som D tyagi | Drexel University (original) (raw)

Papers by som D tyagi

Proceedings of the National Academy of Sciences, 1977

An important feature of Perutz's trigger mechanism for cooperativity in the reversible oxygen... more An important feature of Perutz's trigger mechanism for cooperativity in the reversible oxygenation of hemoglobin (Hb) is the tension along the histidine--metal linkage in deoxyHb and deoxycobaltohemoglobin (deoxy CoHb), supposedly due to the pull exerted by the globin on the metal atom. We have attempted to verify the existence of this pull by studying the emission Mössbauer spectra of deoxy 57CoHb and oxy 57 CoHb at different temperatures. The emission Mössbauer spectrum for none of the cobalt Hbs agrees with the absorption spectrum of the corresponding iron analog and, moreover, the spectrum of deoxy 57CoHb is characteristic of the intermediate-spin iron. These observations indicate that the daughter 57Fe atom is "frozen" almost in the same spatial situation as that of the parent 57Co. The protein is apparently holding the cobalt atom in position rather rigidly and, after the electron-capture decay of the 57Co atom, the protein does not permit the daughter 57Fe to mo...

Physical Review B

Y 1Ϫx Pr x Ba 2 Cu 3 O 7Ϫ␦ with xϭ0.30 and 0.45 has been studied by emission Mössbauer spectrosco... more Y 1Ϫx Pr x Ba 2 Cu 3 O 7Ϫ␦ with xϭ0.30 and 0.45 has been studied by emission Mössbauer spectroscopy using 57 Co dopant. Cobalt substitutes for copper predominantly at the Cu͑1͒ site, and one can extract information on the local electronic and vibrational state of the Cu͑1͒-O͑4͒ chain from the Mössbauer spectra. The major coordination environments observed for the 57 Co dopant in YBa 2 Cu 3 O 7Ϫ␦ are fourfold square planar ͑species A͒, fivefold trigonal bipyramidal ͑species B͒, and fivefold square pyramidal ͑species C͒. We have found that the reversible interconversion between species B and C, which was found earlier in YBa 2 Cu 3 O 7Ϫ␦ and Y 0.9 Pr 0.1 Ba 2 Cu 3 O 7Ϫ␦ , is still observed in Y 0.7 Pr 0.3 Ba 2 Cu 3 O 7Ϫ␦ with lower intensity, and it further diminishes in Y 0.55 Pr 0.45 Ba 2 Cu 3 O 7Ϫ␦. This tendency shows that the out of the chain vibration of the O͑4͒ oxygens is gradually damped as one approaches the critical Pr content where superconductivity is destroyed. This supports a phonon-assisted mechanism for high-T c superconductivity in 1-2-3 compounds with significant contribution from the chains. We attribute the damping of the chain oxygen vibrations to the increase of the Cu͑1͒-Cu͑1͒ distance along the chain with increasing Pr content. We have also concluded from Mössbauer isomer shifts that up to the critical Pr content where superconductivity disappears, the electronic density at the Cu͑1͒ site seems to remain constant. This indicates that, in this region, any hybridization which takes place between Pr and sheet oxygens has no substantial influence on the chains. ͓S0163-1829͑99͒00417-8͔

Journal of Applied Physics, 1990

Measurements of the magnetic-field-induced microwave absorption, {ital P}({ital H}), in micron-si... more Measurements of the magnetic-field-induced microwave absorption, {ital P}({ital H}), in micron-sized powders of the high-temperature superconductor YâBaâCuâOâ at several microwave frequencies for 1.3 Kâ¤{ital T}â¤{ital T}{sub {ital c}} are reported. The hysteresis loops ({ital P}({ital H}) vs {ital H}) observed at low temperatures ({ital T}â¤16 K) indicate the presence of spontaneous circulating currents within the sample. These hysteresis loops and

