Stacking faults in alpha-RuCl3 revealed by local electric polarization

We present out-of-plane dielectric and magnetodielectric measurements of single-crystalline alpha-RuCl3 with various degrees of stacking faults. A frequency-dependent, but field-independent, dielectric anomaly appears at T-A (f = 100 kHz) similar to 4 K once both magnetic transitions at TN1 similar to 7 K and T-N2 similar to 14 K set in. The observed dielectric anomaly is attributed to the emergence of possible local electric polarizations whose inversion symmetry is broken by inhomogeneously distributed stacking faults. A field-induced intermediate phase is only observed when a magnetic field is applied perpendicular to the Ru-Ru bonds for samples with minimal stacking faults. Less pronounced in-plane anisotropy is found in samples with a sizable contribution from stacking imperfections. Our findings suggest that dielectric measurement is a sensitive probe in detecting the structural and magnetic properties, which may be a promising tool, especially in studying alpha-RuCl3 thin-film devices. Moreover, the stacking details of RuCl3 layers strongly affect the ground state both in the magnetic and electric channels. Such a fragile ground state against stacking faults needs to be overcome for realistic applications utilizing the magnetic and/or electric properties of Kitaev-based physics in alpha-RuCl3.