Short-range Antiferromagnetic Correlations Research Articles

Quantum lattice fluctuations in the one-dimensional molecular-crystal model with Coulomb repulsion.

The effect of quantum lattice fluctuations on the ground state of a half-filled-band one-dimensional molecular-crystal model with on-site (U) and nearest-neighbor (V) Coulomb repulsion is investigated. The nonadiabatic effects due to finite phonon frequency \ensuremath{\omega} are treated through a variational polaron wave function. The electronic correlations are decoupled by a Hartree-Fock approximation. Our variational approach gives a rather good description of the continuous variation of the dimerization, which is in good agreement with that of the Monte Carlo simulations, as functions of U, V, and \ensuremath{\omega}. However, the Hartree-Fock theory predicts a fictitious long-range spin-density-wave order and a discontinuous transition between it and the charge-density-wave (CDW) state. These are somewhat different from the results of numerical simulations. Furthermore, we propose a valence-bonding (VB) state for describing the short-range antiferromagnetic correlation for intermediate values of U and V and determine the transition point between the VB state and the CDW state, which is in good agreement with that of the numerical simulations. The effect of quantum lattice fluctuations on the transition point is also discussed.

Neutron scattering study of the magnetic excitations in metallic and superconductingLa2−xSrxCuO4−y

We report on neutron scattering studies of the dynamic spin correlations in ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{y}}$. Two superconducting and one nonsuperconducting single crystals were studied. All three of the samples exhibited metallic normal-state resistivity modified by corrections consistent with localization effects. The structure factor S(Q,\ensuremath{\omega}) measured in these samples reflects to a first approximation short-range two-dimensional antiferromagnetic spin correlations closely related to those found in insulating ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. In addition, S(Q,\ensuremath{\omega}) measured in the superconducting samples has an incommensurate, double-peaked structure which is not present in the nonsuperconducting crystal. The integrated ( F dQ) intensities of the scattering at both concentrations are nearly independent of temperature over the range ${k}_{B}$T\ensuremath{\gg}\ensuremath{\Elzxh}\ensuremath{\omega} to ${k}_{B}$T\ensuremath{\ll}\ensuremath{\Elzxh}\ensuremath{\omega} for energy transfers greater than \ensuremath{\sim}6 meV. Furthermore, the difference in the temperature dependence of the intensity between the nonsuperconducting and superconducting samples for energy transfers less than \ensuremath{\sim}6 meV suggests that a gap may be forming in the magnetic excitation spectrum of the superconducting samples. The relationship of these results to the superconductivity in ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{y}}$ is discussed.

Magnetic correlations in UPt 3 and U1− xThxPt3

Neutron scattering experiments on UPt 3 and U 1− x Th x Pt 3 are reviewed. At relatively high energies (≈ 5 meV), the magnetic fluctuation spectrum is modulated by the structure factor derived from short-range antiferromagnetic correlations where the two U ions in each unit cell are oppositely polarized. In contrast, at low energies (≤ 1 meV), the diffuse inelastic scattering is associated with antiferromagnetic correlations where the unit cell is doubled. Nominally pure UPt 3 exhibits magnetic order with the wavevector corresponding to this doubling and a static moment of (0.02±0.01)μ B. (U 1− x Th x )Pt 3 with x ≈ 0.05 exhibits ordering of the same type, but with a much larger static moment, (0.65±0.1)μ B. Neutron scattering measurements on UPt 3 with implications for superconductivity are surveyed.

MAGNETIC PROPERTIES OF THE (Ce, La) Ru2Si2 SYSTEM

Magnetization measurements of Cel-ILaxRu2Siz single crystals are reported. Antiferromagnetic order occurs for 0.08 5 x 5 0.9. For H parallel to the tetragonal c-axis, metamagnetic transitions occur below TN as well as in the paramagnetic phase for a: < 0.13. The transition field first decreases exponentially on increasing x, while TN increases; then for x 2 0.4, this field decreases linearly, like TN. The compound CeRuzSiz shows interesting mag- netic properties. For H parallel to the tetragonal c- axis, its low temperature magnetization, M (H // c) , exhibits a metamagnetic-like transition at a, field HM & 8 T (I), although no long range magnetic order could be detected yet. HM is now known (2) to correspond to the quenching of the short range antiferromagnetic (AF) correlations which develop below FZ 60 K. The low temperature specific heat (which for T -+ 0 ex- trapolates to 7 = 0.35 J mol-l~-~ (3, 41) contains a magnetic contribution, Cm, which shows a broad max- imum at 11 K. The latter has been interpreted (3) as resulting from a Kondo-like contribution, leading to TK M 24 K. Although this maximum can result also from the AF interactions, the analysis made in refer- ence (3) seems to give a good approximation of the variations of TK (or of a characteristic temperature T*) on doping: TK decreases almost exponentially for concentrations between = 50 % of Y and 15 % of La substituted to Ce then starts to decrease linearly for higher La content. The measurements of reference (3), performed on polycrystalline samples revealed also the existence of AF order in the Cel-,LaXRuzSiz system, for 0.15 < x 50.9 and of two magnetic transitions in the magnetization at T < TN. In order to better char- acterize these transitions we have performed magnetic measurements on samples cleaved from single crystals grown by the Czochralsky technique. Neutron diffrac- tion experiments performed on similar crystals have shown recently (5) that the AF order already occurs for x = 0.08 and is a modulated one, at least up to x = 0.2.

