Role of Phase Variables in Quarter-Filled Spin Density Wave States

Several kinds of spin density wave (SDW) states with both quarter-filled band and dimerization are reexamined for a one-dimensional system with on-site, nearest-neighbor and next-nearest-neighbor repulsive interactions, which has been investigated by Kobayashi et al. (J. Phys. Soc. Jpn. 67 (1998) 1098). Within the mean-field theory, the ground state and the response to the density variation are calculated in terms of phase variables, $\theta$ and $\phi$, where $\theta$ expresses the charge fluctuation of SDW and $\phi$ describes the relative motion between density wave with up spin and that with down spin respectively. It is shown that the exotic state of coexistence of 2k_F-SDW and 2k_F-charge density wave (CDW) is followed by 4k_F-SDW but not by 4k_F-CDW where k_F denotes a Fermi wave vector. The harmonic potential with respect to the variation of $\theta$ and/or $\phi$ disappears for the interactions, which lead to the boundary between the pure 2k_F-SDW state and the corresponding coexistent state.Comment: 9 pages, 15 figures, to be published in J. Phys. Soc. Jpn. 69 No.3 (2000) 79

Exact-Differential Large-Scale Traffic Simulation

Analyzing large-scale traffics by simulation needs repeating execution many times with various patterns of scenarios or parameters. Such repeating execution brings about big redundancy because the change from a prior scenario to a later scenario is very minor in most cases, for example, blocking only one of roads or changing the speed limit of several roads. In this paper, we propose a new redundancy reduction technique, called exact-differential simulation, which enables to simulate only changing scenarios in later execution while keeping exactly same results as in the case of whole simulation. The paper consists of two main efforts: (i) a key idea and algorithm of the exact-differential simulation, (ii) a method to build large-scale traffic simulation on the top of the exact-differential simulation. In experiments of Tokyo traffic simulation, the exact-differential simulation shows 7.26 times as much elapsed time improvement in average and 2.26 times improvement even in the worst case as the whole simulation

Magnetic response and quantum critical behavior in the doped two-leg extended Hubbard ladder

We have investigated quantum critical behavior in the doped two-leg extended Hubbard ladder, by using a weak-coupling bosonization method. In the ground state, the dominant fluctuation changes from the conventional d-wave-like superconducting (SCd) state into density-wave states, with increasing nearest-neighbor repulsions and/or decreasing doping rate. The competition between the SCd state and the charge-density-wave state coexisting with the p-density-wave state becomes noticeable on the critical point, at which the gap for magnetic excitations vanishes. Based on the Majorana-fermion description of the effective theory, we calculate the temperature dependence of the magnetic response such as the spin susceptibility and the NMR relaxation rate, which exhibit unusual properties due to two kinds of spin excitation modes. On the quantum critical point, the spin susceptibility shows paramagnetic behavior with logarithmic corrections and the NMR relaxation rate also exhibits anomalous power-law behavior. We discuss the commensurability effect due to the umklapp scattering and relevance to the two-leg ladder compounds Sr_{14-x}Ca_xCu_{24}O_{41}.Comment: 18 pages, 9 figures, accepted for publication in Phys. Rev.

Alternative formalism to the slave particle mean field theory of the t-J model without deconfinement

An alternative formalism that does not require the assumption of the deconfinement phase of a U(1) gauge field is proposed for the slave particle mean field theory. Starting form the spin-fermion model, a spinon field, which is either fermion or boson, is introduced to represent the localized spin moment. We find a d-wave superconductive state in the mean field theory in the case of the fermion representation of the localized spin moment that corresponds to the slave boson mean field theory of the t-J model, whereas the d-wave superconductive state is absent in case of the Schwinger boson representation of the localized spin moments.Comment: 8 page

Towards large-scale what-if traffic simulation with exact-differential simulation

To analyze and predict a behavior of large-scale traffics with what-if simulation, it needs to repeat many times with various patterns of what-if scenarios. In this paper, we propose new techniques to efficiently repeat what-if simulation tasks with exact-differential simulation. The paper consists of two main efforts: what-if scenario filtering and exact-differential cloning. The what-if scenario filtering enables to pick up meaningful what-if scenarios and reduces the number of what-if scenarios, which directly decreases total execution time of repeating. The exact-differential cloning enables to execute exact-differential simulation tasks in parallel to improve its total execution time. In our preliminary evaluation in Tokyo bay area's traffic simulation, we show potential of our proposals by estimating how the what-if scenarios filtering reduces the number of meaningless scenarios and also by estimating a performance improvement from our previous works with the exact-differential cloning

Serum factors that suppress cytotoxic effect of methotrexate

To study the phenomenon that human erythroid leukemia K-562 cells are more sensitive to cytotoxic effect of antimetabolites when cultured in a serum-free medium than in a conventional medium containing fetal calf serum (FCS). Methods: Cytotoxic effects of methotrexate, azaserine and 5-fluorouracil were estimated by accessing the lactate dehydrogenase (LDH) activity of viable tumor cells. Proteins of FCS were separated using two-dimensional electrophoresis followed by mass spectrometry analysis. Results: Addition of 10% FCS attenuated anti-tumor activity of methotrexate and azaserine against K-562 cells compared with serum-free medium. Such an activity of FCS was different for each serum lot. Comparison of the proteins in active serum lot with those in not active one using two-dimensional electrophoresis showed that in the active serum there were proteins 150 kDa, which were absent in the not active serum lot. Mass spectrometry indicated that all those proteins had the amino acid sequence of albumin. Sera of one healthy volunteer and two patients with thyroid cancer also attenuated the activity of the agent. Conclusion: Several lots of FCS and human serum demonstrated the ability to attenuate the cytotoxic effect of methotrexate in vitro, possibly due to the formation of albumin dimers/MTX complexes