Micro Hall probe magnetometry has been used to investigate the magnetisation of various electrodeposited microcrystals. Superconducting tin crystals of almost perfect square cuboid shapes exhibit a strong size dependence of the supercooling of the superconducting state and, for the smallest accessible crystals, the crossover to the mesoscopic regime can be readily explored close to their critical temperatures. Experimental results are in good agreement with Ginzburg-Landau simulations using the exact experimental parameters. Electroplating of the tin cores with another material provides unique core-shell structures of either two superconductors (S-S’: tin-lead) or of a superconducting core, covered with a ferromagnetic shell (S-F: tin/lead-nickel). The critical parameters of the tin core in Sn-Pb core-shell crystals are considerably enhanced and superconductivity in the tin core is detected up to 1:16 TSn c . Little-Parks oscillations in the shell can be analysed to reveal the extent of the superconducting sheath and hence can be utilised to measure the range of the proximity effect close to the critical temperature of the shell. In S-F core-shell structures, field cancellation effects govern the overall behaviour. Under certain conditions it was possible to switch the overall magnetic response from para(ferro-)magnetic to diamagnetic and back at finite applied fields. Micromagnetic simulations qualitatively reproduce the experimentally observed effects. Applications for the core-shell structures include magnetic guidance or memory devices.

Introduction. Defect-induced superconductivity is an important phenomenon manifested in triggering the superconducting state due to defects and disorder in the material lattice. Promising materials for this investigation are carbon-based. Josephson behavior has been reported in 1974 for a disordered graphite powder, which is considered to be the first hint of a room temperature graphite-based superconductor. Theoretical and experimental studies support the idea that certain two-dimensional stacking faults (SFs) in the semiconducting matrix contribute to the granular superconducting-like behavior of graphene-based materials. Hints for the existence of high-temperature superconductivity at certain SFs in graphite were demonstrated. This phenomenon is considered to be caused by flat band regions at the SF. Especially the SFs between Bernal and rhombohedral stacking orders (without any twist angle around the common c-axis) have the largest probability to show robust superconductivity due to an extended and robust flat band behavior. In this work, a permanent current path in graphite, after the application of a magnetic field, is investigated to show clear evidence for the existence of room temperature superconductivity (RTS). Preliminary results for the existence of such permanent current path were obtained with magnetic force microscopy (MFM) and published a few years ago. Thus, the objectives of this work are to investigate trapped magnetic flux with magnetic force microscopy, to reveal the reasons for the difficulties of finding such permanent current path in the remanent state of the sample and to give an additional hints to the semiconducting behavior and energy gaps of an ideal graphite using a new PF-TUNA method. Summary. The experimental pre-characterization of graphite samples was conducted using XRD and Raman spectroscopy. The spectra show well-ordered structure of the samples with a sufficient content of the rhombohedral phase. The grounded samples were examined with PF-TUNA mode at bias voltages applied between the conductive tip and the sample surface. The samples with Bernal phase and with mixed phases showed semiconductor-like behavior. Using the semiconductor model, the obtained simulations of registered I-V curves could estimate the energy gap in a range from 12 to 37 meV. This is in a good agreement with the values of energy gaps, observed in transport measurements. Additionally, the shift in the position of the minimum of the tunneling conductance was explained by the tip-induced band bending. The results of this thesis confirm the existence of the peak in the density of states, that is correlated to the flat band in a sufficiently thick multigraphene flake with a 3R stacking order (thickness should be much greater than 3 nm to observe it) at room temperature and the existence of the trapped magnetic flux, expulsed by the weakly coupled superconducting patches in the natural graphite sample. The trapped flux was identified and examined by MFM measurements at the surface of natural graphite sample in the remanent state. Therefore, we successfully reproduced the results reported in and performed field and time dependent measurements, that prove the superconducting origin of this phenomena. The modeling of the MFM signal was done according to the monopole tip approximation. The value of the permanent current was estimated in the range of 0.2 μA to 6 μA, which is consistent with literature. An accidental scratch on the sample surface allowed