube bundles

An analogue to Raoult's law is determined for the case of a 3He-4He mixture adsorbed in the interstitial channels of a bundle of carbon nanotubes. Unlike the case of He mixtures in other environments, the ratio of the partial pressures of the coexisting vapor is found to be a simple function of the ratio of concentrations within the nanotube bundle.

cond-mat.stat-mech↗