We present measurements of the complete elastic constants of single-crystal FeSi using resonant ultrasound spectroscopy (RUS). FeSi is a narrow-gap (0.05 eV) semiconductor whose physical properties are similar to a class of compounds known as hybridization gap semiconductors or Kondo insulators. The narrow gap is reflected in the temperature dependence of the elastic moduli. © 1994.

We have measured the thermal expansion of the Kondo insulator SmB6 from 5 to 80 K using a sensitive capacitance dilatometer. There is a pronounced negative anomaly in the linear expansion coefficient α, of electronic origin, which achieves a local minimum of -5×10-6 K-1 at 45 K. We interpret this anomaly as a Schottky-like contribution arising from transitions across a semiconducting energy gap Eg140 K. Although there are several energy scales in the problem, a Grüneisen analysis indicates that the thermodynamics is driven by a single energy, which we identify as the gap energy. © 1994 The American Physical Society.

The results of an on-going study of YbIn1-xAgxCu4 are presented. YbInCu4 undergoes a first-order valence transition at 40 K, while YbAgCu4 is an isostructural heavy Fermion compound. We have succeeded in growing single crystals of these compounds, as well as intermediate alloys, using flux-growth techniques. A smooth evolution from YbInCu4 to YbAgCu4 is observed. Results on the influence of sample quality, as well as the effect of other dopants, on the valence transition in YbInCu4 are also discussed.

We present measurements of the linear expansion coefficient, α, on Fe1-xCoxSi from 40 to 300 K and for x = 0, 0.02, 0.05, and 1. For the semiconducting samples (x = 0, 0.02, and 0.05) the results below 200 K are consistent with the presence of a small gap in the quasiparticle excitation spectrum; above 200 K there is an anomalous contribution to the thermal expansion that lacks a detailed explanation, but which is probably associated with the high density of conduction electrons found in FeSi above 200 K. © 1994.

A zero-temperature magnetic-nonmagnetic phase boundary is accessed in CeRh2Si2 by application of pressure and in CeRh2−xRuxSi2 at ambient pressure for x ≈ 1.0. A comparison of specific heat and resistivity measurements in the two cases emphasizes the importance of disorder in producing non-Fermi-liquid-like behavior in these as well as in other Ce-based systems. © 1997 American Physical Society.

We present resonant ultrasound spectroscopy measurements of the complete elastic moduli of La2-xSrxCuO4 (x=0.00 and 0.14). The x dependence of both the absolute values of the moduli in the tetragonal phase and the temperature dependences of the moduli near the structural phase transition is weak. In both materials c44 and 1/2 (c11-c12) are unaffected by the transition, c33 shows a steplike drop broadened by fluctuations, and c11 and c66 decrease as 1/(T-TSPT) for T>TSPT (where TSPT is the structural phase transition temperature). We also present measurements of the anisotropic thermal expansion coefficients in La1.86Sr0.14CuO4 using a high-resolution capacitance dilatometer. These data when taken together raise serious questions about the standard interpretation of the structural phase transition in La2-xSrxCuO4. © 1994 The American Physical Society.

(Formula presented) undergoes a first-order isostructural valence transition near 40 K, whereas (Formula presented) is a moderately heavy (γ=250 mJ/mol (Formula presented)) mixed-valence compound. We have succeeded in growing single crystals of these compounds, as well as many intermediate alloys, using flux-growth techniques. The evolution from (Formula presented) to (Formula presented) has been characterized using electrical resistivity, magnetic susceptibility, and x-ray powder diffraction. The data are interpreted in terms of the evolution of the single-impurity Kondo temperature as a function of Ag concentration. © 1996 The American Physical Society.

We present here resonant ultrasound spectroscopy measurements of the elastic moduli and attenuation for U2Zn17 and UCu5 through the respective TN's of 9.7 and 15 K. For single-crystal U2Zn17, the data are the first modulus measurements for this material, and exhibit a very large softening of the sound velocity and an anomalous increase in attenuation below TN, while no precursors at all are observed above. Polycrystalline UCu5, in contrast, exhibits weak softening above TN, and a weak stiffening below. © 1994.

We present neutron diffraction results for YbIn(Formula presented), and Rietveld refinements of the structure. The ground state has the same structure (C15B) as the high temperature state, so that the first-order phase transition at (Formula presented) K is indeed isomorphic, i.e., the lattice constant changes without a change of crystal symmetry. The site disorder and the diffraction linewidths decrease systematically on going from polycrystal samples with two transitions (at 40 and 70 K) to polycrystals with a single (40 K) transition to flux-grown single crystals with a sharp transition at 40 K. We argue that in site-disordered samples, the effect of doping Yb onto the In site is to increase (Formula presented) and cause the transition to become continuous. © 1996 The American Physical Society.

We present results for the resistivity, the magnetoresistance, and the specific heat of (Formula presented)(Formula presented)(Formula presented) and (Formula presented). The impurity contributions to these measurements follow the predictions of the single-impurity Kondo model for a Kondo temperature (Formula presented)≊65 K, assuming that the impurity behaves as a crystal-field split ((Formula presented)) doublet. Assuming a J=5/2 impurity, the value of (Formula presented) needed to fit these experiments varies from 65 to 125 K. The contribution to the susceptibility may be too small to be explained by the model. These results address whether the nonmagnetic impurity behaves as a Kondo hole. © 1996 The American Physical Society.