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 obse
rved 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.
We studied the temperature and magnetic field dependence of vortex dissipation and critical current in the mixed-state of unconventional superconducting alloys Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($0.044 leq x leq 0.100$) through current-voltage measureme
nts. Our results reveal that all the electric field $E$ vs current density $j$ curves in the Ohmic regime merge to one point ($j_0,E_0$) and that there is a simple relationship between the critical current density $j_c$ and flux-flow resistivity $rho_{rm ff}$: $rho_{rm ff}/rho_{rm n} = (1- j_{c}/j_{0})^{-1}$, where $rho_{rm n}=E_0/j_0$ is the normal-state resistivity just above the superconducting transition. In addition, $E_0$ is positive for all five dopings, reflecting the abnormal behavior of the flux-flow resistivity $rho_{rm ff}$: it increases with decreasing magnetic field. In contrast, $E_0$ is negative for the conventional superconductor Nb since, as expected, $rho_{rm ff}$ decreases with decreasing magnetic field. Furthermore, in the under-doped and over-doped single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, the parameter $E_0$ remains temperature independent, while it decreases with increasing temperature for the single crystals around optimal doping ($ 0.060leq xleq 0.072 $). This result points to the co-existence of superconductivity with some other phase around optimal doping.
Measurements of the current-voltage characteristics were performed on Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals with doping level $0.044 leq x leq 0.1$. An unconventional increase in the flux-flow resistivity $rho_{rm ff}$ with decreasing magnet
ic field was observed across this doping range. Such an abnormal field dependence of flux-flow resistivity is in contrast with the linear field dependence of $rho_{rm ff}$ in conventional type-II superconductors, but is similar to the behavior recently observed in the heavy-fermion superconductor CeCoIn$_5$. A significantly enhanced $rho_{rm ff}$ was found for the x=0.06 single crystals, implying a strong single-particle energy dissipation around the vortex cores. At different temperatures and fields and for a given doping concentration, the normalized $rho_{rm ff}$ scales with normalized field and temperature. The doping level dependence of these parameters strongly suggests that the abnormal upturn flux-flow resisitivity is likely related to the enhancement of spin fluctuations around the vortex cores of the optimally doped samples.
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 cryst
alline 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.
Here we present our experimental and theoretical study of the effects of pressure on the transport properties of the heavy-fermion alloy Ce(1-x)Yb(x)CoIn5 with x~0.07. We specifically choose this value of ytterbium concentration because the magnetic-
field-induced quantum critical point, which separates the antiferromagnetic and paramagnetic states at zero temperature, approaches zero, as has been established in previous studies. Our measurements show that pressure further suppresses quantum fluctuations in this alloy, just as it does in the parent compound CeCoIn5. In contrast, the square-root temperature dependent part of resistivity remains insensitive to pressure, indicating that the heavy-quasiparticles are not involved in the scattering processes leading to such a temperature dependent resistivity. We demonstrate that the growth of the coherence temperature with pressure, as well as the decrease of the residual resistivity, can be accurately described by employing the coherent potential approximation for a disordered Kondo lattice.
We investigated the onset of the many-body coherence in the f-orbital single crystalline alloys Ce(1-x)Yb(x)CoIn5 through thermodynamic and magneto-transport measurements. Our study shows the evolution of the many-body electronic state as the Kondo l
attice of Ce moments is transformed into an array of Ce impurities. Specifically, we observe a smooth crossover from the predominantly localized Ce moment regime to the predominantly itinerant Yb f-electronic states regime for about 50% of Yb doping. Our analysis of the residual resistivity data unveils the presence of correlations between Yb ions, while from our analysis of specific heat data we conclude that for 0.65<x<0.775, ytterbium f-electrons strongly interact with the conduction electrons while the Ce moments remain completely decoupled. The sub-linear temperature dependence of resistivity across the whole range of Yb concentrations suggest the presence of a nontrivial scattering mechanism for the conduction electrons.
