310 research outputs found
Exchange-based CNOT gates for singlet-triplet qubits with spin orbit interaction
We propose a scheme for implementing the CNOT gate over qubits encoded in a
pair of electron spins in a double quantum dot. The scheme is based on exchange
and spin orbit interactions and on local gradients in Zeeman fields. We find
that the optimal device geometry for this implementation involves effective
magnetic fields that are parallel to the symmetry axis of the spin orbit
interaction. We show that the switching times for the CNOT gate can be as fast
as a few nanoseconds for realistic parameter values in GaAs semiconductors.
Guided by recent advances in surface codes, we also consider the perpendicular
geometry. In this case, leakage errors due to spin orbit interaction occur but
can be suppressed in strong magnetic fields
Stacking Faults, Bound States, and Quantum Hall Plateaus in Crystalline Graphite
We analyze the electronic properties of a simple stacking defect in Bernal
graphite. We show that a bound state forms, which disperses as |\bfk-\bfK|^3
in the vicinity of either of the two inequivalent zone corners \bfK. In the
presence of a strong c-axis magnetic field, this bound state develops a Landau
level structure which for low energies behaves as E\nd_n\propto |n B|^{3/2}.
We show that buried stacking faults have observable consequences for surface
spectroscopy, and we discuss the implications for the three-dimensional quantum
Hall effect (3DQHE). We also analyze the Landau level structure and chiral
surface states of rhombohedral graphite, and show that, when doped, it should
exhibit multiple 3DQHE plateaus at modest fields.Comment: 19 page
General Aviation Weather Encounter Case Studies
This study presents a compilation of 24 cases involving general aviation (GA) pilots’ weather encounters over the continental U.S. The project team interviewed pilots who had experienced a weather encounter, and we examined their backgrounds, flight experience, and weather encounter details. Results from meteorological data analysis for each weather encounter were consistent with findings of larger GA weather accident studies in terms of the types of hazards encountered and flight phase during which the encounters occurred. Investigation of pilot weather products and the sources from which they were obtained revealed a lack of uniformity of pre-flight data sources and underutilization of available en route flight information services. The team used these results to develop a set of pilot weather education and training recommendations intended to reduce the number and severity of weather encounters
Quasiparticle spectrum of d-wave superconductors in the mixed state: a large Fermi-velocity anisotropy study
The quasiparticle spectrum of a two-dimensional d-wave superconductor in the
mixed state, H_c1 << H << H_c2, is studied for large values of the ``anisotropy
ratio'' alpha_D = v_F/v_Delta. For a square vortex lattice rotated by 45
degrees from the quasiparticle anisotropy axes (and the usual choice of
Franz--Tesanovic singular gauge transformation) we determine essential features
of the band structure asymptotically for large alpha_D, using an effective
one-dimensional model, and compare them to numerical calculations. We find that
several features of the band structure decay to zero exponentially fast for
large alpha_D. Using a different choice of singular gauge transformation, we
obtain a different band structure, but still find qualitative agreement between
the 1D and full 2D calculations. Finally, we distort the square lattice into a
non-Bravais lattice. Both the one- and two-dimensional numerical calculations
of the energy spectra show a gap around zero-energy, with our gauge choice, and
the two excitation spectra agree reasonably well.Comment: 14 pages, 13 figures, revte
Translational Symmetry Breaking in the Superconducting State of the Cuprates: Analysis of the Quasiparticle Density of States
Motivated by the recent STM experiments of J.E. Hoffman et.al. and C. Howald
et.al., we study the effects of weak translational symmetry breaking on the
quasiparticle spectrum of a d-wave superconductor. We develop a general
formalism to discuss periodic charge order, as well as quasiparticle scattering
off localized defects. We argue that the STM experiments in
cannot be explained using a simple charge density
wave order parameter, but are consistent with the presence of a periodic
modulation in the electron hopping or pairing amplitude. We review the effects
of randomness and pinning of the charge order and compare it to the impurity
scattering of quasiparticles. We also discuss implications of weak
translational symmetry breaking for ARPES experiments.Comment: 12 pages, 9 figs; (v2) minor corrections to formalism, discussions of
dispersion, structure factors and sum rules added; (v3) discussion of
space-dependent normalization added. To be published in PR
Probing d-wave pairing correlations in the pseudogap regime of the cuprate superconductors via low-energy states near impurities
The issue of probing the pseudogap regime of the cuprate superconductors,
specifically with regard to the existence and nature of superconducting pairing
correlations of d-wave symmetry, is explored theoretically. It is shown that if