us to estimate the depth of the aforementioned superconducting patches, ≲ 10 nm, and gave additional evidence to its origin by changing the route due to the superconducting patches nearby. This investigation provides hints for room temperature superconductivity at certain SFs in graphite and clarifies the reasons for the difficulties of the trapped flux identification in graphite. Further research should be focus on the identification of the permanent currents by MFM at lower temperatures. Moreover, it would be helpful to understand, how to artificially produce extended SFs. Finally, it should be noted, that additional measurements should be performed in order to clarify the field dependence of trapped magnetic flux in graphite and the role of Pearl vortices. Collaboration and External Contributions. This work was conducted under the supervision of Prof. Dr. Pablo Esquinazi, Felix-Bloch-Institute for solid state physics, Division of Superconductivity and Magnetism, University of Leipzig. STEM images were made by Dr. W. Bölmann, University of Leipzig. X-ray diffraction was made by Mr. O. Baehre and Mr. T. Muenster at Institute of Mineralogy, Crystallography and Materials Science at the University of Leipzig. The Raman spectra were recorded by Mr. Tom Venus and Dr. Irina Estrela-Lopis, Institute of Medical Physics and Biophysics, University of Leipzig. The natural graphite samples from Brazil were provided by Prof. Dr. Ana Melva Champi Farfan from Universidade Federal do ABC in Santo Andre, Sao Paulo, Brazil. The natural graphite from Sri-Lanka by Mr. Henning Beth from Golden Bowerbird Pty Ltd. in Mullumbimby, Australia. The magnetoresistance measurement of a natural graphite sample from Sri-Lanka was performed by Dr. Christian E. Precker, AIMEN Technology Centre, Smart Systems and Smart Manufacturing, Artificial Intelligence and Data Analytics Laboratory, PI. Cataboi, Pontevedra, Spain. The calculations, related to modeling of the tunneling current based on the tip-induced band bending, were performed by Dr. Michael Schnedler, Peter Gruenberg Institut, Forschungszentrum Juelich.

This work gives insight in some key aspects for the understanding of the origin of high-temperature superconductivity in the newly discovered class of iron-based materials. In particular, thermodynamic methods, such as SQUID magnetometry, specific heat and dilatometry were used, in order to (i) assess the evolution of electronic correlations in a series of transition metal substitutions of the well-known BaFe2As2 as a function of 3d band filling and (ii) to re-investigate the phase diagram of Co-doped LaFeAsO on single crystals, with particular interest in the interplay between the nematic/magnetic phase of the parent compound and superconductivity induced by in-plane electron doping. In the first part of this work, the Sommerfeld coefficient (γ_exp) was extracted from the low temperature specific heat data and compared with the theoretical values obtained by band theory calculations, in order to obtain the mass enhancement (m∗/mb) in the series BaT2As2 (T = Cr, Mn, Fe, Co, Ni, Cu). The results clearly show an overall decrease of the electronic correlations while departing from the half-filled (3d5) to the fully filled configuration (3d10), thus suggesting a highly correlated 3d5 state. The evolution of electronic correlations as a function of 3d band filling for n > 5 is in agreement with previous theoretical calculations, underlining the importance of Hund’s coupling in describing the normal-state properties of iron-based superconductors. In addition, it was found that the decrease in m∗/mb for n > 5 follows an increase of the crystal field splitting (Δ), determined by the progressive distortion of the As-T-As angle (α_bond) from the ideal tetrahedral environment. This study reveals a complex interplay between electronic correlations, band filling and crystal structure in determining the physical properties of 122 systems. In the second part, the phase diagram of Co-doped LaFeAsO was re-investigated using single crystals by thermodynamic methods. From magnetic susceptibility studies we track the doping evolution of the antiferromagnetic phase, revealing a continuous decrease of T_N up to 5% Co doping. In order to study the evolution of the so-called nematic phase, the temperature dependence of the length changes along the a and b orthorhombic directions, ΔL/L_0, was determined by high-resolution capacitance dilatometry. The results clearly show a gradual reduction of the orthorhombic distortion δ and of T_S with increasing Co content up to 4.5%, while it is completely suppressed for 7.5% Co. Bulk superconductivity with T_c = 10.5 K was found in a small doping region around 6% Co content, while both T_c and the superconducting volume fraction rapidly drop in the neighbouring doping regime. Ultimately, no microscopic coexistence between the superconducting and magnetic phases can be assessed within our resolution limit, in sharp contrast with other iron-pnictide families, e.g., electron- and hole-doped BaFe2As2.

