Haldane Gap Compound Research Articles

Giant dielectric response of Haldane gap compound Y2BaNiO5

The dielectric properties of the Haldane gap compound Y2BaNiO5 were studied in the frequency range from 20 Hz to 1 MHz between 20 K and room temperature. This compound exhibits a high dielectric permittivity (ɛ′ ∼ 104) at room temperature below 100 kHz. Both dielectric relaxation and dc resistivity ρ follow the thermally activated process with similar value of the activation energy. This indicates the close correlation between the electric conduction and the dielectric polarization. The modulus spectra reveal evidence for both grain and grain boundary relaxation times. The grain relaxation time follows thermally activated process, whereas the grain boundary relaxation time exhibits variable range hopping (VRH) behavior. The observed giant dielectric response were explained in terms of internal (grain boundary) barrier layer capacitance (IBLC) effects.

Finite temperature calculations on spin correlation functions of the Haldane gap compound

Abstract We present the finite temperature calculations on the spin correlation functions of the Haldane gap compound CsNiCl 3 . Based on the coupled-chain theory, two methods are used to do the calculations: the large- N expansion and the Ginzburg–Landau approach. The comparisons with the inelastic neutron-scattering experiments are discussed.

Impurity induced antiferromagnetic order in Haldane gap compound [formula omitted

The effect of nonmagnetic Mg 2 + doping in SrNi 2 V 2 O 8 , a Haldane gap system with a disordered ground state, was investigated using DC magnetic susceptibility and heat capacity measurements in polycrystalline samples of SrNi 2 - x Mg x V 2 O 8 with x = 0.03 , 0.05, 0.07, 0.1 and 0.14. The results clearly reveal that the substitution of Ni 2 + ( S = 1 ) ion by Mg 2 + ( S = 0 ) ion induces a magnetic phase transition with the ordering temperatures lying in the range 3.4–4.3 K, for the samples with lowest and highest value of x. The intrachain exchange constant ( J / k B ) and the Haldane gap ( Δ ) for all the compounds were estimated to be ∼ 98 ± 2 and 25 K, respectively, which are close to that of the undoped compound. The magnetization data further suggest that the compounds exhibit metamagnetic behavior below T N , supporting a picture of antiferromagnet with significant magnetic anisotropy and competing intrachain and interchain interactions.

Dynamical properties ofS=1bond-alternating Heisenberg chains in transverse magnetic fields

We calculate dynamical structure factors of the S=1 bond-alternating Heisenberg chain with a single-ion anisotropy in transverse magnetic fields, using a continued fraction method based on the Lanczos algorithm. In the Haldane-gap phase and the dimer phase, dynamical structure factors show characteristic field dependence. Possible interpretations are discussed. The numerical results are in qualitative agreement with recent results for inelastic neutron-scattering experiments on the S=1 bond-alternating Heisenberg-chain compound $\rm{Ni(C_{9}D_{24}N_{4})(NO_{2})ClO_{4}}$ and the S=1 Haldane-gap compound $\rm{Ni(C_{5}D_{14}N_{2})_{2}N_{3}(PF_{6})}$ in transverse magnetic fields.

Spin Dynamics of NTENP and NDMAP in Transverse Magnetic Fields

We calculate the field dependence of dynamical structure factors of the S = 1 bond-alternating Heisenberg-chain compound Ni(C 9 D 24 N 4 )(NO 2 )ClO 4 and the S = 1 Haldane-gap compound Ni(C 5 Di 4 N 2 ) 2 N 3 (PF 6 ). We show that in the former system the highest magnon branch vanishes at H ∼ 0.63H c and one gapped branch emerges in H > H c , where H c is the critical field for the energy-gap closing. In the latter system, by contrast, three distinct magnon branches appear below and above H c . The results are in qualitative agreement with recent experimental findings.

Haldane-gap chains in a magnetic field

We consider quasi-one-dimensional spin-1 Heisenberg chains with crystal fieldanisotropy in a uniform magnetic field. We determine the dynamical structurefactor in various limits and obtain a fairly complete qualitative picture of how itchanges with the applied field. In particular, we discuss how the width of the higherenergy single magnon modes depends on the field. We consider the effects ofa weak interchain coupling. We discuss the relevance of our results for recentneutron scattering experiments on the quasi-1D Haldane-gap compound NDMAP.

Spin-gap and antiferromagnetic correlations in low-dimensional PbNi 2-x A x V 2 O 8 compounds (A=Mg, Co)

The effect of the impurity doping in the quantum-spin chains of the PbNi2V2O8 Haldane-gap compound is studied. We report elastic neutron scattering and bulk magnetic susceptibility measurements on the quasi-one-dimensional system PbNi2-xAxV2O8 doped at the Ni2+ (S=1) sites by either non-magnetic A=Mg2+ (S=0) or magnetic A=Co2+ (S=3/2) ions, with x=0.12 and 0.08, respectively. The results reveal that the magnitude of the Neel temperature depends strongly on the spin value of the A cation.

