Superconducting-like Phase Transition in an Insulating S = 1/2 1-D Chain

Spin 1/2 1D chains are ideal for realizations of quantum spin liquids (QSLs). Here we report the discovery of a QSL in Cs4CuSb2Cl12. Compared to previously reported S = 1/2 1D chains, it possesses a wider temperature range over which the QSL state is stabilized. We find Fermionic-like excitations extending from T = 0.8 K up to at least T = 5 K, with a large T-linear contribution to the specific heat, gamma = 31.5(2) mJ mol-1 K-2. At T = 0.7 K, there is a second-order phase transition, reminiscent of a superconducting transition, with a specific heat jump of delta C / (gammaTc) = 0.8(1), that is unchanged by a 5 T magnetic field. Thus Cs4CuSb2Cl12 reveals new phenomenology deep in the 1D QSL regime.