arXiv subjects

T. Shimada

Publications and source records attributed to T. Shimada.

24 records · Page 2Linked to original sources

arXiv2010

Exciton diffusion in air-suspended single-walled carbon nanotubes

Direct measurements of the diffusion length of excitons in air-suspended single-walled carbon nanotubes are reported. Photoluminescence microscopy is used to identify individual nanotubes and to determine their lengths and chiral indices. Exciton diffusion length is obtained by comparing the dependence of photoluminescence intensity on the nanotube length to numerical solutions of diffusion equations. We find that the diffusion length in these clean, as-grown nanotubes is significantly longer than those reported for micelle-encapsulated nanotubes.

cond-mat.mes-hall↗

arXiv2003

Kinetics and thermodynamics of the degree of order of the B cations in double-perovskite Sr2FeMoO6

Here we show that the B-site cation (Fe/Mo) ordering in the double-perovskite magnetoresistor, Sr2FeMoO6, is controlled by either kinetics or thermal equilibrium depending on the temperature range. In order to enhance the ordering, long synthesis periods at moderate temperatures are thus required. Employment of a special oxygen-getter-controlled low-O2-pressure encapsulation technique for the sample synthesis enabled us to use long heating periods without evident evaporation of Mo, and thereby reach the thermal equilibrium state and a high degree of B-site cation order. For samples fired at 1150C for long periods (> 36 hours), record-high Ms values of 3.7 ~ 3.9 muB were obtained.

cond-mat.str-el↗

arXiv2003

Iron and molybdenum valences in double-perovskite (Sr,Nd)2FeMoO6: electron-doping effect

Double perovskite, (Sr1-xNdx)2FeMoO6, was doped with electrons through partial substitution of divalent Sr by trivalent Nd (0 < x < 0.2). The Fe valence and the degree of B-site order were probed by 57Fe Mossbauer spectroscopy. Replacing Sr by Nd increased the fraction of Fe and Mo atoms occupying wrong sites, i.e. antisite disorder. It had very little effect on the Fe valence: a small but visible increase in the isomer shift was seen for the mixed-valent FeII/III atoms occupying the right site indicating a slight movement towards divalency of these atoms, which was more than counterbalanced by the increase in the fraction of antisite Fe atoms with III valence state. It is therefore argued that the bulk of the electron doping is received by antisite Mo atoms, which - being surrounded by six MoV/VI atoms - prefer the lower IV/V valence state. Thus under Nd substitution, the charge-neutrality requirement inflicts a lattice disorder such that low-valent MoIV/V can exist.

cond-mat.str-el↗

arXiv2000

A Resolution of the Puzzle of the Posi-nega Switch Mechanism in the Globally Coupled Map Lattice

We revisit the globally coupled map lattice (GCML). We use a family of universal 'curves of balance' between two conflicting tendencies in the model- the randomness in each map and the coherence due to an averaging interaction - and we locate the posi-nega switch region in the parameter space of the model. We clarify the mechanism of a basic posi-nega switch in the two-cluster regime, which guarantees no mixing of maps across their mean field in the chaotic transient process. We stress that a special attention must be paid for rounding-off errors; otherwise, there fatally occurs an artifitial numerical degeneracy of maps in a cluster due to a highly negative Lyapunov exponent of the clustor attractor.

nlin.CD↗

arXiv1997

Rapidity Gaps in e+e- Annihilation and Parton Hadron Duality

We calculate the probability for rapidity gaps in the parton cascade for different approximations within the perturbative QCD and compare the results with recent measurements. The aim is to find out whether the dual connection between the parton and hadron final states -- observed so far in various inclusive measurements -- holds as well for the extreme kinematic configurations with colour sources separated by large rapidity gaps. A description of the data is possible indeed choosing the parameters of the cascade in the range suggested by recent analyses of the energy spectra (the $k_\perp$ cutoff $Q_0~\gsim$ QCD scale $Λ\sim 250$ MeV).

hep-ph↗

arXiv1993

String-Based Methods in Perturbative Gravity

String theory implies a relatively modest growth in computational complexity for perturbative gravity calculations as compared to gauge theory calculations, contrary to field theory expectations. An explicit string-based calculation, which would be extremely difficult using conventional techniques, is presented to illustrate this.

hep-th↗