1,948 research outputs found
The motif problem
Fix a choice and ordering of four pairwise non-adjacent vertices of a
parallelepiped, and call a motif a sequence of four points in R^3 that coincide
with these vertices for some, possibly degenerate, parallelepiped whose edges
are parallel to the axes. We show that a set of r points can contain at most
r^2 motifs. Generalizing the notion of motif to a sequence of L points in R^p,
we show that the maximum number of motifs that can occur in a point set of a
given size is related to a linear programming problem arising from hypergraph
theory, and discuss some related questions.Comment: 17 pages, 1 figur
Unusual temperature dependence of band dispersion in Ba(Fe(1-x)Ru(x))2As2 and its consequences for antiferromagnetic ordering
We have performed detailed studies of the temperature evolution of the
electronic structure in Ba(Fe(1-x)Ru(x))2As2 using Angle Resolved Photoemission
Spectroscopy (ARPES). Surprisingly, we find that the binding energy of both
hole and electron bands changes significantly with temperature in pure and Ru
substituted samples. The hole and electron pockets are well nested at low
temperature in unsubstituted (BaFe2As2) samples, which likely drives the spin
density wave (SDW) and resulting antiferromagnetic order. Upon warming, this
nesting is degraded as the hole pocket shrinks and the electron pocket expands.
Our results demonstrate that the temperature dependent nesting may play an
important role in driving the antiferromagnetic/paramagnetic phase transition.Comment: 5 pages, 6 figure
Thermal expansion and effect of pressure on superconductivity in CuxTiSe2
We report measurements of thermal expansion on a number of polycrystalline
CuxTiSe2 samples corresponding to the parts of x - T phase diagram with
different ground states, as well as the pressure dependence of the
superconducting transition temperature for samples with three different values
of Cu-doping. Thermal expansion data suggest that the x - T phase diagram may
be more complex than initially reported. T_c data at elevated pressure can be
scaled to the ambient pressure CuxTiSe2 phase diagram, however, significantly
different scaling factors are needed to accommodate the literature data on the
charge density wave transition suppression under pressure
Effect of tensile stress on the in-plane resistivity anisotropy in BaFe2As2
The effect of uniaxial tensile stress and the resultant strain on the
structural/magnetic transition in the parent compound of the iron arsenide
superconductor, BaFeAs, is characterized by temperature-dependent
electrical resistivity, x-ray diffraction and quantitative polarized light
imaging. We show that strain induces a measurable uniaxial structural
distortion above the first-order magnetic transition and significantly smears
the structural transition. This response is different from that found in
another parent compound, SrFeAs, where the coupled structural and
magnetic transitions are strongly first order. This difference in the
structural responses explains the in-plain resistivity anisotropy above the
transition in BaFeAs. This conclusion is supported by the
Ginzburg-Landau - type phenomenological model for the effect of the uniaxial
strain on the resistivity anisotropy
Remarkably robust and correlated coherence and antiferromagnetism in (CeLa)CuGe
We present magnetic susceptibility, resistivity, specific heat, and
thermoelectric power measurements on (CeLa)CuGe single
crystals (0 1). With La substitution, the antiferromagnetic
temperature is suppressed in an almost linear fashion and moves below
0.36 K, the base temperature of our measurements for 0.8. Surprisingly, in
addition to robust antiferromagnetism, the system also shows low temperature
coherent scattering below up to 0.9 of La, indicating a small
percolation limit 9 of Ce that separates a coherent regime from a
single-ion Kondo impurity regime. as a function of magnetic field was
found to have different behavior for 0.9. Remarkably,
at = 0 was found to be linearly proportional to . The
jump in the magnetic specific heat at as a function of
for (CeLa)CuGe follows the theoretical prediction
based on the molecular field calculation for the = 1/2 resonant level
model
Unpaired Electrons in the Heavy-Fermion Superconductor CeCoIn_{5}
Thermal conductivity and specific heat were measured in the superconducting
state of the heavy fermion material Ce_{1-x}La_{x}CoIn_{5}. With increasing
impurity concentration x, the suppression of T_{c} is accompanied by the
increase in the residual electronic specific heat expected of a d-wave
