709 research outputs found
Superconductivity in a Hubbard-Froehlich Model and in cuprates
Using the variational Monte-Carlo method we find that a relatively weak
long-range electron-phonon interaction induces a d-wave superconducting state
of doped Mott-Hubbard insulators and/or strongly-correlated metals with a
condensation energy significantly larger than can be obtained with Coulomb
repulsion only. Moreover, the superconductivity is shown to exist for infinite
on-site Coulomb repulsion, removing the requirement for additional mechanisms
such as spin fluctuations to mediate d-wave superconductivity. We argue that
the superconducting state is robust with respect to a more intricate choice of
the trial function and that the true origin of high-temperature
superconductivity lies in a proper combination of strong electron-electron
correlations with poorly screened Froehlich electron-phonon interaction.Comment: 4 pages, 4 figure
Integer Vector Addition Systems with States
This paper studies reachability, coverability and inclusion problems for
Integer Vector Addition Systems with States (ZVASS) and extensions and
restrictions thereof. A ZVASS comprises a finite-state controller with a finite
number of counters ranging over the integers. Although it is folklore that
reachability in ZVASS is NP-complete, it turns out that despite their
naturalness, from a complexity point of view this class has received little
attention in the literature. We fill this gap by providing an in-depth analysis
of the computational complexity of the aforementioned decision problems. Most
interestingly, it turns out that while the addition of reset operations to
ordinary VASS leads to undecidability and Ackermann-hardness of reachability
and coverability, respectively, they can be added to ZVASS while retaining
NP-completness of both coverability and reachability.Comment: 17 pages, 2 figure
Electrically pumped single-defect light emitters in WSe
Recent developments in fabrication of van der Waals heterostructures enable
new type of devices assembled by stacking atomically thin layers of
two-dimensional materials. Using this approach, we fabricate light-emitting
devices based on a monolayer WSe, and also comprising boron nitride
tunnelling barriers and graphene electrodes, and observe sharp luminescence
spectra from individual defects in WSe under both optical and electrical
excitation. This paves the way towards the realization of electrically-pumped
quantum emitters in atomically thin semiconductors. In addition we demonstrate
tuning by more than 1 meV of the emission energy of the defect luminescence by
applying a vertical electric field. This provides an estimate of the permanent
electric dipole created by the corresponding electron-hole pair. The
light-emitting devices investigated in our work can be assembled on a variety
of substrates enabling a route to integration of electrically pumped single
quantum emitters with existing technologies in nano-photonics and
optoelectronics
âBad Mum Guiltâ: The Representation of âWork-Life Balanceâ in UK Womenâs Magazines
The social policy climate, labour market trends and gendered arrangements for paid and family work mean that âwork-life balanceâ remains a key social issue in the UK. Media representations of âwork-life balanceâ are a key source for the construction of gender and working motherhood. Despite evidence of gendered representations in media coverage of other social issues, little attention has been paid to the construction of work-life balance in UK women's magazines. Articles from the highest circulating UK women's magazines are analysed using a discursive approach to explicate constructions of work-life balance and working motherhood. The analysis reveals that multiple roles are constructed as a problematic choice leading to stress and guilt. Problems associated with multiple roles are constructed as individual problems, in a way that decontextualises and depoliticises them and normalises gendered assumptions and a gendered division of labour. Parallels can be drawn between this and wider discourses about women's daily lives and to the UK social policy context
A Three-Point Cosmic Ray Anisotropy Method
The two-point angular correlation function is a traditional method used to
search for deviations from expectations of isotropy. In this paper we develop
and explore a statistically descriptive three-point method with the intended
application being the search for deviations from isotropy in the highest energy
cosmic rays. We compare the sensitivity of a two-point method and a
"shape-strength" method for a variety of Monte-Carlo simulated anisotropic
signals. Studies are done with anisotropic source signals diluted by an
isotropic background. Type I and II errors for rejecting the hypothesis of
isotropic cosmic ray arrival directions are evaluated for four different event
sample sizes: 27, 40, 60 and 80 events, consistent with near term data
expectations from the Pierre Auger Observatory. In all cases the ability to
reject the isotropic hypothesis improves with event size and with the fraction
of anisotropic signal. While ~40 event data sets should be sufficient for
reliable identification of anisotropy in cases of rather extreme (highly
