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Register a new userCoupling between heavy fermion superconductor CeCoIn$_5$ and antiferromagnetic metal CeIn$_3$ through the atomic interface
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To study the mutual interaction between unconventional superconductivity and magnetic order through an interface, we fabricate Kondo superlattices consisting of alternating layers of heavy-fermion superconductor CeCoIn$_5$ and antiferromagnetic (AFM) heavy-fermion metal CeIn$_3$. The strength of the AFM fluctuations is tuned by applying hydrostatic pressure to CeCoIn$_5(m)$/CeIn$_3(n)$ superlattices with $m$ and $n$ unit-cell-thick layers of CeCoIn$_5$ and CeIn$_3$, respectively. Superconductivity in CeCoIn$_5$ and AFM order in CeIn$_3$ coexist in spatially separated layers. At ambient pressure, N{e}el temperature $T_N$ of the CeIn$_3$ block layers (BLs) of CeCoIn$_5$(7)/CeIn$_3(n)$ shows little dependence on $n$, in contrast to CeIn$_3(n)$/LaIn$_3$(4) superlattices where $T_N$ is strongly suppressed with decreasing $n$. This suggests that each CeIn$_3$ BL is magnetically coupled by the RKKY interaction through the adjacent CeCoIn$_5$ BL and a 3D magnetic state is formed. With applying pressure to CeCoIn$_5$(7)/CeIn$_3$(13), $T_N$ of the CeIn$_3$ BLs is suppressed up to 2.4 GPa, showing a similar pressure dependence as CeIn$_3$ single crystals. An analysis of upper critical field reveals that the superconductivity in the CeCoIn$_5$ BLs is barely influenced by the AFM fluctuations in the CeIn$_3$ BLs, even when the CeIn$_3$ BLs are in the vicinity of the AFM quantum critical point. This is in stark contrast to CeCoIn$_5$/CeRhIn$_5$ superlattice where the superconductivity in the CeCoIn$_5$ BLs is profoundly affected by AFM fluctuations in the CeRhIn$_5$ BLs. The present results show that although AFM fluctuations are injected into the CeCoIn$_5$ BLs from the CeIn$_3$ BLs through the interface, they barely affect the force which binds superconducting electron pairs. These results demonstrate that 2D AFM fluctuations are essentially important for the pairing interactions in CeCoIn$_5$.
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We present scanning tunneling spectroscopy measurements of the local quasiparticles excitation spectra of CeCoIn$_5$ between 440mK and 3K in samples with a bulk $T_{rm c}=2.25$K. The spectral shape of our low-temperature tunneling data, quite textbook nodal-gap conductance, allow us to confidently fit the spectra with a d-wave density of states considering also a shortening of quasiparticles lifetime term $Gamma$. The $Delta(0)$ value obtained from the fits yields a BCS ratio $2Delta/kT_{rm c} =7.73$ suggesting that CeCoIn$_5$ is an unconventional superconductor in the strong coupling limit. The fits also suggest that the height of coherence peaks in CeCoIn$_5$ is reduced with respect to a pure BCS spectra and therefore the coupling of quasiparticles with spin excitations should play a relevant role. In addition, the tunneling conductance shows a depletion at energies smaller than $Delta$ for temperatures larger than the bulk $T_{rm c}$, giving further support to the existence of a pseudogap phase that in our samples span up to $T^{*}sim 1.2 T_{rm c}$. The phenomenological scaling of the pseudogap temperature observed in various families of cuprates, $2Delta/kT^{*} sim 4.3 $, is not fulfilled in our measurements. This suggests that in CeCoIn$_5$ the strong magnetic fluctuations might conspire to close the local superconducting gap at a smaller pesudogap temperature-scale than in cuprates.
