101 research outputs found
From Clinical Diagnosis to the Discovery of Multigene Rare Sequence Variants in Pseudoxanthoma elasticum: A Case Report
Pseudoxanthoma elasticum (PXE) is a rare autosomal recessive disease clinically characterised by early cutaneous alterations, and by late clinically relevant ocular, and cardiovascular manifestations. ABCC6 genetic tests are used to confirm clinical PXE diagnosis, but this strategy may be rather challenging when only one ABCC6 pathogenic variant is found. A next-generation sequencing approach focusing on 362 genes related to the calcification process and/or to inherited retinal diseases was performed on a patient with clinical PXE diagnosis (skin papules and laxity, angioid streaks, and atrophy) who was carrier of only one ABCC6 rare sequence variant. Beside ABCC6, several rare sequence variants were detected which can contribute either to the occurrence of calcification (GGCX and SERPINF1 genes) and/or to ophthalmological manifestations (ABCA4, AGBL5, CLUAP1, and KCNV2 genes). This wide-spectrum analysis approach facilitates the identification of rare variants possibly involved in PXE, thus avoiding invasive skin biopsy as well as expensive and time-consuming diagnostic odyssey and allows to broaden and to deepen the knowledge on this complex rare disease and to improve patients' counselling, also with a future perspective of personalised medicine
Excitations and Quantum Fluctuations in Site Diluted Two-Dimensional Antiferromagnets
We study the effect of site dilution and quantum fluctuations in an
antiferromagnetic spin system on a square lattice within the linear spin-wave
approximation. By performing numerical diagonalization in real space and
finite-size scaling, we characterize the nature of the low-energy spin
excitations for different dilution fractions up to the classical percolation
threshold. We find nontrivial signatures of fractonlike excitations at high
frequencies. Our simulations also confirm the existence of an upper bound for
the amount of quantum fluctuations in the ground state of the system, leading
to the persistence of long-range order up to the percolation threshold. This
result is in agreement with recent neutron-scattering experimental data and
quantum Monte Carlo numerical calculations. We also show that the absence of a
quantum critical point below the classical percolation threshold holds for a
large class of systems whose Hamiltonians can be mapped onto a system of
coupled noninteracting massless bosons.Comment: RevTex 4, 16 pages, 8 EPS figures, typos corrected, data from Ref. 9
added, few minor changes in the text, to appear in Phys. Rev.
Hamiltonian Formulation of Quantum Error Correction and Correlated Noise: The Effects Of Syndrome Extraction in the Long Time Limit
We analyze the long time behavior of a quantum computer running a quantum
error correction (QEC) code in the presence of a correlated environment.
Starting from a Hamiltonian formulation of realistic noise models, and assuming
that QEC is indeed possible, we find formal expressions for the probability of
a faulty path and the residual decoherence encoded in the reduced density
matrix. Systems with non-zero gate times (``long gates'') are included in our
analysis by using an upper bound on the noise. In order to introduce the local
error probability for a qubit, we assume that propagation of signals through
the environment is slower than the QEC period (hypercube assumption). This
allows an explicit calculation in the case of a generalized spin-boson model
and a quantum frustration model. The key result is a dimensional criterion: If
the correlations decay sufficiently fast, the system evolves toward a
stochastic error model for which the threshold theorem of fault-tolerant
quantum computation has been proven. On the other hand, if the correlations
decay slowly, the traditional proof of this threshold theorem does not hold.
This dimensional criterion bears many similarities to criteria that occur in
the theory of quantum phase transitions.Comment: 19 pages, 5 figures. Includes response to arXiv:quant-ph/0702050. New
title and an additional exampl
Embedding method for the scattering phase in strongly correlated quantum dots
The embedding method for the calculation of the conductance through
interacting systems connected to single channel leads is generalized to obtain
the full complex transmission amplitude that completely characterizes the
effective scattering matrix of the system at the Fermi energy. We calculate the
transmission amplitude as a function of the gate potential for simple
diamond-shaped lattice models of quantum dots with nearest neighbor
interactions. In our simple models we do not generally observe an interaction
dependent change in the number of zeroes or phase lapses that depend only on
the symmetry properties of the underlying lattice. Strong correlations separate
and reduce the widths of the resonant peaks while preserving the qualitative
properites of the scattering phase.Comment: 11 pages, 3 figures. Proceedings of the Workshop on Advanced
Many-Body and Statistical Methods in Mesoscopic Systems, Constanta, Romania,
June 27th - July 2nd 2011. To appear in Journal of Physics: Conference Serie
Effect of oxygen plasma etching on graphene studied with Raman spectroscopy and electronic transport
We report a study of graphene and graphene field effect devices after
exposure to a series of short pulses of oxygen plasma. We present data from
Raman spectroscopy, back-gated field-effect and magneto-transport measurements.
