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Register a new userQuasi-2D superconductivity and Fermi-liquid behavior in bulk CaC$_6$
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The intercalated graphite superconductor CaC6 with Tc ~ 11.5 K has been synthesized and characterized with magnetoresistance measurements. Above the transition, the resistivity follows a T^2 dependence up to 50 K, which suggests Fermi liquid behavior. Above 50 K, the data can be fit to the Bloch-Gruneisen model providing a Debye temperature of theta = 263 K. By using McMillan formula, we estimate the electron-phonon coupling constant of lambda = 0.85 which places this material in the intermediate-coupling regime. The upper critical field is determined parallel and perpendicular to the superconducting planes, and the dependence of the upper critical field as a function of angle suggests that this is a quasi-2D superconductivity. All of these measurements are consistent with BCS-like superconductivity.
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We study the graphite intercalated compound CaC$_6$ by means of Eliashberg theory, focusing on the anisotropy properties. An analysis of the electron-phonon coupling is performed, and we define a minimal 6-band anisotropy structure. Comparing with Superconducting Density Functional Theory (SCDFT) the condition under which Eliashberg theory is able to reproduce the SCDFT gap structure is determined, and we discuss the role of Coulomb interactions. The Engelsberg-Schrieffer polaron structure is computed by solving the Eliashberg equation on the Matsubara axis and analytically continuing it to the full complex plane. This reveals the polaronic quasiparticle bands anisotropic features as well as the interplay with superconductivity.
The temperature dependence of the in-plane magnetic penetration depth, $lambda_{ab}(T)$, has been measured in a c-axis oriented polycrystalline CaC$_{6}$ bulk sample using a high-resolution mutual inductance technique. A clear exponential behavior of $lambda_{ab}(T)$ has been observed at low temperatures, strongly suggesting isotropic s-wave pairing. Data fit using the standard BCS theory yields $lambda_{ab}(0)=(720pm 80)$ Angstroem and $Delta(0)=(1.79pm 0.08)$ meV. The ratio $2Delta(0)/k_{_B}T_{c}=(3.6pm 0.2)$ gives indication for a conventional weakly coupled superconductor.
Nuclear quadrupole resonance measurements were performed on the heavy fermion superconductor Ce2PdIn8. Above the Kondo coherence temperature T_coh simeq 30K, the spin-lattice relaxation rate 1/T_1 is temperature independent, whereas at lower temperatures, down to the onset of superconductivity at T_c = 0.64K, it is nearly proportional to T^{1/2}. Below T_c, 1/T_1 shows no coherence peak and decreases as T^3 down to 75mK. All these findings indicate that Ce2PdIn8 is close to the antiferromagnetic quantum critical point, and the superconducting state has an unconventional character with line nodes in the superconducting gap.
Advances in low-dimensional superconductivity are often realized through improvements in material quality. Apart from a small group of organic materials, there is a near absence of clean-limit two-dimensional (2D) superconductors, which presents an impediment to the pursuit of numerous long-standing predictions for exotic superconductivity with fragile pairing symmetries. Here, we report the development of a bulk superlattice consisting of the transition metal dichalcogenide (TMD) superconductor 2$H$-niobium disulfide (2$H$-NbS$_2$) and a commensurate block layer that yields dramatically enhanced two-dimensionality, high electronic quality, and clean-limit inorganic 2D superconductivity. The structure of this material may naturally be extended to generate a distinct family of 2D superconductors, topological insulators, and excitonic systems based on TMDs with improved material properties.
While multiband systems are usually considered for flat-band physics, here we study one-band models that have flat portions in the dispersion to explore correlation effects in the 2D repulsive Hubbard model in an intermediate coupling regime. The FLEX+DMFT~(the dynamical mean-field theory combined with the fluctuation exchange approximation) is used to show that we have a crossover from ferromagnetic to antiferromagnetic spin fluctuations as the band filling is varied, which triggers a crossover from triplet to singlet pairings with a peculiar filling dependence that is dominated by the size of the flat region in the dispersion. A curious manifestation of the flat part appears as larger numbers of nodal lines associated with pairs extended in real space. We further detect non-Fermi liquid behavior in the momentum distribution function, frequency dependence of the self-energy and spectral function. These indicate correlation physics peculiar to flat-band systems.
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