In a topological insulator (TI), transport is mediated by Dirac-like fermions which exhibit a helical spin polarization. When a TI is coupled to superconductors in a Josephson junction, topologically protected gapless Andreev bound states are predicted. Their energy varies 4π-periodically with the superconducting phase difference, resulting in a fractional 4π-periodic Josephson current. The non-ambiguous observation of such gapless states is regarded as an important experimental signature of the unconventional superconductivity in topological insulators, but no robust evidence has been reported yet.
In topological Josephson junctions based on strained HgTe (three-dimensional TI), we observe a systematic suppression of the first Shapiro step at low frequencies. We attribute it to the existence of a 4π-periodic component in the supercurrent, as an experimental signature of fractional Josephson effect. Recent results obtained on a narrow HgTe quantum wells (two-dimensional TI) also support this interpretation. Our experimental observations thus provide indications of the presence of 4π-periodic zero-energy Andreev bound states.