Browsing M.Sc. Physics by Title
Now showing items 5776 of 110

La1xSrxMnO3 as a candidate for a room temperature pressure sensor /Perovskite manganite compounds, LaixDxMnOs (Ddivalent alkaline earth Ca, Sr or Ba), whose electrical and magnetic properties were first investigated nearly a half century ago, have attracted a great deal of attention due to their rich phase diagram. From the point of view of designing a future application, the strong pressure dependence of the resistivity and the accompanying effects in thin films have potential for application in pressure sensing and electronic devices. In this study we report our experimental investigations of pressure dependence of the resistivity of Lao.siSvo^iQMnOs and LaixSvxMnOs (LSMO) epitaxial films with x= 0.15, 0.20, 0.25, 0.30, 0.35, on SrTiOs substrates.

Low frequency Raman scattering in amorphous materials: fused quartz, "pyrex" borosilicate glass and sodalime silicate glassRaman scattering in the region 20 to 100 cm 1 for fused quartz, "pyrex" borosilicate glass, and soft sodalime silicate glass was investigated. The Raman spectra for the fused quartz and the pyrex glass were obtained at room temperature using the 488 nm exciting line of a Coherent Radiation argonion laser at powers up to 550 mW. For the soft sodalime glass the 514.5 nm exciting line at powers up to 660 mW was used because of a weak fluorescence which masked the Stokes Raman spectrum. In addition it is demonstrated that the lowfrequency Raman coupling constant can be described by a model proposed by Martin and Brenig (MB). By fitting the predicted spectra based on the model with a Gaussian, Poisson, and Lorentzian forms of the correlation function, the structural correlation radius (SCR) was determined for each glass. It was found that to achieve the best possible fit· from each of the three correlation functions a value of the SCR between 0.80 and 0.90 nm was required for both quartz and pyrex glass but for the soft sodalime silicate glass the required value of the SCR. was between 0.50 and 0.60 nm .. Our results support the claim of Malinovsky and Sokolov (1986) that the MB model based on a Poisson correlation function provides a universal fit to the experimental VH (vertical and horizontal polarizations) spectrum for any glass regardless of its chemical composition. The only deficiency of the MB model is its failure to fit the experimental depolarization spectra.

Magnetic and Dielectric Properties of Cu3xNixWO6 and Cu3W1xMoxO6Cu3WO6 is a compound with an interesting coordination chemistry for both Cu and W. In this research, all samples were made by using the standard Solid Phase Reaction method with and without any doping. Using Powder xray diffraction and Rietveld analysis, we did not observe any distortion of the cubic crystal. Ni substitution for Cu, and Mo substitution for W, will act as a negative pressure on the lattice parameter of Cu3WO6. Magnetization measurements of Cu3xNixWO6 indicate that all compounds undergo an antiferromagnetic phase transition at the Néel temperature. However, a significant change was observed in Néel temperature with Ni’s concentration. All compounds show Curie–Weiss antiferromagnetic behavior at high temperatures. The value of the 𝛍eff is close to the theoretical calculation in Cu3WO6. And the magnitude of 𝛍eff (exp) increases with Ni’s concentration. A spinsinglet ground state with energy gap at low temperatures was observed for all compounds. The energy gap 𝚫 is decreasing with the increasing concentration of Ni. The dielectric permittivity as a function of temperature and different frequency from1 kHz to 20 kHz for all samples, were investigated. A peak in dielectric loss ε'' appears between 150 K to 260 K in all samples of Cu3xNixWO6. The peak position has a linear relationship with log10(𝑓) as a function of temperaure. The doping of Ni causes a gradual shift in the peak position. The activation energy Ea is decreasing with the increasing of Ni’s concentration.

