Calorimetry under extreme conditions

Abstract: This licentiate thesis presents developments of nanocalorimetry systems tailored for use under extreme conditions such as high pressure, intense magnetic fields, and low temperature. Nanocalorimetry is a powerful approach to study strongly correlated systems like superconductors, heavy fermions, and quantum materials with non-trivial magnetic or electronic properties, materials with emergent magnetic orders, as well as quasicrystals. Introducing high pressure or magnetic fields as tuning parameters in specific heat measurements at low temperatures can enhance the understanding of underlying physical properties of such materials.The key component of calorimeters is the thermometer. A thin-film thermometer based on a composite ceramic metal oxide has been developed. It shows high sensitivity and negligible magnetoresistance over a broad temperature range. Two different nanocalorimeters are fabricated starting from an existing nanocalorimeter design, a high-pressure nanocalorimeter and a calorimeter for sample rotations in high magnetic fields. The high-pressure nanocalorimetry setup involves a nanocalorimeter built on a robust substrate combined with a diamond anvil cell, a gasket sandwich with electric leads, and an optical setup for pressure detection through ruby fluorescence spectroscopy. The high-field nanocalorimeters are fabricated on SiNx membranes for specific heat measurements down to 30 mK. Miniaturization is performed to extend their use for angular-dependent measurements in high magnetic fields, so far used up to 41 T. Reducing the calorimeter platform size in both calorimeters is achieved by a method of plasma etching performed after device fabrication.Specific heat measurements of Eu-doped GdCd7.88 quasicrystals and GdCd6 approximant systems are performed in fields up to 12 T. The preliminary results show the presence of spin-glass behavior in the quasicrystals and an antiferromagnetic transition in the approximant crystals at low temperatures.