84 research outputs found
Thermal annealing study of swift heavy-ion irradiated zirconia
Sintered samples of monoclinic zirconia (alpha-ZrO2) have been irradiated at
room temperature with 6.0-GeV Pb ions in the electronic slowing down regime.
X-ray diffraction (XRD) and micro-Raman spectroscopy measurements showed
unambiguously that a transition to the 'metastable' tetragonal phase
(beta-ZrO2) occurred at a fluence of 6.5x10^12 cm-2 for a large electronic
stopping power value (approx 32.5 MeV m-1). At a lower fluence of
1.0x10^12 cm-2, no such phase transformation was detected. The
back-transformation from beta- to alpha-ZrO2 induced by isothermal or
isochronal thermal annealing was followed by XRD analysis. The
back-transformation started at an onset temperature around 500 K and was
completed by 973 K. Plots of the residual tetragonal phase fraction deduced
from XRD measurements versus annealing temperature or time are analyzed with
first- or second-order kinetic models. An activation energy close to 1 eV for
the back-transformation process is derived either from isothermal annealing
curves, using the so-called "cross-cut" method, or from the isochronal
annealing curve, using a second-order kinetic law. Correlation with the thermal
recovery of ion-induced paramagnetic centers monitored by EPR spectroscopy is
discussed. Effects of crystallite size evolution and oxygen migration upon
annealing are also addressed
Thermal recovery of colour centres induced in cubic yttria-stabilized zirconia by charged particle irradiations
We have used electron paramagnetic resonance to study the thermal annealing
of colour centres induced in cubic yttria-stabilized zirconia by swift electron
and heavy ion-irradiations. Single crystals were irradiated with 1 or 2-MeV
electrons, and 200-MeV 127I, or 200-MeV 197Au ions. Electron and ion beams
produce the same colour centres: namely i) an F+-like centre, ii) the so-called
T-centre (Zr3+ in a trigonal oxygen local environment), and iii) a hole center.
Isochronal annealing was performed up to 973 K. Isothermal annealing was
performed at various temperatures on samples irradiated with 2-MeV electrons.
The stability of paramagnetic centres increases with fluence and with a TCR
treatment at 1373 K under vacuum prior to the irradiations. Two distinct
recovery processes are observed depending on fluence and/or thermal treatment.
The single-stage type I process occurs for F+-like centres at low fluences in
as-received samples, and is probably linked to electron-hole recombination.
T-centres are also annealed according to a single-stage process regardless of
fluence. The annealing curves allow one to obtain activation energies for
recovery. The two-stage type II process is observed only for the F+-like
centres in as-received samples, at higher fluences, or in reduced samples.
These centres are first annealed in a first stage below 550 K, like in type I,
then transform into new paramagnetic centres in a second stage above 550 K. A
simple kinetics model is proposed for this process. Complete colour centre
bleaching is achieved at about 1000 K
Phonon-induced spin relaxation of conduction electrons in aluminum
Spin-flip Eliashberg function and temperature-dependent spin
relaxation time are calculated for aluminum using realistic
pseudopotentials. The spin-flip electron-phonon coupling constant
is found to be . The calculations agree with experiments
validating the Elliott-Yafet theory and the spin-hot-spot picture of spin
relaxation for polyvalent metals.Comment: 4 pages; submitted to PR
Spin injection and spin accumulation in all-metal mesoscopic spin valves
We study the electrical injection and detection of spin accumulation in
lateral ferromagnetic metal-nonmagnetic metal-ferromagnetic metal (F/N/F) spin
valve devices with transparent interfaces. Different ferromagnetic metals,
permalloy (Py), cobalt (Co) and nickel (Ni), are used as electrical spin
injectors and detectors. For the nonmagnetic metal both aluminium (Al) and
copper (Cu) are used. Our multi-terminal geometry allows us to experimentally
separate the spin valve effect from other magneto resistance signals such as
the anomalous magneto resistance (AMR) and Hall effects. We find that the AMR
contribution of the ferromagnetic contacts can dominate the amplitude of the
spin valve effect, making it impossible to observe the spin valve effect in a
'conventional' measurement geometry. In a 'non local' spin valve measurement we
are able to completely isolate the spin valve signal and observe clear spin
accumulation signals at T=4.2 K as well as at room temperature (RT). For
aluminum we obtain spin relaxation lengths (lambda_{sf}) of 1.2 mu m and 600 nm
at T=4.2 K and RT respectively, whereas for copper we obtain 1.0 mu m and 350
nm. The spin relaxation times tau_{sf} in Al and Cu are compared with theory
and results obtained from giant magneto resistance (GMR), conduction electron
spin resonance (CESR), anti-weak localization and superconducting tunneling
experiments. The spin valve signals generated by the Py electrodes (alpha_F
lambda_F=0.5 [1.2] nm at RT [T=4.2 K]) are larger than the Co electrodes
(alpha_F lambda_F=0.3 [0.7] nm at RT [T=4.2 K]), whereas for Ni (alpha_F
lambda_F<0.3 nm at RT and T=4.2 K) no spin signal is observed. These values are
compared to the results obtained from GMR experiments.Comment: 16 pages, 12 figures, submitted to PR
Spintronics: Fundamentals and applications
Spintronics, or spin electronics, involves the study of active control and
manipulation of spin degrees of freedom in solid-state systems. This article
reviews the current status of this subject, including both recent advances and
well-established results. The primary focus is on the basic physical principles
underlying the generation of carrier spin polarization, spin dynamics, and
spin-polarized transport in semiconductors and metals. Spin transport differs
from charge transport in that spin is a nonconserved quantity in solids due to
spin-orbit and hyperfine coupling. The authors discuss in detail spin
decoherence mechanisms in metals and semiconductors. Various theories of spin
injection and spin-polarized transport are applied to hybrid structures
relevant to spin-based devices and fundamental studies of materials properties.
