89 research outputs found
Magnetoelastic coupling in RETiO3 (RE = La, Nd, Sm, Gd, Y)
A detailed analysis of the crystal structure in RETiO3 with RE = La, Nd, Sm,
Gd, and Y reveals an intrinsic coupling between orbital degrees of freedom and
the lattice which cannot be fully attributed to the structural deformation
arising from bond-length mismatch. The TiO6 octahedra in this series are all
irregular with the shape of the distortion depending on the RE ionic radius.
These octahedron distortions vary more strongly with temperature than the tilt
and rotation angles. Around the Ti magnetic ordering all compounds exhibit
strong anomalies in the thermal-expansion coefficients, these anomalies exhibit
opposite signs for the antiferromagnetic and ferromagnetic compounds.
Furthermore the strongest effects are observed in the materials close to the
magnetic cross-over from antiferromagnetic to ferromagnetic order
Crystal and magnetic structure of LaTiO3 : evidence for non-degenerate -orbitals
The crystal and magnetic structure of LaTiO3 ~ has been studied by x-ray and
neutron diffraction techniques using nearly stoichiometric samples. We find a
strong structural anomaly near the antiferromagnetic ordering, T=146 K. In
addition, the octahedra in LaTiO3 exhibit an intrinsic distortion which implies
a splitting of the t2g-levels. Our results indicate that LaTiO3 should be
considered as a Jahn-Teller system where the structural distortion and the
resulting level splitting are enhanced by the magnetic ordering.Comment: 4 pages 5 figure
Optical study of orbital excitations in transition-metal oxides
The orbital excitations of a series of transition-metal compounds are studied
by means of optical spectroscopy. Our aim was to identify signatures of
collective orbital excitations by comparison with experimental and theoretical
results for predominantly local crystal-field excitations. To this end, we have
studied TiOCl, RTiO3 (R=La, Sm, Y), LaMnO3, Y2BaNiO5, CaCu2O3, and K4Cu4OCl10,
ranging from early to late transition-metal ions, from t_2g to e_g systems, and
including systems in which the exchange coupling is predominantly
three-dimensional, one-dimensional or zero-dimensional. With the exception of
LaMnO3, we find orbital excitations in all compounds. We discuss the
competition between orbital fluctuations (for dominant exchange coupling) and
crystal-field splitting (for dominant coupling to the lattice). Comparison of
our experimental results with configuration-interaction cluster calculations in
general yield good agreement, demonstrating that the coupling to the lattice is
important for a quantitative description of the orbital excitations in these
compounds. However, detailed theoretical predictions for the contribution of
collective orbital modes to the optical conductivity (e.g., the line shape or
the polarization dependence) are required to decide on a possible contribution
of orbital fluctuations at low energies, in particular in case of the orbital
excitations at about 0.25 eV in RTiO3. Further calculations are called for
which take into account the exchange interactions between the orbitals and the
coupling to the lattice on an equal footing.Comment: published version, discussion of TiOCl extended to low T, improved
calculation of orbital excitation energies in TiOCl, figure 16 improved,
references updated, 33 pages, 20 figure
Theoretical analysis of neutron scattering results for quasi-two dimensional ferromagnets
A theoretical study has been carried out to analyse the available results
from the inelastic neutron scattering experiment performed on a quasi-two
dimensional spin-1/2 ferromagnetic material . Our formalism is based
on a conventional semi-classical like treatment involving a model of an ideal
gas of vortices/anti-vortices corresponding to an anisotropic XY Heisenberg
ferromagnet on a square lattice. The results for dynamical structure functions
for our model corresponding to spin-1/2, show occurrence of negative values in
a large range of energy transfer even encompassing the experimental range, when
convoluted with a realistic spectral window function. This result indicates
failure of the conventional theoretical framework to be applicable to the
experimental situation corresponding to low spin systems. A full quantum
formalism seems essential for treating such systems.Comment: 16 pages, 6 figures, 1 Table Submitted for publicatio
Colloquium: Light scattering by particle and hole arrays
This colloquium analyzes the interaction of light with two-dimensional
periodic arrays of particles and holes. The enhanced optical transmission
observed in the latter and the presence of surface modes in patterned metal
surfaces are thoroughly discussed. A review of the most significant discoveries
in this area is presented first. A simple tutorial model is then formulated to
capture the essential physics involved in these phenomena, while allowing
analytical derivations that provide deeper insight. Comparison with more
