43 research outputs found
Observation of Vortex Matching Phenomena in Antidot Array of NbN Thin Film
We report vortex matching phenomenon in rectangular antidot array fabricated
on epitaxial NbN thin film. The antidot array was fabricated using Focussed Ion
Beam milling technique. The magneto-transport measurements points to a period
doubling transition at higher magnetic field for rectangular lattices. The
results are discussed within the light of several models including the
multi-vortex model, the matched lattice model and the super-matched lattice
model.Comment: Added references, modified abstract and discussions and corrected
typo-graphic errors. Accepted for proceedings of M2S-IX 2009, Tokyo (Physica
C
Superconducting properties and Hall Effect of epitaxial NbN thin films
We have measured the magnetotransport properties and Hall effect of a series
of epitaxial NbN films grown on (100) oriented single crystalline MgO substrate
under different conditions using reactive magnetron sputtering. Hall effect
measurements reveal that the carrier density in NbN thin films is sensitive to
the growth condition. The carrier density increases by a factor of 3 between
the film with highest normal state resistivity (rho_n~3.83micro-Ohm-m) and
lowest transition temperature (Tc~9.99K) and the film with lowest normal state
resistivity (rho_n~0.94micro-Ohm-m) and highest transition temperature
(Tc~16.11K) while the mobility of carriers does not change significantly except
for the most resistive films. Our results show that the Tc of NbN is governed
primarily by the carrier density rather than disorder scattering. By varying
the carrier concentration during growth we can vary the effective disorder
(kF_l) from the moderately clean limit to the dirty limit which makes this
system ideal to study the interplay of carrier density and disorder on the
superconducting properties of an s-wave superconductor.Comment: ps files with figures (accepted for publication in Phys. Rev. B
Transport signatures of fragile glass dynamics in the melting of the two-dimensional vortex lattice
In a two-dimensional superconducting vortex lattice, the melting from the solid to the isotropic liquid can occur via an intermediate phase that retains orientational correlations. The effect of such correlations on transport and their interplay with the quenched disorder remain open questions. We perform magnetotransport measurements in a wide range of temperatures and magnetic fields on a weakly pinned two-dimensional vortex system in amorphous MoGe films. While at high fields, where quenched disorder dominates, we recover the typical strong-glass behavior of a vortex liquid, at low fields the resistivity shows a clear crossover to a fragile vortex glass. Our findings, supported by numerical simulations, suggest that this is a signature of heterogeneous dynamics that arises from the presence of orientational correlations
Tunneling studies in a homogeneously disordered s-wave superconductor: NbN
We report the evolution of superconducting properties as a function of
disorder in homogeneously disordered epitaxial NbN thin films grown on (100)
MgO substrates, studied through a combination of electrical transport, Hall
Effect and tunneling measurements. The thickness of all our films are >50nm
much larger than the coherence length ~5nm. The effective disorder in different
films encompasses a large range, with the Ioffe-Regel parameter varying in the
range kFl~1.38-8.77. Tunneling measurements on films with different disorder
reveals that for films with large disorder the bulk superconducting transition
temperature (Tc) is not associated with a vanishing of the superconducting
energy gap, but rather a large broadening of the superconducting density of
states. Our results provide strong evidence of the loss of superconductivity
via phase-fluctuations in a disordered s-wave superconductor.Comment: pdf file including figure
Multi-vortex versus interstitial vortices scenario in superconducting antidot arrays
In superconducting thin films, engineered lattice of antidots (holes) act as
an array of columnar pinning sites for the vortices and thus lead to vortex
matching phenomena at commensurate fields guided by the lattice spacing. The
strength and nature of vortex pinning is determined by the geometrical
characteristics of the antidot lattice (such as the lattice spacing ,
antidot diameter , lattice symmetry, orientation, etc) along with the
characteristic length scales of the superconducting thin films, viz., the
coherence length () and the penetration depth (). There are at
least two competing scenarios: (i) multiple vortices sit on each of the
antidots at a higher matching period, and, (ii) there is nucleation of vortices
at the interstitial sites at higher matching periods. Furthermore it is also
possible for the nucleated interstitial vortices to reorder under suitable
conditions. We present our experimental results on NbN antidot arrays in the
light of the above scenarios.Comment: Submitted to ISS2009 proceedings; references adde
Peripherally derived macrophages modulate microglial function to reduce inflammation after CNS injury
Infiltrating monocyte-derived macrophages (MDMs) and resident microglia dominate central nervous system (CNS) injury sites. Differential roles for these cell populations after injury are beginning to be uncovered. Here, we show evidence that MDMs and microglia directly communicate with one another and differentially modulate each other's functions. Importantly, microglia-mediated phagocytosis and inflammation are suppressed by infiltrating macrophages. In the context of spinal cord injury (SCI), preventing such communication increases microglial activation and worsens functional recovery. We suggest that macrophages entering the CNS provide a regulatory mechanism that controls acute and long-term microglia-mediated inflammation, which may drive damage in a variety of CNS conditions
Leprosy among Patient Contacts: A Multilevel Study of Risk Factors
Leprosy is an infectious disease that can lead to physical disabilities, social stigma, and great hardship. Transmitted from person to person, it is still endemic in developing countries, like Brazil and India. Effective treatment has been available since 1960, but early diagnosis of the disease remains the most effective way to stop the transmission chain and avoid late diagnoses and subsequent disabilities. Knowledge of the risk factors for leprosy can facilitate early detection; therefore, our study aimed to investigate the factors presented by leprosy patients and their contacts, who are considered at highest risk of contracting the disease. We studied 6,158 contacts of 1,201 patients under surveillance from 1987 to 2007 in a Public Health Care Center in the City of Rio de Janeiro, Brazil. We evaluated the ways patient and contact demographics and epidemiological characteristics were associated with the detection of leprosy. Statistical analyses took into account both individual and group characteristics and their interrelationships. The main characteristics facilitating the contraction of leprosy among contacts were shown to be consanguinity and household association. Conversely, the bacillary load index of leprosy patients was the principle factor leading to disease among their contacts