296 research outputs found
Search for Cooper-pair Fluctuations in Severely Underdoped YBCO Films
The preformed-pairs theory of pseudogap physics in high- superconductors
predicts a nonanalytic -dependence for the -plane superfluid fraction,
, at low temperatures in underdoped cuprates. We report high-precision
measurements of on severely underdoped YBaCuO and
YCaBaCuO films. At low , looks more
like than , in disagreement with theory.Comment: 3 pages, 2 figure
Correlation between superfluid density and Tc of underdoped YBa2Cu3O6+x near the superconductor-insulator transition
We report measurements of the ab-plane superfluid density Ns (magnetic
penetration depth, \lambda) of severely underdoped films of YBa2Cu3O6+x, with
Tc's from 6 to 50 K. Tc is not proportional to Ns(0); instead, we find Tc ~
Ns^{1/2.3 +/- 0.4}. At the lowest dopings, Tc is as much as 5 times larger than
the upper limit set by the KTB transition temperature of individual CuO2
bilayers.Comment: 4 pages, 2 figures, Submitted to Phys. Rev. Let
Anomalously Sharp Superconducting Transitions in Overdoped Films
We present measurements of -plane resistivity and
superfluid density [, = magnetic penetration
depth] in films. As Sr concentration exceeds about
0.22, the superconducting transition sharpens dramatically, becoming as narrow
as 200 mK near the super-to-normal metal quantum critical point. At the same
time, , , and transition temperature
decrease, and upward curvature develops in . Given the sharp
transitions, we interpret these results in the context of a homogeneous d-wave
superconducting state, with elastic scattering that is enhanced relative to
underdoped LSCO due to weaker electron correlations. This interpretation
conflicts with the viewpoint that the overdoped state is inhomogeneous due to
phase separation into superconducting and normal metal regions.Comment: 21 pages including 3 figures and 56 references. This version includes
responses to referees and slight correction of data on two films. Conclusions
the same as befor
Growth control of GaAs nanowires using pulsed laser deposition with arsenic over pressure
Using pulsed laser ablation with arsenic over pressure, the growth conditions
for GaAs nanowires have been systematically investigated and optimized. Arsenic
over pressure with As molecules was introduced to the system by thermal
decomposition of polycrystalline GaAs to control the stoichiometry and shape of
the nanowires during growth. GaAs nanowires exhibit a variety of geometries
under varying arsenic over pressure, which can be understood by different
growth processes via vapor-liquid-solid mechanism. Single-crystal GaAs
nanowires with uniform diameter, lengths over 20 m, and thin surface oxide
layer were obtained and can potentially be used for further electronic
characterization
Robustness of the Berezinskii-Kosterlitz-Thouless Transition in Ultrathin NbN Films near the Superconductor-Insulator Transition
Occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition is
investigated by superfluid density measurements for two-dimensional (2D)
disordered NbN films with disorder level very close to a
superconductor-insulator transition (SIT). Our data show a robust BKT
transition even near this 2D disorder-tuned quantum critical point (QCP). This
observation is in direct contrast with previous data on deeply underdoped
quasi-2D cuprates near the SIT. As our NbN films approach the QCP, the
vortex-core energy, an important energy scale in the BKT transition, scales
with the superconducting gap, not with the superfluid density, as expected
within the standard 2D-XY model description of BKT physics.Comment: 8 pages, 6 figure
Influence of the Fermi Surface Morphology on the Magnetic Field-Driven Vortex Lattice Structure Transitions in YBaCuO0, 0.15
We report small-angle neutron scattering measurements of the vortex lattice
(VL) structure in single crystals of the lightly underdoped cuprate
superconductor YBa2Cu3O6.85. At 2 K, and for fields of up to 16 T applied
parallel to the crystal c-axis, we observe a sequence of field-driven and
first-order transitions between different VL structures. By rotating the field
away from the c-axis, we observe each structure transition to shift to either
higher or lower field dependent on whether the field is rotated towards the
[100] or [010] direction. We use this latter observation to argue that the
Fermi surface morphology must play a key role in the mechanisms that drive the
VL structure transitions. Furthermore, we show this interpretation is
compatible with analogous results obtained previously on lightly overdoped
YBa2Cu3O7. In that material, it has long-been suggested that the high field VL
structure transition is driven by the nodal gap anisotropy. In contrast, the
results and discussion presented here bring into question the role, if any, of
a nodal gap anisotropy on the VL structure transitions in both YBa2Cu3O6.85 and
YBa2Cu3O7
Fluoxetine: a case history of its discovery and preclinical development
Introduction: Depression is a multifactorial mood disorder with a high prevalence worldwide. Until now, treatments for depression have focused on the inhibition of monoaminergic reuptake sites, which augment the bioavailability of monoamines in the CNS. Advances in drug discovery have widened the therapeutic options with the synthesis of so-called selective serotonin reuptake inhibitors (SSRIs), such as fluoxetine.
