569 research outputs found
Diamond electro-optomechanical resonators integrated in nanophotonic circuits
Diamond integrated photonic devices are promising candidates for emerging
applications in nanophotonics and quantum optics. Here we demonstrate active
modulation of diamond nanophotonic circuits by exploiting mechanical degrees of
freedom in free-standing diamond electro-optomechanical resonators. We obtain
high quality factors up to 9600, allowing us to read out the driven
nanomechanical response with integrated optical interferometers with high
sensitivity. We are able to excite higher order mechanical modes up to 115 MHz
and observe the nanomechanical response also under ambient conditions.Comment: 15 pages, 4 figure
Non-dissipative Thermal Transport and Magnetothermal Effect for the Spin-1/2 Heisenberg Chain
Anomalous magnetothermal effects are discussed in the spin-1/2 Heisenberg
chain. The energy current is related to one of the non-trivial conserved
quantities underlying integrability and therefore both the diagonal and off
diagonal dynamical correlations of spin and energy current diverge. The
energy-energy and spin-energy current correlations at finite temperatures are
exactly calculated by a lattice path integral formulation. The low-temperature
behavior of the thermomagnetic (magnetic Seebeck) coefficient is also
discussed. Due to effects of strong correlations, we observe the magnetic
Seebeck coefficient changes sign at certain interaction strengths and magnetic
fields.Comment: 4 pages, references added, typos corrected, Conference proceedings of
SPQS 2004, Sendai, Japa
Thermomagnetic Power and Figure of Merit for Spin-1/2 Heisenberg Chain
Transport properties in the presence of magnetic fields are numerically
studied for the spin-1/2 Heisenberg XXZ chain. The breakdown of the
spin-reversal symmetry due to the magnetic field induces the magnetothermal
effect. In analogy with the thermoelectric effect in electron systems, the
thermomagnetic power (magnetic Seebeck coefficient) is provided, and is
numerically evaluated by the exact diagonalization for wide ranges of
temperatures and various magnetic fields. For the antiferromagnetic regime, we
find the magnetic Seebeck coefficient changes sign at certain temperatures,
which is interpreted as an effect of strong correlations. We also compute the
thermomagnetic figure of merit determining the efficiency of the thermomagnetic
devices for cooling or power generation.Comment: 8 page
Finite size effects on transport coefficients for models of atomic wires coupled to phonons
We consider models of quasi-1-d, planar atomic wires consisting of several,
laterally coupled rows of atoms, with mutually non-interacting electrons. This
electronic wire system is coupled to phonons, corresponding, e.g., to some
substrate. We aim at computing diffusion coefficients in dependence on the wire
widths and the lateral coupling. To this end we firstly construct a numerically
manageable linear collision term for the dynamics of the electronic occupation
numbers by following a certain projection operator approach. By means of this
collision term we set up a linear Boltzmann equation. A formula for extracting
diffusion coefficients from such Boltzmann equations is given. We find in the
regime of a few atomic rows and intermediate lateral coupling a significant and
non-trivial dependence of the diffusion coefficient on both, the width and the
lateral coupling. These results, in principle, suggest the possible
applicability of such atomic wires as electronic devices, such as, e.g.,
switches.Comment: 9 pages, 5 figures, accepted for publication in Eur. Phys. J.