AIP Conference Proceedings, 1992

The point-contact (Pt-Ir probe) tunneling measurements on high-Tc superconducting Bi2Sr2CaCu2O8 s... more The point-contact (Pt-Ir probe) tunneling measurements on high-Tc superconducting Bi2Sr2CaCu2O8 single crystals (Tc=82° K) have been performed from 4.2 K to 77 K. A high conductance region around the origin, which is attributed to the junction of the pure metallic-bridge type of point-contact tunneling, has been observed. there are also several current steps beyond the high conductance region in the I-V curves, corresponding to conductance troughs, which are either zero or negative, in the dI/dV characteristics of the tunneling spectra. These are pronounced structures indicative of bound states in tunneling spectroscopy. A superconducting energy gap of Δ=26±2 meV has been identified from tunneling spectra taken at T=4.2 K and as a function of temperature. Current drops with some regularity in voltage intervals between drops, occur at high applied voltage (≳80 mV). This resonance tunneling is the result of the effects of quantization due to the layered-structure of the crystal. A model based on the superlattice-like nature of the crystal has been developed to interpret the tunneling observation of the bound states and the energy gap. When a normal-metal probe is brought into contact with the superconducting crystal, the region of the first CaCuO slab under that contact will be most likely reduced to a low Tc region with a smaller energy gap Δ1, or even completely reduced to the normal state, due to the work function difference between the normal metal and the superconductor and/or the proximity effect. However, the second CaCuO slab remains in the intrinsic superconducting state with energy gap Δ2≳Δ1. Thus, when the energy of a tunneling quasiparticle is in the range Δ1≳E≳0, Andreev reflection takes place, giving rise to the high conductance near the origin (up to V=Δ1/e). If the energy of the quasiparticle is greater than Δ1 but less than Δ2 (or even slightly greater than Δ2), quasiparticle reflections take place at the boundary of the first and second CaCuO slabs, resulting in the virtual bound states observed in the tunneling spectra. The drops in current at higher voltages result from a combination of resonant scattering and pair breaking.

Proceedings of Spie the International Society For Optical Engineering, Feb 1, 2010

ABSTRACT Surface enhanced Raman scattering (SERS) is now a well-established technique to greatly ... more ABSTRACT Surface enhanced Raman scattering (SERS) is now a well-established technique to greatly amplify the normally weak Raman scattering signals. The amplification is achieved by using SERS substrates - specially structured metallic substrates with nano-scale morphological features. One of the most widely used methods for SERS amplification employs nanoparticles of silver or gold either in colloidal suspension or in dry-drop form. In such substrates SERS amplification factors (AF) exceeding 1012 have been reported. The reproducibility of the colloid-based substrates, however, is a problem. The lack of reproducibility can be caused by a variety of factors that can change the interparticle distances. In this paper we show that thermal annealing of SERS substrates fabricated using commercially available nano-particle inks can be used to create thermally stable substrates with high reproducibility. It appears that thermal annealing destroys the unstable hot-spots with very high AF's but still leaves the sample with high AF sites yielding spatially averaged substrate AF's exceeding 108.

ABSTRACT Item from the Biomedical Technology Showcase held July 20, 2006 at Drexel University&amp... more ABSTRACT Item from the Biomedical Technology Showcase held July 20, 2006 at Drexel University's Bossone Research Enterprise Center. Inflammatory Bowel Disease affects nearly 1.5 million people. Currently, there are no efficient and reliable methods to quantify the degree of inflammation in these patients. The objective here was to image and quantify in an experimental model of colitis, MPO, IL1_ and TNF_ (proinflammatory cytokines) using Quantum Dots (QDs) conjugated with specific antibodies. The resulting fluorescence intensity was then used as a measure of concentration of MPO and in turn inflammation. The fluorescent images obtained from animals showed sequential increase in fluorescence intensity of MPO correlating (R = 0.96) with clinical disease. Fluorescent images also showed co-localization of all the three markers in both acute as well as chronic inflammation. These observations suggest that QD bioconjugates can be used nanotools to image biomarkers of inflammation.

