Spectral functions in doped transition metal oxides

We present experimental photoemission and inverse photoemission spectra of SrTiO$_{3- \delta}$ representing electron doped $d^0$ systems. Photoemission spectra in presence of electron doping exhibit prominent features arising from electron correlation effects, while the inverse photoemssion spectra are dominated by spectral features explainable within single-particle approaches. We show that such a spectral evolution in chemically doped correlated systems is not compatible with expectations based on Hubbard or any other similar model. We present a new theoretical approach taking into account the inhomogeneity of the `real' system which gives qualitatively different results compared to standard `homogeneous' models and is in quantitative agreement with experiments.Comment: 10 pages; 1 tex file+4 postscript files (to appear in Europhysics Letters

Electron-spectroscopic investigation of metal-insulator transition in Sr2Ru1-xTixO4 (x=0.0-0.6)

We investigate the nature and origin of the metal-insulator transition in Sr2Ru1-xTixO4 as a function of increasing Ti content (x). Employing detailed core, valence, and conduction band studies with x-ray and ultraviolet photoelectron spectroscopies along with Bremsstrahlung isochromat spectroscopy, it is shown that a hard gap opens up for Ti content greater than equal to 0.2, while compositions with x<0.2 exhibit finite intensity at the Fermi energy. This establishes that the metal-insulator transition in this homovalent substituted series of compounds is driven by Coulomb interaction leading to the formation of a Mott gap, in contrast to transitions driven by disorder effects or band flling.Comment: Accepted for publication in Phys. Rev.

Regge behaviour of distribution functions and t and x-evolutions of gluon distribution function at low-x

In this paper t and x-evolutions of gluon distribution function from Dokshitzer-Gribov-Lipatov-Altarelli-Parisi(DGLAP) evolution equation in leading order(LO) at low-x, assuming the Regge behaviour of quark and gluon at this limit, are presented. We compare our results of gluon distribution function with MRST 2001, MRST 2004 and GRV '98 parameterizations and show the compatibility of Regge behaviour of quark and gluon distribution functions with perturbative quantum chromodynamics(PQCD) at low-x. We also discuss the limitations of Taylor series expansion method used earlier to solve DGLAP evolution equations, in the Regge behaviour of distribution functions.Comment: 19 pages, 7 figure

Extended Self-similarity in Kinetic Surface Roughening

We show from numerical simulations that a limited mobility solid-on-solid model of kinetically rough surface growth exhibits extended self-similarity analogous to that found in fluid turbulence. The range over which scale-independent power-law behavior is observed is significantly enhanced if two correlation functions of different order, such as those representing two different moments of the difference in height between two points, are plotted against each other. This behavior, found in both one and two dimensions, suggests that the `relative' exponents may be more fundamental than the `absolute' ones.Comment: 4 pages, 4 postscript figures included (some changes made according to referees' comments. accepted for publication in PRE Rapid Communication

Near room-temperature colossal magnetodielectricity and multiglass properties in partially-disordered La2NiMnO6

We report magnetic, dielectric and magnetodielectric responses of pure monoclinic bulk phase of partially-disordered La2NiMnO6, exhibiting a spectrum of unusual properties and establish that this system intrinsically is a true multiglass with a large magnetodielectric coupling (8-20%) over a wide range of temperatures (150 - 300 K). Specifically, our results establish a unique way to obtain colossal magnetodielectricity, independent of any striction effects, by engineering the asymmetric hopping contribution to the dielectric constant via the tuning of the relative spin orientations between neighboring magnetic ions in a transition metal oxide system. We discuss the role of anti-site (Ni-Mn) disorder in emergence of these unusual properties.Comment: 5 pages, 4 figures, Slightly revised version of previous article in condmat: arXiv:1202.4319v

Raman Scattering Spectra of Elementary Electronic Excitations in Coupled Double-Quantum Well Structures

Using the time-dependent-local-density-approximation (TDLDA) within a self-consistent linear response theory, we calculate the elementary excitation energies and the associated inelastic light-scattering spectra of a strongly coupled two-component plasma in a double-quantum well system with electron occupation of symmetric and antisymmetric subbands. We find, consistent with the results of a recent experimental Raman scattering study, that the intersubband spin density excitations tend to merge with the single particle excitations (i.e. the excitonic shift decreases monotonically) as the Fermi energy increases beyond the symmetric-antisymmetric energy gap $\bigtriangleup_{SAS}$. However, our TDLDA calculation does not show the abrupt suppresion of the excitonic shift seen experimentally at a finite value of the subband occupancy parameter $\eta \equiv \bigtriangleup_{\text{SAS}} / E_{\text{F}}$.Comment: 9 pages, RevTeX, 5 figures available upon request, PIT-SDS-00

Disorder and Interaction in 2D: Exact diagonalization study of the Anderson-Hubbard-Mott model

We investigate, by numerically calculating the charge stiffness, the effects of random diagonal disorder and electron-electron interaction on the nature of the ground state in the 2D Hubbard model through the finite size exact diagonalization technique. By comparing with the corresponding 1D Hubbard model results and by using heuristic arguments we conclude that it is \QTR{it}{unlikely} that there is a 2D metal-insulator quantum phase transition although the effect of interaction in some range of parameters is to substantially enhance the non-interacting charge stiffness.Comment: 13 pages, 2 figures Revised version. Accepted for publication in Phys. Rev. Let

Estimates of electronic interaction parameters for LaMMO3_3 compounds (MM=Ti-Ni) from ab-initio approaches

We have analyzed the ab-initio local density approximation band structure calculations for the family of perovskite oxides, La$M$O$_3$ with $M$=Ti-Ni within a parametrized nearest neighbor tight-binding model and extracted various interaction strengths. We study the systematics in these interaction parameters across the transition metal series and discuss the relevance of these in a many-body description of these oxides. The results obtained here compare well with estimates of these parameters obtained via analysis of electron spectroscopic results in conjunction with the Anderson impurity model. The dependence of the hopping interaction strength, t, is found to be approximately $r^{-3}$.Comment: 18 pages; 1 tex file+9 postscript files (appeared in Phys Rev B Oct 15,1996

1/f noise in a dilute GaAs two-dimensional hole system in the insulating phase

We have measured the resistance and the 1/f resistance noise of a two-dimensional low density hole system in a high mobility GaAs quantum well at low temperature. At densities lower than the metal-insulator transition one, the temperature dependence of the resistance is either power-like or simply activated. The noise decreases when the temperature or the density increase. These results contradict the standard description of independent particles in the strong localization regime. On the contrary, they agree with the percolation picture suggested by higher density results. The physical nature of the system could be a mixture of a conducting and an insulating phase. We compare our results with those of composite thin films.Comment: 4 pages, 3 figures; to appear in Physica E (EP2DS-16 proceedings

Carrier relaxation due to electron-electron interaction in coupled double quantum well structures

We calculate the electron-electron interaction induced energy-dependent inelastic carrier relaxation rate in doped semiconductor coupled double quantum well nanostructures within the two subband approximation at zero temperature. In particular, we calculate, using many-body theory, the imaginary part of the full self-energy matrix by expanding in the dynamically RPA screened Coulomb interaction, obtaining the intrasubband and intersubband electron relaxation rates in the ground and excited subbands as a function of electron energy. We separate out the single particle and the collective excitation contributions, and comment on the effects of structural asymmetry in the quantum well on the relaxation rate. Effects of dynamical screening and Fermi statistics are automatically included in our many body formalism rather than being incorporated in an ad-hoc manner as one must do in the Boltzman theory.Comment: 26 pages, 5 figure