238 research outputs found
Critical exponents of the semimetal-insulator transition in graphene: A Monte Carlo study
The low-energy theory of graphene exhibits spontaneous chiral symmetry
breaking due to pairing of quasiparticles and holes, corresponding to a
semimetal-insulator transition at strong Coulomb coupling. We report a Lattice
Monte Carlo study of the critical exponents of this transition as a function of
the number of Dirac flavors , finding for
, for and for , with throughout. We compare our
results with recent analytical work for graphene and closely related systems,
and discuss scenarios for the fate of the chiral transition at finite
temperature and carrier density, an issue of relevance for upcoming experiments
with suspended graphene samples.Comment: 5 pages, 5 figures. Published versio
Graphene: from materials science to particle physics
Since its discovery in 2004, graphene, a two-dimensional hexagonal carbon
allotrope, has generated great interest and spurred research activity from
materials science to particle physics and vice versa. In particular, graphene
has been found to exhibit outstanding electronic and mechanical properties, as
well as an unusual low-energy spectrum of Dirac quasiparticles giving rise to a
fractional quantum Hall effect when freely suspended and immersed in a magnetic
field. One of the most intriguing puzzles of graphene involves the
low-temperature conductivity at zero density, a central issue in the design of
graphene-based nanoelectronic components. While suspended graphene experiments
have shown a trend reminiscent of semiconductors, with rising resistivity at
low temperatures, most theories predict a constant or even decreasing
resistivity. However, lattice field theory calculations have revealed that
suspended graphene is at or near the critical coupling for excitonic gap
formation due to strong Coulomb interactions, which suggests a simple and
straightforward explanation for the experimental data. In this contribution we
review the current status of the field with emphasis on the issue of gap
formation, and outline recent progress and future points of contact between
condensed matter physics and Lattice QCD.Comment: 14 pages, 6 figures. Plenary talk given at the XXVIII International
Symposium on Lattice Field Theory (Lattice 2010), June 14-19, 2010,
Villasimius, Sardinia, Ital
Is graphene in vacuum an insulator?
We present evidence, from Lattice Monte Carlo simulations of the phase
diagram of graphene as a function of the Coulomb coupling between
quasiparticles, that graphene in vacuum is likely to be an insulator. We find a
semimetal-insulator transition at , where
in vacuum, and on a
SiO substrate. Our analysis uses the logarithmic derivative of the order
parameter, supplemented by an equation of state. The insulating phase
disappears above a critical number of four-component fermion flavors . Our data are consistent with a second-order transition.Comment: 4 pages, 3 figures, published versio
Graphene: From materials science to particle physics
Since its discovery in 2004, graphene, a two-dimensional hexagonal carbon allotrope, has generated great interest and spurred research activity from materials science to particle physics and vice versa. In particular, graphene has been found to exhibit outstanding electronic and mechanical properties, as well as an unusual low-energy spectrum of Dirac quasiparticles giving rise to a fractional quantum Hall effect when freely suspended and immersed in a magnetic field. One of the most intriguing puzzles of graphene involves the low-temperature conductivity at zero density, a central issue in the design of graphene-based nanoelectronic components. While suspended graphene experiments have shown a trend reminiscent of semiconductors, with rising resistivity at low temperatures, most theories predict a constant or even decreasing resistivity. However, lattice field theory calculations have revealed that suspended graphene is at or near the critical coupling for excitonic gap formation due to strong Coulomb interactions, which suggests a simple and straightforward explanation for the experimental data. In this contribution we review the current status of the field with emphasis on the issue of gap formation, and outline recent progress and future points of contact between condensed matter physics and Lattice QCD
The Equation of State of the Unitary Fermi Gas: An Update on Lattice Calculations
The thermodynamic properties of the unitary Fermi gas (UFG) have recently
been measured to unprecedented accuracy at the MIT. In particular, these
measurements provide an improved understanding of the regime below T/eF ~ 0.20,
where a transition into a superfluid phase occurs. In light of this
development, we present an overview of state-of-the-art auxiliary field quantum
Monte Carlo (AFQMC) results for the UFG at finite temperature and compare them
with the MIT data for the energy, chemical potential, and density. These AFQMC
results have been obtained using methods based on the hybrid Monte Carlo (HMC)
algorithm, which was first introduced within the context of lattice QCD.Comment: 4 pages, 3 figure
Ab initio alpha-alpha scattering
Processes involving alpha particles and alpha-like nuclei comprise a major
part of stellar nucleosynthesis and hypothesized mechanisms for thermonuclear
supernovae. In an effort towards understanding alpha processes from first
principles, we describe in this letter the first ab initio calculation of
alpha-alpha scattering. We use lattice effective field theory to describe the
low-energy interactions of nucleons and apply a technique called the adiabatic
projection method to reduce the eight-body system to an effective two-cluster
system. We find good agreement between lattice results and experimental phase
shifts for S-wave and D-wave scattering. The computational scaling with
particle number suggests that alpha processes involving heavier nuclei are also
within reach in the near future.Comment: 6 pages, 6 figure
Structure and rotations of the Hoyle state
The excited state of the 12C nucleus known as the "Hoyle state" constitutes
one of the most interesting, difficult and timely challenges in nuclear
physics, as it plays a key role in the production of carbon via fusion of three
alpha particles in red giant stars. In this letter, we present ab initio
lattice calculations which unravel the structure of the Hoyle state, along with
evidence for a low-lying spin-2 rotational excitation. For the 12C ground state
and the first excited spin-2 state, we find a compact triangular configuration
of alpha clusters. For the Hoyle state and the second excited spin-2 state, we
find a "bent-arm" or obtuse triangular configuration of alpha clusters. We also
calculate the electromagnetic transition rates between the low-lying states of
12C.Comment: 4 pages, 3 figures, 4 tables, version to be published in Physical
Review Letter
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