Quantum spin liquid behavior and ferroelectricity in PbCuTe₂O₆

The quantum spin liquid is a highly entangled magnetic state characterized by the absence of static magnetism in its ground state, instead, the spins fluctuate in a highly correlated way down to the lowest temperatures. Three-dimensional spin liquids are rare and have mostly been sought in materials where the magnetic ions form pyrochlore or hyperkagome lattices. Here we study the compound PbCuTe₂O₆ which is a rare example of the highly frustrated, three-dimensional defect windmill or hyper-hyperkagome lattice. Using a combination of experiment and theory we show that this system exhibits features consistent with being a quantum spin liquid with no detectable static magnetism together with the presence of diffuse continua in the magnetic spectrum suggestive of fractional spinon excitations. More recently ferroelectric order was discovered in single crystal samples below T_FE≈1K, which is suppressed by small grain size in powder samples. This transition is accompanied by a lattice distortion and a modified magnetic response which however is still consistent with a quantum spin liquid ground state.