the d-wave correlations believed to describe the superconducting state persist
into the pseudogap regime, but with pair-potential phase-fluctuations that
destroy their long-range nature, then the low-energy quasiparticle states
observed near extended impurities in the truly superconducting state should
also persist as resonances in the pseudogap regime. The scattering of
quasiparticles by these phase-fluctuations broadens what was (in the
superconducting state) a sharp peak in the single-particle spectral function at
low energy, as we demonstrate within the context of a simple model. This peak
and its broadening is, in principle, accessible via scanning tunneling
spectroscopy near extended impurities in the pseudogap regime. If so, such
experiments would provide a probe of the extent to which d-wave superconducting
correlations persist upon entering the pseudogap regime, thus providing a
stringent diagnostic of the phase-fluctuation scenario.Comment: 8 pages, 2 figure
Singularity in the boundary resistance between superfluid He and a solid surface
We report new measurements in four cells of the thermal boundary resistance
between copper and He below but near the superfluid-transition
temperature . For fits of to the data yielded ,
whereas a fit to theoretical values based on the renormalization-group theory
yielded . Alternatively, a good fit of the theory to the data could
be obtained if the {\it amplitude} of the prediction was reduced by a factor
close to two. The results raise the question whether the boundary conditions
used in the theory should be modified.Comment: 4 pages, 4 figures, revte
Vortex states in 2D superconductor at high magnetic field in a periodic pinning potential
The effect of a periodic pinning array on the vortex state in a 2D
superconductor at low temperatures is studied within the framework of the
Ginzburg-Landau approach. It is shown that attractive interaction of vortex
cores to a commensurate pin lattice stabilizes vortex solid phases with long
range positional order against violent shear fluctuations. Exploiting a simple
analytical method, based on the Landau orbitals description, we derive a rather
detailed picture of the low temperatures vortex state phase diagram. It is
predicted that for sufficiently clean samples application of an artificial
periodic pinning array would enable one to directly detect the intrinsic shear
stiffness anisotropy characterizing the ideal vortex lattice.Comment: 8 pages, 5 figure
The Democratic Biopolitics of PrEP
PrEP (Pre-Exposure Prophylaxis) is a relatively new drug-based HIV prevention technique and an important means to lower the HIV risk of gay men who are especially vulnerable to HIV. From the perspective of biopolitics, PrEP inscribes itself in a larger trend of medicalization and the rise of pharmapower. This article reconstructs and evaluates contemporary literature on biopolitical theory as it applies to PrEP, by bringing it in a dialogue with a mapping of the political debate on PrEP. As PrEP changes sexual norms and subjectification, for example condom use and its meaning for gay subjectivity, it is highly contested. The article shows that the debate on PrEP can be best described with the concepts ‘sexual-somatic ethics’ and ‘democratic biopolitics’, which I develop based on the biopolitical approach of Nikolas Rose and Paul Rabinow. In contrast, interpretations of PrEP which are following governmentality studies or Italian Theory amount to either farfetched or trivial positions on PrEP, when seen in light of the political debate. Furthermore, the article is a contribution to the scholarship on gay subjectivity, highlighting how homophobia and homonormativity haunts gay sex even in liberal environments, and how PrEP can serve as an entry point for the destigmatization of gay sexuality and transformation of gay subjectivity. ‘Biopolitical democratization’ entails making explicit how medical technology and health care relates to sexual subjectification and ethics, to strengthen the voice of (potential) PrEP users in health politics, and to renegotiate the profit and power of Big Pharma
Uganda's HIV Prevention Success: The Role of Sexual Behavior Change and the National Response
There has been considerable interest in understanding what may have led to Uganda's dramatic decline in HIV prevalence, one of the world's earliest and most compelling AIDS prevention successes. Survey and other data suggest that a decline in multi-partner sexual behavior is the behavioral change most likely associated with HIV decline. It appears that behavior change programs, particularly involving extensive promotion of “zero grazing” (faithfulness and partner reduction), largely developed by the Ugandan government and local NGOs including faith-based, women’s, people-living-with-AIDS and other community-based groups, contributed to the early declines in casual/multiple sexual partnerships and HIV incidence and, along with other factors including condom use, to the subsequent sharp decline in HIV prevalence. Yet the debate over “what happened in Uganda” continues, often involving divisive abstinence-versus-condoms rhetoric, which appears more related to the culture wars in the USA than to African social reality
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