BaCo2V2O8 is a remarkable example of a quasi-1D Ising-like antiferromagnet, that can be described, in its gapless phase induced by a longitudinal magnetic field, in terms of Tomonaga-Luttinger liquid physics . It consists of Co2+ effective spin-1/2 screw chains running along the Ising c axis. The quasi-1D and Ising-like characters of this system yields very exotic static and dynamical properties.
At zero field, a Néel antiferromagnetic ordering occurs below TN=5.4 K. At very low temperature, the application of a longitudinal magnetic field (H∥c) induces a quantum phase transition at Hc=3.9 T, where the energy gap closes. In a usual Heisenberg antiferromagnet, this would cause the magnetic moments to flip perpendicularly to the field. However, as BaCo2V2O8 is of the Ising-like type, the incommensurate (IC) longitudinal correlations are first expected to dominate the transverse ones above Hc, before an inversion occurs above H*, yielding the establishment of a transverse staggered ordering. Concerning the zero-field magnetic excitations, they consist in a gapless continuum of transverse spinons in a Heisenberg 1D system. Nevertheless, in BaCo2V2O8, these excitations are predicted to be gapped, because of the Ising-like character, and to be discretized, because of the spinon confinement caused by the interchain attractive linear potential.
I will first present a complete exploration of the magnetic field-temperature H-T phase diagram of BaCo2V2O8, up to H=12 T and down to T=50 mK, by single-crystal neutron diffraction [2,3]. Our phase diagram, together with the magnetic structures determined in the three low temperature magnetic structures (below Hc, between Hc and H*, and above H*) will be discussed with respect to NMR results and to the theoretical predictions.
I will then present our inelastic neutron scattering study in the Néel phase of BaCo2V2O8 [4,5]. This study does reveal the expected unconventional discrete spin excitations, so called Zeeman ladders. But, in addition to the transverse ones, a series of longitudinal modes, interlaced to the first one, was also observed. These results will be discussed in the light of various theoretical works.
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