2,848 research outputs found
A Universal Phase Diagram for PMN-xPT and PZN-xPT
The phase diagram of the Pb(Mg1/3Nb2/3)O3 and PbTiO3 solid solution (PMN-xPT)
indicates a rhombohedral ground state for x < 0.32. X-ray powder measurements
by Dkhil et al. show a rhombohedrally split (222) Bragg peak for PMN-10%PT at
80 K. Remarkably, neutron data taken on a single crystal of the same compound
with comparable q-resolution reveal a single resolution-limited (111) peak down
to 50 K, and thus no rhombohedral distortion. Our results suggest that the
structure of the outer layer of these relaxors differs from that of the bulk,
which is nearly cubic, as observed in PZN by Xu et al.Comment: Replaced Fig. 3 with better versio
Fault‐Slip Distribution of the 1999 M_w 7.1 Hector Mine Earthquake, California, Estimated from Postearthquake Airborne LiDAR Data
The 16 October 1999 Hector Mine earthquake (M_w 7.1) was the first large earthquake for which postearthquake airborne Light Detection and Ranging (LiDAR) data were collected to image the fault surface rupture. In this work, we present measurements of both vertical and horizontal slip along the entire surface rupture of this earthquake based on airborne LiDAR data acquired in April 2000. We examine the details of the along‐fault slip distribution of this earthquake based on 255 horizontal and 85 vertical displacements using a 0.5 m digital elevation model derived from the LiDAR imagery. The slip measurements based on the LiDAR dataset are highest in the epicentral region, and taper in both directions, consistent with earlier findings by other works. The maximum dextral displacement measured from LiDAR imagery is 6.60±1.10 m, located about 700 m south of the highest field measurement (5.25±0.85 m). Our results also illustrate the difficulty in resolving displacements smaller than 1 m using LiDAR imagery alone. We analyze slip variation to see if it is affected by rock type and whether variations are statistically significant. This study demonstrates that a postearthquake airborne LiDAR survey can produce an along‐fault horizontal and vertical offset distribution plot of a quality comparable to a reconnaissance field survey. Although LiDAR data can provide a higher sampling density and enable rapid data analysis for documenting slip distributions, we find that, relative to field methods, it has a limited ability to resolve slip that is distributed over several fault strands across a zone. We recommend a combined approach that merges field observation with LiDAR analysis, so that the best attributes of both quantitative topographic and geological insight are utilized in concert to make best estimates of offsets and their uncertainties
Magnetism and superconductivity driven by identical 4 states in a heavy-fermion metal
The apparently inimical relationship between magnetism and superconductivity
has come under increasing scrutiny in a wide range of material classes, where
the free energy landscape conspires to bring them in close proximity to each
other. This is particularly the case when these phases microscopically
interpenetrate, though the manner in which this can be accomplished remains to
be fully comprehended. Here, we present combined measurements of elastic
neutron scattering, magnetotransport, and heat capacity on a prototypical heavy
fermion system, in which antiferromagnetism and superconductivity are observed.
Monitoring the response of these states to the presence of the other, as well
as to external thermal and magnetic perturbations, points to the possibility
that they emerge from different parts of the Fermi surface. This enables a
single 4 state to be both localized and itinerant, thus accounting for the
coexistence of magnetism and superconductivity.Comment: 4 pages, 4 figure
Ground State of Relaxor Ferroelectric
High energy x-ray diffraction measurements on Pb(ZnNb)O
(PZN) single crystals show that the system does not have a rhombohedral
symmetry at room temperature as previously believed. The new phase (X) in the
bulk of the crystal gives Bragg peaks similar to that of a nearly cubic lattice
with a slight tetragonal distortion. The Bragg profile remains sharp with no
evidence of size broadening due to the polar micro crystals (MC). However, in
our preliminary studies of the skin, we have found the expected rhombohedral
(R) phase as a surface state. On the other hand, studies on an electric-field
poled PZN single crystal clearly indicate a rhombohedral phase at room
temperature.Comment: 11 pages with 3 figure
Neutron and X-ray diffraction study of cubic [111] field cooled Pb(Mg1/3Nb2/3)O3
Neutron and x-ray diffraction techniques have been used to study the
competing long and short-range polar order in the relaxor ferroelectric
Pb(MgNb)O (PMN) under a [111] applied electric field.
