Magnetic bound states in the quarter-filled ladder systemα′−NaV2O5

Abstract

Raman scattering in the quarter-filled spin ladder system α ′ -NaV2O5 shows in the dimerized singlet ground state (T≤TSP =35K) an unexpected sequence of three magnetic bound states. Our results suggest that the recently proposed mapping onto an effective spin chain for T>TSP has to be given up in favor of the full topology and exchange paths of a ladder in the dimerized phase for T < TSP. As the new ground state we propose a dynamic superposition of energetically nearly degenerate dimer configurations on the ladder.Low-dimensional quantum spin systems like spin chains, ladders or plaquettes received considerable attention from both theoretical and experimental points of view due to their manifold of unconventional spin excitation spectra. Of particular interest is the so-called spin-Peierls transition. The degeneracy of the ground state of an isotropic one-dimensional (1d) spin-1/2 system is lifted due to the coupling to the lattice, leading to a dimerized singlet ground state and the opening of a singlet-triplet gap.The magnetic excitation spectrum of a dimerized spin chain consists of a triplet branch at ∆ 01 , a corresponding two-particle continuum of triplet excitations starting at 2∆ 01 , and well defined magnetic bound states 1-3 . The latter consist of strongly interacting triplet excitations with a high energy cutoff at 2∆ 01 . The existence of a singlet bound state at √ 3∆ 01 is predicted by a 1d model with frustrated next-nearest neighbor interaction J 2 = 0.24J 1 , acting as a binding potential. A further triplet bound state should appear for higher values of J 2 2 . A study of magnetic bound states in quantum spin systems therefore gives valuable insight into the low energy spin excitations which govern the physics of these systems.Half-filled spin ladder systems attracted enormous interest recently due to the surprising changes of the ground state and excitation spectrum interpolating between one-and two-dimensional quantum spin systems 4 . For even-leg ladders the doping by holes is found to lead to superconductivity. On the other hand, for the quarterfilled ladder system as the corresponding diluted system the ground state and spin excitation spectrum are not well understood until now. However it is expected that they show a similar rich excitation scheme as the halffilled system 5 . The inorganic compound α ′ -NaV 2 O 5 with double chains of edge-sharing distorted tetragonal VO 5 and a spin-Peierls like transition at T SP =35K 6,7 has been interpreted as a quarter-filled spin ladder system 8 . Recent X-ray diffraction data at room temperature are in favor of a centrosymmetric (D 13 2h ) structure with only one type of V site of average valence +4.5. The spins are therefore attached to a V-O-V molecular orbital on the rungs 8-10 . The exchange interaction across the rung is by a factor of five larger compared to that along the legs 8 . Hence the ladder can be described by an effective spin chain.

Experiments on ladders reveal a complex interplay between a spin-gapped normal state and superconductivity

“…Since this compound and CaV 2 O 5 are isostructural, it was also argued that the latter should also be regarded as a (half-filled) ladder system instead of only having isolated dimers (see discussion in previous paragraph). Very recent results by Lemmens et al (1998) using Raman scattering applied to NaV 2 O 5 showed an interesting sequence of magnetic bound states at low temperatures. The importance of doping NaV 2 O 5 away from quarterfilling was remarked by Smolinski et al (1998).…”

Lattice vibrations ofα′−NaV2O5Popova1, Sushkov2, Климин3 et al. 2002Phys. Rev. B215120View full textAdd to dashboardCite

“…Observed FIR doublets and close frequencies in different polarizations are naturally explained by their origin from the Q-point modes folded into the zone center. Prior to analyse the temperature dependences of frequencies and widths of new modes and to discuss alternative interpretations, we note that published FIR [20,23,26,5,32] and Raman [9,21,27,28,19,39,29] data agree with each other, within the precision of measurements (typically, ±0.5 cm −1 for FIR and ±1.5 cm −1 for Raman measurements).…”

“…[28] communicated that the 67 cm −1 Raman line neither shifts nor splits nor even broadens in a magnetic field up to 7 T. They have attributed this line and the 107 and 134 cm −1 Raman modes to magnetic singlet bound states, on the basis of (i) the temperature dependence of the intensity, frequency shift, and linewidth, and (ii) the frequency dependence on a sample quality. All the three modes rapidly loose their intensity, soften and broaden on approaching T c from below [27,28]. It is worth mentioning that very recently two more Raman modes with similar properties have been reported (namely, 86 and 126 cm −1 modes in (ab) geometry) [29].…”

Spin-gap behavior and charge ordering inα′−NaV2O5Fischer1, Lemmens2, Els3 et al. 1999Phys. Rev. BSelf Cite448480View full textAdd to dashboardCite

“…3), and perpendicular to them (aa) as well as in crossed polarization (ab) 33 . A survey of the frequencies of the additional modes is shown in Table 2.…”