Crystal Symmetry Breaking in Few-Quintuple Bismuth Telluride Films: Applications in Nanometrology of Topological Insulators

We report results of micro-Raman spectroscopy investigation of the "graphene-like" mechanically exfoliated single-crystal bismuth telluride films with the thickness ranging from a few-nm-range to bulk limit. It is found that the optical phonon mode A1u, which is not-Raman active in bulk bismuth telluride crystals, appears in the atomically-thin films due to crystal-symmetry breaking. The intensity ratios of the out-of-plane A1u and A1g modes to the in-plane Eg mode grow with decreasing film thickness. The evolution of Raman signatures with the film thickness can be used for identification of bismuth telluride crystals with the thickness of few-quintuple layers, which are important for topological insulator and thermoelectric applications.Comment: 13 pages, 2 tables, 3 figures; to be presented at MRS Spring Meeting, 201

Response biomarkers: re-envisioning the approach to tailoring drug therapy for cancer

BACKGROUND: The rapidly expanding arsenal of chemotherapeutic agents approved in the past 5 years represents significant progress in the field. However, this poses a challenge for oncologists to choose which drug or combination of drugs is best for any individual. Because only a fraction of patients respond to any drug, efforts have been made to devise strategies to personalize care. The majority of efforts have involved development of predictive biomarkers. While there are notable successes, there are no predictive biomarkers for most drugs. Moreover, predictive biomarkers enrich the cohort of individuals likely to benefit; they do not guarantee benefit. MAIN TEXT: There is a need to devise alternate strategies to tailor cancer care. One alternative approach is to enhance the current adaptive approach, which involves administration of a drug and cessation of treatment once progression is documented. This currently involves radiographic tests for the most part, which are expensive, inconvenient and imperfect in their ability to categorize patients who are and are not benefiting from treatment. A biomarker approach to categorizing response may have advantages. CONCLUSION: Herein, we discuss the state of the art on treatment response assessment. While the most mature technologies for response assessment involve radiographic tests such as CT and PET, reports are emerging on biomarkers used to monitor therapeutic efficacy. Potentially, response biomarkers represent a less expensive and more convenient means of monitoring therapy, although an ideal response biomarker has not yet been described. A framework for future response biomarker discovery is described

Oil Price Shocks and American Depositary Receipt Stock Returns

In this paper we examine the impact of oil price shocks on twelve countries American Depositary Receipt (ADR) returns using monthly data from 1999.01 to 2014.12. The results show that oil price shocks have a positive and statistically significant impact on ADR return in all twelve countries. These results are robust to the inclusion of other explanatory variables such as oil price volatility and the spillover of the United States stock market. Further analysis shows that this effect is stronger in the post financial crisis time period compared to the pre-financial crisis time period

Essays on Financial Markets and Investments

Essay I examine regime-switching in the response of U.S. stock price to oil price shocks. We find statistically significant, time-varying, and dynamic state dependent response that resembles to high and low response regimes. Furthermore, we examine whether the timing of the regime shifts are consistent with three (direction, magnitude, and business cycle) manifestations of asymmetry by modeling the transition probabilities governing the switching process as functions of state variables. We observe strong and statistically significant support that stock price responds more aggressively to oil specific shocks during recessions than during expansions, while findings suggest weaker evidence of asymmetry pertaining to the magnitude or direction of the oil price shocks. Essay II, on the other hand, investigates financial contagion during periods of bubble in developed economies from January 1, 1999 to December 29, 2017. First, we test for explosive behavior and bubbles in six stock index series using the recursive flexible window right-tailed ADF-based procedure. Second, we obtain dynamic conditional correlation by using a DCC multivariate GARCH model to analyze daily stock return data in six developed economies. Third, we pool the time-series data on periods of bubbles obtained from GSADF model and time-varying conditional correlations obtained from DCC-GARCH model. Finally, we use various dynamic panel specifications that takes into account the endogenous nature of the bubble. We find statistically significant decrease in the dynamic correlations during periods of bubble, which suggests that, the financial contagion between pair of countries diminishes when any of the two countries in the pair is going through periods of bubble

The relationship between the metal dusting mechanism and the synthesis of carbon nanofilaments using toluene and a nickel based alloy

