View All Items

Title: POWER DISTRIBUTION OF TERAHERTZ EMISSION FROM HEXAGONAL BSCCO MICROSTRIP ANTENNAS.
Creator: Davis, Andrew E, Klemm, Richard, University of Central Florida
Abstract / Description: We analyze the distribution of coherent terahertz radiation from a regular hexagonal microstrip antenna (MSA) made from the high-Tc superconductor Bi2Sr2CaCu2O8+x (BSCCO). We discuss the C6v symmetry of the solutions of the wave equation on a hexagonal domain and distinguish between the closed-form and non-closed-form solutions. The closed-form wavefunctions of the transverse magnetic (TM) electromagnetic cavity modes are presented and formulas for the radiated power arising from the uniform...
Show moreWe analyze the distribution of coherent terahertz radiation from a regular hexagonal microstrip antenna (MSA) made from the high-Tc superconductor Bi2Sr2CaCu2O8+x (BSCCO). We discuss the C6v symmetry of the solutions of the wave equation on a hexagonal domain and distinguish between the closed-form and non-closed-form solutions. The closed-form wavefunctions of the transverse magnetic (TM) electromagnetic cavity modes are presented and formulas for the radiated power arising from the uniform part of the AC Josephson current and from the resonant cavity modes are derived. The wavefunctions and angular distribution of radiation from both sources are plotted for sixteen of the lowest-energy modes. Finally, we comment on the relevance of these power distributions to hexagonal arrays of equilateral triangular MSAs and propose a strategy for studying the non-closed-form modes.
Show less
Date Issued: 2017
Identifier: CFH2000241, ucf:46016
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFH2000241

Title: ANGULAR DEPENDENCE OF THE EMISSION FROM THE INTRINSIC JOSEPHSON JUNCTION IN PIE-SHAPED WEDGE TRIANGULAR BSCCO MESAS.
Creator: Morales, Manuel, Klemm, Richard, University of Central Florida
Abstract / Description: The purpose of this thesis is to determine the radiation patterns from an acute isosceles triangular superconducting mesa modeled by a pie-shaped geometry. The terahertz band lies between the microwave and infrared regions of the electromagnetic spec- trum. The terahertz radiation from atomic-scale layered superconducting mesas is caused by the tunneling of electron pairs in the ac-Josephson effect. To determine the terahertz power radiated per unit solid angle of an acute isosceles...
Show moreThe purpose of this thesis is to determine the radiation patterns from an acute isosceles triangular superconducting mesa modeled by a pie-shaped geometry. The terahertz band lies between the microwave and infrared regions of the electromagnetic spec- trum. The terahertz radiation from atomic-scale layered superconducting mesas is caused by the tunneling of electron pairs in the ac-Josephson effect. To determine the terahertz power radiated per unit solid angle of an acute isosceles triangular superconducting mesa, a model was employed in which the shape of the mesa is approximated as a pie-shaped wedge. This model is shown to have an accuracy of about 1%. Using Love's Equivalency Principle, the current caused by the Josephson effect is then assumed to be on the edges of the mesa. Since in the mesas used for experiments the electric field is in the direction of the current, it is parallel to the boundary of the sample. Hence, we want the TM modes, and that requires the magnetic field to be transverse to the boundary. We thus require that the tangential component of the magnetic field parallel at the boundary vanishes. Love's equivalency principle provides the easiest and most straight forward way to satisfy this condition. The surface electric current density was modeled by comparing the magnetic vector potential re- sulting from the modeled edge current with that given by a standard volume average integration technique. The surface current density that provided the best approximation to the bulk average was used and the radiation patterns were plotted using Mathematica software.
Show less
Date Issued: 2015
Identifier: CFH0004762, ucf:45345
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFH0004762

Title: THE EFFECT OF IMPURITIES ON THE SUPERCONDUCTIVITY OF BSCCO-2212.
Creator: Vastola, John, Klemm, Richard, University of Central Florida
Abstract / Description: BSCCO-2212 is a high-temperature cuprate superconductor whose microscopic behavior is currently poorly understood. In particular, it is unclear whether its order parameter is consistent with s-wave or d-wave symmetry. It has been suggested that its order parameter might take one of several forms that are consistent with d-wave behavior. We present some calculations using the many-body theory approach to superconductivity that suggest that such order parameters would lead to a suppression of...
Show moreBSCCO-2212 is a high-temperature cuprate superconductor whose microscopic behavior is currently poorly understood. In particular, it is unclear whether its order parameter is consistent with s-wave or d-wave symmetry. It has been suggested that its order parameter might take one of several forms that are consistent with d-wave behavior. We present some calculations using the many-body theory approach to superconductivity that suggest that such order parameters would lead to a suppression of the critical temperature in the presence of impurities. Because some experiments have suggested the critical temperature of BSCCO-2212 is relatively independent of the concentration of impurities, this lends support to the hypothesis that its order parameter has s-wave symmetry.
Show less
Date Issued: 2016
Identifier: CFH2000073, ucf:45541
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFH2000073

