2,848 research outputs found
Soft Mode Dynamics Above and Below the Burns Temperature in the Relaxor Pb(Mg_1/3Nb_2/3)O_3
We report neutron inelastic scattering measurements of the lowest-energy
transverse optic (TO) phonon branch in the relaxor Pb(Mg_1/3Nb_2/3)O_3 from 400
to 1100 K. Far above the Burns temperature T_d ~ 620 K we observe well-defined
propagating TO modes at all wave vectors q, and a zone center TO mode that
softens in a manner consistent with that of a ferroelectric soft mode. Below
T_d the zone center TO mode is overdamped. This damping extends up to, but not
above, the waterfall wave vector q_wf, which is a measure of the average size
of the PNR.Comment: 4 pages, 4 figures; modified discussion of Fig. 3, shortened
captions, added reference, corrected typos, accepted by Phys. Rev. Let
Static magnetic correlations near the insulating-superconducting phase boundary in LaSrCuO
An elastic neutron scattering study has been performed on several single
crystals of LaSrCuO for {\it x} near the lower critical
concentration {\it x} for superconductivity. % In the insulating
spin-glass phase ({\it x} = 0.04 and 0.053), the previously reported
one-dimensional spin modulation along the orthorhombic {\it b}-axis is
confirmed. % Just inside the superconducting phase ({\it x} = 0.06), however,
two pairs of incommensurate magnetic peaks are additionally observed
corresponding to the spin modulation parallel to the tetragonal axes. % These
two types of spin modulations with similar incommensurabilities coexist near
the boundary. % The peak-width along spin-modulation direction
exhibits an anomalous maximum in the superconducting phase near {\it x},
where the incommensurability monotonically increases upon doping
across the phase boundary. % These results are discussed in connection with the
doping-induced superconducting phase transition.Comment: 9pages, 9figure
Borealis sound an interactive wall for situational awareness: The impact of responsive architecture on users
This paper refers to the presence of visual communication elements in public spaces using interactive multimedia surfaces. The state of the art, the development of a proposal, and its evaluation are presented. The interactive surface applies to school of music “Escola de Música do Conservatório Nacional,” to which a design project was done in an academic context. The proposal aims to explore the incorporation of digital multimedia surfaces enhancing new dynamics while users walk in the interior of the building. Visual content will be displayed, and the interaction between users and the multimedia surfaces is done by the sounds produce by people and instruments. To assess the intuitive nature and relevance of the proposal, satisfaction and usability tests were conducted with the potential users. It was concluded that users appreciated the proposal and was able to perceive the effect people’s presence have in the interaction with the multimedia surface.info:eu-repo/semantics/acceptedVersio
Neutron Diffuse Scattering from Polar Nanoregions in the Relaxor Pb(Mg1/3Nb2/3)O3
We have studied the neutron diffuse scattering in the relaxor PMN. The
diffuse scattering appears around the Burns temperature (~620K), indicating its
origin from the polar nanoregions (PNR). While the relative diffuse intensities
are consistent with previous reports, they are entirely different from those of
the lowest-energy TO phonon. Because of that, it has been considered that this
TO mode could not be the ferroelectric soft mode. Recently, a neutron
scattering study has unambiguously shown that the TO mode does soften on
cooling. If the diffuse scattering in PMN originates from the soft mode
condensation, then the atomic displacements must satisfy the center of mass
condition. But, the atomic displacements determined from diffuse scattering
intensities do not fulfill this condition. To resolve this contradiction, we
propose a simple model in which the total atomic displacement consists of two
components: is created by the soft mode condensation, satisfying
the center of mass condition, and, represents a uniform
displacement of the PNR along their polar direction relative to the surrounding
(unpolarized) cubic matrix. Within this framework, we can successfully describe
the neutron diffuse scattering intensities observed in PMN.Comment: 7 pages, 7 figures (Revised: 11-16-2001
