3,063 research outputs found
Fermionic full counting statistics with smooth boundaries: from discrete particles to bosonization
We revisit the problem of full counting statistics of particles on a segment
of a one-dimensional gas of free fermions. Using a combination of analytical
and numerical methods, we study the crossover between the counting of discrete
particles and of the continuous particle density as a function of smoothing in
the counting procedure. In the discrete-particle limit, the result is given by
the Fisher--Hartwig expansion for Toeplitz determinants, while in the
continuous limit we recover the bosonization results. This example of full
counting statistics with smoothing is also related to orthogonality
catastrophe, Fermi-edge singularity and non-equilibrium bosonization.Comment: 7 pages, 4 figure
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Cas9 interrogates DNA in discrete steps modulated by mismatches and supercoiling.
The CRISPR-Cas9 nuclease has been widely repurposed as a molecular and cell biology tool for its ability to programmably target and cleave DNA. Cas9 recognizes its target site by unwinding the DNA double helix and hybridizing a 20-nucleotide section of its associated guide RNA to one DNA strand, forming an R-loop structure. A dynamic and mechanical description of R-loop formation is needed to understand the biophysics of target searching and develop rational approaches for mitigating off-target activity while accounting for the influence of torsional strain in the genome. Here we investigate the dynamics of Cas9 R-loop formation and collapse using rotor bead tracking (RBT), a single-molecule technique that can simultaneously monitor DNA unwinding with base-pair resolution and binding of fluorescently labeled macromolecules in real time. By measuring changes in torque upon unwinding of the double helix, we find that R-loop formation and collapse proceed via a transient discrete intermediate, consistent with DNA:RNA hybridization within an initial seed region. Using systematic measurements of target and off-target sequences under controlled mechanical perturbations, we characterize position-dependent effects of sequence mismatches and show how DNA supercoiling modulates the energy landscape of R-loop formation and dictates access to states competent for stable binding and cleavage. Consistent with this energy landscape model, in bulk experiments we observe promiscuous cleavage under physiological negative supercoiling. The detailed description of DNA interrogation presented here suggests strategies for improving the specificity and kinetics of Cas9 as a genome engineering tool and may inspire expanded applications that exploit sensitivity to DNA supercoiling
Changes in the structure and geography of tourist flows during the COVID-19 pandemic
Tourism suffered more than many other sectors amid Covid-19 travel restrictions. This article looks at the impact of the pandemic on the territorial redistribution of tourist flows in the Baltic region. The study draws on regional annual and monthly data on international and domestic tourist arrivals between 2019 and 2021, available on the websites of the statistical offices of the Baltic Region countries. The regions earlier dependent on tourist arrivals from Russia experienced the largest drop in inbound tourism during the Covid-19 pandemic. Yet, domestic tourism burgeoned in most of the region’s countries. In 2021, the most dramatic increase in domestic arrivals occurred, year on year, in the border regions of Russia’s Northwest Federal District; the growth was also notable in Finland and the Baltic States. Domestic tourist flow compensated, partially or even fully, for the decrease in inbound tourist flow in most regions. The structure of inbound tourism changed markedly during the pandemic, with the share of tourists from the Baltic States growing. The strongest rise was in some regions of southern Finland and more modest in Latvia and Lithuania
Selection rules in the excitation of the divacancy and the nitrogen-vacancy pair in 4H- and 6H-SiC
In this study, we address the selection rules with respect to the
polarization of the optical excitation of two colour centres in 4H-SiC and
6H-SiC with potential for applications in quantum technology, the divacancy and
the nitrogen-vacancy pair. We show that the photoluminescence (PL) of the axial
configurations of higher symmetry (C3v) than the basal ones (C1h) can be
cancelled using any excitation (resonant or non-resonant) with polarization
parallel to the crystal axis (EL||c). The polarization selection rules are
determined using group-theoretical analysis and simple physical arguments
showing that phonon-assisted absorption with EL||c is prohibited despite being
formally allowed by group theory. A comparison with the selection rules for the
silicon vacancy, another defect with C3v symmetry, is also carried out. Using
the selection rules, we demonstrate selective excitation of only one basal
divacancy configuration in 4H-SiC, the P3 line and discuss the higher contrast
and increased Debye-Waller factor in the selectively excited spectrum
Exact Lagrangian submanifolds in simply-connected cotangent bundles
We consider exact Lagrangian submanifolds in cotangent bundles. Under certain
additional restrictions (triviality of the fundamental group of the cotangent
bundle, and of the Maslov class and second Stiefel-Whitney class of the
Lagrangian submanifold) we prove such submanifolds are Floer-cohomologically
indistinguishable from the zero-section. This implies strong restrictions on
their topology. An essentially equivalent result was recently proved
independently by Nadler, using a different approach.Comment: 28 pages, 3 figures. Version 2 -- derivation and discussion of the
spectral sequence considerably expanded. Other minor change
Identification and tunable optical coherent control of transition-metal spins in silicon carbide
Color centers in wide-bandgap semiconductors are attractive systems for
quantum technologies since they can combine long-coherent electronic spin and
bright optical properties. Several suitable centers have been identified, most
famously the nitrogen-vacancy defect in diamond. However, integration in
communication technology is hindered by the fact that their optical transitions
lie outside telecom wavelength bands. Several transition-metal impurities in
silicon carbide do emit at and near telecom wavelengths, but knowledge about
their spin and optical properties is incomplete. We present all-optical
identification and coherent control of molybdenum-impurity spins in silicon
carbide with transitions at near-infrared wavelengths. Our results identify
spin for both the electronic ground and excited state, with highly
anisotropic spin properties that we apply for implementing optical control of
ground-state spin coherence. Our results show optical lifetimes of 60 ns
and inhomogeneous spin dephasing times of 0.3 s, establishing
relevance for quantum spin-photon interfacing.Comment: Updated version with minor correction, full Supplementary Information
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Determination of reducing carbohydrates in natural honey samples by optical micrometry method
The present article deals with the determination of the total amount of glucose and fructose in natural honey samples by the optical micrometry (OM) method. (Poly)vinyl alcohol spherical granules impregnated with a 0.05 mol/l borax solution were chosen as a sensitive element. It was shown that the formation of chelate esters of boron with polymer and carbohydrates is a pH-dependent process, and that the pH range 8.5–10.0 is the most appropriate for quantifying the total amount of reducing carbohydrates because glucose and fructose are undiscriminated. The impregnated polymer is not sensitive to the sucrose presence in the solution due to the absence of cis-diol fragments in it. Subsequently, the OM method was tested in the analysis of natural honey samples. The relative standart deviation in the case of OM method is less than 6%, and the results are similar to those obtained by the iodometric titration method. This makes the OM method suitable for laboratory-scale applications
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