An increasing number of low carrier density materials exhibit a surprisingly large transport mean free path due to inefficient momentum relaxation. Consequently, charge transport in these systems is markedly non-ohmic but rather ballistic or hydrodynamic, features which can be explored by driving current through narrow channels. Using a kinetic equation approach we theoretically investigate how a non-quantizing magnetic field discerns ballistic and hydrodynamic transport, in particular in the spatial dependence of the transverse electric field, Ey: We find that at weak magnetic fields, the curvature of Ey at the middle of the channel has an opposite sign in the ballistic and hydrodynamic regimes; Moreover, at a magnetic field corresponding to a cyclotron radius near one quarter of the channel width, the spatial profile of Ey not only reflects the transport regime but it can also be used to diagnose the specularity of the boundary. Our results demonstrate that a purely hydrodynamic approach is insufficient in the Gurzhi regime once a magnetic field is introduced. arXiv:1901.08546v1 [cond-mat.mes-hall]

No abstract

We study the Lyapunov exponent λ L in quantum field theories with spacetimeindependent disorder interactions. Generically λ L can only be computed at isolated points in parameter space, and little is known about the way in which chaos grows as we deform the theory away from weak coupling. In this paper we describe families of theories in which the disorder coupling is an exactly marginal deformation, allowing us to follow λ L from weak to strong coupling. We find surprising behaviors in some cases, including a discontinuous transition into chaos. We also derive self-consistency equations for the two-and four-point functions for products of N nontrivial CFTs deformed by disorder at leading order in 1/N .

No abstract

We study the quantum Lyapunov exponent λL in theories with spacetime-independent disorder. We first derive self-consistency equations for the two-and four-point functions for products of N models coupled by disorder at large N , generalizing the equations appearing in SYK-like models. We then study families of theories in which the disorder coupling is an exactly marginal deformation, allowing us to follow λL from weak to strong coupling. We find interesting behaviors, including a discontinuous transition into chaos, mimicking classical KAM theory.