Title: TERAHERTZ RADIATION FROM HIGH-TEMPERATURE SUPERCONDUCTING BSCCO MESAS OF VARIOUS GEOMETRIES.
Creator: Cerkoney, Daniel, Klemm, Richard, University of Central Florida
Abstract / Description: The purpose of this thesis is to examine the radiation from high-temperature superconducting mesas of Bi2Sr2CaCu2O8+d (BSCCO). This is motivated by the need for coherent sources of continuous wave terahertz (THz) emission capable of radiating practically in the THz frequency band. As BSCCO has been shown to be tunable from 0.5-2.4 THz (i.e., through the entire so-called terahertz gap centered about 1 THz), and has a higher peak operating temperature near 1 THz than most alternative sources,...
Show moreThe purpose of this thesis is to examine the radiation from high-temperature superconducting mesas of Bi2Sr2CaCu2O8+d (BSCCO). This is motivated by the need for coherent sources of continuous wave terahertz (THz) emission capable of radiating practically in the THz frequency band. As BSCCO has been shown to be tunable from 0.5-2.4 THz (i.e., through the entire so-called terahertz gap centered about 1 THz), and has a higher peak operating temperature near 1 THz than most alternative sources, it is a good candidate for imaging and spectroscopy device applications. When a static DC voltage is applied to a BSCCO mesa, the stack of Josephson junctions intrinsic to this type-II layered superconductor synchronously radiate. Adjustment of the bath temperature and applied voltage allows for the high degree of tunability observed for such an emitter. To determine the angular dependence of radiation from BSCCO mesas, the dual source model from antenna theory is employed, and Love's equivalence principle is used to simplify this framework. The total emission power obtained in this manner for the pie-shaped wedge is then fit to experimental results for a thin isosceles triangular mesa using the method of least squares, resulting in a standard deviation of 0.4657. Additionally, symmetry is shown to play a significant role in the emissions for the transverse magnetic (TM) cavity modes of the equilateral triangular mesa. When the full group symmetry is imposed, the density of allowed modes is heavily diminished, and the original first excited even mode becomes the C3v symmetric ground state. These results for the equilateral triangle suggest, along with earlier experiments on the regular pentagonal mesa, that symmetry breaking effects can be used for purposes of tuning the characteristic frequency and angular dependence of the power radiated from BSCCO mesas with a high degree of symmetry.
Show less
Date Issued: 2015
Identifier: CFH0004898, ucf:45429
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFH0004898

Title: Terahertz Emission from the Intrinsic Josephson Junctions of High-Symmetry Thermally-Managed Bi2Sr2CaCu2O8+d Annular Microstrip Antennas.
Creator: Bonnough, Sheila, Klemm, Richard, Peale, Robert, Shivamoggi, Bhimsen, University of Central Florida
Abstract / Description: The intrinsic Josephson junctions in the high transition temperature superconductor Bi2Sr2CaCu2O8+? (BSCCO) have shown great potential for oscillators emitting in the terahertz frequency. The radiation frequency satisfies the conditions for both the ac Josephson effect and for a mesa cavity resonance mode. The observed angular dependence of the emissions from some mesa imply that the ac Josephson effect plays the primary role in a dual source radiation mechanism. But the integrated emission...
Show moreThe intrinsic Josephson junctions in the high transition temperature superconductor Bi2Sr2CaCu2O8+? (BSCCO) have shown great potential for oscillators emitting in the terahertz frequency. The radiation frequency satisfies the conditions for both the ac Josephson effect and for a mesa cavity resonance mode. The observed angular dependence of the emissions from some mesa imply that the ac Josephson effect plays the primary role in a dual source radiation mechanism. But the integrated emission power had generally been significantly below the 1 mW level suitable for many applications. This output power can be enhanced by a suitable design of an array of suitably shaped mesas that are all within a wavelength of each other so that their combined output is coherent. One such tightly packed array consists of concentric annuli. Here we calculate the angularly independent modes of thin annular microstrip antennas, with the ratio of the inner to the outer radii varying from 0.1 to 0.9. We then calculate the angular distribution of the emission power arising from the annular cavity modes and from the uniform ac Josephson current source at the frequencies of the cavity modes. We also calculate the five leading wavefunctions with the lowest order angular dependence for those annuli.
Show less
Date Issued: 2018
Identifier: CFE0007314, ucf:52137
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007314

Title: Light Matter Interaction in Single Molecule Magnets.
Creator: Cebulka, Rebecca, Del Barco, Enrique, Klemm, Richard, Mucciolo, Eduardo, Luis, Fernando, University of Central Florida
Abstract / Description: This dissertation includes a series of experimental realizations which focus on studying the coupling between photons and single-molecule magnets (SMMs) in both the weak and strong coupling regimes. In the weak coupling regime, the aim is to achieve coherent control over the time evolution of the spin of SMMs while applying rapid microwave pulses at sub-Kelvin temperatures, where polarization of the spin bath may be achieved without large magnetic fields, allowing the suppression of dipolar...
Show moreThis dissertation includes a series of experimental realizations which focus on studying the coupling between photons and single-molecule magnets (SMMs) in both the weak and strong coupling regimes. In the weak coupling regime, the aim is to achieve coherent control over the time evolution of the spin of SMMs while applying rapid microwave pulses at sub-Kelvin temperatures, where polarization of the spin bath may be achieved without large magnetic fields, allowing the suppression of dipolar dephasing. The continuing results of this experiment will be to provide a window into fundamental sources of decoherence in single-crystal SMMs in an energy range not thoroughly investigated. We expect that these conditions would allow us to study the quantum dynamics of the spins as governed by the intrinsic molecular magnetic anisotropy, which should give rise to non-well-defined Rabi oscillations of the spin state, including metastable precessional spin states. In the strong coupling regime, high quality factor superconducting CPW resonators have been designed and fabricated to investigate the vacuum Rabi splitting between a photon and the SMM spin. The proposed setup will permit measurements of coherent collective coupling between molecular spins and a low number of photons, ideally down to a single photon. This experiment may ultimately provide the opportunity for reaching the strong coupling regime with a single spin. Finally, this thesis also documents a research study into the impact of service-learning methodology on students' depth of learning and critical thinking skills during a novel nanoscale science and technology course offered in the UCF Physics Dept. The overall learning of students was assessed and results clearly showed improvement in both multiple choice pre/post-tests and critical reflection papers. We associate this improvement at least partially to the service-learning experience.
Show less
Date Issued: 2019
Identifier: CFE0007442, ucf:52728
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007442

