# 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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# Finite-temperature elastic constants of paramagnetic materials within the disordered local moment picture from ab initio molecular dynamics calculations

**E. Mozafari, N. Shulumba, P. Steneteg, B. Alling, and Igor A. Abrikosov **

**PHYSICAL REVIEW B 94 , 054111 (2016)**

We present a theoretical scheme to calculate the elastic constants of magnetic materials in the high-temperature paramagnetic state. Our approach is based on a combination of disordered local moments picture and ab initio molecular dynamics (DLM-MD). Moreover, we investigate a possibility to enhance the efficiency of the simulations of elastic properties using the recently introduced method: symmetry imposed force constant temperature-dependent effective potential (SIFC-TDEP). We have chosen cubic paramagnetic CrN as a model system. This is done due to its technological importance and its demonstrated strong coupling between magnetic and lattice degrees of freedom. We have studied the temperature-dependent single-crystal and polycrystalline elastic constants of paramagentic CrN up to 1200 K. The obtained results at T = 300 K agree well with the experimental values of polycrystalline elastic constants as well as the Poisson ratio at room temperature. We observe that the Young’s modulus is strongly dependent on temperature, decreasing by ∼ 14% from T = 300 K to 1200 K. In addition we have studied the elastic anisotropy of CrN as a function of temperature and we observe that CrN becomes substantially more isotropic as the temperature increases. We demonstrate that the use of Birch law may lead to substantial errors for calculations of temperature induced changes of elastic moduli. The proposed methodology can be used for accurate predictions of mechanical properties of magnetic materials at temperatures above their magnetic order-disorder phase transition.

CalculatedVoigt-Reuss-Hillaverages[Eqs.( 13 )–( 15 )]of polycrystalline elastic constants of PM CrN from top to bottom (a) bulk modulus, (b) Young’s modulus, (c) shear modulus, and (d) Poisson ratio, as a function of temperature. The error bars correspond to the standard deviation of the molecular dynamics simulations with a 95% confidence interval.

# Earlier Highlights

- Finite-temperature elastic constants of paramagnetic materials within the disordered local moment picture from ab initio molecular dynamics calculations (22 August 2016)
- Probing dopants in wide semiconductor quantum point contacts (7 March 2016)
- Relativistic Doppler reflection as a probe for the initial relaxation of a non-equilibrium electron-hole plasma in silicon (25 November 2015)
- The most incompressible metal osmium at static pressures above 750 gigapascals (22 September 2015)
- Electronic correlations in Fe at Earth's inner core conditions: Effects of alloying with Ni (16 September 2015)
- Magnetoresistance of doped silicon (15 June 2015)
- Casimir effects in systems containing 2D layers such as graphene and 2D electron gases (25 May 2015)
- Role of N defects in paramagnetic CrN at finite temperatures from first-principles (13 March 2015)
- Non-perturbative theory of dispersion interactions (6 March 2015)
- Ultrafast dynamic conductivity and scattering rate saturation of photoexcited charge carriers in silicon investigated with a midinfrared continuum probe (17 February 2015)

Responsible for this page:
Fei Wang

Last updated:
08/22/16