Multiple superconducting transitions in the Sr3Ru2O7 region of Sr3Ru2O7-Sr2RuO4 eutectic crystals

We report superconducting properties of Sr3Ru2O7-Sr2RuO4 eutectic crystals, consisting of the spin-triplet superconductor Sr2RuO4 with a monolayer stacking of RuO2 planes and the metamagnetic normal metal Sr3Ru2O7 with a bilayer stacking. Although Sr3Ru2O7 has not been reported to exhibit superconductivity so far, our AC susceptibility measurements revealed multiple superconducting transitions occurring in the Sr3Ru2O7 region of the eutectic crystals. The diamagnetic shielding essentially reached the full fraction at low AC fields parallel to the c axis. However, both the shielding fraction and the onset temperature are easily suppressed by AC fields of larger than 0.1 mT-rms and no anomaly was observed in the specific heat. Moreover, the critical field curves of these transitions have a positive curvature near zero fields, which is different from the upper critical field curve of the bulk Sr2RuO4. These facts suggest that the superconductivity observed in the Sr3Ru2O7 region is not a bulk property. To explain these experimental results, we propose the scenario that stacking RuO2 planes, the building block of superconducting Sr2RuO4, are contained in the Sr3Ru2O7 region as stacking faults.Comment: 8 pages, 10 figures, accepted for publication in Phys. Rev.

Magnetic field of an in-plane vortex outside a layered superconductor

We present the solution to London's equations for the magnetic fields of a vortex oriented parallel to the plane, and normal to a crystal face, of a layered superconductor. These expressions account for flux spreading at the superconducting surface, which can change the apparent size of the vortex along the planes by as much as 30%. We compare these expressions with experimental results.Comment: 13 pages, 5 figure

Detection of the Vortex Dynamic Regimes in MgB2 by Third Harmonic AC Susceptibility Measurements

In a type-II superconductor the generation of higher harmonics in the magnetic response to an alternating magnetic field is a consequence of the non-linearity in the I-V relationship. The shape of the current-voltage (I-V) curve is determined by the current dependence of the thermal activation energy U(J) and is thus related to the dynamical regimes governing the vortex motion. In order to investigate the vortex dynamics in MgB2 bulk superconductors we have studied the fundamental (chi1) and third (chi3) harmonics of the ac magnetic susceptibility. Measurements have been performed as a function of the temperature and the dc magnetic field, up to 9 T, for various frequencies and amplitudes of the ac field. We show that the analysis of the behaviour in frequency of chi3(T) and chi3(B) curves can provide clear information about the non-linearity in different regions of the I-V characteristic. By comparing the experimental curves with numerical simulations of the non-linear diffusion equation for the magnetic field we are able to resolve the crossover between a dissipative regime governed by flux creep and one dominated by flux flow phenomena.Comment: to be published in "Horizons in Superconductivity Research" (Nova Science Publishers, Inc., NY, 2003

Field Induced Nodal Order Parameter in the Tunneling Spectrum of YBa2_2Cu3_3O7−x_{7-x} Superconductor

We report planar tunneling measurements on thin films of YBa$_2$Cu$_3$O$_{7-x}$ at various doping levels under magnetic fields. By choosing a special setup configuration, we have probed a field induced energy scale that dominates in the vicinity of a node of the d-wave superconducting order parameter. We found a high doping sensitivity for this energy scale. At Optimum doping this energy scale is in agreement with an induced $id_{xy}$ order parameter. We found that it can be followed down to low fields at optimum doping, but not away from it.Comment: 9 pages, 8 figures, accepted for publication in Phys. Rev.

Critical state theory for nonparallel flux line lattices in type-II superconductors

Coarse-grained flux density profiles in type-II superconductors with non-parallel vortex configurations are obtained by a proposed phenomenological least action principle. We introduce a functional $C[H(x)]$, which is minimized under a constraint of the kind $J$ belongs to $Delta$ for the current density vector, where $Delta$ is a bounded set. This generalizes the concept of critical current density introduced by C. P. Bean for parallel vortex configurations. In particular, we choose the isotropic case ($Delta$ is a circle), for which the field penetration profiles $H(x,t)$ are derived when a changing external excitation is applied. Faraday's law, and the principle of minimum entropy production rate for stationary thermodynamic processes dictate the evolution of the system. Calculations based on the model can reproduce the physical phenomena of flux transport and consumption, and the striking effect of magnetization collapse in crossed field measurements.Comment: The compiled TeX document length is 10 pages. Two figures (one page each) are also included The paper is accepted for publication in Phys. Rev. Let

The Performance of the Robo-AO Laser Guide Star Adaptive Optics System at the Kitt Peak 2.1-m Telescope

