This thesis tells the story of the Balkanization of the theory community in High Temperature Superconductivity (HTS) and of the many roles experimental evidence has been playing in the battles there. In the twenty-five years that followed the discovery of HTS, the Condensed Matter Physics (CMP) community has experienced extreme difficulty in trying to reach a consensus on a 'final' theory. I will explore some of the reasons for such dissent, starting from testimonies that I collected through personal interviews with HTS physicists. I will focus on the way experiments actively contribute to the formulation of theories. I claim that there is a tension between the different methods and aims of two scientific traditions as they implement the contribution from experiments. This tension will be illustrated through the discussion of several episodes from the history of Superconductivity and CMP research. In particular the paradigmatic quarrels between two of the major players in the history of superconductivity, physicists PW Anderson and B Matthias, will be presented to explore the meeting of theoretical and experimental driving forces and their impact on the evaluation of theories and research programmes. I will also argue that the ambiguity in the theories of evidence employed by the warring camps in HTS allows each of them to claim empirical adequacy for itself and deny it to the opponents, and I shall raise questions about whether the standards of evidence employed there are consistently applied and grounded.

No description available.

Existing as much more than just a gemstone or refractory material, boron-doped diamond is a semiconductor with enormous potential as an electronic material. Diamond will form the basis of future high-power electronic devices, radiation-hard electronics and radiation detectors. It is also a highly effective electron emitter and a unique, biocompatible material for biomedical devices. Still, many questions remain surrounding the versatile, easy to grow polycrystalline form of boron-doped diamond. What is the surface atomic structure of these films after growth? How uniform is the boron-induced conductivity? How and why does the work function vary across the films? All of these properties affect how diamond electronic devices can be designed, fabricated and used. In this thesis we investigate nanoscale variation in properties across the surfaces of a number of differently grown boron-doped diamond films under ultra-high vacuum and evaluate the potential impact of the changes in these properties to surface electronic applications of diamond. Kelvin probe force microscopy results demonstrate significant variation in work function across the diamond surface, with a step change in work function of 0.8 eV measured between hydrogen and oxygen surface terminations, and variations across a single diamond surface are calculated to correspond to a 310% change in dopant concentration . The effect of this dopant variation is demonstrated by conductive atomic force microscopy studies in which entire crystallites exhibit insulating behaviour, with significant variation in conduction also observed across the surface. Finally, scanning tunneling microscopy studies of the diamond surface demonstrate that nanoscale roughness on large microcrystals is caused by the existence of a layer of nanocrystalline diamond at the surface. These nanocrystals persist throughout the growth process and exhibit many different surface reconstructions. The implications of all of these discoveries are discussed, with possibilities and suggestions for further work given also.

This thesis presents the results obtained from de Haas-van Alphen experiments in ironbased superconductors. Measurements of the quasi particle mass in the quantum critical system BaFe2(Asl- xPxh are presented, which show strong enhancement towards the critical composition Xc = 0.3. This is found to be in good agreement with the prediction of a diverging behaviour of the effective mass close to a quantum critical point. Further results obtained on the lower and upper superconducting critical field of this system will be presented, which are found to contradict the expectations from Ginzburg-Landau theory based on results of the quasi particle mass and London penetration depth. However we can reconcile the different experimental findings on superconducting and normal state properties, in this quantum critical system, by considering a significant contribution from Abrikosov vortex core states. The importance of understanding the normal state electronic structure and interactions is shown to be essential for an understanding of the superconducting ground state of a quantum critical system. Further we will show a detailed de Haas-van Alphen study of the il1-type iron-based superconductor LiFeP and its isovalent partner LiFeAs. To understand the formation of nodes on the superconducting gap structure in systems with little change in their Fermi surface topology, is essential for the formulation of a microscopic theory of the pairing mechanism in pnictide-superconductors. 'While we find both systems to be close to the geometric nesting condition, we are able to point to different possible scenarios of the origin of nodes based on quasi particle mass enhancement. Extending the study of quasi particle mass and its relation to the superconducting properties to the stoichiometric high-temperature cuprate superconductor YBa2 CU4 Os, we aim to study the Fermi surface evolution under hydrostatic pressure. As the system has a very stable oxygen stoichiometry which does not allow it to be doped by oxygen ordering, we use hydrostatic pressure to tune the system unexpectedly leading to an increase in the superconducting critical temperature with almost no change to the quasi particle mass .

