416 research outputs found
The Influence of Halide Substituents on the Structural and Magnetic Properties of FeDy Rings
We report the synthesis and magnetic properties of three new nine-membered Fe(III)-Dy(III) cyclic coordination clusters (CCCs), with a core motif of [FeDy(μ-OMe)(vanox)(X-benz)] where the benzoate ligands are substituted in the para-position with X = F (1), Cl (2), Br (3). Single crystal X-ray diffraction structure analyses show that for the smaller fluorine or chlorine substituents the resulting structures exhibit an isostructural FeDy core, whilst the 4-bromobenzoate ligand leads to structural distortions which affect the dynamic magnetic behavior. The magnetic susceptibility and magnetization of 1-3 were investigated and show similar behavior in the dc (direct current) magnetic data. Additional ac (alternating current) magnetic measurements show that all compounds exhibit frequency-dependent and temperature-dependent signals in the in-phase and out-of-phase component of the susceptibility and can therefore be described as field-induced SMMs. The fluoro-substituted benzoate cluster 1 shows a magnetic behavior closely similar to that of the corresponding unsubstituted FeDy cluster, with U = 21.3 K within the Orbach process. By increasing the size of the substituent toward 4-chlorobenzoate within 2, an increase of the energy barrier to U = 36.1 K was observed. While the energy barrier becomes higher from 1 to 2, highlighting that the introduction of different substituents on the benzoate ligand in the para-position has an impact on the magnetic properties, cluster 3 shows a significantly different SMM behavior where U is reduced in the Orbach regime to only 4.9 K
Electronic excitations and electron-phonon coupling in bulk graphite through Raman scattering in high magnetic fields
We use polarized magneto-Raman scattering to study purely electronic
excitations and the electron-phonon coupling in bulk graphite. At a temperature
of 4.2 K and in magnetic fields up to 28 T we observe -point electronic
excitations involving Landau bands with and with that can be selected by controlling the angular momentum of the
excitation laser and of the scattered light. The magneto-phonon effect
involving the optical phonon and -point inter Landau bands
electronic excitations with is revealed and analyzed within a
model taking into account the full dispersion. These polarization
resolved results are explained in the frame of the Slonczewski-Weiss-McClure
(SWM) model which directly allows to quantify the electron-hole asymmetry.Comment: 13 pages, 10 figure
Modeling and Simulation of Multi-Lane Traffic Flow
A most important aspect in the field of traffic modeling is the simulation of
bottleneck situations. For their realistic description a macroscopic multi-lane
model for uni-directional freeways including acceleration, deceleration,
velocity fluctuations, overtaking and lane-changing maneuvers is systematically
deduced from a gas-kinetic (Boltzmann-like) approach. The resulting equations
contain corrections with respect to previous models. For efficient computer
simulations, a reduced model delineating the coarse-grained temporal behavior
is derived and applied to bottleneck situations.Comment: For related work see
http://www.theo2.physik.uni-stuttgart.de/helbing.htm
Photocatalytic water splitting reaction catalyzed by ion-exchanged salts of potassium poly(heptazine imide) 2D materials
Potassium poly (heptazine imide) (K-PHI), a crystalline two-dimensional carbon–nitride material, is an active photocatalyst for water splitting. The potassium ions in K-PHI can be exchanged with other ions to change the properties of the material and eventually to design the catalysts. We report here the electronic structures of several ion-exchanged salts of K-PHI (K, H, Au, Ru, and Mg) and their feasibility as water splitting photocatalysts, which were determined by density functional theory (DFT) calculations. The DFT results are complemented by experiments where the performances in the photocatalytic hydrogen evolution reaction (HER) were recorded. We show that due to its narrow band gap, Ru-PHI is not a suitable photocatalyst. The water oxidation potentials are straddled between the band edge potentials of H-PHI, Au-PHI, and Mg-PHI; thus, these are active photocatalysts for both the oxygen and hydrogen evolution reactions, whereas K-PHI is active only for the HER. The experimental data show that these are active HER photocatalysts, in agreement with the DFT results. Furthermore, Mg-PHI has shown remarkable performance in the HER, with a rate of 539 μmol/(h·g) and a quantum efficiency of 7.14% at 410 nm light irradiation, which could be due to activation of the water molecule upon adsorption, as predicted by our DFT calculations
