On the Kinetic Roughening in Polymer Film Growth by Vapor Deposition

This is a Comment on a recent publication: Y.-P. Zhao et al., Phys. Rev. Lett. 85, 3229 (2000). In the Letter, the authors report on an experimental investigation of polymeric (p-xylene) thin film growth and propose a new universality class not previously known. Here, we point out that the critical exponents reported in the Letter are consistent with the critical exponents of Das Sarma-Tamborenea growth model.Comment: 2 pages, 1 figure include

Transport properties of diluted magnetic semiconductors: Dynamical mean field theory and Boltzmann theory

The transport properties of diluted magnetic semiconductors (DMS) are calculated using dynamical mean field theory (DMFT) and Boltzmann transport theory. Within DMFT we study the density of states and the dc-resistivity, which are strongly parameter dependent such as temperature, doping, density of the carriers, and the strength of the carrier-local impurity spin exchange coupling. Characteristic qualitative features are found distinguishing weak, intermediate, and strong carrier-spin coupling and allowing quantitative determination of important parameters defining the underlying ferromagnetic mechanism. We find that spin-disorder scattering, formation of bound state, and the population of the minority spin band are all operational in DMFT in different parameter range. We also develop a complementary Boltzmann transport theory for scattering by screened ionized impurities. The difference in the screening properties between paramagnetic ($T>T_c$) and ferromagnetic ($T<T_c$) states gives rise to the temperature dependence (increase or decrease) of resistivity, depending on the carrier density, as the system goes from the paramagnetic phase to the ferromagnetic phase. The metallic behavior below $T_c$ for optimally doped DMS samples can be explained in the Boltzmann theory by temperature dependent screening and thermal change of carrier spin polarization.Comment: 15 pages, 15 figure

Inhomogenous electronic structure, transport gap, and percolation threshold in disordered bilayer graphene

The inhomogenous real-space electronic structure of gapless and gapped disordered bilayer graphene is calculated in the presence of quenched charge impurities. For gapped bilayer graphene we find that for current experimental conditions the amplitude of the fluctuations of the screened disorder potential is of the order of (or often larger than) the intrinsic gap $\Delta$ induced by the application of a perpendicular electric field. We calculate the crossover chemical potential, $\Delta_{\rm cr}$, separating the insulating regime from a percolative regime in which less than half of the area of the bilayer graphene sample is insulating. We find that most of the current experiments are in the percolative regime with $\Delta_{\rm cr}<<\Delta$. The huge suppression of $\Delta_{\rm cr}$ compared with $\Delta$ provides a possible explanation for the large difference between the theoretical band gap $\Delta$ and the experimentally extracted transport gap.Comment: 5 Pages, 2 figures. Published versio

A Discrete Model for Nonequilibrium Growth Under Surface Diffusion Bias

A limited mobility nonequilibrium solid-on-solid dynamical model for kinetic surface growth is introduced as a simple description for the morphological evolution of a growing interface under random vapor deposition and surface diffusion bias conditions. Simulations using a local coordination dependent instantaneous relaxation of the deposited atoms produce complex surface mound morphologies whose dynamical evolution is inconsistent with all the proposed continuum surface growth equations. For any finite bias, mound coarsening is found to be only an initial transient which vanishes asymptotically, with the asymptotic growth exponent being 0.5 in both 1+1 and 2+1 dimensions. Possible experimental implications of the proposed limited mobility nonequilibrium model for real interface growth under a surface diffusion bias are critically discussed.Comment: 6 pages, 3 figures included, to appear in Surf. Sci. Let

Persistent Current in an Artificial Quantum Dot Molecule

Using an exact diagonalization technique within a generalized Mott-Hubbard Hamiltonian, we predict the existence of a ground state persistent current in coherent two-dimensional semiconductor quantum dot arrays pierced by an external magnetic flux. The calculated persistent current, which arises from the nontrivial dependence of the ground state energy on the external flux, exists in isolated arrays without any periodic boundary condition. The sensitivity of the calculated persistent current to interaction and disorder is shown to reflect the intricacies of various Anderson-Mott-Hubbard quantum phase transitions in two dimensional systems.Comment: 4 pages, 3 figure

Absence of damping of low energy excitations in a quasi-2D dipolar Bose gas

We develop a theory of damping of low energy, collective excitations in a quasi-2D, homogenous, dipolar Bose gas at zero temperature, via processes whereby an excitation decays into two excitations with lower energy. We find that owing to the nature of the low energy spectrum of a quasi-2D dipolar gas, such processes cannot occur unless the momentum of the incoming quasi-particle exceeds a critical value k_{crit}. We find that as the dipolar interaction strength is increased, this critical value shifts to larger momenta. Our predictions can be directly verified in current experiments on dipolar Bose condensates using Bragg spectroscopy, and provide valuable insight into the quantum many-body physics of dipolar gases.Comment: 4 pages, 2 figure

Many-body effects and possible superconductivity in the 2D metallic surface states of 3D topological insulators

We theoretically consider temperature and density-dependent electron-phonon interaction induced many-body effects in the two-dimensional (2D) metallic carriers confined on the surface of the 3D topological insulator (e.g. Bi$_2$Se$_3$). We calculate the temperature and the carrier density dependence of the real and imaginary parts of the electronic self-energy, the interacting spectral function, and the phonon-induced velocity renormalization, enabling us to obtain a simple density and temperature dependent effective dimensionless electron-phonon coupling constant parameter, which increases (decreases) strongly with increasing density (temperature). Our theoretical results can be directly and quantitatively compared with experimental ARPES or STS studies of the 2D spectral function of topological insulator surface carriers. In particular, we predict the possible existence of surface superconductivity in Bi$_2$Se$_3$ induced by strong electron-phonon interaction.Comment: 5 pages, 4 figure