Microcrystalline indium(III) selenide was prepared from a diphenyl diselenide precursor and a range of chloroindate(III) ionic liquids via a microwave-assisted ionothermal route. A mixture of nano- and micro-sized zinc(lI) selenide materials were also prepared using the same ionothermal procedure and chlorozincate ionic liquids. Influence of the reaction temperature and variation of the cation and the anion of the ionic liquid on the product morphology and composition were investigated. All products formed were characterised using PXRO, SEM and EOX. Additional characterisation was carried out using Raman spectroscopy and photoluminescence measurements. The investigation into the production of indium(III) selenide, zinc(II) selenide and gallium(III) selenide semiconductor materials using conventional heating methods was subsequently carried out and the products were characterized using SEM, EOX and ESI-MS. Electrochemical investigations into some of the stable homogeneous liquids that were formed between the chloroindatel chlorzincate ionic liquids with the diphenyl diselenide with conventional heating, have been carried out. An additional selenium precursor, selenium tetrachloride, was also investigated with the depositions analysed using SEM and EOX. Finally, the ternary compound copper indium selenide (CIS) has been prepared as nano- and micro-sized materials through colloidal synthesis using an indium(III) selenide precursor and copper(I) chloride via a microwave-assisted ionothermal route. The crystal structures of three intermediate structures were determined after formation through an ionothermal procedure utilising metal-containing imidazolium ionic liquids and a selenium precursor with conventional heating. A comparative study into the use of microwave irradiation over conventional heating with different ionic liquids on the stoichiometry of the resulting products was carried out. The influence of the reaction temperature, reaction time, order of addition of reagents and variation of ionic liquids on the final products was investigated, which have been characterized using PXRO, SEM and EOX. I

As a multipotent tool for scientific exploration, semiconductor nanoparticles, or quantum dots (QDs), have gained enormous interest in nanoscience in the past two decades. The research presented here focused on cadmium telluride (CdTe) QDs: novel synthetic methodologies were used to prepare previously inaccessible nanomaterials based on CdTe QDs. -- CdTe/CdSe/ZnSe core/shell/shell QDs were prepared by a one-pot synthesis. The resulting QDs exhibited near infrared emission, were readily dispersed in aqueous media and applied to deep tissue imaging where emission through the skin indicated the gradual transition of the QDs via the lymphatic tract. -- Using a different synthetic approach, CdTe QDs, which were dispersed in organic media, were exposed to mercury cations in a toluene/methanol solution, resulting in CdHgTe nanoalloy formation. The optical characteristics of the resulting materials were substantially red-shifted from those of the original CdTe QDs. Structural changes were also examined and the influence of the addition of metal cations to other colloidal QDs. -- The organometallic compound Cd(TeC6H5)2 was synthesised and used as a single-source precursor for CdTe QDs. Products isolated after thermal decomposition of the single-source precursors showed strong emission of various wavelengths depending on the reaction time. The underlying chemistry on QDs formation was investigated, and CdTe/ZnS QDs were prepared using only single-source precursors. -- To make the QDs useful in biology, the surface of organically synthesised CdTe/ZnS QDs was modified with an amphiphilic protein (hydrophobin) to phase transfer the particles into aqueous solution. The QDs exhibited bright emission after phase transfer, and were applied to cell imaging in order to examine the validity as a fluorophore and the influence on a cell.

