Modalities of Self-Organized Charge Response in Low Dimensional Systems S. Tomić 1 , T. Vuletić 1 , M. Pinterić 1,2 , B. Korin-Hamzić 1 1 Institut za fiziku, P.O.Box 304, HR-10001 Zagreb, Croatia 2 Faculty of Civil Engineering, University of Maribor, Maribor, Slovenia, Abstract. We present modalities of self-organized charge response in low dimensional systems, like diverse organic and quantum spin systems, studied by the low-frequency (10 mHz – 1 MHz) dielectric spectroscopy. Density wave structures with the order of commensurability N ≈ 4 can be recognized as phasons in a random impurity potential, whereas those with N ≈ 3 can be viewed as topological defects like charge domain wall pairs in the background domain structure. 1. INTRODUCTION Density wave (DW) is a complex, deformable object, which exhibits a self-organized response to externally applied fields. Density waves, mostly studied so far are incommensurate (IC) with respect to the underlying lattice with the order of commensurability N close to 4. The spatial variation of the DW phase in an IC structure is referred to as a phason. A dynamical mode associated with this DW variation, also referred to as the phason, is theoretically gapless long wavelength excitation. However, in the real crystals there is a small gap in the phason spectrum due to defects. The phason mode, as long as free carrier excitations are present, couples to an applied dc and ac electric field and gives novel contributions to the electrical conductivity. The coupling to the former gives rise to the non-linear conduction of electrical current above a finite electric field accompanied by narrow and broad band noise, whereas the coupling to the latter leads to a frequency dependent conductivity [1]. Theoretically, transverse phason mode couples to electromagnetic radiation and yields a narrow absorption close to the pinning frequency, whereas longitudinal screened mode results in a broad low frequency relaxation at frequencies typically lower than 1MHz. The latter is observed due to inherent randomness of density waves. This feature results also in that the observed absorptions are neither purely longitudinal nor transverse [2]. In this paper we summarize modalities of DW dielectric response observed in diverse systems. Section 2 describes the well established phason response for N ≈ 4 DW whose randomness is due to inhomogeneously distributed positions of impurities in the real crystals. Section 3 presents results that show the charge domain wall relaxation for commensurate DW whose randomness is associated with domain structure of the ground state. A DW relaxation observed recently in a low-dimensional quantum spin system with chains and ladders is presented in Section 4. 2. PHASON RELAXATION IN RANDOM IMPURITY POTENTIAL The response of N ≈ 4 DW is characterized by dielectric constants of the order 10 6 -10 9 , and is broader than the Debye one, which is expected for the system with a single degree of freedom [3]. This feature reflects a distribution of relaxation times associated with a single process due to a distribution of metastable states around the equilibrium position. These metastable states correspond to local changes of the phase of the pinned DW in a random impurity potential. Dielectric relaxation is strongly influenced by the free-carrier screening so that the response gradually slows down with temperature, with the activation energy equal to the single-particle activation energy. As an example, we show in Fig.1 and 2 dielectric relaxation observed in N ≈ 4 SDW, established below metal-semiconductor phase transition at T C = 12 K in (TMSTF) 2 PF 6 [4]. The observed behaviour indicates that SDW responds to the outer ac perturbation field by its long-wavelength phason excitations, and that the interaction with free carriers yields the dominant dissipation. In order to get an insight what happens when there is not enough