142 research outputs found
Non-monotonic field-dependence of the ZFC magnetization peak in some systems of magnetic nanoparticles
We have performed magnetic measurements on a diluted system of gamma-Fe2O3
nanoparticles (~7nm), and on a ferritin sample. In both cases, the ZFC-peak
presents a non-monotonic field dependence, as has already been reported in some
experiments,and discussed as a possible evidence of resonant tunneling. Within
simple assumptions, we derive expressions for the magnetization obtained in the
usual ZFC, FC, TRM procedures. We point out that the ZFC-peak position is
extremely sensitive to the width of the particle size distribution, and give
some numerical estimates of this effect. We propose to combine the FC
magnetization with a modified TRM measurement, a procedure which allows a more
direct access to the barrier distribution in a field. The typical barrier
values which are obtained with this method show a monotonic decrease for
increasing fields, as expected from the simple effect of anisotropy barrier
lowering, in contrast with the ZFC results. From our measurements on
gamma-Fe2O3 particles, we show that the width of the effective barrier
distribution is slightly increasing with the field, an effect which is
sufficient for causing the observed initial increase of the ZFC-peak
temperatures.Comment: LaTeX file 19 pages, 9 postscript figures. To appear in Phys. Rev. B
(tentative schedule: Dec.97
Relaxation and Landau-Zener experiments down to 100 mK in ferritin
Temperature-independent magnetic viscosity in ferritin has been observed from
2 K down to 100 mK, proving that quantum tunneling plays the main role in these
particles at low temperature. Magnetic relaxation has also been studied using
the Landau-Zener method making the system crossing zero resonant field at
different rates, alpha=dH/dt, ranging from 10^{-5} to 10^{-3} T/s, and at
different temperatures, from 150 mK up to the blocking temperature. We propose
a new Tln(Delta H_{eff}/tau_0 alpha) scaling law for the Landau-Zener
probability in a system distributed in volumes, where Delta H_{eff} is the
effective width of the zero field resonance.Comment: 13 pages, 4 postscript figure
Real-time non-equilibrium dynamics of quantum glassy systems
We develop a systematic analytic approach to aging effects in quantum
disordered systems in contact with an environment. Within the closed-time
path-integral formalism we include dissipation by coupling the system to a set
of independent harmonic oscillators that mimic a quantum thermal bath. After
integrating over the bath variables and averaging over disorder we obtain an
effective action that determines the real-time dynamics of the system. The
classical limit yields the Martin-Siggia-Rose generating functional associated
to a colored noise. We apply this general formalism to a prototype model
related to the spin-glass. We show that the model has a dynamic phase
transition separating the paramagnetic from the spin-glass phase and that
quantum fluctuations depress the transition temperature until a quantum
critical point is reached. We show that the dynamics in the paramagnetic phase
is stationary but presents an interesting crossover from a region controlled by
the classical critical point to another one controlled by the quantum critical
point. The most characteristic property of the dynamics in a glassy phase,
namely aging, survives the quantum fluctuations. In the sub-critical region the
quantum fluctuation-dissipation theorem is modified in a way that is consistent
with the notion of effective temperatures introduced for the classical case. We
discuss these results in connection with recent experiments in dipolar quantum
spin-glasses and the relevance of the effective temperatures with respect to
the understanding of the low temperature dynamics.Comment: 56 pages, Revtex, 17 figures include
Retrieving C and O Abundance of HR 8799 c by Combining High- and Low-Resolution Data
The formation and evolution pathway for the directly-imaged multi-planetary
system HR 8799 remains mysterious. Accurate constraints on the chemical
composition of the planetary atmosphere(s) are key to solving the mystery. We
perform a detailed atmospheric retrieval on HR 8799~c to infer the chemical
abundances and abundance ratios using a combination of photometric data along
with low- and high-resolution spectroscopic data (R20-35,000). We
specifically retrieve [C/H], [O/H], and C/O and find them to be
0.55, 0.47, and 0.67 at
68\% confidence. The super-stellar C and O abundances, yet a stellar C/O ratio,
reveal a potential formation pathway for HR 8799~c. Planet c, and likely the
other gas giant planets in the system, formed early on (likely within 1
