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

Abstract

We present a new type of quantum critical material YbCo2Ge4, having the largest quantumcritical pseudospin size ever. The YbCo2Ge4-type structure is new, forms in the orthorhombic Cmcm system, and is related to the well-known ThCr2Si2 structure. Heavy rare earth (Tm,Yb,Lu, or Y) members are also possible to be grown. YbCo2Ge4 possesses the Ising-type ground-state doublet, namely the simplest ones of uniaxially up or down, |± ∼ 7/2 . 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.

Classification of materials with divergent magnetic Grüneisen parameter

“…Next we discuss recent results on tetragonal YbCo 2 Ge 4 . In this stoichiometric paramagnetic HF metal the electrical resistivity shows a non-Fermi ∆ρ ∼ T 1.4 dependence between 0.1 and 1.4 K hinting at NFL behavior, possibly related to quantum criticality [45]. This is corroborated by a logarithmic divergence of the heat capacity coefficient and local spin susceptibility (from nuclear magnetic and quadrupole resonance) at elevated temperatures.…”