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Quantum spins in magnets show a remarkable diversity of fascinating strongly correlated behaviour. Examples include clean realizations of magnetic quantum critical points (QCP) and generic quantum ground states as in Bose-Einstein Condensates (BEC), Luttinger-liquids, and spin supersolids [1]. The phase diagram of quasi-1D arrays of spins in temperature and magnetic field can be particularly rich, with two QCPs, related phase transitions, and crossovers triggered by the effective dimensionality of the spin system. Quantum disordered, quantum critical, spin Luttinger-liquid, BEC, and classically saturated phases can in principle be studied. However, model materials are rare, where these phases can be explored experimentally – with the exception of novel metal-organic compounds. We have studied such a material, a quasi-1D two-leg spin ladder, by neutron scattering and measurements of the magnetostriction, magnetocaloric effect, and specific heat [2-5], and compared the results with elaborate predictions by theory. The static and dynamic signatures of these exciting states of quantum mater will be presented and discussed in the context of current research in ultra-cold gases of atoms and quantum many-body theory. Furthermore, some recent theoretical and experimental results on BaCuSi2O6 will be discussed [6,7], which consists of quasi-2D bilayers of spins showing exotic dimensional reduction at a QCP [8].


[1] T. Giamarchi, Ch. Rüegg, O. Tchernyshyov, Nature Physics 4, 198 (2008).

[2] Ch. Rüegg et al., Phys. Rev. Lett. 100, 205701 (2008).

[3] Ch. Rüegg et al., Phys. Rev. Lett. 101, 247202 (2008).

[4] B. Thielemann et al., Phys. Rev. B 79, 020408(R) (2009).

[5] B. Thielemann et al., Phys. Rev. Lett. 102, 107204 (2009).

[6] Ch. Rüegg et al., Phys. Rev. Lett. 98, 017202 (2007).

[7] N. Laflorencie et al., Phys. Rev. Lett. 102, 060602 (2009).

[8] S.E. Sebastian et al., Nature 441, 617 (2006).


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