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We have investigated the effect of Yb substitution on the Pauli limited, heavy fermion superconductor, CeCoIn$_5$. Yb acts as a non-magnetic divalent substituent for Ce throughout the entire doping range, equivalent to hole doping on the rare earth site. We found that the upper critical field in (Ce,Yb)CoIn$_5$ is Pauli limited, yet the reduced (H,T) phase diagram is insensitive to disorder, as expected in the purely orbitally limited case. We use the Pauli limiting field, the superconducting condensation energy and the electronic specific heat coefficient to determine the Wilson ratio ($R_{W}$), the ratio of the specific heat coefficient to the Pauli susceptibility in CeCoIn$_5$. The method is applicable to any Pauli limited superconductor in the clean limit.
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The recent observation of fully-gapped superconductivity in Yb doped CeCoIn5 poses a paradox, for the disappearance of nodes suggests that they are accidental, yet d-wave symmetry with protected nodes is we ll established by experiment. Here, we show that composite pairing provides a natural resolution: in this scenario, Yb doping drives a Lifshitz transition of the nodal Fermi surface, forming a fully-gapped d-wave molecular superfluid of composite pairs. The T4 dependence of the penetration depth associated with the sound mode of this condensate is in accord with observation.
We report the observation of heavy-fermion superconducitivity in CeCoIn5 at Tc =2.3 K. When compared to the pressure-induced Tc of its cubic relative CeIn3 (Tc ~200 mK), the Tc of CeCoIn5 is remarkably high. We suggest that this difference may arise from magnetically mediated superconductivity in the layered crystal structure of CeCoIn5 .
We present a detailed analysis of the upper critical field for CeCoIn5 under high pressure. We show that, consistently with other measurements, this system shows a decoupling between maximum of the superconducting transition temperature Tc and maximum pairing strength. This puts forward CeCoIn5 as an important paradigm for this class of unconventional, strongly correlated superconductors.
The formation of heavy fermion bands can occur by means of the conversion of a periodic array of local moments into itinerant electrons via the Kondo effect and the huge consequent Fermi-liquid renormalizations. Leggett predicted for liquid $^3$He that Fermi-liquid renormalizations change in the superconducting state, leading to a temperature dependence of the London penetration depth~$Lambda$ quite different from that in the BCS theory. Using Leggetts theory, as modified for heavy fermions, it is possible to extract from the measured temperature dependence of $Lambda$ in high quality samples both Landau parameters $F_0^s$ and $F_1^s$; this has never been accomplished before. A modification of the temperature dependence of the specific heat $C_mathrm{el}$, related to that of $Lambda$, is also expected. We have carefully determined the magnitude and temperature dependence of $Lambda$ in CeCoIn$_5$ by muon spin relaxation rate measurements to obtain $F_0^s = 36 pm 1$ and $F_1^s = 1.2 pm 0.3$, and find a consistent change in the temperature dependence of electronic specific heat $C_mathrm{el}$. This, the first determination of $F_1^s$ with a value~$ll F_0^s$ in a heavy fermion compound, tests the basic assumption of the theory of heavy fermions, that the frequency dependence of the self-energy is much more important than its momentum dependence.
The heavy-fermion superconductor CeCoIn5 is the first material, where different experimental probes show strong evidence pointing to the realization of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The inhomogeneous superconducting FFLO state with a periodically modulated order parameter was predicted to appear in Pauli-limited, sufficiently clean type-II superconductors already more than 40 years ago. On the other hand, CeCoIn5 is supposed to be close to a magnetic quantum critical point (QCP) showing strong antiferromagnetic (AFM) spin fluctuations (SF) at atmospheric pressure. We studied the evolution of the FFLO phase away from the influence of the strong AFM-SF by heat capacity experiments under pressure (0 GPa <= P <= 1.5 GPa, 0 T <= mu_0 H <= 14 T, and 100 mK <= T <= 4 K). Our results prove the stability of the the FFLO phase under pressure. It even expands, while the Pauli-limiting becomes weaker and the AFM-SF are suppressed. This shows the intriguing influence of the AFM-SF on the FFLO state.
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