Magnetic field effects on spin excitations in the spin-Peierls compoundCuGeO3

Abstract

Neutron inelastic scattering measurements on an undoped single crystal of ${\mathrm{CuGeO}}_{3}$ and on a doped crystal ${\mathrm{CuGe}}_{0.997}{\mathrm{Si}}_{0.003}{\mathrm{O}}_{3}$ are performed in the presence of a magnetic field H up to 12 T. In the present work, a particular attention is given to the effect of H on the low-energy elementary excitations. This investigation is performed in the three different phases of the spin-Peierls system. In the dimerized D phase, the Zeeman splitting of the acoustic magnon branch is evaluated all along the dispersion branch $(0<q<~1/2).$ In the uniform U phase, it is established first that, at finite temperature $T\ensuremath{\ne}0,$ the low wave vector $(\stackrel{\ensuremath{\rightarrow}}{q}0)$ spinon modes are little affected by the thermal fluctuations $\ensuremath{\delta}{E}_{T}\ensuremath{\simeq}T.$ Second, the effect of H on the same low-$q$ mode results essentially in a transfer of the fluctuations towards the Zeeman-shifted lower boundary of the spinon continuum. Finally, our measurements on the doped compound establishes that an energy gap occurs in the elementary excitation spectrum of the incommensurate I phase.