In searching for novel optimal superconductors, three strategic routes based on theoretical and experimental knowledge from the known high-Tc superconductors are followed. CaFe4As3 is a newly discovered 3D compound, with Fe2+ in tetrahedral coordination, similar to that in the parent compounds of the known superconductors. The thermodynamic and transport properties reveal a spin density wave (SDW) transition at TN = 88 K, and an incommensurate-to-commensurate SDW transition at T2=26.4 K. A large electronic specific heat coefficient γ=0.02 J/molK^2 and an unusually high Kadowaki-Woods (KW) ratio A/γ^2=55×10E−5 μΩcm mol^2K^2/mJ^2 point to strong electron correlations. While the commensurate SDW state below T2 is suppressed in Co-doped CaFe4As3, neither doping with P, Yb, Co and Cu, nor application of hydrostatic pressures up to 5 GPa, is able to fully suppress the robust incommensurate SDW order in this system. The new layered compound SrMnBi2 has been studied as a promising candidate for high Tc superconductivity as suggested by theoretical calculations. We found that SrMnBi2 is structurally similar to, but more two dimensional than the known Fe superconductors. Two phase transitions at T1=292 K and T2=252 K have been observed. A large electronic specific heat coefficient γ=36.5 mJ/molK^2 and a KW ratio of 9.38×10E−5 μΩcm mol^2K^2/mJ^2 indicate enhanced electron correlations. DFT calculations have revealed metallic Sr-Bi layers in SrMnBi2, as well as Dirac-cone like features in the band structure. Doping experiments on the Mott insulator Sr2F2Fe2OS2 have been carried out to search for superconductivity at the localized-to-itinerant moment crossover. Increasing amounts of T=Mn in Sr2F2(Fe1−xTx)2OS2 suppress the long range magnetic ordering at x≈0.2, and the subsequent increase in x results in a spin glass behavior for 0.2≤x≤0.5, and possibly a new magnetic order for x≥0.5. By contrast, Co-doping increases the AFM transition from TN=106 K for x=0 up to 124 K for x=0.3. The excitation gap determined from the electrical resistivity is minimized but remains finite around x=0.5 for T=Mn. In addition, a study has been done on a rare binary type I superconductor YbSb2. Besides the superconducting transition at Tc=1.30 K, a possible second superconducting phase is observed below Tc(2)=0.41 K. From thermodynamic and transport measurements, there is strong, unambiguous evidence for the type I nature of the superconductivity in YbSb2.

Mn12-acetate single-molecule magnets (SMMs) are nano-scale magnets showing a strong magnetic anisotropy, slow relaxation and stepwise magnetic hysteresis curves. Possible applications of Mn12-acetate, e.g. for ultra high density magnetic information storage device, quantum computation, and magnetic molecular electronics, have been suggested due to the unusual magnetic behavior. It is an important prerequisite for the applications to develop a reliable technique to organize the molecules on a surface and to detect the magnetic signals of the molecules. A solution evaporation technique combined with conventional lithography is a simple but reliable method to create Mn12-acetate thin film patterns on the micro/nano-scale. The method is demonstrated with a series of analysis. A superconducting quantum interference device (SQUID) shows a non-linear I-V (Current vs. Voltage) characteristic that is modulated by a magnetic flux inside the loop, allowing one to sense and analyze an extremely weak magnetic field. The miniaturized SQUID is appropriate for sensing the magnetic flux from the film structure of the molecular magnets. Theoretical ideas, fabrication, and a measurement technique of the device are presented. A new interesting system, the so-called superconductor/SMM hybrid, results from the experimental configuration. Understanding this new type of hybrid system is important not only because of the expectation of new phenomena affecting the functionality of superconducting devices, but also because the two coupled substances are fundamentally incompatible phases. The first experimental attempt to investigate the interaction between an aluminum superconducting film and Mn12-acetate SMMs will be discussed.

