Determining the Short-Range Spin Correlations in the Spin-ChainLi2CuO2andCuGeO3Compounds Using Resonant Inelastic X-Ray Scattering

Abstract

We report a high-resolution resonant inelastic soft x-ray scattering study of the quantum magnetic spin-chain materials Li2CuO2 and CuGeO3. By tuning the incoming photon energy to the oxygen Kedge, a strong excitation around 3.5 eV energy loss is clearly resolved for both materials. Comparing the experimental data to many-body calculations, we identify this excitation as a Zhang-Rice singlet exciton on neighboring CuO4-plaquettes. We demonstrate that the strong temperature dependence of the inelastic scattering related to this high-energy exciton enables to probe short-range spin correlations on the 1 meV scale with outstanding sensitivity.PACS numbers: 78.70. En,71.27.+a,74.72.Cj Two-dimensional cuprate materials play an essential role in condensed matter physics as they show high temperature superconductivity upon charge doping. For better understanding these complex materials, it is important to tackle simpler model systems sharing similar key components and showing reduced complexity, namely one dimensional cuprate chains made out of CuO 4 plaquettes. In this context, Zhang-Rice singlets (ZRS) are fundamental elementary excitations being composite objects generic to hole doped or photon excited strongly correlated charge transfer insulators, and that are especially well-known in the cuprates [1]. However, more than two decades after their theoretical discovery and numerous observations afterwards, there is still significant theoretical and experimental activity aimed at clarifying their complex details e.g. with respect to additional orbitals [2] or specific magnetic correlations beyond their centers [3].Edge-shared cuprate chains represent a particular class of quantum magnets in which the local geometry gives rise to competing nearest ferromagnetic (FM) or antiferromagnetic (AFM) exchange coupling J 1 and frustrating next-nearest neighbor AFM J 2 superexchange couplings. The AFM one-dimensional spin-1/2 J 1 -J 2 Heisenberg model describes such frustrated magnetic interactions, due to which quantum fluctuations can alter both * Deceased ground state and spin correlations [4]. Generalizing this model by varying the signs and ratio of J 1 and J 2 gives rise to a rich phase diagram with ground states spanning FM, AFM, helical, and gapped singlet states. In real materials, the presence of interchain coupling and occasionally coupling to the lattice adds complexity to this behavior, rendering theoretical treatment more difficult [5]. It is therefore desirable to obtain experimental access to nearest neighbor spin correlations both within the ground state probed at very low-temperature and in thermally occupied excited spin-states as a function of temperature (T ) [6]. Both Li 2 CuO 2 and CuGeO 3 realize frustrated edge-shared chain systems that exhibit ground states with completely different intrachain spin correlations. While CuGeO 3 , on the one hand, displays the well-established spin-Peierls phase below T SP = 14 K resulting in a gapped singlet state with pronounced AFM nearest neighbor spin correlations ...

Comment on ‘Oxygen vacancy-induced magnetic moment in edge-sharing CuO2 chains of Li2CuO2’

“…To summarize, the frustrated model for Li 2 CuO 2 , refined by the INS data [6,16,17] (see the last row in table 1), our DFT and five-band pd-Hubbard model calculations [6], are consistent with the experimental data, including the χ (T) [6], the field dependent magnetization, the saturation field value [27], and the T-dependent RIXS spectra [7]. This is in contrast to the SLT-model, which does not account for these data.…”

“…As already noted, the approximate expression equation (3) is relevant for a quasi-1D system, where the inchain J's dominates. This is true also for the condition (7). Let us establish now a general condition for the applicability of a CW-law.…”

“…Thus, Shu et al's J value is at odds with the high-field data reported in 2011 [27]. Shu et al have also ignored the T-dependence of the RIXS (resonant inelastic x-ray spectroscopy) spectra of Li 2 CuO 2 reported in 2013 [7], which were explained within a pd-model consistent with the INS data. The detection of an intrachain Zhang-Rice singlet exciton at T>T N and significant increase of its weight at 300K 16 The corresponding HTE package used in [25] we have employed obtaining the results shown here, too.…”

“…Li 2 CuO 2 is well studied in both experiment and theory (see e.g. [2][3][4][5][6][7][8][9][10][11]) and serves nowadays as a reference system for more complex and structurally less ideal systems. In particular, it is accepted in the quantum magnetism community that the leading FM coupling is the NN inchain coupling J 1 .…”

Orbital superexchange and crystal field simultaneously at play in YVO3: Resonant inelastic x-ray scattering at the VLedge and the OKedge

“…It is known to be sensitive to both orbital (or d-d) excitations and to intersite charge-transfer excitations. 40,44,[75][76][77][78][79][80] To the best of our knowledge, this is the first report on the observation of Mott-Hubbard excitations in RIXS at the O K edge. The differences between the RIXS data from the O K edge and from the V L 3 edge originate from the different microscopic excitation processes.…”