Depairing critical current achieved in superconducting thin films with through-thickness arrays of artificial pinning centers

Large area arrays of through-thickness nanoscale pores have been milled into superconducting Nb thin films via a process utilizing anodized aluminum oxide thin film templates. These pores act as artificial flux pinning centers, increasing the superconducting critical current, Jc, of the Nb films. By optimizing the process conditions including anodization time, pore size and milling time, Jc values approaching and in some cases matching the Ginzburg-Landau depairing current of 30 MA/cm^2 at 5 K have been achieved - a Jc enhancement over as-deposited films of more than 50 times. In the field dependence of Jc, a matching field corresponding to the areal pore density has also been clearly observed. The effect of back-filling the pores with magnetic material has then been investigated. While back-filling with Co has been successfully achieved, the effect of the magnetic material on Jc has been found to be largely detrimental compared to voids, although a distinct influence of the magnetic material in producing a hysteretic Jc versus applied field behavior has been observed. This behavior has been tested for compatibility with currently proposed models of magnetic pinning and found to be most closely explained by a model describing the magnetic attraction between the flux vortices and the magnetic inclusions.Comment: 9 pages, 10 figure

Comparison of dynamic height measurements from an inverted echo sounder and an island tide gauge in the central Pacific

An inverted echo sounder (IES) and deep pressure sensor were deployed within 70 km of a shallow pressure sensor at Palmyra Island (6°N, 162°W) in the central Pacific. These instruments provided yearlong records of acoustic travel time, deep pressure and sea level. Two independent time series of dynamic height are derived from travel time and sea surface elevation, respectively. The spectra of these time series are similar, and at the spectral peaks the coherence between them exceeds 99.9% confidence levels, indicating that travel time can be used to record dynamic height fluctuations. This investigation provides a frequency dependent calibration for the IES in this region. At the energetic low frequencies (periods ∼ 1 month), this calibration agrees with a calibration by the standard method using conductivity‐temperature‐depth (CTD) casts. At higher frequencies (periods of ∼3 days), using the CTD‐derived calibration may underestimate the amplitude of some processes by as much as 30%

Variability in the central equatorial Pacific, 1985–1989

We describe variability in the equatorial Pacific Ocean near 160°W during the 5‐year period 1985–1989, encompassing “normal”, El Niño, and La Niña conditions. This description is based on conductivity‐temperature‐depth and acoustic Doppler current profiler data acquired during five cruises between 21°N and 4°S and on dynamic‐height time series from an array based mainly on the Line Islands. At Jarvis Island, near the equator, the time series of dynamic height and near‐surface temperature go back to 1981 and show the 1986–1987 El Niño anomalies starting later in the year and having longer duration than those of the 1982–1983 El Niño. Dynamic‐height anomaly was less strong for the 1986–1987 event, but the near‐surface temperature anomaly was of similar magnitude for the two El Niños. The Jarvis near‐surface temperature drop from 1986–1987 El Niño maximum to 1988–1989 La Niña minimum was 8°C. Empirical orthogonal function analysis of the time series shows that interannual and interseasonal variability in dynamic height was dominated by a mode with meridional form similar to a first‐vertical‐mode Kelvin wave, while intraseasonal variability had a primary mode with a single peak at 6°N and a secondary mode with peak at 6°N and trough at 2°N. While the equatorial thermocline deepened to the east and shoaled to the west during the 1986–1987 El Niño, at 160°W it did not change depth during either this El Niño or the subsequent La Niña. Nevertheless, just before El Niño and just after La Niña, the thermocline was observed to be about 50 m deeper than at other times. The South Equatorial Current and North Equatorial Countercurrent had markedly reduced (increased) transports during this El Niño (La Niña). However, the Northern Tsuchiya Jet strengthened during El Niño and weakened during La Niña

Vertical structure of bottom Ekman tidal flows: Observations, theory, and modeling from the northern Adriatic

From September 2002 to May 2003, fifteen bottom‐ mounted, acoustic Doppler current profilers measured currents of the northern Adriatic basin. Tidal fluctuations at all seven of the major Adriatic frequencies were synthesized from a response tidal analysis of these measurements. Most observed tidal current ellipses were nearly reversing, but near the bottom, tidal current ellipses all shortened and broadened, semidiurnal currents led upper water column currents, and diurnal tidal current ellipse orientations rotated counterclockwise toward the bottom. Theoretical solutions for a tidally forced, bottom Ekman layer with vertical eddy viscosity of the form A z = β z + k were least squares fit to the observations. Average values were β = 3 · 10− 4 m/s and k = 5 · 10− 4 m2/s. The value of k was important in matching tidal orientation and phase changes, and a nonzero β was important in matching tidal amplitude changes. The Navy Coastal Ocean Model (NCOM) and the Quoddy model were also compared to the observations. The average RMS errors for the bottom Ekman layer were 0.22 cm/s for the best fit theory, 0.35 cm/s for NCOM, and 0.36 cm/s for Quoddy. A z structures from NCOM and Quoddy show that time variation in A z is relatively unimportant for Adriatic tides. The bottom shear stresses from theory were larger in magnitude than those from the bottom drag formulations in NCOM and Quoddy

South Atlantic mass transports obtained from subsurface float and hydrographic data

