Evidence of Chiral Order in the Charge-Ordered Phase of Superconducting ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{Cuo}}_4$ Single Crystals Using Polar Kerr-Effect Measurements

High resolution polar Kerr effect measurements were performed on ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4$ single crystals revealing that a finite Kerr signal is measured below an onset temperature $T_K$ that coincides with the charge ordering transition temperature $T_{CO}$. We further show that the sign of the Kerr signal cannot be trained with the magnetic field, is found to be the same on opposite sides of the same crystal, and is odd with respect to strain in the diagonal direction of the unit cell. These observations are consistent with a chiral “gyrotropic” order above $T_c$ for ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4$; similarities to other cuprates suggest that it is a universal property in the pseudogap regime.

Figure 1

Kerr angle data of a typical $1/8$-doped LBCO single crystal. For all LBCO samples the measured $T_K$ coincides with the charge-order transition $T_{CO}\sim 54\mathrm{K}$ (see text). (a) ZFW Kerr data taken on two parallel opposite $ab$ surfaces after ZFC. Also plotted is the intensity of the interfering light (i.e., second harmonic) in the vicinity of the Kerr transition [11], which clearly shows the anomaly at the first-order structural transition from low-temperature orthorhombic to low-temperature tetragonal $T_{LT}\sim T_{CO}$ [18]. Inset is a scan of a $20\times 20$ array of pixels, each $10\mu \mathrm{m}$ in size, of the local Kerr angle at $T=4\mathrm{K}$ in some typical part of a crystal. (b) ZFW data taken on the “first surface,” after $\pm 1\mathrm{kG}$ FC.

Figure 2

Kerr angle data of a strained $1/8$-doped LBCO single crystal. (a) The experimental setup. (b) The Kerr response as a function of temperature. Here we also mark the two temperatures at which strain-induced changes in the Kerr angle are measured. The two horizontal lines indicate the range of change of the Kerr effect due to strain at 23 K. (c)–(d) Kerr angle vs strain at 60 and 23 K. Positive strain is applied in leg 1, than polarity is reversed in leg 2, and then reversed again in leg 3 (see text). Strain is measured through change in reflectivity [see Eq. (1)]. While the Kerr angle did not change within this cycle at 60 K, it did change at 23 K.