Browsing by Subject "Nuclear magnetic resonance spectroscopy"
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Item Construction of a low temperature nuclear magnetic resonance force microscope(2006) Lee, Yong J.; Markert, John T.All aspects of a nuclear magnetic resonance force microscope (NMRFM) have been constructed. The development and testing of each aspect area are discussed in detail. In particular, two sets of three-axis piezoelectric positioners based on a stick-slip motion have been successfully incorporated and tested for a wide temperature range. How each component of the microscope contributes to the signal measurement process is discussed, and possible solutions to unsuccessful attempts in detecting a spin-induced NMRFM signal are presented. Also, a novel, two-step phase cycling technique to eliminate the contribution to the spin signal from a spurious frequency modulated (FM) radiofrequency (RF) field, which can be utilized in future experiments, is discussed in detail.Item NMR structure and preliminary crystallographic studies of small protein B (SmpB) from Aquifex aeolicus(2002) Dong, Gang; Hoffman, David W., Ph. D.The tmRNA-SmpB system is a highly conserved quality control system in all prokaryotes. It has the dual function of rescuing ribosomes stalled on defective mRNA templates and tagging proteins resulting from problematic messages for degradation. Small protein B (SmpB) is an essential component of this system, lacking significant homology with any known proteins other than the same proteins from different species. The structure of the SmpB from Aquifex aeolicus was determined by multidimensional NMR techniques. It consists of an antiparallel b-barrel, with three helices packed outside the core of the barrel. While the overall structure of SmpB appears to be unique, the protein does contain an embedded oligonucleotide binding (OB) fold; in this respect SmpB has similarity to several other RNA-binding proteins that are known to be associated with translation, including initiation factor 1 (IF1), ribosomal protein S17 and the N-terminal domain of aspartyltRNA synthetase (DRS). Conserved amino acids on the protein surface that are likely candidates for direct interactions with the tmRNA and other components of the translational apparatus were identified. The presence of the two widely separated clusters of conserved surface amino acids suggests that SmpB could function either by stabilizing two distal regions of the tmRNA, or by facilitating an interaction between the tmRNA and another component of the translational apparatus. While the C-terminal ~20 amino acids appear to be unstructured, their presence may be essential for the function of SmpB in the trans-translation process. The structural model reported in this dissertation will be essential in ultimately determining the detailed mechanism by which the tmRNASmpB system performs its functions. Results of preliminary NMR perturbation studies on the complexes between SmpB and several RNA molecules are described. In addition, single crystals of the core fragment of the SmpB were obtained by the vapor diffusion method in sitting-drops at room temperature. Preliminary crystallographic analysis reveals that the crystal belongs to a tetragonal lattice, with unit cell parameters a = b = 55.0 Å, c = 65.9 Å, a = b = g = 90°. Further structural determinations by molecular rep.Item Nuclear magnetic resonance force microscopy of ammonium dihydrogen phosphate and magnetism of cobalt nanocrystals(2005) Mirsaidov, Utkur; Markert, John T.A Nuclear Magnetic Resonance Force Microscopy (NMR-FM) technique utilizing a somewhat uncommon experimental geometry has been developed. Characterization of external field effects on soft permalloy micromagnets on double torsional oscillators was performed. We showed that at high enough fields (above 1 Tesla), the quality factor for each mode is comparable to the zero field value. The changes in resonance frequency fit well with our model, and permitted high-sensitivity magnetic moment and magnetic anisotropy measurement. Effects of laser power on cantilevers used for NMR-FM has been studied in detail. The origins of the observed self-sustained oscillations has been addressed by our model. NMR-FM detection has been shown in an ammonium dihydrogen phosphate sample. Imaging and spin manipulation techniques were used for the first time to detect the nuclear spins in a sample with short relaxation times. A magnetic study of epitaxially grown cobalt nanocrystals on a Si(111) substrate has been performed. Enhancement of the magnetic moment and anisotropy energy have been observed and data are consistent with single domain model. Experimental evidence indicates small inter-nanocrystal