Unconventional quantum criticality emerging as a new common language of transition-metal compounds, heavy-fermion systems, and organic conductors

TL;DR

This paper reviews various types of unconventional quantum criticalities in transition-metal compounds, heavy-fermion systems, and organic conductors, highlighting new universality classes and their experimental signatures.

Contribution

It introduces the concept of quantum tricritical points and marginal quantum critical points, expanding understanding of quantum criticality beyond conventional models.

Findings

Divergence of order-parameter and uniform fluctuations at QTCP

Identification of the marginal quantum critical point (MQCP) with unique universality

Experimental support from studies on V2-xCrxO3 and organic conductors

Abstract

We analyze and overview several different unconventional quantum criticalities. One origin of the unconventionality is the proximity to first-order transitions. The border between the first-order and continuous transitions is described by a quantum tricritical point (QTCP) for symmetry-breaking transitions. One of the characteristic features is the concomitant divergence of order-parameter and uniform fluctuations in contrast to the conventional quantum critical point (QCP). Several puzzling non-Fermi-liquid properties are referred to be accounted for as in the cases of YbRh2Si2, CeRu2Si2 and beta-YbAlB4. Another more dramatic unconventionality appears in this case for topological transitions such as metal-insulator and Lifshitz transitions. This border, the marginal quantum critical point (MQCP), belongs to an unprecedented universality class with diverging uniform fluctuations at zero temperature. The MQCP has a unique feature by a combined character of symmetry-breaking and topological transitions. The theoretical results are supported by experimental indications for V2-xCrxO3 and an organic conductor kappa-(ET)2Cu[N(CN)2]Cl. Identifying topological transitions also reveals how non-Fermi liquid appears as a phase in metals. The theory also accounts for the criticality of a metamagnetic transition in ZrZn2, by interpreting it as an interplay of Lifshitz transition and correlation effects. We discuss common underlying physics in these examples.