In this work, the electronic phase diagrams of Ca1−xNaxFe2As2 and Fe1+yTe were investigated using muon spin relaxation and Mössbauer spectroscopy. Single crystals of Ca1−xNaxFe2As2 with x = 0.00, 0.35, 0.50, and 0.67 were examined. The undoped 122 parent compound CaFe2As2 is a semi metal and shows antiferromagnetic commensurate spin density wave order below 167 K. By hole doping via Na substitution, the magnetic order is suppressed and superconductivity emerges including a Na-substitution level region, where both phases coexist. Upon Na substitution, a tilting of the magnetic moments out of the ab-plane is found. The interaction of the magnetic and superconducting order parameter in this coexistence region was studied and a nanoscopic coexistence of both order parameters is found. This is proven by a reduction of the magnetic order parameter of 7 % in x = 0.50 below the superconducting transition temperature. This reduction was analysed using Landau theory and a systematic correlation between the reduction of the magnetic order parameter and the ratio of the transition temperatures, Tc/TN, for the 122 family of the iron pnictides is presented. The magnetic phase transition is accompanied by a tetragonal-to-orthorhombic phase transition. The lattice dynamics at temperatures above and below this magneto-structural phase transition were studied and no change in the lattice dynamics were found. However, the lattice for finite x is softer than for the undoped compound. For x = 0.67, diluted magnetic order is found. Therefore, the magnetism in Ca1−xNaxFe2As2 is persistent even at optimal doping. The superconducting state is investigated by measuring the temperature dependence of the magnetic penetration depth, where two superconducting gaps with a weighting of nearly 50:50 are obtained. A temperature independent anisotropy of the magnetic penetration depth γ_λ = 1.5(4) is obtained, which is much smaller compared to other 122 compounds indicating a more three-dimensional behaviour of Ca1−xNaxFe2As2. Powder samples of Fe1+yTe with y = 0.06, 0.12, 0.13, and 0.15 were examined. Fluctuating paramagnetic moments at room temperature were found, which are independent of the excess iron level y. Below 100 K, a magnetic precursor phase is observed, which is independent of y. Fe1.06Te shows a commensurate spin density wave phase below TN, while for y ≥ 0.13 an incommensurate spin density wave phase below TN is found. However, a slowing down of the magnetic fluctuations with decreasing temperature and static magnetic order at lowest temperature are observed. / In dieser Arbeit wurden die elektronischen Phasendiagramme von Ca1−xNaxFe2As2 and Fe1+yTe mit Hilfe der Myonspinrelaxations- und Mössbauerspektroskopie untersucht. Einkristalle von Ca1−xNaxFe2As2 mit x = 0.00, 0.35, 0.50 und 0.67 wurden untersucht. Das undorierte 122-System CaFe2As2 ist ein Halbmetal und zeigt eine antiferromagnetische Spindichtewelle unterhalb von 167 K. Substituiert man Ca durch Na, werden Löcher in das System eingebracht. Die magnetische Ordnung wird mit steigendem Na-Anteil unterdrückt und Supraleitung tritt auf. Dabei existiert ein Na-Substitutionslevelbereich, in welchem Magnetismus und Supraleitung koexistieren. Desweiteren wurde ein herausdrehen der magnetischen Momente aus der ab-Ebene als Funktion von x beobachtet. Die Wechselwirkung des magnetischen mit dem supraleitenden Ordnungsparameter in der Koexistenzregion wurde untersucht und nanoskopische Koexistenz der beiden Ordnungsparameter wurde gefunden. Dies konnte durch eine Reduktion des magnetischen Ordnungsparameteres um 7 % in x = 0.50 unterhalb der supraleitenden Ordnungstemperatur gezeigt werden. Diese Reduktion wurde mit Hilfe der Landautheorie untersucht und es wurden systematische Korrelationen zwischen der Reduktion des magnetischen Ordnungsparamteres und dem Verhältnis der Übergangstemperaturen, Tc/TN, in der 122-Familie der Eisenpniktide gefunden. Der magnetische Phasenübergang wird von einem strukturellen Phasenübergang begleitet. Die Gitterdynamik wurde bei Temperaturen oberhalb und unterhalb dieses magneto-elastischen Phasenübergangs untersucht. Es wurden keine Änderungen in der Gitterdynamik festgestellt. Jedoch konnte festgestellt werden, dass das Gitter für endliche x weicher ist als für das undotierte System. Für x = 0.67 wurde festgestellt, dass der Magnetismus im Ca1−xNaxFe2As2-System auch noch bei optimaler Dotierung zu finden ist. In der supraleitenden Phase wurde die Temperaturabghängigkeit der magnetischen Eindringtiefe untersucht und es wurden zwei supraleitende Bandlücken gefunden. Die Anisotropie der magnetischen Eindringtiefe ist temperaturunabhängig und mit γ_λ = 1.5(4) wesentlich kleiner als in anderen 122- Verbindungen, was für eine erhöhte Dreidimensionalität in Ca1−xNaxFe2As2 spricht. Pulverproben von Fe1+yTe mit y = 0.06, 0.12, 0.13 und 0.15 wurden untersucht. Es wurden fluktuierende paramagnetische Momente bei Raumtemperatur gefunden, welche unabhängig vom Überschusseisenlevel y sind. Unterhalb von 100 K wurde eine magnetische Vorgängerphase gefunden, welche unabhängig von y ist. Mit fallender Temperatur wurde eine Verlangsamung der magnetischen Fluktuationen festgestellt, welche in einer statischen magnetischen Ordnung bei tiefen Temperaturen münden.

