NiCl2-4SC(NH2)2 (DTN) is a quantum S = 1 chain system with strong easy-pane anisotropy and a new candidate for the Bose-Einstein condensation of the spin degrees of freedom. ESR studies of magnetic excitations in DTN in fields up to 25 T are presented. Based on analysis of the single-magnon excitation mode in the high-field spin-polarized phase and previous experimental results [ Phys. Rev. Lett. 96, 077204 (2006)], a revised set of spin-Hamiltonian parameters is obtained. Our results yield D = 8.9 K, Jc = 2.2 K, and J a,b = 0.18 K for the anisotropy, intrachain, and interchain exchange interactions, respectively. These values are used to calculate the antiferromagnetic phase boundary, magnetization and the frequency-field dependence of two-magnon bound-state excitations predicted by theory and observed in DTN for the first time. Excellent quantitative agreement with experimental data is obtained. PACS numbers: 75.40.Gb, 75.10.Jm Antiferromagnetic (AFM) quantum spin-1 chains have been the subject of intensive theoretical and experimental studies, fostered especially by the Haldane conjecture [1]. Due to quantum fluctuations, an isotropic spin-1 chain has a spin-singlet ground state separated from the first excited state by a gap ∆ ∼ 0.41J [2], where J is the exchange interaction. As shown by Golinelli et al. [3], the presence of a strong easy-plane anisotropy D can significantly modify the excitation spectrum, so that the gap size is not determined by the strength of the AFM quantum fluctuations exclusively, but depends on the dimensionless parameter ρ = D/J. The Haldane phase is predicted to survive up to ρ c = 0.93 [4], where the system undergoes a quantum phase transition. For ρ > ρ c the gap reopens, but its origin is dominated by the anisotropy D, and the system is in the so-called large-D regime. While the underlying physics of Haldane chains is fairly well understood, relatively little is known about the magnetic properties (and particularly the elementary excitation spectrum) of nonHaldane S = 1 AFM chains in the large-D phase. Intense theoretical work and numerous predictions [3,4,5,6,7,8,9,10] make the experimental investigation of large-D spin-1 chains a topical problem in low-dimensional magnetism.Recently, weakly-coupled spin-1 chains have attracted renewed interest due to their possible relevance to the fieldinduced Bose-Einstein condensation (BEC) of magnons. When the field H, applied perpendicular to the easy plane, exceeds a critical value H c1 (defined at T = 0), the gap closes and the system undergoes a transition into an XY -like AFM phase with a finite magnetization and AFM magnon excitations. If the spin Hamiltonian has axial symmetry with respect to the applied field, the AFM ordering can be described as BEC of magnons by mapping the spin-1 system into a gas of semi-hard-core bosons [11]. The applied field plays the role of a chemical potential, changing the boson population. In accordance with mean-field BEC theory [12,13,14], the phasediagram boundary for a three-dimensional system sh...

We demonstrate that the short-range spin correlator ͗S i · S j ͘, a fundamental measure of the interaction between adjacent spins, can be directly measured in certain insulating magnets. We present magnetostriction data for the insulating organic compound NiCl 2 -4SC͑NH 2 ͒ 2 , and show that the magnetostriction as a function of field is proportional to the dominant short-range spin correlator. Furthermore, the constant of proportionality between the magnetostriction and the spin correlator gives information about the spin-lattice interaction. Combining these results with the measured Young's modulus, we are able to extract dJ / dz, the dependence of the superexchange constant J on the Ni interionic distance z.where the magnetic Hamiltonian PHYSICAL REVIEW B 77, 020404͑R͒ ͑2008͒

In the classical Ising model on the kagome lattice, there are macroscopically degenerate classical states with exponentially-decayed spin correlation. As the singular quantum perturbation is introduced by applying the transverse magnetic field, the quantum ground state remains disordered, dubbed by "disorder-by-disorder". Here we compute the temperature dependence of the spin susceptibility and the specific heat for difference strength of the field to understand the effect of the singular quantum perturbation in this system. The relevance to the ZnCu3(OH)6Cl2 will be also commented.PACS numbers: 75.30. Gw, 75.40.Cx, 75.40.Mg Macroscopically degenerate states often occur in the classical level in the anti-ferromagnetic systems that the exchange energy can not be perfectly shared by all the nearest neighbor spins. It happens naturally if spins are placed in the triangular geometry in the two-dimensional lattices. In two dimensions, triangles can share the edges to form the triangular network or share the corners to form the kagome lattice. If we consider the classical Ising dynamics in these two systems, both of them show macroscopically degenerate classical ground states. However, the spin correlation in these two systems are very different, which determines the fate of the consequent quantum ground states when the quantum dynamics is introduced. When the transverse magnetic field is introduced by the following Hamiltonian,where S k = σ k /2 and σ k are the Pauli spin matrices and J is taken to be positive, the quantum ground state in the triangular lattice favors an spin-ordered ground state, the maximally-flippable state, because the spin correlation is critical in the classical model. On the other hand, the quantum ground state in the kagome case remains disordered because the spin correlation is exponentiallydecayed [1,2]. In this paper, we focus on the kagome case and compute its thermodynamic properties with various Γ. At Γ = 0, the spin susceptibility shows dramatic upturn in the low temperature and diverges at the zero temperature. For non-zero Γ, we also see the significant upturn but it saturates at the zero temperature. Our calculation is the first results on the thermodynamic quantities to distinguish the classical disordered state and the quantum disorder states. Furthermore, as the macroscopically degenerate classical states lead to the residual entropy at T = 0, the quantum dynamics which lifts the degeneracy results in additional peak in the specific heat in the low temperature at the order of Γ. Due to these properties, our model might be relevant to the recent discovered ZnCu 3 (OH) 6 Cl 2 , which shows the abnormal upturn in the spin susceptibility and saturates at T = 0. There have been several theoretical papers trying to describe this unusual property. A straightforward interpretation is the presence of the impurity[3] that contributes additionally to the susceptibility. However, a naive inclusion of the contribution from the impurity can explain the data only above 20K [4]. Different ang...

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