Finite-Field Ground State of the S=1 Antiferromagnetic-Ferromagnetic Bond-Alternating Chain

We investigate the finite-field ground state of the S=1 antiferromagnetic-ferromagnetic bond-alternating chain described by the Hamiltonian {\calH}=\sum\nolimits_{\ell}\bigl\{\vecS_{2\ell-1}\cdot\vecS_{2\ell} +J\vecS_{2\ell}\cdot\vecS_{2\ell+1}\bigr\} +D\sum\nolimits_{\ell} \bigl(S_{\ell}^z)^2 -H\textstyle\sum\nolimits_\ell S_\ell^z, where \hbox{$J\leq0$} and \hbox{$-\infty<D<\infty$}. We find that two kinds of magnetization plateaux at a half of the saturation magnetization, the 1/2-plateaux, appear in the ground-state magnetization curve; one of them is of the Haldane type and the other is of the large-$D$-type. We determine the 1/2-plateau phase diagram on the $D$ versus $J$ plane, applying the twisted-boundary-condition level spectroscopy methods developed by Kitazawa and Nomura. We also calculate the ground-state magnetization curves and the magnetization phase diagrams by means of the density-matrix renormalization-group method

Ground State Phase Diagram of S=1 XXZ Chains with Uniaxial Single-Ion-Type Anisotropy

One dimensional S=1 XXZ chains with uniaxial single-ion-type anisotropy are studied by numerical exact diagonalization of finite size systems. The numerical data are analyzed using conformal field theory, the level spectroscopy, phenomenological renormalization group and finite size scaling method. We thus present the first quantitatively reliable ground state phase diagram of this model. The ground states of this model contain the Haldane phase, large-D phase, N\'{e}el phase, two XY phases and the ferromagnetic phase. There are four different types of transitions between these phases: the Brezinskii-Kosterlitz-Thouless type transitions, the Gaussian type transitions, the Ising type transitions and the first order transitions. The location of these critical lines are accurately determined.Comment: 8 pages, 19 figure

SU(2)/Z2SU(2)/Z_2 symmetry of the BKT transition and twisted boundary conditio n

Berezinskii-Kosterlitz-Thouless (BKT) transition, the transition of the 2D sine-Gordon model, plays an important role in the low dimensional physics. We relate the operator content of the BKT transition to that of the SU(2) Wess-Zumino-Witten model, using twisted boundary conditions. With this method, in order to determine the BKT critical point, we can use the level crossing of the lower excitations than the periodic boundary case, thus the convergence to the transition point is highly improved. Then we verify the efficiency of this method by applying to the S=1,2 spin chains.Comment: LaTex2e,, 33 pages, 14 figures in eps file

Environmental and Dietary Exposure to Copper and Its Cellular Mechanisms Linking to Alzheimer's Disease.

Metals are commonly found in the environment, household, and workplaces in various forms, and a significant segment of the population is routinely exposed to the trace amount of metals from variety of sources. Exposure to metals, such as aluminum, lead, iron, and copper, from environment has long been debated as a potential environmental risk factor for Alzheimer's disease (AD) for decades, yet results from in vitro, in vivo, and human population remain controversial. In the case of copper, the neurotoxic mechanism of action was classically viewed as its strong affinity to amyloid-beta (Aβ) to help its aggregation and increase oxidative stress via Fenton reaction. Thus, it has been thought that accumulation of copper mediates neurotoxicity, and removing it from the brain prevents or reverse Aβ plaque burden. Recent evidence, however, suggests dyshomeostasis of copper and its valency in the body, instead of the accumulation and interaction with Aβ, are major determinants of its beneficial effects as an essential metal or its neurotoxic counterpart. This notion is also supported by the fact that genetic loss-of-function mutations on copper transporters lead to severe neurological symptoms. Along with its altered distribution, recent studies have also proposed novel mechanisms of copper neurotoxicity mediated by nonneuronal cell lineages in the brain, such as capillary endothelial cells, leading to development of AD neuropathology. This review covers recent findings of multifactorial toxic mechanisms of copper and discusses the risk of environmental exposure as a potential factor in accounting for the variability of AD incidence

Magnetization-plateau state of the S=3/2 spin chain with single ion anisotropy

We reexamine the numerical study of the magnetized state of the S=3/2 spin chain with single ion anisotropy D(> 0) for the magnetization M=M_{S}/3, where M_{S} is the saturation magnetization. We find at this magnetization that for D<D_{c1}=0.387 the system is critical and the magnetization plateau does not appear. For D > D_{c1}, the parameter region is divided into two parts D_{c1} < D < D_{c2}=0.943 and D_{c2} < D. In each region, the system is gapful and the M=M_{S}/3 magnetization plateau appears in the magnetization process. From our numerical calculation, the intermediate region D_{c1} < D < D_{c2} should be characterized by a magnetized valence-bond-solid state.Comment: 6 pages, 8 figure

Distinction of disorder, classical and quantum vibrational contributions to atomic mean-square amplitudes in dielectric pentachloronitrobenzene

