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From the time of the unexpected discovery of the insulating nature of NiO by Verwey half a century ago, Oxide materials have continued to occupy the centre stage of condensed matter physics. The recent discovery of high temperature superconductivity in doped cuprates has given a new impetus to the study of the strongly correlated electron systems. Besides, the occurrence of Colossal Magneto-Resistance (CMR) in doped rare earth manganite has also created renewed interest in these rather old systems. Understanding of the rich and complex phase diagram of these materials and their sensitivity to small perturbations e.g. external magnetic field of a few Tesla, temperature, change in isotope etc. are of great theoretical interest and also these materials have many potential technological applications. A common feature of all these oxide materials is that the transition metal ions have partially filled d-shells. Unlike s and p-electrons which gives rise to hybridized Bloch states, the d-electrons retain their atomic nature in a solid. This gives rise to strong Coulomb interaction among d-electrons which may be comparable or more than its kinetic energy. The strong correlation effects are evident from the experimental fact that the undoped parent compounds are insulators rather than metals as suggested by band theory, which favours a metallic state for systems with one electron per unit cell since this gives rise to partially filled bands (and hence a metallic state). These insulators termed Mott insulators, arise solely due to strong electron-electron correlations as compared to the band insulators which arise due to complete filling of one electron bands thereby giving rise to a gap (band gap)in the excitation spectra. The delicate competition between the kinetic energy and the Coulomb energy for d-electrons is broadly responsible for the wide variety of phenomena like Mott metal-insulator transition (MIT), magnetic transitions, charge ordering, orbital ordering, ferro/antiferroelectricity, and most interestingly the observation of high Tc superconductivity in doped cuprates. In this thesis we will restrict our interest to one such class of oxide materials, namely the doped rare earth manganites. In Chapter 1 we give a brief overview of the structure and basic interactions present in the doped manganites. Also, in the same Chapter we give a brief introduction to the phenomenology of manganites, particularly its phase diagram in the doping and temperature plane and various experimental features, e.g. the wide variety of phase transitions and phenomena particularly the observation of CMR, charge ordering and incipient meso-scale phase separations etc.. Then we briefly introduce a recently proposed microscopic model which is believed to be a minimal model which, for the first time, includes the three most important interactions present in the manganites namely the following -1)coupling of the orbitally degenerate eg electrons to local lattice distortions of Jahn-Teller type which gives rise to two species of electrons. The one denoted by by ℓ is associated with Jahn-Teller effects and hence is localized whereas the other denoted by b is an extended state and propagates through the lattice. 2) The strong Hund’s couplingof ℓ and b electrons to the t2g core spin and 3) the strong Coulomb correlation between the two species of electrons. Additionally, the model includes a new doping dependent ferromagnetic exchange between the t2g core spins which can arise from “virtual double exchange” mechanism which will be discussed in great detail in Chapter 1 . Finally, we give a brief account on Dynamical Mean Field Theory (DMFT) and Numerical Renormalization Group (NRG) as an impurity solver for the single impurity problem arising under single site DMFT approximation. In Chapter 2 we study the effect of inter-site ℓ - b hybridization on the ‘ℓ - b’ model. The single impurity problem arising under DMFT approximation has close connection with the Vigman-Finkelshtein (VF)model. Then we briefly introduce the VF model and bring out its close connection with the impurity problem. We consider both the particle-hole symmetric as well as the U → ∞ particle-hole asymmetric cases. We derive various spectral functions at T = 0K and discuss the nature of fixed points under various circumstances. We explicitly show that for the particle-hole symmetric case the Hamiltonian flows from X-ray edge singularity fixed point to Free Electron fixed point under Renormalization Group transformation. This is evident from the spectral properties of the model. We write down the effective Hamiltonian at the free electron fixed point. For the particle-hole asymmetric case the model flows from X-ray edge singularity fixed point to Free Electron/Strong Coupling fixed point with additional potential scattering terms. We write down the effective Hamiltonian at this fixed point and derive various leading order deviations. We found all of them to be irrelevant in nature also most interestingly the quasi-particles describing the under lying Fermi liquid state are found to be asymptotically non-interacting. We also calculate the Fermi liquid parameter, z, by analyzing the energy level structure of a non-interacting Hamiltonian with effective renormalized parameter. Also, we consider the case of ‘self consistent bath hybridization’ without ℓ - b hybridization for Bethe lattice with infinite coordination. Low energy qualitative features are found to be same but some of the high energy features get qualitatively modified. In Chapter 3 we discuss the transport properties of doped manganites in the insulating phases and also the Hall effect in the metallic phase. In the first part of this chapter we calculate the resistivity based on the ‘ℓ - b’model and try to fit it to the semiconducting form: ρ(T )= ρ0(T /T0)−nexp[Δ(T )/kBT ] and extract the “transport gap”, Δ(T ). This gap can be characterized in terms of the “spectral gap” which can be defined for the ℓ - b model. It is found that the transport gap in the paramagnetic phase can be characterized in terms of the near constant “spectral gap” in this phase whereas the same in the ferromagnetic phase can be characterized in terms of the zero temperature spectral gap. In the last part of this chapter we calculate the Hall resistivity (ρxy) of these materials in the metallic phase. Ρxy is found to be negative and linear in applied field -quite consistent with the experimental findings but this fails to explain the positive linear Hall resistivity at low temperatures and its crossover as a function of field and temperature. We then present a reasonable explanation for this discrepancy and support it by calculating the Hall density of states for a two band “toy model” involving inter species hybridization. In Chapter 4 we calculate the optical conductivity, σ(ω), in ℓ - b model. σ(ω) arises from two independent processes. One of the processes involves ‘b’ electrons only and termed as ‘b - b channel’ and this gives rise to a Drude peak in the low frequency region. another process termed as the ‘ℓ - b channel’ involves hopping of an ℓ-electron to a neighbouring empty site and transforms into a ‘b’like state. This process gives rise to a broad mid-infrared peak. The total conductivity is the sum of contributions from these two incoherent channels. Calculated σ(ω) for metallic systems shows lot of similarities with experimental observations particularly the temperature evolution of the mid-infrared peak and the spectral weight transfer between the two peaks. But for the insulating systems the calculated optical conductivity showed trends similar to more recent experimental observations on some insulating systems (x =0.125) but contradicts with earlier experimental observations on some other insulating system (x =0.1). Finally, in the concluding chapter, we summarize results from all the chapters and also sketch some possible future directions of investigations.