Collars and partitions of hyperbolic cone-surfaces
For compact Riemann surfaces, the collar theorem and Bers' partition theorem
are major tools for working with simple closed geodesics. The main goal of this
paper is to prove similar theorems for hyperbolic cone-surfaces. Hyperbolic
two-dimensional orbifolds are a particular case of such surfaces. We consider
all cone angles to be strictly less than to be able to consider
partitions.Comment: 11 pages, 9 figures; v2: minor changes, to appear in Geometriae
Dedicat
Carrier-induced ferromagnetism in n-type ZnMnAlO and ZnCoAlO thin films at room temperature
The realization of semiconductors that are ferromagnetic above room
temperature will potentially lead to a new generation of spintronic devices
with revolutionary electrical and optical properties. Transition temperatures
in doped ZnO are high but, particularly for Mn doping, the reported moments
have been small. We show that by careful control of both oxygen deficiency and
aluminium doping the ferromagnetic moments measured at room temperature in
n-type ZnMnO and ZnCoO are close to the ideal values of 5mB and 3mB
respectively. Furthermore a clear correlation between the magnetisation per
transition metal ion and the ratio of the number of carriers to the number of
transition metal donors was established as is expected for carrier induced
ferromagnetism for both the Mn and Co doped films. The dependence of the
magnetisation on carrier density is similar to that predicted for the
transition temperature for a dilute magnetic semiconductor in which the
exchange between the transition metal ions is through the free carriers.Comment: 14 pages pd
Dynamical effects of the nanometer-sized polarized domains in Pb(Zn1/3Nb2/3)O3
Recent neutron scattering measurements performed on the relaxor ferroelectric
Pb[(Zn1/3Nb2/3)0.92Ti0.08]O3 (PZN-8%PT) in its cubic phase at 500 K, have
revealed an anomalous ridge of inelastic scattering centered ~0.2 A-1 from the
zone center (Gehring et al., Phys. Rev. Lett. 84, 5216 (2000)). This ridge of
scattering resembles a waterfall when plotted as a phonon dispersion diagram,
and extends vertically from the transverse acoustic (TA) branch near 4 meV to
the transverse optic (TO) branch near 9 meV. No zone center optic mode was
found. We report new results from an extensive neutron scattering study of pure
PZN that exhibits the same waterfall feature. We are able to model the dynamics
of the waterfall using a simple coupled-mode model that assumes a strongly
q-dependent optic mode linewidth Gamma1(q) that increases sharply near 0.2 A-1
as one approaches the zone center. This model was motivated by the results of
Burns and Dacol in 1983, who observed the formation of a randomly-oriented
local polarization in PZN at temperatures far above its ferroelectric phase
transition temperature. The dramatic increase in Gamma1 is believed to occur
when the wavelength of the optic mode becomes comparable to the size of the
small polarized micro-regions (PMR) associated with this randomly-oriented
local polarization, with the consequence that longer wavelength optic modes
cannot propagate and become overdamped. Below Tc=410 K, the intensity of the
waterfall diminishes. At lowest temperatures ~30 K the waterfall is absent, and
we observe the recovery of a zone center transverse optic mode near 10.5 meV.Comment: 8 pages, 9 figures (one color). Submitted to Physical Review
Anomalous transverse acoustic phonon broadening in the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O_3
The intrinsic linewidth of the transverse acoustic (TA) phonon
observed in the relaxor ferroelectric compound
Pb(MgNbTiO (PMN-20%PT) begins to broaden
with decreasing temperature around 650 K, nearly 300 K above the ferroelectric
transition temperature ( K). We speculate that this anomalous
behavior is directly related to the condensation of polarized, nanometer-sized,
regions at the Burns temperature . We also observe the ``waterfall''
anomaly previously seen in pure PMN, in which the transverse optic (TO) branch
appears to drop precipitously into the TA branch at a finite momentum transfer
\AA. The waterfall feature is seen even at
temperatures above . This latter result suggests that the PNR exist as
dynamic entities above .Comment: 6 pages, 4 figure
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