Nature of phase transition in magnetic thin films

We study the critical behavior of magnetic thin films as a function of the film thickness. We use the ferromagnetic Ising model with the high-resolution multiple histogram Monte Carlo (MC) simulation. We show that though the 2D behavior remains dominant at small thicknesses, there is a systematic continuous deviation of the critical exponents from their 2D values. We observe that in the same range of varying thickness the deviation of the exponent $\nu$ is very small from its 2D value, while exponent $\beta$ suffers a larger deviation. Moreover, as long as the film thickness is fixed, i. e. no finite size scaling is done in the $z$ direction perpendicular to the film, the 3D values of the critical exponents cannot be attained even with very large (but fixed) thickness. The crossover to 3D universality class cannot therefore take place without finite size scaling applied in the $z$ direction, in the limit of numerically accessible thicknesses. From values of exponent $\alpha$ obtained by MC, we estimate the effective dimension of the system. We conclude that with regard to the critical behavior, thin films behave as systems with effective dimension between 2 and 3.Comment: 8 pages, 17 figures, submitted to Phys. Rev.

Bipolar Magnetic Semiconductors: A New Class of Spintronics Materials

Electrical control of spin polarization is very desirable in spintronics, since electric field can be easily applied locally in contrast with magnetic field. Here, we propose a new concept of bipolar magnetic semiconductor (BMS) in which completely spin-polarized currents with reversible spin polarization can be created and controlled simply by applying a gate voltage. This is a result of the unique electronic structure of BMS, where the valence and conduction bands possess opposite spin polarization when approaching the Fermi level. Our band structure and spin-polarized electronic transport calculations on semi-hydrogenated single-walled carbon nanotubes confirm the existence of BMS materials and demonstrate the electrical control of spin-polarization in them.Comment: 20 pages, 6 figures, accepted by Nanoscal

Chromatin and DNA methylation dynamics during retinoic acid-induced RET gene transcriptional activation in neuroblastoma cells

Although it is well known that RET gene is strongly activated by retinoic acid (RA) in neuroblastoma cells, the mechanisms underlying such activation are still poorly understood. Here we show that a complex series of molecular events, that include modifications of both chromatin and DNA methylation state, accompany RA-mediated RET activation. Our results indicate that the primary epigenetic determinants of RA-induced RET activation differ between enhancer and promoter regions. At promoter region, the main mark of RET activation was the increase of H3K4me3 levels while no significant changes of the methylation state of H3K27 and H3K9 were observed. At RET enhancer region a bipartite chromatin domain was detected in unstimulated cells and a prompt demethylation of H3K27me3 marked RET gene activation upon RA exposure. Moreover, ChIP experiments demonstrated that EZH2 and MeCP2 repressor complexes were associated to the heavily methylated enhancer region in the absence of RA while both complexes were displaced during RA stimulation. Finally, our data show that a demethylation of a specific CpG site at the enhancer region could favor the displacement of MeCP2 from the heavily methylated RET enhancer region providing a novel potential mechanism for transcriptional regulation of methylated RA-regulated loci

Thermodynamic Assessment of Membrane-Assisted Premixed and Non-Premixed Oxy-Fuel Combustion Power Cycles

Abstract This study focuses on the investigations of gas turbine power generation system that works on oxy-combustion technology utilizing membrane-assisted oxygen separation. The two investigated systems are (i) a premixed oxy-combustion power generation cycle utilizing an ion transport membrane (ITM)-based air separation unit (ASU) which selectively allows oxygen to permeate from the feeding air and (ii) a non-premixed oxy-fuel combustion power cycle, where oxygen separation takes place, with cogeneration of hydrogen in an integrated combustor. A gas turbine combined cycle that works on conventional air–methane combustion was considered as the base case for this work. Commercial software package Hysys V8 was utilized to conduct the process simulation for the proposed cycles. The two novel cycle designs were proposed and evaluated in comparison with that of the conventional cycle. The first law efficiency of the premixed combustion power cycle was calculated to be 45.9%, a loss of 2.4% as an energy penalty for the oxygen separation. The non-premixed cycle had the lowest first law efficiency of 39.6%, which was 8.7% lower than the efficiency of the base cycle. The lower effectiveness of the cycle could be attributed to the highly endothermic H2O splitting reaction for oxygen production. High irreversibility in the H2O-splitter and the reactor was identified as the main cause of exergy losses. The overall second law efficiency of the non-premixed power cycle was around 50% lesser than that of the other cycles. The energy penalty related to air separation is dominated as the parameter that reduces the efficiencies of the oxy-fuel combustion cycles; however, the premixed combustion cycle performance was found to be comparable to that of the conventional air-combustion cycle.</jats:p

Investigation of the Effect of the Top and the Bottom Temperatures on the Performance of Humidification Dehumidification Desalination Systems

Humidification dehumidification process is an attractive small scale water desalination technique in which desalinated water is produced by mimicking the nature’s water cycle. Various modifications to the basic HDH system can be vital in improving the productivity and reducing the production cost of the fresh water. In this study, a closed-air-open-water water-heated (CAOW-WH) cycle and a closed-air-open-water air-heated (CAOW-AH) cycle are modeled and optimized. Effects of mass flow ratio, humidifier and dehumidifier effectiveness, relative humidity, top and bottom temperatures (main concern of study) on the gain output ratio (GOR), the recovery ratio (RR), entropy generation in the system have been analyzed and presented. It has been observed that an optimal mass flow ratio exists for both the cycles, which maximizes the GOR of the system. Moreover, effectiveness of the humidifier and the dehumidifier is an important parameter, which determines the productivity of the systems. Furthermore, a higher GOR can be obtained at low Tmin and high Tmax and at high Tmin and low Tmax for systems heated by a water heater, whereas the GOR of the air heated HDH system increases with increasing both the Tmin and the Tmax for values of humidifier and dehumidifier effectiveness of 0.8. This study provide extended design charts for building an optimum HDH system to produce a pre-determined rate of desalinated water.</jats:p