Conductivity of hard core bosons: A paradigm of a bad metal

Abstract

Two-dimensional hard core bosons suffer strong scattering in the high-temperature resistive state at half filling. The dynamical conductivity σ (ω) is calculated using nonperturbative tools such as continued fractions, series expansions, and exact diagonalization. We find a large temperature range with linearly increasing resistivity and broad dynamical conductivity, signaling a breakdown of Boltzmann-Drude quasiparticle transport theory. At zero temperature, a high-frequency peak in σ (ω) appears above a "Higgs mass" gap and corresponds to order-parameter magnitude fluctuations. We discuss the apparent similarity between conductivity of hard core bosons and phenomenological characteristics of cuprates, including the universal scaling of Homes [Nature (London) 430, 539 (2004)]. © 2010 The American Physical Society.