Методом точек помутнения изучены фазовые переходы систем ПС – метилацетат, ПВА – этанол, ПДМС – бутанон и ПММА – бутанол в магнитном поле и в его отсутствие. Построены фазовые диаграммы систем ПС (Mh = 2,0´105) – метилацетат, ПВА (Mη = 5,8´104) – этанол, ПДМС (Mη = 1,0x103) – бутанон и ПММА (Mh = 5,5´104) – бутанол. Установлено, что магнитное поле не влияет на температуру фазовых переходов растворов гибкоцепных аморфных полимеров. / Phase transitions of the PS – methyl acetate, PVAc – ethanol, PDMS – butanone, PMMA – butanol systems have been studied using the cloud-point method. Phase diagrams of these systems are constructed. It is shown that the magnetic field does not influence on the phase transition temperature of flexible chain amorphous polymer solutions.

Phase transitions of the gelatin – water and agaroza - water systems have been studied using the cloud-point method. Phase diagrams of these systems are constructed at different values of pH of medium. It is shown that pH of medium influences on the melting temperature of gelatin and agaroza gels. / Методом точек помутнения изучены фазовые переходы в системах желатин – вода и агароза – вода. Построены фазовые диаграммы систем при разных pH среды в магнитном поле и вне поля. Установлено, что pH среды влияет на температуру плавления гелей желатина и агарозы.

Изучены фазовые переходы и структура систем: гидроксипропилцеллюлоза-вода и гидроксипропилцеллюлоза -вода-этанол. Показано, что с увеличением содержания этанола в смешанном растворители вода-этанол нижняя критическая температура растворения системы гидроксипропилцеллюлоза -вода-этанол повышается. При содержании эталона свыше 50% нижняя критическая температура растворения не обнаружена в диапазоне температур от 298 до 423 К. Определены размеры макромолекул и их ассоциатов для растворов гидроксипропилцеллюлозы в смешанном растворителе. / Phase transitions and the structure of the hydroxypropylcellulose-water and hydroxypropylcellulose-water-ethanol systems were studied. It has been shown that as the ethanol content in the water-ethanol mixed solvents increases, the lower critical solution temperature of the hydroxypropylcellulose-water-ethanol system increases. With the ethanol content of more than 50%, the lower critical solution temperature was not found in the temperature range from 298 to 423 K. The dimensions of macromolecules and their associations for hydroxypropylcellulose solutions in a mixed solvent were determined.

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<p> Block copolymers are a class of soft materials which can self-assemble into a variety of ordered structures. One method to induce new structures is the application of an external field such as an electric field. Previously, studies of the field-induced phase transitions are based on the assumption that the structural change follows certain symmetry pattern or simply using real-space numerical methods. The goal of the current project is to develop a simple analytic method to predict the structural change. Our approach is based on a linear response theory, in which the external field is taken as a perturbation and the lowest-order contribution to the solution is computed. We applied our method to the Landau-Brazovskii theory which is valid close to the order-disorder transition point of diblock copolymers. The result shows that there will be an additional term to the order parameter as a response to the external field. The structural change can be predicted by a new Fourier expansion of the order parameter. As an example, we examined the structural change of a body-centered cubic phase under an applied electric field.</p> / Thesis / Master of Science (MSc)

In this thesis we introduce a new quantity which we call the dynamic fidelity susceptibility (DFS). We show that it is relevant to out-of-equilibrium dynamics in many-particle quantum systems, taking the problem of an impurity in a Bosonic Josephson junction, and the transverse field Ising model, as examples. Both of these systems feature quantum phase transitions in their ground states and understanding the dynamics near such critical points is currently an active area of research. In particular, sweeping a system through a quantum critical point at finite speed leads to non-adiabatic dynamics. A simple theoretical tool for describing such a scenario is the celebrated Kibble-Zurek theory which predicts that the number of excitations is related to the speed of sweep via the phase transition’s critical exponents at equilibrium. Another theoretical tool, useful in describing the static properties of quantum phase transitions, is the fidelity susceptibility. Our DFS generalizes the concept of fidelity susceptibility to nonequilibrium dynamics, reproducing its results in the static limit, whilst also displaying universal scaling properties, akin to those found in Kibble-Zurek theory, in the non-adiabatic regime. Furthermore, we show that the DFS is the same quantity as the time-dependent quantum Fisher information which provides a measure of multi-partite entanglement, as well as being closely related to out-of-time-order correlators (OTOCs). / Thesis / Master of Science (MSc)

