6,938 research outputs found
Decrypting the cyclotron effect in graphite using Kerr rotation spectroscopy
We measure the far-infrared magneto-optical Kerr rotation and reflectivity
spectra in graphite and achieve a highly accurate unified microscopic
description of all data in a broad range of magnetic fields by taking
rigorously the c-axis band dispersion and the trigonal warping into account. We
find that the second- and the forth-order cyclotron harmonics are optically
almost as strong as the fundamental resonance even at high fields. They must
play, therefore, a major role in magneto-optical and magneto-plasmonic
applications based on Bernal stacked graphite and multilayer graphene.Comment: 4 pages, 3 figures + Supplemental Materia
The Quantum Compass Model on the Square Lattice
Using exact diagonalizations, Green's function Monte Carlo simulations and
high-order perturbation theory, we study the low-energy properties of the
two-dimensional spin-1/2 compass model on the square lattice defined by the
Hamiltonian . When
, we show that, on clusters of dimension , the
low-energy spectrum consists of states which collapse onto each other
exponentially fast with , a conclusion that remains true arbitrarily close
to . At that point, we show that an even larger number of states
collapse exponentially fast with onto the ground state, and we present
numerical evidence that this number is precisely . We also extend
the symmetry analysis of the model to arbitrary spins and show that the
two-fold degeneracy of all eigenstates remains true for arbitrary half-integer
spins but does not apply to integer spins, in which cases eigenstates are
generically non degenerate, a result confirmed by exact diagonalizations in the
spin-1 case. Implications for Mott insulators and Josephson junction arrays are
briefly discussed.Comment: 8 pages, 8 figure
Analisis Ability to Pay Dan Willingness to Pay Pengguna Jasa Kereta Api Bandara Kualanamu (Airport Railink Service)
The objectives of this research are to analyze the users' ability and willingness to pay the Kuala Namu airport railink services, to find the ideal fare of the train services and to investigate the basic consideration in determining the services fare from PT.Railink. Data is collected by interviews based on the questionnaire. Moreover, in analyzing the data, household budget method and stated preference method are used to measure the ability and the willingness respectively. The results shows that the average respondent's ability to pay is Rp 78,375 and the average respondent's willingness to pay is Rp 60,375. However, after the increase in quality of priority, the average respondent's ability to pay will become Rp 71,375 with the average amount of money that is willing to be added by respondent is Rp 22,632. In determining the fare, PT. Railink has considered the operation fee and the quality of services given to the customers as its primary reasons. Based on the current fare, respondent who have the ability to pay is only 50%. The research reveal that the ideal fare for the train services is Rp 69.375, which will increase the percentage of respondents who have the ability to pay to 72.5
Learning Optimal Control of Synchronization in Networks of Coupled Oscillators using Genetic Programming-based Symbolic Regression
Networks of coupled dynamical systems provide a powerful way to model systems
with enormously complex dynamics, such as the human brain. Control of
synchronization in such networked systems has far reaching applications in many
domains, including engineering and medicine. In this paper, we formulate the
synchronization control in dynamical systems as an optimization problem and
present a multi-objective genetic programming-based approach to infer optimal
control functions that drive the system from a synchronized to a
non-synchronized state and vice-versa. The genetic programming-based controller
allows learning optimal control functions in an interpretable symbolic form.
The effectiveness of the proposed approach is demonstrated in controlling
synchronization in coupled oscillator systems linked in networks of increasing
order complexity, ranging from a simple coupled oscillator system to a
hierarchical network of coupled oscillators. The results show that the proposed
method can learn highly-effective and interpretable control functions for such
systems.Comment: Submitted to nonlinear dynamic
Computational study of the hydrodefluorination of fluoroarenes at [Ru(NHC)(PR<sub>3</sub>)<sub style="vertical-align: sub;">2</sub>(CO)(H)<sub style="vertical-align: sub;">2</sub>]: predicted scope and regioselectivities
Density functional theory calculations have been employed to investigate the scope and selectivity of the hydrodefluorination (HDF) of fluoroarenes, C6F6-nHn (n = 0-5), at catalysts of the type [Ru(NHC)(PR3)(2)(CO)(H)(2)]. Based on our previous study (Angew. Chem., Int. Ed., 2011, 50, 2783) two mechanisms featuring the nucleophilic attack of a hydride ligand at a fluoroarene substrate were considered: (i) a concerted process with Ru-H/C-F exchange occurring in one step; and (ii) a stepwise pathway in which the rate-determining transition state involves formation of HF and a Ru-sigma-fluoroaryl complex. The nature of the metal coordination environment and, in particular, the NHC ligand was found to play an important role in both promoting the HDF reaction and determining the regioselectivity of this process. Thus for the reaction of C6F5H, the full experimental system (NHC = IMes, R = Ph) promotes HDF through (i) more facile initial PR3/fluoroarene substitution and (ii) the ability of the NHC N-aryl