Interactions between electrons in solids are often behind exciting novel effects such as ferromagnetism, antiferromagnetism and superconductivity. All these phenomena break away from the single-electron picture, instead having to take into account the collective, correlated behaviour of the system as a whole. In this chapter we look at how tunnelling spectroscopy can be used as the experimental tool of choice for probing correlation and interaction effects in one-dimensional (1D) electron systems. We start by introducing the Tomonaga-Luttinger Liquid (TLL) model, showing how it marks a clear departure from Fermi-liquid theory. We then present some early experimental results obtained using tunnelling devices and how they contributed to the decisive observation of both spin-charge separation and power-law behaviour. Other experimental techniques, such as photoemission and transport measurements, are also discussed. In the second half of the chapter we introduce two nonlinear models that are counterparts to the TLL theory, known as the mobile-impurity and the mode-hierarchy pictures, and present some of the most recent experimental evidence in support of both.
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