Tadashi Takamasu

Field-induced magnetic ordering in the quantum spin system KCuCl3

KCuCl 3 is a three-dimensional coupled spin-dimer system and has a singlet ground state with an excitation gap Δ/k B =31 K. High-field magnetization measurements for KCuCl 3 have been performed in static magnetic fields ofup to 30 T and in pulsed magnetic fields of up to 60 T. The entire magnetization curve including the saturation region was obtained at T= 1.3 K. From the analysis of the magnetization curve, it was found that the exchange parameters determined from the dispersion relations of the magnetic excitations should be reduced, which suggests the importance of the renormalization effect in the magnetic excitations. The field-induced magnetic ordering accompanied by the cusplike minimum of the magnetization was observed, as in the isomorphous compound TlCuCl 3 . The phase boundary was almost independent of the field direction, and is represented by the power law. These results are consistent with the magnon Bose-Einstein condensation picture for field-induced magnetic ordering.

Micrometer-Scale Photonic Circuit Components Based on Propagation of Exciton Polaritons in Organic Dye Nanofibers

Since photonic circuits possess advantages over electronic circuits in bandwidth and resistance to electromagnetic wave interference, miniaturized photonic circuits offer promising applications in various fi elds. [ 1 ] However, the diffraction limit of light restricts the degree to which conventional optical waveguide circuits can be miniaturized. Hence, plasmon waveguides, [ 2 , 3 ] photonic crystal waveguides, [ 4 , 5 ] and semiconductor nanofi bers [ 6 , 7 ] have been extensively developed to guide optical signals and to manipulate them below the diffraction limit. In this communication, we report a novel approach to create micrometer-scale photonic circuits by using the propagation of exciton polaritons (EPs) in organic dye nanofi bers. EPs are quasi-particles formed by strong coupling between photons and excitons. This coupling leads to a signifi cantly large refractive index of the crystal, which may allow quasi-one-dimensional structures (nanofi bers) to guide EPs and to manipulate them below the diffraction limit of light. This in turn enables one to create EP-based photonic circuits that can be highly miniaturized as compared to conventional optical waveguide circuits. Organic dye thiacyanine (TC, Figure 1 a) self-assembles into nanofi bers with lengths of up to ∼ 250 μ m in solution. [ 8–10 ]

Micro‐demultiplexer of Coupled Resonator Optical Waveguide Fabricated by Microspheres

How does light propagate within a chain of transparent spheres? In the case of microspheres with diameters of a few micrometers, the feature of an optical resonator should be included in the discussion because the diameters are nearly the same order of magnitude as the wavelength of light. In microspheres, light goes around the circumference of those with whispering gallery modes (WGMs), and indicates a spectrum that has sharp and discrete peaks. [ 1–4 ] Moreover, waveguides that utilize a weak coupling between resonators, such as microrings, microdisks, or microspheres, are called coupled-resonator optical waveguides (CROWs). [ 3 , 5 , 6 ] By controlling the coupling effi cient, this concept can be applied to on-chip optical buffer memory. [ 7–9 ] Another particularly appealing feature of CROWs is the possibility of making loss-less and refl ection-less bends with a wavelength-scale curvature because the microresonators can couple at an arbitrary point on the circumference of light going around. When we use microspheres as the microresonators, CROWs allow us to utilize a self-assembly phenomenon in colloidal suspension to align and connect microspheres with non-elaborate processing. [ 10–15 ] Moreover, if we use a lithographically patterned substrate which is formed lines of dimples in order to trap the microspheres, we can fabricate arbitraryshape waveguides for on-chip optical circuits. Since the prevalent waveguide concepts [ 16–19 ] require very elaborate technology to fabricate such sharp bends, or require a delicate assembling technique, the CROW concept might partly replace the prevalent concepts. In this article, we utilize a self-assembly phenomenon

Possible phase transition deep inside the hidden order phase of ultraclean URu2Si2.

