I. Introduction
In the real world, the dynamic behaviors of systems in many branches of science and industry are depicted by various integro-differential equations because of memory behavior, such as [2], [8], [9]. Over the past several decades, a variety of researchers have paid attention to integro-differential equations with viscoelasticity or thermoviscoelasticity, see for instance [1], [6], [7], [10]. In [13], [14], a special basis property is proved: a sequence of generalized eigenfunctions of the system exists, which constitutes a Riesz basis for the state space. Other different studies for materials with viscoelasticity or thermoviscoelasticity can also be found in [3], [12].