Mean total (barotropic + baroclinic) mass transports of the oceanic top 1000 dbar are estimated for two regions of the South Atlantic between 18°S and 47°S. These transports are obtained by using Gravest Empirical Mode (GEM) fields calculated from historical hydrography with temperature and position data from quasi-isobaric subsurface floats deployed from 1992 through 2001. The float-GEM-estimated total mass transports reveal a Brazil Current with a southward flow of 20.9 Sv at 30°S and 46 Sv at 35°S (1 Sverdrup, Sv = 106 m3 s–1). Two recirculation cells are identified in the southwest corner of the subtropical gyre north of 40°S, one centered at 48°W, 37°S recirculating 28.5 Sv and another centered at 40°W, 38°S recirculating 13.9 Sv. The South Atlantic Current (SAC) flows eastward with 50 Sv at 30°W and splits into two branches in the east, one north of 38°S transporting 19 Sv and one south of 41°S transporting 31 Sv. Of the 39.7 Sv of SAC transport that comes from the Malvinas Current/Antarctic Circumpolar Current (ACC) system in the western basin, only 8.7 Sv flow with the northern branch and the remaining 31 Sv flow as the southern branch out of the South Atlantic rejoining the ACC directly (20.6 Sv) or interacting with the Agulhas Current Retroflection (10.4 Sv). From the northern branch, only 4.7 Sv of Malvinas Current/ACC origin and 10.3 Sv of Brazil Current origin (a total of 15 Sv) stays in the South Atlantic forming the Benguela Current, recirculating within the subtropical gyre. The Agulhas Current Retroflection reaches westward as far as 10°E with a transport of 48 Sv. In terms of mean total transport, the cold-water route carries 4.7 Sv in the upper 1000 dbar whereas the warm-water route carries 8.5 Sv. However, considering the interaction between waters from both origins, there is a total of 19.1 Sv of waters entering the Cape Basin from the Pacific Ocean and 18.5 Sv from the Indian Ocean

The Dok Cold Eddy

Current and temperature patterns in the Ulleung Basin of the Japan/East Sea are examined using acoustic travel-time measurements from an array of pressure-gauge-equipped inverted echo sounders moored between June 1999 and July 2001. The focus here is the formation and behavior of a persistent cold eddy observed south of Dok Island, referred to as the Dok Cold Eddy (DCE), and meandering of the Subpolar Front. The DCE is typically about 60 km in diameter and originates from the pinching off of a Subpolar Front meander between Ulleung and Dok Islands. After formation, the DCE dwells southwest of Dok Island for 1–6 months before propagating westward toward Korea, where it deflects the path of the East Korean Warm Current (EKWC). Four such DCE propagation events between January and June 2000 each deflected the EKWC, and after the fourth deflection the EKWC changed paths and flowed westward along the Japanese shelf as the “Offshore Branch” from June through November 2000. Beginning in March 2001, a deep, persistent meander of the Subpolar Front developed and oscillated with a period near 60 days, resulting in the deformation and northwestward displacement of the Ulleung Eddy. Satellite-altimeter data suggest that the Ulleung Eddy may have entered the northern Japan/East Sea. The evolution of this meander is compared with thin-jet nonlinear dynamics described by the modified Korteweg–deVries equation

p-wave superconductivity in iron-based superconductors

The possibility of p-wave pairing in superconductors has been proposed more than five decades ago, but has not yet been convincingly demonstrated. One difficulty is that some p-wave states are thermodynamically indistinguishable from s-wave, while others are very similar to d-wave states. Here we studied the self-field critical current of NdFeAs(O,F) thin films in order to extract absolute values of the London penetration depth, the superconducting energy gap, and the relative jump in specific heat at the superconducting transition temperature, and find that all the deduced physical parameters strongly indicate that NdFeAs(O,F) is a bulk p-wave superconductor. Further investigation revealed that single atomic layer FeSe also shows p-wave pairing. In an attempt to generalize these findings, we re-examined the whole inventory of superfluid density measurements in iron-based superconductors and show quite generally that single-band weak-coupling p-wave superconductivity is exhibited in iron-based superconductors. © 2019, The Author(s).Japan Science and Technology Corporation, JST: JPMJCR18J4Government Council on Grants, Russian FederationJapan Society for the Promotion of Science, JSPS: 16H04646АААА-А18-118020190104-3The authors thank Prof. Jeffery L. Tallon (Victoria University of Wellington, New Zealand) and Prof. Christian Bernhard (University of Fribourg, Switzerland) for helpful discussions, and also for reading and commenting on the manuscript. EFT is grateful for financial support provided by the state assignment of Minobrnauki of Russia (theme “Pressure” No. АААА-А18-118020190104-3) and by Act 211 of the Government of the Russian Federation, contract No. 02.A03.21.0006. KI and HI acknowledge support by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (B) Grant Number 16H04646, as well as JST CREST Grant Number JPMJCR18J4. The Article Processing Charge for this publication was provided by Ural Federal University, Russia

The onset of dissipation in high-temperature superconductors: magnetic hysteresis and field dependence

Recently, we showed that the self-field transport critical current, Ic(sf), of a superconducting wire can be defined in a more fundamental way than the conventional (and arbitrary) electric field criterion, Ec = 1 microV/cm. We defined Ic(sf) as the threshold current, Ic,B, at which the perpendicular component of the local magnetic flux density, measured at any point on the surface of a high-temperature superconducting tape, abruptly crosses over from a non-linear to a linear dependence with increasing transport current. This effect results from the current distribution across the tape width progressively transitioning from non-uniform to uniform. The completion of this progressive transition was found to be singular. It coincides with the first discernible onset of dissipation and immediately precedes the formation of a measureable electric field. Here, we show that the same Ic,B definition of critical currents applies in the presence of an external applied magnetic field. In all experimental data presented here Ic,B is found to be significantly (10-30%) lower than Ic,E determined by the common electric field criterion of Ec = 1 microV/cm, and Ec to be up to 50 times lower at Ic,B than at Ic,E.Comment: 14 pages, 10 figure