Effect of a Compressive Uniaxial Strain on the Critical Current Density of Grain Boundaries in Superconducting ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathbf{O}}_{7-\delta }$ Films

The mechanism by which grain boundaries impede current flow in high-temperature superconductors has resisted explanation for over two decades. We provide evidence that the strain fields around grain boundary dislocations in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ thin films substantially suppress the local critical current density $J_c$. The removal of strain from the superconducting grain boundary channels by the application of compressive strain causes a remarkable increase in $J_c$. Contrary to previous understanding, the strain-free $J_c$ of the grain boundary channels is comparable to the intrinsic $J_c$ of the grains themselves.

Figure 1

The reversible change in $J_c$ with uniaxial compressive strain that is measured in two YBCO thin films that were deposited on single-crystalline STO substrates. The solid lines are the strain dependence of the intragrain $J_c$, as described by Eq. (1).

Figure 2

The reversible change in $J_{c,\mathrm{GB}}$ with uniaxial compressive strain that is measured in four thin film YBCO grain boundaries, with angles of 4°, 6° (film 6° GB-1 in Table ), 7.5° (film 7.5° GB-1 in Table ), and 12°. The solid lines are the critical current density according to Eq. (3).

Figure 3

The reversible change in $J_{c,\mathrm{GB}}$ with uniaxial compressive strain that is measured in a thin film YBCO grain boundary of 12°. The solid line represents the critical current density according to Eq. (3). The error bars represent a 0.5% uncertainty in $J_c$. The inset shows an exponential decrease in $J_{c,\mathrm{GB}}$ as a function of grain boundary angle. The solid line represents Eq. (2), with $J_c(0)=4.55\mathrm{MA}/{\mathrm{cm}}^2$ and ${\theta }_0=3.2°$.

Figure 4

Dislocations with nonsuperconducting cores reduce the superconducting cross section at grain boundaries, while strain fields that originate from the dislocations extend into the superconducting channels.

Figure 5

Universal strain dependence of $J_c$ as a function of intrinsic strain ${\epsilon }_0$. The solid line represents Eq. (1), with ${\epsilon }_{m,G}=0$. The arrows that follow the curve indicate how existing lattice strain in the YBCO film is reduced when compressive strain is applied, and how $J_c$ changes accordingly.