Myr), followed by further atmospheric enrichment in C and O through the
accretion of solids beyond the CO iceline. The enrichment either preceded or
took place during the early phase of the inward migration to the planet current
locations.Comment: 19 pages, 6 figures, 3 tables, accepted to AAS journal
Does visual cortex lactate increase following photic stimulation in migraine without aura patients? A functional 1H-MRS study
Proton magnetic resonance spectroscopy (1H-MRS) has been used in a number of studies to assess noninvasively the temporal changes of lactate (Lac) in the activated human brain. Migraine neurobiology involves lack of cortical habituation to repetitive stimuli and a mitochondrial component has been put forward. Our group has recently demonstrated a reduction in the high-energy phosphates adenosine triphosphate (ATP) and phosphocreatine (PCr) in the occipital lobe of migraine without aura (MwoA) patients, at least in a subgroup, in a phosphorus MRS (31P-MRS) study. In previous studies, basal Lac levels or photic stimulation (PS)-induced Lac levels were found to be increased in patients with migraine with aura (MwA) and migraine patients with visual symptoms and paraesthesia, paresia and/or dysphasia, respectively. The aim of this study was to perform functional 1H-MRS at 3 T in 20 MwoA patients and 20 control subjects. Repetitive visual stimulation was applied using MR-compatible goggles with 8 Hz checkerboard stimulation during 12 min. We did not observe any significant differences in signal integrals, ratios and absolute metabolite concentrations, including Lac, between MwoA patients and controls before PS. Lac also did not increase significantly during and following PS, both for MwoA patients and controls. Subtle Lac changes, smaller than the sensitivity threshold (i.e. estimated at 0.1–0.2 μmol/g at 3 T), cannot be detected by MRS. Our study does, however, argue against a significant switch to non-aerobic glucose metabolism during long-lasting PS of the visual cortex in MwoA patients
Retrieving the C and O Abundances of HR 7672~AB: a Solar-Type Primary Star with a Benchmark Brown Dwarf
A benchmark brown dwarf (BD) is a BD whose properties (e.g., mass and
chemical composition) are precisely and independently measured. Benchmark BDs
are valuable in testing theoretical evolutionary tracks, spectral synthesis,
and atmospheric retrievals for sub-stellar objects. Here, we report results of
atmospheric retrieval on a synthetic spectrum and a benchmark BD -- HR 7672~B
-- with \petit. First, we test the retrieval framework on a synthetic PHOENIX
BT-Settl spectrum with a solar composition. We show that the retrieved C and O
abundances are consistent with solar values, but the retrieved C/O is
overestimated by 0.13-0.18, which is 4 times higher than the formal error
bar. Second, we perform retrieval on HR 7672~B using high spectral resolution
data (R=35,000) from the Keck Planet Imager and Characterizer (KPIC) and near
infrared photometry. We retrieve [C/H], [O/H], and C/O to be ,
, and . These values are consistent with those of HR
7672~A within 1.5-. As such, HR 7672~B is among only a few benchmark
BDs (along with Gl 570~D and HD 3651~B) that have been demonstrated to have
consistent elemental abundances with their primary stars. Our work provides a
practical procedure of testing and performing atmospheric retrieval, and sheds
light on potential systematics of future retrievals using high- and
low-resolution data.Comment: 29 pages, 17 figures, 5 tables, resubmitted to AAS journals after
first revisio
Vortex Fiber Nulling for Exoplanet Observations: Implementation and First Light
Vortex fiber nulling (VFN) is a single-aperture interferometric technique for
detecting and characterizing exoplanets separated from their host star by less
than a diffracted beam width. VFN uses a vortex mask and single mode fiber to
selectively reject starlight while coupling off-axis planet light with a simple
optical design that can be readily implemented on existing direct imaging
instruments that can feed light to an optical fiber. With its axially symmetric
coupling region peaking within the inner working angle of conventional
coronagraphs, VFN is more efficient at detecting new companions at small
separations than conventional direct imaging, thereby increasing the yield of
on-going exoplanet search campaigns. We deployed a VFN mode operating in K band
(m) on the Keck Planet Imager and Characterizer (KPIC)
instrument at the Keck II Telescope. In this paper we present the instrument
design of this first on-sky demonstration of VFN and the results from on-sky
commissioning, including planet and star throughput measurements and predicted
flux-ratio detection limits for close-in companions. The instrument performance
