The Nature of Grain Boundaries in the High-Tc Superconductors

Babcock, S. E.; Vargas, J.L.

doi:10.1146/annurev.ms.25.080195.001205

Cited by 101 publications

(68 citation statements)

References 46 publications

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“…[10][11][12][13] Samples of HTS's are often granular in composition, consisting of superconducting grains coupled to one another by grain-boundary junctions. 14 This granularity leads to nonlinear effects such as harmonic generation ͑HG͒ and twofrequency intermodulation ͑IM͒, and also to a dependence of the surface impedance on the amplitude of the ac field. 15,16 These nonlinearities can cause various problems in device applications.…”

Section: Introductionmentioning

confidence: 99%

Microwave response and surface impedance of weak links

McDonald

Clem

1997

Phys. Rev. B

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The behavior of superconducting weak links in an ac magnetic field is considered. Both small and long uniform junctions are analyzed. Analytical results are presented for various limiting cases. The general case is solved numerically for different parameter choices and the results are presented. Both similarities and significant differences are found between the small junction and the long uniform junction. ͓S0163-1829͑97͒02845-2͔

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Section: Introductionmentioning

confidence: 99%

Microwave response and surface impedance of weak links

McDonald

Clem

1997

Phys. Rev. B

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“…However, since YBa 2 Cu 3 O x can only be slightly overdoped from the optimal composition of xϭ6.94 up to xϭ7, their GBs are generally thought to be hole deficient. 10,11 Recently, Schmehl et al 12 have made Cadoped YBa 2 Cu 3 O x bicrystals where greater overdoping is possible, 13 finding that the GB resistance could be strongly decreased and J b ͑0 T, 4.2 K͒ of some 24°͓001͔ tilt GBs increased by as much as eight times. More recent work with YBa 2 Cu 3 O x /Y 0.7 Ca 0.3 Ba 2 Cu 3 O x multilayers showed that J b ͑0 T, 77 K͒ could be raised by up to 3.5 times.…”

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confidence: 99%

Improved strong magnetic field performance of low angle grain boundaries of calcium and oxygen overdoped YBa2Cu3Ox

Daniels

Gurevich

Larbalestier

2000

Applied Physics Letters

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Critical currents and extended voltage-current characteristics of ͓001͔ tilt, low angle boundaries in thin film bicrystals of pure YBa 2 Cu 3 O x and Y 0.7 Ca 0.3 Ba 2 Cu 3 O x in strong magnetic fields (HϽ10 T͒ have been measured. For 5°͓001͔ tilt boundaries, there are clear benefits to Ca additions, particularly when the sample is oxygen treated to further overdope the sample after growth. Ca-doping decreases excess grain boundary resistance and diminishes the gap between inter-and intragrain current density over a wide field and temperature range. In spite of the lower T c of the 0.3 Ca-doped samples, Ca doping increases the intergrain critical current density at 44 K as high as 30% over 0-3 T and threefold at 5 T. © 2000 American Institute of Physics. ͓S0003-6951͑00͒03644-5͔Mechanisms of current transport through grain boundaries ͑GB͒ in high-temperature superconductors and the critical misorientation angle c at which a GB obstructs supercurrent flow are of great interest.1-3 These issues are particularly important for biaxially textured coated conductors, 4-6 where recent magneto-optical studies suggest that the intergranular critical current density J b falls below the intragrain current density J c for у4°.7 A low angle ͑Ͻ5°-10°͒ ͓001͔ tilt GB can be modeled as an array of edge dislocations with insulating cores of the order of the Burgers' vector ͑0.4 nm͒ in diameter with relatively undisturbed channels between the cores. The quality of the channels and the charge imbalance on the GB play a large role in determining J b . 8,9 Screening of the excess GB charge leads to a depression of the superconducting order parameter in the channels and over a region of 1-2 nm near the GB. A model, 8 which incorporates these characteristics, explains the quasiexponential decline of J b () observed for large in YBCO bicrystals, 1-3 indicating that J b () can be improved by increasing the GB carrier density ͑for example, by overdoping the sample͒. However, since YBa 2 Cu 3 O x can only be slightly overdoped from the optimal composition of xϭ6.94 up to xϭ7, their GBs are generally thought to be hole deficient. 14 However, these dramatic increases in J b () were observed in zero magnetic field on high angle GBs which remained weakly coupled even after Ca overdoping. In the present work we report on the benefits of overdoping with Ca and O for low angle GBs. We measured the extended voltage-current (V -I) characteristics of 5°and 7°͓001͔ tilt Y 0.7 Ca 0.3 Ba 2 Cu 3 O x bicrystals in magnetic fields up to 10 T. We found considerable increase of J b () due to the Ca overdoping.Thin films were grown on ͓001͔-oriented SrTiO 3 bicrystals by pulsed laser deposition under identical conditions using both pure YBa 2 Cu 3 O x and Y 0.7 Ca 0.3 Ba 2 Cu 3 O x targets. 13Our films were grown at 210 mTorr oxygen pressure and a substrate temperature of 810°C, followed by a postgrowth annealing in oxygen at 830 Torr and 520°C for 30 min before final cooldown. Most data was obtained on two 5°͓001͔ tilt bicrystals. Cross-sectional scanning el...

