1,462 research outputs found
Structural Critical Scattering Study of Mg-Doped CuGeO3
We report a synchrotron x-ray scattering study of the diluted spin-Peierls
(SP) material Cu_(1-x)Mg_xGeO_3. We find that for x>0 the temperature T_m at
which the spin gap is established is significantly higher than the temperature
T_s at which the SP dimerization attains long-range order. The latter is
observed only for xx_c the SP correlation length
quickly decreases with increasing x. We argue that impurity-induced competing
interactions play a central role in these phenomena.Comment: 5 pages, 4 embedded eps figures, to appear in PR
The first-order phase transition between dimerized-antiferromagnetic and uniform-antiferromagnetic phases in Cu_(1-x)M_xGeO_3
We have performed detailed magnetic susceptibility measurements as well as
synchrotron x-ray diffraction studies to determine the temperature vs
concentration ( - ) phase diagram of CuMgGeO. We
observe clear double peaks in the magnetic susceptibility implying two
antiferromagnetic (AF) transition temperatures in samples with Mg
concentrations in the range 0.0237 0.0271. We also observe a
drastic change in the inverse correlation length in this concentration range by
x-ray diffraction. The drastic change of the AF transition temperature as well
as the disappearance of the spin-Peierls (SP) phase have been clarified; these
results are consistent with a first-order phase transition between dimerized AF
(D-AF) and uniform AF (U-AF) phases as reported by T. Masuda {\it et al.}
\lbrack Phys. Rev. Lett. {\bf 80}, 4566 (1998)\rbrack. The - phase
diagram of CuZnGeO is similar to that of
CuMgGeO, which suggests that the present phase transition
is universal for CuGeO.Comment: 7 pages, 5 figures. submitted to PR
Bond-Dilution-Induced Quantum Phase Transitions in Heisenberg Antiferromagnets
Bond-dilution effects on the ground state of the square-lattice
antiferromagnetic Heisenberg model, consisting of coupled bond-alternating
chains, are investigated by means of the quantum Monte Carlo simulation. It is
found that, when the ground state of the non-diluted system is a non-magnetic
state with a finite spin gap, a sufficiently weak bond dilution induces a
disordered state with a mid gap in the original spin gap, and under a further
stronger bond dilution an antiferromagnetic long-range order emerges. While the
site-dilution-induced long-range order is induced by an infinitesimal
concentration of dilution, there exists a finite critical concentration in the
case of bond dilution. We argue that this essential difference is due to the
occurrence of two types of effective interactions between induced magnetic
moments in the case of bond dilution, and that the antiferromagnetic
long-range-ordered phase does not appear until the magnitudes of the two
interactions become comparable.Comment: 7 pages, 13 figure
Using the past to constrain the future: how the palaeorecord can improve estimates of global warming
Climate sensitivity is defined as the change in global mean equilibrium
temperature after a doubling of atmospheric CO2 concentration and provides a
simple measure of global warming. An early estimate of climate sensitivity,
1.5-4.5{\deg}C, has changed little subsequently, including the latest
assessment by the Intergovernmental Panel on Climate Change.
The persistence of such large uncertainties in this simple measure casts
doubt on our understanding of the mechanisms of climate change and our ability
to predict the response of the climate system to future perturbations. This has
motivated continued attempts to constrain the range with climate data, alone or
in conjunction with models. The majority of studies use data from the
instrumental period (post-1850) but recent work has made use of information
about the large climate changes experienced in the geological past.
In this review, we first outline approaches that estimate climate sensitivity
using instrumental climate observations and then summarise attempts to use the
record of climate change on geological timescales. We examine the limitations
of these studies and suggest ways in which the power of the palaeoclimate
record could be better used to reduce uncertainties in our predictions of
climate sensitivity.Comment: The final, definitive version of this paper has been published in
Progress in Physical Geography, 31(5), 2007 by SAGE Publications Ltd, All
rights reserved. \c{opyright} 2007 Edwards, Crucifix and Harriso
Quantal phases, disorder effects and superconductivity in spin-Peierls systems
In view of recent developments in the investigation on cuprate high-T superconductors and the spin-Peierls compound CuGeO, we study the
effect of dilute impurity doping on the spin-Peierls state in quasi-one
dimensional systems. We identify a common origin for the emergence of
antiferromagnetic order upon the introduction of static vacancies, and
superconductivity for mobile holes.Comment: 4 pages revtex; revised versio
A quantum isomonodromy equation and its application to N=2 SU(N) gauge theories
We give an explicit differential equation which is expected to determine the
instanton partition function in the presence of the full surface operator in
N=2 SU(N) gauge theory. The differential equation arises as a quantization of a
certain Hamiltonian system of isomonodromy type discovered by Fuji, Suzuki and
Tsuda.Comment: 15 pages, v2: typos corrected and references added, v3: discussion,
appendix and references adde
Nonlinear regional warming with increasing CO₂ concentration
When considering adaptation measures and global climate mitigation goals, stakeholders need regional-scale climate projections, including the range of plausible warming rates. To assist these stakeholders, it is important to understand whether some locations may see disproportionately high or low warming from additional forcing above targets such as 2 K (ref. 1). There is a need to narrow uncertainty2 in this nonlinear warming, which requires understanding how climate changes as forcings increase from medium to high levels. However, quantifying and understanding regional nonlinear processes is challenging. Here we show that regional-scale warming can be strongly superlinear to successive CO2 doublings, using five different climate models. Ensemble-mean warming is superlinear over most land locations. Further, the inter-model spread tends to be amplified at higher forcing levels, as nonlinearities grow—especially when considering changes per kelvin of global warming. Regional nonlinearities in surface warming arise from nonlinearities in global-mean radiative balance, the Atlantic meridional overturning circulation, surface snow/ice cover and evapotranspiration. For robust adaptation and mitigation advice, therefore, potentially avoidable climate change (the difference between business-as-usual and mitigation scenarios) and unavoidable climate change (change under strong mitigation scenarios) may need different analysis methods
Crystal Structure of a Complex of DNA with One AT-Hook of HMGA1
We present here for the first time the crystal structure of an AT-hook domain. We show the structure of an AT-hook of the ubiquitous nuclear protein HMGA1, combined with the oligonucleotide d(CGAATTAATTCG)2, which has two potential AATT interacting groups. Interaction with only one of them is found. The structure presents analogies and significant differences with previous NMR studies: the AT-hook forms hydrogen bonds between main-chain NH groups and thymines in the minor groove, DNA is bent and the minor groove is widened
On "Dotsenko-Fateev" representation of the toric conformal blocks
We demonstrate that the recent ansatz of arXiv:1009.5553, inspired by the
original remark due to R.Dijkgraaf and C.Vafa, reproduces the toric conformal
blocks in the same sense that the spherical blocks are given by the integral
representation of arXiv:1001.0563 with a peculiar choice of open integration
contours for screening insertions. In other words, we provide some evidence
that the toric conformal blocks are reproduced by appropriate beta-ensembles
not only in the large-N limit, but also at finite N. The check is explicitly
performed at the first two levels for the 1-point toric functions.
Generalizations to higher genera are briefly discussed.Comment: 10 page
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