The compound Sr3Ru2O7 of the strontium ruthenate family has been intensely studied because experimental evidence suggests that quantum fluctuations dominate the magnetic phase diagram in the vicinity of a novel low-temperature phase. In order to understand the interplay between the quantum critical fluctuations and the phase formation, comprehensive thermodynamic information is essential. This thesis reports the results of both specific-heat and magnetocaloric experiments carried out with a bespoke experimental apparatus whose design particularly addresses the demanding constraints of the low-temperature, high-magnetic-field environment. The experimental data give evidence for unusual thermodynamic properties of the novel phase and its bounding phase transitions. Furthermore they show that the phase formation takes place against a background of strongly peaking entropy, suggesting that quantum criticality plays a key role in the physics of this system.

Zusammenfassung
Our department nominated this thesis for a Springer award because we regard it as an outstanding piece of work, carried out with a remarkable level of independence. Andreas Rost joined us in 2005, as one of the inaugural Prize Students of the Scottish Universities Physics Alliance. Our research group has been working on Sr Ru O , in collaboration with our colleagues in the group of Professor Y. Maeno 3 2 7 at Kyoto, since 1998. By early 2005 we had tantalising evidence that a novel phase was forming at very low temperatures, in an overall phase diagram dominated by quantum ?uctuations. We knew that comprehensive thermodynamic information would be needed in order to understand how this was happening, and that the demanding constraints of low temperature and high magnetic ?eld meant that bespoke apparatus would need to be constructed. Andreas had studied the speci?c heat of glasses below 50 mK during his diploma thesis work at Heidelberg, and was brimming with ideas about how to proceed. We gave him advice, and constantly discussed the physics with him, but quickly realised that the best way to proceed practically was to give him a budget, and let him take the main design decisions, double-checking with us from time to time.

Inhalt
Background Physics.- Thermodynamic Measurements of Entropy.- Design and Characterisation of Novel Experimental Setup.- Experimental Results and Discussion.- Conclusions and Future Work.- Appendices.