Platforms for new quantum technologies - addressing the challenges in cooling and exploring the properties of strongly correlated electron systems. / van der Vliet, Harriet.

2018. 249 p.

Research output: ThesisDoctoral Thesis

Unpublished

Abstract

The work presented in this thesis is three separate experiments in the ultra-low temperature regime, linked by the use of current sensing noise thermometry, used as a probe to investigate the properties of strongly correlated electron systems. The three experiments are to measure the Kapitza boundary resistance between liquid 3He and solids, successfully cool and measure the temperature of a two dimensional electron gas, and to investigate the superconducting properties of YbRh2Si2, a new heavy fermion superconductor. The work was predominantly aimed towards creating the environment to cool a two dimensional electron gas (2DEG) and to successfully measure the temperature of the electrons in the 2DEG using a novel current sensing noise thermometer.
A leak tight immersion cell was made in order for the 2DEG to be immersed in liquid 3He and the temperature was inferred using a low heat capacity gold wire bond current sensing noise thermometer, directly coupled to the electrons in the 2DEG. The immersion cell was mounted on the nuclear stage of an adiabatic demagnetisation dilution refrigerator. Due to the Kapitza boundary resistance between the gold wire and the liquid helium being lower than assumed, the temperature of the liquid helium was more accurately being measured than the electrons in the semiconductor. Various ways to reduce the thermal boundary resistance between the 2DEG and the helium were implemented and a thermal
model to extract the temperature of the electrons was developed. The heat leak to the 2DEG was decreased significantly in iterations of the immersion cell, with the final version being metallic, filtered, and with many light tight and leak tight joints, including novel silver epoxy leak tight connections. A heat leak of 6 fW to the 2DEG was characterised and a record electron temperature of 1.4±0.1 mK was achieved. An experiment to measure the Kapitza boundary resistance and redefine it was designed, expanding on work started decades ago. An immersion cell was made to immerse a variety of foils of different thicknesses and materials in liquid helium so that the boundary resistance between the solid and the helium could be measured. The gold wire bond noise thermometer developed in the 2DEG experiment was used to measure the temperature of the gold foil as it was heated in situ with a gold wire bond heater.
A third experiment that was completed based on knowledge attained from novel noise thermometry techniques, was that the temperature and resistance of any substrate could be measured using this novel wire bond noise thermometer. A sample of Ytterbium Rhodium 2 Silicon 2 was placed in a noise thermometer configuration with the YbRh2Si2 in thermal contact with the nuclear
demagnetisation stage of the cryostat. The current sensing noise thermometer was again a gold wire bond, spot welded to the high quality single crystal sample of YbRh2Si2. Both the temperature and the resistance of the sample were measured in order to verify the transition temperature of the heavy fermion superconductor. Superconductivity was found below 4 mK and an intermediate superconducting state was found below 12 mK.
Original languageEnglish
QualificationPh.D.
Awarding Institution
Supervisors/Advisors
Award date1 Jun 2018
Publication statusUnpublished - 2018

Research outputs

ID: 29930136