Putative quantum critical point in locally noncentrosymmetric CeCoGe$_2$ crystals

TL;DR

This study synthesizes CeCoGe$_2$ crystals to explore quantum criticality and potential superconductivity, revealing the influence of intrinsic defects on electronic properties and suggesting pathways to observe superconductivity in improved samples.

Contribution

The paper reports the synthesis of CeCoGe$_2$ crystals with controlled defect levels, identifying a possible quantum critical point and highlighting the impact of vacancies on superconductivity suppression.

Findings

Heavy-fermion ground state with high Sommerfeld coefficient

Proximity to quantum critical point indicated by non-Fermi-liquid behavior

Intrinsic Co vacancies suppress superconductivity and affect resistivity

Abstract

Locally noncentrosymmetric heavy-fermion compounds may produce long-sought correlated quantum phases, such as spin-triplet superconductivity with non-Abelian quasiparticles, but identifying the right candidate systems is challenging. Here, using the In flux method, we synthesize CeCoGe$_2$ single crystals, belonging to the highly tunable pseudotetragonal ($Cmcm$) Ce$TX_2$ family, which allows for substitutions at both the transition metal $T$ and at the $X$ sites. We identify a heavy-fermion ground state with a Sommerfeld coefficient $\gamma\approx 120$ mJ mol$^{-1}$ K$^{-2}$ and a non-Fermi-liquid exponent of the electrical resistivity, which may indicate its proximity to the putative quantum critical point. However, no signs of superconductivity or magnetic order are detected down to 20 mK. Our analysis of electrical transport and structural properties indicates that coherent charge transport and the emergence of superconductivity observed under hydrostatic pressure in related compounds (CePtSi$_2$ and CeRhGe$_2$) are suppressed in CeCoGe$_2$ by strong random potential scattering due to intrinsic Co vacancies (approximately 4% even in the highest-quality crystals). By tuning the growth stoichiometry and temperature profile, we demonstrate that the defect concentration can be controlled and has a pronounced effect on the residual resistivity. We hypothesize that superconductivity may be found in higher-quality CeCoGe$_2$ crystals grown by different techniques.