Research

Superconductivity

Dr Swails has developed sophisticated expertise in areas that include nanotechnology, material science, electrical engineering, thin films, lithography, spectroscopy, and computer programming. A superconducting state is formed when electrons in a metal form pairs and condense into a single macroscopic quantum state. Several peculiar properties can arise from the formation of the superconducting state such as electrical conduction without resistance, perfect diamagnetism, and the quantization of magnetic flux into elementary fluxons. The properties of perfect conduction and diamagnetism hold only for low currents and fields, and at higher current densities a superconductor becomes resistive and can dissipate energy intensely. Our group was the first to demonstrate current induced pair-breaking in a high-temperature superconductor. She studies properties of nanowires and micropatterned films of superconductors, looking at phenomena such as quantized flux vortices, phase coherence in superconducting microbridges, localization and metal-insulator transitions. The experimental facilities include a comprehensive thin-film fabrication laboratory and facilities for measurements at sub-Kelvin temperatures, high magnetic fields (the highest in the state), and extreme power densities (the highest in the world for any superconductor). In the area of dissipation in superconductors, this is one of the leading groups in the world and has discovered several new effects over the years.

Superconductivity in NbTiN superconducting films

Dr Swails and her research group systematically investigate the superconducting properties of Niobium Titanium Nitride (NbTiN) superconducting films with different geometries, which at low temperature remain superconducting. NbTiN superconducting films with a few nm thicknesses are widely used in devices such as superconductor-insulator superconductor (SIS) mixers, superconducting cavities and resonators and superconducting nanowire single-photon detectors (SNSPD). In all these applications, films of varying dimensions are required to achieve optimal performance. The initial thickness of our samples was 125 nm deposited on Silicon substrate. They changed the thickness of their samples using lithography and ion milling. They measured and analyzed the thickness dependence of various superconducting properties of these films. They have studied the pair-breaking effect in all three thicknesses and the measured calculated j_d(T) functions are confirming the T_c ∝ j^2/3 Ginzburg-Landau form. The measurements of j_d(T)and Bc₂(T)combined, provide a useful method to obtain λ based on our transport measurements. This is a special value in the situations in which the geometry of the sample or the volume area makes it not suitable for the penetration depth measurements using inductive method. By decreasing the thickness, we see an increase in the values of Bc₂(0), which can be since scattering is increasing (disorder). The value of ξ₀ in each magnetic field orientation is not changing for different thicknesses is which agrees with what is expected that this value is independent of the geometry of the sample.

Observation of Induced Superconductivity in Bi₂Te₃/FeTe

In research she conducted from January of 2015 through May of 2015, Dr Swails investigated superconductivity occurring at the interface between a topological insulator (Bi₂Te₃) and an iron chalcogenide (FeTe). This represents the first realization of superconductivity at the interface between a topological insulator and an iron chalcogenide, when neither material is in and of itself a superconductor. She was, for example, an essential member of the team to study superconductivity at the interface of Bi₂Te₃ and FeTe. Briefly, superconductors are materials that offer zero electrical resistance, making them extremely useful in large number of applications including electric motors, power transmission and digital circuits. However, neither of these two materials exhibit superconductive properties on their own. Therefore, Dr Swails’s observation of superconductivity at this interface is the first instance of superconductivity found between a topological insulator and an iron chalcogenide. Using several types of measurements, Dr Swails and her team found superconductivity within a layer of 7 nanometers thickness. Further observations using two bridges of differing dimensions showed that the length and width of a bridge of this material do not affect the transition temperature (at which point superconductivity disappears). Through a series of complex calculations, she was also able to determine the electrical and magnetic properties of this material including the free mean path of the electrons, yielding important information for further study of this material.

Depairing Current Density of Cuprate Superconductor Nd_2-xCe_xCuO4_-δ

From September of 2013 through November of 2013, Dr Swails fabricated and then investigated various properties of Nd_2-x Ce_xCuO_4-δ (NCCO) films. NCCO is a high temperature cuprate superconductor where electrons function as carriers, and Dr Swails’s work represents some of the first in the field to clarify the specific differences and similarities between electron- and hole-doped high-temperature superconductors.