In this thesis, the growth of c-axis oriented Sr2RuO4 thin films using pulsed laser deposition and their electrical transport properties are systematically discussed. The deposition and optimization process involved several progressive steps. Specifically, the first focus was on the Sr2RuO4 phase optimization in films grown on lattice-matched (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates. Film composition was found to be greatly influenced by changes in oxygen pressure, substrate temperature, target to substrate distance, and laser fluence. High oxygen pressure, low substrate temperature, large target to substrate distance, and high laser fluence increased the tendency to form the Ru-rich SrRuO3 phase in the film. The second focus was on improving the electrical transport properties of Sr2RuO4 from metal-insulating to fully metallic and eventually to superconducting behavior. It was observed that the full width at half maximum (FWHM) of the Sr2RuO4 (006) rocking curves in x-ray diffraction (XRD) scan was related to the quality of the electrical transport response. By fine tuning the deposition parameters to obtain low FWHM values, the electrical transport behavior of the Sr2RuO4 thin films was consistently improved from metal-insulating to fully metallic. In addition, localized superconductivity with enhanced superconducting transition temperature Tc onset was also observed among the fully metallic film. An in-depth study of the XRD results in fully metallic films indicated the existence of defects (intergrowths) along the c-axis direction, which caused localized c-axis tensile strain. The existence of structural defects within the film was likely to be responsible for the fact that only localized superconductivity was observed in the films. Furthermore, the enhanced superconducting transition temperature (Tc) relative to bulk single crystals is likely to be associated to localized strain in the film. Finally, Nb doped SrTiO3 substrates were used to achieve better quality growth of partial superconducting Sr2RuO4 thin films. Sr2RuO4 films grown on Nb doped SrTiO3 substrates had smaller FWHM values and lower level of c-axis tensile strain compared to those on LSAT substrates. Various partially superconducting films with different thicknesses and different superconducting Tc values are presented, and correlations between fabrication process, film crystalline quality as well as transport properties are discussed. This work provides better understanding of the importance of maximizing crystalline quality by delicate fine tuning of PLD deposition parameters to achieve high quality superconducting films.

The correlated electron systems which are the subject of this thesis are the strong electron–phonon coupling superconductor HfV2, and the localised moment magnetic systems of the alloy series Pd2REIn (RE = Gd, Tb, Ho, Er and Yb).

The nonlinear electrodynamic characteristics and presence of vortex dynamics in pseudo 2-dimensional microbridges make them attractive to design novel passive and active microwave circuits. Before such applications could be feasibly accomplished, a greater understanding of the the these devices are necessary, by a complete DC, microwave and optoelectronic characterization. A cryostat design and construction is discussed including the creation of test beds for DC characterization. Coplanar waveguide (CPW) design methodology is presented and used for the creation of CPWs for microwave characterization. Microbridges and meander lines are also embedded into the CPWs for determining the microwave performance of said devices and for optoelectronic characterizations. Results are compared against accepted results from theory and simulations, introducing vortices as explanations for device behaviour. Feasibility of these devices as single photon detectors is discussed.

The nonlinear electrodynamic characteristics and presence of vortex dynamics in pseudo 2-dimensional microbridges make them attractive to design novel passive and active microwave circuits. Before such applications could be feasibly accomplished, a greater understanding of the the these devices are necessary, by a complete DC, microwave and optoelectronic characterization. A cryostat design and construction is discussed including the creation of test beds for DC characterization. Coplanar waveguide (CPW) design methodology is presented and used for the creation of CPWs for microwave characterization. Microbridges and meander lines are also embedded into the CPWs for determining the microwave performance of said devices and for optoelectronic characterizations. Results are compared against accepted results from theory and simulations, introducing vortices as explanations for device behaviour. Feasibility of these devices as single photon detectors is discussed.

