Universiẗat, Diss., 2002--Mainz.

As one of the major mid-rapidity tracking devices of the STAR detector at the Relativistic Heavy-Ion Collider (RHIC), the Time Projection Chamber (TPC) plays an important role in measuring trajectory and energy of high energy charged particles in polarized proton-proton collision experiments. TPC's in-jet tracking efficiency represents the largest systematic uncertainty on jet energy scale at high transverse momentum, whose measurement contributes to the understanding of the spin structure of protons. The objective of this analysis is to get a better estimation of this systematic uncertainty, through methods of pure Monte-Carlo simulation and real- data embedding, in which simulated tracks are embedded into real-data events. Be- sides, simulated tracks are also embedded into Monte-Carlo events, to make a strict comparison for the uncertainty estimation. The result indicates that the unexplained part of the systematic uncertainty is reduced to 3.3%, from a previous quoted value of 5%. This analysis also suggests that future analysis, such as embedding jets into zero-bias real data and analysis with much higher event statistics, will benefit the understanding of the systematic uncertainty of the in-jet TPC tracking efficiency.

Universiẗat, Diss., 2003--Regensburg.

In this work we studied magnetic properties of molecular magnets of the new heteropolyanion {Cu20}, dodecanuclear cluster {Ni12}, and the heterometallic {Cr7M} wheels, in which one of the CrIII ions of Cr8 has been replaced by a Fe, Cu, Zn, Ni, ion with this extra-spin acts as local probe for the spin dynamics.Such systems have been synthesized recently and they are well described using the Heisenberg spin Hamiltonian with a Zeeman term of an applied magnetic field along the z-axis. Using the numerical exact diagonalization method, we have calculated the energy spectrum and the eigenstates for different compounds,and we have used them for reexamining the available experimental susceptibility data to determine the values of exchange parameters.We have studied the thermodynamic properties such magnetization, susceptibility, heat-capacity. At low temperature regions molecular magnets act as individual quantum nanomagnets and can display super-paramagnetic phenomena like macroscopic quantum tunneling, ground state degeneracy, level-crossing. A crucial issue for understanding these phenomena is the coupling between magnetic molecular levels and the environment such as nuclear spins. We have modeled the behavior of the proton spin lattice relaxation rate as a function of applied magnetic field for low temperatures as it is measured in Nuclear Magnetic Resonance (NMR) experiments.

Conducting and semiconducting polymers, consisting of delocalized π-electrons, have been studied for the past three decades. These materials have shown novel physical properties with interesting applications in batteries, detectors, light emitting diodes, field effect transistors, solar cells, biosensors etc. Nevertheless the charge transport properties are yet to be understood in detail due to the complexity of the system, especially due to the interplay of quasi-one dimensionality (q-1D), disorder, localization and electron-electron interactions(EEI). A combined investigation of both conductivity and spin lattice relaxation time, especially at very low temperatures and high magnetic fields, is really lacking in conducting polymers. In this thesis a set of experiments – dc conductivity, magnetoresistance (MR), Nuclear Magnetic Resonance (NMR) spin lattice relaxation time (T1) measurements, magnetic susceptibility amd ac conductivity have been carried out in conducting polymers. NMR being a local probe it is possible to get the nanoscopic scale charge transport mechanism. Further, this helps to develop a consistent understanding among a wide range of the physical properties in conducting polymers. In this thesis author has reported the results of experiments at ultra low temperature (mk) and ultra high magnetic field which give more insight about the roles of electron-electron interaction(EEI) and disorderin charge transport properties. This thesis describes a detailed study of charge transport and NMR relaxation in three representative conducting polymers namely polypyrrole(PPy)., poly-3-methylthiophene(P3MT) and poly3-hexylthiophene(P3HT). The emphasis is to understand the charge transport phenomena and NMR relaxation, especially at ultra low temperatures (down to 20 mk) and high magnetic field (up to 23.4 T). The NMR T1 relaxation mechanisms are discussed in terms of (i) Korringa relaxation, (ii) relaxation due to spin diffusion to paramagnetic centers (SDPC) amd (iii) reorientation of symmetric groups, depending upon the temperature range.