Quantum spin Hall density wave insulator of correlated fermions

We present the theory of a new type of topological quantum order which is driven by the spin-orbit density wave order parameter, and distinguished by $Z_2$ topological invariant. We show that when two oppositely polarized chiral bands [resulting from the Rashba-type spin-orbit coupling $\alpha_k$, $k$ is crystal momentum] are significantly nested by a special wavevector ${\bf Q}\sim(\pi,0)/(0,\pi)$, it induces a spatially modulated inversion of the chirality ($\alpha_{k+Q}=\alpha_k^*$) between different sublattices. The resulting quantum order parameters break translational symmetry, but preserve time-reversal symmetry. It is inherently associated with a $Z_2$-topological invariant along each density wave propagation direction. Hence it gives a weak topological insulator in two dimensions, with even number of spin-polarized boundary states. This phase is analogous to the quantum spin-Hall state, except here the time-reversal polarization is spatially modulated, and thus it is dubbed quantum spin-Hall density wave (QSHDW) state. This order parameter can be realized or engineered in quantum wires, or quasi-2D systems, by tuning the spin-orbit couping strength and chemical potential to achieve the special nesting condition.Comment: 8 pages, 4 figure

Doctor of Philosophy

dissertationDuring nervous system development, progenitor cells multiply under the control of the cell cycle pathway. When they are poised to differentiate, they withdraw from the cell cycle to form appropriate neuronal cell types. Cell cycle regulation is therefore closely intertwined with progenitor proliferation and neurogenesis in the developing nervous system and often, common factors and pathways are utilized in these processes. Two properties of neuronal progenitors can be critical for proper nervous system development: the time they take to complete one cell cycle, and the exact timing of their withdrawal/exit from the cell cycle to form neurons. Understanding how these properties can be manipulated to influence progenitor cell proliferation and neurogenesis can be invaluable for devising therapeutic strategies involving neuronal stem/progenitor cells. Retina, the primary tissue for vision, is an excellent model system for studying nervous system development and neuronal progenitor cell biology. To gain potential insights into the issues described above, this dissertation focuses on the role of the cell cycle regulators the D-cyclins, Cyclin D1 (Ccnd1) and Cyclin D3 (Ccnd3), during retinal development and characterizes the retinal phenotypes of Ccnd1 and Ccnd3 knockout mice. Chapter 1 is an introduction to retinal development and sets up the relevant questions addressed here. Chapter 2 is a reprint of a published journal article titled, "Cyclin D1 fine-tunes iv the neurogenic output of embryonic retinal progenitor cells." The study shows that during mouse embryonic development, CCND1 expression in retinal progenitor cells (RPCs) is critical for maintenance of their cell cycle time and also for their timing of exit from the cell cycle. Further, CCND1 ensures that the correct complements of early-born retinal neurons are generated from progenitors. Chapter 3 deals with the role of D-cyclins during postnatal retina development. The study shows that CCND1 also influences the production rate of late-born retinal cell types. Unexpectedly, although Ccnd1 null retinas experience progenitor cell depletion during development, proliferation, and neurogenesis persist well beyond the normal period of retinal histogenesis in mutant retinas. Further, Ccnd3 is unable to compensate for Ccnd1's role in regulation of cell cycle time and cell cycle withdrawal. Chapter 4 discusses the implications and relevance of the above studies. Future directions for these studies are also outlined

Synthesis and characterization of zeolite from waste coal flyash for tailored application in bio-refining and process water cleaning: An innovative approach towards a cleaner circular economy

The purpose of the investigation was to assess if Finnish coal flyash (CFA) waste could be used to synthesize zeolites. The world produces 750 million tonnes of CFA annually which is also the largest quantity waste produced. This figure will only increase as India, China, South America and Africa charges ahead with industrialization. The global recycle rate is 15% annually. Finland produces about 750,000 tonnes of CFA per year. It is also estimated that millions of tonnes of CFA is backfilled globally. Hence there is great potential to use it for high value applications as the raw material security exists. There are also disposal and environmental issues related to CFA which makes it our obligation to find a great solution. The recent trend towards circular economy, waste to value, sustainability, EU and National environmental legislations also provides a great platform to find a solution. Other reasons have been analysed from engineering, policy making, markets, scientific & innovation, national and regional impact, international trade, geographic location and future study perspectives. The literature section provides insights into CFA, structure, transformation, mechanism, combustion process, applications, environmental issues etc. The Zeolite section gives a deep understanding of origin, history, classification, trends in development, structure, morphology, applications, properties, circular economy context of zeolites, synthesis methods, raw material variations etc. There are 174 zeolite framework types in zeolite families and we put special emphasis on NaX (Faujasite framework) with tailored descriptions. The literature highlights CFA Zeolites, its differences with pure chemical ones, synthesis methods, previous works and global pilot projects. Conversion of Finnish CFA to Zeolite was a grand success. The overall process involved sieving, batch preparation, ageing, hydrothermal treatment, washing/filtration, drying and grinding. We sieved CFA to collect unburnt carbon (0.2% weight basis) and obtain consistent particle range. Creation of appropriate chemical composition, ageing for 24 hours (650 rpm at 21℃), hydrothermal treatment for 24 hours at 60℃-85℃. Washing is followed by drying the product for 16h and grinding it with mortar and pestle. CFA Zeolites have been made for the first time in Finland and Northern Europe. Both the CFA and CFA Zeolites were analysed using XRD, EDX, SEM and BET. CFA consisted of amorphous SiO2 and Al2O3 along with crystalline quartz (SiO2) and mullite (SiO2.Al2O3). The LOI was 4.57% (weight basis). The BET value for CFA was 366. 73 m2/g. The CFA Zeolite was pure phase NaX and crystalline without competing GIS, SOD, LTA phases. The BET surface area of CFA Zeolite was approx. 1800-2000 m2/g. This is the first time such high values have been reported in the world. The process was scaled up from lab to bench scales. Various repetitive tests were conducted in lab and bench scales to have consistent results. Statistical analysis was conducted to obtain quality control guidelines

