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

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 $γ\approx 120$ 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.

Figures (7)

• Figure 1: (a) Pseudotetragonal crystal structure ($a \approx c \ll b$) of Ce$TX$2 ($T$ being a transition metal and $X=$ Si, Ge) with the locally noncentrosymmetric CeNiSi2 structure ($Cmcm$). (b) Ce atoms are positioned around an inversion center (IC) located in the middle of the unit cell.
• Figure 2: Phase diagram and putative quantum critical point in locally noncentrosymmetric Ce$TX$2. Chemical (filled symbols) and hydrostatic (open symbols) pressure tune the magnetic ground state from antiferromagnetic order for a unit-cell volume $V\gtrsim 300$ Å$^3$ to paramagnetic states for $V\lesssim 300$ Å$^3$. A superconducting dome emerges for CePtSi2 and CeRhGe2 under hydrostatic pressure, centered around $V_\text{c}\approx 300$ Å$^3$, which corresponds to the unit-cell volume of CeCoGe2.
• Figure 3: Temperature-dependent (a) magnetic susceptibility and (b) inverse susceptibility of a plate-like CeCoGe2 single crystal as grown from indium flux. There exists magnetic anisotropy with $\chi$ being about two times larger perpendicular to the crystallographic $b$ axis compared to measurements with an applied field parallel to $b$. Modified Curie-Weiss fits of $\chi(T)$ at temperatures above $\approx 150$ K yield effective magnetic moments of $2.54\,\mu_\text{B}$ and $2.33\,\mu_\text{B}$ for $H\perp b$ and $H \parallel b$, respectively. Inset in (b) shows the single crystal average $\chi_\text{ave}=\frac{2}{3}\chi_{\perp b}+\frac{1}{3}\chi_{\parallel b}$ compared to data for polycrystalline arc-melted samples by Mun et al.mun2004kondo. Scaling the polycrystalline data by a factor 0.83, which corresponds to the phase purity of polycrystalline samples synthesized in the course of this work following the same recipe as in Ref. mun2004kondo, yields near-perfect agreement with $\chi_\text{ave}$.
• Figure 4: Low-temperature specific heat of CeCoGe2. (a) Comparison of as-grown CeCoGe2 single crystals from this work and arc-melted polycrystalline samples from Ref. mun2004kondo. (b) The low-temperature slope of the specific heat $C/T$, measured down to 0.37 K, reveals a heavy-fermion ground state with a Sommerfeld coefficient of around 120 mJ mol$^{-1}$ K$^{-2}$ and an additional upturn of $C/T$ at the lowest temperatures, which, as shown in (c), roughly follows a logarithmic behavior.
• Figure 5: Temperature-dependent electrical resistivity of CeCoGe2. (a) shows a comparison between our single crystals, where current was applied perpendicular to $b$, and the results for arc-melted polycrystalline CeCoGe2 by Mun et al.mun2004kondo (black dashed-dotted line), which is about a factor of 1.4 smaller, while showing a comparable residual resistivity ratio RRR $\approx2.5$. Inset shows negligible field dependence of the magnetoresistance at $T=2\,$K. (b) Low-temperature scaling of $\rho(T)$ follows a non-Fermi-liquid (NFL) behavior with $\rho(T)=\rho_0 + AT^{n}$ ($n\approx 1.5$). Inset shows unit-cell volume dependence of the resistivity exponent $n$ for hydrostatic-pressure-tuned CePtSi2 nakano2009pressure and CeCoGe2 at ambient pressure (this work).