Forum de Physique Statistique à l'École Normale Supérieure

Next seminar

28 Janurary, 11h, L378, Mikhail Feigelman

Evidence for a Two-Dimensional Quantum Glass State at High Temperatures

 Disorder in quantum many-body systems can drive transitions between ergodic and non-ergodic phases, yet the nature of these transitions remains intensely debated. Using a two-dimensional array of superconducting qubits (their number n was up to 70), we study an interacting spin-1/2 model with XY coupling at high temperature in a strong random field along Z direction. Crucially, we were tracking dynamics both in real space and in Hilbert space. Over a broad disorder range, 10 < W < 35, we observe an intermediate non-ergodic regime with glass-like characteristics: physical observables become broadly distributed and some - but not all - degrees of freedom are effectively frozen. The Hilbert space return probability shows slow power-law decay, R(t)∼ t^(−η) consistent with finite-temperature quantum glassiness. Exponent η is found to show unusual dependence on the system size, η(W)∼ κ(W) n^2. In the same regime, we detect the onset of a finite Edwards-Anderson order parameter and the disappearance of spin diffusion. By contrast, at lower disorder, spin transport persists with a nonzero diffusion coefficient. Our experimental results are supplemented by semi-quantitative theory and by numerical simulations on smaller systems. In particular, we demonstrate the existence of an intermediate quantum state with de-phasing but without relaxation - within a model of spins-1/2 residing at the Cayley tree. Thus we demonstrate the presence of non-ergodic glass-like state in two-dimensional quantum spin-1/2 system - that is not, however, fully localized in the sense of many-body-localization (MBL). The glassy phase we found is characterized by intrinsic spin de-phasing rate that is much larger than typical spin relaxation rate. At very strong disorder W > 30, the exponent η(W) drops sharply towards zero for all studied system sizes, indicating plausible second transition into fully MBL state.

Upcoming seminars

4 February, Bastien Lapierre

Nonequilibrium probes of quantum geometry in gapless systems

11 February, Pierfrancesco Urbani

18 February, Max McGinley 

25 March, Laura Foini

1 April, Clément Hongler

8 April, Jean-Noël Fuchs

15 April, Sarah Loos