The solid-state molecular disorder of pentachloronitrobenzene (PCNB) and its role in causing anomalous dielectric properties are investigated. Normal coordinate analysis (NCA) of atomic mean-square displacement parameters (ADPs) is employed to distinguish disorder contributions from classical and quantum-mechanical vibrational contributions. The analysis relies on multitemperature (5-295 K) single-crystal neutron-diffraction data. Vibrational frequencies extracted from the temperature dependence of the ADPs are in good agreement with THz spectroscopic data. Aspects of the static disorder revealed by this work, primarily tilting and displacement of the molecules, are compared with corresponding results from previous, much more in-depth and time-consuming Monte Carlo simulations; their salient findings are reproduced by this work, demonstrating that the faster NCA approach provides reliable constraints for the interpretation of diffuse scattering. The dielectric properties of PCNB can thus be rationalized by an interpretation of the temperature-dependent ADPs in terms of thermal motion and molecular disorder. The use of atomic displacement parameters in the NCA approach is nonetheless hostage to reliable neutron data. The success of this study demonstrates that state-of-the-art single-crystal Laue neutron diffraction affords sufficiently fast the accurate data for this type of study. In general terms, the validation of this work opens up the field for numerous studies of solid-state molecular disorder in organic materials.Comment: Now published in Physical Review

Confirmation of a one-dimensional spin-1/2 Heisenberg system with ferromagnetic first-nearest-neighbor and antiferromagnetic second-nearest-neighbor interactions in Rb2{}_{2}Cu2{}_{2}Mo3{}_{3}O12{}_{12}

We have investigated magnetic properties of Rb$_2$Cu$_2$Mo$_3$O$_{12}$ powder. Temperature dependence of magnetic susceptibility and magnetic-field dependence of magnetization have shown that this cuprate is a model compound of a one-dimensional spin-1/2 Heisenberg system with ferromagnetic first-nearest-neighbor (1NN) and antiferromagnetic second-nearest-neighbor (2NN) competing interactions (competing system). Values of the 1NN and 2NN interactions are estimated as $J_1 = -138$ K and $J_2 = 51$ K ($\alpha \equiv J_2 / J_1 = -0.37$). This value of $\alpha$ suggests that the ground state is a spin-singlet incommensurate state. In spite of relatively large $J_1$ and $J_2$, no magnetic phase transition appears down to 2 K, while an antiferromagnetic transition occurs in other model compounds of the competing system with ferromagnetic 1NN interaction. For that reason, Rb$_2$Cu$_2$Mo$_3$O$_{12}$ is an ideal model compound to study properties of the incommensurate ground state that are unconfirmed experimentally.Comment: 6 pages, 4 figure

Phase Diagrams of S=3/2, 2 XXZ Spin Chains with Bond-Alternation

We study the phase diagram of S=3/2 and S=2 bond-alternating spin chains numerically. In previous papers, the phase diagram of S=1 XXZ spin chain with bond-alternation was shown to reflect the hidden $Z_{2}\times Z_{2}$ symmetry. But for the higher S Heisenberg spin chain, the successive dimerization transition occurs, and for anisotropic spin chains the phase structure will be more colorful than the S=1 case. Using recently developed methods, we show directly that the phase structure of the anisotropic spin chains relates to the $Z_{2}\times Z_{2}$ symmetry.Comment: 13 pages, 6 figures(eps), RevTe

Bifurcation at the c=3/2 Takhtajan-Babujian point to the c=1 critical lines

We study the S=1 quantum spin chains with bilinear, biquadratic, plus bond alternation in the vicinity of the S=1 Takhtajan-Babujian model. Transition line between the Haldane and the dimer phases are determined numerically. To see the crossover behavior from c=3/2 (k=2 SU(2) WZW model) at the Takhtajan-Babujian point to c=1 (k=1 SU(2) WZW model), we calculate the conformal anomaly c and scaling dimensions of the primary fields on the transition line.Comment: 10 pages, 8 figure

Tunneling and rattling in clathrate crystal

We present tunneling and rattling motions of an off-center guest atom in cage referring to a clathrate crystal La_3Pd_{20}Ge_6. The elastic constant C_{44} of La_3Pd_{20}Ge_6 shows a Debye-type dispersion around 20 K obeying a relaxation time tau = tau_0exp(E/k_{B}T) with an attempt time tau_0 = 2.0*10^{-12} sec and an activation energy E = 197 K. At low temperatures below 3 K down to 20 mK, the C_{44} shows a softening of C_{44} = C_{44}^0(T-T_C^0)/(T-Theta) with T_C^0 = -337.970 mK and Theta = -338.044 mK. These facts are attributed to two different types of the off-center motions with Gamma_5 symmetry in 4a-site cage of La_3Pd_{20}Ge_6, a thermally activated rattling motion over the potential hill and a tunneling motion through the potential hill at low temperatures.Comment: 5 pages, 4 figures, to be published Phys. Rev.