Quantum fluctuations lead to glassy electron dynamics in the good metal regime of electron doped KTaO3.

One of the central challenges in condensed matter physics is to comprehend systems that have strong disorder and strong interactions. In the strongly localized regime, their subtle competition leads to glassy electron dynamics which ceases to exist well before the insulator-to-metal transition is approached as a function of doping. Here, we report on the discovery of glassy electron dynamics deep inside the good metal regime of an electron-doped quantum paraelectric system: KTaO3. We reveal that upon excitation of electrons from defect states to the conduction band, the excess injected carriers in the conduction band relax in a stretched exponential manner with a large relaxation time, and the system evinces simple aging phenomena-a telltale sign of glassy dynamics. Most significantly, we observe a critical slowing down of carrier dynamics below 35 K, concomitant with the onset of quantum paraelectricity in the undoped KTaO3. Our combined investigation using second harmonic generation technique, density functional theory and phenomenological modeling demonstrates quantum fluctuation-stabilized soft polar modes as the impetus for the glassy behavior. This study addresses one of the most fundamental questions regarding the potential promotion of glassiness by quantum fluctuations and opens a route for exploring glassy dynamics of electrons in a well-delocalized regime.

Quantum-to-Classical Crossover in Many-Body Chaos and Scrambling from Relaxation in a Glass.

Chaotic quantum systems with Lyapunov exponent λ_{L} obey an upper bound λ_{L}≤2πk_{B}T/ℏ at temperature T, implying a divergence of the bound in the classical limit ℏ→0. Following this trend, does a quantum system necessarily become "more chaotic" when quantum fluctuations are reduced? Moreover, how do symmetry breaking and associated nontrivial dynamics influence the interplay of quantum mechanics and chaos? We explore these questions by computing λ_{L}(ℏ,T) in the quantum spherical p-spin glass model, where ℏ can be continuously varied. We find that quantum fluctuations, in general, make paramagnetic phase less and the replica symmetry-broken spin glass phase more chaotic. We show that the approach to the classical limit could be nontrivial, with nonmonotonic dependence of λ_{L} on ℏ close to the dynamical glass transition temperature T_{d}. Our results in the classical limit (ℏ→0) naturally describe chaos in supercooled liquid in structural glasses. We find a maximum in λ_{L}(T) substantially above T_{d}, concomitant with the crossover from simple to slow glassy relaxation. We further show that λ_{L}∼T^{α}, with the exponent α varying between 2 and 1 from quantum to classical limit, at low temperatures in the spin glass phase.