Tuning the antiferromagnetic ground state of Ce2RhIn8 by Ga substitution

We explore the effect of Ga substitution on the physical properties of single crystals of ${\mathrm{Ce}}_{2}{\mathrm{RhIn}}_{8}$ through measurements of temperature-dependent specific heat, magnetic susceptibility, and electrical resistivity. Our data on ${\mathrm{Ce}}_{2}{\mathrm{RhIn}}_{8\ensuremath{-}x}{\mathrm{Ga}}_{x}$, single crystals reveal a monotonic decrease of the antiferromagnetic transition temperature from ${T}_{N}=2.77$ K for the parent compound to ${T}_{N}$ = 1.96 K for the highest Ga concentration achieved ($x=0.55$). To understand the evolution of the crystalline electric field (CEF) scheme and exchange interactions as a function of Ga substitution, we fit our thermodynamic data to a Hamiltonian containing a tetragonal CEF term and mean-field isotropic exchange terms. Our results show a systematic increase in the energies of the first and second excited states as a function of Ga concentration. Moreover, we find significant changes to the CEF ground-state (GS) wave function ($\mathrm{\ensuremath{\Gamma}}=\ensuremath{\alpha}\ensuremath{\mid}\ifmmode\pm\else\textpm\fi{}5/2\ensuremath{\rangle}+\sqrt{1\ensuremath{-}{\ensuremath{\alpha}}^{2}}\ensuremath{\mid}\ensuremath{\mp}3/2\ensuremath{\rangle}$), including an increase in the $\ensuremath{\alpha}$ coefficient and in the $\ensuremath{\mid}{B}_{2}^{0}\ensuremath{\mid}$ CEF parameter, with increasing Ga concentration. This evolution of the GS wave function drives the system away from the quantum critical point, whereas the chemical pressure associated with the Ga substitution drives the system towards it. Such behavior raises the question of whether pressure-induced superconductivity can be stabilized in this system. Our electrical resistivity and heat capacity measurements on ${\mathrm{Ce}}_{2}{\mathrm{RhIn}}_{7.64}{\mathrm{Ga}}_{0.36}$ under hydrostatic pressure reveal a zero-resistance state; however, no bulk superconductivity is detected. These results suggest that the occurrence of bulk superconductivity may be unfavorable in this system due to the observed CEF GS wave-function evolution and/or to the pair-breaking mechanism in the ${\mathrm{Ce}}_{2}{\mathrm{RhIn}}_{8\ensuremath{-}x}{\mathrm{Ga}}_{x}$ series.