Super-heavy fermion material as metallic refrigerant for adiabatic demagnetization cooling

Low-temperature refrigeration is of crucial importance in fundamental research of condensed matter physics, as the investigations of fascinating quantum phenomena, such as superconductivity, superfluidity and quantum criticality, often require refrigeration down to very low temperatures. Currently, cryogenic refrigerators with $^3$He gas are widely used for cooling below 1 Kelvin. However, usage of the gas is being increasingly difficult due to the current world-wide shortage. Therefore, it is important to consider alternative methods of refrigeration. Here, we show that a new type of refrigerant, super-heavy electron metal, YbCo$_2$Zn$_{20}$, can be used for adiabatic demagnetization refrigeration, which does not require 3He gas. A number of advantages includes much better metallic thermal conductivity compared to the conventional insulating refrigerants. We also demonstrate that the cooling performance is optimized in Yb$_{1-x}$Sc$_x$Co$_2$Zn$_{20}$ by partial Sc substitution with $x\sim$0.19. The substitution induces chemical pressure which drives the materials close to a zero-field quantum critical point. This leads to an additional enhancement of the magnetocaloric effect in low fields and low temperatures enabling final temperatures well below 100 mK. Such performance has up to now been restricted to insulators. Since nearly a century the same principle of using local magnetic moments has been applied for adiabatic demagnetization cooling. This study opens new possibilities of using itinerant magnetic moments for the cryogen-free refrigeration

Syntheses of chlorin, benzoporphyrin and bacteriochlorin derivatives

The syntheses of chlorin, benzoporphyrin, and bacteriochlorin derivatives are presented in this thesis. The key step in each synthesis involved a Diels-Alder reaction of a vinylporphyrin with an appropriate dienophile. The vinylporphyrins 81,101 and 106 were prepared in high yield using a variation of Johnson's regioselective synthesis employing dipyrromethenes 93,94,104, and 144 as crucial building blocks. As the first objective, the Diels-Alder reactions of 81 with 1,2-disubstituted vinyl sulfones were investigated in order to provide a route to chlorin derivatives which could act as intermediates in a proposed synthetic pathway for a model compound of factor 1. The regio-and stereoselectivity of the cycloadditions were examined and the appropriate regioisomers were considered for the continuation of the proposed synthetic plan. The second objective of the work was to provide a general strategy directed towards the synthesis of benzoporphyrin derivatives via a common intermediate, namely the β-unsubstituted-β'-vinylporphyrin 101. Evidence is presented that suggests an isomerization of the initial cycloadduct to another porphyrin en route to the benzoporphyrin 171. The final objective of the work was to synthesize stable bacteriochlorin derivatives to be used as photosensitizers in photodynamic therapy. The key intermediate, an A,C-divinylporphyrin 106, was synthesized via two routes, and its chemistry with olefinic and acetylenic dienophiles was studied. The resulting bis-adducts (e.g. 179 and 183) were isolated in moderate yields and were found to be stable compounds absorbing light in the 730-800 nm region.[See Thesis for Diagrams]Science, Faculty ofChemistry, Department ofGraduat