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NMR investigation of the quasi-one-dimensional superconducter class R2Cr3As3 (R = K, Rb or Cs)Zhi, Haizhao January 2016 (has links)
Since the high $T_c$ superconductivity was discovered in iron pnictides in 2008, the interplay between the reduced dimensionality, magnetism and unconventional superconductivity has been attracting renewed interest. Recently, Bao et al. and Tang et al. discovered a series of quasi-one-dimensional (quasi 1D) superconductors: \K($T_c=6.1 K$), \Rb($T_c=4.8 K$), and \Cs($T_c=2.2 K$). In this thesis, we will discuss microscopic investigation of \Cs based on nuclear magnetic resonance techniques. The first chapter is a brief introduction to this series of superconductors. The second chapter is a summary of NMR techniques and theory. In the third part, I summarize $^{133}$Cs NMR and $^{75}$As Nuclear Quadrupole Resonance (NQR) measurements on a powder sample of \Cs ($T_c < 1.6$~K). From the $^{133}$Cs NMR Knight shift $^{133}K$ measured at the Cs1 site, we show that the uniform spin susceptibility $\chi_{spin}$ increases from 295~K to $\sim$ 60~K, followed by a mild suppression; $\chi_{spin}$ then levels off below $\sim$10~K. Low frequency Cr spin dynamics, reflected on $^{75}$As $1/T_1T$ (the nuclear spin-lattice relaxation rate $1/T_1$ divided by temperature $T$), shows an analogous trend as $\chi_{spin}$. Comparison with the results of $1/T_1T$ near $T_c$ with \K($T_c=6.1$~K) and \Rb($T_c=4.8$~K) establishes a systematic trend that substitution of K$^{+}$ ions with larger alkali ions progressively suppresses Cr spin fluctuations together with $T_c$. / Thesis / Master of Science (MSc)
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Local Fluctuations in the Relaxation Rate in Glassy SystemsPandit, Rajib K. 11 June 2019 (has links)
No description available.
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Characterizing Whole-Brain Age-Associated Trajectories of R1 in Early Adulthood Using Multi-Site MRIHeo, Stella (In Kyung) January 2023 (has links)
Myelin facilitates efficient neural signal transmission along axons. While predominantly present in white matter, myelin is also found in smaller amounts in the outermost layer of gray matter, known as the cortex, as well as deep subcortical structures. Quantitative magnetic resonance imaging (qMRI) metrics, including longitudinal relaxation rate (R1), can report myelin levels in vivo, although these measures are often estimated and can vary depending on pulse sequences and acquisition parameters used to collect the data. Multi-site approaches can increase the statistical power and ecological validity of studies by recruiting participants from larger and more diverse samples, but considerable variability has previously been reported in qMRI estimates of myelin computed using data from different MRI scanners. Here, we assessed intra- and inter-vendor variability in R1 maps acquired at three sites using scanners from two manufacturers, General Electric (GE) and Siemens. Two ‘traveling heads’ scans, where a participant visits and completes a scan at each site, were acquired to directly assess site effects independent of biological variability. Histogram-based site-specific scaling factors determined from the ‘traveling heads’ scans were then applied to harmonize the main dataset. Before site correction, comparable inter- and intra-vendor differences were observed in both the main dataset and the ‘traveling heads’ scans. Our method of harmonization reduced the inter-site variability considerably, while remaining sensitive to age effects in R1. We then characterized age trajectories of R1 across the whole brain using linear regressions. In the age range of 16-36, age was significantly associated with mean R1s of bilateral putamen, left pallidum, and left amygdala, in line with past research. Cortical analyses also replicated earlier findings, with higher age associations in regions of the motor and lateral frontal cortices. The findings contribute to the growing research in multi-site harmonization, while providing normative estimates and trajectories of whole-brain R1s that can be referred to in future clinical studies. / Thesis / Master of Science (MSc) / Myelin is crucial for efficient signal transmission in nerve cells, and advancements in magnetic resonance imaging (MRI) have allowed us to indirectly estimate myelin levels in the brain. Collaborative multi-site investigations have become increasingly popular due to their ability to recruit larger and more diverse samples, making their findings more generalizable to the public. However, differences in scanner hardware and configurations across sites can introduce site-dependent biases to MRI scans. In this thesis, we found considerable inter-site variability in our whole-brain MRI-based estimate of myelin based on longitudinal relaxation rate (R1). We thus developed a method that can remove unwanted site-dependent differences from images by comparing repeated scans across sites. Using the corrected data, we showed how myelin amounts in different brain structures changes with age during early adulthood. Overall, we highlight the importance of carefully considering site-specific differences in multi-site MRI data and characterized how myelin changes with age in young adults.
