TLS Spectral Density and Order-Disorder Transition Temperature in (KBr)_1−x(KCN)_x

Abstract

Low-temperature measurements (T ≃ 0.1 to 10K) of the thermal conductivity and the specific heat [1–3] as well as the ultrasonic attenuation and dispersion [4] on (KBr)_1-x (KCN)_x single crystals with 0 ∠ x ∠ 1 have revealed the well-known thermal and acoustic anomalies found in a variety of structurally disordered systems [5]. For x = 0.25, 0.50 and 0.70, the spectral density P of two-level-systems (TLS) has been determined independently from the time-dependence of the specific heat [3, 6] on a time scale texp ≃ 100µsec to 10sec and from ultrasound data [4] for longitudinal and transverse modes (v ≃ 400MHz). By re-evaluating these ultrasound data we can now show that the P(x)-values obtained by this method are in good agreement with those evaluated from the time-dependence of the specific heat. From this analysis we evidence a variation on the CN^- concentration as approximately P ∝ (1-x)³.