227 research outputs found
First principles theoretical studies of half-metallic ferromagnetism in CrTe
Using full-potential linear augmented plane wave method (FP-LAPW) and the
density functional theory, we have carried out a systematic investigation of
the electronic, magnetic, and cohesive properties of the chalcogenide CrTe in
three competing structures: rock-salt (RS), zinc blende (ZB) and the NiAs-type
(NA) hexagonal. Although the ground state is of NA structure, RS and ZB are
interesting in that these fcc-based structures, which can possibly be grown on
many semiconductor substrates, exhibit half-metallic phases above some critical
values of the lattice parameter. We find that the NA structure is not
half-metallic at its equilibrium volume, while both ZB and RS structures are.
The RS structure is more stable than the ZB, with an energy that is lower by
0.25 eV/atom. While confirming previous results on the half-metallic phase in
ZB structure, we provide hitherto unreported results on the half-metallic RS
phase, with a gap in the minority channel and a magnetic moment of 4.0
per formula unit. A comparison of total energies for the
ferromagnetic (FM), non-magnetic (NM), and antiferromagnetic (AFM)
configurations shows the lowest energy configuration to be FM for CrTe in all
the three structures. The FP-LAPW calculations are supplemented by linear
muffin-tin orbital (LMTO) calculations using both local density approximation
(LDA) and LDA+U method. The exchange interactions and the Curie temperatures
calculated via the linear response method in ZB and RS CrTe are compared over a
wide range of the lattice parameter. The calculated Curie temperatures for the
RS phase are consistently higher than those for the ZB phase.Comment: 11 pages, 14 figure
Nonpolar resistance switching of metal/binary-transition-metal oxides/metal sandwiches: homogeneous/inhomogeneous transition of current distribution
Exotic features of a metal/oxide/metal (MOM) sandwich, which will be the
basis for a drastically innovative nonvolatile memory device, is brought to
light from a physical point of view. Here the insulator is one of the
ubiquitous and classic binary-transition-metal oxides (TMO), such as Fe2O3,
NiO, and CoO. The sandwich exhibits a resistance that reversibly switches
between two states: one is a highly resistive off-state and the other is a
conductive on-state. Several distinct features were universally observed in
these binary TMO sandwiches: namely, nonpolar switching, non-volatile threshold
switching, and current--voltage duality. From the systematic sample-size
dependence of the resistance in on- and off-states, we conclude that the
resistance switching is due to the homogeneous/inhomogeneous transition of the
current distribution at the interface.Comment: 7 pages, 5 figures, REVTeX4, submitted to Phys. Rev. B (Feb. 23,
2007). If you can't download a PDF file of this manscript, an alternative one
can be found on the author's website: http://staff.aist.go.jp/i.inoue
Spin-polarized surface state of MnSb(0001)
Knowledge of the spin-dependent electronic structure at surfaces and interfaces plays an increasingly important role when assessing possible use of novel magnetic materials for spintronic applications. It is shown that spin- and angle-resolved photoelectron spectroscopy together with ab initio electronic structure methods provides a full characterization of the surface electronic structure of ferromagnetic MnSb(0 0 0 1). Two different surface reconstructions have been compared in spin- and angle-resolved valence-band photoemission. For annealing at elevated temperatures, the ( 1 x 1)-structure transforms into 2 x 2 and a majority-spin peak appears at - 1.7 eV inside a majority-spin bulk band gap at the surface Brillouin zone centre. Its sensitivity to oxygen supports an interpretation as magnetic compound surface state. Local spin density calculations predict at the same energy (- 1.75 eV) a prominent d(z)2 surface state of majority spin for ( 1 x 1)- Mn terminated MnSb(0 0 0 1) but no such feature for ( 1 x 1)-Sb termination. The calculation shows that neither the bulk nor the surface is half-metallic, in agreement with the expectation for the hexagonal NiAs structure
Appearance of Half-Metallicity in the Quaternary Heusler Alloys
I report systematic first-principle calculations of the quaternary Heusler
alloys like Co[CrMn]Al, CoMn[AlSn] and
[FeCo]MnAl. I show that when the two limiting cases (x=0 or 1)
correspond to a half-metallic compound, so do the intermediate cases. Moreover
the total spin moment in scales linearly with the total number of
valence electrons (and thus with the concentration ) following the
relation , independently of the origin of the extra valence
electrons, confirming the Slater-Pauling behavior of the normal Heusler alloys.
