102 research outputs found
Excitonic gap, phase transition, and quantum Hall effect in graphene
We suggest that physics underlying the recently observed removal of
sublattice and spin degeneracies in graphene in a strong magnetic field
describes a phase transition connected with the generation of an excitonic gap.
The experimental form of the Hall conductivity is reproduced and the main
characteristics of the dynamics are described. Predictions of the behavior of
the gap as a function of temperature and a gate voltage are made.Comment: Revtex4, 10 pages, 4 figures, text essentially extended, one figure
and references added; v3: to match PRB versio
Indication of Superconductivity at 35 K in Graphite-Sulfur Composites
We report magnetization measurements performed on graphite--sulfur composites
which demonstrate a clear superconducting behavior below the critical
temperature T = 35 K. The Meissner-Ochsenfeld effect, screening
supercurrents, and magnetization hysteresis loops characteristic of type-II
superconductors were measured. The results indicate that the superconductivity
occurs in a small sample fraction, possibly related to the sample surface.Comment: published versio
Bose-Einstein Condensation of Excitons: Reply to Tikhodeev's Criticism
The extended version of our reply to Comment on ``Critical Velocities in
Exciton Superfluidity'' by S. G. Tikhodeev (Phys. Rev. Lett., 84 (2000), 3502
or from http://prl.aps.org/) is presented here. The principal question is
discussed: does the moving exciton-phonon packet contain the coherent
`nucleus', or the exciton-phonon condensate?Comment: 3 pages in LaTe
Local and Global Superconductivity in Bismuth
We performed magnetization M(H,T) and magnetoresistance R(T,H) measurements
on powdered (grain size ~ 149 micrometers) as well as highly oriented
rhombohedral (A7) bismuth (Bi) samples consisting of single crystalline blocks
of size ~ 1x1 mm2 in the plane perpendicular to the trigonal c-axis. The
obtained results revealed the occurrence of (1) local superconductivity in
powdered samples with Tc(0) = 8.75 \pm 0.05 K, and (2) global superconductivity
at Tc(0) = 7.3 \pm 0.1 K in polycrystalline Bi triggered by low-resistance
Ohmic contacts with silver (Ag) normal metal. The results provide evidence that
the superconductivity in Bi is localized in a tiny volume fraction, probably at
intergrain or Ag/Bi interfaces. On the other hand, the occurrence of global
superconductivity observed for polycrystalline Bi can be accounted for by
enhancement of the superconducting order parameter phase stiffness induced by
the normal metal contacts, the scenario proposed in the context of "pseudogap
regime" in cuprates [E. Berg et al., PRB 78, 094509 (2008)].Comment: 12 pages including 9 figures and 1 table, Special Issue to the 80th
birthday anniversary of V. G. Peschansky, Electronic Properties of Conducting
System
Magnetothermal Conductivity of Highly Oriented Pyrolytic Graphite in the Quantum Limit
We report on the magnetic field (0TT) dependence of the
longitudinal thermal conductivity of highly oriented pyrolytic
graphite in the temperature range 5 K 20 K for fields parallel to
the axis. We show that shows large oscillations in the
high-field region (B > 2 T) where clear signs of the Quantum-Hall effect are
observed in the Hall resistance. With the measured longitudinal electrical
resistivity we show that the Wiedemann-Franz law is violated in the high-field
regime.Comment: 4 Figures, to be published in Physical Review B (2003
Nernst effect in semi-metals: the meritorious heaviness of electrons
We present a study of electric, thermal and thermoelectric transport in
elemental Bismuth, which presents a Nernst coefficient much larger than what
was found in correlated metals. We argue that this is due to the combination of
an exceptionally low carrier density with a very long electronic
mean-free-path. The low thermomagnetic figure of merit is traced to the
lightness of electrons. Heavy-electron semi-metals, which keep a metallic
behavior in presence of a magnetic field, emerge as promising candidates for
thermomagnetic cooling at low temperatures.Comment: 4 pages, including 4 figure
Interaction Between Superconducting and Ferromagnetic Order Parameters in Graphite-Sulfur Composites
The superconductivity of graphite-sulfur composites is highly anisotropic and
associated with the graphite planes. The superconducting state coexists with
the ferromagnetism of pure graphite, and a continuous crossover from
superconducting to ferromagnetic-like behavior could be achieved by increasing
the magnetic field or the temperature. The angular dependence of the magnetic
moment m(alpha) provides evidence for an interaction between the ferromagnetic
and the superconducting order parameters.Comment: 11 pages, 4 figures, to be published in Phys. Rev.
Magnetic field-induced insulating behavior in highly oriented pyrolitic graphite
We propose an explanation for the apparent semimetal-insulator transition
observed in highly oriented pyrolitic graphite in the presence of magnetic
field perpendicular to the layers. We show that the magnetic field opens an
excitonic gap in the linear spectrum of the Coulomb interacting quasiparticles,
in a close analogy with the phenomenon of dynamical chiral symmetry breaking in
the relativistic theories of the 2+1-dimensional Dirac fermions. Our
strong-coupling appoach allows for a non-perturbative description of the
corresponding critical behavior
Low energy excitations in graphite: The role of dimensionality and lattice defects
In this paper, we present a high resolution angle resolved photoemission
spectroscopy (ARPES) study of the electronic properties of graphite. We found
that the nature of the low energy excitations in graphite is particularly
sensitive to interlayer coupling as well as lattice disorder. As a consequence
of the interlayer coupling, we observed for the first time the splitting of the
bands by 0.7 eV near the Brillouin zone corner K. At low
binding energy, we observed signatures of massless Dirac fermions with linear
dispersion (as in the case of graphene), coexisting with quasiparticles
characterized by parabolic dispersion and finite effective mass. We also report
the first ARPES signatures of electron-phonon interaction in graphite: a kink
in the dispersion and a sudden increase in the scattering rate. Moreover, the
lattice disorder strongly affects the low energy excitations, giving rise to
new localized states near the Fermi level. These results provide new insights
on the unusual nature of the electronic and transport properties of graphite.Comment: 10 pages, 15 figure
Edge states, mass and spin gaps, and quantum Hall effect in graphene
Motivated by recent experiments and a theoretical analysis of the gap
equation for the propagator of Dirac quasiparticles, we assume that the physics
underlying the recently observed removal of sublattice and spin degeneracies in
graphene in a strong magnetic field is connected with the generation of both
Dirac masses and spin gaps. The consequences of such a scenario for the
existence of the gapless edge states with zigzag and armchair boundary
conditions are discussed. In the case of graphene on a half-plane with a zigzag
edge, there are gapless edge states in the spectrum only when the spin gap
dominates over the mass gap. In the case of an armchair edge, however, the
existence of the gapless edge states depends on the specific type of mass gaps.Comment: 14 pages, 7 figure
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