is shown to be sufficient for detecting a companion times fainter than a
magnitude host star in 1 hour at a separation of 50 mas
(1.1). This makes the instrument capable of efficiently detecting
substellar companions around young stars. We also discuss several routes for
improvement that will reduce the required integration time for a detection by a
factor 3.Comment: 26 pages, 5 figures; Accepted to JATI
Phase II of the Keck Planet Imager and characterizer: system-level laboratory characterization and preliminary on-sky commissioning
Multifocality and multicentricity are not contraindications for sentinel lymph node biopsy in breast cancer surgery
BACKGROUND: After the availability of the results of validation studies, the sentinel lymph node biopsy (SLNB) has replaced routine axillary dissection (AD) as the new standard of care in early unifocal breast cancers. Multifocal (MF) and multicentric (MC) tumors have been considered a contraindication for this technique due to the possible incidence of a higher false-negative rate. This prospective study evaluates the lymphatic drainage from different tumoral foci of the breast and assesses the accuracy of SLNB in MF-MC breast cancer. PATIENTS AND METHODS: Patients with preoperative diagnosis of MF or MC infiltrating and clinically node-negative (cN0) breast carcinoma were enrolled in this study. Two consecutive groups of patients underwent SLN mapping using a different site of injection of the radioisotope tracer: a) "2ID" Group received two intradermal (ID) injections over the site of the two dominant neoplastic nodules. A lymphoscintigraphic study was performed after each injection to evaluate the route of lymphatic spreading from different sites of the breast. b) "A" Group had periareolar (A) injection followed by a conventional lymphoscintigraphy. At surgery, both radioguided SLNB (with frozen section exam) and subsequent AD were planned, regardless the SLN status. RESULTS: A total 31 patients with MF (n = 12) or MC (n = 19) invasive, cN0 cancer of the breast fulfil the selection criteria. In 2 ID Group (n = 15) the lymphoscintigraphic study showed the lymphatic pathways from two different sites of the breast which converged into one major lymphatic trunk affering to the same SLN(s) in 14 (93.3%) cases. In one (6.7%) MC cancer two different pathways were found, each of them affering to a different SLN. In A Group (n = 16) lymphoscintigraphy showed one (93.7%) or two (6.3%) lymphatic channels, each connecting areola with one or more SLN(s). Identification rate of SLN was 100% in both Groups. Accuracy of frozen section exam on SLN was 96.8% (1 case of micrometastasis was missed). SLN was positive in 13 (41.9%) of 31 patients, including 4 cases (30.7%) of micrometastasis. In 7 of 13 (53.8%) patients the SLN was the only site of axillary metastasis. SLNB accuracy was 96.8% (30 of 31), sensitivity 92.8 (13 of 14), and false-negative rate 7.1% (1 of 14). Since the case of skip metastasis was identified by the surgeon intraoperatively, it would have been no impact in the clinical practice. CONCLUSION: Our lymphoscintigraphic study shows that axillary SLN represents the whole breast regardless of tumor location within the parenchyma. The high accuracy of SLNB in MF and MC breast cancer demonstrates, according with the results of other series published in the literature, that both MF and MC tumors do not represent a contraindication for SLNB anymore
Frontal GABA Levels Change during Working Memory
Functional neuroimaging metrics are thought to reflect changes in neurotransmitter flux, but changes in neurotransmitter levels have not been demonstrated in humans during a cognitive task, and the relationship between neurotransmitter dynamics and hemodynamic activity during cognition has not yet been established. We evaluate the concentration of the major inhibitory (GABA) and excitatory (glutamate + glutamine: Glx) neurotransmitters and the cerebral perfusion at rest and during a prolonged delayed match-to-sample working memory task. Resting GABA levels in the dorsolateral prefrontal cortex correlated positively with the resting perfusion and inversely with the change in perfusion during the task. Further, only GABA increased significantly during the first working memory run and then decreased continuously across subsequent task runs. The decrease of GABA over time was paralleled by a trend towards decreased reaction times and higher task accuracy. These results demonstrate a link between neurotransmitter dynamics and hemodynamic activity during working memory, indicating that functional neuroimaging metrics depend on the balance of excitation and inhibition required for cognitive processing
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