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. Johannesburg, 2016.Metal dusting (MD) is a severe type of corrosion that occurs mainly in petrochemical industries. The occurrence of MD is mainly due to syngas attacking Fe-, Ni- and Co-based alloys at elevated temperatures. More recently, literature has shown that apart from syngas, liquid hydrocarbon sources have been causing problems on platformer units in refineries. In the first part of this study a highly corrosion resistant Ni-based alloy (Hastelloy C276), in its polished form, was subjected to MD conditions at 800 °C using a liquid hydrocarbon (toluene) and helium (carrier gas) for 1 h. Exposure to these conditions revealed the formation of carbon nanofilaments and graphite layers which were confirmed by laser Raman spectroscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Burning off the carbon nanofilaments and the graphite layers in laboratory air for 1 h at 800 °C revealed that pits were formed on the Hastelloy C276. These same pits were not evident when Hastelloy C276 was exposed to either the carrier gas (helium) or laboratory air alone. Besides MD being a continuous problem in industry, this mechanism has been shown to be beneficial in the synthesis of carbon nanofilaments viz., carbon nanofibers (CNTs) and nanotubes (CNFs). In the second part of this study, unpolished Hastelloy C276 blocks (as opposed to polished blocks) were used to synthesize carbon nanofilaments. This was done as prior studies had shown that carbon nanofilaments were produced with better quality and greater yields this way. Here the flow rate (80, 160 and 240 mL/min) and reaction duration (10, 15, 30, 45, 60, 120 and 240 min) were studied using toluene (a liquid hydrocarbon). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to assess the quality and quantity of the carbon nanofilaments synthesized. Besides the formation of carbon nanofilaments, a less important material known as graphite particle structures (GPSs) were also synthesized. These studies collectively showed that MD had taken place on the surface of Hastelloy C276 when exposed to toluene at 800 °C.TG201

Phase transitional ferroelectric like behaviour of ammonium tetroxalate dihydrate single crystal

Dielectric characterisation on ammonium tetroxalate dihydrate (ATOXAL) single crystal has been carried out. The dielectric constant and dielectric loss studied as a function of temperature and frequency have been found to have anomalous variations in the vicinity of the phase transitions at 100 °C and 140 °C which have been duly supported by thermal studies. An Arrhenius shift in the temperature variation of relaxation frequency has estimated the activation energy and the means of conduction. The ferroelectric and paraelectric phase activation energies have been determined to be 1.772 eV and 3.477 eV, respectively. The Penn band gap is 1.892 eV and the polarizability has been evaluated. The piezoelectric coefficient d33 has been found to be 1.5 pCN-1. The ferroelectric hysteresis behaviour has been investigated at room temperature. From the hysteresis loop it is found that the grown crystal exhibites a remanent polarization (Pr) ∼ 0.367 μC/cm2, saturation polarisation (Ps) ∼ 1.789 μC/cm2 and coercive field (Ec) ∼ 6.599 kV/cm

Explaining the nonlinear response of stock markets to oil price shocks

This paper is set to reconcile the existent conflicting empirical evidence on the effect of oil prices on stock prices. We estimate various nonlinear models where the response changes according to a first-order Markov switching process. More importantly, we model the transition probabilities between the high- and low-response regimes to depend on state variables to allow us to explain the forces behind the asymmetry in the response. The results show statistically significant asymmetries that can be explained by economic recessions and to a lower extent depend on the magnitude of the oil price shift and on whether the shift is positive or negative. In the high response regime, the effect is positive and lasts longer. We also find evidence of asymmetries in the response of stock prices to crude oil supply shocks, global aggregate demand shocks, and oil-specific demand shocks

Financial Contagion During Stock Market Bubbles

We investigate the role of bubbles on financial contagion using a set of developed economies. First, using the recursive flexible window right-tailed ADF-based procedure, we date stamp bubble periods in stock index series. Second, we capture contagion with a DCC multivariate GARCH framework. In a third step, we construct a panel by pooling across the time-series dynamic conditional correlations and bubbles to estimate various dynamic panel specifications that consider the endogenous nature of bubbles. We find statistically significant decreases in the dynamic correlations during periods of bubbles, which shows that the financial contagion between pair of countries diminishes when any of the two countries in the pair is going through a bubble period. This implies that during bubble periods investors are looking for an investment opportunity within their economy and rely less on international diversification. However, decrease in contagion between two economies could provide ample diversification opportunities for portfolio managers

Identifying price bubble periods in the energy sector

In this paper we test for the existence of single and multiple episodes of explosive behavior in three energy sector indices (crude oil, heating oil, and natural gas) and five energy sector spot prices (West Texas Intermediate (WTI), Brent, heating oil, natural gas, and jet fuel). The results from the Supremum Augmented Dickey-Fuller (SADF) and the Generalized SADF tests provide strong statistical evidence of explosive behavior in all of our energy series. A simple theoretical framework of commodity pricing allows us to understand the assumptions to interpret explosive behavior as bubbles. By constructing implied convenience yields using futures prices we test the key assumption and we are able to identify the beginning and the end of bubble periods for the WTI, Brent, heating oil, and natural gas spot prices