Spin-gapped Ground State Research Articles

Magnetic modeling and effect of biaxial strain on the Haldane chain compoundSrNi2V2O8

Using combination of first-principles density-functional theory and quantum Monte Carlo technique we study the magnetic behavior of $S=1$ spin chain compound ${\mathrm{SrNi}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$. The magnetic model, derived in a first-principles Wannier function basis, shows the presence of finite next-neighbor intrachain coupling, in addition to interchain couplings, thereby deviating from the ideal Haldane chain description. The computed single-ion anisotropy turns out to be of easy axis type and of appreciable magnitude. Solution of the first-principles derived spin model confirms the spin-gapped ground state of the compound. The spin gap is found to close at a critical value of magnetic field, which is estimated to be much reduced compared to that of an ideal Haldane chain. Our study highlights the effectiveness of the next-nearest-neighbor intrachain coupling in reducing the spin gap from the ideal Haldane chain limit. Further study on the effect of biaxial strain on the magnetic behavior predicts that the application of tensile strain should enable the closing of the spin gap in the ground state.

Charge Order Induced by Cation Order in δ-Ag2/3V2O5

Several compounds of vanadium bronze have drawn much interest as a stage of various quantum phenomena. The δ-phase of vanadium bronze has been investigated, focusing on the phase transition in δ-Ag2/3V2O5 which shows characteristic supercooling effects. In this study, we reveal that the phase transition accompanied by jumps of magnetic susceptibility and resistivity is caused by the Ag ions ordering. Furthermore, we also reveal that the Ag ion ordering triggers the V4+/V5+ charge ordering. The phase transition in δ-Ag2/3V2O5 is a novel charge ordering phenomenon induced by the cation ordering. A model of the Ag ion ordering is proposed from X-ray diffraction and electron diffraction analyses. A V4+/V5+ charge ordering model is also proposed and the spin-gapped ground state is discussed on the basis of the charge ordering model.

Synthesis, structure and magnetic properties of V 4O 9—A missing link in binary vanadium oxides

V 4O 9: A missing link of Wadsley phases has been successfully synthesized by using sulfur as a reducing agent at a low temperature and its structure has been determined by combining electron, X-ray and neutron diffractions. V 4O 9 has an orthorhombic Cmcm structure and the lattice parameters are a=10.356(2) Å, b=8.174(1) Å and c=16.559(3) Å at room temperature. The structure is composed of shared edges and corners of three types of polyhedra; a VO 6 distorted octahedron, a VO 5 pyramid and a VO 4 tetrahedron. The structure of V 4O 9 is very similar to that of vanadyl pyrophosphate (VO) 2P 2O 7 which has PO 4 tetrahedra instead of VO 4 tetrahedra. This indicates that V 4O 9 is a salt of pyro-ion [V 2O 7] 4-; (VO) 2V 2O 7. The magnetic properties of V 4O 9 have been investigated by magnetic susceptibility, high-field magnetization and inelastic neutron scattering measurements. V 4O 9 is a quantum spin system with a spin-gapped ground state. The excitation gap between the singlet ground state and the excited triplet state is approximately 73 K. The magnetic susceptibility behavior suggests that V 4O 9 is a spin-1/2 dimer system with significant interdimer interactions, as opposed to (VO) 2P 2O 7, which is an alternating spin-1/2 chain system. This difference is thought to be due to the fact that VO 4-mediated interactions are considerably weaker than PO 4-mediated interactions.

Magnetic Study of Pressure-Induced Superconductivity in the [Pd(dmit)2] Salt with Spin-Gapped Ground State

Static magnetic measurements have been applied for the pressure-induced superconducting state in the P 2 1 / m phase of EtMe 3 P[Pd(dmit) 2 ] 2 (Et = C 2 H 5 , Me = CH 3 and dmit 2- = 1,3-dithiol-2...

Randomness-driven Quantum Phase Transition in Bond-alternating Haldane Chain

The effect of bond randomness on the spin-gapped ground state of the spin-1 bond-alternating antiferromagnetic Heisenberg chain is discussed. By using the loop cluster quantum Monte Carlo method, we investigate the stability of topological order in terms of the recently proposed twist order parameter [M. Nakamura and S. Todo: Phys. Rev. Lett. 89 (2002) 077204]. It is observed that the dimer phases as well as the Haldane phase of the spin-1 Heisenberg chain are robust against a weak randomness, though the valence-bond-solid-like topological order in the latter phase is destroyed by introducing a disorder stronger than the critical value.

Dilution Effects on Two-Dimensional Heisenberg Antiferromagnets with Non-Magnetic Spin-Gapped Ground State

Dilution effects on spin-1/2 quantum Heisenberg antiferromagnets with a non-magnetic spin-gapped ground state are studied by means of the qunatum Monte Carlo simulation. In the site-diluted system, an antiferromagnetic long-range order (AF-LRO) is induced at an infinitesimal concentration of dilution due to an effective coupling $\tilde{J}_{mn}$ between induced magnetic moments. In the bond-diluted case, on the other hand, the AF-LRO is not induced up to a certain concentration of dilution due to singlet pairs reformed by an AF coupling $\tilde{J}_{\rm af}$ between the edge spins of the diluted bond through the two-dimensional shortest paths. The competition between $\tilde{J}_{mn}$ and $\tilde{J}_{\rm af}$ yields peculiar phenomena in the bond-diluted system.

Pseudo gap formation and related phenomena in Cu-oxides and other systems

Detailed process of the singlet formation in high- T c oxides and quantum spin systems with a spin gapped ground state have been studied by various kinds of measurements including neutron inelastic scattering. The results can provide a physical picture how the anomalous metallic phase is formed and how the superconductivity appears in high- T c oxides as the temperature is lowered.

Effects of Short Range Spin Correlation on ESR Lines of Low-Dimensional Antiferromagnetic and Spin-Gapped Systems

ESR measurements have been performed on (CH 3 ) 4 NMnCl 3 (TMMC) and Cu 1- x Zn x Nb 2 O 6 with varying temperature T down to the region where significant spin correlation can be observed. For TMMC which is a typical example of systems with quasi-one-dimensional spin chains, the ESR linewidth Δ H pp and the dynamical shift, the resonance-field-shift which depends on the direction of the microwave field increase similarly with decreasing temperature below T ∼ T 0 , where the uniform magnetic susceptibility has a maximum. For a Zn-doped crystal of monoclinic CuNb 2 O 6 which has a spin-gapped ground state, the dynamical shift and Δ H pp exhibit similar decrease with decreasing temperature below T ∼Δ/ k B , where Δ is the spin-gap energy. For orthorhombic CuNb 2 O 6 , the dynamical shift has not been observed at room temperature. Rather large value of the dynamical shift observed in TMMC in the T -region where the antiferromagnetic correlation is significant suggests that it becomes important in the discussi...