Multiscale insight into the Cd1-xZnxTe vibrational-mechanical properties -- High-pressure experiments and ab initio calculations
The Cd1-xZnxTe semiconductor alloy is a regular system regarding its macroscopic mechanic properties in that its experimental bulk modulus exhibits a linear x-dependence, in line with ab initio predictions. Complexity arises at the bond scale, referring to the intricate Cd1-xZnxTe percolation-type Raman pattern [T. Alhaddad et al., Journal of Applied Physics 133, 065701 (2023)]. This offers an appealing benchmark to test various phonon coupling processes at diverse length scales in a compact multi-oscillator assembly, presently tuned by pressure. At x around 0, an inter-bond long-range/macro electric coupling between the matrix and impurity polar phonons is detuned under pressure. Inversely, at x around 1, an intra-bond short-range/nano mechanic coupling is enforced between the two Zn Te apolar sub-phonons stemming from same and alien percolation-type environments. The pressure-induced macro/nano polar/apolar coupling/decoupling processes are compared within a model of two coupled electric/mechanic harmonic oscillators in terms of a compromise between proximity to resonance and strength of coupling, impacting the degree of mode mixing, with ab initio (apolar case) and analytical (polar case) Raman calculations in support. Notably, the free mechanic coupling at x around 1 opposes the achievement of a phonon exceptional point, manifesting the inhibition of mechanic coupling, earlier evidenced with similar bonds for x smaller than 0.5. Hence, the pressure dependence of a given bond vibration in a disordered alloy basically differs depending on whether the bond is matrix-like, i.e., self-connected in bulk (free coupling), or dispersed, i.e., self-connected in a chain (inhibited coupling). This features pressure-tunable percolation-based on-off phonon switches in complex media.