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

The many-body theory of interacting electrons in solids establishes the existence 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 spin and charge, called spinons and holons, respectively(2). This approach predicts power-law behaviour for the various properties of one-dimensional metals which is experimentally testable using a wide variety of methods, such as transport measurements(3,4) and optical conductivity measurements(5). Photoemission, on the other hand, provides a means by which the spin and charge 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), consistent with theoretical expectations. Recently, signatures of the existence of spin-charge separation have been inferred from line-shape analyses in a metal with different bands(7) and in an insulator(8). Here we present photoemission data from a genuine one-dimensional metal constructed on an insulating substrate. The spectra contain structures indicative of the excitation of spin and charge collective modes.