Giant spin canting in the S=1/2 antiferromagnetic chain [CuPM(NO3)2(H2O)2]n observed by 13C-NMR

We present a combined experimental and theoretical study on copper pyrimidine dinitrate [CuPM(NO3)2(H2O)2]n, a one-dimensional S=1/2 antiferromagnet with alternating local symmetry. From the local susceptibility measured by NMR at the three inequivalent carbon sites in the pyrimidine molecule we deduce a giant spin canting, i.e., an additional staggered magnetization perpendicular to the applied external field at low temperatures. The magnitude of the transverse magnetization, the spin canting of (52+/-4) degrees at 10 K and 9.3 T, and its temperature dependence are in excellent agreement with exact diagonalization calculations.     

Striped phase in a quantum XY model with ring exchange

We present quantum Monte Carlo results for a square-lattice S=1/2 XY model with a standard nearest-neighbor coupling J and a four-spin ring exchange term K. Increasing K/J, we find that the ground state spin stiffness vanishes at a critical point at which a spin gap opens and a striped bond-plaquette order emerges. At still higher K/J, this phase becomes unstable and the system develops a staggered magnetization. We discuss the quantum phase transitions between these phases.     

Order by disorder from nonmagnetic impurities in a two-dimensional quantum spin liquid

We consider doping of nonmagnetic impurities in the spin-1/2, 1/5-depleted square lattice. This structure, whose undoped phase diagram offers both magnetically ordered and spin-liquid ground states, is realized physically in CaV4O9. Doping into the ordered phase results in a progressive loss of order, which becomes complete at the percolation threshold. By contrast, doping into the spin liquids creates a phase of weak but long-ranged antiferromagnetic order, a true order-by-disorder phenomenon. We study the phase diagram of the doped system by computing the static susceptibility and staggered magnetization using a stochastic series-expansion quantum Monte Carlo technique.     

Fermion-Spin Approach to Study the Doping Dependence of Antiferromagnetism in the Copper Oxide Material

Within the fermion-spin theory, we study the ground-state properties of the copper oxide materials by considering quantum fluctuations of spinons in the random-phaseapproximation (RPA). The RPA ground-state at half-filling is the magnetized π-flux state with the energy E₀=-0.332J per bond and the staggered magnetization M = 0.327. Away from half-fiing this staggered magnetization vanishes around doping δ= 5% for the reasonable parameter t/J = 5, which is in very good agreement with the experiments on copper oxide materials. Our results indicate that both hole dopings and quantum fluctuations of spinons lead to a strong suppression of the antiferromagnetic long-range order.     

States of local moment induced by nonmagnetic impurities in cuprate superconductors

By using a model Hamiltonian with d-wave superconductivity and competing antiferromagnetic (AF) orders, the local staggered magnetization distribution due to nonmagnetic impurities in cuprate superconductors is investigated. We show that the net moment induced by a single impurity corresponds to a local spin with S(z)=0 or 1/2 depending on the strength of the AF interaction U and the impurity scattering strength epsilon. Phase diagram of epsilon versus U for the moment formation is presented. We discuss the connection of this result with the Kondo problem. When two impurities are placed at the nearest neighboring sites, the net moment is always zero, unusually robust to parameter changes. For two neighboring strong impurities, separated by a Cu-ion site, the induced net moment has S(z)=0, 1/2, or 1.     

On the uniqueness of the equilibrium state for Ising spin systems

We show that for an Ising spin system of arbitrary spin with a ferromagnetic pair interaction and a periodic external magnetic field there is a unique equilibrium state if and only if the magnetization is continuous with respect to a uniform change in the external field. Hence, if the critical temperatureT _c is defined as the temperature where the spontaneous magnetization (which is a non-increasing function of the temperature) becomes positive, then the equilibrium state is unique forT>T _c and is non-unique forT<T _c (when the external field is zero). This implies that the correlation functions have a cluster property forT>T _c .We also show that for an anti-ferromagnet consisting of two sublattices there is a unique equilibrium state if and only if the staggered magnetization is continuous with respect to a change in the staggered field.Supported in part by U.S.A.F.O.S.R. under contract F 44620-71-C-0013, P001.     

Probing anomalous longitudinal fluctuations of the interacting Bose gas via Bose-Einstein condensation of magnons

The emergence of a finite staggered magnetization in quantum Heisenberg antiferromagnets subject to a uniform magnetic field can be viewed as Bose-Einstein condensation of magnons. Using nonperturbative results for the infrared behavior of the interacting Bose gas, we present exact results for the staggered spin-spin correlation functions of quantum antiferromagnets in a magnetic field at zero temperature. In particular, we show that in dimensions 1相似文献   

Field-induced order-disorder transition in antiferromagnetic BaCo(2)V(2)O(8) driven by a softening of spinon excitation

High field magnetization and ESR measurements on the quasi-one-dimensional (1D) antiferromagnet BaCo(2)V(2)O(8) have been performed in magnetic fields up to 50 T along the chain. The experimental results are explained well in terms of a 1D S=1/2 antiferromagnetic XXZ model in longitudinal fields. We show that the quantum phase transition from the Néel ordered phase to the spin liquid one in the model is responsible for a peculiar order to disorder transition in BaCo(2)V(2)O(8).     

