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.     

New polarization effect and collective excitation in S = 1/2 quasi-one-dimensional antiferromagnetic quantum spin chain

Anomalous polarization characteristics of magnetic resonance in CuGeO₃ doped with 2% Co impurity are reported. For the Faraday geometry, this mode is damped for the microwave field B _ω aligned along a certain crystallographic direction, showing that the character of magnetic oscillation differs from the standard spin precession. The observed resonance coexists with the ESR on Cu²⁺ chains; it is argued not to be caused by “impurity” EPR, as previously claimed, but to correspond to a previously unknown collective mode of magnetic oscillations in an S = 1/2 AF quantum spin chain. The text was submitted by the authors in English.     

Low-temperature spin diffusion in a highly ideal S=1/2 Heisenberg antiferromagnetic chain studied by muon spin relaxation

The organic radical-ion salt DEOCC-TCNQF4 contains linear chains of stacked molecules with significant Heisenberg antiferromagnet interactions along the chain and extremely weak interactions between the chains. Zero-field muSR has confirmed the absence of long-range magnetic order down to 20 mK and field-dependent muSR is found to be consistent with diffusive motion of the spin excitations. The anisotropic spin dynamics and the upper boundary for magnetic ordering temperature both indicate interchain magnetic coupling /J'/<7 mK. As the intrachain coupling J is 110 K, /J'/J/ is significantly less than 10(-4). This system therefore provides one of the most ideal examples of the one-dimensional S=1/2 Heisenberg antiferromagnet yet discovered.     

Frustration-induced eta inversion in the S=1/2 bond-alternating spin chain

We study the frustration-induced enhancement of the incommensurate correlation for a bond-alternating quantum spin chain in a magnetic field, which is associated with a quasi-one-dimensional organic compound F5PNN. We investigate the temperature dependence of the staggered susceptibilities by using the density matrix renormalization group, and then find that the incommensurate correlation becomes dominant in a certain range of the magnetic field. We also discuss the mechanism of this enhancement on the basis of the mapping to the effective S=1/2 XXZ chain and a possibility of the field-induced incommensurate long-range order.     

Electron spin resonance in the spin-1/2 quasi-one-dimensional antiferromagnet with Dzyaloshinskii-Moriya interaction BaCu2Ge2O7

We have investigated the electron spin resonance (ESR) on single crystals of BaCu2Ge2O7 at temperatures between 300 and 2 K and in a large frequency band, 9.6-134 GHz, in order to test the predictions of a recent theory, proposed by Oshikawa and Affleck (OA) [Phys. Rev. Lett. 82, 5136 (1999)]], which describes the ESR in a spin-1/2 Heisenberg chain with the Dzyaloshinskii-Moriya interaction. We find, in particular, that the ESR linewidth, Delta H, displays a rich temperature behavior. As the temperature decreases from T(max)/2 approximately 170 to 50 K, Delta H shows a rapid and linear decrease, Delta H approximately T. At low temperatures, below 50 K, Delta H acquires a strong dependence on the magnetic field orientation and for H axially c it shows a (h/T)(2) behavior which is due to an induced staggered field h, according to OA's prediction.     

Low-lying excitations in the S = 1 antiferromagnetic Heisenberg chain

In order to confirm the picture of domain-wall excitations in the hidden antiferromagnetic order of the Haldane phase, the structure of the low-lying excitations in the S = 1 antiferromagnetic Heisenberg chain is studied by a quantum Monte Carlo method. It is confirmed that there exists a finite energy gap between the first- and the second-excited states at k = π as well as between the ground state and the first-excited state at k = π. In the thermodynamic limit, the second-excited state at k = π is separated from the ground state by the gap which is three times as large as the Haldane gap. From the size dependences of the low-lying-excitation energies, the interactions between the elementary excitations in the excited states are concluded to be repulsive.     

Low-dimensional spin S = 1/2 system at the quantum critical limit: Na2V3O7

We report the results of measurements of the dc susceptibility and the 23Na-NMR response of Na2V3O7, a recently synthesized, nonmetallic low dimensional spin system. Our results indicate that, upon reducing the temperature to below 100 K, the V4+ moments are gradually quenched, leaving only one moment out of nine active. The NMR data reveal a phase transition at very low temperatures. With decreasing applied field H, the critical temperature shifts towards T=0 K, suggesting that Na2V3O7 may be regarded as an insulator reaching a quantum critical point at H=0.     

Structure and magnetic properties of the S=3/2 zigzag spin chain antiferromagnet BaCoTe_2O_7

We report a study of the structure and magnetic properties of the S=3/2 zigzag spin chain compound BaCoTe_2O_7.Neutron diffraction measurements show that it crystallizes in the noncentrosymmetric space group Ama2 with a canted↑↑↓↓spin structure along the quasi-one-dimensional zigzag chain and a moment size of 1.89(2)μ_Bat 2 K.Both magnetic susceptibility and specific heat measurements yield an antiferromagnetic phase transition at T_N=6.2 K.A negative Curie-Weiss temperature,Θ_(CW)=-74.7(2) K,and an empirical frustration parameter,f=|Θ_(CW)|/T_N≈12,are obtained by fitting the magnetic susceptibility,indicating antiferromagnetic interactions and strong magnetic frustration.From ultraviolet-visible absorption spectroscopy and first-principles calculations,an indirect band gap of 2.68(2) eV is determined.We propose that the canted zigzag spin chain of BaCoTe_2O_7 may produce a change in the polarization via the exchange-striction mechanism.     

Magnetic properties of the spin S = 1/2 Heisenberg chain with hexamer modulation of exchange

We consider the spin-1/2 Heisenberg chain with alternating spin exchange in the presence of additional modulation of exchange on odd bonds with period 3. We study the ground state magnetic phase diagram of this hexamer spin chain in the limit of very strong antiferromagnetic (AF) exchange on odd bonds using the numerical Lanczos method and bosonization approach. In the limit of strong magnetic field commensurate with the dominating AF exchange, the model is mapped onto an effective XXZ Heisenberg chain in the presence of uniform and spatially modulated fields, which is studied using the standard continuum-limit bosonization approach. In the absence of additional hexamer modulation, the model undergoes a quantum phase transition from a gapped phase into the only one gapless Lüttinger liquid (LL) phase by increasing the magnetic field. In the presence of hexamer modulation, two new gapped phases are identified in the ground state at magnetization equal to [Formula: see text] and [Formula: see text] of the saturation value. These phases reveal themselves also in the magnetization curve as plateaus at corresponding values of magnetization. As a result, the magnetic phase diagram of the hexamer chain shows seven different quantum phases, four gapped and three gapless, and the system is characterized by six critical fields which mark quantum phase transitions between the ordered gapped and the LL gapless phases.