Emergent spin-1 Haldane gap and ferroelectricity in a frustrated spin- ladder

Ueda, Hiroshi*; Onoda, Shigeki*; Yamaguchi, Yasuhiro*; Kimura, Tsuyoshi*; Yoshizawa, Daichi*; Morioka, Toshiaki*; Hagiwara, Masayuki*; Hagihara, Masato*   ; Soda, Minoru*; Masuda, Takatsugu*; Sakakibara, Toshiro*; Tomiyasu, Keisuke*; Kawamura, Seiko   ; Nakajima, Kenji  ; Kajimoto, Ryoichi  ; Nakamura, Mitsutaka   ; Inamura, Yasuhiro  ; Reynolds, N.*; Frontzek, M.*; White, J. S.*; Hase, Masashi*; Yasui, Yukio*

We report experimental and theoretical evidence that RbCuMoO has a nonmagnetic tetramer ground state of a two-leg ladder comprising antiferromagnetically coupled frustrated spin- chains and exhibits a Haldane spin gap of emergent spin-1 pairs. Three spin excitations split from the spin-1 triplet by a Dzyaloshinskii-Moriya interaction are identified in inelastic neutron-scattering and electron spin resonance spectra. A tiny magnetic field generates ferroelectricity without closing the spin gap, indicating a unique class of ferroelectricity induced by a vector spin chirality order.