Disorder instability of the magnon condensate in a frustrated spin ladder

Abstract

The effect of disorder is studied on the field-induced quantum phase transition in the frustrated spin-ladder compound H${}_{8}$C${}_{4}$SO${}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}$Cu${}_{2}$(Cl${}_{1\ensuremath{-}x}$Br${}_{x}$)${}_{4}$ using bulk magnetic and thermodynamic measurements. The parent material ($x=0$) is a quantum spin liquid, which in applied fields is known to form a magnon condensate with long-range helimagnetic order. We show that bond randomness introduced by a chemical substitution on the nonmagnetic halogen site destroys this phase transition at very low concentrations, already for $x=0.01$. The extreme fragility of the magnon condensate is attributed to random frustration in the incommensurate state.