Observation and determination of abnormal rolls and abnormal zigzag rolls in electroconvection in homeotropic liquid crystals

Direct evidence for two different types of normal rolls and of zigzag rolls in homeotropically aligned nematic liquid crystals in a magnetic field is reported. The conventional normal rolls have the reflection symmetry in the xy plane. The instability, however, breaks the reflection symmetry [Formula Presented] on the director and then the abnormal rolls are expected to be observed. We have investigated the instability experimentally and discussed it in terms of the recent numerical results by Plaut et al. [Phys. Rev. Lett. 79, 2367 (1997)]. Due to the new instability, the abnormal zigzag rolls are also found below the Lifshitz frequency.