Magnetic and high pressure studies in the YPd5B3C3 systemThe macroscopic properties of the superconducting phase in the multiphase compound YPd5B3 C.3 have been investigated. The onset of superconductivity was observed at 22.6 K, zero resistance at 21.2 K, the lower critical field Hel at 5 K was determined to be Hel (5) rv 310 Gauss and the compound was found to be an extreme typeII superconductor with the upper critical field in excess of 55000 Gauss at 15 K. From the upper and lower critical field values obtained, several important parameters of the superconducting state were determined at T = 15 K. The GinzburgLandau paramater was determined to be ~ > 9 corresponding to a coherence length ~ rv 80A and magnetic penetration depth of 800A. In addition measurements of the superconducting transition temperature Te(P) under purely hydrostatically applied pressure have been carried out. Te(P) of YPd5B3 C.3 decreases linearly with dTe/dP rv 8.814 X 105 J</bar. The discussion of Te(P) will focus on the influence pressure has on the phonon spectrum and the density of states near the Fermi level.

Magnetic and transport properties of (Ba1xKx)Fe2As2Single crystals of (Bal  xKx)Fe2As2 were prepared using the Sn flux method. Two heating methods were used to prepare the single crystals: the slow heating and rapid heating methods. It was found that the single crystals grown using the slow heating method were not superconducting due to a significant loss of potassium. When the rapid heating method was used, the single crystals were observed to be superconducting with the desired potassium concentration. The energy dispersive Xray spectroscopy analysis indicated the presence of multiple phases in the single crystals. Using single crystal Xray diffraction, the crystal structure of the single crystals was found to be 14/mmm tetragonal at room temperature. The magnetic measurements on the single crystals indicated the presence of multiple phases and magnetic impurities.

Magnetic properties of the Biâ Srâ CaCuâ Oâ single crystalThe Bi2Sr2CaCu20g single crystal with a superconducting transition temperature equal to 90 ± 2 K was prepared. The irreversibility line of the single crystal for a mgnetic field direction along the caxis and T* in the abplane was determined. The reduced temperature (l  T ) is proportional to H 1.1 for fields below 004 T and proportional to HO.09 for fields above 0.4 T. The zero temperature upper critical field Hc2(0) and coherence length ~ (0) were determined from the magnetization meaurements to be HlC2=35.9T , H//C2=31.2T, ~c(0)=35.0 A, and ~ab(0)=32.5A,and from the magnetoresistance measurements to be Hlc2 = 134.6T , H//C2=55.5T '~c(0)=38.1 A, and ~ab(0)=2404 A for both directions of the applied magnetic field. The results obtained for Hc2(0) and ~(O) are not reliable due to the rounding that the single crystal exhibits in the magnetization and magnetoresistance curves. The magnetization relaxation of the single crystal was investigated, and was found to be logarithmic in time, and the relaxation rate increases with temperature up to 50 60 K, then decreases at higher temperatures.

The measured variation of the DebyeWaller factor of aluminum from 295K to 815K by using the energy dispersive xray diffraction techniqueThe Energy Dispersive Xray Diffraction System at Brock University has been used to measure the intensities of the diffraction lines of aluminum powder sample as a function of temperature. At first, intensity measurements at high temperature were not reproducible. After some modifications have been made, we were able to measure the intensities of the diffraction lines to 815K, with good accuracy and reproducibility. Therefore the changes of the DebyeWaller factor from room temperature up to 815K for aluminum were determined with precision. Our results are in good agreement with those previously published.

Molecular dynamics calculation of mean square displacement in alkali metals and rare gas solids and comparison with lattice dynamicsMolec ul ar dynamics calculations of the mean sq ua re displacement have been carried out for the alkali metals Na, K and Cs and for an fcc nearest neighbour LennardJones model applicable to rare gas solids. The computations for the alkalis were done for several temperatures for temperature vol ume a swell as for the the ze r 0 pressure ze ro zero pressure volume corresponding to each temperature. In the fcc case, results were obtained for a wide range of both the temperature and density. Lattice dynamics calculations of the harmonic and the lowe s t order anharmonic (cubic and quartic) contributions to the mean square displacement were performed for the same potential models as in the molecular dynamics calculations. The Brillouin zone sums arising in the harmonic and the quartic terms were computed for very large numbers of points in qspace, and were extrapolated to obtain results ful converged with respect to the number of points in the Brillouin zone.An excellent agreement between the lattice dynamics results was observed molecular dynamics and in the case of all the alkali metals, e~ept for the zero pressure case of CSt where the difference is about 15 % near the melting temperature. It was concluded that for the alkalis, the lowest order perturbation theory works well even at temperat ures close to the melting temperat ure. For the fcc nearest neighbour model it was found that the number of particles (256) used for the molecular dynamics calculations, produces a result which is somewhere between 10 and 20 % smaller than the value converged with respect to the number of particles. However, the general temperature dependence of the mean square displacement is the same in molecular dynamics and lattice dynamics for all temperatures at the highest densities examined, while at higher volumes and high temperatures the results diverge. This indicates the importance of the higher order (eg. ~* ) perturbation theory contributions in these cases.

