High-temperature criticality in strongly constrained quantum systems

Abstract

The exotic nature of many strongly correlated materials at reasonably high temperatures---for instance, cuprate superconductors in their normal state---has led to the suggestion that such behavior occurs within a quantum-critical region where the physics is controlled by the influence of a phase transition down at zero temperature. Such a scenario can be thought of as a bottom-up approach, with the zero-temperature mechanisms finding a way to manifest critical behavior at high temperatures. Here we propose an alternative, top-down, mechanism by which strong kinematic constraints that can only be broken at extremely high temperatures are responsible for critical behavior at intermediate but still high temperatures. This critical behavior may extend all the way down to zero temperature, but this outcome is not one of necessity, and the system may order at low temperatures. We provide explicit examples of such high-temperature criticality when additional strong interactions are introduced in quantum Heisenberg, transverse-field Ising, and some bosonic lattice models.