# Theoretical Physics

The research in the Theoretical Physics group is primarily focused on condensed matter physics/theoretical materials science, nanoscience and electromagnetic modeling.

The group also gives a large number of courses on graduate as well as undergraduate levels.

# Highlights

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# First-principles study of configurational disorder in B4C using a superatom-special quasirandom structure method

**A. Ektarawong, S. I. Simak, L. Hultman, J. Birch, and B. Alling**

**PHYSICAL REVIEW B 90, 024204 (2014)**

Configurationally disordered crystalline boron carbide, with the composition B_{4}C, is studied using first- principles calculations. We investigate both dilute and high concentrations of carbon-boron substitutional defects. For the latter purpose, we suggest a superatom’s picture of the complex structure and combine it with a special quasirandom structure approach for disorder. In this way, we model a random distribution of high concentrations of the identified low-energy defects: (1) bipolar defects and (2) substitution of icosahedral carbon among the three polar-up sites. Additionally, the substitutional disorder of the icosahedral carbon at all six polar sites, as previously discussed in the literature, is also considered. Two configurational phase transitions from the ordered to the disordered configurations are predicted to take place upon an increase in temperature using a mean-field approximation for the entropy. The first transition, at 870 K, induces substitutional disorder of the icosahedral carbon atoms among the three polar-up sites; meanwhile the second transition, at 2325 K, reveals the random substitution of the icosahedral carbon atoms at all six polar sites coexisting with bipolar defects. Already the first transition removes the monoclinic distortion existing in the ordered ground-state configuration and restore the rhombohedral system (*R*3*m*). The restoration of inversion symmetry yielding the full rhombohedral symmetry (*R*3-*m*) on average, corresponding to what is reported in the literature, is achieved after the second transition. Investigating the effects of high pressure on the configurational stability of the disordered B_{4}C phases reveals a tendency to stabilize the ordered ground-state configuration as the configurationally ordering/disordering transition temperature increases with pressure exerted on B_{4}C. The electronic density of states, obtained from the disordered phases, indicates a sensitivity of the band gap to the degree of configurational disorder in B_{4}C.

A model of a substitutionally disordered B_{4}C represented in (a) a normal-atom picture and (b) a superatom picture. (a) Green and brown spheres represent boron and carbon atoms, respectively. (b) Black, white, and gray spheres represent superatom types *A*, *B*, and C, respectively.

# Earlier Highlights

## First-principles study of configurational disorder in B4C using a superatom-special quasirandom structure method

(21 August 2014)...

## Strongly localized moving discrete dissipative breather-solitons in Kerr nonlinear media supported by intrinsic gain

(4 August 2014)...

## Phonon Self-Energy and Origin of Anomalous Neutron Scattering Spectra in SnTe and PbTe Thermoelectrics

(15 July 2014)...

## High-throughput screening of perovskite alloys for piezoelectric performance and thermodynamic stability

(20 June 2014)...

## Finite-size–dependent dispersion potentials between atoms and ions dissolved in water

(13 June 2014)...

## A silicon carbide room-temperature single-photon source

(26 March 2014)...

## Two approaches for describing the Casimir interaction in graphene: Density-density correlation function versus polarization tensor

(10 March 2014)...

## Lithium atom storage in nanoporous cellulose via surface-induced Li2 breakage

(3 February 2014)...

## Pressure-Induced Hydrogen Bond Symmetrization in Iron Oxyhydroxide

(16 December 2013)...

## Temperature-dependent effective third-order interatomic force constants from first principles

(1 November 2013)...

Responsible for this page: Fei Wang

Last updated:08/21/14