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تسجيل مستخدم جديدFrom local moment to mixed-valence regime in Ce(1-x)Yb(x)CoIn5 alloys
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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 lattice 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.
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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.
The electronic structure of (Ce,Yb)CoIn5 has been studied by a combination of photoemission, x-ray absorption and bulk property measurements. Previous findings of a Ce valence near 3+ for all x and of an Yb valence near 2.3+ for x>0.3 were confirmed.
One new result of this study is that the Yb valence for x<0.2 increases rapidly with decreasing x from 2.3+ toward 3+, which correlates well with de Haas van Alphen results showing a change of Fermi surface around x=0.2. Another new result is the direct observation by angle resolved photoemission Fermi surface maps of about 50% cross sectional area reductions of the alpha- and beta-sheets for x=1 compared to x=0, and a smaller, essentially proportionate, size change of the alpha-sheet for x=0.2. These changes are found to be in good general agreement with expectations from simple electron counting. The implications of these results for the unusual robustness of superconductivity and Kondo coherence with increasing x in this alloy system are discussed.
Magnetic susceptibility of the isostructural Ce(Ni{1-x}Cu{x})5 alloys (0< x <0.9) was studied as a function of the hydrostatic pressure up to 2 kbar at fixed temperatures 77.3 and 300 K, using a pendulum-type magnetometer. A pronounced magnitude of t
he pressure effect is found to be negative in sign and to depend strongly and non-monotonously on the Cu content, showing a sharp maximum in vicinity of x = 0.4. The experimental results are discussed in terms of the Ce valence change under pressure. It has been concluded that the fractional occupation of the f-states, which corresponds to the half-integer valence of Ce ion (3.5), is favorable for the valence instability in alloys studied. For the reference CeNi5 compound the main contributions to magnetic susceptibility and their volume dependence are calculated ab initio within the local spin density approximation (LSDA), and appeared to be in close agreement with experimental data.
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.
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.
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