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Hydrogen and Muonium behaviour in diamondJoseph, Sithole Makgamathe 31 March 2005 (has links)
This work is aimed at the understanding of the dynamical behavior of hydrogen
in diamond. The investigation was carried out using Transverse Field muon Spin
Rotation (TF-SR) and the Longitudinal Field muon Spin Relaxation (LF-SR)
techniques. The chemical analogy between hydrogen (p+e-) and muonium (u+e-)
enabled the study of the indirect dynamical behavior of hydrogen in diamond.
The TF-SR and LF-SR measurements were carried out in an isotopically pure 13C
diamond in the temperature ranges 11 mK - 320 K and 10 K - 400 K, respectively.
In the TF-SR results, the Prompt Absolute Fraction (PAF) of both diamagnetic
and the paramagnetic states are temperature independent. The spin
relaxation rate for the state is non-zero and temperature independent,
while that of the Mux state is non-zero and temperature dependent. The
behavior of indicates that the u+d is immobile, while that of MuT indicates the
mobility in diamond. The LF-SR results provide hop rate and associated nuclear
hyperfine interaction parameters of the Mut state in diamond. / Physics / M.Sc. (Physics)
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In vivo measurement and imaging of ferrimagnetic particle concentrations in biological tissuesPardoe, Heath January 2005 (has links)
[Truncated abstract] Clinical magnetic resonance imaging (MRI) scanners were used to investigate the measurement and imaging of ferrimagnetic particle concentrations in biological tissues in vivo. The presence of ferrimagnetic particles tends to increase the proton transverse relaxation rate (R2) of water protons in tissue. A quantitative image of R2 can be generated using a series of single spin echo magnetic resonance images acquired using clinical MRI scanners and analysing the images using techniques based on that reported by Clark and St. Pierre (2000). If ferrimagnetic particles have a high enough concentration, there is a monotonic relationship between particle concentration and R2; therefore an image of R2 gives a map of the ferrimagnetic particle concentration in the tissue. These techniques were used to investigate the feasibility of in vivo measurement of the concentration and distribution of both synthetic and biogenic ferrimagnetic particles in tissue. Rabbit liver was loaded with ferrimagnetic particles of ?-Fe2O3 (designed for magnetic hyperthermia treatment of liver tumours) by injecting various doses of a suspension of the particles into the hepatic artery in vivo. R2 images of the livers in vivo, excised, and dissected were generated from a series of single spin-echo images. Mean R2 values for samples of ferrimagnetic-particle-loaded liver dissected into approximate 1 cm cubes were found to linearly correlate with tissue iron concentration over the range from approximately 0.1 to at least 2.7 mg Fe/g dry tissue when measured at room temperature. Changing the temperature of ferrimagnetic-particle-loaded samples of liver from 1?C to 37?C had no observable effect on tissue R2 values. However, a small but significant decrease in R2 was found for control samples containing no ferrimagnetic material on raising the temperature from 1?C to 37?C. Both chemically measured iron ii concentrations and mean R2 values for rabbit livers with implanted tumours tended to be higher than those measured for tumour-free liver. This study indicates that tissue R2 measurement and imaging by nuclear magnetic resonance may have a useful role in magnetic hyperthermia therapy protocols for the treatment of liver cancer. In order to investigate the use of clinical MRI scanners to measure biogenic ferrimagnetic particle concentrations in human brain tissue, agar gel based phantoms containing ferrimagnetic particles were made in order to determine the lower concentration detection limit for such particles in a homogenous medium. Magnetite/maghemite nanoparticles were synthesized in the presence of either dextran or polyvinyl alcohol, yielding cluster- and necklace-like aggregates, respectively. Magnetization, Mossbauer spectroscopy, and microscopy measurements indicated that the arrangement of the particles within the aggregates affects the magnetic properties of the particles resulting in smaller particles in the clusters having higher superparamagnetic blocking temperatures than larger particles in the necklaces.