Title: The Consequences of a Reduced Superlattice Thickness on Quantum Cascade LASER Performance.
Creator: Figueiredo, Pedro, Lyakh, Arkadiy, Peale, Robert, Klemm, Richard, Fathpour, Sasan, University of Central Florida
Abstract / Description: Coherent infrared radiation sources are essential for the operability of a wide range of scientific, industrial, military and commercial systems. The importance of the mid-infrared spectral region cannot be understated. Numerous molecules have some vibrational band in this range, allowing for identification of species by means of absorption, emission or some other form of spectroscopy. As such, spectroscopy alone has numerous applications ranging from industrial process control to disease...
Show moreCoherent infrared radiation sources are essential for the operability of a wide range of scientific, industrial, military and commercial systems. The importance of the mid-infrared spectral region cannot be understated. Numerous molecules have some vibrational band in this range, allowing for identification of species by means of absorption, emission or some other form of spectroscopy. As such, spectroscopy alone has numerous applications ranging from industrial process control to disease diagnosis utilizing breath analysis. However, despite the discovery of the LASER in the 60s, to this day the amount of coherent sources in this range is limited. It is for this reason that the quantum cascade laser has gained such momentum over the past 23 years.Quantum Cascade LASERS (QCL) are semiconductor LASERS which are based on the principle of bandgap engineering. This incredible technique is a testament to the technological maturity of the semiconductor industry. It has been demonstrated that by having precise control of individual material composition (band gap control), thicknesses on the order of monolayers, and doping levels for each individual layer in a superlattice, we have unprecedented flexibility in designing a LASER or detector in the infrared. And although the technology has matured since it's discovery, there still remain fundamental limitations on device performance. In particular, active region overheating limits QCL performance in a high duty cycle mode of operation.In this dissertation, along with general discussion on the background of the QCL, we propose a solution of where by limiting the growth of the superlattice to a fraction of typical devices, we allow for reduction of the average superlattice temperature under full operational conditions. The consequences of this reduction are explored in theory, experiment and system level applications.
Show less
Date Issued: 2017
Identifier: CFE0006592, ucf:51273
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006592

Title: Saturn's Rings: Measuring Particle Size Distributions Using Cassini UVIS Occultation Data.
Creator: Becker, Tracy, Colwell, Joshua, Fernandez, Yan, Campins, Humberto, Showalter, Mark, Klemm, Richard, University of Central Florida
Abstract / Description: Since its arrival to Saturn in 2004, the Cassini spacecraft has utilized its suite of sophisticated instruments to further our understanding of the Saturnian ring system. We analyze occultation data from Cassini's Ultraviolet Imaging Spectrograph (UVIS) in order to measure the particle size distribution and place limits on the minimum particle sizes in Saturn's rings.Throughout the ring system, particle accretion is countered by collisional and tidal disruption and Keplerian shear. Therefore,...
Show moreSince its arrival to Saturn in 2004, the Cassini spacecraft has utilized its suite of sophisticated instruments to further our understanding of the Saturnian ring system. We analyze occultation data from Cassini's Ultraviolet Imaging Spectrograph (UVIS) in order to measure the particle size distribution and place limits on the minimum particle sizes in Saturn's rings.Throughout the ring system, particle accretion is countered by collisional and tidal disruption and Keplerian shear. Therefore, the particle size distribution of the rings is continually evolving. The presence of sub-centimeter particles, which have short lifetimes due to these processes, is indicative of ongoing dynamics in the rings. Sub-centimeter-sized particles efficiently diffract light at ultraviolet wavelengths, and thus produce signatures of diffraction in the occultation data. The shape and intensity of the diffraction signatures are indicative of the sizes of the particles that produce them. The UVIS wavelength bandpass, 51.2 - 180 nm, contains the shortest wavelengths of the Cassini instruments, making it most sensitive to the smallest particles in the rings. We have developed a computational model that reconstructs the geometry of a UVIS observation and produces a synthetic diffraction signal for a given truncated power-law particle size distribution, which we compare with the observed signal. We implement this model for two sets of observations: (1) diffraction spikes at sharp ring edges during stellar occultations and (2) the light curve due to attenuated and diffracted sunlight by particles in Saturn's F ring during solar occultations. Near sharp ring edges, ring particles can diffract light such that there is a measurable increase in the signal of an unocculted star exterior to the ring. In Saturn's A ring, diffracted light can augment the stellar signal by up to 6% and can be detected tens of kilometers radially beyond the edge. The radial profile of the diffraction signal is dependent on the size distribution of the particle population near the ring edge. These diffraction signals are observed at sharp edges throughout Saturn's rings, although in this work we focus on diffraction at the outer edge of Saturn's A ring and at the edges of the Encke Gap. We find an overall steepening of the power-law size distribution and a decrease in the minimum particle size at the outer edge of the A ring when compared with the Encke Gap edges. This suggests that interparticle collisions caused by satellite perturbations in the region result in more shedding of regolith or fragmentation of particles in the outermost parts of the A ring. We rule out any significant population of sub-millimeter-sized particles in Saturn's A ring, placing a lower limitation of 1-mm on the minimum particle size in the ring.We also model the light curves produced as Saturn's F ring occults the Sun. We consider both the attenuated signal and the light diffracted by the particles in the ring during the occultation. Five of the eleven solar occultations analyzed show a clear signature of diffracted light that surpasses the unocculted solar signal. This includes a misaligned solar occultation that placed most of the solar disk outside of the instrument's field of view, reducing the solar signal by 97.5% and resulting in the serendipitous detection of diffracted light. We measure a large variation in the the size distribution of the particles that fill the broad, ~500 km region surrounding the F ring core. We find that smaller particles ((<) 50 micrometers) are present during solar occultations for which diffraction was detected, and place a lower limit on the minimum particle size of 100 micrometers for occultations during which diffraction was not detected. A comparison with images of the F ring observed by the Cassini Imaging Science Subsystem near the times of the occultations reveals that the detections of small particles in the UVIS data correspond with locations of collisional events in the F ring. This implies that collisions within the F ring core replenish the sub-millimeter-sized dust in the 500-km region that encompasses the F ring core.
Show less
Date Issued: 2016
Identifier: CFE0006073, ucf:50940
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006073