Despite reports of a structural transition from a cubic phase to a rhombohedral
phase for fields E 1.7 kV/cm, we find that the bulk unit cell remains cubic
(within a sensitivity of 90- =0.03)for fields up to
8 kV/cm. Furthermore, we observe a structural transition confined to the near
surface volume or `skin' of the crystal where the cubic cell is transformed to
a rhombohedral unit cell at T=210 K for E 4 kV/cm, for which
90-=0.08 0.03 below 50 K. While the bulk unit
cell remains cubic, a suppression of the diffuse scattering and concomitant
enhancement of the Bragg peak intensity is observed below T=210 K,
indicating a more ordered structure with increasing electric field yet an
absence of a long-range ferroelectric ground state in the bulk. The electric
field strength has little effect on the diffuse scattering above T,
however below T the diffuse scattering is reduced in intensity and adopts
an asymmetric lineshape in reciprocal space. The absence of hysteresis in our
neutron measurements (on the bulk) and the presence of two distinct temperature
scales suggests that the ground state of PMN is not a frozen glassy phase as
suggested by some theories but is better understood in terms of random fields
introduced through the presence of structural disorder. Based on these results,
we also suggest that PMN represents an extreme example of the two-length scale
problem, and that the presence of a distinct skin maybe necessary for a relaxor
ground state.Comment: 12 pages, 9 figure
Evidence for anisotropic polar nanoregions in relaxor PMN: A neutron study of the elastic constants and anomalous TA phonon damping
We use neutron scattering to characterize the acoustic phonons in the relaxor
PMN and demonstrate the presence of an anisotropic damping mechanism directly
related to short-range, polar correlations. For a large range of temperatures
above Tc ~ 210, K, where dynamic polar correlations exist, acoustic phonons
propagating along [1\bar{1}0] and polarized along [110] (TA2 phonons) are
overdamped and softened across most of the Brillouin zone. By contrast,
acoustic phonons propagating along [100] and polarized along [001] (TA1
phonons) are overdamped and softened for only a limited range of wavevectors.
The anisotropy and temperature dependence of the acoustic phonon energy
linewidth are directly correlated with the elastic diffuse scattering,
indicating that polar nanoregions are the cause of the anomalous behavior. The
damping and softening vanish for q -> 0, i.e. for long-wavelength acoustic
phonons, which supports the notion that the anomalous damping is a result of
the coupling between the relaxational component of the diffuse scattering and
the harmonic TA phonons. Therefore, these effects are not due to large changes
in the elastic constants with temperature because the elastic constants
correspond to the long-wavelength limit. We compare the elastic constants we
measure to those from Brillouin scattering and to values reported for pure PT.
We show that while the values of C44 are quite similar, those for C11 and C12
are significantly less in PMN and result in a softening of (C11-C12) over PT.
There is also an increased elastic anisotropy (2C44/(C11-C12)) versus that in
PT. These results suggest an instability to TA2 acoustic fluctuations in
relaxors. We discuss our results in the context of the debate over the
"waterfall" effect and show that they are inconsistent with TA-TO phonon
coupling or other models that invoke the presence of a second optic mode.Comment: (21 pages, 16 figures, to be published in Physical Review B
Competing orders in PZN-xPT and PMN-xPT relaxor ferroelectrics
Neutron and x-ray scattering studies on relaxor ferroelectric systems
Pb(ZnNb)O (PZN), Pb(MgNb)O (PMN), and
their solid solutions with PbTiO (PT) have shown that inhomogeneities and
disorder play important roles in the materials properties. Although a
long-range polar order can be established at low temperature - sometimes with
the help of an external electric field; short-range local structures called the
``polar nano-regions'' (PNR) still persist. Both the bulk structure and the PNR
have been studied in details. The coexistence and competition of long- and
short-range polar orders and how they affect the structural and dynamical
properties of relaxor materials are discussed.Comment: Article submitted for JPSJ Special Topics (Novel States of Matter
Induced by Frustration
Bose-Einstein Correlations of Pion Wavepackets
A wavepacket model for a system of free pions, which takes into account the
full permutation symmetry of the wavefunction and which is suitable for any
phase space parametrization is developed. The properties of the resulting mixed
ensembles and the two-particle correlation function are discussed. A physical
interpretation of the chaoticity lambda as localizat of the pions in the source
is presented.
Two techniques to generate test-particles, which satisfy the probability
densities of the wavepacket state, are studied:
1. A Monte Carlo procedure in momentum space based on the standard Metropolis
technique.
2. A molecular dynamic procedure using Bohm's quantum theory of motion.
In order to reduce the numerical complexity, the separation of the
wavefunction into momentum space clusters is discussed. In this context th
influence of an unauthorized factorization of the state, i. e. the omissio of
interference terms, is investigated. It is shown that the correlation radius
remains almost uneffected, but the chaoticity parameter decreases
substantially. A similar effect is observed in systems with high multiplic
where the omission of higher order corrections in the analysis of two-part
correlations causes a reduction of the chaoticity and the radius.
The approximative treatment of the Coulomb interaction between pions and
source is investigated. The results suggest that Coulomb effects on the co
radii are not symmetric for pion pairs of different charges. For negative the
radius, integrated over the whole momentum spectrum, increases substan while
for positive pions the radius remains almost unchanged.Comment: 15 pages, 8 figures, 0.8 Mb, uses ljour2-macro, Submitted to Z. Phys.
A (1997
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