The Impact of Molecular Polarization on the Electronic Properties of Molecular Semiconductors

In a molecular semiconductor, the carrier is dressed with a polarization cloud that we treat as a quantum field of Frenkel excitons coupled to it. The consequences of the existence of this electronic polaron on the dynamics of an extra charge in a material like pentacene can thus be evaluated.Comment: 7 pages, 1 figure, LaTe

Polarization effects in the channel of an organic field-effect transistor

We present the results of our calculation of the effects of dynamical coupling of a charge-carrier to the electronic polarization and the field-induced lattice displacements at the gate-interface of an organic field-effect transistor (OFET). We find that these interactions reduce the effective bandwidth of the charge-carrier in the quasi-two dimensional channel of a pentacene transistor by a factor of two from its bulk value when the gate is a high-permittivity dielectric such as $(\textrm{Ta}_{2}\textrm{O}_{5})$ while this reduction essentially vanishes using a polymer gate-insulator. These results demonstrate that carrier mass renormalization triggers the dielectric effects on the mobility reported recently in OFETs.Comment: 19 pages, 3 figure

Developing strategies for the control of Ichthyophthirius multifiliis Fouquet, 1876 (Ciliophora)

The intensification of freshwater aquaculture worldwide has facilitated the propagation of the parasitic ciliate protozoan Ichthyophthirius multifiliis Fouquet, 1876 commonly known as “fish white spot” or “Ich”. Ichthyophthirius multifiliis infections lead to high mortalities, generating significant economic losses in most cultured freshwater fish species worldwide. Until recently, malachite green was the chemical treatment traditionally used to control I. multifiliis infections. Its reclassification as carcinogenic to humans and its subsequent ban for use in food fish has left the industry without any suitable treatments. Currently, in-bath formaldehyde and sodium chloride treatments are the most common option used in farm systems to control I. multifliis infections. Given their low efficacy, however, they are not considered as sustainable long–term options. There is, therefore, an urgent necessity to find efficacious alternatives for controlling I. multifliis infections. The general aim of this research project was to improve the management of I. multifiliis infections in order to develop more comprehensive, environmentally friendly and sustainable therapeutic strategies for use in freshwater food fish aquaculture. The present PhD-thesis present first a literature review chapter providing an overview and critical assessment of chemotherapeutants and physical interventions tested within the last 30 years against I. multifiliis infections. The experimental worked consisted of a number of in vitro and in vivo trials were conducted using experimental scale flow-through, static tank systems and commercial scale raceways within a rainbow trout hatchery, in addition to molecular work on different isolates of the parasite. The results of this research are organised into three experimental chapters which describe the testing of chemical and non-chemical treatments against I. multifiliis infections and work undertaken to determine the most suitable molecular markers to identify I. multifiliis isolates. In the first experimental chapters, the possibility of efficiently controlling I. multifliis infections through the administration of novel environmentally-friendly chemical treatments (e.g. bronopol and peracetic acid-based products) was investigated. The results clearly showed that bronopol and peractic acid-based products have a strong biocidal/cytotoxic effect against all free-living stages of I. multifiliis (e.g. tomonts, cysts and theronts). The administration of high concentrations of bronopol (e.g. 20, 50 and 100 mg L-1) over short periods of exposure (e.g. 30 min) significantly reduced the survival of tomonts, cysts and theronts and delayed the development of I. multifiliis tomonts and cysts. Prolonged low concentrations of bronopol (e.g. 1 mg L-1) greatly reduced the survival of infective theronts, although such treatment did not affect the ability of surviving theronts to subsequently infect a host. When tested in vivo, the continuous prolonged exposure (e.g. 27 days) of low concentrations of bronopol (e.g. 2 and 5 mg L-1) had an impact on the population dynamics of I. multifiliis, this being demonstrated by a significant reduction in the number of trophonts developing within the fish. Low concentrations of bronopol (e.g. 2 mg L-1) administrated as a preventive treatment prior to infection also proved to be very successful at reducing the colonisation success of I. multifiliis. Peracetic acid administrated at low concentrations (e.g. 8, 12 and 15 mg L-1) over a short window of exposure (e.g. 1 h) displayed a strong biocidal effect against all the free-living stages of I. multifiliis (e.g. tomonts, cysts and theronts). The bronopol and peracetic acid-based products tested here both appear to be capable of disrupting the development of the cyst stage of I. multifiliis which is seldom reported for chemotherapeutants currently used against this parasite. These results suggest that bronopol and peracetic acid-based products have a place in the arsenal of treatment options for controlling I. multifiliis infections in commercial aquaculture systems. The use of a mechanical device or a biological control agent to remove the cyst stage of I. multifiliis and the impact of such control on the population dynamics and the levels of infection of fish were also investigated. The results revealed that tomonts preferentially settle and encyst on the base of culture systems and on biofilm–covered substrates. The survival of the tomont stage is greatly affected by the composition of the substrate upon which it settles and is significantly lower on polypropylene-based plastic. The lining of raceways in a commercial rainbow trout hatchery with a low-adhesion polymer created a smooth surface facilitating the dislodgement and elimination of the cyst stage of I. multifilis by natural flushing or brushing. The physical removal of the cyst stage alone, through the use of a mechanical device or substrate detrivorous/algae feeder as a biological control agent, significantly reduced the propagation of I. multifiliis to a low level of infection without the need to deploy an additional chemical treatment. These studies demonstrate that the cyst is a key stage in the dynamics of I. multifiliis infection and its removal from the fish culture systems could constitute an effective and simple mean of managing I. multifiliis infections. The third experimental chapter explores the utilisation of molecular marker to characterise different isolates of I. multifiliis. The results highlight the unsuitability of the rDNA region (ITS-1 and ITS-2) and the strong potential of the mtDNA (COI) as molecular markers to discriminate isolates of I. multifiliis from distant geographical locations. It is suggested that genetic “barcoding” using mtDNA is the most effective method to identify I. multifiliis isolates. Importantly, genetic “barcoding” could allow associating I. multifiliis strains or geographical isolates with particular properties as regards their ecophysiology, pathogenicity and sensitivity to treatment, in order to improve the management of I. multifiliis infections according to the specific genetic isolate encountered. This research project demonstrates the efficacy of a range of new approaches against the propagation of I. multifiliis. Together, our findings contribute towards the development of a more effective and integrated system for managing I. multifliis infections in farm systems. The utilisation of physical methods and of environmentally friendly chemotherapeutants holds great potential for the control of I. multifiliis infections in organic fish production and in a broader context to any freshwater food fish farms affected by I. multifiliis

