7,822 research outputs found
Collective Edge Modes near the onset of a graphene quantum spin Hall state
Graphene subject to a strong, tilted magnetic field exhibits an
insulator-metal transition tunable by tilt-angle, attributed to the transition
from a canted antiferromagnetic (CAF) to a ferromagnetic (FM) bulk state at
filling factor zero. We develop a theoretical description for the spin and
valley edge textures in the two phases, and the implied evolution in the nature
of edge modes through the transition. In particular, we show that the CAF has
gapless neutral modes in the bulk, but supports gapped charged edge modes. At
the transition to the FM state the charged edge modes become gapless and are
smoothly connected to the helical edge modes of the FM state. Possible
experimental consequences are discussed.Comment: 5 pages, 2 figure
Collective Bulk and Edge Modes through the Quantum Phase Transition in Graphene at
Undoped graphene in a strong, tilted magnetic field exhibits a radical change
in conduction upon changing the tilt-angle, which can be attributed to a
quantum phase transition from a canted antiferromagnetic (CAF) to a
ferromagnetic (FM) bulk state at filling factor . This behavior
signifies a change in the nature of the collective ground state and excitations
across the transition. Using the time-dependent Hartree-Fock approximation, we
study the collective neutral (particle-hole) excitations in the two phases,
both in the bulk and on the edge of the system. The CAF has gapless neutral
modes in the bulk, whereas the FM state supports only gapped modes in its bulk.
At the edge, however, only the FM state supports gapless charge-carrying
states. Linear response functions are computed to elucidate their sensitivity
to the various modes. The response functions demonstrate that the two phases
can be distinguished by the evolution of a local charge pulse at the edge.Comment: 15 pages, 23 figure
Emergence of helical edge conduction in graphene at the \nu=0 quantum Hall state
The conductance of graphene subject to a strong, tilted magnetic field
exhibits a dramatic change from insulating to conducting behavior with
tilt-angle, regarded as evidence for the transition from a canted
antiferromagnetic (CAF) to a ferromagnetic (FM) \nu=0 quantum Hall state. We
develop a theory for the electric transport in this system based on the
spin-charge connection, whereby the evolution in the nature of collective spin
excitations is reflected in the charge-carrying modes. To this end, we derive
an effective field theoretical description of the low-energy excitations,
associated with quantum fluctuations of the spin-valley domain wall
ground-state configuration which characterizes the two-dimensional (2D) system
with an edge. This analysis yields a model describing a one-dimensional charged
edge mode coupled to charge-neutral spin-wave excitations in the 2D bulk.
Focusing particularly on the FM phase, naively expected to exhibit perfect
conductance, we study a mechanism whereby the coupling to these bulk
excitations assists in generating back-scattering. Our theory yields the
conductance as a function of temperature and the Zeeman energy - the parameter
that tunes the transition between the FM and CAF phases - with behavior in
qualitative agreement with experiment.Comment: 16 pages, 1 figur
Design of optimized three-dimensional thrust nozzle contours
Design of optimized three-dimensional thrust nozzle contour
METCAN updates for high temperature composite behavior: Simulation/verification
The continued verification (comparisons with experimental data) of the METCAN (Metal Matrix Composite Analyzer) computer code is updated. Verification includes comparisons at room and high temperatures for two composites, SiC/Ti-15-3 and SiC/Ti-6-4. Specifically, verification of the SiC/Ti-15-3 composite includes comparisons of strength, modulus, and Poisson's ratio as well as stress-strain curves for four laminates at room temperature. High temperature verification includes comparisons of strength and stress-strain curves for two laminates. Verification of SiC/Ti-6-4 is for a transverse room temperature stress-strain curve and comparisons for transverse strength at three temperatures. Results of the verification indicates that METCAN can be used with confidence to simulate the high temperature nonlinear behavior of metal matrix composites
Spin-excitations of the quantum Hall ferromagnet of composite fermions
The spin-excitations of a fractional quantum Hall system are evaluated within
a bosonization approach. In a first step, we generalize Murthy and Shankar's
Hamiltonian theory of the fractional quantum Hall effect to the case of
composite fermions with an extra discrete degree of freedom. Here, we mainly
investigate the spin degrees of freedom, but the proposed formalism may be
useful also in the study of bilayer quantum-Hall systems, where the layer index
may formally be treated as an isospin. In a second step, we apply a
bosonization scheme, recently developed for the study of the two-dimensional
electron gas, to the interacting composite-fermion Hamiltonian. The dispersion
of the bosons, which represent quasiparticle-quasihole excitations, is
analytically evaluated for fractional quantum Hall systems at \nu = 1/3 and \nu
= 1/5. The finite width of the two-dimensional electron gas is also taken into
account explicitly. In addition, we consider the interacting bosonic model and
calculate the lowest-energy state for two bosons. Besides a continuum
describing scattering states, we find a bound-state of two bosons. This state
is interpreted as a pair excitation, which consists of a skyrmion of composite
fermions and an antiskyrmion of composite fermions. The dispersion relation of
the two-boson state is evaluated for \nu = 1/3 and \nu = 1/5. Finally, we show
that our theory provides the microscopic basis for a phenomenological
non-linear sigma-model for studying the skyrmion of composite fermions.Comment: Revised version, 14 pages, 4 figures, accepted to Phys. Rev.
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