Magnons in tetragonal CuO

Abstract

We investigate by resonant inelastic x-ray scattering the magnetic excitations in thin films of tetragonal CuO. We identify a spin wave excitation, dispersing on two cupratelike antiferromagnetic sublattices. Its energy at the boundary of the Brillouin zone (220 meV), is significantly lower than typical values (E ∼ 300 meV) found in two-dimensional cuprates. A spin wave expansion starting from an extended Hubbard model suggests two possible scenarios for this energy lowering. DOI: 10.1103/PhysRevB.92.140404 PACS number(s): 74.72.Cj, 74.25.Jb, 75.30.Ds, 78.70.Ck Magnetism in the undoped, insulating quasi-twodimensional (2D) cuprates is determined by the 180• Cu-O-Cu bonds of their corner-sharing copper oxide layers [ Fig. 1(a) [3]. Phenomenologically, these systems can be described by extended spin-1/2 Heisenberg models with antiferromagnetic coupling on a square lattice [4,5], in agreement with secondorder superexchange theory. However, to make contact with other spectroscopies one needs to adopt a more microscopic description in terms of an extended Hubbard model. To be quantitative, it is necessary to go beyond second-order perturbation theory and to consider the effective spin model generated by the one-band Hubbard model up to fourth order in hopping. Such a procedure renormalizes the two-spin interactions and also leads to four-spin interactions. This approach yields more realistic values for the on-site Coulomb repulsion U , is consistent with higher-energy resonant inelastic x-ray scattering (RIXS) and angle-resolved photoemission spectroscopy (ARPES) data, and provides good fits to the experimentally observed spin excitation spectra [6,7].A recent exciting development was the discovery of a new tetragonal form of the simple binary oxide CuO, with an alternative structure to that of the cuprates. Tetragonal CuO (T-CuO) can be grown epitaxially on a SrTiO 3 (001) substrate up to a thickness of several unit cells [8][9][10]. In this material CuO 6 octahedra give rise to infinite CuO layers, consisting of edge-sharing CuO 4 plaquettes, stacked along the c axis. At variance with the CuO 2 cuprate layers, oxygen ions do not bridge nearest-neighbor (NN) but next-NN copper ions. Electronically, the edge-sharing structure is well described by two interpenetrating corner-sharing sublattices [ Fig. 1(b) • Cu-O-Cu bonds, the coupling could be either ferromagnetic or antiferromagnetic but, in any case, it introduces frustration. In the classical limit, the system would still develop independent AFM Heisenberg order on each sublattice. However, if quantum fluctuations are taken into account, J d is expected to lock the relative orientation of the two sublattices and to break the fourfold rotational C 4 symmetry, leading to an additional Ising order parameter [12][13][14].To examine these speculations, we have studied the magnetic excitation spectrum of T-CuO. Since inelastic neutron scattering is inadequate due to the limited film thickness, we employ RIXS at the Cu L 3 (Cu 2p 3/2 → 3d) edge [15]. We find a magnon...

Magnetic order of tetragonal CuO ultrathin films

“…Although our simplistic model explains all the phenomena observed in our study, an AFM ordering of FM planes along the (001) direction does not agree with the theoretical prediction in Ref. [8]. The magnetic exchange interaction between the Cu 2+ ions avoids having ferromagnetic single layers as the orange layer sketched in our model (Fig.…”

“…We calculated T z using the Crispy software [25] that is based on the Quanty code [27]. The crystal field parameters were taken from Moser et al [8] with 10Dq = 1.5 eV, Ds= 0.25 eV and Dt= 0.13 eV. The calculated X-ray Linear Dichroism (XLD) spectra is in a good agreement with our experimental XLD data (Figure 4).…”

“…Experimentally, resonant inelastic x-ray scattering (RIXS) measurements reported spin-wave excitations dispersing through two cupratelike antiferromagnetic (AFM) sublattices. Two possible scenarios for the magnetic structure have been proposed with AFM orderings along the film plane [8]. However, as of yet there is no direct experimental confirmation of a magnetic order in T-CuO and its possible spin structure, since magnetic excitations away from the zone center have also been found in superconducting cuprates by RIXS at doping levels far from the antiferromagnetically ordered state [14].…”

“…The electronic structure of T-CuO is determined by the properties of its 2D edge-sharing lattice planes, each conceptually built from two interpenetrating corner-sharing sub-lattices, with lattice parameters typical of HTSCs and 180° Cu-O-Cu bonds. A small coupling between these frustrated sub-lattices gives rise to slight quasiparticle renormalization [8]. In addition, T-CuO extends the simple rock salt transition metal oxides series from FeO, CoO, NiO to CuO and has been predicted to have a very high magnetic ordering temperature, with Néel temperature (T N ) close to 800 K [9]- [10].…”

Magnetic excitations from the two-dimensional interpenetrating Cu framework in Ba2Cu3O4Cl2

“…Along the antiferromagnetic zone-boundary there exists: (i) an anomalous dispersion of spin-waves and (ii) a wavevector-dependent continuum that results in a redistribution of the spectral weight. The former has been attributed to quantum corrections to linear spin-wave theory, second neighbor exchange interactions, or four-spin interactions [1][2][3][4][5][6][7][8][9][10][11][12]. The latter * peter.babkevich@gmail.com phenomena has recently been proposed to originate from spinon deconfinement [13] or strong attractive magnonmagnon interaction, leading to two-magnon-bound states as well as enhanced multi-magnon continua [14].…”