Title: Type-II thin film superconductors studied by terahertz radiation Author: RNDr. Roman Tesař Department: Department of Low Temperature Physics Supervisor: prof. RNDr. Ladislav Skrbek, DrSc. Consultant: RNDr. Jan Koláček, CSc. Abstract: Utilization of type-II superconductors for future practical applications such as fluxonics requires detailed knowledge of their physical properties, espe- cially at high frequencies within the THz spectral region. We have investigated interactions of thin-film NbN samples deposited on Si substrate and of a high quality epitaxial film of the NbN superconductor grown on a birefringent R-cut sapphire substrate with monochromatic linearly polarized laser beam both below and above the critical temperature Tc. For photon energies lower than the optical gap, detailed measurements of transmission in zero field provide BCS-like tem- perature curves with a pronounced peak below Tc which disappears as the energy of incident radiation is increased above the gap. In externally applied magnetic fields up to 10 T oriented perpendicularly to the sample, i.e., in the Faraday exper- imental geometry, the temperature behavior of transmission is modified because the gap is suppressed and vanishes at the upper critical field and, additionally, the presence of quantized vortices changes the shape...

In this thesis, I firstly report high field measurements of graphene/NbN junctions, in which NbN makes edge contact to graphene. Transport measurements at zero field demonstrate clear features associated with both retro and specular Andreev reflection. By applying perpendicular magnetic field, field dependence of junction transparency at Quantum Hall (QH) / superconductor (SC) interface is calculated and explained by a picture of superposition of electron and hole edge excitation. Zeeman splitting is induced in graphene by applying in plane magnetic field. We observe changes in the Andreev reflection spectrum that are consisting with spin splitting of the graphene band structure. This edge contact technique provides the opportunity to create hybrid SC/graphene or SC/QH system to illustrate new physics such as non-Abelian zero modes of Majorana physics. Secondly, other potential material candidates for SC/graphene junctions are discussed, high field transport measurement of FeSeTe/graphene junction is discussed, Superconductor/quantum spin Hall (QSH) interface and superconductor-graphene-superconductor weak link are also discussed, respectively. At last, via contact, a new contact method for two-dimensional materials, especially air-sensitive materials is discussed, the via contact method provides a new and reliable fabrication technique for two dimensional materials.

Electron-phonon interactions (EPIs) are ubiquitous in condensed matter physics and materials science. They are crucial for understanding numerous phenomena, including conventional superconductivity, charge-transport and, most pertinent for this thesis, polaron formation. A polaron is a charge carrier (electron or hole) dressed with a “cloud” of phonons. The polaronic quasiparticle may have vastly different ground- and excited-state properties from that of the bare, constituent charge carrier. While polarons are well-studied and largely understood in canonical model Hamiltonians, recent advances have made it possible to study more complex, fully ab initio systems. Here, the numerically exact methods which are available for some model systems become much more challenging to apply, so accurate approximate methods are a necessity. In this dissertation, we present several advancements in approximate but accurate methods for different polaronic problems and polaron observables. With respect to polaron dynamics, we focus on low-scaling methods to produce wave vector-dependent single-particle spectral function. We present a thorough study of the accuracy of the second- and fourth-order cumulant expansions (CE) of the electronic Green’s function by comparing them against numerically-exact reference data for the one-dimensional Holstein model. We find that the second-order CE is accurate at zero electronic-momentum across a wide range of temperatures, while for non-zero electronic momenta, the CE is only accurate at high-temperatures. The fourth-order cumulant expansion improves on the dynamics at short times and can improve the spectra; however, it can also introduce non-physical divergences and negative spectral weight. The second-order cumulant expansion is thus a useful tool for determining spectral functions in some instances. However, increasing the order of the CE introduces pathologies that may persist at arbitrarily high-order. As an alternate approach to improving the CE, we introduce a new self-consistent cumulant expansion (SC-CE) which remedies many of the deficits of the CE. We compare the results for this new approximation against those from the second-order cumulant expansion as well as reference data for the one-dimensional Holstein model. Unlike the CE, the SC-CE can produce accurate spectra across the entire Brillouin-zone, and captures non-perturbative features excellently. The trade-off for this increased accuracy is the introduction of some degree of negative spectral-weight and the potential for rapid divergences in time in some instances. We find that these problems can be minimized, but not completely eliminated in the thermodynamic limit and in more realistic cases where phonon dispersion exists. We also demonstrate how the SC-CE fits into the greater scheme of Green’s function methods which approximate the self-energy non-diagrammatically as has recently been proposed by Pandey and Littlewood, and we note the potential applications of the SC-CE both in ab initio polaron problems and in general many-body problems. We finally consider a new method to determine the ground-state structure of the polaron in ab initio materials, a topic which has only recently appeared in the literature. We present a new all-coupling variational method based on the Nagy-Markoš variational ansatz for the Fröhlich model. The ansatz is a projected unitary transform which naturally interpolates between the weak-coupling (Lee-Low-Pines) ansatz and the strong-coupling adiabatic ansatz by modulating the momentum conservation of the electron-phonon scattering processes. We demonstrate our ab initio Nagy-Markoš ansatz on the Holstein model and the Fröhlich model, and show that it always improves upon the better of the weak or strong coupling result. We consider the ab initio case of lithium fluoride (LiF), and find that the ansatz provides accurate polaron binding energies for both the hole-polaron and the electron-polaron which are classical cases of small and large polarons, respectively. We note how our flexible variational ansatz is an ideal starting point for perturbative energy corrections and cumulant Green’s function methods. Future developments and applications of the efficient methodologies presented in this dissertation may enable quantitative calculations of polarons in large-intermediately coupled ab initio systems, such as the lead-halide perovskites and other systems where it has hitherto been difficult to fully understand the effects of the electron-phonon interactions.