Many advances in technology ranging from biology and medicine through engineering and computer science to fundamental physics and chemistry depend upon the capability to control the fabrication of materials and devices at the submicron scale. Quantum mechanical effects become increasingly important to atomic and molecular interactions as the distances between neighbors decrease. These effects will provide materials and device designers with additional flexibility to establish properties of the designers choice, but the cost of this additional flexibility must be paid in the complexity of nonlinearities entering the interactions and the design process. The work presented here has provided several early results on three such interactions among closely-spaced submicron material structures: 1) the properties of superconductivity have been studied, 2) the properties of ferromagnetism have been studied, and 3) the interactions between superconductivity and ferromagnetism have been studied. Since our work was published, there has been considerable interest in all three of these wide-open areas and hundreds or thousands of additional results are now in the literature. We have used standard methods from the semiconductor industry as well as innovative methods to fabricate micron and submicron devices for observation. Standard optical lithography and standard electron beam lithography have been implemented to shape micron and submicron structures, respectively. Additionally, a laser interferometric lithography method has been invented and used to shape submicron structures. The materials used were vanadium, niobium, nickel, and/or permalloy. We have utilized SQUID magnetometry and Hall effect magnetometry to observe the properties of superconductor structures and superconductorferromagnetic mixed systems. We have used SQUID magnetometry and ferromagnetic resonance to observe the physical properties of ferromagnetic structures and the interactions between adjacent structures. Using these materials and methods we have discovered an unusual paramagnetic Meissner effect in thin Nb films that exists at igh-applied magnetic fields. We have discovered fluxoid matching anomalies at low sample temperature. And we have discovered interactions between electron exchange and magnetic dipole forces. Additionally, we have found clear evidence to support several past hypotheses advanced by other authors.

Thesis advisor: Hong Ding / High temperature superconductivity has been one of the most challenging problems in condensed matter physics since its discovery. This dissertation presents systematic studies on electronic structures of single layer high temperature supconductor Bi2Sr2CuO6+[delta] by angle-resolved photoemission spectroscopy. A high binding energy band anomaly is observed in PbxBi2−xSr2CuO6+[delta]. Comparing with LDA calculation, the band is highly remonetized by a factor of 3, the incoherent part coexists and forms a band anomaly as a non-dispersive dive structure. Systematic studies are performed on Bi2+xSr2−xCuO6+[delta] with a wide doping range, revealing a linear doping nature of Bi substitution. An unusual Coulomb gap is observed in the heavily substituted samples. Our results reveal the dual role of off-plane chemical substitution in high-TC cuprates and elucidate the nature of the quantum electronic nature due to strong correlation and disorder. High resolution ARPES on LaxBi2Sr2−xCuO6+[delta] observes a large gap and a small gap coexisting at antinodal region below TC. The small gap is d-wave like and attributed to superconductivity, while the large gap is attributed to a CDW order. A strong short-ranged correlation between the small and large gap magnitude suggesting that superconductivity and charge ordering are driven by similar physical mechanism. / Thesis (PhD) — Boston College, 2008. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Thesis advisor: Vidya Madhavan / It has been long established that the phenomenon of superconductivity is administered by lattice deformations (phonons) which act to pair electrons into spinless bosons free to condense into a coherent ground state. This superconducting phase is protected up to a critical temperature above which thermal fluctuations are potent enough to destroy the resistance free phase. The strength of this phonon mediation has been calculated by strong coupling theory and found to be capable of accommodating pairing up to near 40 K. So with the advent of copper-oxide (cuprate) superconductors boasting transitions temperatures exceeding 90 K it was clear that these material represented a new breed of superconducting physics. More than twenty years after the initial discovery of these high-transition temperatures the most basic questions are yet to be answered, the most fundamental of which is by what mechanism does pairing occur? The field splits between those who feel that a boson mediator is still necessary to act as the virtual glue which binds electrons into cooper pairs while others insist that really the Coulomb force alone is enough to induce pairing physics. Even within the boson-seeker community there is no consensus on what particular type of boson is contained in this system whether it be a lattice excitation or spin excitation. This answer has been clouded by previous experimental results on the hole-doped cuprates which have made strong cases for every scenario rendering them largely inconclusive. For this answer though it is possible to explore materials that have not yet been clouded by conflicting results by performing the first high resolution ultra-high-vacuum low-temperature scanning tunneling microscopy (STM) study of an electron-doped cuprate. A distinct and unambiguous bosonic mode is found at energy near 10.5 meV. Through comparison with other experimental data it is found that this mode does not fit the characteristics of a phonon. It is found, through comparison with neutron scattering experiments on the same sample, that this mode is consistent with a spin collective mode. Further more it is found that this mode is linked with the strength of superconductivity nominating it as the possible electron pairing mechanism. Doping and temperature dependence studies are performed to investigate this possibility. Finally the same procedures developed can be applied to the newly discovered iron based superconductors which may represent yet another type of new superconductor physics. Initial results on the first bosonic mode STM study of SrFe$_{2-x}$Co$_{x}$As$_{2}$ and BaFe$_{2-x}$Co$_{x}$As$_{2}$ are presented. / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