Title: Studying Short-Period Comets and Long-Period Comets Detected by WISE/NEOWISE.
Creator: Kramer, Emily, Fernandez, Yan, Colwell, Joshua, Kokoouline, Viatcheslav, Klemm, Richard, Lisse, Carey, University of Central Florida
Abstract / Description: The Wide-field Infrared Survey Explorer (WISE) mission surveyed the sky in four infrared wavelength bands (3.4, 4.6, 12 and 22 ?m) between January 2010 and February 2011. During the mission, WISE serendipitously observed 160 comets, including 21 newly discovered objects. About 89 of the comets observed by WISE displayed a significant dust tail in the 12 and 22 ?m (thermal emission) bands, showing a wide range of activity levels and dust morphology. Since the observed objects are a mix of both...
Show moreThe Wide-field Infrared Survey Explorer (WISE) mission surveyed the sky in four infrared wavelength bands (3.4, 4.6, 12 and 22 ?m) between January 2010 and February 2011. During the mission, WISE serendipitously observed 160 comets, including 21 newly discovered objects. About 89 of the comets observed by WISE displayed a significant dust tail in the 12 and 22 ?m (thermal emission) bands, showing a wide range of activity levels and dust morphology. Since the observed objects are a mix of both long-period comets (LPCs) and short-period comets (SPCs), differences in their activity can be used to better understand the thermal evolution that each of these populations has undergone. For the comets that displayed a significant dust tail, we have estimated the sizes and ages of the particles using dynamical models based on the Finson-Probstein method [Finson and Probstein, 1968]. For a selection of 40 comets, we have then compared these models to the data using a novel tail-fitting method that allows the best-fit model to be chosen analytically rather than subjectively. For comets that were observed multiple times by WISE, the particle properties were estimated separately, and then compared. We find that the dust tails of both LPCs and SPCs are primarily comprised of ?mm-cm sized particles, which were the result of emission that occurred several months to several years prior to the observations. The LPCs nearly all have strong emission close to the comet's perihelion distance, and the SPCs mostly have strong emission close to perihelion, but some have strong emission well before perihelion.
Show less
Date Issued: 2014
Identifier: CFE0005823, ucf:50938
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005823

Title: Microscopic Theory of the Knight Shift.
Creator: Hall, Bianca, Klemm, Richard, Fernandez, Yan, Rahman, Talat, Del Barco, Enrique, Shivamoggi, Bhimsen, University of Central Florida
Abstract / Description: This dissertation is the beginning of the development of a microscopic theory of the Knight shift. The Knight shift experiment has been used in superconductivity research throughout history, however, a complete understanding of the Knight shift in conventional as well as unconventional superconductors does not yet exist. Motivated by the results of a literature review, which discusses Knight shift anomalies in multiple superconducting materials, this research studies a new model of the Knight...
Show moreThis dissertation is the beginning of the development of a microscopic theory of the Knight shift. The Knight shift experiment has been used in superconductivity research throughout history, however, a complete understanding of the Knight shift in conventional as well as unconventional superconductors does not yet exist. Motivated by the results of a literature review, which discusses Knight shift anomalies in multiple superconducting materials, this research studies a new model of the Knight shift, which involves the processes involved in nuclear magnetic resonance measurements in metals.The result of this study is a microscopic model of nuclear magnetic resonance in metals. The spins of the spin-1/2 local nucleus and its surrounding orbital electrons interact with the arbitrary constant ${\bf B}_0$ and perpendicular time-oscillatory magnetic inductions ${\bf B}_1(t)$ and with each other via an anisotropic hyperfine interaction. An Anderson-like Hamiltonian describes the excitations of the relevant occupied local orbital electrons into the conduction bands, each described by an anisotropic effective mass with corresponding Landau orbits and an anisotropic spin ${\bf g}$ tensor. Local orbital electron correlation effects are included using the mean-field decoupling procedure of Lacroix. The metallic contributions to the Knight shift resonance frequency and linewidth shifts are evaluated to leading orders in the hyperfine and Anderson excitation interactions. While respectively proportional to $(B_1/B_0)^2$ and a constant for weak $B_0(>)(>)B_1$, both shifts are shown to depend strongly upon ${\bf B}_0$ when a Landau level is near the Fermi energy.
Show less
Date Issued: 2015
Identifier: CFE0005954, ucf:50808
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005954