Impurity-induced antiferromagnetic order in the Haldane-gap compoundPbNi2−xMgxV2O8(x=0.24)

An investigation of the effect of nonmagnetic Mg doping in the Haldane gap system ${\mathrm{PbNi}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ is presented. Specifically, the magnetic properties of the quasi-one-dimensional compound ${\mathrm{PbNi}}_{2\ensuremath{-}x}{\mathrm{Mg}}_{x}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ $(x=0.24),$ located in the heavily doped region of the ${T}_{N},$ x phase diagram, have been examined using neutron powder diffraction, muon-spin relaxation $({\ensuremath{\mu}}^{+}\mathrm{SR})$ and magnetic susceptibility. Neutron-diffraction measurements show that antiferromagnetic long-range order (propagation vector $\mathbf{\ensuremath{\kappa}}=[000])$ is established below ${T}_{N}=3.2\mathrm{K}.$ The spins in the Ni sublattice lie either parallel or antiparallel to the c axis, which is the magnetic easy axis of the system. The spin configuration is antiferromagnetic both along the ${\mathrm{NiO}}_{6}$ chains, i.e., the c axis of the crystal structure, and within the ab-plane for the nearest-neighbor Ni atoms. The refined ordered moment at the ${\mathrm{Ni}}^{2+}$-site is $\ensuremath{\mu}=0.9(1){\ensuremath{\mu}}_{B}.$ To our knowledge, this is the first diffraction evidence of the antiferromagnetic ordering in this system. The static magnetic correlations of the doping-induced magnetic moments along the Ni chains were probed by zero-field ${\ensuremath{\mu}}^{+}\mathrm{SR}$ at $T<{T}_{N}.$ The large number of detected spontaneous muon-precession frequencies $(0.5<{\ensuremath{\nu}}_{\ensuremath{\mu}}<3\mathrm{MHz})$ implies a nonuniform distribution in the magnitude of the ordered moments along the spiral ${\mathrm{NiO}}_{6}$ chains. Bulk susceptibility measurements reveal metamagnetic behavior below ${T}_{N},$ consistent with the picture of an antiferromagnet with magnetic anisotropy and competing exchange interactions.

Unconventional antiferromagnetic correlations of the doped Haldane gapsystem Y 2 BaNi 1 - x Zn x O 5

We make a new proposal to describe the very low temperature susceptibility of the doped Haldane gap compound Y$_2$BaNi$_{1-x}$Zn$_x$O$_5$. We propose a new mean field model relevant for this compound. The ground state of this mean field model is unconventional because antiferromagnetism coexists with random dimers. We present new susceptibility experiments at very low temperature. We obtain a Curie-Weiss susceptibility $\chi(T) \sim C / (\Theta+T)$ as expected for antiferromagnetic correlations but we do not obtain a direct signature of antiferromagnetic long range order. We explain how to obtain the ``impurity'' susceptibility $\chi_{imp}(T)$ by subtracting the Haldane gap contribution to the total susceptibility. In the temperature range [1 K, 300 K] the experimental data are well fitted by $T \chi_{imp}(T) = C_{imp} (1 + T_{imp}/T )^{-\gamma}$. In the temperature range [100 mK, 1 K] the experimental data are well fitted by $T \chi_{imp}(T) = A \ln{(T/T_c)}$, where $T_c$ increases with $x$. This fit suggests the existence of a finite N\'eel temperature which is however too small to be probed directly in our experiments. We also obtain a maximum in the temperature dependence of the ac-susceptibility $\chi'(T)$ which suggests the existence of antiferromagnetic correlations at very low temperature.

Impurity-induced antiferromagnetic phase in a new Haldane-gap compound PbNi 2V 2O 8

Recently, it was reported that a new Haldane compound PbNi 2V 2O 8 was discovered and the spin-vacancy-induced antiferromagnetism was found with substitution of Mg 2+ion ( S=0) for Ni 2+ ions ( S=1) [Uchiyama et al., Phys. Rev. Lett. 83 (1999) 632]. In this paper, the effect of substitutions of Cu 2+ions ( S= 1 2 ) and Co 2+ions ( S= 3 2 ) was studied and it was revealed that this effect varied with the values of the spins of the impurity ions.

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.