Sol-gel titania particles were investigated, primarily by optical techniques, by systematically varying synthesis, sample handling, and annealing variables. The material phases investigated were amorphous titania, anatase TiO2, and rutile TiO2. Annealing-induced phase transformations from amorphous TiO2 to anatase to rutile were studied by Raman scattering, infrared reflectivity, infrared absorption, x-ray diffraction, and electron energy-loss spectroscopy (EELS). Detailed experiments were carried out on the effects of annealing on the Raman and infrared spectra of anatase nanocrystals. The frequencies of the zone-center transverse optical (TO) and longitudinal-optical (LO) phonons of anatase were determined and were used in analyzing the results obtained on composites consisting of annealed solgel particles. The TO and LO frequencies of anatase were obtained from polarization-dependent far-infrared reflectivity measurements on single crystals. These results, which determined the dielectric functions of anatase, were used to explain infrared (IR) reflectivity spectra of titania nanoparticles pressed into pellets, as well as the grazing-incidence IR reflectivity observed for titania thin films. Because of the polycrystalline character of the titania nanoparticles, the surface roughness of the pressed pellets, and the island-structure character of the thin films, effective-medium theories (appropriate for composites) were used, along with the anatase dielectric functions, to interpret the experimental results. The titania nanoparticles were prepared by the hydrolysis/condensation of Ti(OC2H5)4. A polymeric steric stabilizer was used in the sol-gel synthesis in order to prevent continued agglomeration during the condensation process. This yielded particles with a relatively narrow size distribution. The amount of water used in the reaction determines the final particle size. Particles as small as 80 nm and as large as 300 nm were used throughout this work. From the colloidal suspension, loose powders, pressed pellets, and thin films were formed. These samples were subjected to different annealing processes at temperatures ranging from room temperature up to 1000 C. Two different annealing atmospheres were used: air (oxygen-containing) and argon (no oxygen). The amorphous to anatase transformation was followed by in-situ IR transmission measurements carried out during annealing. The particles as prepared are amorphous and the anatase phase could be detected, using this sensitive IR technique, at temperatures as low as 150 C. This phase transition was shown to be particle size dependent. It was also shown that introducing the stabilizer by means of the alkoxide flask instead of the water flask (during the sol-gel synthesis) decreases the anatase to rutile transformation temperature. Loose powders were found to transform more readily than dense pellets, while island-structure films were found to be the hardest to transform. Even at 1000 C, most of these films did not transform to rutile. X-ray diffraction experiments were used to determine nanocrystal sizes in anatase samples obtained by air and argon anneals at temperatures from 300 to 800 C. A correlation was found between Raman band shape (peak position and linewidth) and crystallite size, but this correlation was different for air anneals and for argon anneals. These experiments called for an interpretation based on a stoichiometric effect rather than a finite size effect. Based on this interpretation, the as-prepared particles are slightly oxygen-deficient, with a stoichiometry corresponding to TiO1.98. In the electron energy-loss experiments, a special data-analysis technique was used to extract the EELS spectrum of the titania nanoparticles from the observed substrate-plus-particles signal. This technique successfully resolved the titania absorption-edge peak. Which was found to be momentum independent. For low electron momentum, the results were consistent with the reported optical absorption edge. / Ph. D.