interactions. Finally, future directions in achieving the single-spin detection limit is addressed.Item Nuclear magnetic resonance force microscopy: adiabaticity, external field effects, and demonstration of magnet-on-oscillator detection with sub-micron resolution(2003) Miller, Casey William; Markert, John T.Investigations of the adiabatic condition governing nuclear magnetic resonance force microscopy (NMR-FM) have been performed. It has been determined that the adiabatic conditional factor for protons in ammonium sulfate must have a value of 1.5 or greater to optimize the NMR-FM signal. A theoretical formalism is presented that describes the data. The characteristics of CoPt-capped single-crystal-silicon micro-oscillators with a magnetic field applied perpendicular to the magnetic film have been determined. The resonance frequency of the oscillators show two distinct regimes, one of softening and one of stiffening of the oscillator. A model is developed to describe the previously unseen softening. This work suggests that using NMR-FM with a magnetic particle on the oscillator is experimentally feasible. Magnet-on-oscillator NMR-FM has been demonstrated for the first time with our NMR-FM microscope using resonance slice thicknesses as small as ∼150 nm. The sample investigated was a semi-infinite slab of ammonium sulfate. The resonance slice of the microscope was scanned from vacuum into the sample by changing the tuned carrier frequency of the AC magnetic field H1. The resulting signal-to-noise ratio of ∼ 4 is slightly better than what was expected from conservative calculations. Finally, feasibility calculations and an experimental plan are set forth for the future measurement of relaxation times of single crystals of the superconductor magnesium diboride.Item Strongly correlated systems: magnetic measurements of magnesium diboride and group IV magnetic semiconductor alloys(2007-12) Guchhait, Samaresh, 1976-; Markert, John T.Nuclear Magnetic Resonance Force Microscopy (NMRFM) is a unique quantum microscopy technique, which combines the three-dimensional imaging capabilities of magnetic resonance imaging (MRI) with the high sensitivity and resolution of atomic force microscopy (AFM). It has potential applications in many different fields. This novel scanning probe instrument holds potential for atomic-scale resolution. MgB2 is a classic example of two-band superconductor. However, the behavior of these two bands below the superconducting transition temperature is not well understood yet. Also, the anisotropic relaxation times of single crystal MgB2 have not been measured because it is not yet possible to grow large enough MgB2 single crystals for conventional NMR. Using our homemade NMRFM probe, we have set out to measure the relaxation times of micron size MgB2 single crystals to anix swer several questions relating to the anisotropy, multiband behavior, and coherence effects in this unusual superconductor. The goal of a second project is to study the effects of doping on the critical current of MgB2 superconducting wires. Ti-sheathed MgB2 wires doped with nanosize crystalline-SiC up to a concentration of 15 wt% SiC have been fabricated, and the effects of the SiC doping on the critical current density (Jc) and other superconducting properties studied. In contrast with the previously reported results, our measurements show that SiC doping decreases Jc over almost the whole field range from 0 to 7.3 tesla at all temperatures. Furthermore, it is found that the degradation of Jc becomes stronger at higher SiC doping levels. Our results indicate that these negative effects on Jc could be attributed to the absence of significant effective pinning centers (mainly Mg2Si) due to the high chemical stability of the crystalline-SiC particles. The principle goal of a third project, the study of magnetic semiconductors, is to investigate magnetic properties of Mn-implanted GeC thin films. 20 keV energy Mn ions were implanted in two samples: 1) bulk Ge (100) and 2) a 250 nm thick epitaxial GeC film, grown on a Si (100) wafer by UHV chemical vapor deposition using a mixture of germane (GeH4) and methylgermane (CH3GeH3) gases. A SQUID magnetometer study shows granular ferromagnetism in both samples. While the Curie temperature for both samples is about 180 K, the in-plane saturated magnetic moment per unit area for the first sample is about 2.2×10−5 emu/cm2 and that for the second sample is about 3.0 × 10−5 emu/cm2 . The external field necessary to saturate the magnetic moment is also larger for the second sample. These results show clear enhancement of magnetic properties of the Mn-implanted GeC thin film over the identically implanted Ge layer due to the presence of a small amount of non-magnetic element carbon.