Extremely strong coupling superconductivity in artificial two-dimensional Kondo lattices

Nature Physics (Impact Factor: 20.15). 09/2011; 7(11). DOI: 10.1038/nphys2112
Source: arXiv


When interacting electrons are confined to low-dimensions, the
electron-electron correlation effect is enhanced dramatically, which often
drives the system into exhibiting behaviors that are otherwise highly
improbable. Superconductivity with the strongest electron correlations is
achieved in heavy-fermion compounds, which contain a dense lattice of localized
magnetic moments interacting with a sea of conduction electrons to form a 3D
Kondo lattice. It had remained an unanswered question whether superconductivity
would persist upon effectively reducing the dimensionality of these materials
from three to two. Here we report on the observation of superconductivity in
such an ultimately strongly-correlated system of heavy electrons confined
within a 2D square-lattice of Ce-atoms (2D Kondo lattice), which was realized
by fabricating epitaxial superlattices built of alternating layers of
heavy-fermion CeCoIn5 and conventional metal YbCoIn5. The field-temperature
phase diagram of the superlattices exhibits highly unusual behaviors, including
a striking enhancement of the upper critical field relative to the transition
temperature. This implies that the force holding together the superconducting
electron-pairs takes on an extremely strong coupled nature as a result of

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    • "Thus, dimensional reduction 3D→2D may affect optical, electronic, and thermodynamic properties of the system. In superconductivity , studying quasi-2D systems turned out to be a particularly fruitful field and led to remarkable findings such as insulators featuring a superconducting gap [1], pseudogap in conventional s-wave superconductors [2], or extremely strongcoupling superconductivity in Kondo lattices [3]. Besides pure academic interest, thin-film superconductors play a key U. S. Pracht, E. Heintze, C. Clauss, D. Hafner, R. Bek, D. Werner, S. Gelhorn, M. Scheffler, and M. Dressel are with 1. "
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    ABSTRACT: We report on terahertz frequency-domain spectroscopy (THz-FDS) experiments in which we measure charge carrier dynamics and excitations of thin-film superconducting systems at low temperatures in the THz spectral range. The characteristics of the set-up and the experimental procedures are described comprehensively. We discuss the single-particle density of states and a theory of electrodynamic absorption and optical conductivity of conventional superconductors. We present the experimental performance of the setup at low temperatures for a broad spectral range from 0.1 - 1.1 THz by the example of ultra-thin films of weakly disordered superconductors niobium nitride (NbN) and tantalum nitride (TaN) with different values of critical temperatures. Furthermore, we analyze and interpret our experimental data within the framework of conventional Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity.
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    ABSTRACT: Cooper pairing between a conduction electron ($c$ electron) and an $f$ electron, referred to as the "$c$-$f$ pairing," is examined to explain s-wave superconductivity in heavy-fermion systems. We first apply the Schrieffer-Wolff transformation to the periodic Anderson model assuming deep $f$ level and strong Coulomb repulsion. The resulting effective Hamiltonian contains direct and spin-exchange interactions between $c$ and $f$ electrons, which are responsible for the formation of the $c$-$f$ Cooper pairs. The mean-field analysis shows that the fully gapped $c$-$f$ pairing phase with anisotropic s-wave symmetry appears in a large region of the phase diagram. We also find two different types of exotic $c$-$f$ pairing phases, the Fulde-Ferrell and breached pairing phases. The formation of the $c$-$f$ Cooper pairs is attributed to the fact that the strong Coulomb repulsion makes a quasiparticle $f$ band near the center of the conduction band.
    Full-text · Article · May 2012 · Physical review. B, Condensed matter
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    ABSTRACT: To study the nature of partially substituted Yb-ions in a Ce-based Kondo lattice, we fabricated high quality Ce_{1-x}Yb_xCoIn_5 epitaxial thin films using molecular beam epitaxy. We find that the Yb-substitution leads to a linear decrease of the unit cell volume, indicating that Yb-ions are divalent forming Kondo-holes in Ce_{1-x}Yb_xCoIn_5, and leads to a strong suppression of the superconductivity and Kondo coherence. These results, combined with the measurements of Hall effect, indicate that Yb-ions act as nonmagnetic impurity scatters in the coherent Kondo lattice without serious suppression of the antiferromagnetic fluctuations. These are in stark contrast to previous studies performed using bulk single crystals, which claim the importance of valence fluctuations of Yb-ions. The present work also highlights the suitability of epitaxial films in the study of the impurity effect on the Kondo lattice.
    Preview · Article · Aug 2012 · Physical review. B, Condensed matter
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