Excitations of the Field-Induced Quantum Soliton Lattice inCuGeO3

Abstract

Here we report the first inelastic neutron scattering study of the magnetic excitations in the incommensurate high-field phase of a spin-Peierls material. The results on CuGeO3 provide direct evidence for a finite excitation gap, two sharp magnetic excitation branches and a very low-lying excitation which is identified as a phason mode, the Goldstone mode of the incommensurate soliton lattice.A one-dimensional (1D) spin 1 2 Heisenberg antiferromagnet is unstable with respect to dimerization as well as long-range antiferromagnetic order. Coupled to a three-dimensional (3D) phonon field, it can undergo a second-order phase transition at a finite temperature into a dimerized (spin-Peierls) ground state with total spin S tot = i S i = 0 (D-phase). The dimerization is supported by antiferromagnetic next-nearest-neighbor exchange in the 1D chain direction. Antiferromagnetic (AF) interchain couplings prefer a Néel-type ground state and compete with dimerization.In the D-phase of a spin-Peierls material, the lowest magnetic excitation is an isolated triplet branch (S tot = 1). This excitation is a bound pair of domain walls with respect to the dimer order parameter. The two domain walls, or solitons, are created by breaking and delocalizing one dimer bond. The binding energy depends on the magnetic and elastic interchain interactions. The energy of the triplet is finite over the entire Brillouin zone and achieves its minima at wave vectors k = 0 and k = π c (the AF zone center, c being the average distance between two spins in the chain direction.) In a magnetic field, the S tot = 1 branch becomes Zeeman split, and a magnetized incommensurate (IC) phase is entered above a critical field H c , which corresponds approximately to the field where the lowest mode should soften to zero energy. In the IC-phase, the magnetization is generated by introducing more and more domain walls into the dimerized ground state. The unpaired spin at the center of a domain wall aligns parallel to the magnetic field (assumed to be to z). The transverse parts of the spin Hamiltonian, S + i S − j , delocalize the solitons. This results in an equal spacing of solitons, and thus leads to an IC superlattice of distortive and magnetic solitons. Their finite width implies a staggered magnetic polarization close to the soliton center.For a spin-Peierls system, the magnetic excitation spectrum in the IC-phase is expected to have finite excitation gaps, because at each given magnetic field the lattice, and hence the intrachain exchange, adapts to the magnetization and the number of solitons. Two excitation branches with polarization perpendicular to H are predicted, ∆ ± , corresponding to an increase or decrease of the total spin by 1, with minima at k = m CuGeO 3 is the first spin-Peierls compound where large single crystals are available [6], thus allowing the complete phonon and magnetic excitation spectra to be measured using neutron scattering techniques. The IC phase is also accessible, since magnetic fields up to 14.5 T have become available for...

Magnetic excitations and phonons in the spin-chain compoundNaCu2O2

“…15,16,17 One-magnon modes of comparable energies have also been observed in Raman scattering experiments on the two-dimensional antiferromagnet La 2 CuO 4 .…”

Diffusion neutronique sous conditions extrêmes

“…Ils montrent l'existence, dans la nouvelle phase (notée "A" sur le diagramme de phase de la Figure 8 Les champs forts jouent aussi un rôle essentiel pour l'étude des excitations dans les systèmes magnétiques à fluctuations quantiques. L'exemple choisi est tiré d'expériences de diffusion inélastique sur monocristal réalisées à HMI sur un spectromètre trois-axes [11]. Le composé CuGeO 3 ) (d'après la référence [11]).…”

“…L'exemple choisi est tiré d'expériences de diffusion inélastique sur monocristal réalisées à HMI sur un spectromètre trois-axes [11]. Le composé CuGeO 3 ) (d'après la référence [11]). …”

Enhancement of Magnetic Order in the Incommensurate Phase of Mg-doped CuGeO3

“…16,17 Excitations above the incommensurate transition have been studied in the pure system and, in contrast to the soft modes in classical incommensurate structures, three gapped modes were clearly observed at high magnetic fields. 18 In this work, we study the effect of impurities on the high-magnetic field incommensurate phase using single crystals of Cu 1−x Mg x GeO 3 with x = 0.02 and 0.04. We find that the solitons formed at high magnetic fields essentially act as impurities breaking the chains and enhancing the magnetic order.…”