245 research outputs found
Implementation Grant Project
A Preservation and Access: Sustaining Cultural Heritage Collections implementation grant application to fund a capital improvement project to upgrade the heat and air conditioning system software and replace the two networks of sensors and controllers that automate the system that provides a preservation storage environment for the Center's 93,750 square feet of collection storage space
Static magnetic susceptibility, crystal field and exchange interactions in rare earth titanate pyrochlores
The experimental temperature dependence (T = 2–300 K) of single crystal bulk and site susceptibilities of rare earth titanate pyrochlores R2Ti2O7 (R = Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) is analyzed in the framework of crystal field theory and a mean field approximation. Analytical expressions for the site and bulk susceptibilities of the pyrochlore lattice are derived taking into account long range dipole–dipole interactions and anisotropic exchange interactions between the nearest neighbor rare earth ions. The sets of crystal field parameters and anisotropic exchange coupling constants have been determined and their variations along the lanthanide series are discussed.
Mapping the B,T phase diagram of frustrated metamagnet CuFeO2
The magnetic phase diagram of CuFeO2 as a function of applied magnetic field
and temperature is thoroughly explored and expanded, both for magnetic fields
applied parallel and perpendicular to the material's c-axis. Pulsed field
magnetization measurements extend the typical magnetic staircase of CuFeO2 at
various temperatures, demonstrating the persistence of the recently discovered
high field metamagnetic transition up to Tn2 ~ 11 K in both field
configurations. An extension of the previously introduced phenomenological spin
model used to describe the high field magnetization process (Phys. Rev. B, 80,
012406 (2009)) is applied to each of the consecutive low-field commensurate
spin structures, yielding a semi-quantitative simulation and intuitive
description of the entire experimental magnetization process in both relevant
field directions with a single set of parameters.Comment: 14 pages, 11 figures, submitted to Phys. Rev.
High-field recovery of the undistorted triangular lattice in the frustrated metamagnet CuFeO2
Pulsed field magnetization experiments extend the typical metamagnetic
staircase of CuFeO2 up to 58 T to reveal an additional first order phase
transition at high field for both the parallel and perpendicular field
configuration. Virtually complete isotropic behavior is retrieved only above
this transition, indicating the high-field recovery of the undistorted
triangular lattice. A consistent phenomenological rationalization for the field
dependence and metamagnetism crossover of the system is provided, demonstrating
the importance of both spin-phonon coupling and a small field-dependent
easy-axis anisotropy in accurately describing the magnetization process of
CuFeO2.Comment: 4 pages, 4 figure
Thermotropic Phase Boundaries in Classic Ferroelectrics
High-performance piezoelectrics are lead-based solid solutions that exhibit a so-called morphotropic phase boundary, which separates two competing phases as a function of chemical composition; as a consequence, an intermediate low-symmetry phase with a strong piezoelectric effect arises. In search for environmentally sustainable lead-free alternatives that exhibit analogous characteristics, we use a network of competing domains to create similar conditions across thermal inter-ferroelectric transitions in simple, lead-free ferroelectrics such as BaTiO3 and KNbO3. Here we report the experimental observation of thermotropic phase boundaries in these classic ferroelectrics, through direct imaging of low-symmetry intermediate phases that exhibit large enhancements in the existing nonlinear optical and piezoelectric property coefficients. Furthermore, the symmetry lowering in these phases allows for new property coefficients that exceed all the existing coefficients in both parent phases. Discovering the thermotropic nature of thermal phase transitions in simple ferroelectrics thus presents unique opportunities for the design of \u27green\u27 high-performance materials
Phonon and crystal field excitations in geometrically frustrated rare earth titanates
The phonon and crystal field excitations in several rare earth titanate
pyrochlores are investigated. Magnetic measurements on single crystals of
Gd2Ti2O7, Tb2Ti2O7, Dy2Ti2O7 and Ho2Ti2O7 are used for characterization, while
Raman spectroscopy and terahertz time domain spectroscopy are employed to probe
the excitations of the materials. The lattice excitations are found to be