Monte Carlo study of the XYmodel on quasiperiodic latticesMonte Carlo Simulations were carried out using a nearest neighbour ferromagnetic XYmodel, on both 2D and 3D quasiperiodic lattices. In the case of 2D, both the unfrustrated and frustrated XVmodel were studied. For the unfrustrated 2D XVmodel, we have examined the magnetization, specific heat, linear susceptibility, helicity modulus and the derivative of the helicity modulus with respect to inverse temperature. The behaviour of all these quatities point to a KosterlitzThouless transition occuring in temperature range Te == (1.0 1.05) JlkB and with critical exponents that are consistent with previous results (obtained for crystalline lattices) . However, in the frustrated case, analysis of the spin glass susceptibility and EdwardsAnderson order parameter, in addition to the magnetization, specific heat and linear susceptibility, support a spin glass transition. In the case where the 'thin' rhombus is fully frustrated, a freezing transition occurs at Tf == 0.137 JlkB , which contradicts previous work suggesting the critical dimension of spin glasses to be de > 2 . In the 3D systems, examination of the magnetization, specific heat and linear susceptibility reveal a conventional second order phase transition. Through a cumulant analysis and finite size scaling, a critical temperature of Te == (2.292 ± 0.003) JI kB and critical exponents of 0:' == 0.03 ± 0.03, f3 == 0.30 ± 0.01 and I == 1.31 ± 0.02 have been obtained.

NMR characterization of chlorhexidine in lipidbased formulations /A mixture of Chlorhexidine digluconate (CHG) with glycerophospholipid 1,2dimyristoyl <^54glycero3phospocholine (DMPCrf54) was analysed using ^H nuclear magnetic resonance. To analyze powder spectra, the dePakeing technique was used. The method is able to extract simultaneously both the orientation distribution function and the anisotropy distribution function. The spectral moments, average order parameter profiles, and longitudinal and transverse relaxation times were used to explore the structural phase behaviour of various DMPC/CHG mixtures in the temperature range 560°C.

Numerical Solutions of Laplace's Equation for Various Physical SituationsThere are two projects in this thesis. In the first project, a general method is introduced to numerically calculate the resistance of truncated resistors in cylindrical coordinates, with nonconstant crosssectional area. The problem of finding the resistance of a truncated conical resistor is given in some introductory textbooks as a simple problem. The textbook method is flawed however, and leads to the wrong answer. The textbook method assumes that the electric potential distribution inside the truncated cone is approximately equivalent to a cylindrical resistor. This assumption ignores the constricting affect that the boundary of the truncated conical resistor has on the electric potential inside. The deformation of the electric field is not accounted for by excess charge or changing magnetic fields, instead it is the result of a derivative operation called the shear of the field. Numerical solutions for the resistance of truncated conical, ellipsoidal, and hyperboloidal resistors are presented as a function of a/b, where a is the radius of the smallest crosssectional area and b is radius of the largest. It was found that the textbook solution always underestimates the numerical value of the resistance. In the second project, dielectric breakdown clusters were grown with a stochastic two dimensional Dielectric Breakdown Model (DBM) on a honeycomb, square, and triangle lattice, as well as on a random distribution of nodes. On the regular lattices the number of nearest neighbours was a constant at all lattice sites. For a random distribution of nodes there was variation in the number of nearest neighbours at different nodes. Some percentage of the nodes were isolated from the rest of the distribution, because they had 0 nearest neighbours. Distributions of nodes in which many of the nodes had 0 nearest neighbours indicated a medium with high density fluctuations. The motivation for this work was to study the relationship between the fractal dimension of the dielectric breakdown clusters and the number of nearest neighbours, and the density variation of the medium. The singularity spectra were calculated for the clusters, as well as their fractal dimension using box counting, and sandbox methods. It was found that the dielectric breakdown model produces monofractal clusters. As such, the dimension of the clusters can be represented by a single fractal dimension. In the DBM, the probability of a perimeter site connecting to the cluster is proportional to the strength of the local electric field raised to an exponent. If the exponent is a large positive number then perimeter sites which feel a stronger electric field are more likely to connect to the cluster. Increasing the exponent produces clusters which resemble lightning, with a fractal dimension lower than the dimension of the lattice. Similarly increasing the percentage of isolated nodes decreases the fractal dimension.

