High Resolution Polar Kerr Effect Studies of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$: Tests for Time-Reversal Symmetry Breaking below the Charge-Order Transition

We report high resolution polar Kerr effect measurements on ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ single crystals in search of signatures of spontaneous time-reversal symmetry breaking below the charge-order transition at $T^{*}\approx 94\mathrm{K}$. Utilizing two different versions of zero-area loop Sagnac interferometers operating at 1550 nm wavelength, each with the fundamental attribute that without a time-reversal symmetry breaking sample at its path, the interferometer is perfectly reciprocal, we find no observable Kerr effect to within the noise floor limit of the apparatus at 30 nanoradians. Simultaneous coherent reflection ratio measurements confirm the sharpness of the charge-order transition in the same optical volume as the Kerr measurements. At finite magnetic field we observe a sharp onset of a diamagnetic shift in the Kerr signal at $T^{*}$, which persists down to the lowest temperature without change in trend. Since 1550 nm is an energy that was shown to capture all features of the optical properties of the material that interact with the charge-order transition, we are led to conclude that it is highly unlikely that time-reversal symmetry is broken in the charge ordered state in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$.

Figure 1

(a) Crystal structure of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. (b) Photo of “sample 1” from Dresden, measured at Stanford and “sample 2” from UCSB, measured at UC Irvine. (c) Schematic cross section of probe end optics assembly and sample (not to scale).

Figure 2

Kerr signal in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. (a) Sample 1 measured at Stanford. Relative Kerr angle $\mathrm{\Delta }\theta =\theta -\theta (120K)$ (left axes) in external magnetic fields of 20 mT directed along $\pm \widehat{z}$, showing the onset of diamagnetic shift when the specific heat $C_V$ (right axes) develops a peak at $T^{*}\approx 94\mathrm{K}$. (b) Sample 2 measured at Irvine. Absolute Kerr signal ${\theta }_K$ (left axes) during $\pm 0.34\mathrm{T}$ cooldowns showing the same onset of diamagnetic shift that coincides with the anomalous peak in specific heat $C_p$ (right axes). (c) Sample 1: Kerr angle $\mathrm{\Delta }\theta$ measured during zero field warmups after $+30\mathrm{mT}$ and $-20\mathrm{mT}$ trainings. (d) Sample 2: Kerr signal ${\theta }_K$ measured during zero field warmups after $\pm 0.34\mathrm{T}$ trainings. There is no discernible spontaneous Kerr effect below $T^{*}$ to within 30 nrad in either sample.