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.

The work presented in this thesis was broadly focussed on the investigation of the magnetic behaviour of different superconducting materials in the form of bulk (singe crystals and pellets) and thin films (nanomagnetic devices like superconducting spin valves etc). Neutrons and muons were extensively used to probe the structural and magnetic behaviour of these systems at the nanoscale along with bulk characterisation techniques like high-sensitive magnetic property measurements, scanning probe microscopy and magneto-transport measurements etc. The nanoscale interplay of Superconductivity and Ferromagnetism was studied in the thin film structures using a combination of Polarised Neutron Reflectivity (PNR) and Low Energy Muon Spin Rotation (LE-µSR) techniques while bulk Muon Spin Rotation (µSR) technique was used for microscopic magnetic investigation in the bulk materials. In the Fe/Pb heterostructure, evidence of the Proximity Effect was observed in the form of an enhancement of the superconducting penetration depth (λs) with an increase in the ferromagnetic layer thickness (dF) in both the bilayered and the trilayered structures. The existence of an Inverted Magnetic Region was also detected at the Ferromagnet-Superconductor (F/S) interface in the normal state possibly originating from the induced spin polarisation within the Pb layer in the presence of the neighbouring Fe layer(s). The spatial size (height and width) of the Inverted Magnetic Region did not change much while cooling the sample below the superconducting transition temperature(Tc)and it also stayed unaffected by an increase in the Fe layer thickness and by a change of the applied magnetic field. In the superconducting spin valve structure containing Permalloy (Py) as ferromagnetic layer and Nb as the superconducting layer, LE-µSR measurements revealed the evidence of the decay of magnetic flux density (as a function of thickness) within the Nb layer symmetrically from the Py/Nb interfaces towards the centre of the Nb layer in the normal state. The thickness dependent magnetisation decay occurred over two characteristic length scales in the normal state that stayed of similar values in the superconducting state also. In the superconducting state, an additional contribution towards the magnetisation was found in the vicinity of the Py/Nb interfaces possibly originating from the spin polarisation of the singlet Cooper pairs in these areas. The nanoscale magnetic investigation on a highly engineered F/S/F structure (where each of the F blocks made of multiple Co/Pd layers with magnetic moments aligned perpendicular to the plane of these layers and neighbouring magnetic blocks separated by Ru layers giving rise to antiferromagnetic alignment) using LE-µSR showed an antisymmetric thickness dependent magnetic flux density profile with two characteristic length scales. In the superconducting state, the magnetic flux density profile got modified within the superconducting Nb₆₇Ti₃₃ layer near the F/S interfaces in a way similar to that of observed in the case of Py/Nb system, most likely because of the spin polarisation of the superconducting electron pairs. The vortex magnetic phase diagram of Bi₂Sr₂Ca₂Cu₃O10-δ was studied using the Muon Spin Rotation (µSR) technique to explore the effects of vortex lattice melting and rearrangements for vortex transitions and crossover as a function of magnetic field and temperatures. At low magnetic fields, the flux vortices undergo a first order melting transition from a vortex lattice to a vortex liquid state with increasing temperature while another transition also occurred with increasing field at fixed temperature to a vortex glass phase at the lowest temperatures. Evidence of a frozen liquid phase was found in the intermediate field region at low temperature in the form of a lagoon in the superconducting vortex state which is in agreement with earlier observations made in BiSCCO-2212. The magnetic behaviour of the unconventional superconductor Sr₂RuO₄ was investigated using µSR to find the evidence of normal state magnetism and the nature of the vortex state. In the normal state, a weak hysteretic magnetic signal was detected over a wide temperature and field range believed to be supporting the evidence of a chiral order parameter. The nature of the vortex lattice structure was obtained in different parts of the magnetic phase diagram and the evidence of magnetic field driven transition in the lattice structure was detected from a Triangular→Square structure while the vortex lattice stayed Triangular over the entire temperature region below Tc at low fields with a disappearance of pinning at higher temperatures.

