296 research outputs found

    Search for Cooper-pair Fluctuations in Severely Underdoped YBCO Films

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    The preformed-pairs theory of pseudogap physics in high-TCT_C superconductors predicts a nonanalytic TT-dependence for the abab-plane superfluid fraction, ρS\rho_S, at low temperatures in underdoped cuprates. We report high-precision measurements of ρS(T)\rho_S(T) on severely underdoped YBa2_2Cu3_3O6+x_{6+x} and Y0.8_{0.8}Ca0.2_{0.2}Ba2_2Cu3_3O6+x_{6+x} films. At low TT, ρS\rho_S looks more like 1T21 - T^2 than 1T3/21 - T^{3/2}, in disagreement with theory.Comment: 3 pages, 2 figure

    Correlation between superfluid density and Tc of underdoped YBa2Cu3O6+x near the superconductor-insulator transition

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    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 La2xSrxCuO4La_{2-x}Sr_{x}CuO_{4} Films

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    We present measurements of abab-plane resistivity ρab(T)\rho_{ab}(T) and superfluid density [λ2\propto \lambda^{-2}, λ\lambda = magnetic penetration depth] in La2xSrxCuO4La_{2-x}Sr_{x}CuO_{4} films. As Sr concentration xx 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, ρab(T)\rho_{ab}(T), λ2(T)\lambda^{-2}(T), and transition temperature TcT_c decrease, and upward curvature develops in λ2(T)\lambda^{-2}(T). 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

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    Using pulsed laser ablation with arsenic over pressure, the growth conditions for GaAs nanowires have been systematically investigated and optimized. Arsenic over pressure with As2_2 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 μ\mum, 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

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    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 YBa2_{2}Cu3_{3}O7δ:δ=_{7-\delta}:\delta=0, 0.15

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    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

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    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

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    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

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    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

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    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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