To elucidate the underlying nature of the hidden order (HO) state in heavy-fermion compound URu(2)Si(2), we measure electrical transport properties of ultraclean crystals in a high field, low temperature regime. Unlike previous studies, the present system with much less impurity scattering resolves a distinct anomaly of the Hall resistivity at H;{*} = 22.5 T, well below the destruction field of the HO phase = or approximately 36 T. In addition, a novel quantum oscillation appears above a magnetic field slightly below H;{*}. These results indicate an abrupt reconstruction of the Fermi surface, which implies a possible phase transition well within the HO phase caused by a band-dependent destruction of the HO parameter.

Large anisotropy in the magnetic susceptibility of metallic carbon nanotubes.

T. A. Searles, Y. Imanaka, T. Takamasu, H. Ajiki, J. A. Fagan, E. K. Hobbie, and J. Kono ∗ Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, USA National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan Photon Pioneers Center, Osaka University, Suita 565-0871, Japan National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA (Dated: January 4, 2010)

Anomalous magnetic field effects on photochemical reactions in ionic liquid under ultrahigh fields of up to 28 T.

The magnetic field effects (MFEs) on photoinduced hydrogen abstraction reactions between benzophenone and thiophenol in an ionic liquid, N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl) imide (TMPA TFSI), were studied by a nanosecond laser flash photolysis technique under ultrahigh fields of up to 28 T. Extremely large and anomalous stepwise MFEs were observed for the first time. The escape yield of benzophenone ketyl radical decreased with increasing magnetic field strength (B) at 0 T<B<or=2 T. The decrease was almost saturated at 2 T<B<or=10 T. At much higher fields (10 T<B<or=28 T), the yield decreased again with increasing B, producing a 25% decrease at 28 T.

Influence of micro-joints formed between spheres in coupled-resonator optical waveguide.

Light propagation is simulated through coupled-resonator optical waveguides (CROWs) composed of seven transparent polystyrene microspheres, including micro-joints formed between the spheres. In nanojet-induced mode (NIM) light propagation, the micro-joints increased the optical coupling between microspheres drastically, and the light confinement by individual microspheres weakened as the micro-joint diameter increases. These results suggest that we can control NIM light propagation by changing the micro-joint diameter; this amounts to a nanojet throttle valve.

InAs Quantum Dots Growth by Modified Droplet Epitaxy Using Sulfur Termination.

We have applied the modified droplet epitaxy method using sulfur termination to fabricate InAs quantum dots on GaAs(001) substrates for the first time. It is observed that the S-terminated surface effectively prevents the two-dimensional growth of InAs, forming InAs nanocrystals. From the magneto-photoluminescence measurements of this structure, three-dimensional quantum confinement effect was confirmed. This modified droplet epitaxy method is promising for the fabrication of quantum dots, not only in the lattice-matched system but also in the lattice-mismatched system.

Anisotropic elasticity of magnetically ordered agarose gel.

Abstract The physical properties of agarose gel prepared under strong magnetic fields were investigated. The storage modulus was measured by the reflection method with an ultrasonic pulse. The measurement results of the gels elasticity indicate that agarose gel has anisotropic properties. The elasticity and its anisotropy depend on the concentration of the gel and the magnetic field to which it is exposed. The experimental results indicate that the anisotropic network structure of the gel is induced by the exposure to the magnetic field during gelation. The gelation mechanism under a magnetic field is discussed.

The anisotropic properties of magnetically ordered gel

Abstract The preparation of gels under strong magnetic fields causes the molecular ordering of the gels. The relationships between the magnetic ordering and various physical properties of agarose gel at various concentrations were investigated. It was found that the ordering of the gel molecules is related to the phase transition temperature. Our measurements of the electrophoretic velocity of DNA, the elasticity of the gel and its ratio of shrinkage in acetone–water systems indicate that agarose gel has anisotropic properties. The experimental results indicate the presence of an anisotropic network structure in the gel.