884 research outputs found
NASA-FAA helicopter Microwave Landing System curved path flight test
An ongoing series of joint NASA/FAA helicopter Microwave Landing System (MLS) flight tests was conducted at Ames Research Center. This paper deals with tests done from the spring through the fall of 1983. This flight test investigated and developed solutions to the problem of manually flying curved-path and steep glide slope approaches into the terminal area using the MLS and flight director guidance. An MLS-equipped Bell UH-1H helicopter flown by NASA test pilots was used to develop approaches and procedures for flying these approaches. The approaches took the form of Straight-in, U-turn, and S-turn flightpaths with glide slopes of 6 deg, 9 deg, and 12 deg. These procedures were evaluated by 18 pilots from various elements of the helicopter community, flying a total of 221 hooded instrument approaches. Flying these curved path and steep glide slopes was found to be operationally acceptable with flight director guidance using the MLS
Pressure-induced superconductivity in the giant Rashba system BiTeI
At ambient pressure, BiTeI is the first material found to exhibit a giant
Rashba splitting of the bulk electronic bands. At low pressures, BiTeI
undergoes a transition from trivial insulator to topological insulator. At
still higher pressures, two structural transitions are known to occur. We have
carried out a series of electrical resistivity and AC magnetic susceptibility
measurements on BiTeI at pressure up to ~40 GPa in an effort to characterize
the properties of the high-pressure phases. A previous calculation found that
the high-pressure orthorhombic P4/nmm structure BiTeI is a metal. We find that
this structure is superconducting with Tc values as high as 6 K. AC magnetic
susceptibility measurements support the bulk nature of the superconductivity.
Using electronic structure and phonon calculations, we compute Tc and find that
our data is consistent with phonon-mediated superconductivity.Comment: 7 pages, 7 figure
Structural transitions, octahedral rotations, and electronic properties of NiO rare-earth nickelates under high pressure
Motivated by the recent observation of superconductivity with K
in pressurized La3Ni2O7 [Nature 621, 493 (2023)], we explore the structural and
electronic properties in A3Ni2O7 bilayer nickelates (A=La-Lu, Y, Sc) as a
function of hydrostatic pressure (0-150 GPa) from first principles including a
Coulomb repulsion term. At GPa, we observe an
orthorhombic-to-tetragonal transition in LaNiO at variance with
recent x-ray diffraction data, which points to so-far unresolved complexities
at the onset of superconductivity, e.g., charge doping by variations in the
oxygen stoichiometry. We compile a structural phase diagram with particular
emphasis on the ratio, octahedral anisotropy, and octahedral rotations.
Intriguingly, chemical and external pressure emerge as two distinct and
counteracting control parameters. We find unexpected correlations between
and the in-plane Ni-O-Ni bond angles for LaNiO. Moreover, two novel
structural phases with significant octahedral rotations and in-plane bond
disproportionations are uncovered for A=Nd-Lu, Y, Sc that exhibit a surprising
pressure-driven electronic reconstruction in the Ni manifold. By
disentangling the involvement of basal versus apical oxygen states at the Fermi
surface, we identify TbNiO as an interesting candidate for
superconductivity at ambient pressure. These results suggest a profound
tunability of the structural and electronic phases in this novel materials
class and are key for a fundamental understanding of the superconductivity
mechanism.Comment: 12 pages, 5 figure
Alternative route to charge density wave formation in multiband systems
Charge and spin density waves, periodic modulations of the electron and
magnetization densities, respectively, are among the most abundant and
non-trivial low-temperature ordered phases in condensed matter. The ordering
direction is widely believed to result from the Fermi surface topology.
