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.

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We present results of low-temperature calorimetric and resistive measurements on the isostructural heavy-fermion compounds CeCu 2 Si 2 and CeNi 2 Ge 2 . 'Non-Fermi-liquid' effects are established which suggest the nearness of an antiferromagnetic quantum critical point (QCP) in both systems. The observed deviations from the properties of a Landau Fermi liquid (FL) may be related to anomalous energy dependences of both the quasiparticle mass and the quasiparticlequasiparticle scattering cross section. For CeNi 2 Ge 2 , a moderately heavy FL can be recovered by application of moderate values of either magnetic field or hydrostatic pressure. For p = 1.7 GPa a novel, non-superconducting, phase transition has been discovered at T 1 1 K.

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Inelastic neutron scattering measurements show the existence of a strong two-spinon continuum in the frustrated ferromagnetic spin-1/2 chain compound LiCuVO4. The dynamic magnetic susceptibility is well described by a mean-field model of two coupled interpenetrating antiferromagnetic Heisenberg chains. The extracted values of the exchange integrals are in good agreement with the static magnetic susceptibility data and an earlier spin-wave description of the bound state near the lower boundary of the two-spinon continuum. In addition, there is clear evidence for a four-spinon continuum at high energies.

We report on magnetic resonance studies within the magnetically ordered phase of the quasi-onedimensional antiferromagnet LiCuVO 4 . Our studies reveal a spin reorientational transition at a magnetic field H c1 Ϸ 25 kOe applied within the crystallographic ab plane in addition to the recently observed one at H c2 Ϸ 75 kOe ͓M. G. Banks et al., J. Phys.: Condens. Matter 19, 145227 ͑2007͔͒. Spectra of the antiferromagnetic resonance along low-frequency branches can be described in the framework of a macroscopic theory of exchange-rigid planar magnetic structures. These data allow us to obtain the parameter of the anisotropy of the exchange susceptibility together with a constant of the uniaxial anisotropy. Spectra of 7 Li nuclear magnetic resonance ͑NMR͒ show that, within the magnetically ordered phase of LiCuVO 4 in the low-field range H Ͻ H c1 , a planar spiral spin structure is realized with the spins lying in the ab plane, in agreement with neutron-scattering studies of Gibson et al. ͓Physica B 350, 253 ͑2004͔͒. Based on NMR spectra simulations, the transition at H c1 can well be described as a spin-flop transition, where the spin plane of the magnetically ordered structure rotates to be perpendicular to the direction of the applied magnetic field. For H Ͼ H c2 Ϸ 75 kOe, our NMR spectra simulations show that the magnetically ordered structure exhibits a modulation of the spin projections along the direction of the applied magnetic field H.

We investigated the paramagnetic resonance in single crystals of LiCuVO 4 with special attention to the angular variation of the absorption spectrum. To explain the large resonance linewidth of the order of 1 kOe, we analyzed the anisotropic exchange interaction in the chains of edge-sharing CuO 6 octahedra, taking into account the ring-exchange geometry of the nearest-neighbor coupling via two symmetric rectangular Cu-O bonds. The exchange parameters, which can be estimated from theoretical considerations, nicely agree with the parameters obtained from the angular dependence of the linewidth.The anisotropy of this magnetic ring exchange is found to be much larger than it is usually expected from conventional estimations which neglect the bonding geometry. Hence, the data yield the evidence that in copper oxides with edge-sharing structures the role of the orbital degrees of freedom is strongly enhanced. These findings establish LiCuVO 4 as one-dimensional compound 1 at high temperatures.

The presence of a quantum-critical point (QCP) can significantly affect the thermodynamic properties of a material at finite temperatures T . This is reflected, e.g., in the entropy landscape SðT,rÞ in the vicinity of a QCP, yielding particularly strong variations for varying the tuning parameter r such as pressure or magnetic field B. Here we report on the determination of the critical enhancement of ∂S∕∂B near a B-induced QCP via absolute measurements of the magnetocaloric effect (MCE), ð∂T ∕∂BÞ S and demonstrate that the accumulation of entropy around the QCP can be used for efficient low-temperature magnetic cooling. Our proof of principle is based on measurements and theoretical calculations of the MCE and the cooling performance for a Cu 2þ -containing coordination polymer, which is a very good realization of a spin-½ antiferromagnetic Heisenberg chain-one of the simplest quantum-critical systems.quantum criticality | quantum magnetism | low-dimensional spin systems | magnetothermal effect T he magnetocaloric effect (MCE), i.e., a temperature change in response to an adiabatic change of the magnetic field, has been widely used for refrigeration. Although up until now applications have focused on cryogenic temperatures (1-3), possible extensions to room temperature have been discussed (4). The MCE is an intrinsic property of all magnetic materials in which the entropy S changes with magnetic field B. Paramagnetic salts have been the materials of choice for low-temperature refrigeration (1), including space applications (5-7), with an area of operation ranging from about one or two degrees Kelvin down to some hundredths or even thousandths degree Kelvin. Owing to their large ΔS∕ΔB values, the ease of operation, and the applicability under microgravity conditions, paramagnets have matured to a valuable alternative to 3 He-4 He dilution refrigerators, the standard cooling technology for reaching sub-Kelvin temperatures.A large MCE also characterizes a distinctly different class of materials, where the low-temperature properties are governed by pronounced quantum many-body effects. These materials exhibit a B-induced quantum-critical point (QCP)-a zero-temperature phase transition-and the MCE has been used to study their quantum criticality (8)(9)(10)(11)(12)(13)(14) or to determine their B-T phase diagrams (15)(16)(17)(18)(19). The aim of the present work is to provide an accurate determination of the enhanced MCE upon approaching a B-induced QCP both as a function of B and T and to explore the potential of this effect for magnetic cooling.Materials in the vicinity of a QCP have been of particular current interest, as their properties reflect critical behavior arising from quantum fluctuations instead of thermal fluctuations that govern classical critical points (20). Prominent examples of findings made here include the intriguing low-temperature behaviors encountered in some heavy-fermion metals, itinerant transition metal magnets (21 and references cited therein, 22), or magnetic insulators (23, 24) and the ...