719 research outputs found
Our Part in These Times by J. O. Christianson, UND Mid-Year Commencement: January 31, 1943
Text of speech delivered by Dr. J. O. Christianson at the UND Mid-Year Commencement on January 31, 1943. Dr. Christianson was the superintendent for the School of Agriculture at the University of Minnesota - Twin Cities. He entitled his remarks: Our Part in These Times
Phonon self-energy and origin of anomalous neutron scattering spectra in SnTe and PbTe thermoelectrics
The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe
and PbTe are investigated with inelastic neutron scattering (INS) and
first-principles calculations. The experiments show that, surprisingly,
although SnTe is closer to the ferroelectric instability, phonon spectra in
PbTe exhibit a more anharmonic character. This behavior is reproduced in
first-principles calculations of the temperature-dependent phonon self-energy.
Our simulations reveal how the nesting of phonon dispersions induces prominent
features in the self-energy, which account for the measured INS spectra and
their temperature dependence. We establish that the phase-space for
three-phonon scattering processes, rather than just the proximity to the
lattice instability, is the mechanism determining the complex spectrum of the
transverse-optical ferroelectric mode
A low-cost autonomous rover for polar science
We present the developmental considerations, design, and deployment of an autonomous modular terrestrial rover for ice-sheet exploration that is inexpensive, easy to construct, and allows for instrumentation customization. The total construction cost for this rover is less than USD 3000, approximately one-tenth the cost of existing platforms, and it can be built using facilities frequently available at academic institutions (machine shop, 3-D printer, open-source hardware and software). Instrumentation deployed on this rover can be customized; the rover presented in this study was equipped with a dual-frequency GPS receiver and a digital SLR camera for constructing digital elevation models using structure-from-motion (SfM) photogrammetry. We deployed this prototype rover on the Northeast Greenland Ice Stream to map local variations in snow accumulation and surface topography. The rover conducted four autonomous missions based out of the East Greenland Ice-Core Project (EastGRIP) camp during July 2017, measuring surface elevation transects across the hazardous ice-stream shear margins. During these missions, the rover proved capable of driving over 20 km on a single charge with a drawbar pull of 250 N, sufficient to tow instrumentation of up to 100 kg. The rover also acquired photographs that were subsequently used to construct digital elevation models of a site monitored for spatiotemporal variability in snow accumulation, demonstrating adequate stability for high-resolution imaging applications. Due to its low cost, low-power requirements, and simple modular design, mass deployments of this rover design are practicable. Operation of the rover in hazardous areas circumvents the substantial expense and risk to personnel associated with conventional, crewed deployments. Thus, this rover is an investigatory platform that enables direct exploration of polar environments considered too hazardous for conventional field expeditions.publishedVersio
Crystalline Electric Field Excitations in the Heavy Fermion Superconductor CeCoIn_5
The crystalline electric field (CEF) energy level scheme of the heavy fermion
superconductor CeCoIn_5 has been determined by means of inelastic neutron
scattering (INS). Peaks observed in the INS spectra at 8 meV and 27 meV with
incident neutron energies between E_i=30-60 meV and at a temperature T = 10 K
correspond to transitions from the ground state to the two excited states,
respectively. The wavevector and temperature dependence of these peaks are
consistent with CEF excitations. Fits of the data to a CEF model yield the CEF
parameters B^0_2=-0.80 meV, B^0_4=0.059 meV, and |B^4_4|= 0.137 meV
corresponding to an energy level scheme: Gamma_7^(1) (0)[=0.487|+/-5/2> -
0.873|-/+3/2>], Gamma_7^(2) (8.6 meV, 100 K), and Gamma_6 (24.4 meV, 283 K).Comment: uses latex packages revtex4,amsmath,graphicx,natbib, 9th Annual
MMM-Intermag Conference, (Accepted for publication in J. Appl. Phys.) 7
pages, 2 figure
Enhanced low-energy magnetic excitations via suppression of the itinerancy in Fe0.98-zCuzTe0.5Se0.5
We have performed resistivity and inelastic neutron scattering measurements
on three samples of Fe0.98-zCuzTe0.5Se0.5 with z = 0, 0.02, and 0.1. It is
found that with increasing Cu doping the sample's resistivity deviates
progressively from that of a metal. However, in contrast to expectations that
replacing Fe with Cu would suppress the magnetic correlations, the low-energy
(no larger than 12 meV) magnetic scattering is enhanced in strength, with
greater spectral weight and longer dynamical spin-spin correlation lengths.
Such enhancements can be a consequence of either enlarged local moments or a
slowing down of the spin fluctuations. In either case, the localization of the
conduction states induced by the Cu doping should play a critical role. Our
results are not applicable to models that treat 3d transition metal dopants
simply as effective electron donors.Comment: 5 pages, 5 figures. To appear in PR
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