Areas covered: The aim of this case history is to describe and discuss the pharmacokinetic and pharmacodynamic profiles of fluoxetine, including its acute effects and the adaptive changes induced after long-term treatment.
Furthermore, the authors review the effect of fluoxetine on neuroplasticity and adult neurogenesis. In addition, the article summarises the preclinical behavioural data available on fluoxetine’s effects on depressive-like behaviour,
anxiety and cognition as well as its effects on other diseases. Finally, the article describes the seminal studies validating the antidepressant effects of fluoxetine.
Expert opinion: Fluoxetine is the first selective SSRI that has a recognised clinical efficacy and safety profile. Since its discovery, other molecules that mimic its mechanism of action have been developed, commencing a new
age in the treatment of depression. Fluoxetine has also demonstrated utility in the treatment of other disorders for which its prescription has now been approved
Vortex lattice structure in BaFe2(As0.67P0.33)2 by the small-angle neutron scattering technique
We have observed a magnetic vortex lattice (VL) in BaFe2(As_{0.67}P_{0.33})2
(BFAP) single crystals by small-angle neutron scattering (SANS). With the field
along the c-axis, a nearly isotropic hexagonal VL was formed in the field range
from 1 to 16 T, which is a record for this technique in the pnictides, and no
symmetry changes in the VL were observed. The temperature-dependence of the VL
signal was measured and confirms the presence of (non d-wave) nodes in the
superconducting gap structure for measurements at 5 T and below. The nodal
effects were suppressed at high fields. At low fields, a VL reorientation
transition was observed between 1 T and 3 T, with the VL orientation changing
by 45{\deg}. Below 1 T, the VL structure was strongly affected by pinning and
the diffraction pattern had a fourfold symmetry. We suggest that this (and
possibly also the VL reorientation) is due to pinning to defects aligned with
the crystal structure, rather than being intrinsic.Comment: 9 pages, 9 figure
Implication of oxygen vacancies on current conduction mechanisms in TiN/Zr1−xAlxO2/TiN metal-insulator-metal structures
The dominating conduction mechanisms through TiN/Zr1−xAlxO2/TiN capacitors have been investigated over a wide temperature range (25 K to 430 K) in order to obtain information about the traps which cause the current transport. Single positive charged oxygen vacancies are the principal transport sites which participate in all mechanisms observed. However, the conduction mostly defined by intrinsic traps could also be strongly influenced by defects originating from undesirable high-k/metal gate interface reactions which could act as real traps or as transport sites
Universal relationship between the penetration depth and the normal-state conductivity in YBaCuO
The absolute values of the conductivity in the normal state sigma_n and of
the low temperature penetration depths lambda(0) were measured for a number of
different samples of the YBaCuO family. We found a striking correlation between
sigma_n and 1/lambda^2, regardless of doping, oxygen reduction or defects, thus
providing a simple method to predict the superconducting penetration depth and
to have an estimate of the sample quality by measuring the normal-state
conductivity.Comment: 7 pages, 1 figure, Europhys. Lett., accepte
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