Abstract
We report inelastic neutron scattering experiments in Ca2Y2Cu5O10 and map out the full one-magnon dispersion which extends up to a record value of 53 meV for frustrated ferromagnetic (FM) edge-sharing CuO2 chain (FFESC) cuprates. A homogeneous spin-1/2 chain model with a FM nearest-neighbor (NN), an antiferromagnetic (AFM) next-nearest-neighbor (NNN) inchain, and two diagonal AFM interchain couplings (ICs) analyzed within linear spin-wave theory (LSWT) reproduces well the observed strong dispersion along the chains and a weak one perpendicularly. The ratio α=|Ja2/Ja1| of the FM NN and the AFM NNN couplings is found as ∼0.23, close to the critical point αc=1/4 which separates ferromagnetically and antiferromagnetically correlated spiral magnetic ground states in single chains, whereas αc>0.25 for coupled chains is considerably upshifted even for relatively weak IC. Although the measured dispersion can be described by homogeneous LSWT, the scattering intensity appears to be considerably reduced at ∼11.5 and ∼28meV. The gaplike feature at 11.5 meV is attributed to magnon-phonon coupling whereas based on density matrix renormalization group simulations of the dynamical structure factor the gap at 28 meV is considered to stem partly from quantum effects due to the AFM IC. Another contribution to that gap is ascribed to the intrinsic superstructure from the distorting incommensurate pattern of CaY cationic chains adjacent to the CuO2 ones. It gives rise to nonequivalent CuO4 units and Cu-O-Cu bond angles φ and a resulting distribution of all exchange integrals. The J's fitted by homogeneous LSWT are regarded as average values. The record value of the FM NN integral J1=24meV among FFESC cuprates can be explained by a nonuniversal φ(≠90°) and Cu-O bond length dependent anisotropic mean direct FM Cu-O exchange K̄pd∼120meV, similar to a value of 105 meV for Li2CuO2, in accord with larger values for La2CuO4 and CuGeO3 (∼110meV) reported by Braden et al. [Phys. Rev. B 54, 1105 (1996)PRBMDO0163-182910.1103/PhysRevB.54.1105] phenomenologically. Enhanced Kpd values are also needed to compensate a significant AFM Jdd≥ 6 meV from the dd channel, generic for FFESC cuprates but ignored so far.
| Original language | English |
|---|---|
| Article number | 104415 |
| Journal | Physical Review B |
| Volume | 100 |
| Issue number | 10 |
| DOIs | |
| State | Published - Sep 12 2019 |
Funding
This work has been partially supported by U.S. Department of Energy (DOE) Grant No. DE-FG02-13ER41967. ORNL is managed by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 for the DOE. The United States Government retains, and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. We used resources at the High Flux Isotope Reactor and Spallation Neutron Source, DOE Office of Science User Facilities, operated by the Oak Ridge National Laboratory. Support by the SFB 1143 of the Deutsche Forschungsgemeinschaft is acknowledged (S.N.). We thank U. Nitzsche for technical assistance and S. Johnston, D. Miloslavlevic, A. Tsirlin, O. Janson, J. Málek, R. Klingeler, W. E. A. Lorenz, T. Schmitt, C. Monney, J. van den Brink, U. Rößler, A. S. Moskvin, D. Khomskii, A. M. Oles, G. Sawatzky, K. Wohlfeld, A. Yaresko, S. Lebernegg, G. Roth, and J. Thar for discussions. Special thanks to G.R. and J.T. for providing figures of the CuO 2 chains in CYCO according to their structure model as well as to D.M., A.T., O.J., S.L., and U.N. for providing unpublished theoretical results included partly in Table I of the Supplemental Material [7] . We also thank J. Richter and A. Hauser for help with their HTE package code used here.