Quantum order-by-disorder near criticality and the secret of partial order in MnSi

TL;DR

This paper introduces a unifying framework based on quantum order-by-disorder to explain phase reconstruction near metallic quantum-critical points, specifically applied to MnSi, revealing how fluctuations influence magnetic order and match experimental data.

Contribution

It applies the quantum order-by-disorder concept to MnSi, explaining partial magnetic order and fluctuation effects near quantum criticality.

Findings

Fluctuations increase the spiral wave vector near criticality.

Reorientation of magnetic order away from lattice directions occurs.

Theoretical results agree with neutron scattering data.

Abstract

The vicinity of quantum phase transitions has proven fertile ground in the search for new quantum phases. We propose a physically motivated and unifying description of phase reconstruction near metallic quantum-critical points using the idea of quantum order-by-disorder. Certain deformations of the Fermi surface associated with the onset of competing order enhance the phase space available for low-energy, particle-hole fluctuations and self-consistently lower the free energy. Applying the notion of quantum order-by-disorder to the itinerant helimagnet MnSi, we show that near to the quantum critical point, fluctuations lead to an increase of the spiral ordering wave vector and a reorientation away from the lattice favored directions. The magnetic ordering pattern in this fluctuation-driven phase is found to be in excellent agreement with the neutron scattering data in the partially ordered phase of MnSi.