Thesis (Ed.M.)--Boston University

Thesis (Ed.M.)--Boston University

Thesis (Ed.M.)--Boston University. Missing page 49.

Thesis (Ed.M.)--Boston University

Thesis (Ed.M.)--Boston University

Thesis (Ed.M.)--Boston University

Bismuth is among the most studied of all elements, but its behaviour under pressure exhibits myriad unexpected puzzles even after many decades of research. Bismuth narrowly avoids being an insulator: a Peierls-type distortion almost completely gaps the electronic energy bands, producing a rhombohedral metal with a tiny overlap of conduction and valence bands. The resulting solitary free electron per 100,000 atoms can travel large distances in high-purity crystals, leading to a host of unusual properties. We show that the rhombohedral structure can be tuned with pressure, driving the carrier concentration to nearly zero. We compare our measurements to recent experimental advances implying the formation of novel electronic order driven by the pairing of low-density electrons and holes, and show evidence for a previously unseen phase at very low temperatures in the semiconducting state. We also present a method for calculating the carrier density and resistivity as a function of pressure, based on phenomenological band parameters and a simple charge-balance argument, and demonstrate that this approach can quite well describe most - but not all - of the observed behaviour of the resistivity. At higher pressures, bismuth undergoes a transition into a quasiperiodic host-guest structure. Here, two distinct crystal lattices coexist and interpenetrate, but the lattice parameters are incommensurate. This crystal thus lacks a single unit cell - an unexpected complexity for a simple element. The discovery of such unusual structures in elements is a new phenomenon and their physical properties are rather unexplored. We present experimental measurements of the resistivity and magnetic susceptibility in the incommensurate host-guest state. We argue that the experimental data (in particular, the shape of the normal-state electrical resistivity, and the high value of the low-temperature upper critical field) may be evidence for strong electron-phonon coupling. This strong coupling is consistent with theoretical predictions which suggest the presence of a low-energy phonon mode arising due to the vanishing energy cost of moving guest atoms through the host lattice.

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A wide gap streamer chamber has been designed and constructed. Its performance in various modes has been investigated. It is shown that an angular accuracy of 1.5 m rad can be achieved for a 20 centimetre track. The chamber has been applied to the study of multiple scattering of 100 Mev muons in one inch of lead. A large departure from theory has been observed, and the analysis has indicated a second narrow component of the distribution of the particles plotted against the projected angles of scattering. The chamber has been modified to study low energy particles.

A new model for two-body high energy scattering is presented as part of an investigation into the phenomenological consequences of the non-planar structure of Reggeon-particle scattering. The model is a modification of the weak cut reggeized Absorption model for Pion-Nucleon scattering and is developed in form of a correlation modified quasi eikonal where the Reggeon and an arbitrary number of Pomerons are allowed to change the projection of the nucleon spin. A correlation parameter - the "Gribov c" - which has its origin in Gribov's theory, provides an indication about the failure of the traditional weak cut reggeized absorption model and restores its most profound shortcoming - the prediction of an incorrect phase behaviour of the helicity isovector nonflip amplitude in the reaction while retaining the model's attractive simplicity. The vertices of the Reggeon-calculus depend in general on the angle between the momenta of the exchanged reggepoles. By parameterizing this dependence we take into account the effective contribution of inelastic intermediate states in the unitarity expansion of the Regge-particle scattering amplitude. We obtain a reasonable phase energy description of the isovector amplitude. We demonstrate in detail the mechanism by which the correct phase behaviour is restored. The spin-structure of the amplitudes is investigat ed and observables of N scattering between 6 and 200 GeV/c within a range of momentum transfer of are being produced.