Spin-waves in the mid-infrared spectrum of antiferromagnetic YBa ₂ Cu ₃ O _6.0

Abstract

The mid-infrared spin-wave spectrum of antiferromagnetic YBa 2 Cu 3 O 6.0 was determined by infrared transmission and reflection measurements (k c) at T = 10 K. Excitation of single magnons of the optical branch was observed at E op = 178.0 meV. Two further peaks at 346 meV (≈ 1.94 E op ) and 470 meV (≈ 2.6 E op ) both belong to the two-magnon spectrum. Linear spin wave theory is in good agreement with the measured two-magnon spectrum, and allows to determine the exchange constant J to be about 120 meV, whereas the intrabilayer coupling J 12 is approximately 0.55 J. 74.25.Gz, 74.25.Ha, 75.30.Ds High temperature superconductors are basically layered copper-oxide materials. It is widely accepted that the relevant electronic degrees of freedom are confined to copper-oxide planes. The number of CuO 2 planes per unit cell varies: e.g., La 2−x Sr x CuO 4 exists in a single plane form with a large spacing between planes of ≈ 13.2Å, and YBa 2 Cu 3 O 6+x has a double layer structure with intra-and interbilayer spacings of ≈ 3.3Å and 8.5Å, respectively. Electronic correlations, and hence spin dynamics [1], may depend on the type of stacking of the planes. More specifically, a sizable coupling J 12 between spins on adjacent planes of a bilayer will influence the spin excitation spectrum as well as the nature of the ground state. This may have been seen already in doped compounds: the normal state spin susceptibility of La 2−x Sr x CuO 4 extrapolates to a finite value at zero temperature, whereas it extrapolates to zero for YBa 2 Cu 3 O 6.6 [2]. This may be interpreted as a signature for the opening of a spin excitation gap in YBa 2 Cu 3 O 6.6 at low temperatures [3]-a behavior certainly not encountered in Fermi liquids. Further, a spin density wave ordering for La 2−x Sr x CuO 4 has been proposed, but for YBa 2 Cu 3 O 6.6 a singlet pairing of spins in adjacent CuO 2 planes with strong antiferromagnetic fluctuations within a plane [4,2,5]. Such a scenario seems to require an unrealistically large J 12 2.5J [6], where J is the in-plane exchange coupling of the Heisenberg Hamiltonian supposed to describe the low energy spin dynamics of a single bilayer for zero doping (x = 0). However it was argued that, for finite doping, the itinerant carriers destroy the antiferromagnetism of the insulating phase and, therefore, much smaller values of J 12 will produce a singlet interplane pairing in the conducting phase of YBa 2 Cu 3 O 6.6 . 74Up to now, no experimental evidence has been given of a sizable bilayer coupling (J 12 ∼ J). In neutron-scattering experiments on YBa 2 Cu 3 O 6+x , the in-plane coupling was determined from the dispersion of acoustic spin-waves and was found to be extremely large (J = 120 ± 20 meV [7], J = 150 meV [8], both for x = 0.15). Yet, no optical modes have been found for energies up to 60 meV [7,9], suggesting a bilayer coupling of J 12 8 meV. In Raman-1

Dynamic susceptibility and photoemission in thet-t’-Jmodel

“…The calculated result that interplane contributions are stronger than intraplane ones over most of the energy range (Fig. 7) arises primarily from their smaller enhancing denominator (36), and is consistent with the results of experiment in YBCO [40]. In the absence of coherent interplane hopping [25], a different explanation of this modulation would be required.…”

“…We note, however, that recent infrared transmission and reflectometry measurements [36] suggest a considerably larger value of this parameter in the insulating YBCO system, and so we do not discount this possibility.…”

“…6(a) is shown the same quantity for q z = π (in "bilayer units", r c = 1), so that only bonding to antibonding band scattering processes are sampled. Here the scattered intensity is considerably higher, indicating a greater enhancement from the relevant denominator in (36), and is largely commensurate as a result of poor nesting between the two bands. In Fig.…”

Conductivity in Cuprates Arises from Two Different Sources: One-Electron Exchange and Disproportionation

“…Perkins et al [52] and Grüninger et al [53,54] have studied the MIR absorption at about 0.4 eV, equivalent to about 3000 cm −1 . Remarkably enough, the Raman and MIR absorptions are at the same energy.…”

Magnon–phonon coupling and implications for charge-density wave states and superconductivity in cuprates

“…The first sideband, centered near 4000 cm -1 (~0.5 eV), dominates their spectral weight. These bands were shown to be intrinsic excitations of the CuO 2 layers, and have since been observed in bilayer cuprates [29]. To our knowledge, only cuprates are known to display this full set of excitations.…”