analogous across the compounds over the whole temperature range investigated
(295-4 K). The resulting full phononic characterization of the R2Ti2O7
pyrochlore structure is then used to identify crystal field excitations
observed in the materials. Several crystal field excitations have been observed
in Tb2Ti2O7 in Raman spectroscopy for the first time, among which all of the
previously reported excitations. The presence of additional crystal field
excitations, however, suggests the presence of two inequivalent Tb3+ sites in
the low temperature structure. Furthermore, the crystal field level at
approximately 13 cm-1 is found to be both Raman and dipole active, indicating
broken inversion symmetry in the system and thus undermining its current
symmetry interpretation. In addition, evidence is found for a significant
crystal field-phonon coupling in Tb2Ti2O7. These findings call for a careful
reassessment of the low temperature structure of Tb2Ti2O7, which may serve to
improve its theoretical understanding.Comment: 13 pages, 7 figure
Shaping, imaging and controlling plasmonic interference fields at buried interfaces
Filming and controlling plasmons at buried interfaces with nanometer (nm) and
femtosecond (fs) resolution has yet to be achieved and is critical for next
generation plasmonic/electronic devices. In this work, we use light to excite
and shape a plasmonic interference pattern at a buried metal-dielectric
interface in a nanostructured thin film. Plasmons are launched from a
photoexcited array of nanocavities and their propagation is filmed via
photon-induced near-field electron microscopy (PINEM). The resulting movie
directly captures the plasmon dynamics, allowing quantification of their group
velocity at approximately 0.3c, consistent with our theoretical predictions.
Furthermore, we show that the light polarization and nanocavity design can be
tailored to shape transient plasmonic gratings at the nanoscale. These results,
demonstrating dynamical imaging with PINEM, pave the way for the fs/nm
visualization and control of plasmonic fields in advanced heterostructures
based on novel 2D materials such as graphene, MoS, and ultrathin metal
films.Comment: 16 pages, 5 figures, 3 supplementary figure
Ultrafast Imaging of Plasmons in a Transmission Electron Microscope
Miniaturized plasmonic and photonic integrated circuits are generally considered as the core of future generations of optoelectronic devices, due to their potential to bridge the size-compatibility gap between photonics and electronics. However, as the nanoscale is approached in increasingly small plasmonic and photonic systems, experimentally observing their behavior involves ever more stringent requirements in terms of both temporal and spatial resolution. This talk focuses on the use of time-resolved Photon-Induced Near-Field Electron Microscopy (PINEM) to study the excitation, propagation, (self-)interference and dynamics of surface plasmon polaritons (SPPs) in various plasmonic nanostructures with both nanometer and ultrafast resolution in a transmission electron microscope. Using this field-ofview technique, we directly show how photo-excited plasmonic interference patterns are controlled through the combination of excitation polarization and nanostructure geometry. Moreover, we capture the propagation of the photoinduced self-interfering plasmonic wave, clearly demonstrating the effects of axial confinement in nanostructured plasmonic thin film stacks
Flood Risk Management: An Illustrative Approach
Widespread flooding with significant damage in many countries, such as the Philippines in 2013, highlights the ongoing need for effective flood risk management (FRM). This hinges on comprehensive access to and dissemination of information about the elements and the people at risk. Simulations, real-time graphs, and maps illustrate the spatial distribution of flood risks, spatial allocation and dissemination of flood effects, if flood risk reduction measures are not implemented, as well as the benefits to be derived from the effective implementation and maintenance of flood risk management measures not realized. Using precipitation, river water, and tide levels, a real-time monitoring site was set up for the Shirakawa River, Kumamoto, Japan. The data gathered from the July 2012 flood event is used as a demonstrator, illustrating a flood event as well as how to utilize the information provided on this site to determine the future time and possibility of flooding. Additionally, an electronically generated flood hazard map making process is being developed for distribution across Japan. These illustrative approaches can be utilized in cities and communities around the globe
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