Dimer liquid state in the quantum antiferromagnet compoundLiCu2O2

Results of systematic structural, magnetic, electron-spin resonance, and specific-heat investigations of the $S=1/2\mathrm{}$ quantum antiferromagnet compound ${\mathrm{LiCu}}_2{\mathrm{O}}_2$ are presented. A spin-singlet ground state is revealed at temperatures above $T\sim 23\mathrm{K}.$ In this temperature region ${\mathrm{LiCu}}_2{\mathrm{O}}_2$ may be regarded as a quantum $S=1/2\mathrm{}$ chain with competing nearest- and next-nearest-neighbor interactions in the intermediate range of the $J_2{/J}_1$ ratio $({\alpha }_{c1\mathrm{}}<J_2{/J}_1<{\alpha }_{c2\mathrm{}}).\mathrm{}$ The size of the energy gap between the spin-singlet ground state and the first excited triplet, Δ, is found to be ∼72 K. The study confirms a magnetic phase transition in ${\mathrm{LiCu}}_2{\mathrm{O}}_2,$ which results in the collapse of the spin-singlet ground state at a temperature below 23 K. In addition, a second low-temperature transition was found at $T\sim 9\mathrm{K}.$ Peculiarities of the magnetic excitation spectrum in the dimer phase of ${\mathrm{LiCu}}_2{\mathrm{O}}_2$ and its magnetic structure are discussed.