Inelastic neutron scattering, susceptibility, and high-field magnetization identify LiCuVO4 as a nearest-neighbour ferromagnetic, next-nearest-neighbour frustrated, quasi-onedimensional helimagnet, which is largely influenced by quantum fluctuations. Complementary band structure calculations provide a microscopic model with the correct sign and magnitude of the major exchange integrals.

How ground states of quantum matter transform between one another reveals deep insights into the mechanisms stabilizing them. Correspondingly, quantum phase transitions are explored in numerous materials classes, with heavy fermion compounds being among the most prominent ones. Recent studies in an anisotropic heavy fermion compound have shown that different types of transitions are induced by variations of chemical or external pressure 1-3 , raising the question of the extent to which heavy fermion quantum criticality is universal.To make progress, it is essential to broaden both the materials basis and the microscopic parameter variety. Here, we identify a cubic heavy fermion material as exhibiting a field-induced quantum phase transition, and show how the material can be used to explore one extreme of the dimensionality axis. The transition between two different ordered phases is accompanied by an abrupt change of Fermi surface, reminiscent of what happens across the field-induced antiferromagnetic to paramagnetic transition in the anisotropic YbRh 2 Si 2 . This finding leads to a materials-based global phase diagram -a precondition for a unified theoretical description.1 Quantum phase transitions arise in matter at zero temperature due to competing interactions. When they are continuous, the associated quantum critical points (QCPs) give rise to collective excitations which influence the physical properties over a wide range of parameters. As such, they are being explored in a variety of electronic materials, ranging from high T c cuprates to insulating magnets and quantum Hall systems 4,5 .Heavy fermion compounds are prototype materials to study quantum phase transitions. Their low energy scales allow to induce such transitions deliberately, by the variation of external parameters such as pressure or magnetic field. Microscopically, electrons in partiallyfilled f shells behave as localized magnetic moments. They interact with conduction electrons through a Kondo exchange interaction, which favors a non-magnetic ground state that entangles the local moments and the spins of the conduction electrons. They also interact among themselves through an RKKY exchange interaction, which typically induces antiferromagnetic order. It has been known that tuning external parameters changes the ratio of the Kondo coupling to the RKKY interaction. Recently, the importance of a second microscopic quantity has been suggested. This is the degree of quantum fluctuations of the local moments, parameterized by G: magnetic order weakens with increasing G, as it would with enhancing the Kondo coupling J K . These two quantities define a two-dimensional parameter space, which allows the consideration of a global phase diagram 10 . This global phase diagram is most clearly specified via the energy scale T * associated with the breakdown of the Kondo entanglement between the local moments and conduction electrons. So far T * has only been identified in tetragonal YbRh 2 Si 2 (refs. 8,11,12 ). It is believed that this energy scale no...

Magnetization of the frustrated S = 1/2 chain compound LiCuVO4, focusing on high magnetic field phases, is reported. Besides a spin-flop transition and the transition from a planar spiral to a spin modulated structure observed recently, an additional transition was observed just below the saturation field. This newly observed magnetic phase is considered as a spin nematic phase, which was predicted theoretically but was not observed experimentally. The critical fields of this phase and its dM/dH curve are in good agreement with calculations performed in a microscopic model (M. E. Zhitomirsky and H. Tsunetsugu, preprint, arXiv:1003.4096v2).PACS numbers: 75.50. Ee, 75.10.Jm, 75.10.Pq Unconventional magnetic orders and phases in frustrated quantum spin chains are attractive issues, because they appear under a fine balance of the exchange interactions and are sometimes caused by much weaker interactions or fluctuations. 1-4

Polarized and unpolarized neutron scattering experiments on the frustrated ferromagnetic spin-1/2 chain LiCuVO4 show that the phase transition at H(Q) of 8 T is driven by quadrupolar fluctuations and that dipolar correlations are short range with moments parallel to the applied magnetic field in the high-field phase. Heat-capacity measurements evidence a phase transition into this high-field phase, with an anomaly clearly different from that at low magnetic fields. Our experimental data are consistent with a picture where the ground state above H(Q) has a next-nearest neighbor bond-nematic order along the chains with a fluidlike coherence between weakly coupled chains.

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