A quantum magnet, LiCuSbO 4 , with chains of edge-sharing spin-½ CuO 6 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-½ chains with competing ferro-and antiferromagnetic interactions (J 1 = -75 K, J 2 = 34 K) accounts for the T > 2 K data. PACS: 75.10.Jm, 75.30.Kz, 75.40.Cx, 75.30.Et 2 The Heisenberg spin-½ chain is one of very few quantum critical systems to be realized in a crystalline solid. An element of frustration is added by next-nearest-neighbor (NNN) interactions (J 2 ). [1][2][3] In such systems, theoretical work [4][5][6][7][8][9] indicates that qualitatively different quantum phases are possible as a function of α = J 2 /J 1 , axial exchange anisotropy, ∆, and the applied field h = gµ B H/|J 1 |. Finite values of α are observed in copper oxide spin-chains formed by corner-or edge-sharing Jahn-Teller distorted CuO 6 polyhedra. [10][11][12][13][14][15][16][17] While the sign and magnitude of J 1 is dependent on the α c ) from a short-range ordered (SRO) phase with a small yet finite gap (α < α c ). 20Dimer, spiral, vector-chiral and multipolar phases have been predicted as a function of α, ∆ and h. 4,6,8 Interest has been further amplified by the observation that ferroelectricity accompanies magnetic ordering in some of these systems. 14,21,22 Known spin-½ FM-AFM chains span a wide range of α and include Li 2 CuO 2 , 17,21,22 Here we present the structure and magnetic properties of the one-dimensional (1D) spin chain compound LiCuSbO 4 . Due to weak inter-chain interactions it has no 3D ordering down to 100 mK and can be driven to a saturated-FM phase above the experimentally accessible field <µ o H s > = 12 T. The magnetic susceptibility, isothermal magnetization and specific heat can be simultaneously modelled for T > 2 K using the approximate spin Hamiltonian:Here θ is the angle between the applied field, h, and the unique molecular axis, z. The parameters J 1 = -75K, α = -0.45, and ∆ = 0.83 account for the main features in the thermomagnetic data and are also consistent with the magnetic excitations measured by inelastic neutron scattering.Powder samples of LiCuSbO 4 were prepared using a ceramic synthesis. 25LiCuSbO 4 has an orthorhombic structure 26, 27 related to that of LiFeSnO 4 . 28Rietveld analysis 29 of powder neutron diffraction data acquired on the BT1 diffractometer at NIST 30 was carried out using FULLPROF. 31The final fit, with the corr...

We propose a measure of non-Gaussianity for quantum states of a system of n oscillator modes. Our measure is based on the quasi-probability Q(α), α ∈ C n . Since any measure of non-Gaussianity is necessarily an attempt at making a quantitative statement on the departure of the shape of the Q function from Gaussian, any good measure of non-Gaussianity should be invariant under transformations which do not alter the shape of the Q functions, namely displacements, passage through passive linear systems, and uniform scaling of all the phase space variables: Q(α) → λ 2n Q(λα). Our measure which meets this 'shape criterion' is computed for a few families of states, and the results are contrasted with existing measures of non-Gaussianity. The shape criterion implies, in particular, that the non-Gaussianity of the photon-added thermal states should be independent of temperature.

A modified density matrix renormalization group ͑DMRG͒ algorithm is applied to the zigzag spin-1 2 chain with frustrated antiferromagnetic exchange J 1 and J 2 between first and second neighbors. The modified algorithm yields accurate results up to J 2 / J 1 Ϸ 4 for the magnetic gap ⌬ to the lowest triplet state, the amplitude B of the bond order wave phase, the wavelength of the spiral phase, and the spin correlation length . The J 2 / J 1 dependences of ⌬, B, , and provide multiple comparisons to field theories of the zigzag chain. The twist angle of the spiral phase and the spin structure factor yield additional comparisons between DMRG and field theory. Attention is given to the numerical accuracy required to obtain exponentially small gaps or exponentially long correlations near a quantum phase transition.

Theoretical and computational studies of the quantum phase diagram of the one-dimensional half-filled extended Hubbard model (EHM) indicate a narrow bond order wave (BOW) phase with finite magnetic gap Em for on-site repulsion U < U * , the critical point, and nearest neighbor interaction Vc ≈ U/2 near the boundary of the charge density wave (CDW) phase. Potentials with more extended interactions that retain the EHM symmetry are shown to have a less cooperative CDW transition with higher U * and wider BOW phase. Density matrix renormalization group (DMRG) is used to obtain Em directly as the singlet-triplet gap, with finite Em marking the BOW boundary Vs(U ). The BOW/CDW boundary Vc(U ) is obtained from exact finite-size calculations that are consistent with previous EHM determinations. The kinetic energy or bond order provides a convenient new estimate of U * based on a metallic point at Vc(U ) for U < U * . Tuning the BOW phase of half-filled Hubbard models with different intersite potentials indicates a ground state with large charge fluctuations and magnetic frustration. The possibility of physical realizations of a BOW phase is raised for Coulomb interactions. PACS 71.10.Fd, 71.30.+h, 71.45.Lr PACS numbers: I.

No abstract

The spin-1/2 chain with isotropic Heisenberg exchange J 1 , J 2 > 0 between first and second neighbors is frustrated for either sign of J 1 . Its quantum phase diagram has critical points at fixed J 1 /J 2 between gapless phases with nondegenerate ground state (GS) and

The linear spin-1/2 Heisenberg antiferromagnet with exchanges J1, J2 between first and second neighbors has a bond-order wave (BOW) phase that starts at the fluid-dimer transition at J2/J1 = 0.2411 and is particularly simple at J2/J1 = 1/2. The BOW phase has a doubly degenerate singlet ground state, broken inversion symmetry and a finite energy gap Em to the lowest triplet state. The interval 0.4 < J2/J1 < 1.0 has large Em and small finite size corrections. Exact solutions are presented up to N = 28 spins with either periodic or open boundary conditions and for thermodynamics up to N = 18. The elementary excitations of the BOW phase with large Em are topological spin-1/2 solitons that separate BOWs with opposite phase in a regular array of spins. The molar spin susceptibility χM (T ) is exponentially small for T ≪ Em and increases nearly linearly with T to a broad maximum. J1, J2 spin chains approximate the magnetic properties of the BOW phase of Hubbard-type models and provide a starting point for modeling alkali-TCNQ salts.

No abstract