Das Forschungsgebiet der Kuprat- oder Hochtemperatursupraleiter (HTSL) ist bis heute einer der lebendigsten Bereiche der Physik kondensierter Materie. Ein besonderer Stellenwert kommt dem normalleitenden Zustand oberhalb TC zu, dessen Verständnis wesentlich zu einer Theorie der Hochtemperatur-Supraleitung beitragen könnte. Gegenstand dieser Arbeit ist die Untersuchung der Elektronenstruktur von HTSLn der Bi2Sr2CaCu2O8+[delta](Bi2212)-Familie nahe der Fermifläche im normalleitenden Zustand. Die Experimente wurden mittels winkelaufgelöster Photoemissionsspektroskopie (ARPES) durchgeführt, wobei die hohe Auflösung in Energie und Impuls recht genaue Rückschlüsse auf die Spektralfunktion und die Übergangs-Matrixelemente erlaubt. Die wichtigsten experimentellen Ergebnisse sind: 1) Hochaufgelöste ARPES-Fermiflächenkarten von Bi2212 und (Pb,Bi)2212 zeigen folgendes Bild: Die Hauptfermifläche ist lochartig und um die Ecken der Brillouinzone zentriert. Weiterer Bestandteil der Elektronenstruktur ist die Schattenfermifläche. Bei reinem Bi2212 treten außerdem extrinsische Beugungskopien der Fermifläche auf. 2) ARPES-Messungen entlang der -M-Richtung von Bi2212 zeigen eine starke Abhängigkeit von der Anregungsenergie, die auf starken Matrixelement-Effekten beruht. Verschiedene Methoden zur Bestimmung von kF zeigen, dass die Daten konsistent mit einer lochartigen Topologie der Hauptfermifläche sind. Des Weiteren wird die Qualität verschiedener Methoden zur Bestimmung von kF bei starken Matrixelement-Effekten bewertet. 3) Die Hauptfermifläche von (Pb,Bi)2212 behält ihre lochartige Topologie über einen großen Dotierungsbereich nahe optimaler Dotierung. 4) Erste hochaufgelöste ARPES-Messungen des Zirkulardichroismus wurden an (Pb,Bi)2212 durchgeführt. In der verwendeten nicht-chiralen Messanordnung wird ein CDAD (Circular Dichroism in the Angular Distribution of Photoelectrons)-Effekt beobachtet, dessen Asymmetrie antisymmetrisch bezüglich der Zweischicht-aufgespaltenen Zustände ist.

