Direct observation of the Aharonov-Casher phase*

We report the direct observation of Aharonov-Casher effect, which can occur when electrons propagate in a ring structure in the presence of spin-orbit interactions and external magnetic field perpendicular to the ring plane. The transport measurements have been conducted on the series of ring structures fabricated from HgTe/HgCdTe quantum wells. We study Aharonov-Bohm type conductance oscillations as a function of Rashba spin-orbit splitting strength. We observe non-monotonic phase changes indicating that an additional phase factor modifies the electron wave function. We associate these observations with the Aharonov-Casher effect and confirm it by numerical calculations of the magneto-conductance for a multichannel ring within the Landauer-B\"uttiker formalism.