Thesis advisor: Xiao Chen / The development of highly controllable quantum coherent simulators such as superconducting qubits and Rydberg atom arrays has stimulated the study of non-equilibrium quantum dynamics, opening the door to exciting topics including dynamical phase transitions, thermalization, transport, and quantum error correction. This thesis addresses various questions from non-equilbrium quantum dynamics, with a concentration on measurement-induced phase transitions (MIPT), adaptive dynamics with feedback mechanism, and Hilbert space fragmentation. In the first part, we study the hybrid quantum automaton (QA) circuits with different symmetries subject to local composite measurements. For $\mathbb{Z}_2$-symmetric hybrid QA circuits, there exists an entanglement phase transition from a volume-law phase to a critical phase by varying the measurement rate. The special feature of QA circuits enables us to interpret the entanglement dynamics in terms of a stochastic particle model. With the help of this stochastic model, we further investigate the entanglement fluctuations and quantum error correcting property of the volume-law phase in QA circuits with no symmetry, and study the entanglement dynamics in QA circuits with U(1) symmetry. Despite being a hallmark of non-unitary quantum dynamics, MIPT is absent in the density matrix averaged over measurement outcomes. In the second part, we introduce an adaptive quantum circuit subject to measurements with feedback. The feedback is applied according to the measurement outcome and steers the system towards a unique state above certain measurement threshold. We show that there exists an absorbing phase transition in both quantum trajectories and quantum channels. In the end, we turn to the phenomenon of Hilbert space fragmentation (HSF), whereby dynamical constraints fragment Hilbert space into many disconnected sectors, providing a simple mechanism by which thermalization can be arrested. However, little is known about how thermalization occurs in situations where the constraints are not exact. To study this, we consider a situation in which a fragmented 1d chain with pair-flip constraints is coupled to a thermal bath at its boundary. For product states quenched under Hamiltonian dynamics, we numerically observe an exponentially long thermalization time, manifested in both entanglement dynamics and the relaxation of local observables. To understand this, we study an analogous model of random unitary circuit dynamics, where we rigorously prove that the thermalization time scales exponentially with system size. Slow thermalization in this model is shown to be a consequence of strong bottlenecks in configuration space, demonstrating a new way of producing anomalously slow thermalization dynamics. / Thesis (PhD) — Boston College, 2024. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Condensed matter systems undergoing phase transitions rarely allow exact solutions. The presence of disorder renders the situation  even worse but collective Monte Carlo methods and parallel algorithms allow numerical descriptions. This thesis considers classical phase transitions in disordered spin systems in general and in effective models of superfluids with disorder and novel interactions in particular. Quantum phase transitions are considered via a quantum to classical mapping. Central questions are if the presence of defects changes universal properties and what qualitative implications follow for experiments. Common to the cases considered is that the disorder maps out correlated structures. All results are obtained using large-scale Monte Carlo simulations of effective models capturing the relevant degrees of freedom at the transition. Considering a model system for superflow aided by a defect network, we find that the onset properties are significantly altered compared to the $\lambda$-transition in $^{4}$He. This has qualitative implications on expected experimental signatures in a defect supersolid scenario. For the Bose glass to superfluid quantum phase transition in 2D we determine the quantum correlation time by an anisotropic finite size scaling approach. Without a priori assumptions on critical parameters, we find the critical exponent $z=1.8 \pm 0.05$ contradicting the long standing result $z=d$. Using a 3D effective model for multi-band type-1.5 superconductors we find that these systems possibly feature a strong first order vortex-driven phase transition. Despite its short-range nature details of the interaction are shown to play an important role. Phase transitions in disordered spin models exposed to correlated defect structures obtained via rapid quenches of critical loop and spin models are investigated. On long length scales the correlations are shown to decay algebraically. The decay exponents are expressed through known critical exponents of the disorder generating models. For cases where the disorder correlations imply the existence of a new long-range-disorder fixed point we determine the critical exponents of the disordered systems via finite size scaling methods of Monte Carlo data and find good agreement with theoretical expectations. / <p>QC 20150306</p>