Odd Frequency Density Waves

Abstract

A new type of hidden order in many body systems is explored. This order appears in states which are analogues to charge density waves, or spin density waves, but involve anomalous particle-hole correlations that are odd in relative time and frequency. These states are shown to be inherently different from the usual states of density waves. We discuss two methods to experimentally observe the new type of pairing where a clear distinction between odd and even correlations can be detected: (i) by measuring the density-density correlation, both in time and space and (ii) via the conductivity which, according to the Kubo formula, is given by the current-current correlation. An order parameter for these states is defined and calculated for a simple model, illuminating the physical nature of this order.