Aspects of religion with science/religion have been covered in the pages of Physics Today and Phy... more Aspects of religion with science/religion have been covered in the pages of Physics Today and Physics News. They reflect wide student interest in these topics. For a decade, two physicists and a campus minister have taught a writing-intensive course ``Issues in Science and Religion'' Physics/Sociology 137. Here we outline our course (open to all students), to encourage others contemplating similar

M edically approved disinfectants and hand washes are used routinely, many of which are not suffi... more M edically approved disinfectants and hand washes are used routinely, many of which are not sufficient to disinfect surfaces, and fail to inactivate a substantial percent of the pathogens found within biofilms. Strong biocides such as benzalkonium chloride, chlorhexidinegluconate, and triclosan were unable to completely inactivate pathogens in their planktonic and embedded biofilm (sessile) forms, and thus the reservoirs of hospital-acquired infections are inefficiently controlled. There is a real need for a disinfectant solution that is less toxic, carries less fear of systemic bacterial resistance, simple, and safe for use in all patient populations including children/neonates. It was demonstrated that water have antimicrobial properties after it is treated with room temperature non-thermal plasma. Liquid medical waste originating in hospitals and pharmaceutical companies is one of the main sources of antibiotics release into the environment worldwide. Release of antibiotics into ...

1992. Digests of Intermag. International Magnetics Conference, 1992

Le Journal de Physique Colloques, 1988

The superconducting transition onset temperature in YBaCuO monitored using the microwave absorpti... more The superconducting transition onset temperature in YBaCuO monitored using the microwave absorption technique shows little change as a function of lattice distortion due to oxygen depletion indicating the presence of the orthorhombic superconducting phase even in highly oxygen-depleted samples.

ABSTRACT Low-field magnetization, ac resistivity, and the magnetic field dependence of microwave ... more ABSTRACT Low-field magnetization, ac resistivity, and the magnetic field dependence of microwave absorption at low fields ( ~ 200 Oe) in thin film and bulk samples of La_0.67Ba_0.33MnO3 (LBMO) is reported. The films were prepared using pulsed laser deposition. A sapphire cavity at ~ 10 GHz was employed for the microwave measurements. As before, we measure DeltaPc = P_c(H) - P_c(0), the change in the power reflected at the cavity frequency, P_c, as a function of the applied magnetic field, H for a fixed input power P_n. The essential results of our study can be summarized as follows: DeltaPc is a function of B and not of H; (2) in the thin film samples, DeltaPc is not simply related to either the magnetization M or the field B. DeltaPc increases almost linearly with H while B and M approach saturation. (3) In bulk LBMO, DeltaPc is mainly a magnetoimpedance effect tied to the ferromagnetic antiresonance, the contribution from the GMR being barely detectible. (4) DeltaP_c/P_c(0) values of 15% for bulk and 4% for thin film samples make LBMO an attractive candidate for device applications.

Physical Review B, 1991

ABSTRACT Using the modulated-microwave-absorption technique, we have studied the flux quantizatio... more ABSTRACT Using the modulated-microwave-absorption technique, we have studied the flux quantization in single-crystal samples of YBa2Cu3O7-δ as a function of an externally applied dc magnetic field (dc scan) at constant amplitude of the microwave magnetic field, and as a function of the microwave-field amplitude (microwave scan) at nominally zero dc field. Periodic oscillations in the modulated microwave absorption due to flux quantization are observed in both scans. In the dc scan an increasing number of junctions are activated as the temperature is increased at a fixed microwave power level or as the power level is increased at a fixed temperature. Surprisingly, only a single junction is activated in the microwave scan in the temperature range 0.1<t<=0.92, where t=T/Tc is the reduced temperature. It is shown that different junctions are activated in the two scans. From the temperature dependence of the microwave scan spectra, the superconducting penetration depth λL is shown to obey the empirical two-fluid temperature dependence λL(T)/λL(0)=(1-t4)-1/2 in the temperature range 10–85 K, which is consistent with the s-wave-pairing mechanism. In the same temperature range the temperature dependence of the junction critical current can be described by Jc(T)/Jc(0)=(1-t)1.9±0.2.

International Journal of Modern Physics B, 1988

Microwave absorption measurements of the high T c ceramic superconductors reveal reproducible fea... more Microwave absorption measurements of the high T c ceramic superconductors reveal reproducible features in weak magnetic fields. These features are qualitatively explained by a quantum network model of these superconducting oxides. The ceramic superconductors are modeled as a set of one-dimensional wires weakly coupled at random nodes. The magnetoconductance, magnetization and susceptibility are computed from the network S-matrix.