We present magnetic torque measurements on the Shastry-Sutherland quantum spin system SrCu$_2$(BO$_3$)$_2$ in fields up to 31 T and temperatures down to 50 mK. A new quantum phase is observed in a 1 T field range above the 1/8 plateau, in agreement w
ith recent NMR results. Since the presence of the DM coupling precludes the existence of a true Bose-Einstein condensation and the formation of a supersolid phase in SrCu$_2$(BO$_3$)$_2$, the exact nature of the new phase in the vicinity of the plateau remains to be explained. Comparison between magnetization and torque data reveals a huge contribution of the Dzyaloshinskii-Moriya interaction to the torque response. Finally, our measurements demonstrate the existence of a supercooling due to adiabatic magnetocaloric effects in pulsed field experiments.
We have performed $^{77}$Se NMR on a single crystal sample of the field induced superconductor $lambda$-(BETS)$_{2}$FeCl$_{4}$. Our results obtained in the paramagnetic state provide a microscopic insight on the exchange interaction $J$ between the s
pins textbf{s} of the BETS $pi$ conduction electrons and the Fe localized $d$ spins textbf{S}. The absolute value of the Knight shift textbf{K} decreases when the polarization of the Fe spins increases. This reflects the ``negative spin polarization of the $pi$ electrons through the exchange interaction $J$. The value of $J$ has been estimated from the temperature and the magnetic field dependence of textbf{K} and found in good agreement with that deduced from transport measurements (L. Balicas textit{et al}. Phys. Rev. Lett. textbf{87}, 067002 (2001)). This provides a direct microscopic evidence that the field induced superconductivity is due to the compensation effect predicted by Jaccarino and Peter (Phys. Rev. Lett. textbf{9}, 290 (1962)). Furthermore, an anomalous broadening of the NMR line has been observed at low temperature, which suggests the existence of charge disproportionation in the metallic state neighboring the superconducting phase.
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 i
ntra-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.
