Unparticles and Anomalous Dimensions in the Cuprates

TL;DR

This paper proposes a unifying framework using unparticles and scale invariance to explain the anomalous optical and transport properties of cuprates, incorporating flavor-dependent parameters and anomalous dimensions.

Contribution

It introduces a scale-invariant unparticle model with flavor-dependent parameters to describe cuprate phenomenology, explaining fractional dynamical exponents and anomalous current dimensions.

Findings

Effective mass variation explains observed transport scaling.

Fractional dynamical exponents are achievable through running mass.

Non-zero anomalous current dimension is necessary regardless of bare exponent.

Abstract

Motivated by the overwhelming evidence some type of quantum criticality underlies the power-law for the optical conductivity and $T-$linear resistivity in the cuprates, we demonstrate here how a scale-invariant or unparticle sector can lead to a unifying description of the observed scaling forms. We adopt the continuous mass formalism or multi band (flavor) formalism of the unparticle sector by letting various microscopic parameters be mass-dependent. In particular, we show that an effective mass that varies with the flavor index as well as a running band edge and lifetime capture the AC and DC transport phenomenology of the cuprates. A key consequence of the running mass is that the effective dynamical exponent can differ from the underlying bare critical exponent, thereby providing a mechanism for realizing the fractional values of the dynamical exponent required in a previous analysis\cite{ Hartnoll:2015sea}. We also predict that regardless of the bare dynamical exponent, $z$, a non-zero anomalous dimension for the current is required. Physically, the anomalous dimension arises because the charge depends on the flavor, mass or energy. The equivalent phenomenon in a $d+1$ gravitational construction is the running of the charge along the radial direction. The nature of the superconducting instability in the presence of scale invariant stuff shows that the transition temperature is not necessarily a monotonic function of the pairing interaction.