36 research outputs found
Application of Photothermal and Photoacoustic Spectroscopy for the Monitoring of Aqueous Dispersions of Carbon Nanomaterials
Photothermal and optoacoustic spectroscopy in their state-of-the-art techniques—multiwavelength, scanning and transient—are used for complex investigation and analysis (chemical analysis and the estimation of physicochemical properties and size) of novel carbon materials—fullerenes and nanodiamonds—and their aqueous dispersions as promising biomedical nanosystems. The estimation of the cluster size and the possibilities to determine subnanogram amounts of both nanodiamonds and fullerenes by these techniques are shown. The comparison of fullerene solutions in various solvents, toluene, N-methylpyrrolydone and water, is made. The advantages of the photothermal and optoacoustic techniques over conventional spectroscopies and the current limitation are discussed. The necessity to develop robust models for transient and imaging photothermal techniques is outlined
Discreteness, periodicity, holomorphy, and factorization
The main topic of the paper is to establish some relations between the solvability of a special kind of discrete equations in certain canonical domains and holomorphy properties of their Fourier analogue
On the magnetism of Ln{2/3}Cu{3}Ti{4}O{12} (Ln = lanthanide)
The magnetic and thermodynamic properties of the complete
LnCuTiO series were investigated. Here stands for
the lanthanides La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. %Most
of the compounds were prepared as single phase polycrystalline powder %without
any traces of impurities. Marginal amounts of %impurities were
detected Gd, Er, and Tm. %Significant amounts of impurity phases were
found for Ce and Yb. All the samples investigated crystallize in the
space group with lattice constants that follow the lanthanide
contraction. The lattice constant of the Ce compound reveals the presence of
Ce leading to the composition CeCuTiO. From
magnetic susceptibility and electron-spin resonance experiments it can be
concluded that the copper ions always carry a spin and order
antiferromagnetically close to 25\,K. The Curie-Weiss temperatures can
approximately be calculated assuming a two-sublattice model corresponding to
the copper and lanthanide ions, respectively. It seems that the magnetic
moments of the heavy rare earths are weakly coupled to the copper spins, while
for the light lanthanides no such coupling was found. The moments remain
paramagnetic down to the lowest temperatures, with the exception of the Tm
compound, which indicates enhanced Van-Vleck magnetism due to a non-magnetic
singlet ground state of the crystal-field split manifold. From
specific-heat measurements we accurately determined the antiferromagnetic
ordering temperature and obtained information on the crystal-field states of
the rare-earth ions. The heat-capacity results also revealed the presence of a
small fraction of Ce in a magnetic state.Comment: 10 pages, 10 figure
Schwinger-Dyson approach to non-equilibrium classical field theory
In this paper we discuss a Schwinger-Dyson [SD] approach for determining the
time evolution of the unequal time correlation functions of a non-equilibrium
classical field theory, where the classical system is described by an initial
density matrix at time . We focus on field theory in 1+1
space time dimensions where we can perform exact numerical simulations by
sampling an ensemble of initial conditions specified by the initial density
matrix. We discuss two approaches. The first, the bare vertex approximation
[BVA], is based on ignoring vertex corrections to the SD equations in the
auxiliary field formalism relevant for 1/N expansions. The second approximation
is a related approximation made to the SD equations of the original formulation
in terms of alone. We compare these SD approximations as well as a
Hartree approximation with exact numerical simulations. We find that both
approximations based on the SD equations yield good agreement with exact
numerical simulations and cure the late time oscillation problem of the Hartree
approximation. We also discuss the relationship between the quantum and
classical SD equations.Comment: 36 pages, 5 figure
Correlations of structural, magnetic, and dielectric properties of undoped and doped CaCu3Ti4O12
The present work reports synthesis, as well as a detailed and careful
characterization of structural, magnetic, and dielectric properties of
differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics. For this
purpose, neutron and x-ray powder diffraction, SQUID measurements, and
dielectric spectroscopy have been performed. Mn-, Fe-, and Ni-doped CCTO
ceramics were investigated in great detail to document the influence of
low-level doping with 3d metals on the antiferromagnetic structure and
dielectric properties. In the light of possible magnetoelectric coupling in
these doped ceramics, the dielectric measurements were also carried out in
external magnetic fields up to 7 T, showing a minor but significant dependence
of the dielectric constant on the applied magnetic field. Undoped CCTO is
well-known for its colossal dielectric constant in a broad frequency and
temperature range. With the present extended characterization of doped as well
as undoped CCTO, we want to address the question why doping with only 1% Mn or
0.5% Fe decreases the room-temperature dielectric constant of CCTO by a factor
of ~100 with a concomitant reduction of the conductivity, whereas 0.5% Ni
doping changes the dielectric properties only slightly. In addition,
diffraction experiments and magnetic investigations were undertaken to check
for possible correlations of the magnitude of the colossal dielectric constants
with structural details or with magnetic properties like the magnetic ordering,
the Curie-Weiss temperatures, or the paramagnetic moment. It is revealed, that
while the magnetic ordering temperature and the effective moment of all
investigated CCTO ceramics are rather similar, there is a dramatic influence of
doping and tempering time on the Curie-Weiss constant.Comment: 10 pages, 11 figure