867 research outputs found
On the ground state energy scaling in quasi-rung-dimerized spin ladders
On the basis of periodic boundary conditions we study perturbatively a large
N asymptotics (N is the number of rungs) for the ground state energy density
and gas parameter of a spin ladder with slightly destroyed rung-dimerization.
Exactly rung-dimerized spin ladder is treated as the reference model. Explicit
perturbative formulas are obtained for three special classes of spin ladders.Comment: 4 page
Towers and fibered products of model categories
Given a left Quillen presheaf of localized model structures, we study the homotopy limit model structure on the associated category of sections. We focus specifically on towers and fibered products of model categories. As applications we consider Postnikov towers of model categories, chromatic towers of spectra and Bousfield arithmetic squares of spectra. For spectral model categories, we show that the homotopy fiber of a stable left Bousfield localization is a stable right Bousfield localization
Phase diagrams of spin ladders with ferromagnetic legs
The low-temperature properties of the spin S=1/2 ladder with anisotropic
ferromagnetic legs are studied using the continuum limit bosonization approach.
The weak-coupling ground state phase diagram of the model is obtained for a
wide range of coupling constants and several unconventional gapless
''spin-liquid'' phases are shown to exist for ferromagnetic coupling. The
behavior of the ladder system in the vicinity of the ferromagnetic instability
point is discussed in detail.Comment: 11 pages, 4 figure
String order in spin liquid phases of spin ladders
Two-leg spin ladders have a rich phase diagram if rung, diagonal and
plaquette couplings are allowed for. Among the possible phases there are two
Haldane-type spin liquid phases without local order parameter, which differ,
however, in the topology of the short range valence bonds. We show that these
phases can be distinguished numerically by two different string order
parameters. We also point out that long range string- and dimer orders can
coexist
Vortex states in superconducting rings
The superconducting state of a thin superconducting disk with a hole is
studied within the non-linear Ginzburg-Landau theory in which the
demagnetization effect is accurately taken into account. We find that the flux
through the hole is not quantized, the superconducting state is stabilized with
increasing size of the hole for fixed radius of the disk, and a transition to a
multi-vortex state is found if the disk is sufficiently large. Breaking the
circular summetry through a non central location of the hole in the disk
enhances the multi-vortex state.Comment: 11 pages, 23 figures (postscript). To appear in Physical Review B,
Vol. 61 (2000
Phase Transitions Between Topologically Distinct Gapped Phases in Isotropic Spin Ladders
We consider various two-leg ladder models exhibiting gapped phases. All of
these phases have short-ranged valence bond ground states, and they all exhibit
string order. However, we show that short-ranged valence bond ground states
divide into two topologically distinct classes, and as a consequence, there
exist two topologically distinct types of string order. Therefore, not all
gapped phases belong to the same universality class. We show that phase
transitions occur when we interpolate between models belonging to different
topological classes, and we study the nature of these transitions.Comment: 11 pages, 16 postscript figure
Augmented Reality in Astrophysics
Augmented Reality consists of merging live images with virtual layers of
information. The rapid growth in the popularity of smartphones and tablets over
recent years has provided a large base of potential users of Augmented Reality
technology, and virtual layers of information can now be attached to a wide
variety of physical objects. In this article, we explore the potential of
Augmented Reality for astrophysical research with two distinct experiments: (1)
Augmented Posters and (2) Augmented Articles. We demonstrate that the emerging
technology of Augmented Reality can already be used and implemented without
expert knowledge using currently available apps. Our experiments highlight the
potential of Augmented Reality to improve the communication of scientific
results in the field of astrophysics. We also present feedback gathered from
the Australian astrophysics community that reveals evidence of some interest in
this technology by astronomers who experimented with Augmented Posters. In
addition, we discuss possible future trends for Augmented Reality applications
in astrophysics, and explore the current limitations associated with the
technology. This Augmented Article, the first of its kind, is designed to allow
the reader to directly experiment with this technology.Comment: 15 pages, 11 figures. Accepted for publication in Ap&SS. The final
publication will be available at link.springer.co
Antiferromagnetically coupled alternating spin chains
The effect of antiferromagnetic interchain coupling in alternating spin
(1,1/2) chains is studied by mean of a spin wave theory and density matrix
renormalization group (DMRG). In particular, two limiting cases are
investigated, the two-leg ladder and its two dimensional (2D) generalization.
Results of the ground state properties like energy, spin gap, magnetizations,
and correlation functions are reported for the whole range of the interchain
coupling . For the 2D case the spin wave results predict a smooth
dimensional crossover from 1D to 2D keeping the ground state always ordered.
For the ladder system, the DMRG results show that any drives the
system to a gapped ground state. Furthermore the behaviour of the correlation
functions closely resemble the uniform spin-1/2 ladder. For lower
than 0.3, however, the gap behaves quadratically as . Finally, it is argued that the behaviour of the spin gap for an
arbitrary number of mixed coupled spin chains is analogous to that of the
uniform spin-1/2 chains.Comment: 5 pages, 7 ps-figure
Exclusive semileptonic B_s decays to excited D_s mesons: Search of D_{sJ}(2317) and D_{sJ}(2460)
We study the exclusive semileptonic decays B_s->D_{s0}^*\ell\bar\nu and
B_s->D_{s1}^*\ell\bar\nu, where p-wave excited D_{s0}^* and D_{s1}^* states are
identified with the newly observed D_{sJ}(2317) and D_{sJ}(2460) states. Within
the framework of HQET the Isgur-Wise functions up to the subleading order of
the heavy quark expansion are calculated by QCD sum rules. The decay rates and
branching ratios are computed with the inclusion of the order of 1/m_Q
corrections. We point out that the investigation of the B_s semileptonic decays
to excited D_s mesons may provide some information about the nature of the new
D_{sJ}^* mesons.Comment: 15 pages, 6 eps figures, RevTeX 4, accepted for publication in Phys.
Rev.
Autophagy in T cells from aged donors is maintained by spermidine and correlates with function and vaccine responses
Vaccines are powerful tools to develop immune memory to infectious diseases and prevent excess mortality. In older adults, however vaccines are generally less efficacious and the molecular mechanisms that underpin this remain largely unknown. Autophagy, a process known to prevent aging, is critical for the maintenance of immune memory in mice. Here, we show that autophagy is specifically induced in vaccine-induced antigen-specific CD8+ T cells in healthy human volunteers. In addition, reduced IFNγ secretion by RSV-induced T cells in older vaccinees correlates with low autophagy levels. We demonstrate that levels of the endogenous autophagy-inducing metabolite spermidine fall in human T cells with age. Spermidine supplementation in T cells from old donors recovers their autophagy level and function, similar to young donors' cells, in which spermidine biosynthesis has been inhibited. Finally, our data show that endogenous spermidine maintains autophagy via the translation factor eIF5A and transcription factor TFEB. In summary, we have provided evidence for the importance of autophagy in vaccine immunogenicity in older humans and uncovered two novel drug targets that may increase vaccination efficiency in the aging context
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