Finally I discuss in all cases the trends in the atomic projected DOSs and in
the atomic spin moments.Comment: 4 pages, 3 figures, 2 Table
Electronic structure, exchange interactions and Curie temperature in diluted III-V magnetic semiconductors: (GaCr)As, (GaMn)As, (GaFe)As
We complete our earlier (Phys. Rev. B, {\bf 66}, 134435 (2002)) study of the
electronic structure, exchange interactions and Curie temperature in (GaMn)As
and extend the study to two other diluted magnetic semiconductors (GaCr)As and
(GaFe)As. Four concentrations of the 3d impurities are studied: 25%, 12.5%,
6.25%, 3.125%. (GaCr)As and (GaMn)As are found to possess a number of similar
features. Both are semi-metallic and ferromagnetic, with similar properties of
the interatomic exchange interactions and the same scale of the Curie
temperature. In both systems the presence of the charge carriers is crucial for
establishing the ferromagnetic order. An important difference between two
systems is in the character of the dependence on the variation of the number of
carriers. The ferromagnetism in (GaMn)As is found to be very sensitive to the
presence of the donor defects, like As antisites. On the other hand,
the Curie temperature of (GaCr)As depends rather weakly on the presence of this
type of defects but decreases strongly with decreasing number of electrons. We
find the exchange interactions between 3d atoms that make a major contribution
into the ferromagnetism of (GaCr)As and (GaMn)As and propose an exchange path
responsible for these interactions. The properties of (GaFe)As are found to
differ crucially from the properties of (GaCr)As and (GaMn)As. (GaFe)As does
not show a trend to ferromagnetism and is not half-metallic that makes this
system unsuitable for the use in spintronic semiconductor devices
Surface Half-Metallicity of CrAs in the Zinc-Blende Structure
The development of new techniques such as the molecular beam epitaxy have
enabled the growth of thin films of materials presenting novel properties.
Recently it was made possible to grow a CrAs thin-film in the zinc-blende
structure. In this contribution, the full-potential screened KKR method is used
to study the electronic and magnetic properties of bulk CrAs in this novel
phase as well as the Cr and As terminated (001) surfaces. Bulk CrAs is found to
be half-ferromagnetic for all three GaAs, AlAs and InAs experimental lattice
constants with a total spin magnetic moment of 3 . The Cr-terminated
surface retains the half-ferromagnetic character of the bulk, while in the case
of the As-termination the surface states destroy the gap in the minority-spin
band.Comment: 4 pages, 2 figures, new text, new titl
Towards New Half-Metallic Systems: Zinc-Blende Compounds of Transition Elements with N, P, As, Sb, S, Se, and Te
We report systematic first-principles calculations for ordered zinc-blende
compounds of the transition metal elements V, Cr, Mn with the sp elements N, P,
As, Sb, S, Se, Te, motivated by recent fabrication of zinc-blende CrAs, CrSb,
and MnAs. They show ferromagnetic half-metallic behavior for a wide range of
lattice constants. We discuss the origin and trends of half-metallicity,
present the calculated equilibrium lattice constants, and examine the
half-metallic behavior of their transition element terminated (001) surfaces.Comment: 2nd Version: lattice constants calculations added, text revise
Retinoblastoma protein-initiated cellular growth arrest overcomes the ability of cotransfected wild-type p53 to induce apoptosis
The retinoblastoma gene, RB, participates in the regulation of the G1/S-phase transition and in p53-mediated apoptosis. We have previously reported that stably transfected RB functions as a growth and tumour suppressor in HTB9 human bladder carcinoma cells, which carry a mutation of the p53 gene at codon 280 and lack RB expression. To elucidate the potential role of RB in the regulation of p53-mediated apoptosis, we transfected a wt p53 expression plasmid under the control of the human cytomegalovirus promoter into parental and RB-transfected HTB9 cells. The p53+/RB– cells were susceptible to apoptosis under various experimental conditions: 1) incubation in serum-free culture for 72 h, 2) short-term (6 h) or long-term (48 h) exposure to etoposide, and 3) culturing in soft agar. In contrast, p53+/RB+ cells were significantly resistant to apoptosis under similar conditions and exhibited efficient growth arrest, as measured by laser scanning cytometry. Tumorigenicity in nude mice of parental HTB9 cells was lost by exogenous expression of wt p53. Likewise, none of mice injected subcutaneously with either p53–/RB+ or p53+/RB+ cells developed tumours, indicating that RB allows suppression of tumorigenesis, regardless of p53 status. These results suggest that the growth-inhibitory function of RB may overcome the ability of wt p53 to induce apoptosis. © 2000 Cancer Research Campaig
Carrier-mediated ferromagnetic ordering in Mn ion-implanted p+GaAs:C
Highly p-type GaAs:C was ion-implanted with Mn at differing doses to produce
Mn concentrations in the 1 - 5 at.% range. In comparison to LT-GaAs and
n+GaAs:Si samples implanted under the same conditions, transport and magnetic
properties show marked differences. Transport measurements show anomalies,
consistent with observed magnetic properties and with epi- LT-(Ga,Mn)As, as
well as the extraordinary Hall Effect up to the observed magnetic ordering
temperature (T_C). Mn ion-implanted p+GaAs:C with as-grown carrier
concentrations > 10^20 cm^-3 show remanent magnetization up to 280 K
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