Observation of spin and charge collective modes in one-dimensional metallic chains

The many-body theory of interacting electrons in solids establishes the exi stence of elementary excitations, named quasiparticles, which show a one-to -one correspondence with noninteracting electrons. But this so-called Fermi liquid approach breaks down spectacularly in one-dimensional metals(1). In this situation, which is described by the Luttinger liquid formalism, the quasiparticles are replaced by distinct collective excitations involving sp in and charge, called spinons and holons, respectively(2). This approach pr edicts power-law behaviour for the various properties of one-dimensional me tals which is experimentally testable using a wide variety of methods, such as transport measurements(3,4) and optical conductivity measurements(5). P hotoemission, on the other hand, provides a means by which the spin and cha rge excitations can be observed directly. Previous photoemission studies of quasi-one-dimensional metals have essentially revealed only the absence of any discontinuity of the spectral function at the Fermi energy(6), consist ent with theoretical expectations. Recently, signatures of the existence of spin-charge separation have been inferred from line-shape analyses in a me tal with different bands(7) and in an insulator(8). Here we present photoem ission data from a genuine one-dimensional metal constructed on an insulati ng substrate. The spectra contain structures indicative of the excitation o f spin and charge collective modes.