M.Sc. Physics
http://hdl.handle.net/10464/2250
2016-05-03T08:44:43ZPhonon spectra and temperature variation of thermodynamic properties of fcc metals via Finnis-Sinclair type many body potentials: Sutton-Chen and improved Sutton-Chen models
http://hdl.handle.net/10464/5484
Phonon spectra and temperature variation of thermodynamic properties of fcc metals via Finnis-Sinclair type many body potentials: Sutton-Chen and improved Sutton-Chen models
Januszko, Aistis
Volume(density)-independent pair-potentials cannot describe metallic cohesion adequately as the presence of the free electron gas renders the total energy strongly dependent on the electron density. The embedded atom method (EAM) addresses this issue by replacing part of the total energy with an explicitly density-dependent term called the embedding function. Finnis and Sinclair proposed a model where the embedding function is taken to be proportional to the square root of the electron density. Models of this type are known as Finnis-Sinclair many body potentials.
In this work we study a particular parametrization of the Finnis-Sinclair type potential, called the "Sutton-Chen" model, and a later version, called the "Quantum Sutton-Chen" model, to study the phonon spectra and the temperature variation thermodynamic properties of fcc metals. Both models give poor results for thermal expansion, which can be traced to rapid softening of transverse phonon frequencies with increasing lattice parameter. We identify the power law decay of the electron density with distance assumed by the model as the main cause of this behaviour and show that an exponentially decaying form of charge density improves the results significantly. Results for Sutton-Chen and our improved version of Sutton-Chen models are compared for four fcc metals: Cu, Ag, Au and Pt. The calculated properties are the phonon spectra, thermal expansion coefficient, isobaric heat capacity, adiabatic and isothermal bulk moduli, atomic root-mean-square displacement and Gr\"{u}neisen parameter. For the sake of comparison we have also considered two other models where the distance-dependence of the charge density is an exponential multiplied by polynomials. None of these models exhibits the instability against thermal expansion (premature melting) as shown by the Sutton-Chen model.
We also present results obtained via pure pair potential models, in order to identify advantages and disadvantages of methods used to obtain the parameters of these potentials.
2014-06-06T00:00:00ZDesign of Resistivity Instrumentation for a He3 Cryostat and its Application to the Charge Density Wave Superconductor CuxTiSe2
http://hdl.handle.net/10464/5238
Design of Resistivity Instrumentation for a He3 Cryostat and its Application to the Charge Density Wave Superconductor CuxTiSe2
Iwachow, Jason
Fermi patches in quasi-two dimensional charge density waves (CDW) have not described the connection to superconductivity (SC) according to theory adequately at this point in time. The connection between CDW and SC in the quasi-two dimensional material CuxTiSe2 is an interesting one which might reveal mechanisms in unconventional superconductors. A previous Brock graduate student grew crystals of CuxTiSe2. The precise doping of the samples was not known. In order to determine the doping parameter x in CuxTiSe2, a sensitive resistivity measurement system was necessary. A new resistivity measurement system was designed and implemented utilizing an Infrared Labs HDL-10 He3 cryostat. By comparing with data from the literature, doping of two samples was investigated using the new measurement system and a Quantum Design Magnetic Property Measurement System (MPMS). Methods for determining the doping revealed that the old resistivity system would not be able to determine the CDW transition temperature of highly doped samples or doping for elongated samples due to electronic noise. Doping in one sample was found to be between x=0.06 and x=0.065. Values of doping in the second sample had a discrepancy but could be explained by incorrect sample orientation.
2014-03-13T00:00:00ZPlanar Topological Defects in Unconventional Superconductors
http://hdl.handle.net/10464/4987
Planar Topological Defects in Unconventional Superconductors
Przedborski, Michelle
In this work, we consider the properties of planar topological defects in unconventional
superconductors. Specifically, we calculate microscopically the interaction energy of domain walls separating degenerate ground states in a chiral p-wave fermionic superfluid. The interaction is mediated by the quasiparticles experiencing Andreev scattering at the domain walls. As a by-product, we derive a useful general expression for the free energy of an arbitrary nonuniform texture of the order parameter in terms of the quasiparticle scattering matrix. The thesis is structured as follows. We begin with a historical review of the theories of superconductivity (Sec. 1.1), which led the way to the celebrated Bardeen-Cooper-
Schrieffer (BCS) theory (Sec. 1.3). Then we proceed to the treatment of superconductors
with so-called "unconventional pairing" in Sec. 1.4, and in Sec. 1.5 we introduce the specific case of chiral p-wave superconductivity. After introducing in Sec. 2 the domain wall (DW) model that will be considered throughout the work, we derive the Bogoliubov-de Gennes (BdG) equations in Sec. 3.1, which determine the quasiparticle excitation spectrum for a nonuniform superconductor. In this work, we use the semiclassical (Andreev) approximation, and solve the Andreev equations (which are a particular case of the BdG equations) in Sec. 4 to determine the quasiparticle spectrum for both the single- and two-DW textures. The Andreev equations are derived in Sec. 3.2, and the formal properties of the Andreev scattering coefficients are discussed in the following subsection. In Sec. 5, we use the transfer matrix method to relate the interaction energy of the DWs to the scattering matrix of the Bogoliubov quasiparticles. This facilitates the derivation of an analytical expression for the interaction energy between the two DWs in Sec. 5.3. Finally, to illustrate the general applicability our method, we apply it in Sec. 6 to the interaction between phase solitons in a two-band s-wave superconductor.
2013-09-12T00:00:00ZInfrared Spectroscopy of Gadolinium
http://hdl.handle.net/10464/4275
Infrared Spectroscopy of Gadolinium
Obied, Laila
Measurements of the optical reflectivity of the normal incident light along c-axis [0001]
have been made on a Gadolinium single crystal, for temperatures between 50 K and
room temperature just above the Curie temperature of Gd, which is 293 K. And
covering the spectrum range between 100 -11000 cm-I . This work is the first study
of Gd in the far infrared range. In fact it fills the gap below 0.2 eV which has never
been measured before.
Extreme attention was paid to the fact that Gadolinium is a very reactive metal
with air. Thus, the sample was mechanically polished and carefully handled during
the measurement. However, temperature dependent optical measurements have been
made in the same frequency range for a sample of Gd2O3. For comparison, both
samples of Gd and Gd2O3 were examined by X-Ray diffraction. XRD analysis showed
that the sample was pure gadolinium and the oxide layer either does not exist, or is
very thin. Furthermore, this fact was supported by the absence of any of Gd2O3
features in the Gd sample reflectivity.
Kramers Kronig analysis was applied to extract the optical functions from the
reflectance data. The optical conductivity shows a strong temperature dependence
feature in the mid-infrared. This feature disappears completely at room temperature
which supports a magnetic origin.
2013-04-10T00:00:00Z