Thermodynamics of Spin-1/2 AF-AF-F and F-F-AF Trimerized Quantum Heisenberg Chains

The magnetization process, the susceptibility and the specific heat of the spin-1/2 AF-AF-F and F-F-AF trimerized quantum Heisenberg chains have been investigated by means of the transfer matrix renormalization group (TMRG) technique as well as the modified spin-wave (MSW) theory. A magnetization plateau at $m=1/6$ for both trimerized chains is observed at low temperature. The susceptibility and the specific heat show various behaviors for different ferromagnetic and antiferromagnetic interactions and in different magnetic fields. The TMRG results of susceptibility and the specific heat can be nicely fitted by a linear superposition of double two-level systems, where two fitting equations are proposed. Three branch excitations, one gapless excitation and two gapful excitations, for both systems are found within the MSW theory. It is observed that the MSW theory captures the main characteristics of the thermodynamic behaviors at low temperatures. The TMRG results are also compared with the possible experimental data.Comment: 11 pages, 10 figure

A semi-proximal-based strictly contractive Peaceman-Rachford splitting method

The Peaceman-Rachford splitting method is very efficient for minimizing sum of two functions each depends on its variable, and the constraint is a linear equality. However, its convergence was not guaranteed without extra requirements. Very recently, He et al. (SIAM J. Optim. 24: 1011 - 1040, 2014) proved the convergence of a strictly contractive Peaceman-Rachford splitting method by employing a suitable underdetermined relaxation factor. In this paper, we further extend the so-called strictly contractive Peaceman-Rachford splitting method by using two different relaxation factors, and to make the method more flexible, we introduce semi-proximal terms to the subproblems. We characterize the relation of these two factors, and show that one factor is always underdetermined while the other one is allowed to be larger than 1. Such a flexible conditions makes it possible to cover the Glowinski's ADMM whith larger stepsize. We show that the proposed modified strictly contractive Peaceman-Rachford splitting method is convergent and also prove $O(1/t)$ convergence rate in ergodic and nonergodic sense, respectively. The numerical tests on an extensive collection of problems demonstrate the efficiency of the proposed method

Inhibition of Subsets of G Protein-coupled Receptors by Empty Mutants of G Protein α Subunits in Go, G11, and G16

We previously reported that the xanthine nucleotide binding Goα mutant, GoαX, inhibited the activation of Gi-coupled receptors. We constructed similar mutations in G11α and G16α and characterized their nucleotide binding and receptor interaction. First, we found that G11αX and G16αX expressed in COS-7 cells bound xanthine 5'-O-(thiotriphosphate) instead of guanosine 5'-O-(thiotriphosphate). Second, we found that G11αX and G16αX interacted with βγ subunits in the presence of xanthine diphosphate. These experiments demonstrated that G11aαX and G16αX were xanthine nucleotide-binding proteins, similar to GoαX. Third, in COS-7 cells, both G11αX and G16αX inhibited the activation of Gq-coupled receptors, whereas only G16αX inhibited the activation of Gi-coupled receptors. Therefore, when in the nucleotide-free state, empty G11αX and G16αX appeared to retain the same receptor binding specificity as their wild-type counterparts. Finally, we found that GoαX, G11αX, and G16αX all inhibited the endogenous thrombin receptors and lysophosphatidic acid receptors in NIH3T3 cells, whereas G11αX and G16αX, but not GoαX, inhibited the activation of transfected m1 muscarinic receptor in these cells. We conclude that these empty G protein mutants of Goα, G11α, and G16α can act as dominant negative inhibitors against specific subsets of G protein-coupled receptors

Ground state properties of a Tonks-Girardeau Gas in a periodic potential

In this paper, we investigate the ground-state properties of a bosonic Tonks-Girardeau gas confined in a one-dimensional periodic potential. The single-particle reduced density matrix is computed numerically for systems up to $N=265$ bosons. Scaling analysis of the occupation number of the lowest orbital shows that there are no Bose-Einstein Condensation(BEC) for the periodically trapped TG gas in both commensurate and incommensurate cases. We find that, in the commensurate case, the scaling exponents of the occupation number of the lowest orbital, the amplitude of the lowest orbital and the zero-momentum peak height with the particle numbers are 0, -0.5 and 1, respectively, while in the incommensurate case, they are 0.5, -0.5 and 1.5, respectively. These exponents are related to each other in a universal relation.Comment: 9 pages, 10 figure

Possible d0 ferromagnetism in MgO doped with nitrogen

We study the possibility of d0 ferromagnetism in the compound MgO doped with nitrogen (N). The Haldane-Anderson impurity model is formulated within the tight-binding approximation for determining the host band-structure and the impurity-host hybridization. Using the quantum Monte Carlo technique, we observe a finite local moment for an N impurity, and long-range ferromagnetic correlations between two N impurities. The ferromagnetic correlations are strongly influenced by the impurity bound state. When the ferromagnetic correlation between a pair of impurities is mapped onto the isotropic Heisenberg model for two spin-1/2 particles, the effective exchange constant J12 is found to increase with increasing temperature. Similar temperature dependence of J12 is also obtained in other diluted magnetic semiconductors, such as zincblende ZnO doped with Mn. The temperature dependence of J12 suggests that the mapping of the full Hamiltonian onto the spin Hamiltonian cannot fully describe the magnetic correlations for the diluted magnetic semiconductors at least in the limit of low impurity spin.Comment: 8 pages, 8 figure