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<p> The phenomena of energy gap and transition temperature enhancement have been studied on very thin (35-80 A° ) aluminum films, using the technique of electron tunneling through a thin insulating barrier. Transition temperatures as high as 2.16°K have been measured, and the corresponding measured energy gaps are found to be unique. In addition, these thin superconducting films exhibit stable transition temperatures when held in a vacuum. However, upon breaking vacuum, oxide growth occurs, and the tunneling barrier becomes impenetrable. Existing theories proposed to explain the observed enhancements, are discussed in the light of present accumulated experimental evidence. However, it is found that none are capable of explaining adequately the observed enhancements. </p> / Thesis / Master of Science (MSc)

<p> Low dimensional quantum magnets which display a collective singlet ground state and a gap in their magnetic excitation spectrum provide a framework for much exotic phase behavior in new materials, with high temperature superconductivity being the best appreciated example. Neutron scattering techniques can be applied to study a wide variety of problems in condensed matter physics. These techniques are particularly useful as applied to understanding the magnetic properties of quantum magnets that display exotic phases.</p> <p> SrCu2(BO3)2, is a rare example of a two-dimensional quantum magnet for which an exact theoretical solution describing its ground state is known to be a collective singlet. Previous high resolution neutron scattering measurements identified the most prominent features of the spin excitation spectrum in SrCu2(BO3)2, including the presence of one and two triplet excitations and weak dispersion characteristic of subleading terms in the spin Hamiltonian.</p> <p> The resemblance between the spin gap behavior in the Mott insulator SrCu2(BO3)2 and that associated with high temperature superconductors motivated the consideration of the significance of doping in order to understand the properties of this quantum magnetic system. For this reason, a series of neutron scattering studies on doped SrCu2(BO3)2 were initiated.</p> <p> These series of investigations began with the performance of neutron scattering measurements on a SrCu(2-x)Mgx(BO3)2 single crystal in order to introduce magnetic vacancies to the system. These results revealed the presence of new spin excitations within the singlet-triplet gap of this system. Application of a magnetic field induces Zeeman-split states associated with un-paired spins which exist as a consequence of doping with quenched non-magnetic impurities. Additional substantial broadening of both the one and two triplet excitations is observed in the doped system as compared to the pure system. Theoretical calculations are shown to qualitatively account for these features.</p> <p> These studies were extended to neutron scattering measurements on Sr(1-x)LaxCu2(BO3)2, with an aim of introducing charged carriers into this system. The broadening of the one and two triplet excitations is observed and compared to the thermally induced finite lifetime of the pure system. The temperature dependence of this broadening in Sr(1-x)LaxCu2(BO3)2 is different compared to that observed in both SrCu2(BO3)2 and SrCu(2-x)Mgx(BO3)2.</p> <p> It has also been suggested that there is a relation between the spin-lattice interaction in SrCu2(BO3)2 and the magnetic dynamics at low temperatures and high magnetic fields. For this reason there has been increased interest in the study of the crystalline structure and vibrational modes of SrCu2(BO3)2. In order to investigate the role of the lattice in the formation of the singlet ground state in SrCu2(BO3)2, a series of low and high energy neutron scattering measurements were carried out on this system to study both the crystalline structure as well as the normal modes of vibration of the lattice, the transverse acoustic and optical phonons. Transverse acoustic phonons with energies comparable to and higher than the onset of the two triplet continuum show substantially increased lifetimes on entering the singlet ground state below ~ 10 K. This may indicate the removal of the decay channel for the phonons due to the gapping of the spin excitation spectrum in SrCu2(BO3)2 at low temperatures. In high energy inelastic neutron scattering we observe broadening of optic phonons in the ~ 52 to 65 meV region on entering the low temperature singlet ground state.</p> <p> Additionally, the magnetic properties of CuMoO4, which is a triclinic quantum magnet system based on S=1/2 moments at the Cu2+ site, were studied using elastic and inelastic neutron scattering experiments. This material exhibits a first order structural phase transition at ~ 190 K as well as a magnetic phase transition at ~ 1.75 K. We were primarily interested in the low temperature magnetic properties of this material. Magnetization and heat capacity measurements as well as elastic and inelastic neutron scattering measurements were conducted on this system within the low temperature ordered phase. These studies confirm that this material has a magnetic phase transition at ~ 1.7 K. Neutron scattering results indicate that this magnetically ordered phase is characterized by a doubling of the a axis. Inelastic neutron scattering measurements revealed a gapped magnetic excitation spectrum in zero magnetic field, which could be filled in by the application of magnetic fields approaching 7 T.</p> / Thesis / Doctor of Philosophy (PhD)

