Low-energy magnetic excitations in the dimerized and incommensurate phase of CuGeO3

Loa, I.; Gronemeyer, S.; Thomsen, C.; Kremer, Reinhard K.

doi:10.1016/s0038-1098(99)00196-9

Cited by 6 publications

(9 citation statements)

References 33 publications

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“…3. The same gap energy (2.3±0.2 cm −1 ≈ 0.29 meV) was observed in a Raman experiment [18], which mainly probes the structural excitations, pointing to a magneto-structural origin for this excitation. We attribute the mode at 0.29 meV to phase oscillations of the soliton lattice, so-called phasons.…”

Section: + δKsp)supporting

confidence: 76%

Excitations of the Field-Induced Quantum Soliton Lattice inCuGeO3

Enderle

Rønnow²,

McMorrow³

et al. 2001

Phys. Rev. Lett.

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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...

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Section: + δKsp)supporting

confidence: 76%

Excitations of the Field-Induced Quantum Soliton Lattice inCuGeO3

Enderle

Rønnow²,

McMorrow³

et al. 2001

Phys. Rev. Lett.

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“…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 .…”

mentioning

confidence: 62%

Magnetic excitations and phonons in the spin-chain compoundNaCu2O2

et al. 2006

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We report an inelastic light scattering study of single-crystalline NaCu2O2, a spin-chain compound known to exhibit a phase with helical magnetic order at low temperatures. Phonon excitations were studied as a function of temperature and light polarization, and the phonon frequencies are compared to the results of ab-initio lattice dynamical calculations, which are also reported here. The good agreement between the observed and calculated modes allows an assignment of the phonon eigenvectors. Two distinct high-energy two-magnon features as well as a sharp low-energy one-magnon peak were also observed. These features are discussed in terms of the magnon modes expected in a helically ordered state. Their polarization dependence provides evidence of substantial exchange interactions between two closely spaced spin chains within a unit cell. At high temperatures, the spectral features attributable to magnetic excitations are replaced by a broad, quasielastic mode due to overdamped spin excitations.

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“…30 He and his coworkers also studied Raman scattering of pure CuGeO 3 under high magnetic fields and reported that the spin-gap mode was activated in the IC phase, while it is Raman inactive in the SP phase. 29,30,33 Moreover, the two-magnetic-excitation bound state gradually disappeared as the magnetic field was increased in the IC phase.…”

Section: Magnetic-field Effectsmentioning

confidence: 99%

“…29,30,33 Moreover, the two-magnetic-excitation bound state gradually disappeared as the magnetic field was increased in the IC phase.…”

Section: Introductionmentioning

confidence: 99%

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Raman scattering from the spin-gap mode in the Ni-doped spin-Peierls compoundCuGeO3

et al. 2008

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By means of Raman scattering we have studied the spin-Peierls ͑SP͒ transition in Ni-doped CuGeO 3 crystals. The folded-phonon peak extremely broadens and the two-magnetic-excitation mode disappears when the Ni concentration is above about 2.0%, indicating that the SP phase collapses. The spin-gap mode is activated in lightly Ni-doped samples. This peak neither splits nor shifts under magnetic fields in the SP phase, but its frequency increases in the incommensurate phase. The activation of the spin-gap mode is interpreted in terms of a mixing between the spin-singlet ground state and the S z = 0 triplet excited one by staggered fields around the doped impurities. Meanwhile, the two-magnetic-excitation mode gradually diminishes in intensity with increasing magnetic field in the incommensurate phase.

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Low-energy magnetic excitations in the dimerized and incommensurate phase of CuGeO3

Cited by 6 publications

References 33 publications

Excitations of the Field-Induced Quantum Soliton Lattice inCuGeO3

Excitations of the Field-Induced Quantum Soliton Lattice inCuGeO3

Magnetic excitations and phonons in the spin-chain compoundNaCu2O2

Raman scattering from the spin-gap mode in the Ni-doped spin-Peierls compoundCuGeO3

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