A large NMR lineshift due to induced-staggered magnetization in paramagnetic state

Antiferromagnetic staggered susceptibility is studied in a new way by the examination of the proton NMR lineshift above T_N in a crystallographic two-sublattice system with different g-tensors. The lineshift has a large contribution from the staggered magnetization which is induced in the paramagnetic state by the action of a uniform external field. A large value of the staggered susceptibility derived from the experiment in a nearly two-dimensional Heisenberg antiferromagnet, Cu(HCOO)₂.4D₂O, can be ascribed to the existence of a highly developed short-range order above T_N in this crystal.     

Impurity induced spin texture in quantum critical 2D antiferromagnets

We describe the uniform and staggered magnetization distributions around a vacancy in a quantum critical two-dimensional S=1/2 antiferromagnet. The distributions are delocalized across the entire sample with a universal functional form arising from an uncompensated Berry phase. The numerical results, obtained using quantum Monte Carlo simulations of the Heisenberg model on bilayer lattices with up to approximately equal to 10(5) spins, are in good agreement with the proposed scaling structure. We determine the exponent eta' = 0.40+/-0.02, which governs both the staggered and uniform magnetic structure away from the impurity and also controls the impurity spin dynamics.     

Antiferromagnetic ordering in superconducting YBa2Cu3O6.5

Commensurate antiferromagnetic ordering has been observed in the superconducting high- Tc cuprate YBa2Cu3O6.5 ( Tc = 55 K) by polarized and unpolarized elastic neutron scattering. The magnetic peak intensity exhibits a marked enhancement at Tc. Zero-field muon-spin-resonance experiments demonstrate that the staggered magnetization is not truly static but fluctuates on a nanosecond time scale. These results point towards an unusual spin density wave state coexisting with superconductivity.     

Effects of the Dzyaloshinskii-Moriya interaction on low-energy magnetic excitations in copper benzoate

We have investigated the physical effects of the Dzyaloshinskii-Moriya (DM) interaction in copper benzoate. In the low-field limit, the spin gap is found to vary as H(2/3)ln((1/6)(J/mu(B)H(s)) (H(s): an effective staggered field induced by the external field H) in agreement with the prediction of conformal field theory, while the staggered magnetization varies as H(1/3) and the ln((1/3)(J/mu(B)H(s)) correction predicted by conformal field theory is not confirmed. The linear scaling relation between the momentum shift and the magnetization is broken. We have determined the coupling constant of the DM interaction and have given a complete quantitative account for the field dependence of the spin gaps along all three principal axes, without resorting to additional interactions such as interchain coupling. A crossover to strong applied field behavior is predicted for further experimental verification.     

63Cu NMR evidence for enhanced antiferromagnetic correlations around Zn impurities in YBa2Cu3O6.7

Doping the high- T(c) superconductor YBa2Cu3O6.7 with 1.5% of nonmagnetic Zn impurities in CuO2 planes is shown to produce a considerable broadening of 63Cu NMR spectra, as well as an increase of low-energy magnetic fluctuations detected in 63Cu spin-lattice relaxation measurements. A model-independent analysis demonstrates that these effects are due to the development of staggered magnetic moments on many Cu sites around each Zn and that the Zn-induced moment in the bulk susceptibility might be explained by this staggered magnetization. Several implications of these enhanced antiferromagnetic correlations are discussed.     

Magnetic Properties of an Antiferromagnetic Spin-1/2 XYZ Model in the Presence of Different Magnetic Fields: Finite-Size Effects of Inhomogeneity Property*

Magnetic and thermodynamic properties of the anisotropic XYZ spin-1/2 finite chain under both homogeneous and inhomogeneous magnetic fields are theoretically studied at low temperature. Using exact diagonalization method (ED), we study the magnetization, magnetic susceptibility, and specific heat of the model characterized in terms of the finite correlation length in the presence of three different magnetic fields including longitudinal, transverse, and transverse staggered magnetic fields. The magnetization, susceptibility, and the specific heat of the model are investigated under two conditions separately: (i) When the model is putted in the presence of homogeneous magnetic fields. (ii) When finite inhomogeneities are considered for all applied magnetic fields in the Hamiltonian. We show that for the finite-size XYZ chains at low temperature, the evident magnetization plateaus gradually convert to their counterpart quasi-plateaus when the transverse magnetic field increases. Moreover, the influence of the transverse and staggered transverse magnetic fields, and their corresponding inhomogeneities on the magnetization process, magnetic susceptibility, and specific heat are reported in detail. Our exact results illustrate that by altering the inhomogeneity parameters, magnetization plateaus gradually convert to their counterpart quasi-plateaus. The specific heat manifests Schottky-type maximum, double-peak, and triple-peak, as well as, transformation between them by varying considered inhomogeneity parameters in the Hamiltonian.     

