Contributions of Phasons and of Low-Energy Excitations to the Specific Heat of the Quasi One-Dimensional Compound(TaSe4)2

Abstract

Measurements of the specific heat of quasi one-dimensional conductors, (TaSe 4 ) 2 I and (NbSe 4 ) 3 I, between 0.1 and 8 K, are reported. Whereas no deviation from the P law occurs at low temperature for (NbSe 4 ) 3 I, we have measured two anomalies in (TaSe^I: an excess heat capacity with a peak at J= 1.75 K and a specific heat which follows a P 2 law below 0.4 K. We ascribe this excess heat capacity to the charge-density-wave phase excitations above their pinning frequency in the insulating Peierls state (rp = 263 K).PACS numbers: 65.40.Em, 63.20.Dj, 63.50, + x, 72.15.Nj It is now well known that many few-dimensional compounds are unstable and undergo a Peierls transition at a critical temperature, T ? } This instability is characterized by the opening of an electronic gap at the Fermi level, by a softening in the phonon spectrum at 0==2/c F with A: F the Fermi wave vector, and by a modulation of the electron charge density with the same periodicity called a charge-density wave (CDW). Below T ? the soft-phonon branch splits into two different modes 2 " 4 : an opticlike amplitude mode and an acousticlike phase mode. For ideal one-dimensional conductors, when Q is incommensurate with the lattice, the phase mode or phason exists at zero frequency and corresponds to the current provided by the sliding motion of the CDW as proposed by Frohlich. 5 In real systems, as shown by Lee, Rice, and Anderson, 3 the gapless character of the phase mode is destroyed through various mechanisms, especially impurity pinning.Recently, the interest has focused on several families of one-dimensional inorganic compounds undergoing Peierls transitions, namely, transition-metal trichalcogenides (NbSe 3 , TaS 3 ), blue bronze, and halogenated transition-metal tetrachalcogenide (TaSe^L 6 In these compounds an extra dc current appears when the applied electric field is higher than a threshold value (typically 0.1-1 V/cm). This extra conductivity has been ascribed to Frohlich-type conduction when the CDW becomes unpinned and moves through the crystal. 6 By complex-impednace measurements in the millimeter-wave range the pinned sliding model for NbSe 3 and TaS 3 is found to be overdamped with a spectral response in the megahertz-gigahertz range, 7 whereas for (TaSe^I this mode is underdamped, indicating a pinning frequency 8 of 35 GHz or 1.7 K. The frequency of this pinned mode is just in the right frequency range to allow for the detection of CDW excitations by thermodynamical measurements.Hereafter we report on heat-capacity measurements on (TaSe^I and, for comparison, on (NbSe 4 ) 3 I. These compounds consist of parallel MSe 4 chains well separated from one another by halogen atoms. In each MSe 4 chain the metal atom is located at the center of a rectangular antiprism of eight Se atoms. The d z2 band filling of the transition metal M can be varied according to the composition. This change in band filling leads to quite different structural and electrical properties: (TaSe 4 ) 2 I and (NbSe 4 )ioI 3 undergo a Peierls transition at T P ...

Towards equilibrium ground state in charge-density-wave systems

“…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][6][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].…”

Incommensurate Systems as Model Compounds for Disorder Revealing Low-Temperature Glasslike Behavior

“…Cp/fiT1 , plots of several IC compounds, with p the coefficient of the Debye law. The left and middle panels are charge density wave compounds [K0 3MoO3 [20], (TaSe4)2I [22], KCP [20], and TaS3 [23] ]. The CP//3T} plot of vitreous silica is displayed as a comparison [19].…”

Phonon Poiseuille Flow in Quasi-One-Dimensional Single Crystals

“…It flattens off over the main part of the Brillouin zone and the corresponding characteristic frequency n 0 (or equivalent energy D 0 ) has a low value: D 0 ഠ 7 9 K. This exceptionally flat branch, which is not affected by Nb doping, reflects the strong anisotropy and quasi-1D character of this lattice structure. This has two important consequences: the effective acoustic Debye temperature estimated from specific heat is only 56 K [9] and the Grüneisen parameter g of these flat branches is extremely high (g ϳ 10 2 at 1 K) [10].…”