### Session Y18: Focus Session: Low D/Frustrated Magnetism - More Frustrated Magnets

8:00 AM–11:00 AM, Friday, March 25, 2011
Room: D172

Chair: Mark Meisel, University of Florida

Abstract ID: BAPS.2011.MAR.Y18.3

### Abstract: Y18.00003 : Bose-Einstein Condensation in Han Purple - a NMR Study

8:24 AM–8:36 AM

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#### Authors:

Raivo Stern
(NICPB, Tallinn, Estonia)

Steffen Kr\"amer
(LNCMI, Grenoble, France)

(LNCMI, Grenoble, France)

Claude Berthier
(LNCMI, Grenoble, France)

Ivo Heinmaa
(NICPB, Tallinn, Estonia)

Enno Joon
(NICPB, Tallinn, Estonia)

Tsuyoshi Kimura
(Osaka University, Japan)

NMR study of the two quasi-2D coupled spin-1/2 dimer compound, BaCuSi$_2$O$_6$ (Han Purple) [1], is presented. $T_{\rm{BEC}}$ varies as $(H-H_{c1})^{2/d}$, where $d$ is the dimensionality of the system, and $H_{c1}$ the critical field which closes the gap. BaCuSi$_2$O$_6$ was claimed to exhibit an reduction of d from 3D to 2D at low T [2]. However, due to a structural transformation at 90 K, different intradimer exchange couplings and different gaps ($\Delta_{\rm{B}}$/$\Delta_{\rm{A}}$=1.16) exist in every second plane along the c axis [3]. In our first NMR experiments [3], we have shown that the population of bosons in the B planes $n_{\rm{B}}$ was much smaller than $n_{\rm{A}}$, but finite in the field range $\Delta_{\rm{A}}/g\mu_{\rm{B}} < H < \Delta_{\rm{B}}/g\mu_{\rm{B}}$ where $n_{\rm{B}}$ = 0 is expected in a naive model of uncoupled planes. Recently, a new model has been presented [4] which takes into account both frustration and quantum fluctuations. This leads to a non-zero population $n_{\rm{B}}$ of \emph{uncondensed} bosons in the $B$ plane, increasing quadratically with $(H-H_{c1})$, as compared to the linear dependence of $n_{\rm{A}}$. We compare our new NMR results to these predictions. $[$1$]$ M. Jaime \textit{et al.}, PRL \textbf{93},087203 (2004). $[$2$]$ S. E. Sebastian \textit{et al.}, Nature \textbf{441}, 617 (2006). $[$3$]$ S. Kr\"{a}mer \textit{et al.}, PRB \textbf{76}, 100406(R) (2007). $[$4$]$ N. Laflorencie and F. Mila, PRL \textbf{102}, 060602 (2009).

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2011.MAR.Y18.3