Ergodicity Breaking Provably Robust to Arbitrary Perturbations.

We present a new route to ergodicity breaking via Hilbert space fragmentation that displays an unprecedented level of robustness. Our construction relies on a single emergent (prethermal) conservation law. In the limit when the conservation law is exact, we prove the emergence of Hilbert space fragmentation with an exponential number of frozen configurations. These configurations are low-entanglement states in the middle of the energy spectrum and therefore constitute examples of quantum many-body scars. We further prove that every frozen configuration is absolutely stable to arbitrary perturbations, to all finite orders in perturbation theory. In contrast to previous constructions, our proof is not limited to symmetric perturbations, or to perturbations with compact support, but also applies to perturbations with long-range tails, and even to arbitrary geometrically nonlocal k-body perturbations, as long as k/L→0 in the thermodynamic limit, where L is linear system size. Additionally, we identify one-form U(1) charges characterizing some nonfrozen sectors, and discuss the dynamics starting from typical initial conditions, which we argue is best interpreted in terms of the magnetohydrodynamics of the emergent one-form symmetry.

Transition between Heavy-Fermion-Strange-Metal and Quantum Spin Liquid in a 4d-Electron Trimer Lattice.

We present experimental evidence that a heavy Fermi surface consisting of itinerant, charge-neutral spinons underpins both heavy-fermion-strange-metal (without f electrons) and quantum-spin-liquid states in the 4d-electron trimer lattice, Ba_{4}Nb_{1-x}Ru_{3+x}O_{12}(|x|<0.20). These two exotic states both exhibit an extraordinarily large entropy, a linear heat capacity extending into the milli-Kelvin regime, a linear thermal conductivity at low temperatures, and separation of charges and spins. Furthermore, the insulating spin liquid is a much better thermal conductor than the heavy-fermion-strange-metal that separately is observed to strongly violate the Wiedemann-Franz law. We propose that at the heart of this 4d system is a universal, heavy spinon Fermi surface that provides a unified framework for explaining the exotic phenomena observed throughout the entire series. The control of such exotic ground states provided by variable Nb concentration offers a new paradigm for studies of correlated quantum matter.

Current-sensitive Hall effect in a chiral-orbital-current state.

Chiral orbital currents (COC) underpin a novel colossal magnetoresistance in ferrimagnetic Mn3Si2Te6. Here we report the Hall effect in the COC state which exhibits the following unprecedented features: (1) A sharp, current-sensitive peak in the magnetic field dependence of the Hall resistivity, and (2) A current-sensitive scaling relation between the Hall conductivity σxy and the longitudinal conductivity σxx, namely, σxy ∝ σxxα with α reaching up to 5, which is exceptionally large compared to α ≤ 2 typical of all solids. The novel Hall responses along with a current-sensitive carrier density and a large Hall angle of 15% point to a giant, current-sensitive Hall effect that is unique to the COC state. Here, we show that a magnetic field induced by the fully developed COC combines with the applied magnetic field to exert the greatly enhanced transverse force on charge carriers, which dictates the COC Hall responses.