However, several recent studies indicate that this common view needs to be
supplemented. Here, we show how an enhanced electron-lattice interaction can
contribute to or even determine the selection of the ordering vector in the
model charge density wave system ErTe3. Our joint experimental and theoretical
study allows us to establish a relation between the selection rules of the
electronic light scattering spectra and the enhanced electron-phonon coupling
in the vicinity of band degeneracy points. This alternative proposal for charge
density wave formation may be of general relevance for driving phase
transitions into other broken-symmetry ground states, particularly in multiband
systems such as the iron based superconductors
Studies on the Weak Itinerant Ferromagnet SrRuO3 under High Pressure to 34 GPa
The dependence of the Curie temperature Tc on nearly hydrostatic pressure has
been determined to 17.2 GPa for the weak itinerant ferromagnetic SrRuO3 in both
polycrystalline and single-crystalline form. Tc is found to decrease under
pressure from 162 K to 42.7 K at 17.2 GPa in nearly linear fashion at the rate
dTc/dP = -6.8 K/GPa. No superconductivity was found above 4 K in the pressure
range 17 to 34 GPa. Room-temperature X-ray diffraction studies to 25.3 GPa
reveal no structural phase transition but indicate that the average Ru-O-Ru
bond angle passes through a minimum near 15 GPa. The bulk modulus and its
pressure derivative were determined to be B =192(3) GPa and B' = 5.0(3),
respectively. Parallel ac susceptibility studies on polycrystalline CaRuO3 at 6
and 8 GPa pressure found no evidence for either ferromagnetism or
superconductivity above 4 K
Remarkable low-energy properties of the pseudogapped semimetal Be5Pt
We report measurements and calculations on the properties of the intermetallic compound Be5Pt. High-quality polycrystalline samples show a nearly constant temperature dependence of the electrical resistivity over a wide temperature range. On the other hand, relativistic electronic structure calculations indicate the existence of a narrow pseudogap in the density of states arising from accidental approximate Dirac cones extremely close to the Fermi level. A small true gap of order 3c3 meV is present at the Fermi level, yet the measured resistivity is nearly constant from low to room temperature. We argue that this unexpected behavior can be understood by a cancellation of the energy dependence of density of states and relaxation time due to disorder, and discuss a model for electronic transport. With applied pressure, the resistivity becomes semiconducting, consistent with theoretical calculations that show that the bandgap increases with applied pressure. We further discuss the role of Be inclusions in the samples
Nb-substitution suppresses the superconducting critical temperature of pressurized MoB
A recent work has demonstrated that MoB, transforming to the same
structure as MgB (), superconducts at temperatures above 30 K near
100 GPa [C. Pei . Natl. Sci. Rev., nwad034 (2023)], and
Nb-substitution in MoB stabilizes the structure down to ambient
pressure [A. C. Hire . Phys. Rev. B 106, 174515 (2022)]. The current
work explores the high pressure superconducting behavior of Nb-substituted
MoB (NbMoB). High pressure x-ray diffraction
measurements show that the sample remains in the ambient pressure
structure to at least 160 GPa. Electrical resistivity measurements demonstrate
that from an ambient pressure of 8 K (confirmed by specific heat to be a
bulk effect), the critical temperature is suppressed to 4 K at 50 GPa, before
gradually rising to 5.5 K at 170 GPa. The critical temperature at high pressure
is thus significantly lower than that found in MoB under pressure (30 K),
revealing that Nb-substitution results in a strong suppression of the
superconducting critical temperature. Our calculations indeed find a reduced
electron-phonon coupling in NbMoB, but do not account
fully for the observed suppression, which may also arise from inhomogeneity and
enhanced spin fluctuations.Comment: 8 pages, 5 figures, 1 tabl
Correlation of fluvial sequences in the Mediterranean basin over the last 200 ka and their relationship to climate change
This paper presents a new correlation of Late and Middle Pleistocene fluvial sedimentary sequences in Greek, Libyan and Spanish river basins and evaluates river response to climate change over the Last Interglacial–Glacial Cycle. Over the past 200,000 years there have been at least 13 major alluviation episodes in the Mediterranean, although the amplitude, frequency and possibly, duration of these events varied significantly across the region. Parts of Oxygen Isotope Stage (OIS) 5 appears to have been periods of pronounced landscape change in many Mediterranean catchments with major river aggradation occurring atB109–111 ka (during OIS 5d) and most notably at B88 ka (OIS 5b/5a boundary). Other parts of OIS 5 appear to have been periods of relative fluvial inactivity. OIS 2 and 3 were both characterised by an apparent increase in the number of alluviation events, and this record of river behaviour parallels many other palaeoenvironmental records in the region which also show more frequent climate fluctuations between B12 and 65 ka. There is evidence for a high degree of synchrony in major river aggradation events across the Mediterranean in catchments with very different sizes, tectonic regimes and histories. Climate-related changes in catchmen
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