Measurement of Magnetic Excitations in the Two-Dimensional AntiferromagneticSr2CuO2Cl2Insulator Using Resonant X-Ray Scattering: Evidence for Extended Interactions

Abstract

We measured the momentum dependence of magnetic excitations in the model spin-1=2 2D antiferromagnetic insulator Sr 2 CuO 2 Cl 2 (SCOC). We identify a single-spin-wave feature and a multimagnon continuum, with different polarization dependences. The spin waves display a large (70 meV) dispersion between the zone-boundary points (, 0) and (=2, =2). Employing an extended t-t 0 -t 00 -U one-band Hubbard model, we find significant electronic hopping beyond nearest-neighbor Cu ions, indicative of extended magnetic interactions. The spectral line shape at (, 0) indicates sizable quantum effects in SCOC and probably more generally in the cuprates. DOI: 10.1103/PhysRevLett.105.157006 PACS numbers: 74.72.Cj, 75.30.Ds, 78.70.Ck Magnetism in low-dimensional cuprates remains of considerable interest, in relation both to the fundamental quest to understand strong electron correlation and quantum spin effects in Mott insulators, and to the search for the mechanism of high-T c superconductivity. To lowest order, the undoped cuprate superconductors can be described by the spin 1=2 two-dimensional (2D) square-lattice nearestneighbor (NN) Heisenberg antiferromagnet, which is among the simplest and most studied models in magnetism [1]. The ground state displays classical order, reduced by quantum fluctuations at zero temperature and destroyed by thermal fluctuations at finite temperature. A possible crossover between renormalized classical [2] and quantum critical [3] scaling was tested experimentally in the undoped cuprates Sr 2 CuO 2 Cl 2 (SCOC) [4] and La 2 CuO 4 (LCO) [5], and in the organometallic salt CuðDCOOÞ 2 Á 4D 2 O (CFTD) [6]. However, while the latter shows only nearest-neighbor coupling, high-energy inelastic neutron scattering (INS) data on LCO [7] suggest that further-neighbor magnetic interactions influence the scaling measurements. Frustrated further-neighbor interactions could also bring the undoped cuprates closer to the valence bond liquid proposed as mechanism for superconductivity [8].It is therefore timely to investigate the excitation spectrum of SCOC, as an important model system. Inelastic neutron scattering (INS) measurements of SCOC have been limited to low energies and small momenta around the ordering wave vector [4]. In this Letter we report the full magnetic excitation spectrum measured by resonant inelastic x-ray scattering (RIXS). We discover a surprisingly large dispersion along the magnetic Brillouin zone boundary (ZB). An analysis of the data in terms of an extended Hubbard model yields a quantitative estimation of sizable further-neighbor electronic hopping. The resulting series of longer-ranged magnetic interactions enhance quantum fluctuations, in agreement with the reduced ordered moment. The importance of quantum fluctuations is further revealed by differences in the spectral line shapes at the (À, 0) and (À=2, =2) ZB points.SCOC is an insulating single-layer parent compound of the high-T c superconducting (SC) materials. It is isostructural to the high-temperature tetragonal phase of...

Anisotropic softening of magnetic excitations along the nodal direction in superconducting cuprates

“…Figure 1c,d shows spectra of insulating BSYCO, measured along the (100) and (110) high-symmetry directions. The line shapes are similar to those of other insulating cuprates, namely, La 2 CuO 4 (LCO) 24 and Sr 2 CuO 2 Cl 2 (SCOC) 25 . One can identify the elastic peak, broadened by non-resolved phonon sidebands, and a sharp magnon peak, dispersing from zero energy at q ¼ 0 to B0.3 eV at the boundary of the magnetic Brillouin zone (BZ).…”

“…Alternatively, the spectra along the (100) direction could be reasonably well described by a suitably broadened magnon peak plus a multi-magnon tail, as in ref. 25, but such an approach would also fail along the nodal direction. Such discrepancy could not be appreciated from the previously published RIXS data.…”

“…1 using the same analysis in ref. 25. The RIXS intensity follows the spin-wave dispersion of the insulator along the (100) direction.…”

“…An independent picture of magnetic and charge excitations has now been gained by resonant inelastic X-ray scattering (RIXS) experiments at the Cu K edge 22 and Cu L 3 edge [23][24][25][26][27][28] in the cuprates and at the Fe L 3 edge in the iron pnictides 29 . RIXS offers some interesting advantages, such as the possibility of measuring small-volume samples, additional flexibility from polarization control and complementary information from the Cu (single magnons) and O (two magnons) edges 30 .…”

Spin and charge excitations in artificial hole- and electron-doped infinite layer cuprate superconductors

“…This class of experiments is particularly valuable because it probes the set of collective and local excitations that are supposedly involved with the basic mechanisms of high temperature superconductivity. In particular, high resolution RIXS at Cu−L 3 edge has been used to measure the magnon dispersion in undoped insulating cuprates [3][4][5][6] , and damped spin excitations (paramagnons) in hole-doped (h-doped) cuprates. It has thus been shown that paramagnons persist throughout the phase diagram, from superconducting underand optimally-doped [7][8][9] to non-superconducting highly-overdoped 10 samples.…”