<p> Simple theoretical expressions for the zero temperature energy gap and the transition temperature of a weak coupling superconductor are derived and applied to an investigation of several phenomena.</p> <p> The anisotropy of the energy gap in aluminium arising from the anisotropy in the phonon spectrum is calculated. The effect of this energy gap anisotropy on some thermodynamic properties of superconducting aluminium is investigated.</p> / Thesis / Doctor of Philosophy (PhD)

Thesis advisor: Krzysztof Kempa / “Simulations are like an experiment but on a computer.” – K. Kempa. Powerful ideas can be explored in immense detail and unmatched flexibility through computational resources. Combined with the beauty of electromagnetics, worlds of situations and problems can be uncovered. Of the many interesting phenomena available to study, a relatively recent explosion of engineered plasmonic materials has benefitted greatly from numerical breakthroughs in simulating Maxwell’s equations. Using these tools on novel metamaterial systems, composite materials with precisely designed structural features, the analysis and optimization probes the unique capabilities they have interacting with light. Example phenomena from this work includes fundamental principle breaking, extraordinary optical transmission, negative refraction, and superconductivity enhancement. The systems that harbor such outstanding feats fall into the umbrella term of metamaterials, each with distinct geometry and contrasting electrical properties that allow for an engineered control of the effective structural dielectric function. As the response to electromagnetic radiation, manipulating the dielectric function is key to creating and discovering the effects that control light, without changing any chemistry. This work scales pedagogically through the different types of metamaterials, beginning first with 2D planar checkerboard structures with highly non-linear percolation. In combination with spoofed plasmonics, the longstanding symmetry of the Babinet principle is challenged. Layers of checkerboards are then stacked and translated to create subwavelength gaps for which plasmonic coupling between layers aids in optical transmission. In fact, there is similar physics controlling other layered quasi-complementary structures shown by comparison to experimental transmittance data. A further stage introduces photonic crystals constructed out of 3D periodic lattice of nanoparticles. Photonic band structure calculations for properly designed systems suggest the possibility of bandwidths of the IR spectrum where the crystal has a negative refractive index. Such a material property allows for the invention of lenses that beat the diffraction limit, applicable to subwavelength imaging. Lastly, non-local extensions to plasmonics are theoretically worked into expressions for superconductivity, creating a resonant anti-shielding effect, in composite topological crystal/superconductor layered arrangements. Applying this to known topics, like Bi2Se3 and MgB2, shows significant boost to electron pairing and thus rises in superconducting critical temperature. Central to all the systems and effects explored are the modifications made to the dielectric function of each effective medium. Supported by electromagnetic simulations and theoretical efforts, the listed engineered materials transform the dielectric environment purposefully to originate the mentioned exotic optical phenomena. / Thesis (PhD) — Boston College, 2023. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Emergent electronic phenomena in iron-based superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origins and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critical temperatures, and evidence for nontrivial band topology, including associated spin-helical surface states and vortex-induced Majorana bound states. Here, we review superconductivity in iron chalcogenide superconductors, including bulk FeSe, doped bulk FeSe, FeTe1−xSex, intercalated FeSe materials, and monolayer FeSe and FeTe1−xSex on SrTiO3. We focus on the superconducting properties, including a survey of the relevant experimental studies, and a discussion of the different proposed theoretical pairing scenarios. In the last part of the paper, we review the growing recent evidence for nontrivial topological effects in FeSe-related materials, focusing again on interesting implications for superconductivity.

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