The First Use of a ReX Synthon to Modulate Fe Spin Crossover via Supramolecular Halogen⋅⋅⋅Halogen Interactions
We have added the {ReX} (X=Br, Cl) synthon to a pocket‐based ligand to provide supramolecular design using halogen⋅⋅⋅halogen interactions within an Fe system that has the potential to undergo spin crossover (SCO). By removing the solvent from the crystal lattice, we “switch on” halogen⋅⋅⋅halogen interactions between neighboring molecules, providing a supramolecular cooperative pathway for SCO. Furthermore, changes to the halogen‐based interaction allow us to modify the temperature and nature of the SCO event
Magneto-Raman scattering of graphene on graphite: Electronic and phonon excitations
Magneto-Raman scattering experiments from the surface of graphite reveal
novel features associated to purely electronic excitations which are observed
in addition to phonon-mediated resonances. Graphene-like and graphite domains
are identified through experiments with spatial resolution
performed in magnetic fields up to 32T. Polarization resolved measurements
emphasize the characteristic selection rules for electronic transitions in
graphene. Graphene on graphite displays the unexpected hybridization between
optical phonon and symmetric across the Dirac point inter Landau level
transitions. The results open new experimental possibilities - to use light
scattering methods in studies of graphene under quantum Hall effect conditions.Comment: 4 pages, 5 figures; revised manuscript with additional dat
Critical exponents and intrinsic broadening of the field-induced transition in NiCl4SC(NH)
The field-induced ordering transition in the quantum spin system
NiCl4SC(NH) is studied by means of neutron diffraction, AC
magnetometry and relaxation calorimetry. The interpretation of the data is
strongly influenced by a finite distribution of transition fields in the
samples, which was present but disregarded in previous studies. Taking this
effect into account, we find that the order-parameter critical exponent is
inconsistent with the BEC universality class even at temperatures below 100 mK.
All results are discussed in comparison with previous measurements and with
recent similar studies of disordered
Ni(ClBr)4SC(NH)
Spin-Peierls transition of the first order in S=1 antiferromagnetic Heisenberg chains
We investigate a one-dimensional S=1 antiferromagnetic Heisenberg model
coupled to a lattice distortion by a quantum Monte Carlo method. Investigating
the ground state energy of the static bond-alternating chain, we find that the
instability to a dimerized chain depends on the value of the spin-phonon
coupling, unlike the case of S=1/2. The spin state is the dimer state or the
uniform Haldane state depending on whether the lattice distorts or not,
respectively. At an intermediate value of the spin-phonon coupling, we find the
first-order transition between the two states. We also find the coexistence of
the two states.Comment: 7 pages, 12 eps figures embedded in the text; corrected typos,
replaced figure
Theoretical approach to two-dimensional traffic flow models
In this paper we present a theoretical analysis of a recently proposed
two-dimensional Cellular Automata model for traffic flow in cities with the
novel ingredient of turning capability. Numerical simulations of this model
show that there is a transition between a freely moving phase with high
velocity to a jammed state with low velocity. We study the dynamics of such a
model starting with the microscopic evolution equation, which will serve as a
basis for further analysis. It is shown that a kinetic approach, based on the
Boltzmann assumption, is able to provide a reasonably good description of the
jamming transition. We further introduce a space-time continuous
phenomenological model leading to a couple of partial differential equations
whose preliminary results agree rather well with the numerical simulations.Comment: 15 pages, REVTeX 3.0, 7 uuencoded figures upon request to
[email protected]
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