La photonique Josephson est un domaine récent de la physique à la croisée entre l’électrodynamique quantique en circuit et le blocage de Coulomb dynamique. Elle explique et étudie la possibilité pour une paire de Cooper de traverser une jonction Josephson polarisée en tension par effet tunnel inélastique, en dissipant la différence de potentiel électrique aux bornes de la jonction sous forme de photons émis dans l’environnement électromagnétique de la jonction.Cette thèse s’arrête sur deux aspects de la photonique Josephson:• La possibilité de contrôler la statistique des photons émis dans l’environnement, en particulier Générer des photons non-classiques;• La possibilité de stimuler l’émission de photons, ce qui permet d’Amplifier avec un bruit ajouté à la limite quantique.Pour fonctionner ces dispositifs ne demandent qu’une simple tension continue servant à polariser la jonction Josephson. A terme ces dispositifs pourraient simplifier certaines mesures quantiques en remplaçant avantageusement des dispositifs micro-ondes existants plus difficiles à utiliser.Nous avons étudié nos dispositifs avec deux théories, la théorie P(E) et celle liant les flux de photons entrant et sortant, pour en tirer les caractéristiques de fonctionnement de nos dispositifs : taux d’émission, gain, bruit, bande passante, point de compression. Les dispositifs expérimentaux mesurés sont réalisés en nitrure de niobium en créant un environnement électromagnétique répondant à nos besoins. La possibilité de contrôler les processus photoniques que l’on veut en réalisant l’environnement électromagnétique adapté laisse la porte ouverte à de futures dispositifs : divers sources non-classiques, amplificateurs large bande, détecteurs de photons. / The recent field of Josephson photonics is about the interplay between circuit quantum electrodynamic and dynamical Coulomb blockade. It explains and studies the ability of a Cooper pair to inelasticity tunnel through a DC-biased Josephson junction by dissipating the Cooper pair energy in the electromagnetic environment of the junction in the form of photons.This thesis focuses on two aspects of the Josephson photonics:• Control over the statistics of the emitted photons with focus on Generation of non-classical photons;• Stimulated emission of photons leading to Amplification with added noise at the quantumlimit.These devices are powered with a simple DC voltage used to biased the Josephson junction. Such devices can be a new solution in a frequencies range where only few simple alternative solutions are now available.We have studied our devices with two theories, P-theory and input output theory, to derive working characteristics of our devices : Photon rate, gain, noise, bandwidth, compression point. The measured samples are made of niobium nitride and the electromagnetic environment of the junction is engineered to fulfil our needs. The possibility to select the photonic processes at will by engineering the electromagnetic environment permits to imagine further devices: other types of sources, wideband amplifiers, photon detectors.