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Hydrogen and Muonium behaviour in diamondJoseph, Sithole Makgamathe 31 March 2005 (has links)
This work is aimed at the understanding of the dynamical behavior of hydrogen
in diamond. The investigation was carried out using Transverse Field muon Spin
Rotation (TF-SR) and the Longitudinal Field muon Spin Relaxation (LF-SR)
techniques. The chemical analogy between hydrogen (p+e-) and muonium (u+e-)
enabled the study of the indirect dynamical behavior of hydrogen in diamond.
The TF-SR and LF-SR measurements were carried out in an isotopically pure 13C
diamond in the temperature ranges 11 mK - 320 K and 10 K - 400 K, respectively.
In the TF-SR results, the Prompt Absolute Fraction (PAF) of both diamagnetic
and the paramagnetic states are temperature independent. The spin
relaxation rate for the state is non-zero and temperature independent,
while that of the Mux state is non-zero and temperature dependent. The
behavior of indicates that the u+d is immobile, while that of MuT indicates the
mobility in diamond. The LF-SR results provide hop rate and associated nuclear
hyperfine interaction parameters of the Mut state in diamond. / Physics / M.Sc. (Physics)
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Collective effects in ultracold neutral plasmasJanuary 2012 (has links)
This thesis describes the measurements of collective effects in strongly coupled ultra-cold neutral plasmas (UNPs). It shows the implementation of experimental techniques that perturb either the density or velocity distribution of the plasma and it describes the subsequent excitation, observation and analysis of the aforementioned collective phenomena. UNPs are interesting in that they display physics of strongly coupled systems. For most plasma systems, collective effects are well described with classical hydrodynamic or kinetic descriptions. However, for strongly coupled systems, the Coulomb interaction energy between nearest neighbors exceeds the kinetic energy, and these descriptions must be modified as the plasma crosses over from a gas-like to liquid-like behavior. Strongly coupling can be found in exotic plasma systems found astrophysics, dusty plasmas, non-neutral trapped ion plasmas, intense-laser/matter interactions and inertial confinement fusion experiments. Compared to other strongly coupled plasmas, UNPs are ideal for studying collective effects in this regime since they have lower timescales, precisely controllable initial conditions and non-invasive diagnostics. Previous studies of UNPs concentrated on plasma expansion dynamics and some collective effects such as disorder induced heating, but little work had been done in relaxation or collision rates and collective modes in UNPs. This thesis presents a method for measuring collision rates by perturbing the velocity distribution of the plasma, observing plasma relaxation and measuring the relaxation rate. It also presents a new technique for observing collective modes in the plasma by perturbing the initial density of the plasma and how this results in the excitation of ion acoustic waves and a measurement of its dispersion relation. Finally, this thesis presents how this last technique can be used to create a gap in the center of the plasma and how this leads to hole propagation and plasma streaming and presents a characterization of both phenomena. The result of these experiments will be valuable for predicting the behavior of collective effects in other strongly coupled plasmas and for comparison with theories that describe them.
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Magnetic properties and proton spin-lattice relaxation in molecular clustersAllalen, Mohammed 06 June 2006 (has links)
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.
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Critical Behavior and Crossover Effects in the Properties of Binary and Ternary Mixtures and Verification of the Dynamic Scaling Conception / Kritisches Verhalten und Crossover Effekte in den Eigenschaften Binärer und Ternärer Gemische sowie Verifizierung des Konzeptes der Dynamischen SkalierungIwanowski, Ireneusz 07 November 2007 (has links)
No description available.
