A Study of Atomic Tunneling Centers in the Mixed Crystal Potassium-Bromide(1-X)(cyanide)(x) Using Dielectric Echo Techniques
Cyanide tunneling centers in the mixed crystal KBr$\sb{1-x}$(CN)$\sb{x}$ have attracted a great deal of experimental interest because of their wide range of behavior. In the dilute limit of low cyanide concentration, the CN molecules behave as isolated impurities in the ordered KBr host lattice. For cyanide concentrations of $x$(CN) $\approx$ 50%, however, disorder due to strong CN-CN interactions produces low-temperature properties that are nearly indistinguishable from those of a glass. In order to better understand the development of this 'glassy' behavior as the cyanide concentration is increased from the dilute-impurity limit to the glassy, high-concentration regime, dielectric-echo measurements were made on five KBr$\sb{1-x}$(CN)$\sb{x}$ crystals that had cyanide concentrations varying from 0.034 to 70%. These experiments were at a frequency of 0.79 GHz and in the temperature range 10 to 60 mK. Three-pulse echo experiments measured the spin-lattice relaxation times, T$\sb1$, of the states in order to determine the coupling strengths, $\gamma$, between the tunneling centers and lattice strain. Two-pulse echo experiments observed variations in the dipole moment, p, and the density of states, P, of tunneling centers with varying cyanide concentration. The two-pulse echoes additionally measured dephasing times, T$\sb2$, which reflect interactions between these low-energy excitations. In addition, the dielectric constants of the KBr$\sb{1-x}$(CN)$\sb{x}$ samples were measured at 0.79 GHz from room temperature down to 10 mK. The quantities p and $\gamma$ were found to be fairly independent of cyanide concentration. This result implies that the basic nature of the tunneling centers probably does not change significantly with changing cyanide concentration. The concentration dependences of P and T$\sbsp{2}{-1}$, however, suggest that the distribution in symmetry of the potential wells characterizing these excitations may vary greatly as the cyanide concentration changes. In particular, at low cyanide concentration the states seem to be fairly symmetric. As the cyanide concentration is increased, however, the distribution of tunneling centers becomes weighted progressively more towards the asymmetric states.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign