Low-temperature properties of CeRu4Sn6 from NMR and specific heat measurements: Heavy fermions emerging from a Kondo-insulating state

E. M. Brüning, M. Brando, M. Baenitz, A. Bentien, A. M. Strydom, R. E. Walstedt, and F. Steglich
Phys. Rev. B 82, 125115 – Published 15 September 2010

Abstract

The combination of low-temperature specific heat and nuclear-magnetic-resonance (NMR) measurements reveals important information on the ground-state properties of CeRu4Sn6, which has been proposed as a rare example of a tetragonal Kondo insulator (KI). The NMR spin-lattice-relaxation rate 1/T1 deviates from the Korringa law below 100 K signaling the onset of an energy gap ΔEg1/kB30K. This gap is stable against magnetic fields up to 10 T. Below 10 K, however, unusual low-energy excitations of in-gap states are observed, which depend strongly on the field H. The specific heat C detects these excitations in the form of an enhanced Sommerfeld coefficient γ=C(T)/T: in zero field, γ increases steeply below 5 K, reaching a maximum at 0.1 K, and then saturates at γ0.6J/K2mol. Upon increasing field, this maximum is shifted to higher temperatures with an overall reduction in γ, suggesting a residual density of states at the Fermi level developing a spin (pseudo-)gap ΔEg2. A simple model, based on two narrow quasiparticle bands located at the Fermi level—which cross the Fermi level in zero field at 0.022 states/meV f.u.—can account qualitatively as well as quantitatively for the measured observables. In particular, it is demonstrated that fitting this model, incorporating a Ce magnetic moment of μ=ΔEg1/μ0H1μB, to our data of both specific heat and NMR leads to the prediction of the field dependence of the gap. Our measurements rule out the presence of a quantum critical point as the origin for the enhanced γ in CeRu4Sn6 and suggest that this arises rather from correlated, residual in-gap states at the Fermi level. This work provides a fundamental route for future investigations into the phenomenon of narrow-gap formation in the strongly correlated class of systems.

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  • Received 26 May 2010

DOI:https://doi.org/10.1103/PhysRevB.82.125115

©2010 American Physical Society

Authors & Affiliations

E. M. Brüning1,*, M. Brando1, M. Baenitz1, A. Bentien1, A. M. Strydom2, R. E. Walstedt3, and F. Steglich1

  • 1Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Straße 40, 01187 Dresden, Germany
  • 2Physics Department, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa
  • 3Physics Department, University of Michigan, Ann Arbor, Michigan 48109, USA

  • *bruening@cpfs.mpg.de

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Vol. 82, Iss. 12 — 15 September 2010

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