The low-energy theory for single-wall carbon nanotubes including Coulomb interactions is derived and analyzed. It describes two fermion chains without interchain hopping but coupled in a specific way by the interaction. The strong-coupling properties are studied by bosonization, and consequences for experiments on single armchair nanotubes are discussed. [S0031-9007(97)04654-1]

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We discuss the properties of interacting electrons on a finite chain with open boundary conditions. We extend the Haldane Luttinger liquid description to these systems and study how the presence of the boundaries modifies various correlation functions. In view of possible experimental applications to quantum wires, we analyse how tunneling measurements can reveal the underlying Luttinger liquid properties. The two terminal conductance is calculated. We also point out possible applications to quasi one dimensional materials and study the effects of magnetic impurities.

We develop a low-energy effective theory for spin-1/2 frustrated two-leg Heisenberg spin ladders. We obtain a new type of interchain coupling that breaks parity symmetry. In the presence of an XXZ-type anisotropy, this interaction gives rise to a novel ground state, characterized by incommensurate correlations. In the case of a single ladder, this state corresponds to a spin nematic phase. For a frustrated quasi-onedimensional system of infinitely many weakly coupled chains, this state develops true three dimensional spiral order. We apply our theory to recent neutron scattering experiments on Cs2CuCl4.PACS No: 75.10.Jm, 75.40.Gb Quantum spin chains have for a long time attracted the attention of both theorists and experimentalists. One of the main reasons for this continuing fascination is the dominant role played by quantum fluctuations in these systems, which lead to a rich variety of observed physical phenomena. In recent neutron scattering experiments [1] on the quasi one-dimensional frustrated Heisenberg antiferromagnet Cs 2 CuCl 4 , an intriguing type of spiral order was observed. The very existence of such spiral order, which is an incommensurate structure, is puzzling from a theoretical point of view. According to the standard lore, three dimensional ordering of quasi-1D systems results from stabilization of the dominant spin correlations in the underlying 1D constituents. Therefore, 3D spiral order would require strong incommensurate 1D spin correlations. This is in contradiction with the known properties of simple antiferromagnetic Heisenberg chains and ladders, where the only known mechanism for generating incommensurabilities is via external magnetic fields.In the present work we propose a novel mechanism that naturally gives rise to incommensurate correlations in spin ladders and quasi 1D materials in the absence of external fields. Interestingly, we find that this phenomenon occurs in a standard model of two coupled spin-1/2 zig-zag Heisenberg chains, which has attracted much recent interest [2,3].In the bulk of this letter we shall concentrate on this simple frustrated ladder system in order to clearly exhibit the precise nature of the mechanism of incommensurate spin correlations. The application of our results to the quasi 1D case relevant for Cs 2 CuCl 4 is briefly discussed at the end. In what follows we derive the low-energy effective theory for the zig-zag ladder system, which we find to contain a parity-breaking interchain interaction. This "twist term", which was not considered in previous studies, is an operator of conformal spin 1 [5] and has important consequences: We will show that, although it alters the position of phase boundaries already in the spin rotationally invariant (SU(2)) case, it leads to incommensurate correlations only in the presence of an easy plane XXZ anisotropy.The Hamiltonian of the anisotropic zig-zag Heisenberg ladder is