Microscopic nature of two-level systems (TLS) in glasses

Determine the microscopic origin and structure of the two-level systems responsible for the universal low-temperature anomalies in insulating non-crystalline solids, specifically identifying the atomic or configurational entities and their distributions that produce a linear-in-temperature specific heat and a quadratic-in-temperature thermal conductivity below 1 K.

Background

Zeller and Pohl observed that below 1 K, a broad class of insulating glassy materials exhibits a specific heat proportional to temperature and a thermal conductivity proportional to the square of temperature, independent of chemical composition. Anderson, Halperin, and Varma (and independently Phillips) proposed that these behaviors arise from quantum tunneling between two nearly degenerate configurations—so-called two-level systems (TLS)—with a broad, approximately flat distribution of energy splittings.

Despite the TLS framework’s success in explaining thermal and ultrasonic measurements, the actual microscopic identity of the TLS—whether they correspond to specific atoms, groups of atoms, or more complex structural motifs in amorphous solids—has not been established in most cases, leaving a fundamental gap between phenomenology and atomic-scale mechanism.