Signature Of Spin-charge Separation Research Articles

Interaction-range effects for fermions in one dimension

Experiments on quasi-one-dimensional systems such as quantum wires and metallic chains on surfaces suggest the existence of electron-electron interactions of substantial range and hence physics beyond the Hubbard model. We therefore investigate one-dimensional, quarter-filled chains with a Coulomb potential with variable screening length by quantum Monte Carlo methods and exact diagonalization. The Luttinger liquid interaction parameter K_rho decreases with increasing interaction strength and range. Experimentally observed values close to 1/4 require strong interactions and/or large screening lengths. As predicted by bosonization, we find a metal-insulator transition at K_rho=1/4. Upon increasing the screening length, the charge and spin correlation functions reveal the crossover from dominant 2k_F spin correlations to dominant 4k_F charge correlations, and a strong enhancement of the charge velocity. In the metallic phase, the signatures of spin-charge separation in the single-particle spectrum, spinon and holon bands, remain robust even for rather long-ranged interactions. The charge-density-wave state exhibits backfolded shadow bands.

Signatures of Spin-Charge Separation in Scanning Probe Microscopy

We analyze the effect of an auxiliary scatterer, such as the potential of a scanning tip, on the conductance of an interacting one-dimensional electron system. We find that the differential conductance for tunneling into the end of a semi-infinite quantum wire reflects the separation of the elementary excitations into spin and charge modes. The separation is revealed as a specific pattern in the dependence of the conductance on bias and on the position of the scatterer.

Signatures of spin-charge separation in the 1D organic conductor TTF-TCNQ from photoelectron spectroscopy

We have studied the electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ by angle-resolved photoelectron spectroscopy (ARPES). The data reveal significant discrepancies to conventional band theory. Instead the experimental dispersions can be quantitatively reproduced by the one-dimensional (1D) Hubbard model, if one allows for a surface-enhancement of the hopping integral induced by a relaxation of the tomost molecular layer. The TCNQ-related conduction band is thus found to display spectroscopic signatures of spin-charge separation on the energy scale of the band width. In contrast, the TTF-derived band seems to be only weakly correlated, at variance with other findings. The important role of electronic correlations in this material is further corrborated by a peculiar temperature dependence of the spectra. While the 1D Hubbard model thus yields a good description at finite excitation energies, it fails concerning the low-energy spectral behavior, most likely due to the additional importance of strong electron-phonon interaction and interchain electronic hopping on small energy scales.

Spectral functions of half-filled one-dimensional Hubbard rings with varying boundary conditions

We study the effect of varying the boundary condition on the spectral function of a finite one-dimensional Hubbard chain, which we compute using direct (Lanczos) diagonalization of the Hamiltonian. By direct comparison with the two-body response functions and with the exact solution of the Bethe ansatz equations, we can identify both spinon and holon features in the spectra. At half-filling the spectra have the well-known structure of a low-energy holon band and its shadow---which spans the whole Brillouin zone---and a spinon band present for momenta less than the Fermi momentum. Features related to the twisted boundary condition are cusps in the spinon band. We show that the spectral building principle, adapted to account for both the finite system size and the twisted boundary condition, describes the spectra well in terms of single spinon and holon excitations. We argue that these finite-size effects are a signature of spin-charge separation and that their study should help establish the existence and nature of spin-charge separation in finite-size systems.

Exact representation of the large-U Hubbard model and its application.

An exact representation of the large-U Hubbard model is developed in terms of fermionic holon, fermionic doublon, and local spin operators which requires no constraint between the charge and spin operators. The Hamiltonian in the new representation explicitly expresses the Nagaoka theorem, and there is an exact one to one correspondence between the new representation and the large-U Lieb-Wu wave function for the case of one dimension. Using the new representation, we further show that the signature of spin-charge separation, in any dimension, is the characteristic wave vector 2${\mathbf{k}}_{\mathit{F}}^{\mathrm{SF}}$ in the static density-density correlation function (${\mathbf{k}}_{\mathit{F}}^{\mathrm{SF}}$ encloses a Fermi volume filled by 1-x spinless fermions per site, where x is the doping concentration). In addition, we derive the corresponding representation for the t-J-J' model, where the J' term describes the indirect hopping of the electrons, and the holon (doublon) propagator which explicitly shows the dependence of charge propagation on the spin state.