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Exploring low-energy landscape of organic semiconductors by heat relaxation and magnetic field

Charge density waves and spin density waves (together DW) are known for their metastabilities. Interaction of DW with impurities or commensurate lattice sets the preferred phase and the residual free carriers screen electrostatically the phase distortion. Pinning and screening are responsible for the glassy behavior of DW, i.e. they are at the origin of energy minima in a system phase space, separated from each other by energy barriers. Glass transition is a generic feature of DW systems which sets at Tg<Tp/4 (Tp being the temperature of DW phase transition) [1]. Collective pinning is frozen below Tg. Here, we present the investigation of the residual degrees of freedom in organic semiconductors at very low temperature. The glassy phenomenology in heat capacity experiment in this T-range is completed with the manifestation of low-energy excitations (LEE) - extra contribution to the heat capacity with huge time dependent effects (Figure 1.). Heat relaxation below 0.5 K, in these systems, has been shown to be nonexponential and to exhibit aging [2], in which the entire wide spectrum of relaxation times is shifted to longer times with increasing duration of the very small heat perturbation. We find a striking difference in this dynamics between the commensurate and incommensurate cases [3]. Contary to homogenous dynamics for incommensurate ground states, commensurability causes heterogenous dynamics, as shown in Figure 2. Finally, we report that LEEs of DW glassy state, soliton-like topological defects, demonstrate rich and unexpected properties in a magnetic field, due to their magnetic moments [4, 5]. Figure 1. Specific heat measured in thermal equilibrium compared with specific heat measured by short heat pulses for four organic salts with various ground states demonstrates huge effect due to very long relaxation times which can reach weeks below 0.1 K [2]. Figure 2. Bimodal energy relaxation rates for commensurate system (TMTTF)2PF6 [3] for various durations of heat perturbation with a weight transfer between well separated slow and fast entities. 1. Starešinić, D. et al. Phys. Rev. B 65, 165109 (2002) ; ibid, Phys. Rev. B 69, 113102 (2004) 2. Lasjaunias, J.C. et al. Phys. Rev. Lett. 72, 1283 (1994) and Phys. Rev. B 53, 7699 (1996) 3. Lasjaunias, J.C. et al. Phys. Rev. Lett. 94, 245701 (2005). 4. Mélin, R. et al., Phys. Rev. Lett. 97, 227203 (2006). 5. Starešinić, D. et al., to be published.

Density waves; glass; low energy excitations; heat relaxation; magnetic field relaxation

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