595 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
Universal pair-breaking in transition metal-substituted iron-pnictide superconductors
The experimental transport scattering rate was determined for a wide range of
optimally doped transition metal-substituted FeAs-based compounds with the
ThCr2Si2 (122) crystal structure. The maximum transition temperature Tc for
several Ba-, Sr-, and Ca-based 122 systems follows a universal rate of
suppression with increasing scattering rate indicative of a common
pair-breaking mechanism. Extraction of standard pair-breaking parameters puts a
limit of \sim26 K on the maximum Tc for all transition metal-substituted 122
systems, in agreement with experimental observations, and sets a critical
scattering rate of 1.5x10^14 s^-1 for the suppression of the superconducting
phase. The observed critical scattering rate is much weaker than that expected
for a sign-changing order parameter, providing important constraints on the
nature of the superconducting gap in the 122 family of iron-based
superconductors.Comment: 4 pages, 3 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
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
Adolescent but not adult-born neurons are critical for susceptibility to chronic social defeat
Recent evidence implicates adult hippocampal neurogenesis in regulating behavioral
and physiologic responses to stress. Hippocampal neurogenesis occurs across the
lifespan, however the rate of cell birth is up to 300% higher in adolescent mice
compared to adults. Adolescence is a sensitive period in development where emotional
circuitry and stress reactivity undergo plasticity establishing life-long set points. Therefore
neurogenesis occurring during adolescence may be particularly important for emotional
behavior. However, little is known about the function of hippocampal neurons born during
adolescence. In order to assess the contribution of neurons born in adolescence to
the adult stress response and depression-related behavior, we transiently reduced cell
proliferation either during adolescence, or during adulthood in GFAP-Tk mice. We found
that the intervention in adolescence did not change adult baseline behavioral response in
the forced swim test, sucrose preference test or social affiliation test, and did not change
adult corticosterone responses to an acute stressor. However following chronic social
defeat, adult mice with reduced adolescent neurogenesis showed a resilient phenotype.
A similar transient reduction in adult neurogenesis did not affect depression-like behaviors
or stress induced corticosterone. Our study demonstrates that hippocampal neurons born
during adolescence, but not in adulthood are important to confer susceptibility to chronic
social defeat
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