On the anharmonic, multiphonon, DebyeWaller contributions to the phononlimited resistivity of metals : applications to Na and KThe anharmonic, multiphonon (MP), and OebyeWaller factor (OW) contributions to the phonon limited resistivity (;0) of metals derived by Shukla and Muller (1979) by the doubletime temperature dependent Green function method have been numerically evaluated for Na and K in the high temperature limit. The anharmonic contributions arise from the cubic and quartic shift of phonons (CS, QS), and phonon width (W) and the interference term (1). The QS, MP and OW contributions to I' are also derived by the matrix element method and the results are in agreement with those of Shukla and Muller (1979). In the high temperature limit, the contributions to;O from each of the above mentioned terms are of the type BT2 For numerical calculations suitable expressions are derived for the anharmonic contributions to ~ in terms of the third and fourth rank tensors obtained by the Ewald procedure. The numerical calculation of the contributions to;O from the OW, MP term and the QS have been done exactly and from the CS, Wand I terms only approximately in the partial and total Einstein approximations (PEA, TEA), using a first principle approach (Shukla and Taylor (1976)). The results obtained indicate that there is a strong pairwise cancellation between the: OW and MP terms, the QS and CS and the Wand I terms. The sum total of these contributions to;O for Na and K amounts to 4 to 11% and 2 to 7%, respectively, in the PEA while in the TEA they amount to 3 to 7% and 1 to 4%, respectively, in the temperature range.

On the equation of state and atomic mean square displacement of crystalsWe have presented a Green's function method for the calculation of the atomic mean square displacement (MSD) for an anharmonic Hamil toni an . This method effectively sums a whole class of anharmonic contributions to MSD in the perturbation expansion in the high temperature limit. Using this formalism we have calculated the MSD for a nearest neighbour fcc Lennard Jones solid. The results show an improvement over the lowest order perturbation theory results, the difference with Monte Carlo calculations at temperatures close to melting is reduced from 11% to 3%. We also calculated the MSD for the Alkali metals Nat K/ Cs where a sixth neighbour interaction potential derived from the pseudopotential theory was employed in the calculations. The MSD by this method increases by 2.5% to 3.5% over the respective perturbation theory results. The MSD was calculated for Aluminum where different pseudopotential functions and a phenomenological Morse potential were used. The results show that the pseudopotentials provide better agreement with experimental data than the Morse potential. An excellent agreement with experiment over the whole temperature range is achieved with the Harrison modified pointion pseudopotential with HubbardSham screening function. We have calculated the thermodynamic properties of solid Kr by minimizing the total energy consisting of static and vibrational components, employing different schemes: The quasiharmonic theory (QH), ).2 and).4 perturbation theory, all terms up to 0 ().4) of the improved self consistent phonon theory (ISC), the ring diagrams up to o ().4) (RING), the iteration scheme (ITER) derived from the Greens's function method and a scheme consisting of ITER plus the remaining contributions of 0 ().4) which are not included in ITER which we call E(FULL). We have calculated the lattice constant, the volume expansion, the isothermal and adiabatic bulk modulus, the specific heat at constant volume and at constant pressure, and the Gruneisen parameter from two different potential functions: LennardJones and Aziz. The Aziz potential gives generally a better agreement with experimental data than the LJ potential for the QH, ).2, ).4 and E(FULL) schemes. When only a partial sum of the).4 diagrams is used in the calculations (e.g. RING and ISC) the LJ results are in better agreement with experiment. The iteration scheme brings a definitive improvement over the).2 PT for both potentials.