Approaching the ground state of the kagomé antiferromagnet

Y0.5Ca0.5BaCo4O7 contains kagomé layers of Co ions, whose spins are strongly coupled, with a Curie-Weiss temperature of -2200 K. At low temperature, T=1.2 K, our diffuse neutron scattering study with polarization analysis reveals characteristic spin correlations close to a predicted two-dimensional coplanar ground state with staggered chirality. The absence of three-dimensional long-range antiferromagnetic order indicates negligible coupling between the kagomé layers. The scattering intensities are consistent with high spin S=3/2 states of Co2+ in the kagomé layers and low spin S=0 states for Co3+ ions on interlayer sites. Our observations agree with previous Monte Carlo simulations indicating a ground state of effectively short range, staggered chiral spin order.     

Ising-like spin anisotropy and competing antiferromagnetic-ferromagnetic orders in GdBaCo2O5.5 single crystals

In RBaCo2O5+x compounds (R is rare earth), a ferromagnetic-antiferromagnetic competition is accompanied by a giant magnetoresistance. We study the magnetization of detwinned GdBaCo2O5.5 single crystals and find a remarkable uniaxial anisotropy of Co3+ spins which is tightly linked with the chain oxygen ordering in GdO0.5 planes. Reflecting the underlying oxygen order, CoO2 planes also develop a spin-state order consisting of Co3+ ions in alternating rows of S=1 and S=0 states. The magnetic structure appears to be composed of weakly coupled ferromagnetic ladders with Ising-like moments, which gives a simple picture for magnetotransport phenomena.     

Supersolid phase in spin dimer XXZ systems under a magnetic field

Using the quantum Monte Carlo method, we study, under external magnetic fields, the ground state phase diagram of the two-dimensional spin S=1/2 dimer model with an anisotropic intraplane antiferromagnetic coupling. With the anisotropy 4 greater/approximately Delta greater/approximately 3, a supersolid phase characterized by a nonuniform Bose condensate density that breaks translational symmetry is found. The rich phase diagram also contains a checkerboard solid, an antiferromagnet in the z axis, and a superfluid phase formed by S(z)= +1 spin triplets which has a finite staggered magnetization in the in-plane direction. As we show, the model can be realized as a consequence of including the next nearest neighbor coupling among dimers and our results suggest that spin dimer systems may be an ideal model system to study the supersolid phase.     

Ferroelectricity induced by spin-dependent metal-ligand hybridization in Ba₂CoGe₂O₇

We have investigated the variation of induced ferroelectric polarization under a magnetic field with various directions and magnitudes in a staggered antiferromagnet Ba?CoGe?O?. While the ferroelectric polarization cannot be explained by the well-accepted spin current model nor the exchange striction mechanism, we have shown that it is induced by the spin-dependent p-d hybridization between the transition metal (Co) and ligand (O) via the spin-orbit interaction. On the basis of the correspondence between the direction of electric polarization and the magnetic state, we have also demonstrated the electrical control of the magnetization direction.     

Field-induced staggered magnetization and magnetic ordering in Cu2(C5H12N2)2Cl4

We present a 2D NMR investigation of the gapped spin-1/2 compound Cu2(C5H10N2D2)2Cl4. Our measurements reveal the presence of a magnetic field-induced transverse staggered magnetization (TSM) which persists well below and above the field-induced 3D long-range magnetically ordered (FIMO) phase. The symmetry of this TSM is different from that of the TSM induced by the order parameter of the FIMO phase. Its origin, field dependence, and symmetry can be explained by an intradimer Dzyaloshinskii-Moriya interaction, as shown by DMRG calculations on a spin-1/2 ladder. This leads us to predict that the transition into the FIMO phase is not in the BEC universality class.     

双模随机晶场对纳米管上Blume-Capel模型磁化强度和相变的影响

近年来, 磁性纳米管的物理性质和相关应用得到了人们的广泛关注. 利用有效场理论研究了纳米管上双模随机晶场中Blume-Capel模型的磁化强度和相变性质, 得到了系统的磁化强度与温度和随机晶场的关系及其相图. 结果表明: 系统在稀释晶场、交错晶场和同向晶场中会表现出不同的磁学性质和相变行为; 稀释晶场和交错晶场会抑制系统的磁化强度, 导致其基态饱和值小于1, 而同向晶场则不会; 随着随机晶场参量的变化, 系统存在多个相变温度, 并呈现出三临界现象和重入现象.