Néel ordered versus quantum disordered behavior in doped spin-Peierls and Haldane gap systems

I consider a theoretical description of recent experiments on doping the spin-Peierls compound CuGeO3 and the Haldane gap compounds PbNi2V2O8 and Y2BaNiO5. The effective theory is the one of randomly distributed spin-1/2 moments interacting with an exchange decaying exponentially with distance. The model has two phases in the (doping, interchain coupling) plane: (i) a Néel ordered phase at small doping; (ii) a quantum disordered phase at larger doping and small interchain interactions. The spin-Peierls compound CuGeO3 and the Haldane gap nickel oxides PbNi2V2O8 and Y2BaNiO5 fit well into this phase diagram. At small temperature, the Néel phase is found to be reentrant into the quantum disordered region. The Néel transition relevant for CuGeO3 and PbNi2V2O8 can be described in terms of a classical disordered model. A simplified version of this model is introduced, and is solved on a hierarchical lattice structure, which allows to discuss the renormalization group flow of the model. It is found that the system looks non disordered at large scale, which is not against available susceptibility experiments. In the quantum disordered regime relevant for Y2BaNiO5, the two spin model and the cluster RG in the 1D regime show a power law susceptibility, in agreement with recent experiments on Y2BaNiO5. It is found that there is a succession of two distinct quantum disordered phases as the temperature is decreased. The classical disordered model of the doped spin-1chain contains already a physics relevant to the quantum disordered phase.

Haldane Gap System: Electronic Structures and Magnetic Properties

The electronic structure of typical s = 1 Haldane gap compounds such as [Ni(NH2(CH2)3NH2)2N3]ClO4 (NINAZ), [Ni(NH2CH2C(CH3)2CH2NH2)2N3]ClO4 (NDMAZ) and [Ni(NH2CH2C(CH3)2CH2NH2)2N3]PF6(NDMAP) have been investigated by means of discrete variational (DV)-Xα molecular orbital calculation. The DV-Xα calculations of these compounds reveal the importance of the overlap population analysis of bridging N3 − ligands, suggesting the tendency of the well-balance intramolecular overlap dispersion in the bridged N3 − molecule is correlated with the larger Haldane gap Eg .

Pressure Effect on Haldane Gap System

The [Ni(dmpn) 2N 3]ClO 4(dmpn=2,2-dimethyl-1,3-propylenediamine) is a Haldane gap compound with Eg=21.6 K and J=−70.6 K. The pressure effect on the magnetic susceptibilities has revealed that with increase of the pressure the absolute J values increase. This is due to the increase of the orbital overlapping between the Ni 2+ ions and the N atoms of bridging azido-ligands.

Coexistence of Haldane-gap excitations and long-range order in R 2BaNiO 5 (R = rare earth)

R 2BaNiO 5 (R = rare earth) quasi-1-D antiferromagnets are structurally equaivalent to the well-studied 1-D S = 1 Haldane-gap compounds Y 2BaNiO 5. Unlike the Y-nickelate though, these materials undergo 3-D magnetic ordering at finite temperatures. Recent inelastic neutron-scattering studies of Pr 2BaNiO 5 and (Nd x Y 1− x ) 2BaNiO 5 revealed purely 1-D gap excitations that propagate exclusively on the Ni-chains and are strikingly similar to Haldane-gap modes in Y 2BaNiO 5. In the ordered phase, these excitations survive and actually coexist with conventional spin waves. The results suggest that the Haldane-singlet ground state of the Ni-chains is not fully destroyed by Neél ordering.

High field heat capacities of some singlet ground state systems at very low temperatures

Several aspects of high field heat capacity observed in different types of singlet ground state system are discussed. The ground state energy gap and its magnetic field dependence are also studied in the Haldane gap compounds NENP, NINO and TMNIN. The origin of the observed minimum gap in applied magnetic field is discussed. In the spin-Peierls compound CuGeO 3, the energy gap of the Cu 2+ spin system in the dimerized phase and the gapless state in the high-field magnetic phase are confirmed.

Low-Dimensional Magnetic Systems With S=1, 3/2 and 2

Abstract New Haldane gap compounds with S=1 have been characterized by single-crystal X-ray analyses and magnetic measurements. From these results, it has been found that the magnitude of the Haldane gaps can be controlled by substituting the bridging groups and in-plane ligands. The magnetic measurements of quasi-one-dimensional halogen-bridged binuclear Ru complexes with S=3/2. Ru2(CH3COO)4X (X=Cl, Br and 1) have revealed that the Br- and I-bridged compounds show one-dimensional antiferromagnetic interactions but no spin-Pelerls transition over the temperature range measured, while the C1-bridged compound shows no magnetic interaction among binuclear RU2 units through the bridging Cl− ions along the chain but spin-crossover behavior between S=3/2 and 1/2 in the binuclear Ru2 unit. The oxalato-bridged Fe2+ compound with S=2, Fe(ox).2Hz0 has no Haldane gap but shows metamagnetism under the high magnetic field.

Haldane gap system: Structure and magnetic properties of azido-bridged nickel complex

A new Haldane gap compound, Ni(dmpn) 2N 3(ClO 4) (dmpn=2,2-dimethyl-1,3-propanediamine) has been synthesized, and its structure and magnetic properties have been investigated. It has an azido-bridged linear-chain structure where each chain is separated to each other by counterions ClO 4 −. The Haldane gap and J value of this compound are estimated to be 21.6 K and −70.6 K, respectively. These absolute values are larger than those of Ni(en) 2(NO 2)(ClO 4) (hereafter abbreviated as NENP) which has a NO 2 −-bridged structure and has been extensively investigated so far.