Title: Design and Engineering of Ultrafast Amplifier Systems.
Creator: Webb, Benjamin, Richardson, Martin, Chang, Zenghu, Delfyett, Peter, Gaume, Romain, Shah, Lawrence, Klemm, Richard, University of Central Florida
Abstract / Description: Recently, the design and engineering of ultrafast laser systems have led to an extraordinary increase in laser power and performance which have brought about advances in many fields such as medicine, material processing, communications, remote sensing, spectroscopy, nonlinear optics, and atomic physics. In this work, several ultrafast amplification techniques -- including chirped-pulse amplification (CPA), optical parametric chirped-pulse amplification (OPCPA), and divided-pulse amplification...
Show moreRecently, the design and engineering of ultrafast laser systems have led to an extraordinary increase in laser power and performance which have brought about advances in many fields such as medicine, material processing, communications, remote sensing, spectroscopy, nonlinear optics, and atomic physics. In this work, several ultrafast amplification techniques -- including chirped-pulse amplification (CPA), optical parametric chirped-pulse amplification (OPCPA), and divided-pulse amplification (DPA) -- are described and demonstrated in the design and construction of two ultrafast laser facilities. An existing Ti:Sapphire laser system was completely redesigned with an increased power of 10 TW for experiments capable of generating hundreds of laser filaments in ordered arrays. The performance of DPA above the Joule-level was investigated in a series of experiments utilizing various DPA schemes with gain-saturated amplifiers at high pulse energy. A new high energy OPCPA facility has been designed and its pump laser system constructed, utilizing the technique of DPA for the first time in a flashlamp-pumped amplifier chain and with a record combined energy of 5 Joules in a 230 ps pulse duration. The demonstrated OPCPA pump performance will allow for the generation of 50 TW quasi-single cycle 5 fs pulses at 2.5 Hz from a table-top OPCPA system.
Show less
Date Issued: 2016
Identifier: CFE0006547, ucf:51349
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006547

Title: Dynamical Formation of Protoplanetesimals.
Creator: Whizin, Akbar, Colwell, Joshua, Fernandez, Yan, Klemm, Richard, Lewis, Mark, Moore, Brian, University of Central Florida
Abstract / Description: The seeds of planetesimals that formed in the gaseous protoplanetary disk (PPD) have many barriers to overcome in their growth from millimeter to meter-sized and larger bodies. Centimeter-sized aggregates are weakly bound and self-gravity is almost non-existent so surface forces play a critical role in holding small loosely-bound rubble-piles together. Their orbital motions and effects form disk processes impart relative velocities leading to collisions so understanding the macroscopic disk...
Show moreThe seeds of planetesimals that formed in the gaseous protoplanetary disk (PPD) have many barriers to overcome in their growth from millimeter to meter-sized and larger bodies. Centimeter-sized aggregates are weakly bound and self-gravity is almost non-existent so surface forces play a critical role in holding small loosely-bound rubble-piles together. Their orbital motions and effects form disk processes impart relative velocities leading to collisions so understanding the macroscopic disk environment is also necessary. To this end we analyze the dynamics of particles in Saturn's F ring as an analogue to understanding the orbital evolution of proto-planetesimals embedded in a PPD. We also study how the mechanical, material, and collisional properties affect the dynamical accretion of cm-sized bodies. The collisional outcomes can be determined by a set of definable collision parameters, and experimental constraints on these parameters will improve formation models for planetesimals. We have carried out a series of microgravity laboratory collision experiments of small aggregates to determine under what conditions collisional growth can occur for protoplanetary aggregates. We measure coefficients of restitution, sticking and fragmentation thresholds, compressive strengths, and sticking probabilities for collision velocities of 1 - 200 cm/s, then compare the results of our experiments with results from a collisional N-body code that includes adhesion between particles. We find that cm-sized aggregates are very weakly bound and require high internal cohesion to avoid fragmentation in agreement with simulations. The threshold for sticking is found to be under 10 cm/s and the fragmentation threshold near 1 m/s. Quiescent regions in the mid-plane of the disk may cultivate abnormally low relative velocities permitting sticking to occur (~1 cm/s), however, without a well-defined path to formation it is difficult to determine whether collisional accretion as a mechanism can overcome low thresholds for sticking and fragmentation. We discuss this research's implications to both the meter-barrier and planetesimal formation.
Show less
Date Issued: 2016
Identifier: CFE0006196, ucf:51103
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006196