<p>High-temperature superconductors have been intensely studiedsince the discovery, almost 20 years ago. Their layeredstructure, extreme type-II behavior, large anisotropy, andstrong fluctuations have led to a large number of new andinteresting problems. In this work the resistive transitionhave been studied in YBa₂Cu₃O_7-δsingle crystals, from the superconductingfluctuations above T<i>c</i>down to the vortex matter near the onset of resistivity.Superconducting fluctuations above T<i>c</i>are suitably studied by measuring themagnetoconductivity Δσ = 1/ρ(B)-1/ρ(0).Such experiments were performed on untwinned, optimally dopedsamples, for<b>I</b>||<b>ab</b>. For<b>H</b>||<b>c</b>, fluctuations in the CuO-chains was not important, andfluctuations in the CuO2 planes seemed isotropic. The in-planecoherence length anisotropy was determined to be close tounity. For<b>H</b>||<b>ab</b>, Δσ<i>b</i>depended on the field direction, indicating an effect ofthe chains on the magnetoconductivity in thisconfiguration.</p><p>The nature of the vortex phase below T<i>c</i>can be examined through vortex-correlation measurements.Using a modified pseudo-flux transformer (PFT) geometry forin-plane experiments, the magnetic field could be appliedparallel to both the a, b, and c axis. A strong Lorentz forcewas detected, and observed to be a requirement for the meltingstep feature. The resistive anisotropy close to T<i>c</i>was strongly field and temperature dependent, which wasexplained by an almost isotropic fluctuation contribution.</p><p>The effect of anisotropy on vortex-liquid correlation infields<b>H</b>||<b>ab</b>, was studied by<i>c</i>-axis transport measurements in oxygen-deficientsamples, using a PFT configuration. Increasing disordersuppressed both longitudinal and transverse correlation. Inheavily underdoped samples, vortex correlation was enhanced formagnetic fields exactly aligned with the<i>ab</i>plane because of the strong anisotropy. Also, thesolid-to-liquid transition temperature became nearly fieldindependent.</p><p>Attempts were also made to connect the fluctuation regime ofthe resistive transition with the vortex region,by accountingfor critical fluctuations close to the transition.</p><p><b>Keywords:</b>high-temperature superconductors, YBa₂Cu₃O_7-δ, untwinned YBCO, oxygen deficiency,superconducting fluctuations, magnetoconductivity, in-planeanisotropy, vortex liquid, vortex solid, vortex correlation</p>

In dieser Arbeit werden Messungen der elektrischen und thermischen Transportkoeffizienten an einem antiferromagnetisch ordnenden Iridat und FeAs-basierten Hochtemperatursupraleitern vorgestellt und analysiert. Iridate sind Materialien mit starker Spin-Bahn-Kopplung. In dem zweidimensionalen Vertreter Sr_2IrO_4 führt diese Kopplung zu isolierendem Mott-Verhalten mit gleichzeitiger antiferromagnetischer Ordnung der gekoppelten Spin-Bahn-Momente. Somit stellt Sr2IrO4 ein Modellsystem für die Untersuchung magnetischer Anregungen dieser Momente in Iridaten dar. Die Analyse der Wärmeleitfähigkeit von Sr_2IrO_4 liefert erstmals klare Hinweise auf magnetische Wärmeleitung in den Iridaten. Die extrahierte magnetische freie Weglänge gibt Aufschluss über die Streuprozesse der zum Wärmetransport beitragenden Magnonen und lässt Schlüsse über die Anregungen des gekoppelten Spin-Bahnsystems zu. Die FeAs-Hochtemperatursupraleiter haben aufgrund ihrer geschichteten Kristallstruktur einen hauptsächlich zweidimensionalen Ladungstransport. Die Phasendiagramme dieser Materialien setzen sich aus Ordnungsphänomenen zusammen, die Magnetismus, Supraleitung und eine Strukturverzerrung umfassen. Das Hauptaugenmerk richtet sich auf die Reaktion der Transportkoeffizienten mit den sich ausbildenden Phasen in Vertretern der 111- und 122-Familien unter chemischer Dotierung innerhalb und außerhalb der Schichtstruktur. Mithilfe von Widerstand und magnetischer Suszeptibilität lassen sich Phasendiagramme der verschiedenen Supraleiterfamilien konstruieren. In ausgewählten Fällen werden der Hall-Koeffizient und elektrothermische Transporteffekte genutzt, um das Phasendiagramm näher zu erforschen. Der Großteil der Untersuchungen zeigt omnipräsente elektrische Ordnungsphänomene, die als nematische Phase bezeichnet werden. Die Messdaten zeigen, dass die Wärmeleitfähigkeit und der Nernst-Koeffizient dominant von Fluktuationen, die der nematischen Phase vorausgehen, beeinflusst werden. Aus den Ergebnissen der Nernst-Daten an dotiertem BaFe_2As_2 werden Schlüsse über die der nematischen Phase zugrunde liegenden Mechanismen des korrelierten Elektronensystems gezogen.

