Magneto-optic far-infrared study of Sr$_{14}$Cu$_{24}$O$_{41}$: triplet excitations in chains

TL;DR

This study uses far-infrared spectroscopy to investigate magnetic excitations in Sr$_{14}$Cu$_{24}$O$_{41}$, revealing two triplet modes with specific polarization and magnetic field dependencies, and suggesting modifications to existing magnetic excitation models.

Contribution

It identifies a previously unobserved triplet mode at 9.65 meV in the center of the Brillouin zone, challenging existing models of magnetic excitations in Sr$_{14}$Cu$_{24}$O$_{41}.

Findings

Two triplet modes at 9.65 meV and 10.86 meV were observed.

The modes' energies and intensities agree with neutron scattering results.

A new triplet mode at the Brillouin zone center requires model revision.

Abstract

Using far-infrared spectroscopy we have studied the magnetic field and temperature dependence of the spin gap modes in the chains of Sr$_{14}$Cu$_{24}$O$_{41}$. Two triplet modes T$_1$ and T$_2$ were found in the center of the Brillouin zone at $\Delta_1=9.65$ meV and $\Delta_2=10.86$ meV in zero magnetic field. The T$_1$ mode was excited when the electric field vector ${\bf E}$ of the light was polarized along the b axis (perpendicular to the planes of chains and ladders) and T$_2$ was excited for ${\bf E}\parallel {\bf a}$ (perpendicular to the chains and along the rungs). Up to the maximum magnetic field of 18 T, applied along the chains, the electron $g$ factors of these two modes were similar, $g_{1c}=2.049$ and $g_{2c}=2.044$. Full linewidth at half maximum for both modes was 1 cm$^{-1}$ (0.12 meV) at 4K and increased with $T$. The temperature dependence of mode energies and line intensities was in agreement with the inelastic neutron scattering results from two groups [Matsuda {\it et al.}, Phys. Rev. B {\bf 59}, 1060 (1999) and Regnault {\it et al.}, Phys. Rev. B {\bf 59}, 1055 (1999)]. The T$_1$ mode has not been observed by inelastic neutron scattering in the points of the $k$-space equivalent to the center of the Brillouin zone. Our study indicates that the zone structure model of magnetic excitations of Sr$_{14}$Cu$_{24}$O$_{41}$ must be modified to include a triplet mode at 9.65 meV in the center of the magnetic Brillouin zone.