Fast relaxation in a fragile liquid under pressure

TL;DR

This study investigates the pressure and temperature dependence of alpha and beta relaxations in ortho-terphenyl, revealing pressure-independent alpha-relaxation shape, a pressure-sensitive beta-relaxation onset, and a mode-coupling description of the dynamics.

Contribution

It provides the first detailed pressure-dependent measurements of relaxation processes in a fragile glass-former, linking relaxation times to viscosity and mode-coupling theory.

Findings

Alpha-relaxation shape is pressure-independent.

Beta-relaxation onset shows a kink in amplitude.

Mode-coupling theory describes beta-relaxation regime.

Abstract

The incoherent dynamic structure factor of ortho-terphenyl has been measured by neutron time-of-flight and backscattering technique in the pressure range from 0.1 MPa to 240 MPa for temperatures between 301 K and 335 K. Tagged-particle correlations in the compressed liquid decay in two steps. The alpha-relaxation lineshape is independent of pressure, and the relaxation time proportional to viscosity. A kink in the amplitude f_Q(P) reveals the onset of beta relaxation. The beta-relaxation regime can be described by the mode-coupling scaling function; amplitudes and time scales allow a consistent determination of the critical pressure P_c(T). alpha and beta relaxation depend in the same way on the thermodynamic state; close to the mode-coupling cross-over, this dependence can be parametrised by an effective coupling Gamma ~ n*T**{-1/4}.