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Register a new userQuantum criticality in Ce$_{1-x}$Sm$_x$CoIn$_5$
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Motivated by the possibility of observing the co-existence between magnetism and unconventional superconductivity in heavy-fermion Ce$_{1-x}$Sm$_x$CoIn$_5$ alloys, we studied how the samarium substitution on the cerium site affects the magnetic field-tuned-quantum criticality of stoicheometric CeCoIn$_5$ by performing specific heat and resistivity measurements. By applying an external magnetic field, we have observed Fermi-liquid to non-Fermi-liquid crossovers in the temperature dependence of the electronic specific heat normalized by temperature and of the resistivity. We obtained the magnetic-field-induced quantum critical point (QCP) by extrapolating to zero temperature the temperature - magnetic field dependence at which the crossovers take place. Furthermore, a scaling analysis of the electronic specific heat is used to confirm the existence of the QCP. We have found that the magnitude of the magnetic-field-induced QCP decreases with increasing samarium concentration. Our analysis of heat capacity and resistivity data reveals a zero-field QCP for $x_textrm{cr} approx 0.15$, which falls inside the region where Sm ions antiferromagnetism and superconductivity co-exist.
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$^{115}$In Nuclear magnetic resonance data are presented for a series of Ce$_{1-x}$La$_x$CoIn$_5$ crystals with different La dilutions, $x$. Multiple In(1) sites associated with different numbers of nearest-neighbor cerium atoms exhibit different Knight shifts and spin lattice relaxation rates. Analysis of the temperature dependence of these sites reveals both an evolution of the heavy electron coherence as a function of dilution, as well as spatial inhomogeneity associated with a complete suppression of antiferromagnetic fluctuations in the vicinity of the La sites. Quantum critical fluctuations persist within disconnected Ce clusters with dilution levels up to 75%, despite the fact that specific heat shows Fermi liquid behavior in dilute samples.
We present results of specific heat, electrical resistance, and magnetoresistivity measurements on single crystals of the heavy-fermion superconducting alloy Ce$_{0.91}$Yb$_{0.09}$CoIn$_5$. Non-Fermi liquid to Fermi liquid crossovers are clearly observed in the temperature dependence of the Sommerfeld coefficient $gamma$ and resistivity data. Furthermore, we show that the Yb-doped sample with $x=0.09$ exhibits universality due to an underlying quantum phase transition without an applied magnetic field by utilizing the scaling analysis of $gamma$. Fitting of the heat capacity and resistivity data based on existing theoretical models indicates that the zero-field quantum critical point is of antiferromagnetic origin. Finally, we found that at zero magnetic field the system undergoes a third-order phase transition at the temperature $T_{c3}approx 7$ K.
In this paper we review some of our recent experimental and theoretical results on transport and thermodynamic properties of heavy-fermion alloys Ce(1-x)Yb(x)CoIn5. Charge transport measurements under magnetic field and pressure on these single crystalline alloys revealed that: (i) relatively small Yb substitution suppresses the field induced quantum critical point, with a complete suppression for nominal Yb doping x>0.20; (ii) the superconducting transition temperature Tc and Kondo lattice coherence temperature T* decrease with x, yet they remain finite over the wide range of Yb concentrations; (iii) both Tc and T* increase with pressure; (iv) there are two contributions to resistivity, which show different temperature and pressure dependences, implying that both heavy and light quasiparticles contribute to inelastic scattering. We also analyzed theoretically the pressure dependence of both T* and Tc within the composite pairing theory. In the purely static limit, when we ignore the lattice dynamics, we find that the composite pairing mechanism necessarily causes opposite behaviors of T* and Tc with pressure: if T* grows with pressure, Tc must decrease with pressure and vice versa.
We have measured the superconducting penetration depth~$Lambda(T)$ in the heavy-fermion/intermediate-valent superconducting alloy series~Ce$_{1-x}$Yb$_x$CoIn$_5$ using transverse-field muon spin relaxation, to study the effect of intermediate-valent Yb doping on Fermi-liquid renormalization. From $Lambda(T)$ we determine the superfluid density $rho_s(T)$, and find that it decreases continuously with increasing nominal Yb concentration~$x$, i.e., with increasing intermediate valence. The temperature-dependent renormalization of the normal fluid density~$rho_N(T) = rho_s(0) - rho_s(T)$ in both the heavy-fermion and intermediate valence limits is proportional to the temperature-dependent renormalization of the specific heat. This indicates that the temperature-dependent Fermi-liquid Landau parameters of the superconducting quasiparticles entering the two different physical quantities are the same. These results represent an important advance in understanding of both intermediate valence and heavy-fermion phenomena in superconductors.
One of the greatest challenges to Landaus Fermi liquid theory - the standard theory of metals - is presented by complex materials with strong electronic correlations. In these materials, non-Fermi liquid transport and thermodynamic properties are often explained by the presence of a continuous quantum phase transition which happens at a quantum critical point (QCP). A QCP can be revealed by applying pressure, magnetic field, or changing the chemical composition. In the heavy-fermion compound CeCoIn$_5$, the QCP is assumed to play a decisive role in defining the microscopic structure of both normal and superconducting states. However, the question of whether QCP must be present in the materials phase diagram to induce non-Fermi liquid behavior and trigger superconductivity remains open. Here we show that the full suppression of the field-induced QCP in CeCoIn$_5$ by doping with Yb has surprisingly little impact on both unconventional superconductivity and non-Fermi liquid behavior. This implies that the non-Fermi liquid metallic behavior could be a new state of matter in its own right rather then a consequence of the underlying quantum phase transition.
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