Polar Kerr-Effect Measurements of the High-Temperature ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ Superconductor: Evidence for Broken Symmetry near the Pseudogap Temperature

The polar Kerr effect in the high-$T_c$ superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ was measured at zero magnetic field with high precision using a cyogenic Sagnac fiber interferometer. We observed nonzero Kerr rotations of order $\sim 1\mu \mathrm{rad}$ appearing near the pseudogap temperature $T^{*}$ and marking what appears to be a true phase transition. Anomalous magnetic behavior in magnetic-field training of the effect suggests that time reversal symmetry is already broken above room temperature.

Figure 1

The onset of the Kerr-effect signal, $T_s$ (circles), and $T_c$ (red squares) for the ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ samples reported in this Letter. Also shown are $T_c(p)$ (from 12) and $T_N(p)$ (from 22).

Figure 2

The Kerr effect of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.67}$ crystal. The sample was first cooled to 4.2 K in a $+4\mathrm{T}$ field. The field was turned off at 4.2 K, and measurements were taken while warming the sample. Note the large vortex contribution that disappears just before $T_c=65\mathrm{K}$. The inset shows the region above $T_c$ with its zero baseline, indicating a finite Kerr signal that disappears at $T_s=155\mathrm{K}$. Dashed lines are guides to the eye.

Figure 3

The Kerr effect of zero-field warm-up for $x=0.92$, 0.75, 0.67, and 0.5. All samples were cooled and measured in a magnetically shielded environment (field $<3$ mOe). Dashed lines are guides to the eye.

Figure 4

The Kerr-effect measurement of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.67}$ crystal taken while warming the sample after cooling it in fields of $-60$ Oe and $+60$ Oe, and switching the field off at 4.2 K. These measurements were taken after the sample was trained in a field of $-4\mathrm{T}$ (left) and $+4\mathrm{T}$ (right), as shown in Fig. 2 (see text). Note the much smaller vortex contribution and the fact that it tracks the sign of the field in which it was cooled in.

Figure 5

The Kerr effect of a $c$-axis film with estimated average oxygen stoichiometry of $0.5<x<0.67$. The inset shows the resistive transition of the sample. We also note the position of $T_c$ and the range of $T_s$ (see text) for this sample.