Spin correlations of quantum spin liquid and quadrupole-ordered states of TbTiO

量子スピン液体であり四重極秩序状態になっているTbTiOの磁気相関

門脇 広明*; 脇田 美香*; Fk, B.*; Ollivier, J.*; 河村 聖子   ; 中島 健次  ; Lynn, J. W.*

Kadowaki, Hiroaki*; Wakita, Mika*; Fk, B.*; Ollivier, J.*; Kawamura, Seiko; Nakajima, Kenji; Lynn, J. W.*

非弾性中性子散乱を用いてフラストレーションのあるパイロクロア酸化物TbTiO (x = -0.007, 0.000, and 0.003)単結晶試料の磁気相関を測定した。この系は、量子スピン液体と四重極秩序状態を示すものである。実験結果とRPAを使った解析結果を議論する。量子スピン液体状態では対称性の破れが生じており、古典的なRPAの範囲を超えた議論が必要なことが示唆される。

Spin correlations of the frustrated pyrochlore oxide TbTiO have been investigated by using inelastic neutron scattering on single-crystalline samples (x = -0.007, 0.000, and 0.003), which have the putative quantum-spin-liquid (QSL) or electric-quadrupolar ground states. Spin correlations, which are notably observed in nominally elastic scattering, show short-range correlations around points, tiny antiferromagnetic Bragg scattering at L and points, and pinch-point-type structures around points. The short-range spin correlations were analyzed using a random-phase approximation (RPA) assuming the paramagnetic state and two-spin interactions among Ising spins. These analyses have shown that the RPA scattering intensity well reproduces the experimental data using temperature- and x-dependent coupling constants of up to tenth-neighbor site pairs. This suggests that no symmetry breaking occurs in the QSL sample and that a quantum treatment beyond the semiclassical RPA approach is required. Implications of the experimental data and the RPA analyses are discussed.