21 research outputs found
Hidden Orbital Order in
When matter is cooled from high temperatures, collective instabilities
develop amongst its constituent particles that lead to new kinds of order. An
anomaly in the specific heat is a classic signature of this phenomenon. Usually
the associated order is easily identified, but sometimes its nature remains
elusive. The heavy fermion metal is one such example, where the
order responsible for the sharp specific heat anomaly at has
remained unidentified despite more than seventeen years of effort. In
, the coexistence of large electron-electron repulsion and
antiferromagnetic fluctuations in leads to an almost incompressible
heavy electron fluid, where anisotropically paired quasiparticle states are
energetically favored. In this paper we use these insights to develop a
detailed proposal for the hidden order in . We show that
incommensurate orbital antiferromagnetism, associated with circulating currents
between the uranium ions, can account for the local fields and entropy loss
observed at the transition; furthermore we make detailed predictions for
neutron scattering measurements
Bipolar supercurrent in graphene
Graphene -a recently discovered one-atom-thick layer of graphite- constitutes
a new model system in condensed matter physics, because it is the first
material in which charge carriers behave as massless chiral relativistic
particles. The anomalous quantization of the Hall conductance, which is now
understood theoretically, is one of the experimental signatures of the peculiar
transport properties of relativistic electrons in graphene. Other unusual
phenomena, like the finite conductivity of order 4e^2/h at the charge
neutrality (or Dirac) point, have come as a surprise and remain to be
explained. Here, we study the Josephson effect in graphene. Our experiments
rely on mesoscopic superconducting junctions consisting of a graphene layer
contacted by two closely spaced superconducting electrodes, where the charge
density can be controlled by means of a gate electrode. We observe a
supercurrent that, depending on the gate voltage, is carried by either
electrons in the conduction band or by holes in the valence band. More
importantly, we find that not only the normal state conductance of graphene is
finite, but also a finite supercurrent can flow at zero charge density. Our
observations shed light on the special role of time reversal symmetry in
graphene and constitute the first demonstration of phase coherent electronic
transport at the Dirac point.Comment: Under review, 12 pages, 4 Figs., suppl. info (v2 identical, resolved
file problems
Modeling electromagnetic form factors of light and heavy pseudoscalar mesons
The electromagnetic form factors of light and heavy pseudoscalar mesons are
calculated within two covariant constituent-quark models, a light-front and a
dispersion relation approach. We investigate the details and physical origins
of the model dependence of various hadronic observables: the weak decay
constant, the charge radius and the elastic electromagnetic form factor.Comment: 6 pages, 4 figures, use revtex4. Figure 2 and references are
corrected. Acknoledgments are adde
Properties of Graphene: A Theoretical Perspective
In this review, we provide an in-depth description of the physics of
monolayer and bilayer graphene from a theorist's perspective. We discuss the
physical properties of graphene in an external magnetic field, reflecting the
chiral nature of the quasiparticles near the Dirac point with a Landau level at
zero energy. We address the unique integer quantum Hall effects, the role of
electron correlations, and the recent observation of the fractional quantum
Hall effect in the monolayer graphene. The quantum Hall effect in bilayer
graphene is fundamentally different from that of a monolayer, reflecting the
unique band structure of this system. The theory of transport in the absence of
an external magnetic field is discussed in detail, along with the role of
disorder studied in various theoretical models. We highlight the differences
and similarities between monolayer and bilayer graphene, and focus on
thermodynamic properties such as the compressibility, the plasmon spectra, the
weak localization correction, quantum Hall effect, and optical properties.
Confinement of electrons in graphene is nontrivial due to Klein tunneling. We
review various theoretical and experimental studies of quantum confined
structures made from graphene. The band structure of graphene nanoribbons and
the role of the sublattice symmetry, edge geometry and the size of the
nanoribbon on the electronic and magnetic properties are very active areas of
research, and a detailed review of these topics is presented. Also, the effects
of substrate interactions, adsorbed atoms, lattice defects and doping on the
band structure of finite-sized graphene systems are discussed. We also include
a brief description of graphane -- gapped material obtained from graphene by
attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic
Tunable metal-insulator transition in double-layer graphene heterostructures
We report a double-layer electronic system made of two closely-spaced but
electrically isolated graphene monolayers sandwiched in boron nitride. For
large carrier densities in one of the layers, the adjacent layer no longer
exhibits a minimum metallic conductivity at the neutrality point, and its
resistivity diverges at low temperatures. This divergence can be suppressed by
magnetic field or by reducing the carrier density in the adjacent layer. We
believe that the observed localization is intrinsic for neutral graphene with
generic disorder if metallic electron-hole puddles are screened out
Noninertial effects on a Dirac neutral particle inducing an analogue of the Landau quantization in the cosmic string spacetime
We discuss the behaviour of external fields that interact with a Dirac
neutral particle with a permanent electric dipole moment in order to achieve
relativistic bound states solutions in a noninertial frame and in the presence
of a topological defect spacetime. We show that the noninertial effects of the
Fermi-Walker reference frame induce a radial magnetic field even in the absence
of magnetic charges, which is influenced by the topology of the cosmic string
spacetime. We then discuss the conditions that the induced fields must satisfy
to yield the relativistic bound states corresponding to the
Landau-He-McKellar-Wilkens quantization in the cosmic string spacetime. Finally
we obtain the Dirac spinors for positive-energy solutions and the Gordon
decomposition of the Dirac probability current.Comment: 15 pages, no figure, this paper will be published in volume 42 of the
Brazilian Journal of Physic
Carbon nanotubes as excitonic insulators
Fifty years ago Walter Kohn speculated that a zero-gap semiconductor might be unstable against the spontaneous generation of excitons-electron-hole pairs bound together by Coulomb attraction. The reconstructed ground state would then open a gap breaking the symmetry of the underlying lattice, a genuine consequence of electronic correlations. Here we show that this excitonic insulator is realized in zero-gap carbon nanotubes by performing first-principles calculations through many-body perturbation theory as well as quantum Monte Carlo. The excitonic order modulates the charge between the two carbon sublattices opening an experimentally observable gap, which scales as the inverse of the tube radius and weakly depends on the axial magnetic field. Our findings call into question the Luttinger liquid paradigm for nanotubes and provide tests to experimentally discriminate between excitonic and Mott insulators