Soft longitudinal modes in spin-singletCuGeO3

Abstract

We present results from a neutron-scattering study of lattice anomalies associated with the transition to the gapped spin-singlet state observed in the spin-~linear-chain compound Cuoe03. Instead of the lattice dimerization along the chain direction (c axis) expected for a spin-Peierls system, we observe a remarkable softening of the longitudinal acoustic phonons in a direction (along the b axis) perpendicular to the chains. A spontaneous strain appears in the same direction below the magnetic transition temperature of 14 K.Hase, Terasaki, and Uchinokura' recently reported the discovery of an unusual magnetic transition in CuGe03, an inorganic compound containing linear spin--, Cu-0 chains. Below the transition temperature T, of 14 K, the magnetic susceptibility drops isotropically toward zero, indicating the development of an energy gap for spin excitations. Inelastic neutron-scattering experiments by Nishi, Fujita, and Akimitsu have confirmed the existence of a gap, which reaches a value of 2.1 meV well below T, . The singlet nature of the ground state has been directly deinonstrated by Nishi et al. , 3 who have shownthat the minimum-energy excitation splits into three when a magnetic field is applied, as expected for singletriplet excitations. Measurements of the magnetic-field dependence of T"when scaled appropriately, agree quite well with both theoretical predictions and experimental results for organic spin-Peierls systems.It appears that CuGe03 might be an example of an inorganic spin-Peierls system.However, in a proper spin-Peierls transition, the pairing of spins in singlets occurs in the presence of a concomitant structural dimerization of the chains. Nishi, Fujita, and Akimitsu have found no evidence for such a chain distortion, which would double the unit cell along the c axis (parallel to the chains). They have also discovered a significant interchain exchange interaction, with a magnitude of approximately 10% of the intrachain coupling. The coupling between chains may be responsible for the very rapid growth of the gap below T, .We present here the results of a neutron-scattering study on a high-quality CuGe03 single crystal. Contrary to expectations, we observe lattice anomalies along the b direction, perpendicular to the chain direction. In particular, the longitudinal-acoustic (LA) mode in this direction is found, quite surprisingly, to be softer than a transverse-acoustic (TA) mode propagating in the same direction. Furthermore, a spontaneous strain along the b axis appears at the magnetic transition. Whether the distortion is coupled to a modulation of the exchange along the chains or between them is not yet clear.Measurements were performed on the triple-axis spectrometer H8 located in the High-Flux Beam Reactor at Brookhaven National Laboratory. The single crystal used for the experiment was grown by the fioating zone method and has dimensions 4X 3 X 2 mm . Although it is rather small for inelastic neutron studies, it is an exceptionally high-quality crystal, with a mosaic of less than 3 mi...

Magnetic structure of thespin−12frustrated quasi-one-dimensional antiferromagnetCu3Mo2O9Hase1, Kuroe2, Pomjakushin3 et al. 2015Phys. Rev. B171130View full textAdd to dashboardCite

“…In the Heisenberg spin-1/2 chain with antiferromagnetic (AF) nearest-neighbor (NN) and next-nearestneighbor (NNN) exchange interactions (J NN and J NNN interactions, respectively), the ground state(s) are a gapless spin-singlet state designated as Tomonaga-Luttinger liquid and two-folded-degenerated gapped spin-singlet states for 0 ≤ J NNN < 0.241J NN and 0.241J NN < J NNN , respectively [7][8][9]. In the spin-Peierls substance CuGeO 3 [10][11][12], both the interactions are considered to exist [13][14][15]. It is inferred that a large spin gap observed in Raman scattering experiments of 3.5 % Mg-doped CuGeO 3 under high pressures is generated not only by the spinPeierls transition but also by the frustration between the two interactions [16].…”

Magnetic susceptibility in the spin-Peierls systemCuGeO3

“…The possibility of an antiferromagnetic (J ′ > 0) second-neighbor coupling through the Cu-O-O-Cu exchange path was suggested in Ref. [2]. Another possibility is to consider the interchain coupling which would lead to 2D model.…”

“…The purpose of the present study is to describe the spin-Peierls transition in the linear Cu 2+ spin-1/2 chains in the inorganic compound CuGeO 3 which has been recently observed [1,2,3,4]. The transition temperature T c ≈ 14 K has been inferred from the rapid drop of the magnetic susceptibility towards zero, indicating the opening of an energy gap for singlet-triplet spin excitations.…”

Far-infrared spectroscopy in the spin-Peierls compoundCuGeO3under high magnetic fields

“…7 An anomalous spontaneous strain appears with the SP transition and increases with decreasing temperature. 8 This strain is partially removed at the transition from the D to the IC phase and continues to decrease with increasing field, as observed by magnetostriction measurements under high magnetic fields.…”