52 research outputs found
Effect of granularity on the insulator-superconductor transition in ultrathin Bi films
We have studied the insulator-superconductor transition (IST) by tuning the
thickness in quench-condensed films. The resistive transitions of the
superconducting films are smooth and can be considered to represent
"homogeneous" films. The observation of an IST very close to the quantum
resistance for pairs, on several substrates supports
this idea. The relevant length scales here are the localization length, and the
coherence length. However, at the transition, the localization length is much
higher than the superconducting coherence length, contrary to expectation for a
"homogeneous" transition. This suggests the invalidity of a purely fermionic
model for the transition. Furthermore, the current-voltage characteristics of
the superconducting films are hysteretic, and show the films to be granular.
The relevant energy scales here are the Josephson coupling energy and the
charging energy. However, Josephson coupling energies () and the charging
energies () at the IST, they are found to obey the relation .
This is again contrary to expectation, for the IST in a granular or
inhomogeneous, system. Hence, a purely bosonic picture of the transition is
also inconsistent with our observations. We conclude that the IST observed in
our experiments may be either an intermediate case between the fermioinc and
bosonic mechanisms, or in a regime of charge and vortex dynamics for which a
quantitative analysis has not yet been done.Comment: accepted in Physical Review
Vortex dynamics and upper critical fields in ultrathin Bi films
Current-voltage (I-V) characteristics of quench condensed, superconducting,
ultrathin films in a magnetic field are reported. These I-V's show
hysteresis for all films, grown both with and without thin underlayers.
Films on Ge underlayers, close to superconductor-insulator transition (SIT),
show a peak in the critical current, indicating a structural transformation of
the vortex solid (VS). These underlayers, used to make the films more
homogeneous, are found to be more effective in pinning the vortices. The upper
critical fields (B) of these films are determined from the resistive
transitions in perpendicular magnetic field. The temperature dependence of the
upper critical field is found to differ significantly from Ginzburg-Landau
theory, after modifications for disorder.Comment: Phys Rev B, to be published Figure 6 replaced with correct figur
Commensurability oscillations in the rf conductivity of unidirectional lateral superlattices: measurement of anisotropic conductivity by coplanar waveguide
We have measured the rf magnetoconductivity of unidirectional lateral
superlattices (ULSLs) by detecting the attenuation of microwave through a
coplanar waveguide placed on the surface. ULSL samples with the principal axis
of the modulation perpendicular (S_perp) and parallel (S_||) to the microwave
electric field are examined. For low microwave power, we observe expected
anisotropic behavior of the commensurability oscillations (CO), with CO in
samples S_perp and S_|| dominated by the diffusion and the collisional
contributions, respectively. Amplitude modulation of the Shubnikov-de Haas
oscillations is observed to be more prominent in sample S_||. The difference
between the two samples is washed out with the increase of the microwave power,
letting the diffusion contribution govern the CO in both samples. The failure
of the intended directional selectivity in the conductivity measured with high
microwave power is interpreted in terms of large-angle electron-phonon
scattering.Comment: 8 pages, 5 figure
Possible robust insulator-superconductor transition on solid inert gas and other substrates
We present observations of the insulator-superconductor transition in
ultrathin films of Bi on amorphous quartz, quartz coated with Ge, and for the
first time, solid xenon condensed on quartz. The relative permeability
ranges from 1.5 for Xe to 15 for Ge. Though we find screening
effects as expected, the I-S transition is robust, and unmodified by the
substrate. The resistance separatrix is found to be close to h/4e^2 and the
crossover thickness close to for all substrates. I-V studies and
Aslamazov-Larkin analyses indicate superconductivity is inhomogeneous. The
transition can be understood in terms of a percolation model.Comment: accepted in Physical Review
Synthesis, antitubercular activity and mechanism of resistance of highly effective thiacetazone analogues
Defining the pharmacological target(s) of currently used drugs and developing new analogues with greater potency are both important aspects of the search for agents that are effective against drug-sensitive and drug-resistant Mycobacterium tuberculosis. Thiacetazone (TAC) is an anti-tubercular drug that was formerly used in conjunction with isoniazid, but removed from the antitubercular chemotherapeutic arsenal due to toxic side effects. However, several recent studies have linked the mechanisms of action of TAC to mycolic acid metabolism and TAC-derived analogues have shown increased potency against M. tuberculosis. To obtain new insights into the molecular mechanisms of TAC resistance, we isolated and analyzed 10 mutants of M. tuberculosis that were highly resistant to TAC. One strain was found to be mutated in the methyltransferase MmaA4 at Gly101, consistent with its lack of oxygenated mycolic acids. All remaining strains harbored missense mutations