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Estudos das intera??es de quitosana/CTAB/C12E8Santos, Zilvam Melo dos 22 February 2013 (has links)
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Previous issue date: 2013-02-22 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / Surfactant-polymer interactions are widely used when required rheological
properties for specific applications, such as the production of fluids for oil exploration.
Studies of the interactions of chitosan with cationic surfactants has attracted attention by
being able to cause changes in rheological parameters of the systems making room for
new applications. The commercial chitosan represents an interesting alternative to these
systems, since it is obtained from partial deacetylation of chitin: the residues sites
acetylated can then be used for the polymer-surfactant interactions. Alkyl ethoxylated
surfactants can be used in this system, since these non-ionic surfactants can interact
with hydrophobic sites of chitosan, modifying the rheology of solutions or emulsions
resultants, which depends on the relaxation phenomenon occurring in these systems. In
this work, first, inverse emulsions were prepared from chitosan solution as the dispersed
phase and cyclohexane as the continuous phase were, using CTAB as a surfactant. The
rheological analysis of these emulsions showed pronounced pseudoplastic behavior.
This behavior was attributed to interaction of "loops" of chitosan chains. Creep tests
were also performed and gave further support to these discussions. Subsequently, in
order to obtain more information about the interaction of chitosan with non-ionic
surfactants, solutions of chitosan were mixed with C12E8 and and carried out rheological
analysis and dynamic light scattering. The systems showed marked pseudoplastic
behavior, which became less evident when the concentration of surfactant was
increased. Arrhenius and KWW equations were used to obtain parameters of the
apparent activation energy and relaxation rate distribution, respectively, to which were
connected to the content of surfactant and temperature used in this work / As intera??es tensoativo-pol?mero s?o amplamente usadas quando s?o necess?rias
propriedades reol?gicas para aplica??es espec?ficas, como a produ??o de fluidos para
explora??o do petr?leo. Estudos das intera??es de quitosana com tensoativos cati?nicos
tem chamado aten??o por serem capazes de causar mudan?as nos par?metros reol?gicos
dos sistemas abrindo espa?o para novas aplica??es. A quitosana comercial representa
uma alternativa interessante para estes sistemas, uma vez que ela ? obtida a partir da
desacetila??o parcial da quitina: os s?tos acetilados residuais podem, ent?o, ser usados
para as intera??es pol?mero-tensoativo. Tensoativos alquil etoxilados podem ser
utilizados neste sistema, pois estes tensoativos n?o i?nicos podem interagir com s?tios
hidrof?bicos da quitosana, modificando a reologia de solu??es ou emuls?es resultantes,
os quais dependem do fen?meno de relaxa??o ocorrendo nestes sistemas. Neste
trabalho, primeiramente, foram preparadas emuls?es inversas de solu??o de quitosana
como fase dispersa e cicloexano como fase cont?nua usando CTAB como tensoativo. A
an?lise reol?gica destas emuls?es mostrou pronunciado comportamento pseudopl?stico.
Esta pseudoplasticidade foi atribu?da ? intera??o por la?os loops de cadeias de
quitosana. Ensaios de flu?ncia tamb?m foram executados e deram maior suporte a estas
discuss?es. Em seguida, a fim de se obter maiores informa??es sobre as intera??es da
quitosana com tensoativos n?o i?nicos, solu??es de quitosana foram misturadas com
C12E8 e levadas ?s an?lises reol?gica e de espalhamento din?mico de luz. Os sistemas
tiveram elevado comportamento pseudopl?stico, o qual se tornava menos evidente,
quando o teor de tensoativo foi aumentado. Equa??es de Arrhenius e de KWW foram
usadas para obter par?metros de energia de ativa??o aparente e de distribui??o da taxa
de relaxa??o, respectivamente, aos quais foram relacionados em fun??o do teor de
tensoativo e da temperatura, usados neste trabalho
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