386 research outputs found
Superconducting and ferromagnetic phases induced by lattice distortions in SrFe2As2
Single crystals of SrFe2As2 grown using a self-flux solution method were
characterized via x-ray, transport and magnetization studies, revealing a
superconducting phase below T_c = 21 K characterized by a full electrical
resistivity transition and partial diamagnetic screening. The reversible
destruction and reinstatement of this phase by heat treatment and mechanical
deformation studies, along with single-crystal X-ray diffraction measurements,
indicate that internal crystallographic strain originating from c-axis-oriented
planar defects plays a central role in promoting the appearance of
superconductivity under ambient pressure conditions in ~90% of as-grown
crystals. The appearance of a ferromagnetic moment with magnitude proportional
to the tunable superconducting volume fraction suggests that these phenomena
are both stabilized by lattice distortion.Comment: 4 pages, 4 figure
Spatially resolved femtosecond pump-probe study of topological insulator Bi2Se3
Carrier and phonon dynamics in Bi2Se3 crystals are studied by a spatially
resolved ultrafast pump-probe technique. Pronounced oscillations in
differential reflection are observed with two distinct frequencies, and are
attributed to coherent optical and acoustic phonons, respectively. The rising
time of the signal indicates that the thermalization and energy relaxation of
hot carriers are both sub-ps in this material. We found that the thermalization
and relaxation time decreases with the carrier density. The expansion of the
differential reflection profile allows us to estimate an ambipolar carrier
diffusion coefficient on the order of 500 square centimeters per second. A
long-term slow expansion of the profile shows a thermal diffusion coefficient
of 1.2 square centimeters per second.Comment: 8 pages, 6 figure
The suppression of magnetism and the development of superconductivity within the collapsed tetragonal phase of Ca0.67Sr0.33Fe2As2 at high pressure
Structural and electronic characterization of (Ca0.67Sr0.33)Fe2As2 has been
performed as a func- tion of pressure up to 12 GPa using conventional and
designer diamond anvil cells. The compound (Ca0.67Sr0.33)Fe2As2 behaves
intermediate between its end members-CaFe2As2 and SrFe2As2- displaying a
suppression of magnetism and the onset of superconductivity. Like other members
of the AEFe2As2 family, (Ca0.67Sr0.33)Fe2As2 undergoes a pressure-induced
isostructural volume collapse, which we associate with the development of As-As
bonding across the mirror plane of the structure. This collapsed tetragonal
phase abruptly cuts off the magnetic state, giving rise to superconductivity
with a maximum Tc=22.2 K. The maximum Tc of the superconducting phase is not
strongly correlated with any structural parameter, but its proximity to the
abrupt suppression of magnetism as well as the volume collapse transition
suggests that magnetic interactions and structural inhomogeneity may play a
role in its development. The pressure-dependent evolution of the ordered states
and crystal structures in (Ca,Sr)Fe2As2 provides an avenue to understand the
generic behavior of the other members of the AEFe2As2 family.Comment: 9 pages, 9 figure
Evolution of bulk superconductivity in SrFe2As2 with Ni substitution
Single crystals of the Ni-doped FeAs-based superconductor SrFe2-xNixAs2 were
grown using a self-flux solution method and characterized via x-ray
measurements and low temperature transport, magnetization, and specific heat
studies. A doping phase diagram has been established where the
antiferromagnetic order associated with the magnetostructural transition of the
parent compound SrFe2As2 is gradually suppressed with increasing Ni
concentration, giving way to bulk-phase superconductivity with a maximum
transition temperature of 9.8 K. The superconducting phase exists through a
finite range of Ni concentrations centered at x=0.15, with full diamagnetic
screening observed over a narrow range of x coinciding with a sharpening of the
superconducting transition and an absence of magnetic order. An enhancement of
bulk superconducting transition temperatures of up to 20% was found to occur
upon high-temperature annealing of samples.Comment: 10 pages, 9 figure
Parallel suppression of superconductivity and Fe moment in the collapsed tetragonal phase of Ca0.67Sr0.33Fe2As2 under pressure
Using non-resonant Fe K-beta x-ray emission spectroscopy, we reveal that
Sr-doping of CaFe2As2 decouples the Fe moment from the volume collapse
transition, yielding a collapsed-tetragonal, paramagnetic normal state out of
which superconductivity develops. X-ray diffraction measurements implicate the
c-axis lattice parameter as the controlling criterion for the Fe moment,
promoting a generic description for the appearance of pressure-induced
superconductivity in the alkaline-earth-based 122 ferropnictides (AFe2As2). The
evolution of the superconducting critical temperature with pressure lends
support to theories for superconductivity involving unconventional pairing
mediated by magnetic fluctuations
Superconductivity at 23 K in Pt doped BaFe2As2 single crystals
We report superconductivity in single crystals of the new iron-pnictide
system BaFe1.9Pt0.1As2 grown by a self-flux solution method and characterized
via x-ray, transport, magnetic and thermodynamic measurements. The magnetic
ordering associated with a structural transition at 140 K present in BaFe2As2
is completely suppressed by substitution of 5% Fe with Pt and superconductivity
is induced at a critical temperature Tc=23 K. Full diamagnetic screening in the
magnetic susceptibility and a jump in the specific heat at Tc confirm the bulk
nature of the superconducting phase. All properties of the superconducting
state including transition temperature Tc, the lower critical field Hc1=200 mT,
upper critical field Hc2~65 T, and the slope dHc2/dT are comparable in value to
the those found in other transition-metal-substituted BaFe2As2 series,
indicating the robust nature of superconductivity induced by substitution of
Group VIII elements.Comment: 6 pgs, 4 figs, and 1 tbl, slightly revised, updated reference
Discovery of Stable and Selective Antibody Mimetics from Combinatorial Libraries of Polyvalent, Loop-Functionalized Peptoid Nanosheets.
The ability of antibodies to bind a wide variety of analytes with high specificity and high affinity makes them ideal candidates for therapeutic and diagnostic applications. However, the poor stability and high production cost of antibodies have prompted exploration of a variety of synthetic materials capable of specific molecular recognition. Unfortunately, it remains a fundamental challenge to create a chemically diverse population of protein-like, folded synthetic nanostructures with defined molecular conformations in water. Here we report the synthesis and screening of combinatorial libraries of sequence-defined peptoid polymers engineered to fold into ordered, supramolecular nanosheets displaying a high spatial density of diverse, conformationally constrained peptoid loops on their surface. These polyvalent, loop-functionalized nanosheets were screened using a homogeneous Förster resonance energy transfer (FRET) assay for binding to a variety of protein targets. Peptoid sequences were identified that bound to the heptameric protein, anthrax protective antigen, with high avidity and selectivity. These nanosheets were shown to be resistant to proteolytic degradation, and the binding was shown to be dependent on the loop display density. This work demonstrates that key aspects of antibody structure and function-the creation of multivalent, combinatorial chemical diversity within a well-defined folded structure-can be realized with completely synthetic materials. This approach enables the rapid discovery of biomimetic affinity reagents that combine the durability of synthetic materials with the specificity of biomolecular materials
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