Quantum Criticality Based on Large Ising Spins: YbCo$_2$Ge$_4$ with New 1-2-4 Structure Type

TL;DR

This paper introduces YbCo$_2$Ge$_4$, a novel quantum critical material with the largest known Ising pseudospin, revealing quantum criticality in large spin systems and expanding the landscape of quantum materials.

Contribution

It reports the discovery of a new structure type and quantum critical behavior in a large-spin Yb-based compound, providing insights into quantum criticality in uniaxial large-spin systems.

Findings

YbCo$_2$Ge$_4$ exhibits quantum criticality with large Ising pseudospins.

The structure is a new orthorhombic 1-2-4 type related to ThCr$_2$Si$_2$.

Experimental evidence from resistivity, magnetization, specific heat, and NQR/NMR supports quantum critical behavior.

Abstract

We present a new type of quantum critical material YbCo$_2$Ge$_4$, having the largest quantum-critical pseudospin size ever. The YbCo$_2$Ge$_4$-type structure is new, forms in the orthorhombic $Cmcm$ system, and is related to the well-known ThCr$_2$Si$_2$ structure. Heavy rare earth (Tm,Yb,Lu, or Y) members are also possible to be grown. YbCo$_2$Ge$_4$ possesses the Ising-type ground-state doublet, namely the simplest ones of uniaxially up or down, $|\pm \sim 7/2\rangle$. It is clearly manifested through comprehensive resistivity, magnetization, specific heat, and NQR/NMR experiments. Large pseudospin state usually tends to order in simple magnetisms, or hard to be screened by Kondo effect. Therefore, the discovery of the quantum criticality of the fluctuating large spins opens a new door to new-material search and theoretical studies.