2,198 research outputs found
Recent Studies in Superconductivity at Extreme Pressures
Studies of the effect of high pressure on superconductivity began in 1925
with the seminal work of Sizoo and Onnes on Sn to 0.03 GPa and have continued
up to the present day to pressures in the 200 - 300 GPa range. Such enormous
pressures cause profound changes in all condensed matter properties, including
superconductivity. In high pressure experiments metallic elements, Tc values
have been elevated to temperatures as high as 20 K for Y at 115 GPa and 25 K
for Ca at 160 GPa. These pressures are sufficient to turn many insulators into
metals and magnetics into superconductors. The changes will be particularly
dramatic when the pressure is sufficient to break up one or more atomic shells.
Recent results in superconductivity to Mbar pressures wll be discussed which
exemplify the progress made in this field over the past 82 years.Comment: Proceedings of the 21st AIRAPT and 45th EHPRG International
Conference on High Pressure Science and Technology, Catania, Italy, Sept.
17-21, 200
Gallai-Ramsey Number for Classes of Brooms
Given a graph , we consider the problem of finding the minimum number such that any edge colored complete graph on vertices contains either a rainbow colored triangle or a monochromatic copy of the graph , denoted . More precisely we consider where is a broom graph with representing the number of vertices on the handle and representing the number of bristle vertices. We develop a technique to reduce the difficulty of finding , and use the technique to prove a few cases with a fixed handle length, but arbitrarily many bristles. Further, we find upper and lower bounds for any broom
Evolution of the Fermi surface of BiTeCl with pressure
We report measurements of Shubnikov-de Haas oscillations in the giant Rashba
semiconductor BiTeCl under applied pressures up to ~2.5 GPa. We observe two
distinct oscillation frequencies, corresponding to the Rashba-split inner and
outer Fermi surfaces. BiTeCl has a conduction band bottom that is split into
two sub-bands due to the strong Rashba coupling, resulting in two
spin-polarized conduction bands as well as a Dirac point. Our results suggest
that the chemical potential lies above this Dirac point, giving rise to two
Fermi surfaces. We use a simple two-band model to understand the pressure
dependence of our sample parameters. Comparing our results on BiTeCl to
previous results on BiTeI, we observe similar trends in both the chemical
potential and the Rashba splitting with pressure.Comment: 6 pages, 5 figure
Pressure-induced Superconductivity in CaLi2
A search for superconductivity has been carried out on the hexagonal
polymorph of Laves-phase CaLi2, a compound for which Feng, Ashcroft, and
Hoffmann predict highly anomalous behavior under pressure. No superconductivity
is observed above 1.10 K at ambient pressure. However, high-pressure ac
susceptibility and electrical resistivity studies to 81 GPa reveal bulk
superconductivity in CaLi2 at temperatures as high as 13 K. The normal-state
resistivity shows a dramatic increase with pressure.Comment: bulk superconductivity in CaLi2 now confirme
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
Studies of superconductivity and structure for CaC6 to pressures above 15 GPa
The dependence of the superconducting transition temperature Tc of CaC6 has
been determined as a function of hydrostatic pressure in both helium-loaded gas
and diamond-anvil cells to 0.6 and 32 GPa, respectively. Following an initial
increase at the rate +0.39(1) K/GPa, Tc drops abruptly from 15 K to 4 K at 10
GPa. Synchrotron x-ray measurements to 15 GPa point to a structural transition
near 10 GPa from a rhombohedral to a higher symmetry phase
- …