Frustration-induced Raman scattering in CuGeO3

Abstract

We present experimental data for the Raman intensity in the spin-Peierls compound CuGeO3 and theoretical calculations from a one-dimensional frustrated spin model. The theory is based on (a) exact diagonalization and (b) a recently developed solitonic mean field theory. We find good agreement between the 1D-theory in the homogeneous phase and evidence for a novel dimerization of the Raman operator in the spin-Peierls state. Finally we present evidence for a coupling between the interchain exchange, the spin-Peierls order parameter and the magnetic excitations along the chains. 78.20.Ls Low-dimensional spin systems exhibit many unusual properties resulting from quantum and dimensionality effects. An example is the continuum of spin-wave excitations in quantum one-dimensional (1D) spin systems which has been predicted for a long time [1] and has recently been confirmed by neutron scattering experiments [2] on KCuF 3 .Quantum 1D spin systems also show a variety of instabilities. Of particular interest is the spin-Peierls (SP) phase due to residual magnetoelastic couplings [3], which leads to the opening of a gap in the spin excitation spectrum. The discovery [4] of the spin-Peierls transition below T SP = 14 K in an inorganic compound, CuGeO 3 , has attracted widespread attention. This compound consists of chains of spin-1/2 Cu 2+ ions coupled by antiferromagnetic superexchange via the oxygen orbitals [4,5]. The Cu ions lie along the crystallographic c-axis and the exchange along the chains can be modeled by the 1D Hamiltonianwhere δ is the dimerization parameter that vanishes above T SP [6,7]. The special geometry [6,8] of the superexchange path in CuGeO 3 leads to a small value of the exchange integral J ≈ 150K and a substantial n.n.n. frustration term ∼ α which competes with the n.n. antiferromagnetic exchange. The interchain couplings have been estimated to be small, J b ≈ 0.1J and J a ≈ −0.01J for the interchain exchange constants along a-and b-directions, respectively [5]. The phase diagram of H in Eq.(1) has been calculated using the density-matrix renormalization-group method [9]. For δ = 0 and α < α c ≈ 0.2411, the ground state is gapless and renormalizes to the Heisenberg fixed point. For α = 0.5 and δ = 0, the ground state is given by a valence-bond state and a gap of order J/2 induced by frustration is present. While the evaluation of the dynamical properties of (1) is a challenge to theory, the rich phase diagram can be explored by a variety of interesting experiments. In this context, the substantial value of the n.n.n. exchange integral in CuGeO 3 allows the experimental investigation of the effects of competing interactions in a low dimensional magnet, both in the uniform and in the spin-Peierls state.An experimental method particularly suited for the study of magnetic excitations in an antiferromagnet is two magnon Raman scattering. For CuGeO 3 , the Raman operator in A 1g symmetry [10]is proportional toIn the homogeneous state (δ = γ = 0) the interaction Hamiltonian commutes with the Heisenberg...

Raman scattering cross section of spin ladders

“…The theory of Ref. 17 reproduced well the features of the spectrum at T > T SP 4 . The application of Raman scattering to probe the singlet excitations of two leg ladders is more recent [7][8][9] .…”

Spins and phonons in the spin-Peierls compound CuGeO3

“…For a one dimensional nn Heisenberg chain one would not expect any scattering at all in this channel, since the exchange Raman operator commutes with the Hamiltonian itself. Muthukumar et al [39] therefore proposed that the scattering is due to the presence of frustration in CuGeO 3 . Under this assumption they indeed found a remarkable agreement between their theory and the experimental results.…”

Charge kinks as Raman scatterers in quarter-filled ladders

“…The other mechanism is two-magnon Raman scattering due to the frustrated superexchange interaction. Although the scattering intensity depends on many details, 24 an estimation within the Fleury-Loudon theory 25 can be done as magn ϳ(J/E 0 ) 2 a ʈ /J, where J is the superexchange and E 0 is of the order of magnitude of the interband transition energy. By taking into account that s lat a ʈ Ϫ1 ϳ⍀, the ratio of intensities can be estimated as …”

“…1 The best known examples of these systems are half-filled SrCu 2 O 3 and Sr 14 Cu 24 O 41 , where the magnetic properties are determined by sϭ1/2 spins at the Cu sites with a superexchange interaction arising from hopping via oxygen orbitals. Ca substitution on the Sr sites introduces mobile holes and leads to superconductivity in Sr 2.5 Ca 11.5 Cu 24 O 41 under pressure. 2 The pairing interaction has a magnetic origin since it is favorable to build up a local spin singlet of two holes if the exchange interaction overcomes the kinetic energy effects.…”