Title: Quantification of non-stoichiometry and impurities in transparent YAG ceramics by laser-induced breakdown spectroscopy (LIBS).
Creator: Pandey, Sudeep, Gaume, Romain, Coffey, Kevin, Klemm, Richard, Baudelet, Matthieu, Curtarolo, Stefano, University of Central Florida
Abstract / Description: Transparent ceramics are an important class of optical materials with applications in street-lighting, high-strength windows, electro- and magneto-optical isolators, high-power laser gain media and nuclear radiation detectors. Compared to single-crystal growth, ceramic processing enables size scalability, near net-shape forming and prevents issues associated with dopant segregation and inhomogeneity, such as stress-induced birefringence and wavefront distortions. The fabrication of high...
Show moreTransparent ceramics are an important class of optical materials with applications in street-lighting, high-strength windows, electro- and magneto-optical isolators, high-power laser gain media and nuclear radiation detectors. Compared to single-crystal growth, ceramic processing enables size scalability, near net-shape forming and prevents issues associated with dopant segregation and inhomogeneity, such as stress-induced birefringence and wavefront distortions. The fabrication of high optical grade ceramics by route of powder sintering, relies on a controlled set of techniques preventing the formation of scattering centers (pores and secondary phases) and harmful point defects (color centers, charge-carrier trapping sites). This thesis work investigates a novel approach in assisting the fabrication of yttrium aluminum garnet (YAG, Y3Al5O12) transparent ceramics, an important laser material, and minimizing the presence of these defects. As a line compound in the Al2O3-Y2O3 phase diagram, YAG has little tolerance for excess of either yttrium or aluminum oxides. What is more, the estimated compositional range of the garnet phase, (5/3-0.03)(<)Al/Y(<)(5/3+0.008), which is at the root of fabrication inconsistencies, challenges the sensitivity of most analytical techniques. We have evaluated the use of laser-induced breakdown spectroscopy (LIBS), a rapid, cost effective, non-destructive, and versatile technique, in the determination of stoichiometry and impurities at the various stages of the ceramic fabrication, i.e. in powders, green and sintered bodies. It was found that enough sensitivity and accuracy can be achieved on a custom-built system to discern 0.3 mole percent in the Al/Y ratio. To understand the influence of the plasma temperature on the ratio of the atomic emission lines of Al and Y species, simulations of YAG-based laser-induced plasmas were performed. The results have guided our experimental protocol by showing that above 12000 K, the Al/Y intensity ratio and thus the sensitivity of the measurement increases sharply with plasma temperature. In addition, we show that LIBS can be used to monitor the concentrations of unintentional trace impurities along those of sintering additives (SiO2) customarily used for the removal of porosity during firing. Hence, we reveal, for example, that less than 30% of SiO2 remains in the final ceramic due to evaporation during high temperature sintering. This work not only extends the range of capabilities of LIBS by showing how highly sensitive quantification of major elements can be performed in insulating materials, but also provides a new set of tools for estimating the range of solid-state solutions in advanced materials and understanding the densification of ceramics. We foresee that such capability will be invaluable for quality control purposes and in areas where fine and reproducible compositional tuning (defect engineering) is needed.
Show less
Date Issued: 2017
Identifier: CFE0006774, ucf:51855
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006774

Title: Role of internal degrees of freedom in the quantum tunneling of the magnetization in single-molecule magnets.
Creator: Quddusi, Hajrah, Gonzalez Garcia, Enrique, Mucciolo, Eduardo, Klemm, Richard, Schoenfeld, Winston, University of Central Florida
Abstract / Description: The prominent features of single molecule magnets (SMMs), such as the quantum tunneling of the magnetization (QTM), are conventionally understood through the giant spin approximation (GSA) which considers the molecule as a single rigid spin. This model often requires the inclusion of high order anisotropy terms in the Hamiltonian, a manifestation of admixing of low lying excited states that can be more naturally understood by employing a multi-spin (MS) description i.e. considering the...
Show moreThe prominent features of single molecule magnets (SMMs), such as the quantum tunneling of the magnetization (QTM), are conventionally understood through the giant spin approximation (GSA) which considers the molecule as a single rigid spin. This model often requires the inclusion of high order anisotropy terms in the Hamiltonian, a manifestation of admixing of low lying excited states that can be more naturally understood by employing a multi-spin (MS) description i.e. considering the individual spins and the interactions between ions within the molecule. However, solving the MS Hamiltonian for high nuclearity molecules is not feasible due to the enormous dimensions of the associated Hilbert space that put it beyond the capability of existing computational resources. In contrast, low nuclearity systems permit the complete diagonalization of the MS Hamiltonian required to sample the effect of internal degrees of freedom, such as exchange interactions and single ion anisotropies, on the QTM. This dissertation focuses on the study of low nuclearity SMMs in view of understanding these subtle quantum effects. To accomplish this, we have developed a series of magnetic characterization techniques, such as integrated microchip sensors resulting from the combination of two dimensional electron gas (2DEG) Hall-Effect magnetometers and microstrip resonators, capable of performing measurements of magnetization and EPR spectroscopy simultaneously. The thesis bases on a comparative study of two low nuclearity SMMs with identical magnetic cores (Mn4 dicubane) but differing ligands. Notably, one of these SMMs lacked solvent molecules for crystallization; a characteristic that gives rise to extremely sharp resonances in the magnetization loops and whose basic QTM behavior can be well explained with the GSA. On the contrary, the second SMM exhibited mixed energy levels, making a MS description necessary to explain the observations. We have also examined the role of internal degrees of freedom on more subtle QTM phenomena, leading to the explanation of asymmetric Berry-phase interference patterns observed in a Mn4 SMM in terms of a competition between different intermolecular magnetic interactions, i.e. non-collinear zero-field splitting tensors and intramolecular dipolar interactions, resulting in astonishing manifestations of the structural molecular symmetry on the quantum dynamics of the molecular spin.
Show less
Date Issued: 2012
Identifier: CFE0004790, ucf:49722
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004790