in either HadA (at Cys61) or HadC (at Val85, Lys157 or Thr123), which are components of the bhydroxyacyl-ACP dehydratase complex that participates in the mycolic acid elongation step. Separately, a library of 31 new TAC analogues was synthesized and evaluated against M. tuberculosis. Two of these compounds, 15 and 16, exhibited minimal inhibitory concentrations 10-fold lower than the parental molecule, and inhibited mycolic acid biosynthesis in a dose-dependent manner. Moreover, overexpression of HadAB HadBC or HadABC in M. tuberculosis led to high level resistance to these compounds, demonstrating that their mode of action is similar to that of TAC. In summary, this study uncovered new mutations associated with TAC resistance and also demonstrated that simple structural optimization of the TAC scaffold was possible and may lead to a new generation of TAC-derived drug candidates for the potential treatment of tuberculosis as mycolic acid inhibitors
Fluoromycobacteriophages for rapid, specific, and sensitive antibiotic susceptibility testing of Mycobacterium tuberculosis
Rapid antibiotic susceptibility testing of Mycobacterium tuberculosis is of paramount importance as multiple- and extensively- drug resistant strains of M. tuberculosis emerge and spread. We describe here a virus-based assay in which fluoromycobacteriophages are used to deliver a GFP or ZsYellow fluorescent marker gene to M. tuberculosis, which can then be monitored by fluorescent detection approaches including fluorescent microscopy and flow cytometry. Pre-clinical evaluations show that addition of either Rifampicin or Streptomycin at the time of phage addition obliterates fluorescence in susceptible cells but not in isogenic resistant bacteria enabling drug sensitivity determination in less than 24 hours. Detection requires no substrate addition, fewer than 100 cells can be identified, and resistant bacteria can be detected within mixed populations. Fluorescence withstands fixation by paraformaldehyde providing enhanced biosafety for testing MDR-TB and XDR-TB infections. © 2009 Piuri et al
Exploring the Mode of Action of Bioactive Compounds by Microfluidic Transcriptional Profiling in Mycobacteria
10.1371/journal.pone.0069191PLoS ONE87-POLN
Localization and Superconductivity in Doped Semiconductors
Motivated by the discovery of superconductivity in boron-doped (B-doped)
diamond, we investigate the localization and superconductivity in heavily doped
semiconductors. The competition between Anderson localization and s-wave
superconductivity is investigated from the microscopic point of view. The
effect of microscopic inhomogeneity and the thermal fluctuation in
superconductivity are taken into account using the self-consistent 1-loop-order
theory with respect to superconducting fluctuation. The crossover from
superconductivity in the host band to that in the impurity band is described on
the basis of the disordered three-dimensional attractive Hubbard model for
binary alloys. We show that superconductor-insulator transition (SIT)
accompanies the crossover. We point out an enhancement of Cooper pairing in the
crossover regime. Further localization of the electron wave function gives rise
to incoherent Cooper pairs and the pseudogap above T_c. A global phase diagram
is drawn for host band superconductivity, impurity band superconductivity,
Anderson localization, Fermi liquid state, and pseudogap state. A theoretical
interpretation is proposed for superconductivity in the doped diamond, SiC, and
Si.Comment: Final version for publication. To appear in J. Phys. Soc. Jpn. (2009)
No.
An Anaerobic-Type α-Ketoglutarate Ferredoxin Oxidoreductase Completes the Oxidative Tricarboxylic Acid Cycle of Mycobacterium tuberculosis
Aerobic organisms have a tricarboxylic acid (TCA) cycle that is functionally distinct from those found in anaerobic organisms. Previous reports indicate that the aerobic pathogen Mycobacterium tuberculosis lacks detectable α-ketoglutarate (KG) dehydrogenase activity and drives a variant TCA cycle in which succinyl-CoA is replaced by succinic semialdehyde. Here, we show that M. tuberculosis expresses a CoA-dependent KG dehydrogenase activity, albeit one that is typically found in anaerobic bacteria. Unlike most enzymes of this family, the M. tuberculosis KG: ferredoxin oxidoreductase (KOR) is extremely stable under aerobic conditions. This activity is absent in a mutant strain deleted for genes encoding a previously uncharacterized oxidoreductase, and this strain is impaired for aerobic growth in the absence of sufficient amounts of CO2. Interestingly, inhibition of the glyoxylate shunt or exclusion of exogenous fatty acids alleviates this growth defect, indicating the presence of an alternate pathway that operates in the absence of β-oxidation. Simultaneous disruption of KOR and the first enzyme of the succinic semialdehyde pathway (KG decarboxylase; KGD) results in strict dependence upon the glyoxylate shunt for growth, demonstrating that KG decarboxylase is also functional in M. tuberculosis intermediary metabolism. These observations demonstrate that unlike most organisms M. tuberculosis utilizes two distinct TCA pathways from KG, one that functions concurrently with β-oxidation (KOR-dependent), and one that functions in the absence of β-oxidation (KGD-dependent). As these pathways are regulated by metabolic cues, we predict that their differential utilization provides an advantage for growth in different environments within the host
- …