A conserved and essential basic region mediates tRNA binding to the Elp1 subunit of the <em>Saccharomyces cerevisiae</em> Elongator complex

Elongator is a conserved, multi-protein complex discovered in Saccharomyces cerevisiae, loss of which confers a range of pleiotropic phenotypes. Elongator in higher eukaryotes is required for normal growth and development and a mutation in the largest subunit of human Elongator (Elp1) causes familial dysautonomia, a severe recessive neuropathy. Elongator promotes addition of mcm(5) and ncm(5) modifications to uridine in the tRNA anticodon ‘wobble’ position in both yeast and higher eukaryotes. Since these modifications are required for the tRNAs to function efficiently, a translation defect caused by hypomodified tRNAs may therefore underlie the variety of phenotypes associated with Elongator dysfunction. The Elp1 carboxy-terminal domain contains a highly conserved arginine/lysine-rich region that resembles a nuclear localization sequence (NLS). Using alanine substitution mutagenesis, we show that this region is essential for Elongator's function in tRNA wobble uridine modification. However, rather than acting to determine the nucleo-cytoplasmic distribution of Elongator, we find that the basic region plays a critical role in a novel interaction between tRNA and the Elp1 carboxy-terminal domain. Thus the conserved basic region in Elp1 may be essential for tRNA wobble uridine modification by acting as tRNA binding motif

Dynamical mean field theory of optical third harmonic generation

We formulate the third harmonic generation (THG) within the dynamical mean field theory (DMFT) approximation of the Hubbard model. In the limit of large dimensions, where DMFT becomes exact, the vertex corrections to current vertices are identically zero, and hence the calculation of the THG spectrum reduces to a time-ordered convolution, followd by appropriate analytic continuuation. We present the typical THG spectrum of the Hubbard model obtained by this method. Within our DMFT calculation, we observe a nontrivial approximate {\em scaling} function describing the THG spectra in all Mott insulators, independent of the gap magnitude.Comment: 4 eps figure

Monte Carlo Simulation of Secondary Electrons in Solids and its Application for Scanning Electron Microscopy

A new Monte Carlo calculation model is introduced to simulate not only the primary electron behavior but also the secondary electron cascade in a specimen bombarded with an electron beam. Either the primary or the generated electron in a specimen having energy greater than 0.1 keV is defined as a fast electron and the single scattering model is used in the simulation which employs the Mott elastic scattering cross section and the Rao Sahib-Wittry energy loss equation. The electron having energy smaller than 0.1 keV is defined as a slow electron and the cascade model is used which takes into account the classical binary collision with the conduction electrons. The performance of this simulation is verified in comparison with experiments for energy and angular distributions of slow secondary electrons (\u3c50eV). Then, this simulation is applied in a discussion of the quantitative signal variation of the secondary and the backscattered electrons depending on a specimen surface topography. The maximum intensity of the secondary electron signal is obtained where the scanning electron beam reaches around 1nm beside the top edge of a surface step made of Cu with the vertical side wall of 500nm in height

A Simulation of Secondary Electron Trajectories in Solids

A Monte Carlo calculation model is introduced to simulate not only the primary electron behavior but also the secondary electron cascade in a specimen bombarded with an electron beam. Electrons having energy greater than 0.1keV are treated as fast electrons and the single scattering Monte Carlo model is adopted. Electrons having energy smaller than 0.1keV are treated as slow electrons and the electron cascade Monte Carlo model is used. The calculated results for the energy distribution of secondary electrons, and primary electron energy dependence of the total secondary yield and the backscattering yield are in good agreement with experimental results

Nonlinear Optical Response of Spin Density Wave Insulators

We calculate the third order nonlinear optical response in the Hubbard model within the spin density wave (SDW) mean field ansatz in which the gap is due to onsite Coulomb repulsion. We obtain closed-form analytical results in one dimension (1D) and two dimension (2D), which show that nonlinear optical response in SDW insulators in 2D is stronger than both 3D and 1D. We also calculate the two photon absorption (TPA) arising from the stress tensor term. We show that in the SDW, the contribution from stress tensor term to the low-energy peak corresponding to two photon absorption becomes identically zero if we consider the gauge invariant current properly.Comment: we use \psfrag in figur

Charge dynamics of Ca_{2-x}Na_{x}CuO_{2}Cl_{2} as a correlated electron system with the ideal tetragonal lattice

We report the reflectivity and the resistivity measurement of Ca_{2-x}Na_{x}CuO_{2}Cl_{2} (CNCOC), which has a single-CuO2-plane lattice with no orthorhombic distortion. The doping dependence of the in-plane optical conductivity spectra for CNCOC is qualitatively the same to those of other cuprates, but a slight difference between CNCOC and LSCO, i.e., the absence of the 1.5 eV peak in CNCOC, can be attributed to the smaller charge-stripe instability in CNCOC. The temperature dependence of the optical onductivity spectra of CNCOC has been analyzed both by the two-component model (Drude+Lorentzian) and by the one-component model (extended-Drude analysis). The latter analysis gives a universal trend of the scattering rate Gamma(omega) with doping. It was also found that Gamma(omega) shows a saturation behavior at high frequencies, whose origin is the same as that of resistivity saturation at high temperatures.Comment: 8 pages, 11 figures, to be published in Phys. Rev.

Nonlinear Optical Response in two-dimensional Mott Insulators

We study the third-order nonlinear optical susceptibility $\chi^{(3)}$ and photoexcited states of two-dimensional (2D) Mott insulators by using an effective model in the strong-coupling limit of a half-filled Hubbard model. In the numerically exact diagonalization calculations on finite-size clusters, we find that the coupling of charge and spin degrees of freedom plays a crucial role in the distribution of the dipole-allowed states with odd parity and the dipole-forbidden states with even parity in the photoexcited states. This is in contrast with the photoexcited states in one dimension, where the charge and spin degrees of freedom are decoupled. In the third-harmonic generation (THG) spectrum, main contribution is found to come from the process of three-photon resonance associated with the odd-parity states. As a result, the two-photon resonance process is less pronounced in the THG spectrum. The calculated THG spectrum is compared with recent experimental data. We also find that $\chi^{(3)}$ with cross-polarized configuration of pump and probe photons shows spectral distributions similar to $\chi^{(3)}$ with co-polarized configuration, although the weight is small. These findings will help the analyses of the experimental data of $\chi^{(3)}$ in the 2D Mott insulators.Comment: 9 pages,5 figures,RevTeX

Comment on "Origin of Giant Optical Nonlinearity in Charge-Transfer--Mott Insulators: A New Paradigm for Nonlinear Optics"

Comment on Phys. Rev. Lett. 86, 2086 (2001)Comment: 1 page, 1 eps figur

Nonlinear Optical Response Functions of Mott Insulators Based on Dynamical Mean Field Approximation

We investigate the nonlinear optical susceptibilities of Mott insulators with the dynamical mean field approximation. The two-photon absorption (TPA) and the third-harmonic generation (THG) spectra are calculated, and the classification by the types of coupling to external fields shows different behavior from conventional semiconductors. The direct transition terms are predominant both in the TPA and THG spectra, and the importance of taking all types of interaction with the external field into account is illustrated in connection with the THG spectrum and dcKerr effect. The dependence of the TPA and THG spectra on the Coulomb interaction indicate a scaling relation. We apply this relation to the quantitative evaluation and obtain results comparable to those of experiments.Comment: 14 pages, 12 figure