27,884 research outputs found
Coulomb Oscillations in Antidots in the Integer and Fractional Quantum Hall Regimes
We report measurements of resistance oscillations in micron-scale antidots in
both the integer and fractional quantum Hall regimes. In the integer regime, we
conclude that oscillations are of the Coulomb type from the scaling of magnetic
field period with the number of edges bound to the antidot. Based on both
gate-voltage and field periods, we find at filling factor {\nu} = 2 a tunneling
charge of e and two charged edges. Generalizing this picture to the fractional
regime, we find (again, based on field and gate-voltage periods) at {\nu} = 2/3
a tunneling charge of (2/3)e and a single charged edge.Comment: related papers at http://marcuslab.harvard.ed
A Completely Invariant SUSY Transform of Supersymmetric QED
We study the SUSY breaking of the covariant gauge-fixing term in SUSY QED and
observe that this corresponds to a breaking of the Lorentz gauge condition by
SUSY. Reasoning by analogy with SUSY's violation of the Wess-Zumino gauge, we
argue that the SUSY transformation, already modified to preserve Wess-Zumino
gauge, should be further modified by another gauge transformation which
restores the Lorentz gauge condition. We derive this modification and use the
resulting transformation to derive a Ward identitiy relating the photon and
photino propagators without using ghost fields. Our transformation also
fulfills the SUSY algebra, modulo terms that vanish in Lorentz gauge
Distinct Signatures For Coulomb Blockade and Aharonov-Bohm Interference in Electronic Fabry-Perot Interferometers
Two distinct types of magnetoresistance oscillations are observed in two
electronic Fabry-Perot interferometers of different sizes in the integer
quantum Hall regime. Measuring these oscillations as a function of magnetic
field and gate voltages, we observe three signatures that distinguish the two
types. The oscillations observed in a 2.0 square micron device are understood
to arise from the Coulomb blockade mechanism, and those observed in an 18
square micron device from the Aharonov-Bohm mechanism. This work clarifies,
provides ways to distinguish, and demonstrates control over, these distinct
physical origins of resistance oscillations seen in electronic Fabry-Perot
interferometers.Comment: related papers at http://marcuslab.harvard.ed
Observation of pinning mode of stripe phases of 2D systems in high Landau levels
We study the radio-frequency diagonal conductivities of the anisotropic
stripe phases of higher Landau levels near half integer fillings. In the hard
direction, in which larger dc resistivity occurs, the spectrum exhibits a
striking resonance, while in the orthogonal, easy direction, no resonance is
discernable. The resonance is interpreted as a pinning mode of the stripe
phase
Measurements of quasi-particle tunneling in the nu = 5/2 fractional quantum Hall state
Some models of the 5/2 fractional quantum Hall state predict that the
quasi-particles, which carry the charge, have non-Abelian statistics: exchange
of two quasi-particles changes the wave function more dramatically than just
the usual change of phase factor. Such non-Abelian statistics would make the
system less sensitive to decoherence, making it a candidate for implementation
of topological quantum computation. We measure quasi-particle tunneling as a
function of temperature and DC bias between counter-propagating edge states.
Fits to theory give e*, the quasi-particle effective charge, close to the
expected value of e/4 and g, the strength of the interaction between
quasi-particles, close to 3/8. Fits corresponding to the various proposed wave
functions, along with qualitative features of the data, strongly favor the
Abelian 331 state
Onset of Interlayer Phase Coherence in a Bilayer Two-Dimensional Electron System: Effect of Layer Density Imbalance
Tunneling and Coulomb drag are sensitive probes of spontaneous interlayer
phase coherence in bilayer two-dimensional electron systems at total Landau
level filling factor . We find that the phase boundary between the
interlayer phase coherent state and the weakly-coupled compressible phase moves
to larger layer separations as the electron density distribution in the bilayer
is imbalanced. The critical layer separation increases quadratically with layer
density difference.Comment: 4 pages, 3 figure
Interference measurements of non-Abelian e/4 & Abelian e/2 quasiparticle braiding
The quantum Hall states at filling factors and are expected
to have Abelian charge quasiparticles and non-Abelian charge
quasiparticles. For the first time we report experimental evidence for the
non-Abelian nature of excitations at and examine the fermion parity,
a topological quantum number of an even number of non-Abelian quasiparticles,
by measuring resistance oscillations as a function of magnetic field in
Fabry-P\'erot interferometers using new high purity heterostructures. The phase
of observed oscillations is reproducible and stable over long times
(hours) near and , indicating stability of the fermion parity.
When phase fluctuations are observed, they are predominantly phase flips,
consistent with fermion parity change. We also examine lower-frequency
oscillations attributable to Abelian interference processes in both states.
Taken together, these results constitute new evidence for the non-Abelian
nature of quasiparticles; the observed life-time of their combined
fermion parity further strengthens the case for their utility for topological
quantum computation.Comment: A significantly revised version; 54 double-column pages containing 14
pages of main text + Supplementary Materials. The figures, which include a
number of new figures, are now incorporated into the tex
Graphs Identified by Logics with Counting
We classify graphs and, more generally, finite relational structures that are
identified by C2, that is, two-variable first-order logic with counting. Using
this classification, we show that it can be decided in almost linear time
whether a structure is identified by C2. Our classification implies that for
every graph identified by this logic, all vertex-colored versions of it are
also identified. A similar statement is true for finite relational structures.
We provide constructions that solve the inversion problem for finite
structures in linear time. This problem has previously been shown to be
polynomial time solvable by Martin Otto. For graphs, we conclude that every
C2-equivalence class contains a graph whose orbits are exactly the classes of
the C2-partition of its vertex set and which has a single automorphism
witnessing this fact.
For general k, we show that such statements are not true by providing
examples of graphs of size linear in k which are identified by C3 but for which
the orbit partition is strictly finer than the Ck-partition. We also provide
identified graphs which have vertex-colored versions that are not identified by
Ck.Comment: 33 pages, 8 Figure
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