anisotropic) data, much larger data sets are suggested for reliable
identification of more subtle anisotropies. The shape-strength method
consistently performs better than the two point method and can be easily
adapted to an arbitrary experimental exposure on the celestial sphere.Comment: Fixed PDF erro
Bose-Einstein condensation of strongly correlated electrons and phonons in cuprate superconductors
The long-range Froehlich electron-phonon interaction has been identified as
the most essential for pairing in high-temperature superconductors owing to
poor screening, as is now confirmed by optical, isotope substitution, recent
photoemission and some other measurements. I argue that low energy physics in
cuprate superconductors is that of superlight small bipolarons, which are
real-space hole pairs dressed by phonons in doped charge-transfer Mott
insulators. They are itinerant quasiparticles existing in the Bloch states at
low temperatures as also confirmed by continuous-time quantum Monte-Carlo
algorithm (CTQMC) fully taking into account realistic Coulomb and long-range
Froehlich interactions. Here I suggest that a parameter-free evaluation of Tc,
unusual upper critical fields, the normal state Nernst effect, diamagnetism,
the Hall-Lorenz numbers and giant proximity effects strongly support the
three-dimensional (3D) Bose-Einstein condensation of mobile small bipolarons
with zero off-diagonal order parameter above the resistive critical temperature
Tc at variance with phase fluctuation scenarios of cuprates.Comment: 35 pages, 10 figures, to appear in the special volume of Journal of
Physics: Condensed Matte
Regular Strategies in Pushdown Reachability Games
International audienceWe show that positional winning strategies in pushdown reachability games can be implemented by deterministic finite state au-tomata of exponential size. Such automata read the stack and control state of a given pushdown configuration and output the set of winning moves playable from that position. This result can originally be attributed to Kupferman, Piterman and Vardi using an approach based on two-way tree automata. We present a more direct approach that builds upon the popular saturation technique. Saturation for analysing pushdown systems has been successfully implemented by Moped and WALi. Thus, our approach has the potential for practical applications to controller-synthesis problems
The QICKD study protocol: a cluster randomised trial to compare quality improvement interventions to lower systolic BP in chronic kidney disease (CKD) in primary care.
BACKGROUND: Chronic kidney disease (CKD) is a relatively newly recognised but common long-term condition affecting 5 to 10% of the population. Effective management of CKD, with emphasis on strict blood pressure (BP) control, reduces cardiovascular risk and slows the progression of CKD. There is currently an unprecedented rise in referral to specialist renal services, which are often located in tertiary centres, inconvenient for patients, and wasteful of resources. National and international CKD guidelines include quality targets for primary care. However, there have been no rigorous evaluations of strategies to implement these guidelines. This study aims to test whether quality improvement interventions improve primary care management of elevated BP in CKD, reduce cardiovascular risk, and slow renal disease progression DESIGN: Cluster randomised controlled trial (CRT) METHODS: This three-armed CRT compares two well-established quality improvement interventions with usual practice. The two interventions comprise: provision of clinical practice guidelines with prompts and audit-based education. The study population will be all individuals with CKD from general practices in eight localities across England. Randomisation will take place at the level of the general practices. The intended sample (three arms of 25 practices) powers the study to detect a 3 mmHg difference in systolic BP between the different quality improvement interventions. An additional 10 practices per arm will receive a questionnaire to measure any change in confidence in managing CKD. Follow up will take place over two years. Outcomes will be measured using anonymised routinely collected data extracted from practice computer systems. Our primary outcome measure will be reduction of systolic BP in people with CKD and hypertension at two years. Secondary outcomes will include biomedical outcomes and markers of quality, including practitioner confidence in managing CKD. A small group of practices (n = 4) will take part in an in-depth process evaluation. We will use time series data to examine the natural history of CKD in the community. Finally, we will conduct an economic evaluation based on a comparison of the cost effectiveness of each intervention. CLINICAL TRIALS REGISTRATION: ISRCTN56023731. ClinicalTrials.gov identifier
The Pierre Auger Observatory III: Other Astrophysical Observations
Astrophysical observations of ultra-high-energy cosmic rays with the Pierre
Auger ObservatoryComment: Contributions to the 32nd International Cosmic Ray Conference,
Beijing, China, August 201
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