# NMR evidence for a strong modulation of the Bose-Einstein Condensate in BaCuSi$_2$O$_6$

## TL;DR

This study uses NMR to reveal strong modulation of the Bose-Einstein condensate in BaCuSi$_2$O$_6$, showing significant density variations along the c-axis and in each plane, which impacts understanding of its quantum critical behavior.

## Contribution

It provides the first detailed NMR evidence of strong spatial modulation of the Bose-Einstein condensate in BaCuSi$_2$O$_6$, highlighting inhomogeneities in the density near the quantum critical point.

## Key findings

- Strong c-axis modulation of boson density near critical field
- Existence of different intra-dimer exchange couplings and gaps
- Incommensurate density modulation within planes

## Abstract

We present a $^{63,65}$Cu and $^{29}$Si NMR study of the quasi-2D coupled spin 1/2 dimer compound BaCuSi$_2$O$_6$ 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 ($\Delta_{\rm{B}}/\Delta_{\rm{A}}$ = 1.16) exist in every second plane along the c-axis, in addition to a planar incommensurate (IC) modulation. $^{29}$Si spectra in the field induced magnetic ordered phase reveal that close to the quantum critical point at $H_{\rm{c1}}$ = 23.35 T the average boson density $\bar{n}$ of the Bose-Einstein condensate is strongly modulated along the c-axis with a density ratio for every second plane $\bar{n}_{\rm{A}}/\bar{n}_{\rm{B}} \simeq 5$. An IC modulation of the local density is also present in each plane. This adds new constraints for the understanding of the 2D value $\phi$ = 1 of the critical exponent describing the phase boundary.