Although the pairing mechanism in MgB2 is thought to be phonon mediated, there are still many experimental results that lack appropriate explanation. For example, there is no consensus about the magnitude of the energy gap, its temperature dependence, and whether it has only one-gap or not. Many techniques have been used to investigate this, like Raman spectroscopy, farinfrared transmission, specific heat, high-resolution photoemission and tunneling. Most tunneling data on MgB2 are obtained from mechanical junctions. Measurements of energy gap by these junctions have many disadvantages like the instability to temperature and field changes. On the other hand, sandwich-like planar junctions offer a stable and reliable measurement for temperature dependence of the energy gap, where any variation in the tunneling spectra can be interpreted as a direct result from the sample under study. To the best of our knowledge, we report the first energy gap temperature- and magnetic field-dependence of MgB2/Pb planar junctions. Study of the temperature-dependence shows that the small gap value (reported by many groups and explained as a result of surface degradation) is a real bulk property of MgB2. Moreover, our data is in favor of the two-gap model rather than the onegap, multi-gap, or single anisotropic gap models. The study of magnetic field effect on the junctions gave an estimation of the upper critical field of about 5.6 T. The dependence of energy gap on the field has been studied as well. Our junctions show stability against temperature changes, but "collapsed" when the magnetic field (applied normal to the junction barrier) is higher than 3.2 T. The irreversible structural change switched the tunnling mechanism from quisiparticle tunneling into Josephson tunneling. Josephson I-V curves at different temperatures have been studied and the characteristic voltages are calculated. The estimated MgB2 energy gap from supercurrent tunneling in weak link junctions agrees very well with that from quasiparticle tunneling. Reported properties on polycrystalline, single crystal and thin film MgB2 samples are widely varied, depending on the details of preparation procedure. MgB2 single crystals are synthesized mainly by heat treatment at high temperature and pressure. Single crystals prepared by this way have the disadvantages of Mg deficiency and shape irregularity. On the other hand, improving the coupling of grain boundaries in polycrystalline MgB2 (has the lowest normal state resistivity in comparison to many other practical superconductors) will be of practical interest. Consequently, we have been motivated to look for a new heat treatment to prepare high quality polycrystalline and single crystal MgB2 in the same process. The importance of our new method is its simplicity in preparing single crystals (neither high pressure cells nor very high sintering temperatures are required to prepare single crystals) and the quality of the obtained single crystal and polycrystalline MgB2. This method gives high quality and dense polycrystalline MgB2 with very low normal state resistivity (σ(40 ) = 0.28 cm). Single crystals have an average diagonal of 50 m and 10 m thickness with a unique shape that resembles the hexagonal crystal structure. Furthermore, preparing both forms in same process gives a great opportunity to study inconsistencies in their properties. On the other hand, magnesium diboride thin films have also been prepared by magnetron sputtering under new preparation conditions. The prepared thin films have a transition temperature of about 35.2 K and they are promising in fabricating tunnel junctions.

The conduction band of InAs lies lower in energy than the GaSb valence band. In order to preserve continuity of the Fermi level across the interface, charge transfer takes place resulting in a confined quasi two dimensional electron gas (2DEG) in the InAs and a confined quasi two dimensional hole gas (2DHG) in the GaSb. This study is an investigation into the vertical transport in an n-InAs/p-GaSb single heterojunction (SHET). Application of a forward bias (InAs negative with respect to GaSb) increases the 2DEG and 2DHG concentrations and, therefore, their confinement energies. Eventually a critical bias is reached where the electron confinement energy moves above the hole confinement energy (the theoretical voltage induced semimetal/semiconductor transition V_c). Any subsequent increase in voltage is expected to result in a current decrease, and a region of negative differential resistance (NDR) should occur. The SHET can be grown with two distinct interface types, 'InSb-like' and 'GaAs-like'. This in turn affects the vertical transport characteristics of each type. Experimental IV traces at various pressures are compared with the corresponding results from sophisticated self-consistent band profile calculations taking into account band mixing effects for the first time through a k.p theory framework. Experimental IV traces of the SHETs under a magnetic field parallel to the interface are also compared with results from calculations that take into account the coupling of the growth and in-plane electron and heavy hole motions. Both sets of analysis support earlier conclusions that NDR occurs after V_c for both interfaces, and that each interface supports a different conduction mechanism. Evidence of multiple phonon processes occurring in both sample types is observed for the first time and is proposed to reconcile the above experimental observations with theory. This data is found to offer explanations for a number of other observations. Field perpendicular to the interface leads to the observation of features beyond the NDR region in both sample types. In samples with an 'InSb-like' interface, applying additional hydrostatic pressure leads to very strong features beyond the main NDR. Through a complex self-consistent decoupled model taking into account electrons and heavy holes, all these features are proposed to be due to a filling of an integer number of Landau levels. The band profile is predicted to alter dramatically at this point. The same model explains the observation of weaker features at 1 bar at high fields (~ 40T). A variation of NDR position is found with a rotation of an-plane field.

