1,783 research outputs found
Dimensional reduction, quantum Hall effect and layer parity in graphite films
The quantum Hall effect (QHE) originates from discrete Landau levels forming
in a two-dimensional (2D) electron system in a magnetic field. In three
dimensions (3D), the QHE is forbidden because the third dimension spreads
Landau levels into multiple overlapping bands, destroying the quantisation.
Here we report the QHE in graphite crystals that are up to hundreds of atomic
layers thick - thickness at which graphite was believed to behave as a 3D bulk
semimetal. We attribute the observation to a dimensional reduction of electron
dynamics in high magnetic fields, such that the electron spectrum remains
continuous only in the direction of the magnetic field, and only the last two
quasi-one-dimensional (1D) Landau bands cross the Fermi level. In sufficiently
thin graphite films, the formation of standing waves breaks these 1D bands into
a discrete spectrum, giving rise to a multitude of quantum Hall plateaux.
Despite a large number of layers, we observe a profound difference between
films with even and odd numbers of graphene layers. For odd numbers, the
absence of inversion symmetry causes valley polarisation of the standing-wave
states within 1D Landau bands. This reduces QHE gaps, as compared to films of
similar thicknesses but with even layer numbers because the latter retain the
inversion symmetry characteristic of bilayer graphene. High-quality graphite
films present a novel QHE system with a parity-controlled valley polarisation
and intricate interplay between orbital, spin and valley states, and clear
signatures of electron-electron interactions including the fractional QHE below
0.5 K
Transverse Lattice
The transverse lattice approach to non-perturbative light-front hamiltonian
QCD is described. Preliminary results on the pi-rho system are presented, at
fixed DLCQ and Tamm-Dancoff cut-offs. A renormalised, approximately Lorentz
covariant light-front hamiltonian is found to leading order of the
colour-dielectric expansion, compatible with a massless pion. The pi
light-front wavefunction is compared with experiment. Exclusive processes agree
reasonably well, given the approximations, but inclusive processes, sensitive
to higher Fock state structure, still exhibit large cut-off artifacts.Comment: 11 pages, Invited talk at Xth International Light-Cone Meeting on
Non-Perturbative QCD and Hadron Phenomenology, Heidelberg 12-17 June 200
A real parametric characterisation of ex-service compressor blade leading edges
In-service the degradation of compressor blade leading edges can have a disproportional effect on compressor efficiency. The high surface curvature in this region makes quantifying the surface finish of this sensitive and prominent region difficult. An automated technique that characterises the roughness of the leading edge in terms of areal parameters is presented. A set of ex-service blades of differing sizes are used to demonstrate the procedure. Improved characterisation of this blade region will allow engine companies to better understand where in-service deterioration has the greatest effect and inform them as to how they might minimise the effect. The present work shows that the leading edges of compressor blades exhibit a significantly higher characteristic surface roughness than other blade regions, and the spatial distribution of peaks in this characteristic roughness is detailed. In addition it is shown that peak wear and roughness are not uniformly correlated
In-flight calibration and verification of the Planck-LFI instrument
In this paper we discuss the Planck-LFI in-flight calibration campaign. After
a brief overview of the ground test campaigns, we describe in detail the
calibration and performance verification (CPV) phase, carried out in space
during and just after the cool-down of LFI. We discuss in detail the
functionality verification, the tuning of the front-end and warm electronics,
the preliminary performance assessment and the thermal susceptibility tests.
The logic, sequence, goals and results of the in-flight tests are discussed.
All the calibration activities were successfully carried out and the instrument
response was comparable to the one observed on ground. For some channels the
in-flight tuning activity allowed us to improve significantly the noise
performance.Comment: Long technical paper on Planck LFI in flight calibration campaign:
109 pages in this (not final) version, 100 page in the final JINST versio
The confining string beyond the free-string approximation in the gauge dual of percolation
We simulate five different systems belonging to the universality class of the
gauge dual of three-dimensional random percolation to study the underlying
effective string theory at finite temperature. All the data for the finite
temperature string tension, when expressed by means of adimensional variables,
are nicely described by a unique scaling function. We calculate the first few
terms of the string tension up to order and compare to different
theoretical predictions. We obtain unambiguous evidence that the coefficients
of and terms coincide with those of the Nambu-Goto string, as
expected, while the term strongly differs and is characteristic of the
universality class of this specific gauge theory.Comment: 13 pages, 3 figure
The Relativistic Bound State Problem in QCD: Transverse Lattice Methods
The formalism for describing hadrons using a light-cone Hamiltonian of SU(N)
gauge theory on a coarse transverse lattice is reviewed. Physical gauge degrees
of freedom are represented by disordered flux fields on the links of the
lattice. A renormalised light-cone Hamiltonian is obtained by making a
colour-dielectric expansion for the link-field interactions. Parameters in the
Hamiltonian are renormalised non-perturbatively by seeking regions in parameter
space with enhanced Lorentz symmetry. In the case of pure gauge theories to
lowest non-trivial order of the colour-dielectric expansion, this is sufficient
to determine accurately all parameters in the large-N limit. We summarize
results from applications to glueballs. After quarks are added, the Hamiltonian
and Hilbert space are expanded in both dynamical fermion and link fields.
Lorentz and chiral symmetry are not sufficient to accurately determine all
parameters to lowest non-trivial order of these expansions. However, Lorentz
symmetry and one phenomenological input, a chiral symmetry breaking scale, are
enough to fix all parameters unambiguously. Applications to light-light and
heavy-light mesons are described.Comment: 55 pp, revised version, to appear in `Progress in Particle and
Nuclear Physics
Spin and orbital fluctuations in non-equilibrium transport through quantum dots: A renormalisation-group analysis
We study non-equilibrium current and occupation probabilities of a
two-orbital quantum dot. The couplings to the leads are allowed to be
asymmetric and orbital dependent as it is generically the case in transport
experiments on molecules and nanowires. Starting from a two-orbital Anderson
model, we perform a generalised Schrieffer-Wolff transformation to derive an
effective Kondo model. This generates an orbital potential scattering
contribution which is of the same order as the spin exchange interaction. In a
first perturbative analysis we identify a regime of negative differential
conductance and a cascade resonance in the presence of an external magnetic
field, which both originate from the non-equilibrium occupation of the
orbitals. We then study the logarithmic enhancement of these signatures by
means of a renormalisation-group treatment. We find that the orbital potential
scattering qualitatively changes the renormalisation of the spin exchange
couplings and strongly affects the differential conductance for asymmetric
couplings.Comment: 6 pages, 4 figures, revised version as publishe
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