The intensity ratio between Raman "D" and "G" peaks, I(D)/I(G) (commonly used
to characterize disorder in graphene) is observed to increase approximately
linearly with the number (N(e)) of plasma etching pulses initially, but then
decreases at higher Ne. We also discuss implications of our data for extracting
graphene crystalline domain sizes from I(D)/I(G). At the highest Ne measured,
the "2D" peak is found to be nearly suppressed while the "D" peak is still
prominent. Electronic transport measurements in plasma-etched graphene show an
up-shifting of the Dirac point, indicating hole doping. We also characterize
mobility, quantum Hall states, weak localization and various scattering lengths
in a moderately etched sample. Our findings are valuable for understanding the
effects of plasma etching on graphene and the physics of disordered graphene
through artificially generated defects.Comment: 10 pages, 5 figure
Case report of an atypical early onset X-linked retinoschisis in monozygotic twins
Background: X-linked Retinoschisis (XLRS) is one of the most common macular degenerations in young males, with a worldwide prevalence ranging from 1:5000 to 1:20000. Clinical diagnosis of XLRS can be challenging due to the highly variable phenotypic presentation and limited correlation has been identified between mutation type and disease severity or progression.
Case presentation: We report the atypical early onset of XLRS in 3-month-old monozygotic twins. Fundus examination was characterized by severe bullous retinal schisis with pre-retinal and intraretinal haemorrhages. Molecular genetic analysis of the RS1 was performed and the c.288G > A (p. Trp96Ter) mutation was detected in both patients.
Conclusions: Early onset XLRS is associated with a more progressive form of the disease, characterized by large bullous peripheral schisis involving the posterior pole, vascular abnormalities and haemorrhages. The availability of specific technology permitted detailed imaging of the clinical picture of unusual cases of XLRS. The possible relevance of modifying genes should be taken into consideration for the future development of XLRS gene therapy
Free Magnetic Moments in Disordered Metals
The screening of magnetic moments in metals, the Kondo effect, is found to be
quenched with a finite probability in the presence of nonmagnetic disorder.
Numerical results for a disordered electron system show that the distribution
of Kondo temperatures deviates strongly from the result expected from random
matrix theory. A pronounced second peak emerges for small Kondo temperatures,
showing that the probability that magnetic moments remain unscreened at low
temperatures increases with disorder. Analytical calculations, taking into
account correlations between eigenfunction intensities yield a finite width for
the distribution in the thermodynamic limit. Experimental consequences for
disordered mesoscopic metals are discussed.Comment: RevTex 4.0, 4.3 pages, 4 EPS figures; typos fixed, reference added,
final published versio
Manifestation of quantum chaos on scattering techniques: application to low-energy and photo-electron diffraction intensities
Intensities of LEED and PED are analyzed from a statistical point of view.
The probability distribution is compared with a Porter-Thomas law,
characteristic of a chaotic quantum system. The agreement obtained is
understood in terms of analogies between simple models and Berry's conjecture
for a typical wavefunction of a chaotic system. The consequences of this
behaviour on surface structural analysis are qualitatively discussed by looking
at the behaviour of standard correlation factors.Comment: 5 pages, 4 postscript figures, Latex, APS,
http://www.icmm.csic.es/Pandres/pedro.ht
Parametric Conductance Correlation for Irregularly Shaped Quantum Dots
We propose the autocorrelator of conductance peak heights as a signature of
the underlying chaotic dynamics in quantum dots in the Coulomb blockade regime.
This correlation function is directly accessible to experiments and its decay
width contains interesting information about the underlying electron dynamics.
Analytical results are derived in the framework of random matrix theory in the
regime of broken time-reversal symmetry. The final expression, upon rescaling,
becomes independent of the details of the system. For the situation when the
external parameter is a variable magnetic field, the system-dependent,
nonuniversal field scaling factor is obtained by a semiclassical approach. The
validity of our findings is confirmed by a comparison with results of an exact
numerical diagonalization of the conformal billiard threaded by a magnetic flux
line.Comment: Minor corrections added to the text and references (36 pages RevTeX
3, epsf, 10 figure
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