189 research outputs found
Origin of room-temperature ferromagnetism in Mn doped semiconducting CdGeP2
CdGeP2 chalcopyrites doped with Mn have been recently found to exhibit room
temperature ferromagnetism. Isovalent substitution of the Cd site is expected,
however, to create antiferromagnetism, in analogy with the well-known CdTe:Mn
(d^5) case. However, chalcopyrite semiconductors exhibit low-energy intrinsic
defects. We show theoretically how ferromagnetism results from the interaction
of Mn with hole-producing intrinsic defects.Comment: 4 pages, 4 figures. (To appear in Phys. Rev. Lett.
Bulk and nano GaN: Role of Ga d states
We have studied the role of Ga 3d states in determining the properties of
bulk as well as nanoparticles of GaN using PAW potentials. A significant
contribution of the Ga d states in the valence band is found to arise from the
interaction of Ga 4d states with the dominantly N p states making up the
valence band. The errors arising from not treating the Ga 3d states as a part
of the valence are found to be similar, ~ 1%, for bulk as well as for
nanoclusters of GaN.Comment: 17 pages, 7 figure
Trends in ferromagnetism, hole localization, and acceptor level depth for Mn substitution in
We examine the intrinsic mechanism of ferromagnetism in dilute magnetic
semiconductors by analyzing the trends in the electronic structure as the host
is changed from GaN to GaP, GaAs and GaSb, keeping the transition metal
impurity fixed. In contrast with earlier interpretations which depended on the
host semiconductor, we found that a single mechanism is sufficient to explain
the ferromagnetic stabilization energy for the entire series.Comment: 4 figures; To appear in Appl. Phys. Let
Elastic Properties of Carbon nanotubes : An atomistic approach
Energetically the single sheet of graphite (graphene) is more stable than the
nanotube. The energy difference between the two systems can be directly related
to the strain energy involved in rolling up the graphene sheet to form the
nanotube. We have carried out first principle electronic structure calculations
and evaluated the strain energy as a function of the nanotube radius. The
dependence of the strain energy on the diameter of the nanotube has been found
by several groups to be welldescribed by a continuum elasticity model. We
attempt to examine why this is the case and show where atomistics enter the
description.Comment: 10 pages, 4 figure
First-principles investigation of the assumptions underlying Model-Hamiltonian approaches to ferromagnetism of 3d impurities in III-V semiconductors
We use first-principle calculations for transition metal impurities V, Cr,
Mn, Fe, Co and Ni in GaAs as well as Cr and Mn in GaN, GaP and GaSb to identify
the basic features of the electronic structure of these systems. The
microscopic details of the hole state such as the symmetry and the orbital
character, as well as the nature of the coupling between the hole and the
transition metal impurity are determined. This could help in the construction
of model Hamiltonians to obtain a description of various properties beyond what
current first-principle methods are capable of.Comment: 14 figure
Trends in Ferromagnetism in Mn doped dilute III-V alloys from a density functional perspective
Mn doping in dilute III-V alloys has been examined as a route to enhance
ferromagnetic stability. Strong valence band bowing is expected at the dilute
limit, implying a strong modification of the ferromagnetic stability upon
alloying, with even an increase in some cases. Using first principle electronic
structure calculations we show that while codoping with a group V anion
enhances the ferromagnetic stability in some cases when the effects of
relaxation of the lattice are not considered, strong impurity scattering in the
relaxed structure result in a reduction of the ferromagnetic stability.Comment: 12 pages, 3 figures, Accepted in Physical Review
Strain induced metal-insulator transition in ultrathin films of SrRuO
The ultrathin film limit has been shown to be a rich playground for unusual
low dimensional physics. Taking the example of SrRuO which is ferromagnetic
and metallic at the bulk limit, one finds that it becomes antiferromagnetic and
insulating at the three monolayers limit when grown on SrTiO. The origin of
the insulating state is traced to strongly orbital dependent exchange
splittings. A modest compressive strain of 1% of the SrTiO substrate is
then found to drive the system into a highly confined two-dimensional 100% spin
polarized metallic state. This metal-insulator transition driven by a modest
strain could be useful in two state device applications.Comment: Accept in Phys. Rev.
Spin, charge and orbital ordering in La0.5Sr1.5MnO4
We have analyzed the experimental evidence of charge and orbital ordering in
La0.5Sr1.5MnO4 using first principles band structure calculations. Our results
suggest the presence of two types of Mn sites in the system. One of the Mn
sites behaves like an Mn(3+) ion, favoring a Jahn-Teller distortion of the
surrounding oxygen atoms, while the distortion around the other is not a simple
breathing mode kind. Band structure effects are found to dominate the
experimental spectrum for orbital and charge ordering, providing an alternate
explanation for the experimentally observed results.Comment: 4 pages + 3 figures; To appear in Phys. Rev. Let
Origin of transition metal clustering tendencies in GaAs based dilute magnetic semiconductors
While isovalent doping of GaAs (e.g. by In) leads to a repulsion between the
solute atoms, two Cr, Mn, or Fe atoms in GaAs are found to have lower energy
than the well-separated pair, and hence attract each other. The strong bonding
interaction between levels with t2 symmetry on the transition metal (TM) atoms
results in these atoms exhibiting a strong tendency to cluster. Using
first-principles calculations, we show that this attraction is maximal for Cr,
Mn and Fe while it is minimal for V. The difference is attributed to the
symmetry of the highest occupied levels. While the intention is to find
possible choices of spintronic materials that show a reduced tendency to
cluster, one finds that the conditions that minimize clustering tendencies also
minimize the stabilization of the magnetic state.Comment: To appear in Appl. Phys. Let
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