Low-frequency resistance fluctuations in metal films under current stressing at low temperature (T<0.3Tmelting)

In this paper, we report a systematic study of low frequency 1∕fα resistance fluctuation in thin metal films (Ag on Si) at different stages of damage process when the film is subjected to high current stressing. The resistance fluctuation (noise) measurement was carried out in situ using a small ac bias that has been mixed with the dc stressing current. The experiment has been carried out as a function of temperature in the range of 150–350 K. The experiment establishes that the current stressed film, as it undergoes damage due to various migration forces, develops an additional low-frequency noise spectral power that does not have the usual 1∕f spectral shape. The magnitude of extra term has an activated temperature dependence (activation energy of ≈0.1 eV) and has a 1∕f1.5 spectral dependence. The activation energy is the same as seen from the temperature dependence of the lifetime of the film. The extra 1∕f1.5 spectral power changes the spectral shape of the noise power as the damage process progress. The extra term likely arising from diffusion starts in the early stage of the migration process during current stressing and is noticeable much before any change can be detected in simultaneous resistance measurements. The experiment carried out over a large temperature range establish a strong correlation between the evolution of the migration process in a current stressed film and the low-frequency noise component that is not a 1∕f noise.

Molecular Basis for the Role of Staphylococcus aureus Penicillin Binding Protein 4 in Antimicrobial Resistance

Penicillin binding proteins (PBPs) are membrane-associated proteins that catalyze the final step of murein biosynthesis. These proteins function as either transpeptidases or carboxypeptidases and in a few cases demonstrate transglycosylase activity. Both transpeptidase and carboxypeptidase activities of PBPs occur at the D-Ala-D-Ala terminus of a murein precursor containing a disaccharide pentapeptide comprising N-acetyl-glucosamine and N-acetyl-muramic acid-L-Ala-D-Glu-L-Lys-D-Ala-D-Ala. beta-Lactam antibiotics inhibit these enzymes by competing with the pentapeptide precursor for binding to the active site of the enzyme. Here we describe the crystal structure, biochemical characteristics, and expression profile of PBP4, a low-molecular-mass PBP from Staphylococcus aureus strain COL. The crystal structures of PBP4-antibiotic complexes reported here were determined by molecular replacement, using the atomic coordinates deposited by the New York Structural Genomics Consortium. While the pbp4 gene is not essential for the viability of S. aureus, the knockout phenotype of this gene is characterized by a marked reduction in cross-linked muropeptide and increased vancomycin resistance. Unlike other PBPs, we note that expression of PBP4 was not substantially altered under different experimental conditions, nor did it change across representative hospital- or community-associated strains of S. aureus that were examined. In vitro data on purified recombinant S. aureus PBP4 suggest that it is a beta-lactamase and is not trapped as an acyl intermediate with beta-lactam antibiotics. Put together, the expression analysis and biochemical features of PBP4 provide a framework for understanding the function of this protein in S. aureus and its role in antimicrobial resistance.

Voltage delay during constant-current or constant-resistance discharge of Mg-MnO2 dry cells: A comparative study

A theoretical approach has been developed to relate the voltage delay transients of the Mg-MnO2 dry cell observed during discharge by two commonly employed modes, viz., (1) at constant current, and (2) across a constant resistance. The approach has been verified by comparison of experimentally obtained transients with those generated from theory. The method may be used to predict the delay parameters of the Mg-MnO2 dry cell under the two modes of discharge and can, in principle, be extended to lithium batteries.

Tracking Random Walk of Individual Domain Walls in Cylindrical Nanomagnets with Resistance Noise

The stochasticity of domain-wall (DW) motion in magnetic nanowires has been probed by measuring slow fluctuations, or noise, in electrical resistance at small magnetic fields. By controlled injection of DWs into isolated cylindrical nanowires of nickel, we have been able to track the motion of the DWs between the electrical leads by discrete steps in the resistance. Closer inspection of the time dependence of noise reveals a diffusive random walk of the DWs with a universal kinetic exponent. Our experiments outline a method with which electrical resistance is able to detect the kinetic state of the DWs inside the nanowires, which can be useful in DW-based memory designs.

Isobaric electrical resistance along the critical line in nickel: An experimental test of universality

High-precision measurement of the electrical resistance of nickel along its critical line, a first attempt of this kind, as a function of pressure to 47.5 kbar is reported. Our analysis yields the values of the critical exponents α=α’=-0.115±0.005 and the amplitude ratios ‖A/A’‖=1.17±0.07 and ‖D/D’‖=1.2±0.1. These values are in close agreement with those predicted by renormalization-group (RG) theory. Moreover, this investigation provides an unambiguous experimental verification to one of the key consequences of RG theory that the critical exponents and amplitudes ratios are insensitive to pressure variation in nickel, a Heisenberg ferromagnet.