Title: Exchange coupling in molecular magnets: Zero, one and three dimensions.
Creator: Amjad, Asma, Gonzalez Garcia, Enrique, Klemm, Richard, Peale, Robert, Hill, Stephen, University of Central Florida
Abstract / Description: Molecular magnets with different dimensionality, whether they are zero-dimensional single-molecule magnets (SMM) or one-dimensional single-chain magnets (SCM) are very interesting, since they allow probing the fundamental aspects bordering quantum and classical physics at the nanoscale level. This dissertation covers experimental studies of two Mn-based exchange-coupled molecule-based magnets and two Co-based single-chain magnets, using both dc Hall-effect magnetometry and electron paramagnet...
Show moreMolecular magnets with different dimensionality, whether they are zero-dimensional single-molecule magnets (SMM) or one-dimensional single-chain magnets (SCM) are very interesting, since they allow probing the fundamental aspects bordering quantum and classical physics at the nanoscale level. This dissertation covers experimental studies of two Mn-based exchange-coupled molecule-based magnets and two Co-based single-chain magnets, using both dc Hall-effect magnetometry and electron paramagnet resonance (EPR) techniques. In these multi-dimensional systems, the spin of the molecule exhibits quantum mechanical behavior at low temperature. It is quite interesting to observe the effect of magnetic exchange interactions on the magnetic properties of various complexes; hence they strongly affect the magnetic behavior.In this dissertation, the research is initiated with the study of low-magnetic-nuclearity molecules, starting with a spectroscopic study of a significantly anisotropic Mn(IV) monomer. At low temperature the molecule possesses easy-plane type anisotropy of a remarkable magnitude. Although the molecule is not a single-molecule magnet, the remarkable anisotropy can initiate synthesis of newer and better molecular magnets with Mn(IV) as the main building block. Furthermore, the interplay between the magnetic anisotropy and the inter-ion exchange interactions (J) within the molecule are probed for a dimer and a trimer where the magnetic core is comprised of two and three ions respectively. In the Mn-based case of the dimer, the low coupling between the atoms leads to significant state mixing, thus making it impossible to assign the individual spin states to the dimer or to the respective individual Mn(II) ions. In the case of the trimer, lowering of the symmetry achieved by fine tuning of the inter-ion exchange interactions leads to relieving of frustration in the antiferromagnetic (AF) triangular Mn(III) system, resulting in a well defined ground state and significant zero field splitting. Also a clear hysteretic behavior observed in this system demonstrates its SMM nature at low temperature. Finally, high-field high-frequency magnetic and spectroscopic studies performed on two cobalt-based SCMs reveal that formation of magnetic domains by exchange interactions within the chain are strongly influenced by thermal fluctuations. The chain possesses a uniaxial anisotropy with the quantization axis lying along the length of the chain. Moreover it is shown that modulation of the magnitude of inter- and intra-chain interactions results in a three-dimensional dynamics in one of the samples. Interestingly, detailed dc magnetic studies show a tunable crossover between one- and three-dimensional magnetic dynamics as a function of temperature and/or magnetic field sweep rate. Our voyage through several molecular systems of different dimensionality have allowed us to expand our understanding of the role of exchange interactions on the magnetic behavior in molecular magnetism.
Show less
Date Issued: 2013
Identifier: CFE0004806, ucf:49723
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004806

Title: Novel properties of ferromagnetic p-wave superconductors.
Creator: Lorscher, Christopher, Klemm, Richard, Leuenberger, Michael, Rahman, Talat, Schoenfeld, Winston, University of Central Florida
Abstract / Description: This thesis investigates the many extraordinary physical properties of the candidate p-wave ferromagnetic superconductors UCoGe and URhGe, and proposes theoretical predictions for p-wave superconductors yet to be discovered. In particular, we carry out angular dependent quantum field theoretical calculations of the thermodynamic H - T phase diagram known as the upper critical field, or more appropriately for ferromagnetic superconductors the upper critical induction, for various p-wave...
Show moreThis thesis investigates the many extraordinary physical properties of the candidate p-wave ferromagnetic superconductors UCoGe and URhGe, and proposes theoretical predictions for p-wave superconductors yet to be discovered. In particular, we carry out angular dependent quantum field theoretical calculations of the thermodynamic H - T phase diagram known as the upper critical field, or more appropriately for ferromagnetic superconductors the upper critical induction, for various p-wave superconducting order parameter symmetries including: The axial Anderson-Brinkman-Morel(ABM) state, the chiral Scharnberg-Klemm (SK) state, and the completely broken symmetry polar state (CBS), as well as for some other states with partially broken symmetry (PBS) superconducting order parameter symmetries. The most notable contribution of the work presented in this thesis is the application of the Klemm-Clem transformations to analytically calculate the full angular and temperature dependencies of the upper critical field for orthorhombic materials, which may prove to be useful to experimentalists in identifying these exotic states of matter experimentally. Second, this work formulates a double spin-split ellipsoidal Fermi surface (FS) model for ferromagnetic superconductors in the normal state, which introduces a field dependence to the effective mass in one crystallographic direction on the dominant Fermi surface and to the chemical potential, and is subsequently applied to the normal state of URhGe to explain theoretically the anomalous specific heat data of Aoki and Flouquet. Extension of this work to understanding the still elusive reentrant high-field superconducting phase of URhGe and the S-shaped upper critical field curve for external magnetic field parallel to the b-axis direction inUCoGe is discussed. Third, this work also presents theoretical fits to the upper critical field data of Kittika et al. for Sr2RuO4 using the helical p-wave states and including Pauli limiting effects of the three components of the triplet pair-spin fixed to the highly conducting layers by strong spin-orbit coupling.
Show less
Date Issued: 2014
Identifier: CFE0005371, ucf:50451
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005371

