Fractional power-law susceptibility and specific heat in low temperature insulating state of o-TaS_{3}

Measurements of the magnetic susceptibility and its anisotropy in the quasi-one-dimensional system o-TaS_{3} in its low-T charge density wave (CDW) ground state are reported. Both sets of data reveal below 40 K an extra paramagnetic contribution obeying a power-law temperature dependence \chi(T)=AT^{-0.7}. The fact that the extra term measured previously in specific heat in zero field, ascribed to low-energy CDW excitations, also follows a power law C_{LEE}(0,T)=CT^{0.3}, strongly revives the case of random exchange spin chains. Introduced impurities (0.5% Nb) only increase the amplitude C, but do not change essentially the exponent. Within the two-level system (TLS) model, we estimate from the amplitudes A and C that there is one TLS with a spin s=1/2 localized on the chain at the lattice site per cca 900 Ta atoms. We discuss the possibility that it is the charge frozen within a soliton-network below the glass transition T_{g}~40 K determined recently in this system.Comment: 7 pages, 3 figures, submitted to Europhysics Letter

Low-temperature conductivity of quasi-one-dimensional conductors: Luttinger liquid stabilized by impurities

A new non-Fermi-liquid state of quasi-one-dimensional conductors is suggested in which electronic system exists in a form of collection of bounded Luttinger liquids stabilized by impurities. This state is shown to be stable towards interchain electron hopping at low temperatures. Electronic spectrum of the system contains zero modes and collective excitations of the bounded Luttinger liquids in the segments between impurities. Zero modes give rise to randomly distributed localized electronic levels, and long-range interaction generates the Coulomb gap in the density of states at the Fermi energy. Mechanism of conductivity at low temperatures is phonon-assisted hopping via zero-mode states. At higher voltages the excitations of Luttinger liquid are involved in electron transport, and conductivity obeys power-law dependence on voltage. The results provide a qualitative explanation for recent experimental data for NbSe3 and TaS3 crystals.Comment: 12 pages, 1 figur

Variable-range hopping in quasi-one-dimensional electron crystals

We study the effect of impurities on the ground state and the low-temperature dc transport in a 1D chain and quasi-1D systems of many parallel chains. We assume that strong interactions impose a short-range periodicicity of the electron positions. The long-range order of such an electron crystal (or equivalently, a $4 k_F$ charge-density wave) is destroyed by impurities. The 3D array of chains behaves differently at large and at small impurity concentrations $N$. At large $N$, impurities divide the chains into metallic rods. The low-temperature conductivity is due to the variable-range hopping of electrons between the rods. It obeys the Efros-Shklovskii (ES) law and increases exponentially as $N$ decreases. When $N$ is small, the metallic-rod picture of the ground state survives only in the form of rare clusters of atypically short rods. They are the source of low-energy charge excitations. In the bulk the charge excitations are gapped and the electron crystal is pinned collectively. A strongly anisotropic screening of the Coulomb potential produces an unconventional linear in energy Coulomb gap and a new law of the variable-range hopping $-\ln\sigma \sim (T_1 / T)^{2/5}$. $T_1$ remains constant over a finite range of impurity concentrations. At smaller $N$ the 2/5-law is replaced by the Mott law, where the conductivity gets suppressed as $N$ goes down. Thus, the overall dependence of $\sigma$ on $N$ is nonmonotonic. In 1D, the granular-rod picture and the ES apply at all $N$. The conductivity decreases exponentially with $N$. Our theory provides a qualitative explanation for the transport in organic charge-density wave compounds.Comment: 20 pages, 7 figures. (v1) The abstract is abridged to 24 lines. For the full abstract, see the manuscript (v2) several changes in presentation per referee's comments. No change in result

SPT 2004 - Symmetry and Perturbation Theory

SPT 2004 Symmetry and Perturbation Theory 30 May - 6 June 2004, Cala Gonone (Sardinia, Italy) Scientific Committee: S. Abenda (Bologna, I), D. Bambusi (Milano, I), G. Cicogna (Pisa, I), A. Degasperis (Roma, I), G. Gaeta (Milano, I), V. Kuznetsov (Leeds, UK), G. Marmo (Napoli, I), P. Olver (Minneapolis, USA), J.P. Ortega (Besan\ue7on, F), S. Rauch (Linkoping, S), E. Sousa Dias (Lisboa, P), S. Terracini (Milano, I), F. Verhulst (Utrecht, NL), S. Walcher (Aachen, D), B. Zhilinskii (Dunquerque, F) Organizing Commitee: A. Degasperis (Roma), G. Gaeta (Milano), B. Prinari (Lecce), S. Terracini (Milano) The conference is the fifth of a series begun in 1996. The principal aim of the series of conference is to join together researchers from areas of pure and applied mathematics, physics and chemistry to present their most recent and innovative achievements in the field of symmetries, perturbation and integrable systems. Conference proceedings are published by World Scientific