1,291 research outputs found
Application of spectral phase shaping to high resolution CARS spectroscopy
By spectral phase shaping of both the pump and probe pulses in coherent anti-Stokes Raman scattering (CARS) spectroscopy we demonstrate the extraction of the frequencies, bandwidths and relative cross sections of vibrational lines. We employ a tunable broadband Ti:Sapphire laser synchronized to a ps-Nd:YVO mode locked laser. A high resolution spectral phase shaper allows for spectroscopy with a precision better than 1 cm-1 in the high frequency region around 3000 cm-1. We also demonstrate how new spectral phase shaping strategies can amplify the resonant features of isolated vibrations to such an extent that spectroscopy and microscopy can be done at high resolution, on the integrated spectral response without the need for a spectrograph
spl(2,1) dynamical supersymmetry and suppression of ferromagnetism in flat band double-exchange models
The low energy spectrum of the ferromagnetic Kondo lattice model on a N-site
complete graph extended with on-site repulsion is obtained from the underlying
spl(2,1) algebra properties in the strong coupling limit. The ferromagnetic
ground state is realized for 1 and N+1 electrons only. We identify the large
density of states to be responsible for the suppression of the ferromagnetic
state and argue that a similar situation is encountered in the Kagome,
pyrochlore, and other lattices with flat bands in their one-particle density of
states.Comment: 7 pages, 1 figur
Conference Discussion of the Nuclear Force
Discussion of the nuclear force, lead by a round table consisting of T.
Cohen, E. Epelbaum, R. Machleidt, and F. Gross (chair). After an invited talk
by Machleidt, published elsewhere in these proceedings, brief remarks are made
by Epelbaum, Cohen, and Gross, followed by discussion from the floor moderated
by the chair. The chair asked the round table and the participants to focus on
the following issues: (i) What does each approach (chiral effective field
theory, large Nc, and relativistic phenomenology) contribute to our knowledge
of the nuclear force? Do we need them all? Is any one transcendent? (ii) How
important for applications (few body, nuclear structure, EMC effect, for
example) are precise fits to the NN data below 350 MeV? How precise do these
fits have to be? (iii) Can we learn anything about nonperturbative QCD from
these studies of the nuclear force? The discussion presented here is based on a
video recording made at the conference and transcribed afterward.Comment: Discussion at the 21st European Conference on Few Body Problems
(EFP21) held at Salamanca, Spain, 30 Aug - 3 Sept 201
On the stability of the primordial closed string gas
We recast the study of a closed string gas in a toroidal container in the
physical situation in which the single string density of states is independent
of the volume because energy density is very high. This includes the gas for
the well known Brandenberger-Vafa cosmological scenario. We describe the gas in
the grandcanonical and microcanonical ensembles. In the microcanonical
description, we find a result that clearly confronts the Brandenberger-Vafa
calculation to get the specific heat of the system. The important point is that
we use the same approach to the problem but a different regularization. By the
way, we show that, in the complex temperature formalism, at the Hagedorn
singularity, the analytic structure obtained from the so-called
F-representation of the free energy coincides with the one computed using the
S-representation.Comment: 20 pages and 1 figure. The final version that appeared in JHE
Condensing Momentum Modes in 2-d 0A String Theory with Flux
We use a combination of conformal perturbation theory techniques and matrix
model results to study the effects of perturbing by momentum modes two
dimensional type 0A strings with non-vanishing Ramond-Ramond (RR) flux. In the
limit of large RR flux (equivalently, mu=0) we find an explicit analytic form
of the genus zero partition function in terms of the RR flux and the
momentum modes coupling constant alpha. The analyticity of the partition
function enables us to go beyond the perturbative regime and, for alpha>> q,
obtain the partition function in a background corresponding to the momentum
modes condensation. For momenta such that 0<p<2 we find no obstruction to
condensing the momentum modes in the phase diagram of the partition function.Comment: 22 page
Conformal Field Theory and Hyperbolic Geometry
We examine the correspondence between the conformal field theory of boundary
operators and two-dimensional hyperbolic geometry. By consideration of domain
boundaries in two-dimensional critical systems, and the invariance of the
hyperbolic length, we motivate a reformulation of the basic equation of
conformal covariance. The scale factors gain a new, physical interpretation. We
exhibit a fully factored form for the three-point function. A doubly-infinite
discrete series of central charges with limit c=-2 is discovered. A
correspondence between the anomalous dimension and the angle of certain
hyperbolic figures emerges. Note: email after 12/19: [email protected]: 7 pages (PlainTeX
Meson-Baryon-Baryon Vertex Function and the Ward-Takahashi Identity
Ohta proposed a solution for the well-known difficulty of satisfying the
Ward-Takahashi identity for a photo-meson-baryon-baryon amplitude (MBB)
when a dressed meson-baryon-baryon (MBB) vertex function is present. He
obtained a form for the MBB amplitude which contained, in addition to
the usual pole terms, longitudinal seagull terms which were determined entirely
by the MBB vertex function. He arrived at his result by using a Lagrangian
which yields the MBB vertex function at tree level. We show that such a
Lagrangian can be neither hermitian nor charge conjugation invariant. We have
been able to reproduce Ohta's result for the MBB amplitude using the
Ward-Takahashi identity and no other assumption, dynamical or otherwise, and
the most general form for the MBB and MBB vertices. However, contrary
to Ohta's finding, we find that the seagull terms are not robust. The seagull
terms extracted from the MBB vertex occur unchanged in tree graphs,
