Abstract
The combination of low-temperature specific heat and nuclear-magnetic-resonance (NMR) measurements reveals important information on the ground-state properties of , which has been proposed as a rare example of a tetragonal Kondo insulator (KI). The NMR spin-lattice-relaxation rate deviates from the Korringa law below 100 K signaling the onset of an energy gap . 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 . The specific heat detects these excitations in the form of an enhanced Sommerfeld coefficient : in zero field, increases steeply below 5 K, reaching a maximum at 0.1 K, and then saturates at . Upon increasing field, this maximum is shifted to higher temperatures with an overall reduction in , suggesting a residual density of states at the Fermi level developing a spin (pseudo-)gap . 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 , 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 in 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.
6 More- Received 26 May 2010
DOI:https://doi.org/10.1103/PhysRevB.82.125115
©2010 American Physical Society