Quantum Multicriticality

TL;DR

This paper analyzes a quantum multicritical point in an itinerant magnet with competing ferromagnetic and antiferromagnetic orders, revealing that certain thermodynamic properties follow single critical point power laws and that multicriticality does not alter these behaviors.

Contribution

It provides a renormalization group analysis of a quantum multicritical point with two dynamical exponents, showing that thermodynamic scaling laws remain unchanged from single critical points.

Findings

Thermodynamic quantities follow single critical point power laws.

Antiferromagnetic correlations are suppressed by ferromagnetic fluctuations.

No difference between bicritical and tetracritical quantum points.

Abstract

Several quantum critical compounds have been argued to have multiple instabilities towards orders with distinct dynamical exponents. We present an analysis of a quantum multicritical point in an itinerant magnet with competition between ferro- and antiferromagnetic order, modelled using Hertz-Millis theory. We perform a one-loop renormalization group treatment of this action in the presence of two dynamical exponents. In two and in three dimensions, when both incipient orders are quantum critical, we find that the specific heat, thermal expansion and Gr\"{u}neisen parameter obey the same power laws as those expected for a single ferromagnetic quantum critical point. The antiferromagnetic correlation length and boundary of the antiferromagnetic ordered phase are suppressed by the dangerously irrelevant interactions with quantum critical ferromagnetic fluctuations. We find no difference between a quantum bicritical point and a quantum tetracritical point. Our results are compared with experiments on NbFe$_2$.