  We report on the observation of a helical Luttinger-liquid in the edge of
InAs/GaSb quantum spin Hall insulator, which shows characteristic suppression
of conductance at low temperature and low bias voltage. Moreover, the
conductance shows power-law behavior as a function of temperature and bias
voltage. The results underscore the strong electron-electron interaction effect
in transport of InAs/GaSb edge states. Because of the fact that the Fermi
velocity of the edge modes is controlled by gates, the Luttinger parameter can
be fine tuned. Realization of a tunable Luttinger-liquid offers a
one-dimensional model system for future studies of predicted correlation
effects.
