2,961 research outputs found
Magnetic and transport properties of i--Cd icosahedral quasicrystals ( = Y, Gd-Tm)
We present a detailed characterization of the recently discovered i--Cd
( = Y, Gd-Tm) binary quasicrystals by means of x-ray diffraction,
temperature-dependent dc and ac magnetization, temperature-dependent resistance
and temperature-dependent specific heat measurements. Structurally, the
broadening of x-ray diffraction peaks found for i--Cd is dominated by
frozen-in phason strain, which is essentially independent of . i-Y-Cd is
weakly diamagnetic and manifests a temperature-independent susceptibility.
i-Gd-Cd can be characterized as a spin-glass below 4.6 K via dc magnetization
cusp, a third order non-linear magnetic susceptibility peak, a
frequency-dependent freezing temperature and a broad maximum in the specific
heat. i--Cd ( = Ho-Tm) is similar to i-Gd-Cd in terms of features
observed in thermodynamic measurements. i-Tb-Cd and i-Dy-Cd do not show a clear
cusp in their zero-field-cooled dc magnetization data, but instead show a more
rounded, broad local maximum. The resistivity for i--Cd is of order 300 cm and weakly temperature-dependent. The characteristic freezing
temperatures for i--Cd ( = Gd-Tm) deviate from the de Gennes scaling, in
a manner consistent with crystal electric field splitting induced local moment
anisotropy.Comment: 14 page
Ripples and Shear Bands in Plowed Granular Media
Monodisperse packings of dry, air-fluidized granular media typically exist
between volume fractions from = 0.585 to 0.64. We demonstrate that the
dynamics of granular drag are sensitive to volume fraction and their
exists a transition in the drag force and material deformation from smooth to
oscillatory at a critical volume fraction . By dragging a
submerged steel plate (3.81 cm width, 6.98 cm depth) through glass
beads prepared at volume fractions between 0.585 to 0.635 we find that below
the media deformation is smooth and non-localized while above
media fails along distinct shear bands. At high the
generation of these shear bands is periodic resulting in the ripples on the
surface. Work funded by The Burroughs Wellcome Fund and the Army Research Lab
MAST CT
Anisotropy and large magnetoresistance in narrow gap semiconductor FeSb2
A study of the anisotropy in magnetic, transport and magnetotransport
properties of FeSb2 has been made on large single crystals grown from Sb flux.
Magnetic susceptibility of FeSb2 shows diamagnetic to paramagnetic crossover
around 100K. Electrical transport along two axes is semiconducting whereas the
third axis exhibits a metal - semiconductor crossover at temperature Tmin which
is sensitive to current alignment and ranges between 40 and 80K. In H=70kOe
semiconducting transport is restored for T<300K, resulting in large
magnetoresistance [rho(70kOe)-rho(0)]/rho(0)=2200% in the crossover temperature
rangeComment: 4 pages, 4 figures, Submitted to Phys. Rev.
Cotton production in the state of São Paulo, Brazil: a survey of recent trends and problems.
Thesis (M.A.)--Boston UniversitySão Paulo, although Brazil's greatest industrial area, is also one of the country's largest agricultural producers. Its past tradition of monoculture is now giving way to a more diversified economy, cotton becoming the State's (and Brazil's) second largest export crop. São Paulo is well suited for growing cotton although the clay soils of the State tend to erode very rapidly, especially when cultivated to a clean-culture crop such as cotton. This tendency has resulted in a steady shift of cotton and other agricultural production in the State, moving from the old zones of the east to newer, more fertile soils in the west. [TRUNCATED
High-resolution x-ray diffraction study of the heavy-fermion compound YbBiPt
YbBiPt is a heavy-fermion compound possessing significant short-range
antiferromagnetic correlations below a temperature of K,
fragile antiferromagnetic order below K, a Kondo temperature
of K, and crystalline-electric-field splitting on the
order of K. Whereas the compound has a
face-centered-cubic lattice at ambient temperature, certain experimental data,
particularly those from studies aimed at determining its
crystalline-electric-field scheme, suggest that the lattice distorts at lower
temperature. Here, we present results from high-resolution, high-energy x-ray
diffraction experiments which show that, within our experimental resolution of
\AA, no structural phase transition
occurs between and K. In combination with results from dilatometry
measurements, we further show that the compound's thermal expansion has a
minimum at K and a region of negative thermal expansion for
K. Despite diffraction patterns taken at K which indicate that
the lattice is face-centered cubic and that the Yb resides on a
crystallographic site with cubic point symmetry, we demonstrate that the linear
thermal expansion may be modeled using crystalline-electric-field level schemes
appropriate for Yb residing on a site with either cubic or less than
cubic point symmetry.Comment: 7 pages, 3 figures, submitted to Phys. Rev.
Antiferromagnetic order in CaK(Fe[1-x]Ni[x])4As4 and its interplay with superconductivity
The magnetic order in CaK(Fe[1-x]Ni[x])4As4 (1144) single crystals (x = 0.051
and 0.033) has been studied by neutron diffraction. We observe magnetic Bragg
peaks associated to the same propagation vectors as found for the collinear
stripe antiferromagnetic (AFM) order in the related BaFe2As2 (122) compound.
The AFM state in 1144 preserves tetragonal symmetry and only a commensurate,
non-collinear structure with a hedgehog spin-vortex crystal (SVC) arrangement
in the Fe plane and simple AFM stacking along the c direction is consistent
with our observations. The SVC order is promoted by the reduced symmetry in the
FeAs layer in the 1144 structure. The long-range SVC order coexists with
superconductivity, however, similar to the doped 122 compounds, the ordered
magnetic moment is gradually suppressed with the developing superconducting
order parameter. This supports the notion that both collinear and non-collinear
magnetism and superconductivity are competing for the same electrons coupled by
Fermi surface nesting in iron arsenide superconductors.Comment: (5 pages, 5 figures
Unconventional pairing in the iron arsenide superconductors
We use magnetic long range order as a tool to probe the Cooper pair wave
function in the iron arsenide superconductors. We show theoretically that
antiferromagnetism and superconductivity can coexist in these materials only if
Cooper pairs form an unconventional, sign-changing state. The observation of
coexistence in Ba(FeCo)As then demonstrates
unconventional pairing in this material. The detailed agreement between theory
and neutron diffraction experiments, in particular for the unusual behavior of
the magnetic order below , demonstrates the robustness of our
conclusions. Our findings strongly suggest that superconductivity is
unconventional in all members of the iron arsenide family.Comment: 3 figures and 4 pages; final version as published
Pressure induced half-collapsed-tetragonal phase in CaKFeAs
We report the temperature-pressure phase diagram of CaKFeAs
established using high pressure electrical resistivity, magnetization and high
energy x-ray diffraction measurements up to 6 GPa. With increasing pressure,
both resistivity and magnetization data show that the bulk superconducting
transition of CaKFeAs is suppressed and then disappears at
4 GPa. High pressure x-ray data clearly indicate a phase transition
to a collapsed tetragonal phase in CaKFeAs under pressure that
coincides with the abrupt loss of bulk superconductivity near 4 GPa. The x-ray
data, combined with resistivity data, indicate that the collapsed tetragonal
transition line is essentially vertical, occuring at 4.0(5) GPa for
temperatures below 150 K. Band structure calculations also find a sudden
transition to a collapsed tetragonal state near 4 GPa, as As-As bonding takes
place across the Ca-layer. Bonding across the K-layer only occurs for
12 GPa. These findings demonstrate a new type of collapsed tetragonal
phase in CaKFeAs: a half-collapsed-tetragonal phase
- …