LAUSR

The $\mathrm{spin}\text{\ensuremath{-}}\frac{1}{2}$ 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 one-dimensional (1D) QSL in $\mathrm{C}{\mathrm{s}}_{4}\mathrm{CuS}{\mathrm{b}}_{2}\mathrm{C}{\mathrm{l}}_{12}$. Compared to previously reported $S=\frac{1}{2}$ 1D chains, this material possesses a wider temperature range over which the QSL state is stabilized. We identify spinon excitations extending at $Tg0.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, with a large $T$-linear contribution to the specific heat, $\ensuremath{\gamma}=31.5(2)\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}$, which contribute itinerantly to thermal transport up to temperatures as high as $T=35\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. At $T=0.7\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, we find a second-order phase transition that is unchanged by a ${\ensuremath{\mu}}_{0}H=5\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ magnetic field. $\mathrm{C}{\mathrm{s}}_{4}\mathrm{CuS}{\mathrm{b}}_{2}\mathrm{C}{\mathrm{l}}_{12}$ 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.