Spin dynamics in the high-field phases of volborthite

Abstract

In a frustrated magnet with competing ferromagnetic and antiferromagnetic interactions, a spin-nematic state can be realized due to the condensation of two magnon bound stats. Volborthite Cu${}_{3}$V${}_{2}$O${}_{7}$(OH)${}_{2}\ensuremath{\cdot}$2H${}_{2}$O is a promising candidate. The authors study the spin dynamics in the high-field phases of volborthite. In the magnetization plateau state, the nuclear relaxation rates indicate an excitation gap with a large effective $g$ factor, pointing to magnon bound states. The relaxation rates also indicate unusual spin fluctuations with slow time scales. These findings suggest that an exotic spin state caused by the condensation of magnon bound states is realized below the magnetization plateau.