Yb-based heavy-fermion metal situated close to a quantum critical point

We report low-temperature thermodynamic, magnetic, and transport measurements on high-quality single crystals of the tetragonal heavy-fermion system ${\mathrm{YbIr}}_2{\mathrm{Si}}_2$. Depending on synthesis conditions it crystallizes either in the $I$-type body-centered ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$ or $P$-type ${\mathrm{CaBe}}_2{\mathrm{Ge}}_2$ structure. The $P$-type system has a low Kondo scale $T_K=2\mathrm{K}$ and orders magnetically below 0.7 K whereas the $I$-type system remains paramagnetic and shows, above 0.5 K, a temperature dependence of the resistivity and the specific heat similar to that of ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$. In contrast to ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$ which exhibits non-Fermi-liquid behavior, ${\mathrm{YbIr}}_2{\mathrm{Si}}_2$ reaches a Landau Fermi-liquid state below 0.2 K and is thus ideally suited for pressure studies across the quantum critical point.

Figure 1

Specific heat and (inset) resistivity of $P$-type ${\mathrm{YbIr}}_2{\mathrm{Si}}_2$. The specific heat shows a clear anomaly at 0.7 K which we attribute to a phase transition, presumably of antiferromagnetic type. The temperature dependence of the resistivity evidence a pronounced Kondo lattice behavior.

Figure 2

Magnetic susceptibility of $I$-type ${\mathrm{YbIr}}_2{\mathrm{Si}}_2$ with $B∕∕a$ and $B∕∕c$, showing the pronounced easy plane anisotropy. Inset: temperature dependence of the resistivity with $I∕∕ab$ up to 300 K. The resistivity behavior is typical of heavy-fermion compounds.

Figure 3

Electrical resistivity of I-${\mathrm{YbIr}}_2{\mathrm{Si}}_2$ with $I∕∕ab$ at low temperatures in a field of 0.03 T in order to suppress the effect of superconductivity in In inclusions. The inset shows the temperature dependence of the exponent n of a fit $\rho \left(T\right)={\rho }_0+A{T}^n$. $n$ increases towards low temperatures and levels out at the value $n=2$, indicating the formation of a Fermi-liquid ground state. Lower inset: Plot of $\rho \left(T\right)$ versus $T^2$ showing the Fermi-liquid behavior at low temperature

Figure 4

Specific heat of ${\mathrm{YbIr}}_2{\mathrm{Si}}_2$ plotted as delta $C∕T$ vs $\ln T$. $C∕T$ increases logarithmically with decreasing temperature but levels out below 400 mK as expected for a Fermi liquid. Inset: Magnetic susceptibility at low temperatures. Note the tendency to saturation below 400 mK.