Examining latent change classes: An application of factor mixture modeling to change scores

Although change scores are used in a variety of statistical methods (e.g., analysis of variance and regression), there is a lack of application of latent variable modeling methods to change scores. This thesis provides a detailed description of two latent variable modeling methods applied to change scores: factor analysis of change scores and change score factor mixture modeling. To illustrate advantages of these methods, both were applied to change score data from undergraduates. Students responded to sense of identity items during a university-wide assessment day on two occasions, once as incoming freshmen and again as second-semester sophomores. Change scores were computed by subtracting sophomore item responses from freshmen item responses. Factor analysis results indicated sense of identity change scores were best represented by two factors, change in sense of self and purpose and development of morals and beliefs. Factor mixture modeling results suggested two latent classes underlying these factors. The classes differed in both factor means and factor variances, which implied two possible change patterns associated with development of sense of identity. One class contained students who were stable on the two change score factors (i.e. changed minimally on sense of self and purpose and morals and beliefs) and the other class contained students who were fluid on one of the two factors. Classes were somewhat replicated with a second, independent sample, in that two classes were detected, but class means and variances diverged from those in the first sample. Results across the two methods provided insightful information about change processes of sense of identity, particularly how development of sense of identity is not the same across students. The applied example highlights the advantages of applying these methods to change scores. Implications of the two methods are further discussed throughout the thesis

Does the Spiritual Values/Religion Subscale of the Self-Description Questionnaire III Function Differentially Across Heterosexual and Non-Heterosexual Young Adults? A Measurement Invariance Study

We evaluated the dimensionality and measurement invariance of the Spiritual Values/Religion (SVR) subscale from the Self-Description Questionnaire III across heterosexual and non-heterosexual young adults. We found a one-factor model provided adequate fit to the data for each group, with the SVR items exhibiting configural, metric, and scalar invariance across the two groups. Given that we established measurement invariance, we examined the latent mean difference on the construct and found the heterosexual group reported significantly higher levels of spiritual value/ religion than the non-heterosexual group. Our results provided empirical support for the theorized factor structure of the SVR items and the use of the SVR subscale across heterosexual and non-heterosexual young adults, making the scale a viable option for researchers studying religiosity in these specific subpopulations

Affect Variability and Physical Health: The Moderating Role of Mean Affect

Research has only begun to explore how affect variability relates to physical health and has typically not assessed long-term associations nor considered the moderating role of mean affect. Therefore, we used data from the Midlife in the United States Study waves 2 (N = 1512) and 3 (N = 1499) to test how affect variability predicted concurrent and long-term physical health while also testing the moderating role of mean affect. Results indicated that greater negative affect variability was associated concurrently with a greater number of chronic conditions (p = .03) and longitudinally with worse self-rated physical health (p \u3c .01). Greater positive affect variability was associated concurrently with more chronic conditions (p \u3c .01) and medications (p \u3c .01) and longitudinally with worse self-rated physical health (p = .04). Further, mean negative affect played a moderating role such that at lower levels of mean negative affect, as affect variability increased, so did the number of concurrent chronic conditions (p \u3c .01) and medications (p = .03) and the likelihood of reporting worse long-term self-rated physical health (p \u3c .01). Thus, the role of mean affect should be considered when testing short- and long-term associations between affect variability and physical health

Extreme Sensitivity of Superconductivity to Stoichiometry in FeSe (Fe1+dSe)

The recently discovered iron arsenide superconductors, which display superconducting transition temperatures as high as 55 K, appear to share a number of general features with high-Tc cuprates, including proximity to a magnetically ordered state and robustness of the superconductivity in the presence of disorder. Here we show that superconductivity in Fe1+dSe, the parent compound of the superconducting arsenide family, is destroyed by very small changes in stoichiometry. Further, we show that non-superconducting Fe1+dSe is not magnetically ordered down to low temperatures. These results suggest that robust superconductivity and immediate instability against an ordered magnetic state should not be considered as intrinsic characteristics of iron-based superconducting systems, and that Fe1+dSe may present a unique opportunity for determining which materials characteristics are critical to the existence of superconductivity in high Tc iron arsenide superconductors and which are not.Comment: Updated to reflect final version and include journal referenc

