We have measured the diffusion thermopower of a quantum Hall system in a Corbino setup. A concentric electron-temperature gradient is introduced by irradiating microwaves, via a coplanar waveguide, near the outer rim of a circular mesa of a two-dimen
sional electron gas. The resulting radial thermovoltages exhibit sawtooth-like oscillations with the magnetic field, taking large positive (negative) values just below (above) integer fillings with sign reversal at the center of the quantum Hall plateaus. The behavior is in agreement with a recent theory [Y. Barlas and K. Yang: Phys. Rev. B 85 (2012) 195107], which treats disorder within the self-consistent Born approximation.
We have measured the rf magnetoconductivity of unidirectional lateral superlattices (ULSLs) by detecting the attenuation of microwave through a coplanar waveguide placed on the surface. ULSL samples with the principal axis of the modulation perpendic
ular (S_perp) and parallel (S_||) to the microwave electric field are examined. For low microwave power, we observe expected anisotropic behavior of the commensurability oscillations (CO), with CO in samples S_perp and S_|| dominated by the diffusion and the collisional contributions, respectively. Amplitude modulation of the Shubnikov-de Haas oscillations is observed to be more prominent in sample S_||. The difference between the two samples is washed out with the increase of the microwave power, letting the diffusion contribution govern the CO in both samples. The failure of the intended directional selectivity in the conductivity measured with high microwave power is interpreted in terms of large-angle electron-phonon scattering.
We have measured magnetoresistance of hexagonal lateral superlattices. We observe three types of oscillations engendered by periodic potential modulation having hexagonal-lattice symmetry: amplitude modulation of the Shubnikov-de Haas oscillations, c
ommensurability oscillations, and the geometric resonances of open orbits generated by Bragg reflections. The latter two reveal the presence of two characteristic periodicities, sqrt{3} a / 2 and a / 2, inherent in a hexagonal lattice with the lattice constant a. The formation of the hexagonal-superlattice minibands manifested by the observation of open orbits marks the first step toward realizing massless Dirac fermions in semiconductor 2DEGs.
We have investigated the effect of unidirectional periodic potential modulation on the fractional quantum Hall (FQH) state at filling factors nu=5/3 and 4/3. For large enough modulation amplitude, we find that the resistivity minimum at nu=5/3 gives
way to a peak that grows with decreasing temperature. Density matrix renormalization group calculation reveals that phase transition from FQH state to unidirectional striped state having a period sim 4 l (with l the magnetic length) takes place at nu=1/3 (equivalent to nu=5/3 by the particle-hole symmetry) with the increase of the modulation amplitude, suggesting that the observed peak is the manifestation of the stripe phase.
We report on a deep search for CO(J=3-2) line emission from the host galaxy of GRB 980425 with the Atacama Submillimeter Telescope Experiment (ASTE). We observed five points of the galaxy covering the entire region. After combining all of the spectra
, we obtained a global spectrum with the rms noise level of 3.3 mK in T_mb scale at a velocity resolution of 10 km s^-1. No significant emission was detected, though we find a marginal emission feature in the velocity range corresponding to the redshift of the galaxy. We derive 3 sigma upper limits on the global properties: the velocity-integrated CO(3-2) intensity of I_CO(3-2) < 0.26 K km s^-1 by adopting a velocity width of 67 km s^-1; the H_2 column density of N(H_2) < 3 x 10^20 cm^-2; the molecular gas mass of M(H_2) < 3 x 10^8 M_sun, by assuming a CO line luminosity to H_2 molecular gas mass conversion factor of X_CO = 5.0 x 10^20 cm^-2 (K km s^-1)^-1; and the star formation rate of SFR < 0.1 M_sun yr^-1, based on the Schmidt law. The SFR is consistent with the previous results of H_alpha and mid-IR observations, thereby suggesting that there is no significant obscured star formation in the host galaxy of GRB 980425. This result implies that there is a variety of GRB hosts with regard to the presence of obscured star formation.
