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No description available.

Active fluids are an interesting new class of non-equilibrium systems in physics. In such fluids, the system is forced out of equilibrium by the individual active particles - in contrast to driven systems where the system is forced out of equilibrium by some external forces. Some biological examples of active fluids are bacterial suspensions and actomyosin solutions inside eukaryotic cells. In the case of bacterial suspensions, the fluid is stirred internally by the swimming bacteria and as a consequence of this, active fluids can have some interesting physics of their own such as hydrodynamic instabilities and spontaneous symmetry breaking. Here, in particular, we study how such instabilities may arise and how they may lead to a non-equilibrium steady state. We also study numerically a droplet of active matter as a simple representation of cell extract comprising actomyosin solution bounded by a cell membrane. It is widely believed that cell motility is driven only by actin polymerization pushing against the cell membrane. However, we show that even in the absence of actin polymerization, actin-myosin contraction alone can also generate a unidirectional motion. This happens due to the spontaneous breakdown of a discrete symmetry at large enough activity (i.e. actomyosin contraction). This non-equilibrium phase transition from stationary to motile state is somewhat similar to the second order phase transition in equilibrium thermodynamics. Finally, we studied the behaviour of an active droplet on a two-dimensional surface to mimic cell crawling. Whereas cell migration in 3D environment maybe driven mainly by actin-myosin contraction (described above), cell crawling on a 2D surface is driven mainly by actin polymerisation. Here we find that localised actin polymerisation can cause protrusion in the cell membrane which is qualitatively similar to lamellipodium formation in cell crawling.

The perovskite solar cells have attracted much attention recently due to their low fabrication cost and high power conversion efficiency. However, the instability of such devices remained a serious challenge, which is yet to be resolved despite many attempts. In this thesis, the moisture degradation mechanism has been uncovered. It was found that the perovskite structure can be reconstructed when annealed at around 90℃, which leads to a reversible device of a much higher device lifetime. Following the experimental explorations, modelling work has also been conducted to simulate the water diffusion during the degradation and recovery. It was found that the moisture diffusion was initiated at the surface imperfection, where the activation energy for diffusion can be reduced. Based on these discoveries, a fresh structural negotiation method has been proposed to obtain a stable FAPbI3 phase of a suitable bandgap for photovoltaics, which showed a reduced Gibbs free energy of 0.12eV compared with the δ phase. In addition, the activation energy for such phase transition was calculated to be 0.45eV, meaning that the discovered phase is protected by both thermodynamics and kinetics. All this opens an unprecedented avenue in perovskite research, which will hopefully be of intrinsic interest to the broad materials research community as well. / Thesis / Doctor of Philosophy (PhD)

This thesis is missing page vi and is not in any of the copies. -Digitization Centre / The specific heat of (Mg1-xMnx) 2P2O7 was measured in the vicinity of the alpha-beta phase transition for 0 < x < 0.04. The temperature and character of the phase transition were found to depend on the composition of the sample and the method of preparation. The results are discussed with relation to the crystal structures of Mg2P2O7 and Mn2P2O7 . / Thesis / Master of Science (MS)

Various sizes of BaTiO3 and Cr3+-doped BaTiO3 nanocrystals were synthesized through hydrothermal and solvothermal methods. The applied solvents water, ethanol and benzyl alcohol lead to nanoparticles with average sizes of 200, 10 and 5 nm, respectively. The nanocrystals were treated with trioctylphosphine oxide to remove surface-bound dopant ions, and colloidal free-standing nanocrystals smaller than 10 nm were obtained by using oleic acid as a dispersant surfactant. The tetragonal-to-cubic phase transition at room temperature of undoped nanocrystalline BaTiO3 has been investigated by powder X-ray diffraction (XRD) and Raman spectroscopy. The size effect of nanoscale BaTiO3 is observed that the tetragonal phase becomes unstable with decreasing particle size. However, we found that ferroelectric tetragonal structure persists to some extent even for particles at 5 nm. The successful substitution of Ti4+ with Cr3+ in the host BaTiO3 lattice for all three sizes was achieved at different Cr3+/Ti4+ molar ratios. The dopant is found to significantly promote the phase transition, even dominate over the size effect. Ligand-field electronic absorption spectroscopy suggests a subtle change of the octahedral coordinated Cr3+ environments between particles at 5 and 10 nm, confirming the structural differences. Preliminary magnetic measurement indicates Cr3+ as isolated paramagnetic ions without any chromium clusters or oxides. The ability to rationally manipulate the ferroelectric properties of BaTiO3 by size and dopants, in combination with possible ferromagnetism induced by incorporating paramagnetic transition metal ions, opens up new opportunities for modern multiferroic materials in information storage technology.

Various sizes of BaTiO3 and Cr3+-doped BaTiO3 nanocrystals were synthesized through hydrothermal and solvothermal methods. The applied solvents water, ethanol and benzyl alcohol lead to nanoparticles with average sizes of 200, 10 and 5 nm, respectively. The nanocrystals were treated with trioctylphosphine oxide to remove surface-bound dopant ions, and colloidal free-standing nanocrystals smaller than 10 nm were obtained by using oleic acid as a dispersant surfactant. The tetragonal-to-cubic phase transition at room temperature of undoped nanocrystalline BaTiO3 has been investigated by powder X-ray diffraction (XRD) and Raman spectroscopy. The size effect of nanoscale BaTiO3 is observed that the tetragonal phase becomes unstable with decreasing particle size. However, we found that ferroelectric tetragonal structure persists to some extent even for particles at 5 nm. The successful substitution of Ti4+ with Cr3+ in the host BaTiO3 lattice for all three sizes was achieved at different Cr3+/Ti4+ molar ratios. The dopant is found to significantly promote the phase transition, even dominate over the size effect. Ligand-field electronic absorption spectroscopy suggests a subtle change of the octahedral coordinated Cr3+ environments between particles at 5 and 10 nm, confirming the structural differences. Preliminary magnetic measurement indicates Cr3+ as isolated paramagnetic ions without any chromium clusters or oxides. The ability to rationally manipulate the ferroelectric properties of BaTiO3 by size and dopants, in combination with possible ferromagnetism induced by incorporating paramagnetic transition metal ions, opens up new opportunities for modern multiferroic materials in information storage technology.

ABSTRACT The phase transition of the antiferromagnetic-torsional XY model is studied with molecular dynamics. In the process of releasing energy, we find a minimally stable state. The minimally stable state indicates the process of disordered state to ordered state when the 2nd phase transition was derived. The temperature of phase transition for the iferromagnetic-torsional XY model is lower than that for the antiferromagnetic XY model. A increase of the coefficient J2 will delay the temperature of phase transition for the antiferromagnetic-torsional XY model .

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We examine discrete models with hexagonal symmetry to compare the sequence of transitions with the alpha-inc-beta phase transition of quartz. We examine a model by Parlinski which employs interactions of nearest and next-nearest neighbor atoms. We numerically determine the configurations which lead to minimum energy for a range of parameters. We then use Golubitsky's results on systems with hexagonal symmetry to derive the bifurcation diagram for Parlinski's model. Finally, we study a large class of modifications to Parlinski's model and show that all such modifications have the same bifurcation picture as the original model. / Ph. D.