Relaxation due to Incoherent Tunnelling in Dielectric Glasses

Abstract

A new relaxation mechanism is shown to arise from overdamped two-level systems above a critical temperature T * ≈ 5 K, thus yielding an explanation for experimental observations in dielectric glasses in the temperature range between T * and the relaxation peak at 50 K. Using the distribution function of the tunnelling model for the parameters of the two-level systems, both the linear decrease of the sound velocity and the linear increase of the absorption up to the relaxation maximum, are quantitatively accounted for by our theory.PACS. 61.40-amorphous and polymeric materials PACS. 63.50 -disordered solids, vibrational states PACS. 77.22G -relaxation phenomena, dielectrics

Incoherent Tunneling in Glasses at Very Low Temperatures

“…At this point, we note that two other mechanisms provoke incoherent tunneling in disordered solids provided that the temperature is sufficiently high. In metals this arises from the strong interaction between TSs and conduction electrons [31], and in insulating glasses from the interaction with phonons [32]. Whereas these two mechanisms die out at low enough temperature, the process discussed here is driven by the interaction between the TSs and persists down to the lowest temperatures.…”

Acoustic behaviour of normal and densified vitreous GeO2

“…The tunnelling model [7,8] associates these defect modes to single atoms or groups of atoms subjected to quantummechanical tunnelling between two different stable positions available in the glassy network and schematizes the locally mobile "particles" by asymmetric double-well potentials. A more recent extension of the tunnelling model [1,9] includes the case of a strong coupling between tunnelling two-level systems (TLS) and phonons and individuates three temperature regions where different mechanisms are regulating the motion within the two wells and the related acoustic properties: coherent tunnelling at low temperatures (T < 5 K), incoherent tunnelling at intermediate temperatures (5 K < T < 20 K) and thermally activated jumps over the potential barrier at higher temperatures (T >20 K). This theoretical approach covers both tunnelling and classical activation, leading to a coherent description of the acoustic behaviour of glasses over the wide range from very low to room temperature.…”

Tunneling Systems in Amorphous and Crystalline Solids

“…For more details on the mode-coupling approximation we refer to the original work by Neu and Wiirger (1994a); the case of a finite asymmetry energy is treated in Rau et al (1995a). For more details on the mode-coupling approximation we refer to the original work by Neu and Wiirger (1994a); the case of a finite asymmetry energy is treated in Rau et al (1995a).…”