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“…1,4,6,7 Since this range covers a PGBD separation of 4.5-0.6 nm, a more explicit test of the above models is highly desirable. 8 This issue has regained importance because of renewed activity on making conductors with a strong biaxial texture from YBa 2 Cu 3 O 7Ϫx ͑YBCO͒, using an ion beam assisted deposition process. [9][10][11] Since such conductors would be useful for the production of strong fields, provided that the spread in the a/b grain axis misalignment is controlled, knowledge of the extended high field voltage current (V -I) characteristics of YBCO bicrystals of varying misorientations is of both fundamental and applied interest.…”

mentioning

confidence: 99%

Evidence for channel conduction in low misorientation angle [001] tilt YBa2Cu3O7−x bicrystal films

Heinig

Redwing

Tsu

et al. 1996

Applied Physics Letters

100

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Extended voltage-current (V -I) characteristics of ͓001͔ tilt YBa 2 Cu 3 O 7Ϫx bicrystal films having misorientation angles, , of 3°to 20°showed a substantial and progressive change with , when measured in large fields. Transmission electron microscopy of the 10°bicrystal showed the grain boundary to contain edge dislocations separated by channels of relatively undisturbed lattice. A large drop in the intergrain irreversibility field, H*, occurred between 10°and 15°, and the characteristics became qualitatively different by 20°. Both the microscopy and the electromagnetic data support a heterogeneous description of the grain boundary, consisting of strongly coupled channels that close at a misorientation angle of around 15°. © 1996 American Institute of Physics. ͓S0003-6951͑96͒03630-3͔It is well established that the critical current density, J c , of ͓001͔ tilt thin film bicrystals of high temperature superconductors is strongly dependent on the misorientation angle, , of the grain boundary ͑GB͒. 1-4 Dimos et al. 1 proposed that the cores of primary grain boundary dislocations ͑PGBD͒ subdivide low angle grain boundaries. Chisholm and Pennycook 5 additionally proposed that the strain fields of the PGBDs extend the effective size of the dislocation cores. Both models postulate that the supercurrent is restricted to channels of good lattice between the PGBDs. The effective size of the PGBD cores should then determine the magnitude of J b /J c ͑the ratio of the inter-to intragranular critical current density͒, and the critical misorientation angle at which strong coupling properties disappear. Measurements on different kinds of samples suggest that this critical angle can vary over a wide angular range of 5°-20°. 1,4,6,7 Since this range covers a PGBD separation of 4.5-0.6 nm, a more explicit test of the above models is highly desirable. 8 This issue has regained importance because of renewed activity on making conductors with a strong biaxial texture from YBa 2 Cu 3 O 7Ϫx ͑YBCO͒, using an ion beam assisted deposition process. [9][10][11] Since such conductors would be useful for the production of strong fields, provided that the spread in the a/b grain axis misalignment is controlled, knowledge of the extended high field voltage current (V -I) characteristics of YBCO bicrystals of varying misorientations is of both fundamental and applied interest.This letter describes the first systematic measurements of the V -I curves of ͓001͔ tilt YBCO bicrystal thin films with misorientation angles in the low-to-high angle crossover regime of 3°-20°in large magnetic fields. Prior measurements of single crystal thin film 12,13 and bulk 14 high temperature superconductors have established that the irreversibility field, H*, above which J c tends to zero, can be experimentally defined by the change in sign of the curvature of the electric field-current density, E -J, characteristic plotted on a double logarithmic scale. In single crystal YBCO films, this transition occurs near 4 T at 77 K. 12 To test the channel models described...

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The Nature of Grain Boundaries in the High-Tc Superconductors

Cited by 101 publications

References 46 publications

Microwave response and surface impedance of weak links

Microwave response and surface impedance of weak links

Improved strong magnetic field performance of low angle grain boundaries of calcium and oxygen overdoped YBa2Cu3Ox

Evidence for channel conduction in low misorientation angle [001] tilt YBa2Cu3O7−x bicrystal films

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