Abstract

This talk concerns with various aspects of ZnO-based thin films forphotovoltaic applications. In particular, emphasis has been given tounderstand dynamics of thin film growth by studying temporal and spatialevolution of surface roughness within framework of the dynamic scalingtheory. The films deposited at room temperature by RF magnetron sputteringfrom a ceramic target follow anomalous (super-rough) scaling behaviorcharacterized by global roughness and growth exponents clearly distinctfrom the local ones. The observed anomalous scaling and high growthexponents are correlated to the shadowing effect arising due tonon-uniform flux distribution favored by the 30-tilt of thesubstrates. The scaling exponents and hence the growth dynamics are foundto be strongly dependent on the process parameter, namely the RF powerduring sputtering. While the roughness exponent α is stable at1.5±0.2, the growth exponents βlocal and β decrease with thedecrease in the RF power. A dominant anisotropic growth of crystallites athigh powers is believed to be the main reason for the power-dependentroughening behavior. Besides, thickness-dependent film properties, role ofsputtering configuration and influence of the RF power on film properties,which assume much importance from technological point of view, have beendiscussed. It has been shown that thickness-dependent stress cancounteract the typical carrier concentration dependent broadening of bandgap observed in degenerate semiconductors. The process parameters (forexample, RF power) and sputtering configuration are shown to havetremendous impact on film properties. A very brief introduction to ourother research efforts (that includes research in flexibleoptoelectronics, alternate absorber layers for inorganic solar cells,etc.) is also presented.

Abstract

Phase transitions occurring at absolute zero temperature and governedby a critical value of a variable like pressure, magnetic field,doping etc. are called quantum phase transitions and the point of thephase diagram where it happens the quantum critical point (QCP). Ahuge accumulation of entropy at the QCP leads to a competition betweendifferent ground states and hence, unusual behaviour in thethermodynamics and transport properties of systems close/at the QCP. Aone dimensional (1-d) spin ½ antiferromagnetic Heisenberg chain (AfHC)is expected to be quantum critical at a field H_s (saturation field)above which it undergoes a transition to a ferromagnetic state.Recently, it has been theoretically predicted that the divergence andsign change of Grüneisen parameter across the quantum critical pointcan be used as a novel technique to probe quantum criticality. For afield induced quantum critical point, the Grüneisen parameter is, infact, magnetocaloric effect (MCE) which is the heating or cooling of asystem adiabatically, in response to a changing magnetic field. Inthis talk, I will present measurements andtheoretical calculations of the MCE in a metal organic polymer systembuilt from Cu^2+ (S=½ ) ions, which is a very good realization of a1-d AfHC. We verify unambiguously the theoretical predictions anddemonstrate that the 1-d AfHC shows an extraordinarily large MCE and apronounced magnetic cooling effect near the QCP. Our results suggestthat quantum magnets near a H-induced QCP open up new possibilitiesfor realizing very efficient low-temperature coolants. We, therefore,suggest the MCE experiments as a new means of exploring quantumcriticality, one of the most interesting issues in modern condensedmatter physics.

Abstract

Abstract

Mutators are cells with a higher rate of mutations than normalcells. These mutations are base pair substitutions – errors that creep inwhen the genome is being copied during cell division. Mutator cells arepresent in small amounts in natural populations of some bacteria. Some kindsof cancer cells are also known to be mutators. A high mutation rate isusually a disadvantage but sometimes can also be an advantage to thespecies. In this talk, I will present my results on a simple model thattries to capture the evolutionary dynamics of mutator and normal cells. Ourmodel incorporates the conversion of normal cells to mutators and thereduction of fitness due to mutation. The mutators have a higher selectionpressure since a high mutation rate reduces fitness. The interplay ofconversion and the reduced fitness leads to an interesting phase diagram. Wehave predicted a new, all-mutator phase and calculated the phase transitionpoint analytically.

Abstract