Plyšinių jungčių kanalai yra sudaryti iš koneksinų baltymų, kurie yra randami įvairiuose audiniuose ir dėka jų ląstelės gali keistis metabolitais bei elektriniais signalais. Plyšinių jungčių kanalai pasižymi dideliu jautrumu viduląsteliniam pH. Šiame darbe tyrėme įvairių inhibitorių poveikį homotipinių plyšinių jungčių, sudarytų iš Cx26, Cx30.2, Cx36, Cx40, Cx43, Cx45, Cx46, Cx47 ir Cx50 laidumui bei šio poveikio priklausomybę nuo viduląstelinio pH.. Plyšinių jungčių blokatoriais buvo pasirinktos kelias skirtingas farmakologines grupes atstovaujančios medžiagos: ilgagrandžiai alkoholiai (heksanolis, oktanolis, nonanolis), izofluranas, meflokvinas, flufenaminė rūgštis, karbenoksolonas ir arachidono rūgštis. Pademonstravome, kad pHi pakėlimas panaudojant NH4Cl iki ~8 didino plyšinių jungčių laidumą tarp ląstelių, turinčių Cx30.2 ar Cx45, tačiau neturėjo jokios įtakos ląstelėms su Cx26, Cx40, Cx46, Cx47 ir Cx50 ir mažino laidumą tarp ląstelių su Cx36 ar Cx43. pHi padidinimas žymiai susilpnino ilgagrandžių alkoholių, izoflurano ir meflokvino poveikį Cx45 plyšinių jungčių laidumui ir neturėjo įtakos flufenaminės rūgšties, karbenoksolono ir arachidono rūgšties poveikiui. Taip pat parodėme, kad nuo pHi priklauso oktanolio poveikio Cx45 plyšinių jungčių laidumui galingumas, t.y., pusinė blokavimo koncentracija IC50 buvo 0.1, 0.25 ir 2.68 mM atitinkamai prie pHi 6.9, 7.2 ir 8.1. Manoma, kad pH poveikis plyšinių jungčiu laidumui pasireiškia protonams jungiantis su koneksino baltymo... [toliau žr. visą tekstą] / Gap junction channels formed from connexin proteins provide a direct pathway for electrical and metabolic cell-cell communication exhibiting high sensitivity to intracellular pH. We examined pHi-dependent modulation of junctional conductance of GJs formed of Cx26, Cx30.2, Cx36, Cx40, Cx43, Cx45, Cx46, Cx47 and Cx50 by reagents representing several distinct groups of uncouplers, such as long carbon chain alkanols, arachidonic acid, carbenoxolone, isoflurane, flufenamic acid and mefloquine. We demonstrate that alkalization by NH4Cl to pH≈8 increased coupling in cells expressing Cx30.2 and Cx45, yet did not affect coupling of Cx26, Cx40, Cx46, Cx47 and Cx50 and decreased it in Cx43 and Cx36 gap junctions. Unexpectedly, cells expressing Cx45, but not other connexins, exhibited full coupling recovery after alkalization with NH4Cl under the continuous presence of alkanols, isoflurane and mefloquine. There was no coupling recovery by alkalization in the presence of arachidonic acid, carbenoxolone and flufenamic acid. In cells expressing Cx45, IC50 for octanol was 0.1, 0.25 and 2.68 mM at pHis of 6.9, 7.2 and 8.1, respectively. Histidine modification of Cx45 protein by N-bromosuccinimide proportionally reduced the coupling-promoting effect of NH4Cl as well as uncoupling effect of octanol. This suggests that long chain alkanols and some other uncouplers may act through the formation of hydrogen bonds with the as-of-yet unidentified histidine/s of the Cx45 gap junction channel protein... [to full text]

Thesis (Ph. D.)--University of Hawaii at Manoa, 1995. / Includes bibliographical references (leaves 76-93). / Microfiche. / vii, 93 leaves, bound ill. (some col.) 29 cm

Thesis (Ph.D.) -- McMaster University, 1998. / Includes bibliographical references (leaves 195-230). Also available via World Wide Web.

Diss. (sammanfattning) Linköping : Univ., 2001. / Härtill 4 uppsatser.

Thesis (M.Sc.)--University of Wollongong, 2005. / Typescript. Includes bibliographical references: leaf 146-148.

Thesis (Ph.D. in Physiology and Biophysics) -- University of Colorado Denver, 2008. / Typescript. Includes bibliographical references (leaves 89-98). Free to UCD Anschutz Medical Campus. Online version available via ProQuest Digital Dissertations;

Thesis (Ph. D.)--University of Hong Kong, 2008. / Includes bibliographical references (leaf 138-155) Also available in print.

Thesis (Ph. D.)--University of Hong Kong, 2008. / Includes bibliographical references (leaf 138-155) Also available online.