On the formulation and the calculation of the harmonic contributions to the DebyeWaller factor in metals (sodium)The algebraic expressions for the anharmonic contributions to the DebyeWaller factor up to 0(A ) and 0 L% ) £ where ^ is the scattering wavevector] have been derived in a form suitable for cubic metals with small ion cores where the interatomic potential extends to many neighbours. This has been achieved in terms of various wavevector dependent tensors, following the work of Shukla and Taylor (1974) on the cubic anharmonic Helmholtz free energy. The contribution to the various wavevector dependent tensors from the coulomb and the electronion terms in the interatomic metallic potential has been obtained by the Ewald procedure. All the restricted multiple whole B r i l l o u i n zone (B.Z.) sums are reduced to single whole B.Z. sums by using the plane wave representation of the delta function. These single whole B.Z. sums are further reduced to the •%?? portion of the B.Z. following Shukla and Wilk (1974) and Shukla and Taylor (1974). Numerical calculations have been performed for sodium where the BornMayer term in the interatomic potential has been neglected because i t is small £ Vosko (1964)3 • *n o^er to compare our calculated results with the experimental results of Dawton (1937), we have also calculated the r a t io of the intensities at different temperatures for the lowest five reflections (110), (200), (220), (310) and (400) . Our calculated quasiharmonic results agree reasonably well with the experimental results at temperatures (T) of the order of the Debye temperature ( 0 ). For T » © ^ 9 our calculated anharmonic results are found to be in good agreement with the experimental results.The anomalous terms in the DebyeWaller factor are found not to be negligible for certain reflections even for T ^ ©^ . At temperature T yy Op 9 where the temperature is of the order of the melting temperature (Xm) » "the anomalous terms are found to be important almost for all the f i ve reflections.

On the path integral formulation and the evaluation of quantum statistical averagesFour problems of physical interest have been solved in this thesis using the path integral formalism. Using the trigonometric expansion method of Burton and de Borde (1955), we found the kernel for two interacting one dimensional oscillators• The result is the same as one would obtain using a normal coordinate transformation, We next introduced the method of Papadopolous (1969), which is a systematic perturbation type method specifically geared to finding the partition function Z, or equivalently, the Helmholtz free energy F, of a system of interacting oscillators. We applied this method to the next three problems considered• First, by summing the perturbation expansion, we found F for a system of N interacting Einstein oscillators^ The result obtained is the same as the usual result obtained by Shukla and Muller (1972) • Next, we found F to 0(Xi)f where A is the usual Tan Hove ordering parameter* The results obtained are the same as those of Shukla and Oowley (1971), who have used a diagrammatic procedure, and did the necessary sums in Fourier space* We performed the work in temperature space• Finally, slightly modifying the method of Papadopolous, we found the finite temperature expressions for the Debyecaller factor in Bravais lattices, to 0(AZ) and u(/K/ j,where K is the scattering vector* The high temperature limit of the expressions obtained here, are in complete agreement with the classical results of Maradudin and Flinn (1963) .

Optical properties of organic superconductor K(BETS)2FeBr4 //c(BETS)2FeBr4 is the first antiferromagnetic organic superconductor with successive antiferromagnetic and superconducting transitions at Ta^=2.5K and Tc=l.lK respectively at ambient pressure. Polarized reflectance measurements were performed on three single crystalsamples of this material using a Briiker IFS66V/S Interferometer, and a Bolometer detector or an MCT detector, at seven temperatures between 4K and 300K, in both the farinfrared and midinfrared frequency range. After the reflectance results were obtained, the KramersKronig dispersion relation was apphed to determine the optical conductivity of /c(BETS)2FeBr4 at these seven temperatures. Additionally, the optical conductivity spectra were fitted with a Drude/Lorentz Oscillator model in order to study the evolution of the optical conductivity with temperature along the aaxis and caxis. The resistivities calculated from the Drude model parameters along the aaxis and caxis agreed reasonably with previous transport measurements.