Crossover from Poisson to Wigner-Dyson Level Statistics in Spin Chains with Integrability Breaking
We study numerically the evolution of energy-level statistics as an
integrability-breaking term is added to the XXZ Hamiltonian. For finite-length
chains, physical properties exhibit a cross-over from behavior resulting from
the Poisson level statistics characteristic of integrable models to behavior
corresponding to the Wigner-Dyson statistics characteristic of the
random-matrix theory used to describe chaotic systems. Different measures of
the level statistics are observed to follow different crossover patterns. The
range of numerically accessible system sizes is too small to establish with
certainty the scaling with system size, but the evidence suggests that in a
thermodynamically large system an infinitesimal integrability breaking would
lead to Wigner-Dyson behavior.Comment: 8 pages, 8 figures, Revtex
The phase diagram of the extended anisotropic ferromagnetic-antiferromagnetic Heisenberg chain
By using Density Matrix Renormalization Group (DMRG) technique we study the
phase diagram of 1D extended anisotropic Heisenberg model with ferromagnetic
nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions. We
analyze the static correlation functions for the spin operators both in- and
out-of-plane and classify the zero-temperature phases by the range of their
correlations. On clusters of sites with open boundary
conditions we isolate the boundary effects and make finite-size scaling of our
results. Apart from the ferromagnetic phase, we identify two gapless spin-fluid
phases and two ones with massive excitations. Based on our phase diagram and on
estimates for the coupling constants known from literature, we classify the
ground states of several edge-sharing materials.Comment: 12 pages, 13 figure
Thermal conductivity via magnetic excitations in spin-chain materials
We discuss the recent progress and the current status of experimental
investigations of spin-mediated energy transport in spin-chain and spin-ladder
materials with antiferromagnetic coupling. We briefly outline the central
results of theoretical studies on the subject but focus mainly on recent
experimental results that were obtained on materials which may be regarded as
adequate physical realizations of the idealized theoretical model systems. Some
open questions and unsettled issues are also addressed.Comment: 17 pages, 4 figure
Dedicated versus mainstreaming approaches in local climate plans in Europe
Cities are gaining prominence committing to respond to the threat of climate change, e.g., by developing local climate plans or strategies. However, little is known regarding the approaches and processes of plan development and implementation, or the success and effectiveness of proposed measures. Mainstreaming is regarded as one approach associated with (implementation) success, but the extent of integration of local climate policies and plans in ongoing sectoral and/or development planning is unclear. This paper analyses 885 cities across the 28 European countries to create a first reference baseline on the degree of climate mainstreaming in local climate plans. This will help to compare the benefits of mainstreaming versus dedicated climate plans, looking at policy effectiveness and ultimately delivery of much needed climate change efforts at the city level. All core cities of the European Urban Audit sample were analyzed, and their local climate plans classified as dedicated or mainstreamed in other local policy initiatives. It was found that the degree of mainstreaming is low for mitigation (9% of reviewed cities; 12% of the identified plans) and somewhat higher for adaptation (10% of cities; 29% of plans). In particular horizontal mainstreaming is a major effort for local authorities; an effort that does not necessarily pay off in terms of success of action implementation. This study concludes that climate change issues in local municipalities are best tackled by either, developing a dedicated local climate plan in parallel to a mainstreamed plan or by subsequently developing first the dedicated and later a mainstreaming plan (joint or subsequent “dual track approach”). Cities that currently provide dedicated local climate plans (66% of cities for mitigation; 26% of cities for adaptation) may follow-up with a mainstreaming approach. This promises effective implementation of tangible climate actions as well as subsequent diffusion of climate issues into other local sector policies. The development of only broad sustainability or resilience strategies is seen as critical.We thank the many council representatives that supported the datacollection. Special thanks to Birgit Georgi who helped in setting up this large net work of researchers across the EU-28. We also thank the EU COST Action TU 0902 (ledbyRichardDawson) that established the core research network and the positive engagement and interaction of th emembers of this group. OH is Fellow of the Tyndall Centre for Climate Change Research and was funded by the UK EPSRC LC Transforms: Low Carbon Transitions of Fleet Operations in Metropolitan Sites Project (grant number EP/N010612/1). EKL was supported by the Ministry of Education, Youth and Sports, Czechia, within the National Sustainability Program I (NPU I) (grant number LO1415). DG ac-knowledges support by the Ministry of Education, University and Research (MIUR), Italy ("Departments of Excellence" grant L. 232/2016). HO was supported by the Ministry of Education and Research, Estonia (grantnumberIUT34-17). MO acknowledges funding from the Ministry of Economy and Competitiveness (MINECO), Spain (grant number IJCI-2016-28835). SS acknowledges that CENSE's research is partially funded by the Science Foundation, Portugal (grant number UID/AMB/04085/2019). The paper reflects only the views of the authors. The European Union, the European Environment Agency or other supporting bodies are not liable for any use that may be made of the information that is provided in this manuscript
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