elaborated calculations is offered as well. Finally, hole arrays in
plasmon-supporting metals are compared to perforated perfect conductors, thus
assessing the role of plasmons in these types of structures through analytical
considerations.Comment: 19 figure
Piperidinols that show anti-tubercular activity as inhibitors of arylamine N-acetyltransferase: an essential enzyme for mycobacterial survival inside macrophages
Latent M. tuberculosis infection presents one of the major obstacles in the global eradication of tuberculosis (TB). Cholesterol plays a critical role in the persistence of M. tuberculosis within the macrophage during latent infection. Catabolism of cholesterol contributes to the pool of propionyl-CoA, a precursor that is incorporated into cell-wall lipids. Arylamine N-acetyltransferase (NAT) is encoded within a gene cluster that is involved in the cholesterol sterol-ring degradation and is essential for intracellular survival. The ability of the NAT from M. tuberculosis (TBNAT) to utilise propionyl-CoA links it to the cholesterol-catabolism pathway. Deleting the nat gene or inhibiting the NAT enzyme prevents intracellular survival and results in depletion of cell-wall lipids. TBNAT has been investigated as a potential target for TB therapies. From a previous high-throughput screen, 3-benzoyl-4-phenyl-1-methylpiperidinol was identified as a selective inhibitor of prokaryotic NAT that exhibited antimycobacterial activity. The compound resulted in time-dependent irreversible inhibition of the NAT activity when tested against NAT from M. marinum (MMNAT). To further evaluate the antimycobacterial activity and the NAT inhibition of this compound, four piperidinol analogues were tested. All five compounds exert potent antimycobacterial activity against M. tuberculosis with MIC values of 2.3-16.9 µM. Treatment of the MMNAT enzyme with this set of inhibitors resulted in an irreversible time-dependent inhibition of NAT activity. Here we investigate the mechanism of NAT inhibition by studying protein-ligand interactions using mass spectrometry in combination with enzyme analysis and structure determination. We propose a covalent mechanism of NAT inhibition that involves the formation of a reactive intermediate and selective cysteine residue modification. These piperidinols present a unique class of antimycobacterial compounds that have a novel mode of action different from known anti-tubercular drugs
Dielectric properties and dynamical conductivity of LaTiO3: From dc to optical frequencies
We provide a complete and detailed characterization of the
temperature-dependent response to ac electrical fields of LaTiO3, a
Mott-Hubbard insulator close to the metal-insulator transition. We present
combined dc, broadband dielectric, mm-wave, and infrared spectra of ac
conductivity and dielectric constant, covering an overall frequency range of 17
decades. The dc and dielectric measurements reveal information on the
semiconducting charge-transport properties of LaTiO3, indicating the importance
of Anderson localization, and on the dielectric response due to ionic
polarization. In the infrared region, the temperature dependence of the phonon
modes gives strong hints for a structural phase transition at the magnetic
ordering temperature. In addition, a gap-like electronic excitation following
the phonon region is analyzed in detail. We compare the results to the
soft-edge behavior of the optical spectra characteristic for Mott-Hubbard
insulators. Overall a consistent picture of the charge-transport mechanisms in
LaTiO3 emerges.Comment: 11 pages, 8 figures, 1 tabl
Self-control interventions for children under age 10 for improving self-control and delinquency and problem behaviors
Self-control improvement programs are intended to serve many purposes, most
notably improving self-control. Yet, interventions such as these often aim to reduce
delinquency and problem behaviors. However, there is currently no summary
statement available regarding whether or not these programs are effective in
improving self-control and reducing delinquency and problem behaviors. The main objective of this review is to assess the available research evidence on the
effect of self-control improvement programs on self-control and delinquency and
problem behaviors. In addition to investigating the overall effect of early selfcontrol
improvement programs, this review will examine, to the extent possible, the
context in which these programs may be most successful. The studies included in this systematic review indicate that self-control
improvement programs are an effective intervention for improving self-control and
reducing delinquency and problem behaviors, and that the effect of these programs
appears to be rather robust across various weighting procedures, and across context,
outcome source, and based on both published and unpublished data
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