Search for broken time-reversal symmetry near the surface of superconducting YBa${}_2$Cu${}_3$O${}_{7-\delta }$ films using $\beta$-detected nuclear magnetic resonance

Weak spontaneous magnetic fields are observed near the surface of YBa${}_2$Cu${}_3$O${}_{7-\delta }$ films using $\beta$-detected nuclear magnetic resonance. Below $T_c$, the magnetic field distribution in a silver film evaporated onto the superconductor shows additional line broadening, indicating the appearance of small disordered magnetic fields. The line broadening increases linearly with a weak external magnetic field applied parallel to the surface, and is depth independent up to 45 nm from the Ag/YBa${}_2$Cu${}_3$O${}_{7-\delta }$ interface. The magnitude of the line broadening extrapolated to zero applied field is less than 0.2 G, and is close to nuclear dipolar broadening in the Ag. This indicates that any fields due to broken time-reversal symmetry are less than 0.2 G.

Figure 1

Typical $\beta$-NMR spectra taken by implanting 2 keV ${}^8\mathrm{Li}$${}^{+}$ into Ag/YBCO(110), in an external field of $B_0=10$ G (FC) applied along the surface of the film. Solid lines are fits to a Lorentzian of HWHM $\Delta$. Inset: Simulated implantation profile using TRIM.SP for ${}^8\mathrm{Li}$${}^{+}$ of 2 keV in 15 nm of Ag on YBCO (Ref. 26). The ${}^8\mathrm{Li}$${}^{+}$ stops at an average depth of 8 nm away from the Ag/YBCO interface.

Figure 2

$T$ dependence of the HWHM, $\Delta$, of the resonance of 2 keV ${}^8\mathrm{Li}$${}^{+}$ implanted into Ag/YBCO(110), Ag/YBCO(001), and Ag. The data on the Ag/YBCO(110) and Ag film were taken using FC, and ZFC for the Ag/YBCO(001). The widths were independent of FC or ZFC. The dashed lines represent the average Ag width ${\Delta }_0$, the arrows point to the $T_c$ of the YBCO films, and the solid lines are guide to the eye.

Figure 3

HWHM of the resonance in Ag/YBCO(001) at 10 K taken after ZFC. The dashed lines are linear fits. Inset: energy and depth dependence of the excess HWHM at 10 K [$\Delta$(10 K)$-{\Delta }_0$] in the 50 nm Ag/YBCO(001) sample at applied fields 10 and 100 G. The resonance at all energies is due to the fraction of ${}^8\mathrm{Li}$${}^{+}$ landing in Ag film.