The field-orientation dependent thermal conductivity of the heavy-fermion superconductor UPt$_3$ was measured down to very low temperatures and under magnetic fields throughout three distinct superconducting phases: A, B, and C phases. In the C phase, a striking twofold oscillation of the thermal conductivity within the basal plane is resolved reflecting the superconducting gap structure with a line of node along the a axis. Moreover, we find an abrupt vanishing of the oscillation across a transition to the B phase, as a clear indication of a change of gap symmetries. We also identify extra two line nodes below and above the equator in both B and C phases. From these results together with the symmetry consideration, the gap function of UPt$_3$ is conclusively determined as a $E_{1u}$ representation characterized by a combination of two line nodes at the tropics and point nodes at the poles.
Our previous point-contact Andreev reflection studies of the heavy-fermion superconductor CeCoIn$_5$ using Au tips have shown two clear features: reduced Andreev signal and asymmetric background conductance [1]. To explore their physical origins, we have extended our measurements to point-contact junctions between single crystalline heavy-fermion metals and superconducting Nb tips. Differential conductance spectra are taken on junctions with three heavy-fermion metals, CeCoIn$_5$, CeRhIn$_5$, and YbAl$_3$, each with different electron mass. In contrast with Au/CeCoIn$_5$ junctions, Andreev signal is not reduced and no dependence on effective mass is observed. A possible explanation based on a two-fluid picture for heavy fermions is proposed. [1] W. K. Park et al., Phys. Rev. B 72 052509 (2005); W. K. Park et al., Proc. SPIE-Int. Soc. Opt. Eng. 5932 59321Q (2005); W. K. Park et al., Physica C (in press) (cond-mat/0606535).
High--quality single crystals of the heavy fermion superconductors CeCoIn$_5$ and CeIrIn$_5$ have been studied by means of low--temperature Scanning Tunneling Microscopy. Methods were established to facilitate textit{in-situ} sample cleaving. Spectroscopy in CeCoIn$_5$ reveals a gap which persists to above $T_c$, possibly evidencing a precursor state to SC. Atomically resolved topographs show a rearrangement of the atoms at the crystal surface. This modification at the surface might influence the surface properties as detected by tunneling spectroscopy.
In this paper we report the impact of uniaxial strain $varepsilon$ applied along the crystalline $a$ axis on the newly discovered kagome superconductor CsV$_3$Sb$_5$. At ambient conditions, CsV$_3$Sb$_5$ shows a charge-density wave (CDW) transition at $T_{rm CDW}=94.5$ K and superconducts below $T_C = 3.34$ K. In our study, when the uniaxial strain $varepsilon$ is varied from $-0.90%$ to $0.90%$, $T_C$ monotonically increases by $sim 33%$ from 3.0 K to 4.0 K, giving rise to the empirical relation $T_C (varepsilon)=3.4+0.56varepsilon+0.12varepsilon^2$. On the other hand, for $varepsilon$ changing from $-0.76%$ to $1.26%$, $T_{rm CDW}$ decreases monotonically by $sim 10%$ from 97.5 K to 87.5 K with $T_{rm CDW}(varepsilon)=94.5-4.72varepsilon-0.60varepsilon^2$. The opposite response of $T_C$ and $T_{rm CDW}$ to the uniaxial strain suggests strong competition between these two orders. Comparison with hydrostatic pressure measurements indicate that it is the change in the $c$-axis that is responsible for these behaviors of the CDW and superconducting transitions, and that the explicit breaking of the sixfold rotational symmetry by strain has a negligible effect. Combined with our first-principles calculations and phenomenological analysis, we conclude that the enhancement in $T_C$ with decreasing $c$ is caused primarily by the suppression of $T_{rm CDW}$, rather than strain-induced modifications in the bare superconducting parameters. We propose that the sensitivity of $T_{rm CDW}$ with respect to the changes in the $c$-axis arises from the impact of the latter on the trilinear coupling between the $M_1^+$ and $L_2^-$ phonon modes associated with the CDW. Overall, our work reveals that the $c$-axis lattice parameter, which can be controlled by both pressure and uniaxial strain, is a powerful tuning knob for the phase diagram of CsV$_3$Sb$_5$.
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