Dimensional reduction at a quantum critical point

TL;DR

This study provides experimental evidence of dimensional reduction at a quantum critical point in a three-dimensional material, revealing emergent two-dimensional behavior due to frustrated lattice interactions.

Contribution

The paper demonstrates that in BaCuSi2O6, a three-dimensional system exhibits a two-dimensional quantum critical point due to frustration-induced decoupling of layers.

Findings

Decoupling of layers at the QCP causes 2D power law scaling.

Emergent lower-dimensional behavior arises despite 3D lattice structure.

Experimental verification of 2D QCP in a bulk 3D material.

Abstract

Competition between electronic ground states near a quantum critical point (QCP) - the location of a zero-temperature phase transition driven solely by quantum-mechanical fluctuations - is expected to lead to unconventional behaviour in low-dimensional systems. New electronic phases of matter have been predicted to occur in the vicinity of a QCP by two-dimensional theories, and explanations based on these ideas have been proposed for significant unsolved problems in condensed-matter physics, such as non-Fermi-liquid behaviour and high-temperature superconductivity. But the real materials to which these ideas have been applied are usually rendered three-dimensional by a finite electronic coupling between their component layers; a two-dimensional QCP has not been experimentally observed in any bulk three-dimensional system, and mechanisms for dimensional reduction have remained the subject of theoretical conjecture. Here we show evidence that the Bose-Einstein condensate of spin triplets in the three-dimensional Mott insulator BaCuSi2O6 provides an experimentally verifiable example of dimensional reduction at a QCP. The interplay of correlations on a geometrically frustrated lattice causes the individual two-dimensional layers of spin-1/2 Cu2+ pairs (spin dimers) to become decoupled at the QCP, giving rise to a two-dimensional QCP characterized by power law scaling distinctly different from that of its three-dimensional counterpart. Thus the very notion of dimensionality can be said to acquire an 'emergent' nature: although the individual particles move on a three-dimensional lattice, their collective behaviour occurs in lower-dimensional space.