215 research outputs found
Effect of Al doping on the optical phonon spectrum in Mg(1-x)Al(x)B(2)
Raman and infrared absorption spectra of Mg(1-x)Al(x)B(2) have been collected
for 0<x<0.5 in the spectral range of optical phonons. The x-dependence of the
peak frequency, the width and the intensity of the observed Raman lines has
been carefully analized. A peculiar x-dependence of the optical modes is
pointed out for two different Al doping ranges. In particular the onset of the
high-doping structural phase previously observed in diffraction measurements is
marked by the appearence of new spectral components at high frequencies. A
connection between the whole of our results and the observed suppression of
superconductivity in the high doping region is established
Optical study of the vibrational and dielectric properties of BiMnO3
BiMnO3 (BMO), ferromagnetic (FM) below Tc = 100 K, was believed to be also
ferroelectric (FE) due to a non-centro-symmetric C2 structure, until
diffraction data indicated that its space group is the centro-symmetric C2/c.
Here we present infrared phonon spectra of BMO, taken on a mosaic of single
crystals, which are consistent with C2/c at any T > 10 K, as well as
room-temperature Raman data which strongly support this conclusion. We also
find that the infrared intensity of several phonons increases steadily for
decreasing T, causing the relative permittivity of BMO to vary from 18.5 at 300
K to 45 at 10 K. At variance with FE materials of displacive type, no
appreciable softening has been found in the infrared phonons. Both their
frequencies and intensities, moreover, appear insensitive to the FM transition
at Tc
Optical Properties of (SrMnO3)n/(LaMnO3)2n superlattices: an insulator-to-metal transition observed in the absence of disorder
We measure the optical conductivity of (SrMnO3)n/(LaMnO3)2n superlattices
(SL) for n=1,3,5, and 8 and 10 < T < 400 K. Data show a T-dependent insulator
to metal transition (IMT) for n \leq 3, driven by the softening of a polaronic
mid-infrared band. At n = 5 that softening is incomplete, while at the
largest-period n=8 compound the MIR band is independent of T and the SL remains
insulating. One can thus first observe the IMT in a manganite system in the
absence of the disorder due to chemical doping. Unsuccessful reconstruction of
the SL optical properties from those of the original bulk materials suggests
that (SrMnO3)n/(LaMnO3)2n heterostructures give rise to a novel electronic
state.Comment: Published Online in Nano Letters, November 8, 2010;
http://pubs.acs.org/doi/abs/10.1021/nl1022628; 5 pages, 3 figure
Infrared study of the charge-ordered multiferroic LuFe(2)O(4)
The reflectivity of a large LuFe(2)O(4) single crystal has been measured with
the radiation field either perpendicular or parallel to the c axis of its
rhombohedral structure, from 10 to 500K, and from 7 to 16000 cm-1. The
transition between the two-dimensional and the three-dimensional charge order
at T_(CO) = 320 K is found to change dramatically the phonon spectrum in both
polarizations. The number of the observed modes above and below T_(CO),
according to a factor-group analysis, is in good agreement with a transition
from the rhombohedral space group R{bar 3}m to the monoclinic C2/m. In the
sub-THz region a peak becomes evident at low temperature, whose origin is
discussed in relation with previous experiments.Comment: Physical Review B in pres
Design of a beamline for soft and deep lithography on third generation synchrotron radiation source
ELETTRA is a third generation synchrotron radiation source. The energy spectrum allows the design of beamlines suitable for x-ray lithography from soft to hard x-ray wavelengths. An appropriate lithographic window for micro- and nanofabrication can be obtained by a combination of selected filters and mirrors. As the beamline is interfaced to a vertical x-ray stepper, a uniformity in the beam intensity better than 3% (3σ) in the horizontal direction has to be reached. The present beamline is designed by taking into account the main factors which can affect the beam quality, namely, thermal loading on mirrors and filters, slope errors, and surface roughness of the mirrors. The resulting lithographic resolution at soft x-ray wavelengths is better than 100 nm
Effect of Mn substitution by Ga on the optical properties of a metallic manganite
In a metallic manganite like La(2/3)Sr(1/3)MnO(3), the substitution of Mn(+3)
by Ga(+3) dilutes the ferromagnetic order and locally cancels the Jahn-Teller
distortion, without heavily affecting the crystal structure. One can thus
follow the changes in the charge dynamics induced by Ga, until the
ferro-metallic manganite is turned into an insulator. Here this phenomenon is
studied in detail through the infrared reflectivity of five samples of
La(2/3)Sr(1/3)Mn(1-x)Ga(x)O(3), with x increasing from 0 to 0.30 and for 50 < T
< 320 K. A simple model which links the measured optical parameters to the
magnetization M(x, T) well describes the behavior of the plasma frequency, the
scattering rate, and the mid-infrared absorption along the metal-to-insulator
transition.Comment: 8 pages including 7 figure
Observation of charge-density-wave excitations in manganites
In the optical conductivity of four different manganites with commensurate
charge order (CO), strong peaks appear in the meV range below the ordering
temperature T_{CO}. They are similar to those reported for one-dimensional
charge density waves (CDW) and are assigned to pinned phasons. The peaks and
their overtones allow one to obtain, for La{1-n/8}Ca{n/8}$MnO{3} with n = 5, 6,
the electron-phonon coupling, the effective mass of the CO system, and its
contribution to the dielectric constant. These results support a description of
the CO in La-Ca manganites in terms of moderately weak-coupling and of the CDW
theory.Comment: To be published on Phys. Rev. Let
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