300 research outputs found
Ab-initio study of disorder effects on the electronic and magnetic structures of SrFeMoO
We have investigated the electronic structure of ordered and disordered
SrFeMoO using {\it ab-initio} band structure methods. The effect of
disorder was simulated within super-cell calculations to realize several
configurations with mis-site disorders. It is found that such disorder effects
destroy the half-metallic ferro-magnetic state of the ordered compound. It also
leads to a substantial reduction of the magnetic moments at the Fe sites in the
disordered configurations. Most interestingly, it is found for the disordered
configurations, that the magnetic coupling within the Fe sub-lattice as well as
that within the Mo sub-lattice always remain ferro-magnetic, while the two
sub-lattices couple anti-ferromagnetically, in close analogy to the magnetic
structure of the ordered compound, but in contrast to recent suggestions.Comment: 7 pages, 3 figure
X-ray photoemission study of NiS_{2-x}Se_x (x = 0.0 - 1.2)
Electronic structure of NiS_{2-x}Se_x system has been investigated for
various compositions (x) using x-ray photoemission spectroscopy. An analysis of
the core level as well as the valence band spectra of NiS_2 in conjunction with
many-body cluster calculations provides a quantitative description of the
electronic structure of this compound. With increasing Se content, the on-site
Coulomb correlation strength (U) does not change, while the band width W of the
system increases, driving the system from a covalent insulating state to a
pd-metallic state.Comment: 19 pages, 6 figures, To appear in Phys. Rev. B, 200
Magnetoresistance in ordered and disordered double perovskite oxide, SrFeMoO
We have prepared crystallographically ordered and disorder specimens of the
double perovskite, SrFeMoO and investigated their magnetoresistance
behaviour. The extent of ordering between the Fe and Mo sites in the two
samples is determined by Rietveld analysis of powder x-ray diffraction patterns
and reconfirmed by M\"{o}ssbauer studies. While the ordered sample exhibits the
sharp low-field response, followed by moderate changes in the magnetoresistance
at higher fields, the disordered sample is characterised by the absence of the
spectacular low-field response. We argue that the low field response depends
crucially on the half-metallic ferromagnetism, while the high-field response
follows from the overall magnetic nature of the sample, even in absence of the
half-metallic state.Comment: accepted in Solid State Communicatio
XPS evidence for molecular charge-transfer doping of graphene
By employing x-ray photoelectron spectroscopy (XPS), we have been able to
establish the occurrence of charge-transfer doping in few-layer graphene
covered with electron acceptor (TCNE) and donor (TTF) molecules. We have
performed quantitative estimates of the extent of charge transfer in these
complexes and elucidated the origin of unusual shifts of their Raman G bands
and explained the differences in the dependence of conductivity on n- and
p-doping. The study unravels the cause of the apparent difference between the
charge-transfer doping and electrochemical doping.Comment: 15 pages, 5 figure
Electronic structure of NiS_{1-x}Se_x
We investigate the electronic structure of the metallic NiSSe
system using various electron spectroscopic techniques. The band structure
results do not describe the details of the spectral features in the
experimental spectrum, even for this paramagnetic metallic phase. However, a
parameterized many-body multi-band model is found to be successful in
describing the Ni~2 core level and valence band, within the same model. The
asymmetric line shape as well as the weak intensity feature in the Ni~2 core
level spectrum has been ascribed to extrinsic loss processes in the system. The
presence of satellite features in the valence band spectrum shows the existence
of the lower Hubbard band, deep inside the metallic regime, consistent
with the predictions of the dynamical mean field theory.Comment: To be published in Physical Review B, 18 pages and 5 figure
Structural Ordering and Antisite Defect Formation in Double Perovskites
We formulate an effective model for B-B' site ordering in double perovskite
materials A_2BB'O_6. Even within the simple framework of lattice-gas type
models, we are able to address several experimentally observed issues including
nonmonotonic dependence of the degree of order on annealing temperature, and
the rapid decrease of order upon overdoping with either B or B' species. We
also study ordering in the `ternary' compounds A_2BB'_{1-y}B''_yO_6 Although
our emphasis is on the double perovskites, our results are easily generalizable
to a wide variety of binary and ternary alloys.Comment: 9 pages pdflatex, 21 embedded png fig
An experimental and computational investigation of structure and magnetism in pyrite CoFeS: Chemical bonding and half-metallicity
Bulk samples of the pyrite chalcogenide solid solutions CoFeS
0 <= x <= 0.5, have been prepared and their crystal structures and magnetic
properties studied by X-ray diffraction and SQUID magnetization measurements.
Across the solution series, the distance between sulfur atoms in the persulfide
(S) unit remains nearly constant. First principles electronic
structure calculations using experimental crystal structures as inputs point to
the importance of this constant S-S distance, in helping antibonding S-S levels
pin the Fermi energy. In contrast hypothetical rock-salt CoS is not a good half
metal, despite being nearly isostructural and isoelectronic. We use our
understanding of the CoFeS system to make some prescriptions
for new ferromagnetic half-metals.Comment: 8 pages including 9 figure
Phase diagram and influence of defects in the double perovskites
The phase diagram of the double perovskites of the type Sr_{2-x} La_x Fe Mo
O_6 is analyzed, with and without disorder due to antisites. In addition to an
homogeneous half metallic ferrimagnetic phase in the absence of doping and
disorder, we find antiferromagnetic phases at large dopings, and other
ferrimagnetic phases with lower saturation magnetization, in the presence of
disorder.Comment: 4 pages, 3 postscript figures, some errata correcte
Crossover effects in the Wolf-Villain model of epitaxial growth in 1+1 and 2+1 dimensions
A simple model of epitaxial growth proposed by Wolf and Villain is
investigated using extensive computer simulations. We find an unexpectedly
complex crossover behavior of the original model in both 1+1 and 2+1
dimensions. A crossover from the effective growth exponent to is observed in 1+1
dimensions, whereas additional crossovers, which we believe are to the scaling
behavior of an Edwards--Wilkinson type, are observed in both 1+1 and 2+1
dimensions. Anomalous scaling due to power--law growth of the average step
height is found in 1+1 D, and also at short time and length scales in 2+1~D.
The roughness exponents obtained from the
height--height correlation functions in 1+1~D () and 2+1~D
() cannot be simultaneously explained by any of the continuum
equations proposed so far to describe epitaxial growth.Comment: 11 pages, REVTeX 3.0, IC-DDV-93-00
Tuning gaps and phases of a two-subband system in a quantizing magnetic field
In this work we study the properties of a two-subband quasi-two-dimensional
electron system in a strong magnetic field when the electron filling factor is
equal to four. When the cyclotron energy is close to the intersubband splitting
the system can be mapped onto a four-level electron system with an effective
filling factor of two. The ground state is either a ferromagnetic state or a
spin-singlet state, depending on the values of the inter-level splitting and
Zeeman energy. The boundaries between these phases are strongly influenced by
the inter-electron interaction. A significant exchange-mediated enhancement of
the excitation gap results in the suppression of the electron-phonon
interaction. The rate of absorption of non-equilibrium phonons is calculated as
a function of Zeeman energy and inter-subband splitting. The phonon absorption
rate has two peaks as a function of intersubband splitting and has a step-like
structure as a function of Zeeman energy
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