In der vorliegenden Arbeit werden die mikroskopischen Eigenschaften des supraleitenden Zustands von MgCNi3, MgB2 und einigen Seltenerd-Nickel-Borkarbiden anhand von Messungen der spezifischen Wärme untersucht. Der die Supraleitung verursachende Cooper-Paarzustand der Elektronen wird durch eine Wechselwirkung der Elektronen mit Gitterschwingungen erzeugt. Daher wird zusätzlich zur spezifischen Wärme des supraleitenden Zustands auch die des normalleitenden Zustands untersucht. Aus letzterer kann unter Berücksichtigung theoretischer Ergebnisse für die elektronische Zustandsdichte die Elektron-Phonon-Wechselwirkungsstärke bestimmt werden. Mit Hilfe eines selbstentwickelten Computerprogramms wird ausserdem das Frequenzspektrum der Gitterschwingungen abgeschätzt und mit Ergebnissen aus Neutronenstreuexperimenten verglichen. Die Energielücke des supraleitenden Zustands kann aus der spezifischen Wärme des supraleitenden Zustands bestimmt werden, die ebenso wie das obere kritische Magnetfeld Hc2(0) Hinweise auf die Elektron-Phonon-Kopplung liefert. Aus der Analyse dieser Ergebnisse und dem Vergleich mit Ergebnissen aus Transportmessungen wie der Tunnel- oder Punktkontaktspektroskopie kann gefolgert werden, inwieweit das BCS-Modell der Supraleitung modifiziert werden muss, um den supraleitenden Zustand der untersuchten Verbindungen beschreiben zu können. Dazu stehen sowohl bekannte Erweiterungen zur Berücksichtigung von verstärkter Elektron-Phonon-Kopplung als auch im Rahmen dieser Arbeit entwickelte analytische Zweibandformulierungen zur Verfügung. Untersuchungen an MgCNi3, das sich nahe einer magnetischen Instabilität befindet, zeigen, dass auftretende magnetische Fluktuationen eine Halbierung der supraleitende Übergangstemperatur Tc zur Folge haben. Der unter diesem Aspekt relativ hohe Wert von Tc=7 K ist eine Konsequenz starker Elektron-Phonon Kopplung, die im Wesentlichen durch vom Kohlenstoff stabilisierte Nickelschwingungen getragen wird. Mehrbandeffekte sind in diesem System aufgrund der Dominanz eines der Bänder an der Fermi-Kante nur für den konsistenten Vergleich unterschiedlicher Experimente von Bedeutung. So messen Transportexperimente vorrangig die Eigenschaften der schnellen Ladungsträger (Band mit der geringen partiellen Zustandsdichte), während die spezifische Wärme über die Bandanteile mittelt und daher die Eigenschaften der langsamen Ladungsträger (Band mit der hohen partiellen Zustandsdichte) reflektiert. Eine erstmalig beobachtete ausgeprägte Anomalie in der spezifischen Wärme des klassischen Mehrbandsupraleiters MgB2 (hier mit reinem Bor-10) bei etwa Tc/4=10 K kann mittels eines Zweibandmodells in Übereinstimmung mit erst kürzlich gemachten theoretischen Vorhersagen für den Fall besonders schwacher Kopplung zwischen den beiden Bändern verstanden werden. Die Stärke der Interbandkopplung ist auch von praktischem Interesse, da durch das Einbringen von Streuzentren Hc2(0) zwar erhöht wird, gleichzeitig dann aber auch im Allgemeinen die Interbandkopplung ansteigt, was eine Absenkung des gemeinsamen Tc's beider Bänder zur Folge hat. Die Analyse der spezifischen Wärme der supraleitenden Phase der nichtmagnetischen Seltenerd-Nickel-Borkarbide YNi2B2C und LuNi2B2C führt zu dem Schluss, dass sichtbare Effekte des Mehrbandelektronensystems sowohl von der Masse auf dem Platz der Seltenen Erde, als auch des Übergangsmetalls [untersucht an Lu(Ni1-xPtx)2B2C] abhängig sind. Das Signal des in der spezifischen Wärme des antiferromagnetischen HoNi2B2C sichtbaren supraleitenden Phasenübergangs ist kleiner als erwartet. Die Diskrepanz entspricht etwa einem Drittel der elektronischen Zustandsdichte und deckt sich in etwa mit Ergebnissen zu den ebenfalls magnetischen Systemen DyNi2B2C und ErNi2B2C. Im Rahmen des Mehrbandmodells kann das als natürliche Konsequenz des unterschiedlich starken Einflusses des Magnetismus auf die verschiedenen Bänder gedeutet werden.