Infrared spectroscopy under multi-extreme conditions: Direct observation of pseudo gap formation and collapse in CeSb

Infrared reflectivity measurements of CeSb under multi-extreme conditions (low temperatures, high pressures and high magnetic fields) were performed. A pseudo gap structure, which originates from the magnetic band folding effect, responsible for the large enhancement in the electrical resistivity in the single-layered antiferromagnetic structure (AF-1 phase) was found at a pressure of 4 GPa and at temperatures of 35 - 50 K. The optical spectrum of the pseudo gap changes to that of a metallic structure with increasing magnetic field strength and increasing temperature. This change is the result of the magnetic phase transition from the AF-1 phase to other phases as a function of the magnetic field strength and temperature. This result is the first optical observation of the formation and collapse of a pseudo gap under multi-extreme conditions.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev.

Quasi-particle scattering and protected nature of topological states in a parent topological insulator Bi2_2Se3_3

We report on angle resolved photoemission spectroscopic studies on a parent topological insulator (TI), Bi$_2$Se$_3$. The line width of the spectral function (inverse of the quasi-particle lifetime) of the topological metallic (TM) states shows an anomalous behavior. This behavior can be reasonably accounted for by assuming decay of the quasi-particles predominantly into bulk electronic states through electron-electron interaction and defect scattering. Studies on aged surfaces reveal that topological metallic states are very much unaffected by the potentials created by adsorbed atoms or molecules on the surface, indicating that topological states could be indeed protected against weak perturbations.Comment: accepted for publication in Phys. Rev. B(R

Spin-Wave Spectrum in `Single-Domain' Magnetic Ground State of Triangular Lattice Antiferromagnet CuFeO2

By means of neutron scattering measurements, we have investigated spin-wave excitation in a collinear four-sublattice (4SL) magnetic ground state of a triangular lattice antiferromagnet CuFeO2, which has been of recent interest as a strongly frustrated magnet, a spin-lattice coupled system and a multiferroic. To avoid mixing of spin-wave spectrum from magnetic domains having three different orientations reflecting trigonal symmetry of the crystal structure, we have applied uniaxial pressure on [1-10] direction of a single crystal CuFeO2. By elastic neutron scattering measurements, we have found that only 10 MPa of the uniaxial pressure results in almost 'single domain' state in the 4SL phase. We have thus performed inelastic neutron scattering measurements using the single domain sample, and have identified two distinct spin- wave branches. The dispersion relation of the upper spin-wave branch cannot be explained by the previous theoretical model [R. S. Fishman: J. Appl. Phys. 103 (2008) 07B109]. This implies the importance of the lattice degree of freedom in the spin-wave excitation in this system, because the previous calculation neglected the effect of the spin-driven lattice distortion in the 4SL phase. We have also discussed relationship between the present results and the recently discovered "electromagnon" excitation.Comment: 5 pages, 3 figures, accepted for publication in J. Phys. Soc. Jp

Cluster structures in Oxygen isotopes

Cluster structure of 16O,18O and 20O is investigated by the antisymmettrized molecular dynamics (AMD) plus generator coordinate method (GCM). We have found the K^{\pi}=0$_2^+$ and 0$_1^-$ rotational bands of 18O that have the prominent 14C+\alpha cluster structure. Clustering systematics becomes richer in 20O. We suggest the K^{\pi}=0$_2^+$ band that is the mixture of the 12C+\alpha+4n and 14C+6He cluster structures, and the K^{\pi}=0$_1^-$ band that has the 14C+6He cluster structure. The K^{\pi}=0$_3^+$ and 0$_2^-$ bands that have the prominent 16C+\alpha cluster structure are also found.Comment: 9pages, 9figure

Quantum Larmor radiation in conformally flat universe

We investigate the quantum effect on the Larmor radiation from a moving charge in an expanding universe based on the framework of the scalar quantum electrodynamics (SQED). A theoretical formula for the radiation energy is derived at the lowest order of the perturbation theory with respect to the coupling constant of the SQED. We evaluate the radiation energy on the background universe so that the Minkowski spacetime transits to the Milne universe, in which the equation of motion for the mode function of the free complex scalar field can be exactly solved in an analytic way. Then, the result is compared with the WKB approach, in which the equation of motion of the mode function is constructed with the WKB approximation which is valid as long as the Compton wavelength is shorter than the Hubble horizon length. This demonstrates that the quantum effect on the Larmor radiation of the order e^2\hbar is determined by a non-local integration in time depending on the background expansion. We also compare our result with a recent work by Higuchi and Walker [Phys. Rev. D80 105019 (2009)], which investigated the quantum correction to the Larmor radiation from a charged particle in a non-relativistic motion in a homogeneous electric field.Comment: 12 pages, 4 figure, accepted for publication in Physical Review

Leaf area index and topographical effects on turburlent diffusion in a deciduous forest

In order to investigate turbulent diffusion in a deciduous forest canopy, wind velocity measurements were conducted from late autumn of 2009 to early spring of 2010, using an observation tower 20 m in height located in the campus of Kanazawa University. Four sonic anemometers mounted on the tower recorded the average wind velocities and temperatures, as well as their fluctuations, at four different heights simultaneously. Two different types of data sets were selected, in which the wind velocities, wind bearings and atmospheric stabilities were all similar, but the Leaf Area Indexes (LAI's) were different. Vertical profiles of average wind velocities were found to have an approximately exponential profile in each case. The characteristic length scales of turbulence were evaluated by both von Karman's method and the integral time scale deduced from the autocorrelation from time-series analyses. Both methods produced comparable values of eddy diffusivity for the cases with some foliage during late autumn, but some discrepancy in the upper canopy layer was observed when the trees did not have their leaves in early spring. It was also found that the eddy diffusivities generally take greater values at higher positions, where the wind speeds are large. Anisotropy of eddy diffusivities between the vertical and horizontal components was also observed, particularly in the cases when the canopy does not have leaves, when the horizontal eddy diffusivities are generally larger than the vertical ones. On the other hand, the anisotropy is less visible when the trees have some foliage during autumn. The effects of topography on the turbulent diffusion were also investigated, including evaluation of the non-zero time-averaged vertical wind velocities. The results show that the effects are marginal for both cases, and can be neglected as far as diffusion in the canopy is concerned

Novel ordering of an S = 1/2 quasi one-dimensional Ising-like anitiferromagnet in magnetic field

High-field specific heat measurements on BaCo2V2O8, which is a good realization of an S = 1/2 quasi one-dimensional Ising-like antifferomagnet, have been performed in magnetic fields up to 12 T along the chain and at temperature down to 200 mK. We have found a new magnetic ordered state in the field-induced phase above Hc ~ 3.9 T. We suggest that a novel type of the incommensurate order, which has no correspondence to the classical spin system, is realized in the field-induced phase.Comment: 4pages, 4figure

Electronic inhomogeneity in EuO: Possibility of magnetic polaron states

We have observed the spatial inhomogeneity of the electronic structure of a single-crystalline electron-doped EuO thin film with ferromagnetic ordering by employing infrared magneto-optical imaging with synchrotron radiation. The uniform paramagnetic electronic structure changes to a uniform ferromagnetic structure via an inhomogeneous state with decreasing temperature and increasing magnetic field slightly above the ordering temperature. One possibility of the origin of the inhomogeneity is the appearance of magnetic polaron states.Comment: 4 pages, 3 figure