Expression of drug resistance markers by spheroplasts ofMycobacterium smegmatis after fusion

Mycobacterial spheroplasts were prepared by treatment of the glycinesensitized cells with a combination of lipase and lysozyme. They were stable for several hours at room temperature but were lysed on treatment with 0.1% sodium dodecyl sulfate. The spheroplasts could be regenerated on a suitable medium. Fusion and regeneration of the spheroplasts were attempted using drug resistant mutant strains ofM. smegmalis. Recombinants were obtained from spheroplast fusion mediated by polyethylene glycol and dimethyl sulfoxide. Simultaneous expression of rccombinant properties was observed only after an initial lag in the isolated clones. This has been explained as due to “chromosome inactivation” in the fused product.

Resistance fluctuations, phase randomness and noise in one-dimensional conductors with a long-range correlated potential

When the size (L) of a one-dimensional metallic conductor is less than the correlation length λ-1 of the Gaussian random potential, one expects transport properties to show ballistic behaviour. Using an invariant imbedding method, we study the exact distribution of the resistance, of the phase θ of the reflection amplitude of an incident electron of wave number k0, and of dθ/dk0, for λL ll 1. The resistance is non-self-averaging and the n-th resistance moment varies periodically as (1 - cos 2k0L)n. The charge fluctuation noise, determined by the distribution of dθ/dk0, is constant at low frequencies.

The Multifunctional PE_PGRS11 Protein from Mycobacterium tuberculosis Plays a Role in Regulating Resistance to Oxidative Stress

Mycobacterium tuberculosis utilizes unique strategies to survive amid the hostile environment of infected host cells. Infection-specific expression of a unique mycobacterial cell surface antigen that could modulate key signaling cascades can act as a key survival strategy in curtailing host effector responses like oxidative stress. We demonstrate here that hypothetical PE_PGRS11 ORF encodes a functional phosphoglycerate mutase. The transcriptional analysis revealed that PE_PGRS11 is a hypoxia-responsive gene, and enforced expression of PE_PGRS11 by recombinant adenovirus or Mycobacterium smegmatis imparted resistance to alveolar epithelial cells against oxidative stress. PE_PGRS11-induced resistance to oxidative stress necessitated the modulation of genetic signatures like induced expression of Bcl2 or COX-2. This modulation of specific antiapoptotic molecular signatures involved recognition of PE_PGRS11 by TLR2 and subsequent activation of the PI3K-ERK1/ 2-NF-kappa B signaling axis. Furthermore, PE_PGRS11 markedly diminished H2O2-induced p38 MAPK activation. Interestingly, PE_PGRS11 protein was exposed at the mycobacterial cell surface and was involved in survival of mycobacteria under oxidative stress. Furthermore, PE_PGRS11 displayed differential B cell responses during tuberculosis infection. Taken together, our investigation identified PE_PGRS11 as an in vivo expressed immunodominant antigen that plays a crucial role in modulating cellular life span restrictions imposed during oxidative stress by triggering TLR2-dependent expression of COX-2 and Bcl2. These observations clearly provide a mechanistic basis for the rescue of pathogenic Mycobacterium-infected lung epithelial cells from oxidative stress.

Microprocessor-based temperature indicator using platinum resistance as the sensor

A novel microprocessor-based platinum resistance temperature indicator has been developed and described. This indicator provides a linear performance over a wide dynamic temperature range of - 183.0°C to +200°C with an accuracy of better than ±0.05°C. To sasily carry out the linearization of platinum thermometer through software, the modified Callendar-Van Dusen equation is used. Test results are given to support the theory.

Negative differential resistance in doped poly(3-methylthiophene) devices

The current density-voltage (J-V) characteristics of poly(3-methylthiophene) devices show a negative differential resistance (NDR) at room temperature with a large peak to valley current ratio (similar to 507). This NDR can be tuned by two orders of magnitude by controlling the carrier density due to the variation of the space-charge region in the device. The temperature and scan rate dependent J-V measurements infer that the NDR is mainly driven by the trapping and de-trapping of carriers. The photo-generation of carriers is observed to reduce the NDR effect.

Negative differential resistance in Gd0.5Sr0.5MnO3: A consequence of Joule heating

Negative differential resistance (NDR) in current-voltage (I-V) characteristics and apparent colossal electroresistance were observed in Gd0.5Sr0.5MnO3 single crystals at low temperatures. The continuous dc I-V measurements showed a marked thermal drift. In addition, temperature of the sample surface was found to be significantly higher than that of the base at high applied currents. Two different strategies namely estimation and diminution of the Joule heating (pulsed I-V measurements) were employed to investigate its role in the electric transport properties. Our experiments reveal that the NDR in Gd0.5Sr0.5MnO3 is a consequence of Joule heating rather than the melting of charge order. (C) 2010 American Institute of Physics. doi:10.1063/1.3486221]