We present a 63,65Cu and 29Si NMR study of the quasi-2D coupled spin 1/2 dimer compound BaCuSi2O6 in the magnetic field range 13-26 T and at temperatures as low as 50 mK. NMR data in the gapped phase reveal that below 90 K different intra-dimer exchange couplings and different gaps (ΔB/ΔA = 1.16) exist in every second plane along the c-axis, in addition to a planar incommensurate (IC) modulation. 29Si spectra in the field induced magnetic ordered phase reveal that close to the quantum critical point at Hc1 = 23.35 T the average boson density nˉ of the Bose-Einstein condensate is strongly modulated along the c-axis with a density ratio for every second plane nˉA/nˉB≃5. An IC modulation of the local density is also present in each plane. This adds new constraints for the understanding of the 2D value ϕ = 1 of the critical exponent describing the phase boundary.
@article{arxiv.0704.0888,
title = {NMR evidence for a strong modulation of the Bose-Einstein Condensate in BaCuSi$_2$O$_6$},
author = {S. Kramer and R. Stern and M. Horvatic and C. Berthier and T. Kimura and I. R. Fisher},
journal= {arXiv preprint arXiv:0704.0888},
year = {2008}
}