THERMES-2D

Studies of electronic transport in GaAs/AlGaAs two-dimensional electron gases (2 DEGs.) have resulted in great advances in our understanding of mesoscopic and correlated physics. In particular, the exploration of the quantum Hall regime (QHR) has shown itself very advantageous, with its abundant physics of electron correlations, as well as the possibility to make use of the chiral transport in one-dimensional channels. In contrast, only recently have experiments shown the importance of thermodynamic investigations of low-dimensional and mesoscopic systems. With the THERMES-2D project we wish to develop novel, thermodynamic tools for the experimental study of mesoscopic, 2DEG-based systems, focusing on the specific heat cp, a bulk property with contributions from all degrees of freedom of a given thermodynamic system. The electron contribution to cp is a probe sensitive to quantum phase changes, excess of entropy, the electron density of states, and to the electronic effective mass. Within the frame of this project we will search for a better understanding of the correlated physics in the (fractional and integer) QHR The THERMES-2D consortium exhibit an indispensible synergy for this purpose: the world-leading expertise in high sensitive specific heat measurements of the NEEL team, the capability of the C2N team to furnish the very special membrane samples needed, and the expertise in quantum Hall measurements at low temperatures and high magnetic fields of the LNCMI team. The concept is to study 2DEGs on sub-micrometer thin membranes using the unique NEEL nanocalometry setup, which allows us to measure extremely small specific heat variations at low temperature. In such samples the specific heat contribution from the GaAs substrate is limited, making it feasible to study the cp of the dilute electron gas. Combining the skills of the partners makes the project perfectly suited for exploring the rich 2D physics in high magnetic fields. The electrons of a 2DEG condense into a sequence of distinct quantum phases, which makes the system an ideal test bed for specific heat studies of correlated physics. The competition between disorder, interaction and/or spin leads to a number of many-body quantum phases, such as fractional quantum Hall states, Wigner crystals, and ferromagnetic states. Through our cp measurements we will search for signatures of spin collective excitations (skyrmions), specific temperature-dependent features at the various phase boundaries, and the existence of new phase transitions between the quantum Hall states. In addition, the evaluation of the entropy of a 2DEG can provide important physical information. Though entropy is a fundamental thermodynamic quantity, its absolute value cannot be directly measured; only changes in entropy or relative values are physically relevant. The ideal way to gain access to this crucial thermodynamic quantity is by specific heat measurements versus temperature. An excess of entropy is predicted in various 2DEG states; it may be the signature of non-abelian anyon statistics or linked to quantum criticality at a quantum phase transition. There is also a strong experimental motivation for pursuing the thermodynamic measurements on 2DEGs in this project. Following the demands of the compelling physics that we will encounter, we will push to extend this expertise even further, in terms of improvements in measurements at low temperature and low noise, in innovative thermometry, and in sample realisation.