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...

VOLUME 41, NUMBER 9 15 MARCH 1990-II g etoresistance and Hall effect near the metal-insulator tr 't'o «n-type Cdo 95Mno»TeThe magnetoresistance (MR) and Hall coefficient of n-type Cd095Mnp o5Te samples with carrier concentrations 1.2X 10"~n~6 .6X10"cm ' were measured at 1.2~T 4.2 K in fields up to 200 kOe. The results at zero magnetic field show that the carrier concentration at the metal-insulator transition is n, =-2X10" cm ', in rough agreement with Mott's prediction. In fields H 80 kOe the resistivity p first increases with H, then passes through a maximum, and finally decreases. The increase of p at low fields is accompanied by an increase in the magnitude of the Hall coefficient, while the decrease of p above the maximum is accompanied by an increase in the Hall mobility.The MR below -80 kOe is attributed to mechanisms associated with the giant spin splitting of the conduction band. The increase of p at low fields follows the behavior expected from quantum corrections to the conductivity arising from the electron-electron interaction. The decrease of p above the maximum is attributed to the rise of the Fermi energy in the majority-spin subband.Above -80 kOe the qualitative behavior of the MR depends on the carrier concentration. Samples with n & n, exhibit an upturn in the resistivity at high fields. This effect is attributed to the squeezing of the donor-electron wave function. In addition, the MR of these samples shows an anomaly near the first magnetization step. In metallic samples (n & n, ) the MR and Hall coefficient exhibit oscillations at high fields. The oscillations are interpreted as Shubnikovde Haas oscillations arising from the majority-spin subband. This interpretation is supported by model calculations.