Atomic structure of large-angle grain boundariesΣ5andΣ13inYBa2

Abstract

We present the results of a computer simulation of the atomic structures of large-angle symmetrical tilt grain boundaries (GBs) Σ5 (misorientation angles 36.87 • and 53.13 • ), Σ13 (misorientation angles 22.62 • and 67.38 • ). The critical strain level ε crit criterion (phenomenological criterion) of Chisholm and Pennycook is applied to the computer simulation data to estimate the thickness of the nonsuperconducting layer h n enveloping the grain boundaries. The h n is estimated also by a bond-valence-sum analysis. We propose that the phenomenological criterion is caused by the change of the bond lengths and valence of atoms in the GB structure on the atomic level. The macro-and micro-approaches become consistent if the ε crit is greater than in earlier papers. It is predicted that the symmetrical tilt GB Σ5 θ = 53.13 • should demonstrate a largest critical current across the boundary. PACS: 74.50. +r; 61.72. Mm The major problem in the applications of YBa 2 Cu 3 O 7−δ (YBCO) crystals is their low critical current density J c caused by poor current transmission at the grain boundaries (GBs). The first studies of this phenomenon, starting with [1], showed that J c across the GBs decreases drastically with increasing misorientations angle θ between grains. Subsequent studies have indicated that certain specific large-angle GBs (near-Σ1 90 • [010] GBs) do not show this sharp decrease (for details see [2]). Several mechanisms have been suggested to explain the GBs influence on superconducting properties of YBCO (see for details [3,4]). We will examine the explanations based on the reduction of the order parameter caused by strain. Chisholm and Pennycook [5] suggested that cores of dislocations forming low-angle GB are nontransparent for the supercurrent because superconductivity is suppressed in the region of the crystal lattice where strains achieve some critical value ε crit . Based 1

Grain boundaries and pinning in bulk MgB2

“…Some of the angles are equal or close to CSLB angles in other systems. For example, CSLB of 28.1 • is registered in cubic MgO [21] and CSLB with 53.1 • ( 5) is predicted [22] to provide high J c in orthorhombic YBa 2 Cu 3 O x . We see CSLB peaks in all samples.…”

Influence of high-angle grain boundaries on the charge order formation in the $\mathrm{YBa_2Cu_3O_{7-δ}}$

“…The molecular dynamics methods exploited in calculations are described in Refs. [18] and [19]. The substantial feature of the present work is the computer simulation of point defects in models of the ideal crystal lattice of the YBCO.…”

Microstructure design by twinning in high-temperature superconductor YBa2Cu3O7−δ for enhanced Jc at high magnetic fields

“…Using the expression of the stress concentration near the pile-up of dislocations (see, for example, [33]), one can make rough estimate of the radius of the non-superconducting region around the tip of the secondary twin as r n $ ðN 2 d  b 2 Þ=ðe 2 crit  L d Þ where N d is the number of dislocations in the domain and L d is the length of the domain. If we take N d = 10 2 , L d = 10 À6 m, and e crit $ 0.01 (actually, as it was shown in [34], e crit at lattice level in YBCO could be somewhat greater) then we will get r n $ 100 Å . Thus, the r n for twin intersection is pretty large and approximates the radius of damage around the ion track when columnar pinning centers are created by the heavy-ion irradiation [35].…”