  Topological quantum matter is characterized by non-trivial global invariants
of the bulk which induce gapless electronic states at its boundaries. A case in
point are two-dimensional topological insulators (2D-TI) which host
one-dimensional (1D) conducting helical edge states protected by time-reversal
symmetry (TRS) against single-particle backscattering (SPB). However, as
two-particle scattering is not forbidden by TRS [1], the existence of
electronic interactions at the edge and their notoriously strong impact on 1D
states may lead to an intriguing interplay between topology and electronic
correlations. In particular, it is directly relevant to the question in which
parameter regime the quantum spin Hall effect (QSHE) expected for 2D-TIs
becomes obscured by these correlation effects that prevail at low temperatures
[2]. Here we study the problem on bismuthene on SiC(0001) which has recently
been synthesized and proposed to be a candidate material for a room-temperature
QSHE [3]. By utilizing the accessibility of this monolayer-substrate system on
atomic length scales by scanning tunneling microscopy/spectroscopy (STM/STS) we
observe metallic edge channels which display 1D electronic correlation effects.
Specifically, we prove the correspondence with a Tomonaga-Luttinger liquid
(TLL), and, based on the observed universal scaling of the differential
tunneling conductivity (dI/dV), we derive a TLL parameter K reflecting
intermediate electronic interaction strength in the edge states of bismuthene.
This establishes the first spectroscopic identification of 1D electronic
correlation effects in the topological edge states of a 2D-TI.
