Physica B 329–333 (2003) 675–678 The field-induced soliton phase of CuGeO 3 Henrik M. R^nnow a, *, Mechthild Enderle b , Desmond F. McMorrow c , Louis-Pierre Regnault a a MDN/SPSMS/DRFMC/CEA-Grenoble, 38054 Grenoble, France b Institut Laue Langevin, 38042 Grenoble, France c Ris^ National Laboratory, 4000 Roskilde, Denmark Abstract The quasi-1D S ¼ 1=2 antiferromagnet CuGeO 3 undergoes a spin–Peierls transition to a dimerised singlet quantum ground state, with S ¼ 1 carrying pairs of domain walls as elementary excitations. Applying a large magnetic field, the domain walls—solitons—can be condensed into the ground state, forming a static incommensurate soliton lattice, which has been investigated by neutron scattering. The static soliton structure was found to be in good accordance to field theoretical and numerical calculations, while a low energy incommensurate mode is interpreted as a gapped phason. We anticipate that theoretical description of this and higher energy excitations will provide general insight to the effect of structural fluctuations on correlated electron systems. r 2003 Elsevier Science B.V. All rights reserved. Keywords: Soliton; One-dimensional; Spin–Peierls; Magneto–elastic Thereisacontinuinginterestinmany-particlesystems governed by quantum mechanical effects, such as low- dimensional, low-spin quantum magnets. In recent years, additional efforts have been devoted to extending such model systems by introducing e.g. holes or orbital degrees of freedom or by coupling to a deformable lattice. That such small perturbations can have drastic effects is evidenced in the 1D quantum ðS ¼ 1=2Þ Heisenberg antiferromagnet, whose ground state is disordered due to quantum fluctuations. Even weak coupling to a 3D phonon field can cause a so-called spin–Peierls (sP) transition where adjacent spins pair up to form a macroscopic dimerised singlet ground state [1]. The energy cost of the concomitant lattice distortion is compensated for by lowering the zero point energy as quantum fluctuations are suppressed. The sP state is gapped,asittakesafiniteenergytobreakadimerintoa triplet excitation which can be seen as a domain wall pair with respect to the dimer order. Such domain walls are called solitons. The detailed understanding of this phenomenon experienced a break through by the discovery of a sP transition in CuGeO 3 [2] (see Fig.1), which can be synthesised in single crystals sufficiently large to apply many experimental methods—including neutron scatter- ing, which holds a particular position, as it allows to measure static and time-dependent magnetic and struc- tural correlation functions. Hence, neutron scattering has provided amongst other results the form of the sP distortion [3], the behaviour of the phonons through the transition [4], and the magnetic excitation spectrum [5]. Contrary to the original belief that the sP transition shouldarisethroughgradualslowingdownandeventual freezing of a phonon, comparing the static distortion pattern to a complete phonon branch determination led to the insight that the distortion can arrive in a central peaktransitionwithonlyweakphononrenormalisation. Similarly, magnetic neutron inelastic scattering showed that the soliton–anti-soliton pair of an excited triplet forms a bound state, with a second energy gap to a continuum of free soliton pairs [6]. In addition to these *Corresponding author. E-mail address: hmr@research.nj.nec.com (H.M. R^nnow). 0921-4526/03/$-see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4526(02)02628-5