Ground state of the strong-coupling Hubbard Hamiltonian: A numerical diagonalization study.

We exactly diagonalize the effective Hamiltonian obtained from the Hubbard model in the strong-coupling limit for a two-dimensional ..sqrt..10 x ..sqrt..10 size square lattice. The effective Hamiltonian operates on a restricted Hilbert space containing only states with singly occupied sites, which makes the diagonalization possible within reasonable computational time for the above lattice. The ground-state energy and wave function are obtained for several values of the coupling ratio t/U and the doping fraction x. We find three different phases in the (x,t/U) phase diagram: The first is characterized by antiferromagnetic long-range order and the second and third by antiferromagnetic and ferromagnetic short-range correlation, respectively. We comment on the possible relevance of the results to the recently discovered high-temperature copper-oxide superconductors.

Antiferromagnetic correlations in almost-localized Fermi liquids.

A Monte Carlo method is used to calculate various properties of one-band Gutzwiller wave functions which are formed by restricting the charge fluctuations in noninteracting wave functions. Gutzwiller's approximate formula for the kinetic energy is tested both for the ground state and excited states. The ground state is found to have strong antiferromagnetic short-range spin-spin correlations for nearly-half-filled bands, thus extending previous work on the half-filled case. These correlations are very sensitive to the choice of occupied Bloch states and when the occupation is distributed uniformly over the band they disappear. From this fact we conclude that correlations are present only at temperatures low compared to the coherence temperature. In the almost-localized limit it is advantageous to describe the system by an effective Hamiltonian which separates into a term due to the kinetic energy of the charge carriers and one due to the Heisenberg spin-spin coupling. We show that the almost-localized Fermi liquid can gain energy from both terms in the effective Hamiltonian. In other words the restrictions on charge fluctuations can cause spin correlations which in turn can stabilize the Fermi-liquid ground state.

Wave-vector- and magnetic-field-dependent spin fluctuations in the heavy-fermion system CeCu6.

We have performed neutron-scattering measurements on a single crystal of ${\mathrm{CeCu}}_{6}$ in applied magnetic fields from 0 to 6.4 T. The observed momentum-dependent magnetic scattering implies the existence of short-range antiferromagnetic correlations. While it induces no resonances, the field broadens the spectra considerably at T=0.4 K. An effective Hamiltonian which incorporates both single-impurity Kondo properties and antiferromagnetic Ruderman-Kittel-Kasuya-Yosida interactions describes our data.

Synthesis, crystal structure, and magnetism of FeMoO 4Cl, a new two-dimensional antiferromagnet with O [sbnd]Mo [sbnd]O superexchange pathways

FeMoO 4Cl crystallizes in the tetragonal system with unit cell dimensions a = 6.672(3)and c = 5.223(3)A˚in space group P4/nmmwith Z = 2. The structure is built from FeMoO 4Cl layers stacked along the c axis and is closely related to α-VOPO 4-type compounds. Individual layers can be described as sheets of corner-sharing MoO 4 tetrahedra and FeO 4Cl square pyramids. The temperature dependence of χ′ Mfor a powder sample exhibits a broad maximum at 104 K characteristic of extensive short range antiferromagnetic correlation within the layers. This is a result of surprisingly strong Fe(III) Fe(III) exchange interactions transmitted by O Mo O pathways. Long range three-dimensional order is confirmed by the hyperfine splitting of the zero field Mo¨ssbauer spectrum, T Ne´el = 69.2 ± 0.1K. Analysis of the combined quadrupole-Zeeman split hyperfine spectrum of the powder at 1.7 K leads to θ = 90° for the angle between the internal hyperfine field ( H hf) and V zz , the principal axis of the electric field gradient. Single crystal Mo¨ssbauer and X-ray powder diffraction data are also discussed.