Title: Field Theoretic Lagrangian Stencils from Off-Shell Supermultiplet Gauge Quotients.
Creator: Katona, Gregory, Klemm, Richard, Hubsch, Tristan, Peale, Robert, Shivamoggi, Bhimsen, University of Central Florida
Abstract / Description: Recent efforts to classify off-shell representations of supersymmetry without a central charge have focused upon directed, supermultiplet graphs of hypercubic topology known as Adinkras. These encodings of Super Poincare algebras, depict every generator of a chosen supersymmetry as a node-pair transformtion between fermionic / bosonic componentfields. This research thesis is a culmination of investigating novel diagrammatic sums of gauge quotients by supersymmetric images of other Adinkras,...
Show moreRecent efforts to classify off-shell representations of supersymmetry without a central charge have focused upon directed, supermultiplet graphs of hypercubic topology known as Adinkras. These encodings of Super Poincare algebras, depict every generator of a chosen supersymmetry as a node-pair transformtion between fermionic / bosonic componentfields. This research thesis is a culmination of investigating novel diagrammatic sums of gauge quotients by supersymmetric images of other Adinkras, and the correlated building of field theoretic worldline Lagrangians to accommodate both classical and quantum venues. We find Ref [40], that such gauge quotients do not yield other stand alone or (")proper(") Adinkras as afore sighted, nor can they be decomposed into supermultiplet sums, but are rather a connected (")Adinkraic network("). Their iteration, analogous to Weyl's construction for producing all finite-dimensional unitary representations in Lie algebras, sets off chains of algebraic paradigms in discrete-graph and continuous-field variables, the links of which feature distinct, supersymmetric Lagrangian templates. Collectively, these Adiankraic series air new symbolic genera for equation to phase moments in Feynman path integrals. Guided in this light, we proceed by constructing Lagrangians actions for the N = 3 supermultiplet YI /(iDI X) for I = 1, 2, 3, where YI and X are standard, Salam-Strathdee superfields: YI fermionic and X bosonic. The system, bilinear in the component fields exhibits a total of thirteen free parameters, seven of which specify Zeeman-like coupling to external background (magnetic) fluxes. All but special subsets of this parameter space describe aperiodic oscillatory responses, some of which are found to be surprisingly controlled by the golden ratio, ? ? 1.61803, Ref [52]. It is further determined that these Lagrangians allow an N = 3 ? 4 supersymmetric extension to the Chiral-Chiral and Chiral-twisted-Chiral multiplet, while a subset admits two inequivalent such extensions. In a natural progression, a continuum of observably and usefully inequivalent, finite-dimensional off-shellrepresentations of worldline N = 4 extended supersymmetry are explored, that are variatefrom one another but in the value of a tuning parameter, Ref [53]. Their dynamics turnsout to be nontrivial already when restricting to just bilinear Lagrangians. In particular, wefind a 34-parameter family of bilinear Lagrangians that couple two differently tuned supermultiplets to each other and to external magnetic fluxes, where the explicit parameter dependence is unremovable by any field redefinition and is therefore observable. This offers the evaluation of X-phase sensitive, off-shell path integrals with promising correlationsto group product decompositions and to deriving source emergences of higher-order background flux-forms on 2-dimensional manifolds, the stacks of which comprise space-time volumes. Application to nonlinear sigma models would naturally follow, having potential use in M- and F- string theories.
Show less
Date Issued: 2013
Identifier: CFE0005011, ucf:50004
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005011

BSCCO (4)	+ -
superconductivity (3)	+ -
Superconductivity (2)	+ -
Terahertz (2)	+ -
single molecule magnets (2)	+ -

(Numerical) Algebraic Geometry (1)	+ -
(Super) Zeeman (1)	+ -
Acoustic Optic Modulator (1)	+ -
Active Region Temperature (1)	+ -
Adiankraic Tensor Product Decomposition (1)	+ -
Adinkra (1)	+ -
Algebraic Paradigms (1)	+ -
Ansatz Templates (1)	+ -
Automata (1)	+ -
Background Flux Fields (1)	+ -
Bi2Sr2CaCu2O8+? (1)	+ -
Bijective (1)	+ -
Bosonic (1)	+ -
CPA (1)	+ -
Cassini (1)	+ -
Chiral (1)	+ -
Chiral-Twisted-Chiral (1)	+ -
Cometary Dust Tails (1)	+ -
Comets (1)	+ -
Controlled Chaos (1)	+ -
DPA (1)	+ -
Diffraction (1)	+ -
F-Theory (1)	+ -
Fermionic (1)	+ -
Field Theory (1)	+ -

ETD Collection (13)	+ -
Honors Collection (4)	+ -

Klemm, Richard (17)	+ -
University of Central Florida (17)	+ -
Fernandez, Yan (4)	+ -
Peale, Robert (4)	+ -
Colwell, Joshua (3)	+ -

Shivamoggi, Bhimsen (3)	+ -
Del Barco, Enrique (2)	+ -
Gaume, Romain (2)	+ -
Gonzalez Garcia, Enrique (2)	+ -
Mucciolo, Eduardo (2)	+ -
Rahman, Talat (2)	+ -
Schoenfeld, Winston (2)	+ -
Amjad, Asma (1)	+ -
Baudelet, Matthieu (1)	+ -
Becker, Tracy (1)	+ -
Bonnough, Sheila (1)	+ -
Campins, Humberto (1)	+ -
Cebulka, Rebecca (1)	+ -
Cerkoney, Daniel (1)	+ -
Chang, Zenghu (1)	+ -
Coffey, Kevin (1)	+ -
Curtarolo, Stefano (1)	+ -
Davis, Andrew E (1)	+ -
Delfyett, Peter (1)	+ -
Fathpour, Sasan (1)	+ -
Figueiredo, Pedro (1)	+ -
Hall, Bianca (1)	+ -
Hill, Stephen (1)	+ -
Hubsch, Tristan (1)	+ -
Katona, Gregory (1)	+ -

Search In

Sort Results By:

Narrow results by:

Topical Subject

Collection

Type of Resource

Language

Creator

Format

Owner