B. Grenier

Field{Controlled Magnetic Order in the Quantum Spin{Ladder System (Hpip)2CuBr4

Neutron diffraction is used to investigate the field-induced, antiferromagnetically ordered state in the two-leg spin-ladder material (Hpip)2CuBr4. This “classical” phase, a consequence of weak interladder coupling, is nevertheless highly unconventional: its properties are influenced strongly by the spin Luttinger-liquid state of the ladder subunits. We determine directly the order parameter (transverse magnetization), the ordering temperature, the spin structure, and the critical exponents around the transition. We introduce a minimal microscopic model for the interladder coupling and calculate the quantum fluctuation corrections to the mean-field interaction.

The co-existence of superconductivity and ferromagnetism in actinide compounds

Recently superconductivity has been observed in two different 5f-electron ferromagnets, UGe2 and URhGe, well below their Curie temperatures. While superconductivity could be generic to all clean ferromagnets an alternative possibility is that it occurs in these materials, which were after all carefully selected for study, due to some special features related to the participation of strongly correlated 5f electrons in the ferromagnetism. 5f electrons potentially give rise to strong anisotropies, strong spin–orbit interactions and also a strong energy dependence of the electronic density of states. Here we focus on UGe2, and review several of the properties of the ferromagnetic state that could be a consequence of such features and discuss whether they promote superconductivity.

Excitations from a Bose-Einstein Condensate of Magnons in Coupled Spin Ladders

The weakly coupled quasi-one-dimensional spin ladder compound (CH3)2CHNH3CuCl3 is studied by neutron scattering in magnetic fields exceeding the critical field of Bose-Einstein condensation of magnons. Commensurate long-range order and the associated Goldstone mode are detected and found to be similar to those in reference to spin-dimer materials. However, for the upper two massive magnon branches, the observed behavior is totally different, culminating in a drastic collapse of excitation bandwidth beyond the transition point.

Longitudinal and transverse Zeeman ladders in the Ising-like chain antiferromagnet BaCo(2)V(2)O(8).

We explore the spin dynamics emerging from the Néel phase of the chain compound antiferromagnet BaCo(2)V(2)O(8). Our inelastic neutron scattering study reveals unconventional discrete spin excitations, so-called Zeeman ladders, understood in terms of spinon confinement, due to the interchain attractive linear potential. These excitations consist of two interlaced series of modes, respectively, with transverse and longitudinal polarization. The latter, which correspond to a longitudinal fluctuation of the ordered moment, have no classical counterpart and are related to the zero-point fluctuations that weaken the ordered moment in weakly coupled quantum chains. Our analysis reveals that BaCo(2)V(2)O(8), with moderate Ising anisotropy and sizable interchain interactions, remarkably fulfills the conditions necessary for the observation of discrete long-lived longitudinal excitations.

Cooperative Ordering of Gapped and Gapless Spin Networks in Cu2Fe2Ge4O13

The unusual magnetic properties of a novel low-dimensional quantum ferrimagnet Cu2Fe2Ge4O13 are studied using bulk methods, neutron diffraction, and inelastic neutron scattering. It is shown that this material can be described in terms of two low-dimensional quantum spin subsystems, one gapped and the other gapless, characterized by two distinct energy scales. Long-range magnetic ordering observed at low temperatures is a cooperative phenomenon caused by weak coupling of these two spin networks.

Charge Ordering and Spin Dynamics in NaV2O5

We report high-resolution neutron inelastic scattering experiments on the spin excitations of NaV(2)O(5). Below T(c), two branches with distinct energy gaps are identified. From the dispersion and intensity of the spin excitation modes, we deduce the precise zigzag charge distribution on the ladder rungs and the corresponding charge order: Delta(c) approximately 0.6. We argue that the spin gaps observed in the low-T phase of this compound are primarily due to the charge transfer.

Half-ordered state in the anisotropic Haldane-gap antiferromagnet Ni(C5D14N2)2N3(PF6)

Neutron diffraction experiments performed on the Haldane gap material Ni(C{sub 5}D{sub 14}N{sub 2}){sub 2}N{sub 3}(PF{sub 6}) in high magnetic fields applied at an angle to the principal anisotropy axes reveal two consecutive field-induced phase transitions. The low-field phase is the gapped Haldane state, while at high fields the system exhibits a three-dimensional long-range Neel order. In a peculiar phase found in intermediate fields only half of all the spin chains participate in the long-range ordering, while the other half remain disordered and gapped.

Dimensional crossover in a spin liquid to helimagnet quantum phase transition.

Neutron scattering is used to study magnetic-field-induced ordering in the quasi-one-dimensional quantum spin-tube compound

Electron spin resonance of Ni-doped CuGeO3 in the paramagnetic, spin-Peierls, and antiferromagnetic states: Comparison with nonmagnetic impurities

{\text{Sul-Cu}}_{2}{\text{Cl}}_{4}

Giant magnetic field dependence of the coupling between spin chains inBaCo2V2O8

that in zero field has a nonmagnetic spin-liquid ground state. The experiments reveal an incommensurate chiral high-field phase stabilized by a geometric frustration of the magnetic interactions. The measured critical exponents