Towards equilibrium ground state in charge-density-wave systems

Abstract

We present a study of energy relaxation in two representative charge-density-wave compounds: TaS3 and NbSei below r=0.5 K. The evolution of the relaxation indicates a crossover between a nonequilibrium state and the thermodynamical equilibrium state when the system has been allowed to age. In the equilibrium state the relaxation is thermally activated with an activation energy of -1 K. We interpret these results as long-time dynamics in the network of charge-density-wave dislocations with a critical temperature close to 0 K. PACS numbers: 72.15.Nj, 64.60.Ht Anomalous slow relaxations have been actively studied for many years in a wide variety of materials, such as polymers, ionic conductors, amorphous semiconductors, and spin glasses, in response to different excitations [1] (mechanical stresses, external electric and magnetic fields, etc.). Recent experiments show that chargedensity-wave (CDW) compounds [2] can be included in this broad class of physical systems [3,4]. The CDW ground state formed below the Peierls transition results from the competition between elastic deformations of the CDW and pinning potentials caused by randomly distributed impurities. In its pinned state due to randomness and disorder, the CDW exists in many metastable states characterized by local deformations of its phase. Indeed the signature of CDW low-energy excitations (LEE) was recently found [5-7] at very low temperature, as a contribution to the specific heat additional to that of phonons, in the family of inorganic CDW compounds TaS3, (TaSe^I, NbSe3, and K0.3M0O3. Moreover, in this temperature range the energy relaxation shows an anomalous long-time response with, in addition, "aging" effects [8]. This term, borrowed from the terminology used in the study of amorphous materials and spin glasses, means that the relaxation-time dependence of the energy relaxation depends on the time duration (waiting time) of the application of the thermal perturbation.In this Letter we report on energy relaxation measurements for two representative CDW systems, i.e., TaS3 and NbSe3 in the temperature range 0.1-1 K and in the time window 1-10 5 s. The relaxation rate clearly shows a crossover between a nonequilibrium state (with waitingtime effects) and the thermodynamic equilibrium state beyond a temperature-dependent waiting time. The time for reaching the ground state increases when T is reduced and is well described by an Arrhenius law with an activation energy of -1 K. We propose that the barriers dominating the relaxation originate from the dynamics between the defects in the CDW superstructure: phase slips or dislocation loops interacting with impurities. By analogy with dynamical properties in random systems in which a hierarchical procedure governs the time evolution, the nonexponential character of the relaxation and the activated behavior of the relaxation rate at equilibrium may indicate that the critical temperature of CDW glassy systems is -0 K.We have studied the energy relaxation between 0.08 and 8 K in a dilution refrigerator after...

Monte Carlo Studies of Ising Spin Glasses and Random Field Systems

“…In fact a spin glass transition is not a necessary ingredient for this scenario to occur, it has also been reported in experiments with two-dimensional spin glasses like Ru 2 Cu 0.89 Co 0.11 F 4 [78,79] or CuMn-films [175] and in models without frustration [238] or disorder [150,73]. The observation characteristic for aging can also be made in other amorphous or strongly disordered materials like polymers [280], charge-density-wave systems [26,27] or high-temperature superconductors [241].…”

Time-dependent specific heat below 1 K in the spin-density-wave state of (TMTSF)2PF6

“…2,3 A typical example has already been described with the case of the charge-density-wave ͑CDW͒ compound o-TaS 3 . [2][3][4] All these features are reminiscent of the low-T thermodynamical properties of structural glasses ͑SG's͒ or orientationally disordered crystals, i.e., orientational glasses OG's͒, [5][6][7][8][9] which are generally interpreted by the standard tunneling model.…”

Interplay between Phase Defects and Spin Polarization in the Specific Heat of the Spin-Density-Wave Compound(TMTTF)2Brin a Magnetic Field

“…This allows a systematic study of the equilibrium energy relaxation over almost one decade in temperature and a direct comparison to the local model of strong pinning without introducing an additional time scale related to ageing [6,11].…”