Robo-AO is an autonomous laser guide star adaptive optics system recently commissioned at the Kitt Peak 2.1-m telescope. Now operating every clear night, Robo-AO at the 2.1-m telescope is the first dedicated adaptive optics observatory. This paper presents the imaging performance of the adaptive optics system in its first eighteen months of operations. For a median seeing value of $1.31^{\prime\prime}$, the average Strehl ratio is 4\% in the $i^\prime$ band and 29\% in the J band. After post-processing, the contrast ratio under sub-arcsecond seeing for a $2\leq i^{\prime} \leq 16$ primary star is five and seven magnitudes at radial offsets of $0.5^{\prime\prime}$ and $1.0^{\prime\prime}$, respectively. The data processing and archiving pipelines run automatically at the end of each night. The first stage of the processing pipeline shifts and adds the data using techniques alternately optimized for stars with high and low SNRs. The second "high contrast" stage of the pipeline is eponymously well suited to finding faint stellar companions.Comment: 12 pages, 16 figures, to be submitted to PAS

Vector magnetic hysteresis of hard superconductors

Critical state problems which incorporate more than one component for the magnetization vector of hard superconductors are investigated. The theory is based on the minimization of a cost functional ${\cal C}[\vec{H}(\vec{x})]$ which weighs the changes of the magnetic field vector within the sample. We show that Bean's simplest prescription of choosing the correct sign for the critical current density $J_c$ in one dimensional problems is just a particular case of finding the components of the vector $\vec{J}_c$. $\vec{J}_c$ is determined by minimizing ${\cal C}$ under the constraint $\vec{J}\in\Delta (\vec{H},\vec{x})$, with $\Delta$ a bounded set. Upon the selection of different sets $\Delta$ we discuss existing crossed field measurements and predict new observable features. It is shown that a complex behavior in the magnetization curves may be controlled by a single external parameter, i.e.: the maximum value of the applied magnetic field $H_m$.Comment: 10 pages, 9 figures, accepted in Phys. Rev.

Magnetocaloric Studies of the Peak Effect in Nb

We report a magnetocaloric study of the peak effect and Bragg glass transition in a Nb single crystal. The thermomagnetic effects due to vortex flow into and out of the sample are measured. The magnetocaloric signature of the peak effect anomaly is identified. It is found that the peak effect disappears in magnetocaloric measurements at fields significantly higher than those reported in previous ac-susceptometry measurements. Investigation of the superconducting to normal transition reveals that the disappearance of the bulk peak effect is related to inhomogeneity broadening of the superconducting transition. The emerging picture also explains the concurrent disappearance of the peak effect and surface superconductivity, which was reported previously in the sample under investigation. Based on our findings we discuss the possibilities of multicriticality associated with the disappearance of the peak effect.Comment: 30 pages, 10 figure

INTERLAYER VORTICES AND EDGE DISLOCATIONS IN HIGH TEMPERATURE SUPERCONDUCTORS

The interaction of an edge dislocation made of half the superconducting plane with a magnetic interlayer vortex is considered within the framework of the Lawrence-Doniach model with negative as well as positive Josephson interlayer coupling. In the first case the binding energy of the vortex and the dislocation has been calculated by employing a variational procedure. The current distribution around the bound vortex turns out to be asymmetric. In the second case the dislocation carries a spontaneous magnetic half-vortex, whose binding energy with the dislocation turns out to be infinite. The half-vortex energy has been calculated by the same variational procedure. Implications of the possible presence of such half-vortices for the properties of high temperature superconductors are discussed.Comment: 14 Latex pages, 1 figure available upon request

Robo-AO Kepler Survey IV: the effect of nearby stars on 3857 planetary candidate systems

We present the overall statistical results from the Robo-AO Kepler planetary candidate survey, comprising of 3857 high-angular resolution observations of planetary candidate systems with Robo-AO, an automated laser adaptive optics system. These observations reveal previously unknown nearby stars blended with the planetary candidate host star which alter the derived planetary radii or may be the source of an astrophysical false positive transit signal. In the first three papers in the survey, we detected 440 nearby stars around 3313 planetary candidate host stars. In this paper, we present observations of 532 planetary candidate host stars, detecting 94 companions around 88 stars; 84 of these companions have not previously been observed in high-resolution. We also report 50 more-widely-separated companions near 715 targets previously observed by Robo-AO. We derive corrected planetary radius estimates for the 814 planetary candidates in systems with a detected nearby star. If planetary candidates are equally likely to orbit the primary or secondary star, the radius estimates for planetary candidates in systems with likely bound nearby stars increase by a factor of 1.54, on average. We find that 35 previously-believed rocky planet candidates are likely not rocky due to the presence of nearby stars. From the combined data sets from the complete Robo-AO KOI survey, we find that 14.5\pm0.5% of planetary candidate hosts have a nearby star with 4", while 1.2% have two nearby stars and 0.08% have three. We find that 16% of Earth-sized, 13% of Neptune-sized, 14% of Saturn-sized, and 19% of Jupiter-sized planet candidates have detected nearby stars.Comment: Accepted to the Astronomical Journa