superconductor, but it occurs in parallel with a decrease in residual
electronic thermal conductivity. This contrasting behavior reveals the presence
of uncondensed electrons coexisting with nodal quasiparticles. An extreme
multiband scenario is proposed, with a d-wave superconducting gap on the
heavy-electron sheets of the Fermi surface and a negligible gap on the light,
three-dimensional pockets.Comment: 4 pages, 3 figure
A Unique Exposure of Quaternary Deposits in Johnson County, Iowa
The Klein Quarry, in Johnson County, Iowa, exposes a unique section of Quaternary deposits. The section extends along the axis of a Late-Sangamon erosion surface. It is mantled by Wisconsinan loess: a 4-5m upper increment of Late-Wisconsinan loess and a thin increment (0.2 to 0.5m) of mixed loess and Wisconsinan-age pedisediment (\u27basal-loess sediments\u27). Some soil development has taken place in the basal-loess sediments (basal-loess paleosol), and this soil merges with the underlying Late-Sangamon Paleosol. The Late-Sangamon erosion surface is developed on Pre-Illinoian age deposits of the Wolf Creek Formation which include (from top to bottom) an upper basal till (the Aurora Till Member), a thin, laminated diamicton, and an underlying stratified fluvial sequence of sand, silt, and gravel. These overlie till of the Alburnett Formation which is locally preserved in low-relief sags on the underlying bedrock surface of Devonian Cedar Valley Limestone. Sedimentary structures, pebble fabrics, and stratigraphic relations suggest that: the stratified fluvial sequence originated as a proglacial fluvial outwash that evolved into a low-energy slackwater environment; the laminated diamicton was derived from glacial sediments which were resedimented and deposited in this slackwater environment; and this was followed by overriding of glacial ice and deposition of the basal till. The Late-Sangamon erosion surface is marked by a stone line and a relatively thin increment of associated pedisediment which overlies the stone line. Various hillslope components are exposed going down the Late-Sangamon paleohillslope. The erosion surface progressively truncates the Aurora Till Member, the laminated diamicton, and most of the stratified sequence of the Wolf Creek Formation. Properties of the stone line and pedisediment vary in a complex, but systematic way. The characteristics of the stone line and lowermost pedisediment vary downslope directly with textural variations in the different deposits underlying the erosion surface. The uppermost pedisediment, however, shows little relationship to the materials underlying the stone line. The upper, younger pedisediment has resulted from reworking older pedisediment and from transport of sediment from farther upslope. The greater transport distance and reworking results in greater sorting and a less direct relationship to local source materials. The Lare-Sangamon Paleosol formed on this paleohillslope, and is developed in the Late-Sangamon pedisediment, stone line, and the underlying Wolf Creek Formation deposits. Sedimentological variations in the pedisediment affect various paleosol properties. Thickness of the paleosol varies (1.8 to 2.3 m) directly with the thickness of pedisediment, becoming thicker down the paleoslope. The increase in paleosol thickness is also directly matched by an increase in B-horizon thickness. The pedologic and sedimentologic features indicate that the Late-Sangamon erosion surface - pedisediment - paleosol evolved slowly and systematically. Pedisediment muse have accumulated in the lower-slope positions at a slow enough rate that B-horizon soil development kept pace with sediment accumulation
Superconductivity in Dense Wires
becomes superconducting just below 40 K. Whereas porous
polycrystalline samples of can be synthesized from boron powders, in
this letter we demonstrate that dense wires of can be prepared by
exposing boron filaments to vapor. The resulting wires have a diameter of
160 , are better than 80% dense and manifest the full shielding in the superconducting state. Temperature-dependent
resistivity measurements indicate that is a highly conducting metal in
the normal state with = 0.38 -. Using this value, an
electronic mean free path, can be estimated, indicating
that wires are well within the clean limit. , , and
data indicate that manifests comparable or better superconducting
properties in dense wire form than it manifests as a sintered pellet.Comment: Figures' layout fixe
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