Magnetic field effects on spin excitations in the spin-Peierls compound CuGeO3

Neutron inelastic scattering measurements on an undoped single crystal of CuGeO3 and on a doped crystal CuGeo(0.997)Si(0.003)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<less than or equal to>1/2). In the uniform U phase, it is established first that, at finite temperature T not equal0, the low wave vector (q-->0) spinon modes are little affected by the thermal fluctuations deltaE(T)similar or equal toT. 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 an the doped compound establishes that an energy gap occurs in the elementary excitation spectrum of the incommensurate I phase.