959 research outputs found
The Ramond-Ramond sector of string theory beyond leading order
Present knowledge of higher-derivative terms in string e ective actions is, with a few exceptions, restricted to the NS-NS sector, a situation which prevents the development of a variety of interesting applications for which the RR terms are relevant. We here provide the formalism as well as e cient techniques to determine the latter directly from string-amplitude calculations. As an illustration of these methods, we compute the dependence of the type-IIB action on the three- and five-form RR field strengths at four-point, genus-one, order-( 0)3 level. We explicitly verify that our results are in accord with the SL(2,Z) S-duality invariance of type-IIB string theory. Extensions of our method to other bosonic terms in the type-II e ective actions are discussed as well
Monte Carlo Study of the Separation of Energy Scales in Quantum Spin 1/2 Chains with Bond Disorder
One-dimensional Heisenberg spin 1/2 chains with random ferro- and
antiferromagnetic bonds are realized in systems such as . We have investigated numerically the thermodynamic properties of a
generic random bond model and of a realistic model of by the quantum Monte Carlo loop algorithm. For the first time we
demonstrate the separation into three different temperature regimes for the
original Hamiltonian based on an exact treatment, especially we show that the
intermediate temperature regime is well-defined and observable in both the
specific heat and the magnetic susceptibility. The crossover between the
regimes is indicated by peaks in the specific heat. The uniform magnetic
susceptibility shows Curie-like behavior in the high-, intermediate- and
low-temperature regime, with different values of the Curie constant in each
regime. We show that these regimes are overlapping in the realistic model and
give numerical data for the analysis of experimental tests.Comment: 7 pages, 5 eps-figures included, typeset using JPSJ.sty, accepted for
publication in J. Phys. Soc. Jpn. 68, Vol. 3. (1999
Phase-Insensitive Scattering of Terahertz Radiation
The nonlinear interaction between Near-Infrared (NIR) and Terahertz pulses is
principally investigated as a means for the detection of radiation in the
hardly accessible THz spectral region. Most studies have targeted second-order
nonlinear processes, given their higher efficiencies, and only a limited number
have addressed third-order nonlinear interactions, mainly investigating
four-wave mixing in air for broadband THz detection. We have studied the
nonlinear interaction between THz and NIR pulses in solid-state media
(specifically diamond), and we show how the former can be frequency-shifted up
to UV frequencies by the scattering from the nonlinear polarisation induced by
the latter. Such UV emission differs from the well-known electric-field-induced
second harmonic (EFISH) one, as it is generated via a phase-insensitive
scattering, rather than a sum- or difference-frequency four-wave-mixing
process
Aperiodic quantum XXZ chains: Renormalization-group results
We report a comprehensive investigation of the low-energy properties of
antiferromagnetic quantum XXZ spin chains with aperiodic couplings. We use an
adaptation of the Ma-Dasgupta-Hu renormalization-group method to obtain
analytical and numerical results for the low-temperature thermodynamics and the
ground-state correlations of chains with couplings following several two-letter
aperiodic sequences, including the quasiperiodic Fibonacci and other
precious-mean sequences, as well as sequences inducing strong geometrical
fluctuations. For a given aperiodic sequence, we argue that in the easy-plane
anisotropy regime, intermediate between the XX and Heisenberg limits, the
general scaling form of the thermodynamic properties is essentially given by
the exactly-known XX behavior, providing a classification of the effects of
aperiodicity on XXZ chains. We also discuss the nature of the ground-state
structures, and their comparison with the random-singlet phase, characteristic
of random-bond chains.Comment: Minor corrections; published versio
Low-Temperature Scaling Regime of Random Ferromagnetic-Antiferromagnetic Spin Chains
Using the Continuous Time Quantum Monte Carlo Loop algorithm, we calculate
the temperature dependence of the uniform susceptibility, and the specific heat
of a spin-1/2 chain with random antiferromagnetic and ferromagnetic couplings,
down to very low temperatures. Our data show a consistent scaling behavior in
both quantities and support strongly the conjecture drawn from the
approximative real-space renormalization group treatment. A statistical
analysis scheme is developed which will be useful for the search scaling
behavior in numerical and experimental data of random spin chains.Comment: 4 pages and 3 figure
TWINLATIN: Twinning European and Latin-American river basins for research enabling sustainable water resources management. Combined Report D3.1 Hydrological modelling report and D3.2 Evaluation report
Water use has almost tripled over the past 50 years and in some regions the water demand already
exceeds supply (Vorosmarty et al., 2000). The world is facing a “global water crisis”; in many
countries, current levels of water use are unsustainable, with systems vulnerable to collapse from even
small changes in water availability. The need for a scientifically-based assessment of the potential
impacts on water resources of future changes, as a basis for society to adapt to such changes, is strong
for most parts of the world. Although the focus of such assessments has tended to be climate change,
socio-economic changes can have as significant an impact on water availability across the four main
use sectors i.e. domestic, agricultural, industrial (including energy) and environmental. Withdrawal
and consumption of water is expected to continue to grow substantially over the next 20-50 years
(Cosgrove & Rijsberman, 2002), and consequent changes in availability may drastically affect society
and economies.
