Research

We are involved in research on a variety of subjects in condensed matter physics. Because of the breadth of this field, it is important to expose graduate students and postdoctoral researchers to a wide spectrum of problems. A broad view is also important because new breakthroughs occur in different subareas of this field. Since the research projects are chosen because of their inherent scientific importance, we are sometimes working directly with experimentalists and at other times developing new formalisms and techniques to understand or solve a problem. We are often trying to predict the existence of new materials and attempting to explain or predict new properties of condensed matter systems.

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.

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.

Zone center phonon splitting in MnO and NiO. Recent inelastic neutron diffraction studies observe a splitting of the zone-center transverse optical phonons of MnO and NiO. To understand the electronic and magnetic origin of this system of fundamental importance, we computed zone-center transverse optical phonon in MnO and NiO using the LSDA+U method. The result for MnO is in good agreement with a recent experiment, while the result for NiO is somewhat inconsistent with experiment and will require further investigation.

Pressure dependence of the ideal strength of bcc Nb. The pressure dependence of the ideal tensile strength of bcc niobium was computed from first principles. External tensile stresses are predicted to result in higher ideal tensile strength, whereas under external compressive stresses, the ideal tensile strength is computed to decrease.

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.

Electronic properties of Boron Nitride (BN) nanotubes in an STM electric field. In collaboration with the A. Zettl experimental group at UC-Berkeley, we computed STM images of BN nanotubes under an external electric field. The predicted image features are in agreement with experiment, and accurate computation of their details provides a measurement of the chiral angle of the tube.

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.