Experimental work is reviewed with the emphasis on projected applications, in
which external electric and magnetic fields and illumination by light will be
used to control spin and charge dynamics to create new functionalities not
feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes
from the published versio
CD4+ T Cell-Derived IL-2 Signals during Early Priming Advances Primary CD8+ T Cell Responses
Stimulating naïve CD8+ T cells with specific antigens and costimulatory signals is insufficient to induce optimal clonal expansion and effector functions. In this study, we show that the activation and differentiation of CD8+ T cells require IL-2 provided by activated CD4+ T cells at the initial priming stage within 0–2.5 hours after stimulation. This critical IL-2 signal from CD4+ cells is mediated through the IL-2Rβγ of CD8+ cells, which is independent of IL-2Rα. The activation of IL-2 signaling advances the restriction point of the cell cycle, and thereby expedites the entry of antigen-stimulated CD8+ T-cell into the S phase. Besides promoting cell proliferation, IL-2 stimulation increases the amount of IFNγ and granzyme B produced by CD8+ T cells. Furthermore, IL-2 at priming enhances the ability of P14 effector cells generated by antigen activation to eradicate B16.gp33 tumors in vivo. Therefore, our studies demonstrate that a full CD8+ T-cell response is elicited by a critical temporal function of IL-2 released from CD4+ T cells, providing mechanistic insights into the regulation of CD8+ T cell activation and differentiation
Sodium Selenide Toxicity Is Mediated by O2-Dependent DNA Breaks
Hydrogen selenide is a recurrent metabolite of selenium compounds. However, few experiments studied the direct link between this toxic agent and cell death. To address this question, we first screened a systematic collection of Saccharomyces cerevisiae haploid knockout strains for sensitivity to sodium selenide, a donor for hydrogen selenide (H2Se/HSe−/Se2−). Among the genes whose deletion caused hypresensitivity, homologous recombination and DNA damage checkpoint genes were over-represented, suggesting that DNA double-strand breaks are a dominant cause of hydrogen selenide toxicity. Consistent with this hypothesis, treatment of S. cerevisiae cells with sodium selenide triggered G2/M checkpoint activation and induced in vivo chromosome fragmentation. In vitro, sodium selenide directly induced DNA phosphodiester-bond breaks via an O2-dependent reaction. The reaction was inhibited by mannitol, a hydroxyl radical quencher, but not by superoxide dismutase or catalase, strongly suggesting the involvement of hydroxyl radicals and ruling out participations of superoxide anions or hydrogen peroxide. The •OH signature could indeed be detected by electron spin resonance upon exposure of a solution of sodium selenide to O2. Finally we showed that, in vivo, toxicity strictly depended on the presence of O2. Therefore, by combining genome-wide and biochemical approaches, we demonstrated that, in yeast cells, hydrogen selenide induces toxic DNA breaks through an O2-dependent radical-based mechanism
Enhancement of the Electron Spin Resonance of Single-Walled Carbon Nanotubes by Oxygen Removal
We have observed a nearly fourfold increase in the electron spin resonance
(ESR) signal from an ensemble of single-walled carbon nanotubes (SWCNTs) due to
oxygen desorption. By performing temperature-dependent ESR spectroscopy both
before and after thermal annealing, we found that the ESR in SWCNTs can be
reversibly altered via the molecular oxygen content in the samples. Independent
of the presence of adsorbed oxygen, a Curie-law (spin susceptibility ) is seen from 4 K to 300 K, indicating that the probed spins are
finite-level species. For both the pre-annealed and post-annealed sample
conditions, the ESR linewidth decreased as the temperature was increased, a
phenomenon we identify as motional narrowing. From the temperature dependence
of the linewidth, we extracted an estimate of the intertube hopping frequency;
for both sample conditions, we found this hopping frequency to be 100
GHz. Since the spin hopping frequency changes only slightly when oxygen is
desorbed, we conclude that only the spin susceptibility, not spin transport, is
affected by the presence of physisorbed molecular oxygen in SWCNT ensembles.
Surprisingly, no linewidth change is observed when the amount of oxygen in the
SWCNT sample is altered, contrary to other carbonaceous systems and certain 1D
conducting polymers. We hypothesize that physisorbed molecular oxygen acts as
an acceptor (-type), compensating the donor-like (-type) defects that are
responsible for the ESR signal in bulk SWCNTs.Comment: 14 pages, 7 figure
Two-photon microscopy analysis of leukocyte trafficking and motility
During the last several years, live tissue imaging, in particular using two-photon laser microscopy, has advanced our understanding of leukocyte trafficking mechanisms. Studies using this technique are revealing distinct molecular requirements for leukocyte migration in different tissue environments. Also emerging from the studies are the ingenious infrastructures for leukocyte trafficking, which are produced by stromal cells. This review summarizes the recent imaging studies that provided novel mechanistic insights into in vivo leukocyte migration essential for immunosurveillance
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