Field-induced three- and two-dimensional freezing in a quantum spin liquid

Field-induced commensurate transverse magnetic ordering is observed in the Haldane-gap compound Ni(C(5)D(14)N(2))2N(3)(PF(6)) by means of neutron diffraction. Depending on the direction of applied field, the high-field phase is shown to be either a three-dimensional ordered Néel state or a short-range ordered state with dominant two-dimensional spin correlations. The structure of the high-field phase is determined, and properties of the observed quantum phase transition are discussed.     

Divergent effects of static disorder and hole doping in geometrically frustrated β-CaCr2O4

The gallium substituted and calcium deficient variants of geometrically frustrated β-CaCr₂O₄, β-CaCr_2-2xGa_2xO₄ (0.02≤x≤0.25) and β-Ca_1−yCr₂O₄ (0.075≤y≤0.15), have been investigated by X-ray powder diffraction, magnetization and specific heat measurements. This allows for a direct comparison of the effects, in a geometrically frustrated magnet, of the static disorder that arises from non-magnetic substitution and the dynamic disorder that arises from hole doping. In both cases, disturbing the Cr³⁺ lattice results in a reduction in the degree of magnetic frustration. On substitution of Ga, which introduces disorder without creating holes, a gradual release of spins from ordered antiferromagnetic states is observed. In contrast, in the calcium-deficient compounds the introduction of holes induces static ferrimagnetic ordering and much stronger perturbations of the β-CaCr₂O₄ host.     

Competing magnetic phases on a kagomé staircase

We present thermodynamic and neutron data on Ni3V2O8, a spin-1 system on a kagomé staircase. The extreme degeneracy of the kagomé antiferromagnet is lifted to produce two incommensurate phases at finite T--one amplitude modulated, the other helical--plus a commensurate canted antiferromagnet for T-->0. The H-T phase diagram is described by a model of competing first and second neighbor interactions with smaller anisotropic terms. Ni3V2O8 thus provides an elegant example of order from subleading interactions in a highly frustrated system.     

Quasielastic neutron scattering in the high-field phase of a Haldane antiferromagnet

Inelastic neutron scattering experiments on the Haldane-gap quantum antiferromagnet Ni(C5D14N2)2-N3(PF6) are performed in magnetic fields below and above the critical field H(c) at which the gap closes. Quasielastic neutron scattering is found for H>H(c), indicating topological excitations in the high-field phase.     

Quantum spin liquid in frustrated one-dimensional LiCuSbO4

A quantum magnet, LiCuSbO4, with chains of edge-sharing spin-1/2 CuO6 octahedra is reported. While short-range order is observed for T<10 K, no zero-field phase transition or spin freezing occurs down to 100 mK. Specific heat indicates a distinct high-field phase near the 12 T saturation field. Neutron scattering shows incommensurate spin correlations with q=(0.47±0.01)π/a and places an upper limit of 70 μeV on any spin gap. Exact diagonalization of 16-spin easy-plane spin-1/2 chains with competing ferro- and antiferromagnetic interactions (J1=-75 K, J2=34 K) accounts for the T>2 K data.     

Inhomogeneous level splitting in Pr 2-x BixRu2O7

We report that Bi doping drives Pr 2-x BixRu2O7 from an antiferromagnetic insulator (x = 0) to a metallic paramagnet (x approximately 1) with a broad low T maximum in C/T. Neutron scattering reveals local low energy spin excitations (variant Planck's omega approximately 1 meV) with a spectrum that is unaffected by heating to k(B)T > variant Planck's omega. We show that a continuous distribution of splittings of the non-Kramers Pr3+ ground-state doublet such as might result from various types of lattice strain can account for all the data.     

Spin fluctuations in a magnetically frustrated metal LiV(2)O(4).

Inelastic neutron scattering is used to characterize spin fluctuations in the d-electron heavy fermion spinel LiV(2)O(4). The spin-relaxation rate, gamma(Q), for Q = 0.6 A(-1) is 1.4(2) meV at low temperatures and increases linearly with temperature at a rate of 0.46(8)k(B). There is antiferromagnetic short-range order at low temperatures with a characteristic wave vector Q(c) = 0.64(2) A(-1) and a correlation length of 6(1) A. While warming shifts intensity towards lower Q, the staggered susceptibility peaks at a finite wave vector for T < 80 K. The data are compared with conventional heavy fermion systems, geometrically frustrated insulating magnets, and recent theories for LiV(2)O(4).     

Spin resonance in the d-wave superconductor CeCoIn5

Neutron scattering is used to probe antiferromagnetic spin fluctuations in the d-wave heavy fermion superconductor CeCoIn5 (T_(c)=2.3 K). Superconductivity develops from a state with slow (variant Planck's over 2piGamma=0.3+/-0.15 meV) commensurate [Q_(0)=(1/2,1/2,1/2)] antiferromagnetic spin fluctuations and nearly isotropic spin correlations. The characteristic wave vector in CeCoIn5 is the same as CeIn3 but differs from the incommensurate wave vector measured in antiferromagnetically ordered CeRhIn5. A sharp spin resonance (variant Planck's over 2piGamma<0.07 meV) at variant Planck's over 2piomega=0.60+/-0.03 meV develops in the superconducting state removing spectral weight from low-energy transfers. The presence of a resonance peak is indicative of strong coupling between f-electron magnetism and superconductivity and consistent with a d-wave gap order parameter satisfying Delta(q+Q0)=-Delta(q).     

Spinons in the strongly correlated copper oxide chains in SrCuO2

We have investigated the spin dynamics in the strongly correlated chain copper oxide SrCuO2 for energies up to greater, similar 0.6 eV using inelastic neutron scattering. We observe a gapless continuum of magnetic excitations, which is well described by the "Müller ansatz" for the two-spinon continuum in the S=1/2 antiferromagnetic Heisenberg spin chain. The lower boundary of the continuum extends up to approximately 360 meV, which corresponds to an exchange constant J=226(12) meV.     

Structure of end states for a Haldane spin chain

Inelastic neutron scattering was used to probe edge states in a quantum spin liquid. The experiment was performed on finite length antiferromagnetic spin-1 chains in Y2BaNi1-xMgxO5. At finite fields, there is a Zeeman resonance below the Haldane gap. The wave-vector dependence of its intensity provides direct evidence for staggered magnetization at chain ends, which decays exponentially towards the bulk [xi=8(1) at T=0.1 K]. Continuum contributions to the chain-end spectrum indicate interchain segment interactions. We also observe a finite size blueshift of the Haldane gap.