21 research outputs found
Friedel oscillations in a two-band Hubbard model for CuO chains
Friedel oscillations induced by open boundary conditions in a two-band
Hubbard model for CuO chains are numerically studied. We find that for
physically realistic parameters and close to quarter filling, these
oscillations have a 2k_F modulation according with experimental results on
YBa_2Cu_3O_{7-delta}. In addition, we predict that, for the same parameters, as
hole doping is reduced from quarter filling to half filling, Friedel
oscillations would acquire a 4k_F modulation, typical of a strongly correlated
electrons regime. The 4k_F modulation dominates also in the electron doped
region. The range of parameters varied is very broad, and hence the results
reported could apply to other cuprates and other strongly correlated compounds
with quasi-one dimensional structures. On a more theoretical side, we stress
the fact that the copper and oxygen subsystems should be described by two
different Luttinger liquid exponents.Comment: 7 pages, 7 eps figure
A Coulomb gas approach to the anisotropic one-dimensional Kondo lattice model at arbitrary filling
We establish a mapping of a general spin-fermion system in one dimension into
a classical generalized Coulomb gas. This mapping allows a renormalization
group treatment of the anisotropic Kondo chain both at and away from
half-filling. We find that the phase diagram contains regions of paramagnetism,
partial and full ferromagnetic order. We also use the method to analyze the
phases of the Ising-Kondo chain.Comment: 19 pages, 9 figure
Self-completeness and spontaneous dimensional reduction
A viable quantum theory of gravity is one of the biggest challenges facing
physicists. We discuss the confluence of two highly expected features which
might be instrumental in the quest of a finite and renormalizable quantum
gravity -- spontaneous dimensional reduction and self-completeness. The former
suggests the spacetime background at the Planck scale may be effectively
two-dimensional, while the latter implies a condition of maximal compression of
matter by the formation of an event horizon for Planckian scattering. We
generalize such a result to an arbitrary number of dimensions, and show that
gravity in higher than four dimensions remains self-complete, but in lower
dimensions it is not. In such a way we established an "exclusive disjunction"
or "exclusive or" (XOR) between the occurrence of self-completeness and
dimensional reduction, with the goal of actually reducing the unknowns for the
scenario of the physics at the Planck scale. Potential phenomenological
implications of this result are considered by studying the case of a
two-dimensional dilaton gravity model resulting from dimensional reduction of
Einstein gravity.Comment: 12 pages, 3 figures; v3: final version in press on Eur. Phys. J. Plu
Pairing and Density Correlations of Stripe Electrons in a Two-Dimensional Antiferromagnet
We study a one-dimensional electron liquid embedded in a 2D antiferromagnetic
insulator, and coupled to it via a weak antiferromagnetic spin exchange
interaction. We argue that this model may qualitatively capture the physics of
a single charge stripe in the cuprates on length- and time scales shorter than
those set by its fluctuation dynamics. Using a local mean-field approach we
identify the low-energy effective theory that describes the electronic spin
sector of the stripe as that of a sine-Gordon model. We determine its phases
via a perturbative renormalization group analysis. For realistic values of the
model parameters we obtain a phase characterized by enhanced spin density and
composite charge density wave correlations, coexisting with subleading triplet
and composite singlet pairing correlations. This result is shown to be
independent of the spatial orientation of the stripe on the square lattice.
Slow transverse fluctuations of the stripes tend to suppress the density
correlations, thus promoting the pairing instabilities. The largest amplitudes
for the composite instabilities appear when the stripe forms an antiphase
domain wall in the antiferromagnet. For twisted spin alignments the amplitudes
decrease and leave room for a new type of composite pairing correlation,
breaking parity but preserving time reversal symmetry.Comment: Revtex, 28 pages incl. 5 figure
\eta-superconductivity in the Hubbard chain with pair hopping
The ground state phase diagram of the 1D Hubbard chain with pair-hopping
interaction is studied. The analysis of the model is performed using the
continuum-limit field theory approach and exact diagonalization studies. At
half-filling the phase diagram is shown to consist of two superconducting
states with Cooper pair center-of-mass momentum Q=0 (BCS-\eta_0 phase) and
Q=\pi (\eta_\pi-phase) and four insulating phases corresponding to the Mott
antiferromagnet, the Peierls dimerized phase, the charge-density-wave (CDW)
insulator as well as an unconventional insulating phase characterized by the
coexistence of a CDW and a bond-located staggered magnetization. Away from
half-filling the phase diagram consists of the superconducting BCS-\eta_0 and
\eta_\pi phases and the metallic Luttinger-liquid phase. The BCS-\eta_0 phase
exhibits smooth crossover from a weak-coupling BCS type to a strong coupling
local-pair regime. The \eta_\pi phase shows properties of the doublon (zero
size Cooper pair) superconductor with Cooper pair center-of-mass momentum
Q=\pi. The transition into the \eta_\pi- paired state corresponds to an abrupt
change in the groundstate structure. After the transition the conduction band
is completely destroyed and a new \eta_\pi-pair band corresponding to the
strongly correlated doublon motion is created.Comment: 15 pages Revtex, 15 embedded eps figure
The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance
INTRODUCTION
Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic.
RATIONALE
We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs).
RESULTS
Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants.
CONCLUSION
Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century
Nederlandse boseigenaar ervaart gevolgen klimaatverandering en pleit voor steun
Uit een enquĂȘte blijkt dat 80 procent van de boseigenaren steeds meer last heeft van natuurlijke verstoringen zoals droogte, stormschade en insecten. Veel boseigenaren en -beheerders overwegen over te stappen naar andere beheertypen als hier een klimaatslimme variant voor bestaat. De huidige SNL-beheertypen lijken niet toereikend om het Nederlandse bos gezond te houden. In de enquĂȘte geven eigenaren aan zeker ook een impuls nodig te hebben voor het multifunctionele bos