The work is conveniently divided into two sections:- (i) the electrical properties of cadmium sulphide and (ii) the optical properties of cadmium sulphide.(i) Measurements have been made on thin pure platelets of cadmium sulphide from which it is clear that electrical conduction in these samples is governed by two complex defect centres with associated levels within the forbidden gap (0.41 eV - 0.61 eV; 0.25 eV - 0.83). This has been deduced from measurements of the static current- voltage curves made over a wide range of temperatures coupled with an investigation of thermally stimulated current curves obtained after electron injection and optical excitation. The measurements also demonstrate that Lamport theory of Space Charge Limited Current Flow in an insulator with traps is applicable at low current densities but that trap emptying occurs when high current densities are passed through a crystal.(ii) Measurements of the spectral distribution of ultra-violet stimulated fluorescence at 77 K and 300 K, demonstrate the existence of electronic energy levels within the forbidden gap which can be correlated with those determined from the electrical experiments. The spectral distribution of luminescent emission has been measured in pure and doped samples from which it is concluded that edge emission is associated with both sulphur vacancies and interstitials (0.13 eV below the conduction band and 0.16 eV above the valence band respectively.). The two emission series can be separate in suitable crystals. Further work, with better resolution of the emission bands should clarify the position further. Electro-luminescence, due to the recombination of injected electrons and holes, is attributes to bulk processes which produce the edge emission series, surface and impurity recombination.

Specific heat measurements are presented on the unconventional superconductors BaFe2(Asl-xP xb YBa2Cu408 and Lio.gM060 17 · The development of a novel highprecision AC micro-calorimeter is also presented. An inexpensive and simple to fabricate SiN membrane-based AC micro-calorimeter was developed and successfully calibrated to an accuracy of 1 % between 6 K and 200 K. An appropriate thermal model was developed and the device shown to be capable of resolving transitions in heat capacity with an absolute accuracy of 5%, with good resolution of transitions down to 10 pJ /K. The total heat capacity of a sample at any given temperature can be determined to within 10%. Specific heat measurements of the supel'conducting transition in BaFe2(Asl_xP xh show that the quantum critical fluctuations renormalising the electron effective mass are robust to both temperature and magnetic field and that inter-band scattering is dominant giving equal renormalisation across the Fermi surface. The upper critical field has been shown to contradict theoretical predictions through a lack of enhancement around the quantum critical point, possibly due to mixing of antiferromagnetism in the vortex cores with the superconducting state. With reference to lower critical field measurements, the condensation energy is used to infer a strongly renormalised vortex core energy that indicates a direct enhancement of the superconducting state due to quantum criticality. The heat capacity of YBa2Cu408 was studied with magnetic fields applied along each of the orthogonal crystallographic directions. An enhancement in the density of states along the chain direction of 0.52 mJ mol- 1 K-2 was found in relatively low fields «1 T) consistent with the quenching of the superfluid density in the unhybridised region of the chain Fermi surface. Two values of the electron effective mass are deduced, mi = 3.9 ± OAme and m2 = 3 ± 0.3me, in excellent agreement with quantum oscillations. A large increase in the electron effective mass is seen on approach to the pseudogap end point up to a value of m* ~ 9.9me by analysing published data in a similar fashion. The condensation energy of Lio.gM06017 was determined and with reference to magnetoresistance data shows a breaching of any reasonable Pauli-limiting field thereby invoking an unconventional superconducting state. The superconducting critical temperature, the normal state specific heat, and the size of the superconducting transition are found to loosely correlate between different samples. Hall effect measurements on the same samples reveal that the strong upturn seen in the Hall effect cannot be a result of carrier loss thus refuting a density wave gap. A long-localisation scenario is proposed as an alternative explanation. This study also suggests that the sample dependence in Lio.gM06017 is a doping effect at all temperatures, likely due to the removal of electrons by excess oxygen. The Hall coefficient is observed to change sign at low temperatures and moderate fields.

No description available.

No description available.