Research

Our group is engaged in research in the fields of theoretical condensed matter physics and materials science. Our research covers a broad range of materials systems, from bulk materials (metals, semiconductors, and insulators) to those of finite size, such as organic and inorganic nanostructures, to complex multifunctional oxides; phenomena of interest include optical properties, superconductivity, conductance of nanostructures at finite bias, pressure and temperature effects, and dynamics. Particular emphasis is placed on the study of the role of many-particle effects in determining experimentally observed properties.

Our primary goal is to understand and predict materials properties at the most fundamental level using atomistic first principles (or "ab initio" ) quantum-mechanical calculations. A variety of different computational approaches are used that require only the atomic number and positions as input. These first principles methods have, in the past, resulted in excellent quantitative agreement with experiment and have predicted with good accuracy materials properties that were later verified experimentally.

We present below some highlights on recent research projects. For more details, please see our list of publications.

Electronic properties of fullerenes on metallic substrates. As part of a joint effort between experiment and theory, we computed electron-phonon coupling matrix elements that explain high-resolution scanning tunneling spectroscopy (STS) images of Gd@C82 on Ag(100). Our calculations compared well with the measurements over a range of tip voltages. This work was performed in collaboration with the M. E. Crommie group at UC-Berkeley.

Hydrostatic pressure and temperature effects on the structural and electronic properties of carbon nanotubes. The temperature dependence of the band gap Eg(T) of semiconducting SWNTs has been calculated by direct evaluation of electron-phonon couplings within a "frozen-phonon" scheme. A rich diameter and chirality dependence of Eg(T) was obtained, including non-monotonic behavior for certain tubes and a distinct "family" behavior. We have also studied the structural and electronic properties of isolated single-wall carbon nanotubes (SWNTs) under hydrostatic pressure as a function of chirality. A phase transition from a cylindrical shape to a collapsed geometry is computed, with good agreement between calculated and experimental values of the critical pressure Pc. A family behavior of the Kohn-Sham energy gaps of semiconducting tubes is also predicted.

Excitonic effects and optical spectra of single-walled carbon nanotubes. We computed the optical absorption spectra of four SWCNTs with the following (n,m) chiralities: (7,0), (10,0), (11,0) and (12,0). Strong excitonic effects in both semiconducting and metallic nanotubes have been found. In contrast with bulk metallic systems, we discovered bound excitons can exist in metallic SWCNTs. We have also observed a decrease in the exciton binding energy with nanotube diameter which is consistent with the expected absence of exciton binding in graphene.

Negative differential resistance of variable-length carbon chains with different leads. We have calculated the I-V characteristics of carbon chains of varying lengths sandwiched between Au and Al leads, and found that the transport properties are extremely sensitive to atomic positions and the specific chemistry of the lead-molecule interface. We have also predicted negative differential resistance (NDR) for carbon chains of various lengths between both Au and Al nanoleads, with the onset bias depending on the lead chemistry and the number of atoms in the carbon chains.

Structural and electronic properties of nanopeapods. We computed the structure and electronic properties of   "nanopeapods" (NPPs), chains of C60 molecules packed inside boron nitride or carbon nanotubes (BNNTs or CNTs). Both the chemistry of the outer nanotube and the spacing between adjacent C60 molecules were found to influence the electronic properties of the C60 chains. Encapsulation energies in BN and carbon NPP's were computed to be significantly larger than the activation energy required for C60 polymerization.

Electronic and structural properties of NaxCoO2. We have carried out a systematic LSDA+U study of doping effects on the electronic and structural properties of NaxCoO2. Due to the strong interaction between the doped electron and other correlated Co d electrons, the calculated electronic structure of (CoO2)x- depends sensitively on the doping level x. Zone center optical phonon energies are calculated and are in good agreement with measured values. Our calculated Fermi surface agrees well with recent angle-resolved  photoemission spectroscopy experiments. Contrary to previous suggestions, we find no violation of Luttinger's rule in this system. We have also studied the energetics of Na ordering in NaxCoO2. We find different ordered structures as a function of composition in excellent agreement with available experiments.

Electron-phonon coupling and phonon renormalization in metals. We have developed a method for calculating the phonon self-energy in metals arising from the coupling between phonons and electrons near the Fermi surface. The computational advantage of our formalism is that it does not require explicit calculations of the electron-phonon matrix elements.

Thermoelectricity in bismuth telluride. We have performed first-principles quasiparticle calculations of bismuth telluride, a thermoelectric material, within the GW approximation and including spin-orbit effects. We found highly anisotropic effective mass tensors, a large number of valleys at the band extrema, and band gap of 0.17 eV, in almost perfect agreement with the experimental value. Our results verify the features that give bismuth telluride its exceptional thermoelectric properties and they pave the way towards the ultimate goal of designing improved thermoelectric materials.

Ab Initio Studies of Doped SrTiO3. We have calculated the effects of carrier doping on the structural and electronic properties of SrTiO3. Our results indicate that the rigid band model provides a reasonable description of hole doping but is unable to describe the effects of oxygen vacancy-induced electron doping on the electronic properties. We also estimate the electron-phonon coupling parameters and discuss the implications of this study on superconductivity in SrTiO3.

Mechanism for bias-assisted indium mass transport on graphitic surfaces. Motivated by a recent experiment in the A. Zettl group at UC-Berkeley, we have calculated adsorption energetics of indium (as a function of coverage) on graphite-like surfaces. For low surface densities, In becomes positively charge, consistent with experimental observations of bias-assisted In transport opposite to that of electron flow. The In diffusion rate on graphene is also estimated.

Energy dissipation in carbon nanotube bearings. We have undertaken a study of energy dissipation mechanisms in fundamental nanoscale mechanical elements created from carbon nanotubes. This work was motivated by recent work in the group of Prof. Alex Zettl at U. C. Berkeley in which linear and rotational bearings were created using carbon nanotubes. We have simulated at an atomistic level a variety of linear and rotational bearings and a carbon nanotube-based oscillator of gigahertz frequency.