Real-Space Imaging of Alternate Localization and Extension of Quasi Two-Dimensional Electronic States at Graphite Surfaces in Magnetic Fields

We measured the local density of states (LDOS) of a quasi two-dimensional (2D) electron system near point defects on a surface of highly oriented pyrolytic graphite (HOPG) with scanning tunneling microscopy and spectroscopy. Differential tunnel conductance images taken at very low temperatures and in high magnetic fields show a clear contrast between localized and extended spatial distributions of the LDOS at the valley and peak energies of the Landau level spectrum, respectively. The localized electronic state has a single circular distribution around the defects with a radius comparable to the magnetic length. The localized LDOS is in good agreement with a spatial distribution of a calculated wave function for a single electron in 2D in a Coulomb potential in magnetic fields.Comment: 4 pages, 4 figure

Broadening effects due to alloy scattering in Quantum Cascade Lasers

We report on calculations of broadening effects in QCL due to alloy scattering. The output of numerical calculations of alloy broadened Landau levels compare favorably with calculations performed at the self-consistent Born approximation. Results for Landau level width and optical absorption are presented. A disorder activated forbidden transition becomes significant in the vicinity of crossings of Landau levels which belong to different subbands. A study of the time dependent survival probability in the lowest Landau level of the excited subband is performed. It is shown that at resonance the population relaxation occurs in a subpicosecond scale.Comment: 7 pages, 8 figure

Vortex molecules in coherently coupled two-component Bose-Einstein condensates

A vortex molecule is predicted in rotating two-component Bose-Einstein condensates whose internal hyperfine states are coupled coherently by an external field. A vortex in one component and that in the other are connected by a domain wall of the relative phase, constituting a "vortex molecule", which features a nonaxisymmetric (pseudo)spin texture with a pair of merons. The binding mechanism of the vortex molecule is discussed based on a generalized nonlinear sigma model and a variational ansatz. The anisotropy of vortex molecules is caused by the difference in the scattering lengths, yielding a distorted vortex-molecule lattice in fast rotating condensates.Comment: 4 pages, 4 figures, greatly revised versio

Electronic properties of a graphene antidot in magnetic fields

We report on several unusual properties of a graphene antidot created by a piecewise constant potential in a magnetic field. We find that the total probability of finding the electron in the barrier can be nearly one while it is almost zero outside the barrier. In addition, for each electron state of a graphene antidot there is a dot state with exactly the same wavefunction but with a different energy. This symmetry is a consequence of Klein tunneling of Dirac electrons. Moreover, in zigzag nanoribbons we find strong coupling between some antidot states and zigzag edge states. Experimental tests of these effects are proposed

Noncommutative Schwarzschild geometry and generalized uncertainty principle

We discuss a possible link between the deformation parameter $$\Theta ^{\mu \nu }$$ Θμν arising in the framework of noncommutative geometry and the parameter $$\beta$$ β of the generalized uncertainty principle (GUP). We compute the shift of the Hawking temperature induced by the $$\Theta ^{\mu \nu }$$ Θμν -deformed Schwarzschild geometry, and then we relate it to one obtained by GUP. Results suggest a granular structure of specetime at the Planck scales. The current bounds on $$\beta$$ β allow to constraint the noncommutative parameter $$\Theta ^{\mu \nu }$$ Θμν

Pressure-induced change of the pairing symmetry in superconducting CeCu2Si2

Low-temperature (T) heat-capacity measurements under hydrostatic pressure of up to p=2.1 GPa have been performed on single-crystalline CeCu2Si2. A broad superconducting (SC) region exists in the T-p phase diagram. In the low-pressure region antiferromagnetic spin fluctuations and in the high-pressure region valence fluctuations had previously been proposed to mediate Cooper pairing. We could identify these two distinct SC regions. We found different thermodynamic properties of the SC phase in both regions, supporting the proposal that different mechanisms might be implied in the formation of superconductivity.Comment: 4 pages, 5 figure

Possible observation of phase coexistence of the ν=1/3\nu=1/3 fractional quantum Hall liquid and a solid

We have measured the magnetoresistance of a very low density and an extremely high quality two-dimensional hole system. With increasing magnetic field applied perpendicularly to the sample we observe the sequence of insulating, $\nu=1/3$ fractional quantum Hall liquid, and insulating phases. In both of the insulating phases in the vicinity of the $\nu=1/3$ filling the magnetoresistance has an unexpected oscillatory behavior with the magnetic field. These oscillations are not of the Shubnikov-de Haas type and cannot be explained by spin effects. They are most likely the consequence of the formation of a new electronic phase which is intermediate between the correlated Hall liquid and a disorder pinned solid.Comment: 5 pages, 4 figure

Hall Crystal States at ν=2\nu=2 and Moderate Landau Level Mixing

The $\nu=2$ quantum Hall state at low Zeeman coupling is well-known to be a translationally invariant singlet if Landau level mixing is small. At zero Zeeman interaction, as Landau level mixing increases, the translationally invariant state becomes unstable to aninhomogeneous state. This is the first realistic example of a full Hall crystal, which shows the coexistence of quantum Hall order and density wave order. The full Hall crystal differs from the more familiar Wigner crystal by a topological property, which results in it having only linearly dispersing collective modes at small $q$, and no $q^{3/2}$ magnetophonon. I present calculations of the topological number and the collective modes.Comment: Final version to appear in PRL. Two references added, minor changes to figures and tex

QHE of Bilayer Systems in the Presence of Tunneling -- ν=1/q\nu=1/q case --

Transport properties of bilayer quantum Hall systems at $\nu=1/q$, where $q$ is an odd integer, are investigated. The edge theory is used for the investigation, since tunneling between the two layers is assumed to occur on the edge of the sample because of the bulk incompressibility. It is shown that in the case of the independent Laughlin state tunneling is irrelevant when $\nu<1/2$ in the low temperature and long wave length limit. The temperature dependence of two-terminal conductance of the system in which only one of the two layers is contacted with electrode is discussed.Comment: 5 page

Unified hydrodynamics theory of the lowest Landau level

We propose a hydrodynamics theory of collective quantum Hall states, which describes incompressible liquids, hexatic liquid crystals, a bubble solid and a Wigner crystal states within a unified framework. The structure of the theory is uniquely determined by the space-time symmetry, and a symmetry with respect to static shear deformations. In agreement with recent experiments the theory predicts two gapped collective modes for incompressible liquids. We argue that the presence of the above two modes is a universal property of a magnetized two-dimensional collective liquid.Comment: RevTex, 8 pages. Revised and expanded versio