substituents to stabilise the key C-F bond breaking transition state through F center dot center dot center dot HC interactions. This latter effect is maximised along the lower energy stepwise pathway when an ortho-H substituent is present and this accounts for the ortho-selectivity seen in the reaction of C6F5H to give 1,2,3,4-C6F4H2. Computed C-F bond dissociation energies (BDEs) for C6F6-nHn substrates show a general increase with larger n and are most sensitive to the number of ortho-F substituents present. However, HDF is always computed to remain significantly exothermic when a silane such as Me3SiH is included as terminal reductant. Computed barriers to HDF also generally increase with greater n, and for the concerted pathway a good correlation between C-F BDE and barrier height is seen. The two mechanisms were found to have complementary regioselectivities. For the concerted pathway the reaction is directed to sites with two ortho-F substituents, as these have the weakest C-F bonds. In contrast, reaction along the stepwise pathway is directed to sites with only one ortho-F substituent, due to difficulties in accommodating ortho-F substituents in the C-F bond cleavage transition state. Calculations predict that 1,2,3,5-C6F4H2 and 1,2,3,4-C6F4H2 are viable candidates for HDF at [Ru(IMes)(PPh3)(2)(CO)(H)(2)] and that this would proceed selectively to give 1,2,4-C6F3H3 and 1,2,3-C6F3H3, respectively.</p
Contribution of Galaxies to the Background Hydrogen-Ionizing Flux
We estimate the evolution of the contribution of galaxies to the cosmic
background flux at by means of a semi-analytic model of galaxy
formation and evolution. Such a modelling has been quite successful in
reproducing the optical properties of galaxies. We assume hereafter the
high-redshift damped Lyman- (DLA) systems to be the progenitors of
present day galaxies, and we design a series of models which are consistent
with the evolution of cosmic comoving emissivities in the available near
infrared (NIR), optical, ultraviolet (UV), and far infrared (FIR) bands along
with the evolution of the neutral hydrogen content and average metallicity of
damped Lyman- systems (DLA). We use these models to compute the
galactic contribution to the Lyman-limit emissivity and background flux for . We take into account the absorption of Lyman-limit photons by
HI and dust in the interstellar medium (ISM) of the galaxies. We find that the
background Lyman-limit flux due to galaxies might dominate (or be comparable
to) the contribution from quasars at almost all redshifts if the absorption by
HI in the ISM is neglected. The ISM HI absorption results in a severe
diminishing of this flux--by almost three orders of magnitude at high redshifts
to between one and two orders at . Though the resulting galaxy flux
is completely negligible at high redshifts, it is comparable to the quasar flux
at .Comment: 14 pages, 5 figures, requires mn.sty, accepted for publication in
MNRA
SiGMa: Simple Greedy Matching for Aligning Large Knowledge Bases
The Internet has enabled the creation of a growing number of large-scale
knowledge bases in a variety of domains containing complementary information.
Tools for automatically aligning these knowledge bases would make it possible
to unify many sources of structured knowledge and answer complex queries.
However, the efficient alignment of large-scale knowledge bases still poses a
considerable challenge. Here, we present Simple Greedy Matching (SiGMa), a
simple algorithm for aligning knowledge bases with millions of entities and
facts. SiGMa is an iterative propagation algorithm which leverages both the
structural information from the relationship graph as well as flexible
similarity measures between entity properties in a greedy local search, thus
making it scalable. Despite its greedy nature, our experiments indicate that
SiGMa can efficiently match some of the world's largest knowledge bases with
high precision. We provide additional experiments on benchmark datasets which
demonstrate that SiGMa can outperform state-of-the-art approaches both in
accuracy and efficiency.Comment: 10 pages + 2 pages appendix; 5 figures -- initial preprin
Similar glassy features in the NMR response of pure and disordered La1.88Sr0.12CuO4
High Tc superconductivity in La2-xSrxCuO4 coexists with (striped and glassy)
magnetic order. Here, we report NMR measurements of the 139La spin-lattice
relaxation, which displays a stretched-exponential time dependence, in both
pure and disordered x=0.12 single crystals. An analysis in terms of a
distribution of relaxation rates T1^-1 indicates that i) the spin-freezing
temperature is spatially inhomogeneous with an onset at Tg(onset)=20 K for the
pristine samples, and ii) the width of the T1^-1 distribution in the vicinity
of Tg(onset) is insensitive to an ~1% level of atomic disorder in CuO2 planes.
This suggests that the stretched-exponential 139La relaxation, considered as a
manifestation of the systems glassiness, may not arise from quenched disorder.Comment: 7 pages, to be published in Phys. Rev.
Gutzwiller density functional theory for correlated electron systems
We develop a new density functional theory (DFT) and formalism for correlated
electron systems by taking as reference an interacting electron system that has
a ground state wavefunction which obeys exactly the Gutzwiller approximation
for all one particle operators. The solution of the many electron problem is
mapped onto the self-consistent solution of a set of single particle
Schroedinger equations analogous to standard DFT-LDA calculations.Comment: 4 page
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