such as in an exchange current amplitude. But the seagull terms which appear in
a loop graph, as in the calculation of an electromagnetic form factor, are, in
general, different. The whole procedure says nothing about the transverse part
of the (MBB) vertex and its contributions to the amplitudes in
question.Comment: A 20 pages Latex file and 16 Postscript figures in an uuencoded
format. Use epsf.sty to include the figures into the Latex fil
Revealing the Competition between Peeled-Ssdna, Melting Bubbles and S-DNA during DNA Overstretching using Fluorescence Microscopy
Understanding the structural changes occurring in double-stranded (ds)DNA during mechanical strain is essential to build a quantitative picture of how proteins interact and modify DNA. However, the elastic response of dsDNA to tension is only well-understood for forces < 65 pN. Above this force, torsionally unconstrained dsDNA gains ∼70% of its contour length, a process known as overstretching. The structure of overstretched DNA has proved elusive, resulting in a rich and controversial debate in recent years. At the centre of the debate is the question of whether overstretching yields a base-paired elongated structure, known as S-DNA, or instead forms single-stranded (ss)DNA via base-pair cleavage. Here, we show clearly, using a combination of fluorescence microscopy and optical tweezers, that both S-DNA and base-pair melted structures can exist, often concurrently, during overstretching. The balance between the two models is affected strongly by temperature and ionic strength. Moreover, we reveal, for the first time, that base-pair melting can proceed via two entirely different processes: progressive strand unpeeling from a free end in the backbone, or by the formation of ‘bubbles' of ssDNA, nucleating initially in AT-rich regions. We demonstrate that the mechanism of base-pair melting is governed by DNA topology: strand unpeeling is favored when there are free ends in the DNA backbone. Our studies settle a long running debate, and unite the contradictory dogmas of DNA overstretching. These findings have important implications for both medical and biological sciences. Force-induced melting transitions (yielding either peeled-ssDNA or melting bubbles) may play active roles in DNA replication and damage repair. Further, the ability to switch easily from DNA containing melting bubbles to S-DNA may be particularly advantageous in the cell, for instance during the formation of RNA within transcription bubbles. Copyright © 2013 Biophysical Society. Published by Elsevier Inc. All rights reserved
Revealing the Competition between Peeled-Ssdna, Melting Bubbles and S-DNA during DNA Overstretching using Fluorescence Microscopy
Understanding the structural changes occurring in double-stranded (ds)DNA during mechanical strain is essential to build a quantitative picture of how proteins interact and modify DNA. However, the elastic response of dsDNA to tension is only well-understood for forces < 65 pN. Above this force, torsionally unconstrained dsDNA gains ∼70% of its contour length, a process known as overstretching. The structure of overstretched DNA has proved elusive, resulting in a rich and controversial debate in recent years. At the centre of the debate is the question of whether overstretching yields a base-paired elongated structure, known as S-DNA, or instead forms single-stranded (ss)DNA via base-pair cleavage. Here, we show clearly, using a combination of fluorescence microscopy and optical tweezers, that both S-DNA and base-pair melted structures can exist, often concurrently, during overstretching. The balance between the two models is affected strongly by temperature and ionic strength. Moreover, we reveal, for the first time, that base-pair melting can proceed via two entirely different processes: progressive strand unpeeling from a free end in the backbone, or by the formation of ‘bubbles' of ssDNA, nucleating initially in AT-rich regions. We demonstrate that the mechanism of base-pair melting is governed by DNA topology: strand unpeeling is favored when there are free ends in the DNA backbone. Our studies settle a long running debate, and unite the contradictory dogmas of DNA overstretching. These findings have important implications for both medical and biological sciences. Force-induced melting transitions (yielding either peeled-ssDNA or melting bubbles) may play active roles in DNA replication and damage repair. Further, the ability to switch easily from DNA containing melting bubbles to S-DNA may be particularly advantageous in the cell, for instance during the formation of RNA within transcription bubbles. Copyright © 2013 Biophysical Society. Published by Elsevier Inc. All rights reserved
Mutation of the Traj18 gene segment using TALENs to generate Natural Killer T cell deficient mice.
Invariant Natural Killer T (iNKT) cells are a unique subset of T lymphocytes that have been implicated in both promoting and suppressing a multitude of immune responses. In mice, iNKT cells express T cell antigen receptors (TCRs) comprising a unique TCRα rearrangement between the Trav11 and Traj18 gene segments. When paired with certain Trbv TCRβ chains, these TCRs recognize lipid antigens presented by the major histocompatibility complex (MHC) class I-like molecule, CD1d. Until recently, the sole model of iNKT deficiency targeted the Jα18, which is absolutely required to form the TCR with the appropriate antigenic specificity. However, these mice were demonstrated to have a large reduction in TCR repertoire diversity, which could confound results arising from studies using these mice. Here, we have created a new NKT-deficient mouse strain using transcription activator-like effector nuclease (TALEN) technology to only disrupt the expression of Jα18, leaving the remaining Jα repertoire unperturbed. We confirm that these mice lack iNKT cells and do not respond to lipid antigen stimulation while the development of conventional T cells, regulatory T cells, and type Ib NKT cells is normal. This new mouse strain will serve as a new model of iNKT cell deficiency to facilitate our understanding of iNKT biology
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