Optical properties of organic superconductor K(BETS)2FeBr4 /K(BETS)2FeBr4 is a quasi2D charge transfer organic metal with interesting electronic and magnetic properties. It undergoes a transition to an antiferromagnetic (AF) state at ambient pressure at the Neel temperature (T^^) = 2.5 K, as well as to a superconducting (SC) state at 1.1 K [1]. The temperature dependence of the electrical resistivity shows a small decrease at T;v indicating the resistivity drops as a result of the onset of the ordering of Fe'*''" spins. A sharp drop in the resistivity at 1.1 K is due to its superconducting transition. The temperature dependence of the susceptibility indicates an antiferromagnetic spin structure with the easy axis parallel to the aaxis. The specific heat at zerofield shows a large peak at about 2.4 K, which corresponds to the antiferromagnetic transition temperature (Tat) and no anomaly is observed around the superconducting transition temperature (1.1 K) demonstrating that the magnetically ordered state is not destroyed by the appearance of another phase transition (the superconducting transition) in the 7relectron layers [1], [2]. This work presents an investigation of how the low frequency electromagnetic response is affected by the antiferromagnetic and superconducting states, as well as the onset of strong correlation. The location of the easy axis of three samples was determined and polarized thermal reflectance measurements of these «(BETS)2FeBr4 samples oriented with their vertical axis along the a and c axes were then carried out using a *He refrigerator cryostat and a MartinPuplett type polarizing interferometer at various temperatures (T = 0.5 K, 1.4 K. 1.9 K, 2.8 K) above and below the superconducting state and/or antiferromagnetic state. Comparison of the SC state to the normal state along the o and caxes indicates a rising thermal reflectance at low frequencies (below 10 cm"' ) which may be a manifestation of the superconducting energy gap. A dipHke feature is detected at low frequencies (below 15 cm"') in the thermal reflectance plots which probe the antiferromagnetic state along the two axes, and may be due to the opening of a gap in the excitation spectrum as a result of the antiferromagnetism. In another set of experiments, thermal reflectance measurements carried out along the a and caxes at higher temperatures (10 K80 K) show that the reflectivity decreases with increasing temperature to 60 K (the coherence temperature) above which it increases again. Comparison of the thermal reflectance plots along the a and caxes at higher temperatures reveals an anisotropy between these two axes. The HagenRubens thermal reflectance plots corresponding to an average over the acplane were calculated using experimental hterature resistivity values. Comparison of the HagenRubens plots with the experimental thermal reflectance along the a and caxes indicates that both exhibit the general trend of a decrease in thermal reflectance with increasing frequency, however the calculated HagenRubens thermal reflectance at different temperatures is much lower than the experimental curves.

Optical Properties of Sb2Te3, and Dilute Magnetic Semiconductors Sb1.97 VO.03 Te3 and Sb1.94 CrO.06 Te3This thesis reports on the optical properties of the dilute magnetic semiconductors, Sb1.97 V 0.03 Te3 and Sb1.94Cr0.06Te3, along with the parent compound Sb2Te3' These materials develop a ferromagnetic state at low temperature with Curie temperatures of 22 K and 16 K respectively. All three samples were oriented such that the electric field vector of the light was perpendicular to the caxis. The reflectance profile of these samples in the midinfrared (500 to 3000 cm1) shows a pronounced plasma edge which retracts with decreasing temperature. The farinfrared region of these samples exhibits a phonon at ~ 60 cm1 which softens as temperature decreases. KramersKronig analysis and a DrudeLorentz model were employed to determine the optical constants of the bulk samples. The real part of the optical conductivity is shown to consist of intraband contributions at frequencies below the energy gap (~0.26 eV) and interband contributions at frequencies above the energy gap. The temperature dependence of the scattering rate show that a mix of phonon and impurity scattering are present, while the signature of traditional spin disorder (magnetic) scattering was difficult to confirm.

Optimization of the valence energy variance of the CuH moleculeWe developed the concept of split't to deal with the large molecules (in terms of the number of electrons and nuclear charge Z). This naturally leads to partitioning the local energy into components due to each electron shell. The minimization of the variation of the valence shell local energy is used to optimize a simple two parameter CuH wave function. Molecular properties (spectroscopic constants and the dipole moment) are calculated for the optimized and nearly optimized wave functions using the Variational Quantum Monte Carlo method. Our best results are comparable to those from the single and double configuration interaction (SDCI) method.