Large Landau level splitting with tunable one-dimensional graphene superlattice probed by magneto capacitance measurements

The unique zero energy Landau Level of graphene has a particle-hole symmetry in the bulk, which is lifted at the boundary leading to a splitting into two chiral edge modes. It has long been theoretically predicted that the splitting of the zero-energy Landau level inside the {\it bulk} can lead to many interesting physics, such as quantum spin Hall effect, Dirac like singular points of the chiral edge modes, and others. However, so far the obtained splitting with high-magnetic field even on a hBN substrate are not amenable to experimental detection, and functionality. Guided by theoretical calculations, here we produce a large gap zero-energy Landau level splitting ($\sim$ 150 meV) with the usage of a one-dimensional (1D) superlattice potential. We have created tunable 1D superlattice in a hBN encapsulated graphene device using an array of metal gates with a period of $\sim$ 100 nm. The Landau level spectrum is visualized by measuring magneto capacitance spectroscopy. We monitor the splitting of the zeroth Landau level as a function of superlattice potential. The observed splitting energy is an order higher in magnitude compared to the previous studies of splitting due to the symmetry breaking in pristine graphene. The origin of such large Landau level spitting in 1D potential is explained with a degenerate perturbation theory. We find that owing to the periodic potential, the Landau level becomes dispersive, and acquires sharp peaks at the tunable band edges. Our study will pave the way to create the tunable 1D periodic structure for multi-functionalization and device application like graphene electronic circuits from appropriately engineered periodic patterns in near future

Neural Paraphrase Identification of Questions with Noisy Pretraining

We present a solution to the problem of paraphrase identification of questions. We focus on a recent dataset of question pairs annotated with binary paraphrase labels and show that a variant of the decomposable attention model (Parikh et al., 2016) results in accurate performance on this task, while being far simpler than many competing neural architectures. Furthermore, when the model is pretrained on a noisy dataset of automatically collected question paraphrases, it obtains the best reported performance on the dataset

Probing the Fermi surface and magnetotransport properties in MoAs2_{2}

Transition metal dipnictides (TMDs) have recently been identified as possible candidates to host topology protected electronic band structure. These materials belong to an isostructural family and show several exotic transport properties. Especially, the large values of magnetoresistance (MR) and carrier mobility have drawn significant attention from the perspective of technological applications. In this report, we have investigated the magnetotransport and Fermi surface properties of single crystalline MoAs$_{2}$, another member of this group of compounds. Field induced resistivity plateau and a large MR have been observed, which are comparable to several topological systems. Interestingly, in contrast to other isostructural materials, the carrier density in MoAs$_{2}$ is quite high and shows single-band dominated transport. The Fermi pockets, which have been identified from the quantum oscillation, are largest among the members of this group and have significant anisotropy with crystallographic direction. Our first-principles calculations reveal a substantial difference between the band structures of MoAs$_{2}$ and other TMDs. The calculated Fermi surface consists of one electron pocket and another 'open-orbit' hole pocket, which has not been observed in TMDs so far.Comment: 8 pages, 9 figure

Effect of Hygrothermal Treatment on the Tensile Properties of Hot-Pressed Jute Fibre Composites

The growing environment awareness demands the use of nature fibres as reinforcement materials in commercial. The natural fibres however are hydrophilic in nature and their composites undergo environmental degradation during service. Hygrothermal conditioning on as received jute fabric at different temperatures of hotpress have been studied in the present investigation. These composites are usually subjected to various loading conditions. Therefore, an attempt has been made to study the tensile properties of the composites. Fractography studies were carried out to study the fracture surface under SEM. It is noticed that the major mode of failure is due to fibre pullout and matrix cracking. The result from the hygrothermal studies shows the decrease in strength values of the composites on prolong exposure to humid environment.

Discovery of highly spin-polarized conducting surface states in the strong spin-orbit coupling semiconductor Sb2_2Se3_3

Majority of the A$_2$B$_3$ type chalcogenide systems with strong spin-orbit coupling, like Bi$_2$Se$_3$, Bi$_2$Te$_3$ and Sb$_2$Te$_3$ etc., are topological insulators. One important exception is Sb$_2$Se$_3$, where a topological non-trivial phase was argued to be possible under ambient conditions, but such a phase could be detected to exist only under pressure. In this Letter, we show that like Bi$_2$Se$_3$, Sb$_2$Se$_3$, displays generation of highly spin-polarized current under mesoscopic superconducting point contacts as measured by point contact Andreev reflection spectroscopy. In addition, we observe a large negative and anisotropic magnetoresistance in Sb$_2$Se$_3$, when the field is rotated in the basal plane. However, unlike in Bi$_2$Se$_3$, in case of Sb$_2$Se$_3$ a prominent quasiparticle interference (QPI) pattern around the defects could be obtained in STM conductance imaging. Thus, our experiments indicate that Sb$_2$Se$_3$ is a regular band insulator under ambient conditions, but due to it's high spin-orbit coupling, non-trivial spin-texture exists on the surface and the system could be on the verge of a topological insulator phase.Comment: 5 pages, 4 figures, supplemental material not include