The problem of localization of quantum particles by disorder is a long-standing one. It was pioneered by P. W. Anderson in 1958 who discovered that disorder can localize non-interacting degrees of freedom. However little progress was made concerning the effect of mutual interactions, like e.g. the coulomb repulsion between electrons in disordered solids.
Ten years ago a theoretical breakthrough was achieved. It was shown that a system of interacting fermions put in a static disordered potential, totally isolated from any external thermal bath, may experience a transition from an equilibrium « metallic » phase to a localized non ergodic one when the temperature is reduced below a critical T*. The transition was coined many-body localization (MBL).
This prediction has far reaching implications. First it is a fundamental progress in the basic question about the conditions under which isolated quantum systems can thermalize without the help of any external bath, and comply with equilibrium statistical mechanics. It also predicts the existence of an unprecedented kind of insulator which possesses an exactly zero conductivity (in the infinite system limit) in a finite temperature range T < T*.
Although these findings sparked a tremendous theoretical activity, however a clear experimental demonstration of MBL is still missing.
In this project we propose to search for this new phenomenon in Cooper-pair insulators. These are systems possessing Cooper pairs localized by disorder. A prototypical case is amorphous a- InOx under magnetic field. In this system, it was shown by one of the partners that electrons are efficiently decoupled from phonons at low temperature. Moreover in the Cooper-pair insulator phase, the resistance shows a divergence at finite temperature, an expected hallmark of MBL.
These first results are very promising and much broader investigations are now needed to more surely identify and study the various aspects of the MBL. We propose to investigate the different aspects of the transition using dielectric and thermoelectric measurements, as well as to search for the non-ergodic features expected to appear near and across T*. These studies will be complemented by the characterization and measurements of a second promising candidate material: insulating a-YxSi1-x. These investigations will be conducted in close interaction with the theoretical analyses developed by one partner. All partners are leading experts in the experimental and theoretical fields involved.
The project should give the first clear demonstration and rather thorough investigation of this new paradigm of quantum localization physics.
Madame Lara Faoro (Laboratoire de physique théorique et hautes énergies)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
LPTHE Laboratoire de physique théorique et hautes énergies
INEEL Institut Néel - CNRS
CSNSM Centre de Sciences Nucléaires et de Sciences de la Matière
LPEM Laboratoire de Physique et d'Etude des Matériaux
Help of the ANR 638,723 euros
Beginning and duration of the scientific project: September 2019 - 48 Months