One of the most needed improvements in Latin American river basin management is a higher level of
detail in hydrological modelling and erosion risk assessment, as a basis for identification and analysis
of mitigation actions, as well as for analysis of global change scenarios. Flow measurements are too
costly to be realised at more than a few locations, which means that modelled data are required for the
rest of the basin. Hence, TWINLATIN Work Package 3 “Hydrological modelling and extremes” was
formulated to provide methods and tools to be used by other WPs, in particular WP6 on “Pollution
pressure and impact analysis” and WP8 on “Change effects and vulnerability assessment”. With an
emphasis on high and low flows and their impacts, WP3 was originally called “Hydrological
modelling, flooding, erosion, water scarcity and water abstraction”. However, at the TWINLATIN
kick-off meeting it was agreed that some of these issues resided more appropriately in WP6 and WP8,
and so WP3 was renamed to focus on hydrological modelling and hydrological extremes.
The specific objectives of WP3 as set out in the Description of Work are
Spontaneous photon production in time-dependent epsilon-near-zero materials
Quantum field theory predicts that a spatially homogeneous but temporally varying medium will excite photon pairs out of the vacuum state. However, this important theoretical prediction lacks experimental verification due to the difficulty in attaining the required nonadiabatic and large amplitude changes in the medium. Recent work has shown that in epsilon-near-zero (ENZ) materials it is possible to optically induce changes of the refractive index of the order of unity, in femtosecond time scales. By studying the quantum field theory of a spatially homogeneous, time-varying ENZ medium, we theoretically predict photon-pair production that is up to several orders of magnitude larger than in non-ENZ time-varying materials. We also find that while in standard materials the emission spectrum depends on the time scale of the perturbation, in ENZ materials the emission is always peaked at the ENZ wavelength. These studies pave the way to technologically feasible observation of photon-pair emission from a time-varying background with implications for quantum field theories beyond condensed matter systems and with potential applications as a new source of entangled light
Spin Waves in Random Spin Chains
We study quantum spin-1/2 Heisenberg ferromagnetic chains with dilute, random
antiferromagnetic impurity bonds with modified spin-wave theory. By describing
thermal excitations in the language of spin waves, we successfully observe a
low-temperature Curie susceptibility due to formation of large spin clusters
first predicted by the real-space renormalization-group approach, as well as a
crossover to a pure ferromagnetic spin chain behavior at intermediate and high
temperatures. We compare our results of the modified spin-wave theory to
quantum Monte Carlo simulations.Comment: 3 pages, 3 eps figures, submitted to the 47th Conference on Magnetism
and Magnetic Material
On the Dirac-Born-Infeld Action for D-branes
In this note, we consider the reformulation of the Dirac-Born-Infeld action
for a Dirichlet p-brane in Brink-Di Vecchia-Howe-Tucker form, i.e., including
an independent non-propagating world-volume metric. When p>2, the action
becomes non-polynomial. A closed expression is derived for p=3. For selfdual
field-strengths, the DBI action is reproduced by an action with a simple F^2
term. We speculate on supersymmetrization of the D_3-brane action. We also give
the governing equations for arbitrary p, and derive an implicit expression for
the D_4-brane lagrangian.Comment: Plain TeX, 7 pages, TeX file and one postscript figure in compressed
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