Magnetotransport near a quantum critical point in a simple metal

We use geometric considerations to study transport properties, such as the conductivity and Hall coefficient, near the onset of a nesting-driven spin density wave in a simple metal. In particular, motivated by recent experiments on vanadium-doped chromium, we study the variation of transport coefficients with the onset of magnetism within a mean-field treatment of a model that contains nearly nested electron and hole Fermi surfaces. We show that most transport coefficients display a leading dependence that is linear in the energy gap. The coefficient of the linear term, though, can be small. In particular, we find that the Hall conductivity $\sigma_{xy}$ is essentially unchanged, due to electron-hole compensation, as the system goes through the quantum critical point. This conclusion extends a similar observation we made earlier for the case of completely flat Fermi surfaces to the immediate vicinity of the quantum critical point where nesting is present but not perfect.Comment: 11 pages revtex, 4 figure

Nernst Effect and Superconducting Fluctuations in Zn-doped YBa2_{2}Cu3_{3}O7−δ_{7-\delta}

We report the measurements of in-plane resistivity, Hall effect, and Nernst effect in Zn doped YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ epitaxial thin films grown by pulsed laser deposition technique. The pseudogap temperature, $T^*$, determined from the temperature dependence of resistivity, does not change significantly with Zn doping. Meanwhile the onset temperature ($T^{\nu}$) of anomalous Nernst signal above $T_{c0}$, which is interpreted as evidence for vortex-like excitations, decreases sharply as the superconducting transition temperature $T_{c0}$ does. A significant decrease in the maximum of vortex Nernst signal in mixed state is also observed, which is consistent with the scenario that Zn impurities cause a decrease in the superfluid density and therefore suppress the superconductivity. The phase diagram of $T^*$, $T^{\nu}$, and $T_{c0}$ versus Zn content is presented and discussed.Comment: 6 pages, 5 figures, Latex; v2: to be published in PR

In-Plane Magnetic Anisotropy In RF Sputtered Fe-N Thin Films

We have fabricated Fe(N) thin films with varied N2 partial pressure and studied the microstructure, morphology, magnetic properties and resistivity by using X-ray diffraction, atomic force microscopy, transmission electron microscopy, vibrating-sample magnetometer and angle-resolved M-H hysteresis Loop tracer and standard four-point probe method. In the presence of low N2 partial pressure, Fe(N) films showed a basic bcc a-Fe structure with a preferred (110) texture. A variation of in-plane magnetic anisotropy of the Fe(N) films was observed with the changing of N component. The evolution of in-plane anisotropy in the films was attributed to the directional order mechanism. Nitrogen atoms play an important role in refining the a-Fe grains and inducing uniaxial anisotropy.Comment: 11 pages, 6 figure

Episodic Antarctic Shelf Intrusions of Circumpolar Deep Water via Canyons

The structure of the Antarctic Slope Current at the continental shelf is crucial in governing the poleward transport of warm water. Canyons on the continental slope may provide a pathway for warm water to cross the slope current and intrude onto the continental shelf underneath ice shelves, which can increase rates of ice shelf melting, leading to reduced buttressing of ice shelves, accelerating glacial flow and hence increased sea level rise. Observations and modelling studies of the Antarctic Slope Current and cross-shelf warm water intrusions are limited, particularly in the East Antarctica region. To explore this topic, an idealised configuration of the Antarctic Slope Current is developed, using an eddy-resolving isopycnal model that emulates the dynamics and topography of the East Antarctic sector. Warm water intrusions via canyons are found to occur in discrete episodes, with large onshore flow induced by eddies. The episodic nature of cross-shelf warm water transport is demonstrated, with canyon width playing a key role in modulating cross-shelf exchanges; warm water transport through narrower canyons is more irregular than transport through wider canyons. The episodic cross-shelf transport is driven by a cycle of rising and falling rates of eddy generation in the Antarctic Slope Current, a variability intrinsic to the slope current that can be explained without any temporal variability in external forcings. Improved understanding of the intrinsic variability of warm water intrusions can help guide future observational and modelling studies in the analysis of eddy impacts on Antarctic shelf circulation

Superconductivity in CuxBi2Se3 and its implications for pairing in the undoped topological insulator

Bi2Se3 is one of a handful of known topological insulators. Here we show that copper intercalation in the van der Waals gaps between the Bi2Se3 layers, yielding an electron concentration of ~ 2 x 10^20cm-3, results in superconductivity at 3.8 K in CuxBi2Se3 for x between 0.12 and 0.15. This demonstrates that Cooper pairing is possible in Bi2Se3 at accessible temperatures, with implications for study of the physics of topological insulators and potential devices.Comment: 6 pages, 4 figure