Session A15: Metals: 1D, 2D, 3D

8:00 AM–11:00 AM, Monday, March 13, 2006
Baltimore Convention Center - 311

Sponsoring Units: DMP DCMP
Chair: Duane Johnson, University of Illinois, Urbana-Champaign

Abstract: A15.00010 : Optical conductivity of MnN: a combined experimental and theoretical study.

9:48 AM–10:00 AM

Preview Abstract

Authors:

  Walter R. L. Lambrecht
    (Case Western Reserve University)

  S. Granville
  B. J. Ruck
  F. Budde
  A. Koo
  J. E. Downes
  H. J. Trodahl
    (Victoria University of Wellington)

  A. Bittar
  N. Strickland
  G. V. M. Williams
    (Industrial Research Limited, Lower Hutt, NZ)

  Timothy Learmont
  Kevin E. Smith
    (Boston University)

  V. J. Kennedy
  A. Markwitz
    (Inst. Geol. and Nuclear Sciences, Lower Hutt, NZ)

A comparison between measured and calculated optical conductivity is presented for MnN films prepared by ion assisted deposition. X-ray diffraction and extended x-ray absorption fine structure show the films to be nanocrystalline but phase pure. X-ray emission spectroscopy of the N K-edge and X-ray absorption near edge spectroscopy of the N K- and Mn L-edges are used to probe the occupied and empty densities of states, which compare well with the N-2p and Mn-3d partial densities of states calculated using the linearized muffin-tin orbital method. The optical conductivity was measured by spectroscopic ellipsometry/reflectivity in the infared to UV range. The major differences between calculated and measured spectra can be understood on the basis of a limited electron mean-free-path in these nanocrystalline films, which relaxes the momentum conservation requirement. The calculated optical functions are analyzed in terms of their dominant band-to-band contributions including the polarization dependence. The temperature dependent conductivity shows a clear metallic behaviour and a weak Kondo-like low temperature anomaly.

Session Z23: Strongly Correlated Electrons II

11:15 AM–2:15 PM, Friday, March 17, 2006
Baltimore Convention Center - 320

Sponsoring Unit: GMAG
Chair: Patrick Morales

Abstract: Z23.00005 : Electronic structure of CrN: a Mott insulator

12:03 PM–12:15 PM

Preview Abstract

Authors:

  Aditi Herwadkar
  Walter R.L. Lambrecht
    (Case Western Reserve)

  Mark van Schilfgaarde
    (Arizona State University)

It was recently reported by D. Gall et al. [J. Appl. Phys. 91, 5882, 2002] that CrN in the rocksalt structure has an optical band gap of about 0.7 eV, though in local spin density approximation, this materials is predicted to be metallic. We examine this possibility using the LSDA+U approach in the fully localized limit implemented in our full-potential muffin-tin orbital method. Slater integral \(F^{0}=U\) is screened such that the position of the occupied 3$d$ levels agrees well with the photoelectron spectra of CrN. We find that a band gap opens in the band structure. The actual value of the gap obviously depends on the choice of $U$. To understand the origin of the gap it is essential to study how the $d$ states split in cubic symmetry and what their filling is. Cr in forming CrN is trivalent and hence has three $3d$ electrons. The Cr $e_g$, form antibonding states in the conduction band. Adding $U$ tends to push these empty states further up for both spins. The $t_{2g}$ on the other hand form weaker $\pi$ bonds with N $2p$, which in LSDA occur near the Fermi energy. Adding a Hubbard $U$ now shift the majority spin electrons by $-U/2$ and the minority spin ones by $U/2$ and remove them from the Fermi level. This works because the three fold degenerate majority spin state $t_{2g\uparrow}$ becomes completely filled while the minority $t_{2g\downarrow}$ becomes empty. The valence band maximum then has predominantly N$2p$ character, which makes CrN a charge transfer type Mott-insulator.

Session B46: Focus Session: Wide Band Gap Semiconductors II

11:15 AM–2:15 PM, Monday, March 13, 2006
Baltimore Convention Center - 349

Sponsoring Unit: DMP
Chair: Paul Klein, Naval Research Laboratory

Abstract: B46.00007 : Transition metal doped SiC: defect levels and magnetism

12:51 PM–1:03 PM

Preview Abstract

Authors:

  M.S. Miao
  Walter R.L. Lambrecht
    (Case Western Reserve University)

The properties of transition metal substitutions and interstitials in 3C and 4H SiC are studied by first principles supercell calculations. The defect levels change slightly for different polytypes or for different layers in 4H SiC. The calculated defect levels are generally in good agreements with DLTS results. Ti in 4H SiC has two acceptor levels close to the conduction band minimum, corresponding to cubic and hexagonal layers.V, Cr and Mn are amphoteric and have both donor and acceptor levels in the gap. We found Mn has deep trap levels which may be useful to achieve semi-insulating SiC, as is also well-known to be the case for V. The Cr and Mn acceptor levels are deep in the gap. They are unlikely to induce holes at the valence band maximum. Therefore the well-known hole-mediated ferromagnetic mechanism does not apply to Cr and Mn doped SiC. However, our calculations showed ferromagnetic coupling for Cr or Mn doped at neighboring sites. The ferromagnetic coupling is very strong but localized for Cr:SiC. However, it is relatively weak but long range for Mn:SiC. Such features are determined by the nature of the impurity bands. The highest occupied defect state of Cr:SiC is antibonding $e$ which is localized whereas the state of Mn:SiC is $t2$ which is delocalized and strongly couples with the surrounding C dangling bonds.