Quantum Criticality in Quasi-Two Dimensional Itinerant Antiferromagnets

TL;DR

This paper links quantum critical fluctuations in quasi-2D itinerant antiferromagnets to a dissipative quantum XY model, explaining linear resistivity and specific heat behaviors observed in materials like Fe-based superconductors.

Contribution

It introduces a canonical transformation connecting AFM quantum critical fluctuations to a dissipative quantum XY model, highlighting topological excitations and a dynamical critical exponent of infinity.

Findings

Resistivity varies linearly with temperature.

Specific heat shows T ln T dependence.

Predictions for experimental tests are provided.

Abstract

Quasi-two dimensional itinerant fermions in the Anti-Ferro-Magnetic (AFM) quantum-critical region of their phase diagram, such as in the Fe-based superconductors or in some of the heavy-fermion compounds, exhibit a resistivity varying linearly with temperature and a contribution to specific heat or thermopower proportional to $T \ln T$. It is shown here that a generic model of itinerant AFM can be canonically transformed such that its critical fluctuations around the AFM-vector $Q$ can be obtained from the fluctuations in the long wave-length limit of a dissipative quantum XY model. The fluctuations of the dissipative quantum XY model in 2D have been evaluated recently and in a large regime of parameters, they are determined, not by renormalized spin-fluctuations but by topological excitations. In this regime, the fluctuations are separable in their spatial and temporal dependence and have a dynamical critical exponent $z =\infty.$ The time dependence gives $\omega/T$-scaling at criticality. The observed resistivity and entropy then follow directly. Several predictions to test the theory are also given.