  The spin-1/2 Heisenberg antiferromagnetic chain is ideal for realizing one of
the simplest gapless quantum spin-liquids (QSLs), supporting a many-body ground
state whose elementary excitations are fractional fermionic excitations called
spinons. Here we report the discovery of such a 1D QSL in Cs4CuSb2Cl12.
Compared to previously reported S = 1/2 1D chains, this material possesses a
wider temperature range over which the QSL state is stabilized. We identify
spinon excitations extending at T > 0.8 K, with a large T-linear contribution
to the specific heat, gamma = 31.5(2) mJ mol-1 K-2 which contribute itinerantly
to thermal transport up to temperatures as high as T = 35 K. At T = 0.7 K, we
find a second-order phase transition, suggesting a weak spin-Peierls transition
that is unchanged by a 5 T magnetic field. Cs4CuSb2Cl12 reveals new
phenomenology deep in the 1D QSL regime, supporting a gapped QSL phase over a
wide temperature range compared to many other experimental realizations.