Electronic structure calculations constitute a valuable tool to predict the properties of materials. In this study we propose an efficient scheme to study correlated electron systems with essentially only one free parameter, the screening length of the Coulomb potential. A general reformulation of the exchange energy of the correlated electron shell is combined with this method in order to analyze the calculations. The results are interpreted in terms of different polarization channels, due to different multipoles. The method is applied to various actinide compounds, in order to increase the understanding of the complicate behaviour of 5f electrons in these systems. We studied the non-magnetic phase of δ-Pu, where the spin polarization is taken over by a spin-orbit-like term that does not break the time reversal symmetry. We also find that a non-trivial high multipole of the magnetization density, the triakontadipole, constitutes the ordering parameter in the mysterious hidden order phase of the heavy-fermion superconductor URu2Si2. This type of multipolar ordering is also found to play an essential role in the hexagonal-based superconductors UPd2Al3,  UNi2Al3 and UPt3 and in the dioxide insulators UO2, NpO2 and PuO2. The triakontadipole moments are also present in all magnetic actinides we considered, except for Cm. These results led us to formulate a new set of rules for the ground state of a system, that are valid in presence of strong spin-orbit coupling interaction instead of those of Hund; the Katt's rules. Finally, we applied our method to a new class of high-Tc superconductors, the Fe-pnictides, where the Fe 3d electrons are moderately correlated. In these materials we obtain the stabilization of a low spin moment solution, in agreement with experiment, over a large moment solution, due to the gain in exchange energy in the formation of large multipoles of the spin magnetization density. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 705

In dieser Arbeit werden Resultate elektronischer Transportmessungen von supraleitenden Eisenpniktiden und ferromagnetischen Heusler-Verbindungen diskutiert. Die Eisenpniktide sind eine neuartige Klassen von Hochtemperatursupraleitern, deren Eigenschaften sich aus einem Zusammenspiel von Supraleitung und Magnetismus ergeben. Während die sogenannten 122-Pniktide Antiferromagnetismus aufweisen und unter Dotierung in einen supraleitenden Zustand übergehen, konnte in dotiertem LiFeAs Ferromagnetismus beobachtet werden. Undotiert hingegen zeigt dieses Material interessante supraleitende Eigenschaften. Die Heusler-Verbindungen sind u.a. durch ihren Ferromagnetismus bekannt. Das hier untersuchte Co2FeSi ist einer der stärksten Ferromagnete. Der in diesem Material vorhergesagte vollständig spinpolarisierte elektronische Transport, d.h. alle Leitungselektronen besitzen den gleichen Spin, konnte nachgewiesen werden. Die hier genannten Eigenschaften können exzellent mit der Methode der elektronischen Transportmessungen untersucht werden. Deren Ergebnisse aus Messungen an Einkristallen werden in dieser Arbeit diskutiert. / In this work, results of electronic transport measurements are discussed for superconducting iron pnictides as well as for ferromagnetic Heusler compounds. The iron pnictides are a recently discovered class of high temperature superconductors where magnetism might play a crucial role. While the 122-pnictides show antiferromagnetism and migrate to the superconducting state upon doping, ferromagnetism has been observed in doped LiFeAs. On the other hand, in the undoped state this material shows interesting superconducting properties. Among other propierties, Heusler compounds are well known due to their ferromagnetism. Co2FeSi, which was investigated in this work, is one of the strongest ferromagnets. Beside this, one predicts this compound to be a half-metallic ferromagnet with completely spin polarized electronic transport where all conducting electrons have the same spin. The here addressed properties can well be investigated with the method of electronic transport measurements, whose results on single crystals are discussed in this work.

Detailed, accurate measurements of critical current density and resistivity to determine the upper critical field have been made on a technological NbsAl conductor in magnetic fields up to 15 T, temperatures from 4.2 K up to the critical temperature and in the strain range from -1.8% to 0.7%. The uncertainty in temperature above 4.2 K was equivalent to ± 100 mK with a stability during the measurements of < 5 mK up to a limiting current of 80 A and a typical noise level of 1 µ Vm(^-1).When B(_c2){T,ε) is defined at 5%pn, 50%pn or 95%/%pn, an empirical relation is found where and an approximate relation, holds. The Jε data were parameterised using F(_p) = J(_E)B = A(ε)[Bc(_2)](^n)b(^p)(1-b)(^9) where b = B/B(_c2)(T,ε). When B(_c2)(T,ε) is constrained to be the value at 50%pn or 95%pn, the scaling law for F(_p) breaks down such that p and q are strong functions of temperature and q is also a strong function of strain. However, when B(_c2)(T,ε) is defined at 5%pn, there is good scaling where p and q are constants independent of temperature and strain. F(_p) can also be approximated by a Kramer form where the Ginzburg-Landau constant is γ is the electronic density of states and is interpreted as the average B(_c2) for the bulk where percolative current flow occurs. The critical current density of Hot Isostatic Pressed (HIP'ed) and unHIP'ed Nb(_3)Sn Modified Jelly Roll wires has also been measured at 4.2 K. The critical current and upper critical field were decreased for the HIP'ed sample. The reduced upper critical field of the HIP'ed wire was found to be less sensitive to strain than the unHIP'ed wire. The exponent of B(_c2) in the flux pinning scaling law increased from 0.86 to 2.14 as a result of the HIP processing.

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