Dr swails's research was on characterization of the cuprate superconductor, or NCCO. These films were first grown on SrTiO₃ substrate using a sputtering technique and annealed in a vacuum for 30 minutes before being etched with a four-probe bridge pattern for measurement. Using 100 fast electrical pulses of 3 micro seconds duration repeated every second to improve her signal-to-noise ratio, she showed that the temperature at which NCCO shifts from a superconductive to non-superconductive state falls as current increases. By graphing depairing current (the current at which the material is no longer superconductive) and time, Dr Swails and her team were then able to determine the depairing current density of NCCO to be j _d(0)=5.56×10^-3. She also showed the magnetic penetration of this material to be 0.47μm, an essential measurement needed to find the electron density in her sample.

Mathematical modeling Research

Dr Swails carried out a theoretical study of graphene nanoribbon (GNR) Schottky diodes. She was particularly interested in characterizing this type of diode and providing guidance for optimizing its performance, as Schottky diodes are used in power applications for a variety of electronics. Schottky diodes are semiconductor diodes created when a semiconductor is joined with a metal. First, Dr Swails developed an analytical equation to find the potential distribution within the graphene nanoribbon in order to establish the relation for the channel current. Dr Swails then used the WKB approximation to determine the transmission probability through the Schottky barriers. This result then permitted her to find the channel current as a function of electrical and physical parameters, such as the gate insulator thickness, drain bias voltage, graphene nanoribbon width, subband number and Schottky barrier height. Dr Swails then was able to observe the results of the model, which showed the physical effects of the I-V characteristics of the Schottky barrier graphene nanoribbon. She then compared the results of her analytical model to other studies that demonstrated the I-V physical characteristics of the Schottky barrier. She found that her results were in alignment with the results of these previous studies. Dr Swails could then conclude that her model was able to accurately predict the characteristics of Schottky diodes. Thus, her model is useful as a reliable tool in the design process of Schottky diode graphene nanoribbons.

Mathematical modeling of P-N junction

From December of 2010 through May of 2011, Dr Swails studied the electronic properties of p-n junctions. These p-n junction diodes are of immense value to rectifiers and switching circuits, and the graphene p-n junction is recognized as a basic building block for electronic devices. Dr Swails’s research demonstrated that p-n junctions reach equilibrium when electrons flow across the junction, without any need to use applied voltage. When placed in close proximity to one another (p-type materials and n-type materials), the behavior of the junctions changes so that the current flows in a steady one-way direction. Dr swails used ballistic electronic propagation in graphene to create a field-effect p-n junction. She then observed the Fermi energy in the p-n junction, which refers to the difference in energy between the highest and lowest single-particle states within a system featuring non- interacting particles. She found that in the p-n junction, the Fermi energy was occupied by holes and electrons. Dr swails further observed that while the concentration of electrons closest to the conduction band was higher, the concentration of holes closer to the valence band was higher. Therefore, she concluded using a computational approach that the current in p-n junctions is the sum of its electrons and holes. From this conclusion, Dr Swails was able to develop a formula, called the ideal diode, which can model and explain the characteristics of a specific current-voltage relationship in p-n junctions. Her novel formula for evaluating p-n junctions is applicable to design methods for products that use p-n junctions as subcomponents.

Computational Physics Research

In 2007 Dr Swails worked on studying physical properties ( the structural, electronic and optical properties) of Perovskite type material (CaMnO₃, CaTiO₃) by using of density functional theory (DFT). Perovskites show great interest in several applications due to their wide various and useful properties in photochromic, electrochromic, image storage, switching, filtering, and surface acoustic wave signal processing devices. The simulation process was by WIEN2k package. The program package WIEN2k allows to perform electronic structure calculations of solids using density functional theory (DFT). It is based on the full-potential (linearized) augmented plane-wave ((L)APW) + local orbitals (lo) method, one among the most accurate schemes for band structure calculations. WIEN2k is an all-electron scheme including relativistic effects and has many features. The stable phase of the CaTiO3 and CaMnO3 compounds at ordinary is orthorhombic and these compounds have phase transition to tetragonal and cubic phases respectively. This phase transition only happens by increasing the temperature and is not possible by increasing the pressure. The exchange- correlation functional was approximated as a generalized gradient functional (GGA96) and Engel-Vosko (EVGGA). Dr Swails and her team measure the energy gaps for all these three states. They did density of states (DOS) measurements as well. Due to high magnetic moment dipole of these compounds all calculations were spin polarized. The optical constant of these compounds was also calculated with respect to energy of photons.