Solid State Theory, 1FA556, fall 2015
The properties of a system of a large number of particles – in our case, electrons – are determined by two main aspects: the system's ground state and its elementary excitations. From an experimental point of view the ground state is usually inert and information on the system's properties can only be obtained by studying how it responds to external stimuli. This response is governed by the elementary excitations. Experimentally, therefore, the excitations are primary and the ground state is secondary. From the point of view of theory the situation is usually reversed: in order to analyse the elementary excitations we must know the system's ground state since, fundamentally, an excited state is defined through its deviation from the ground state.