Phase separation versus supersolid behavior in frustrated antiferromagnets

We investigate the competition between spin-supersolidity and phase separation in a frustrated spin-half model of weakly coupled dimers. We start by considering systems of hard-core bosons on the square lattice, onto which the low-energy physics of the herein investigated spin model can be mapped, and devise a criterion for gauging the interplay between supersolid order and domain wall formation based on strong coupling arguments. Effective bosonic models for the spin model are derived via the contractor renormalization (CORE) algorithm and we propose to combine a self-consistent cluster mean-field solution with our criterion for the occurrence of phase separation to derive the phase diagram as a function of frustration and magnetic field. In the limit of strong frustration, the model is shown to be unstable toward phase separation, in contradiction with recently published results. However, a region of stable supersolidity is identified for intermediate frustration, in a parameter range not investigated so far and of possible experimental relevance.Comment: 8 pages, 7 figures. Published versio

Supersolid phase with cold polar molecules on a triangular lattice

We study a system of heteronuclear molecules on a triangular lattice and analyze the potential of this system for the experimental realization of a supersolid phase. The ground state phase diagram contains superfluid, solid and supersolid phases. At finite temperatures and strong interactions there is an additional emulsion region, in contrast to similar models with short-range interactions. We derive the maximal critical temperature $T_c$ and the corresponding entropy $S/N = 0.04(1)$ for supersolidity and find feasible experimental conditions for its realization.Comment: 4 pages, 4 figure

Unconventional magnetization plateaus in a Shastry-Sutherland spin tube

Using density matrix renormalization group (DMRG) and perturbative continuous unitary transformations (PCUTs), we study the magnetization process in a magnetic field for all coupling strengths of a quasi-1D version of the 2D Shastry-Sutherland lattice, a frustrated spin tube made of two orthogonal dimer chains. At small inter-dimer coupling, plateaus in the magnetization appear at 1/6, 1/4, 1/3, 3/8, and 1/2. As in 2D, they correspond to a Wigner crystal of triplons. However, close to the boundary of the product singlet phase, plateaus of a new type appear at 1/5 and 3/4. They are stabilized by the localization of {\it bound states} of triplons. Their magnetization profile differs significantly from that of single triplon plateaus and leads to specific NMR signatures. We address the possibility to stabilize such plateaus in further geometries by analyzing small finite clusters using exact diagonalizations and the PCUTs.Comment: Final version as published in EP

On A Cosmological Invariant as an Observational Probe in the Early Universe

k-essence scalar field models are usually taken to have lagrangians of the form ${\mathcal L}=-V(\phi)F(X)$ with $F$ some general function of $X=\nabla_{\mu}\phi\nabla^{\mu}\phi$. Under certain conditions this lagrangian in the context of the early universe can take the form of that of an oscillator with time dependent frequency. The Ermakov invariant for a time dependent oscillator in a cosmological scenario then leads to an invariant quadratic form involving the Hubble parameter and the logarithm of the scale factor. In principle, this invariant can lead to further observational probes for the early universe. Moreover, if such an invariant can be observationally verified then the presence of dark energy will also be indirectly confirmed.Comment: 4 pages, Revte

Mechanisms for Spin-Supersolidity in S=1/2 Spin-Dimer Antiferromagnets

Using perturbative expansions and the contractor renormalization (CORE) algorithm, we obtain effective hard-core bosonic Hamiltonians describing the low-energy physics of $S=1/2$ spin-dimer antiferromagnets known to display supersolid phases under an applied magnetic field. The resulting effective models are investigated by means of mean-field analysis and quantum Monte Carlo simulations. A "leapfrog mechanism", through means of which extra singlets delocalize in a checkerboard-solid environment via correlated hoppings, is unveiled that accounts for the supersolid behavior.Comment: 12 pages, 10 figure