Coupling of the Lattice and Superlattice Deformations and Hysteresis in Thermal Expansion for the Quasi-One-Dimensional ConductorTaS3

Abstract

An original interferometer-based setup for measurements of length of needle-like samples is developed, and thermal expansion of o-TaS3 crystals is studied. Below the Peierls transition the temperature hysteresis of length L is observed, the width of the hysteresis loop δL/L being up to 5 · 10 −5 . The behavior of the loop is anomalous: the length changes so that it is in front of its equilibrium value. The hysteresis loop couples with that of conductivity. The sign and the value of the length hysteresis are consistent with the strain dependence of the charge-density waves (CDW) wave vector. With lowering temperature down to 100 K the CDW elastic modulus grows achieving a value comparable with the lattice Young modulus. Our results could be helpful in consideration of different systems with intrinsic superstructures.PACS Numbers: 71.45.Lr, 65.40.De Internal degrees of freedom is a feature of a random system; in principle, they can give rise to metastable size states resulting, say, to hysteresis in thermal expansion. 1 A special class form the compounds with intrinsic superstructures.Comprising two periodicities, generally incommensurate, the compounds occupy intermediate place between genuine aperiodic and truly periodic systems. 2 In these systems, such as charge-and spindensity waves (CDW and SDW), 3 Wigner cristals, superconductors in magnetic fields, 4 structurally incommensurate crystal phases, 5 the superstructure periodicities could be varied by external fields or temperature changes. The resulting metastable configurations can be reflected back onto the elastic properties and size of the underlying lattice, 3-5 though this question is still poorly understood.Quasi 1-dimensional conductors with CDW belong to a widely studied class of materials, in which intrinsic superstructure develops through the Peierls transition 6 . When electrons condense into CDW they form a deformable medium, -an electronic crystal. Deformation of the CDW affects their main static and dynamic properties and gives rise to metastability and hysteresis.The straightforward treatment of the CDW as a spring, whose strain is just applied to the crystal at the ends or via the impurities is not valid. Moreover, in the simple one-dimensional model the strains of the CDW and the crystal do not couple at all: if initially the CDW are relaxed, any change of the crystal length would not draw the CDW away from the equilibrium, i.e. give rise to a CDW deformation, 7 as it was noticed in Refs. 8-11. Similarly, once the CDW is deformed, any change of the lattice constant, c, would neither decrease nor increase the deviation of the CDW wavelength λ from the equilibrium value, λ eq . So, within this model a CDW deformation would not give rise to a length change.At the same time, the interaction of the CDW and the lattice is clearly seen from the elastic anomalies, including a drop of the Young modulus of the lattice, 8-11 Y l , up to 4%, 10 when the CDW become depinned.Mozurkewich 8 has concluded that the lattice deformation does give rise to a de...

Torsional Strain ofTaS3Whiskers on the Charge-Density Wave Depinning

“…This behavior is quite different from that of the length hysteresis loop [6], whose maximum is achieved around 100 K, and then falls gradually [22] (the measurements [22] were performed down to 35 K). The result indicates that the twisting can couple with the low-temperature anomalies [1], probably the lock-in, or the glass transition [23] (which, in turn, might be related to the commensurability [23]): note that the same activation energy, 400 K, is reported for the so-called -process (T < 60 K) [23].…”

“…Due to the interaction of the two ''springs,'' the crystal will change its dimensions, to enable the CDW to approach its equilibrium. This was experimentally observed as metastable length states resulting from the application of an electric field [11] or thermocycling [6]. Thus, length change on the CDW strain or compression can be directly put into correspondence with the drop of Y on CDW depinning.…”

“…A possible reason could be the variability of the S-S bond lengths with transferring electrons between the Ta and S atoms [26]. This feature of trichalcogenides is plausible for the explanation of coupling of the elastic properties [7] and deformations [6,11] with the longitudinal component of the q-vector. Alternatively, commensurability along the c axis would provide direct coupling of the shear deformations of the CDW and the lattice.…”

Dynamics of the hysteretic voltage-induced torsional strain in tantalum trisulfide

“…Note that coupling of longitudinal strains and CDW deformations, created by temperature changes, have been reported [14], and that Hoen et al [15] have shown that the sample length could be a hysteretic function of voltage as a result of such coupling when the strain response was relaxational. Hoen et al's model was motivated by their observation of small, sluggish hysteresis loops in the sample length (∆L/L ~ 10 -6 with time constants ~ 10 s), very similar in shape to the VITS loops [15].…”

Interferometric setup for measurements of expansion of whisker-like samples

“…For such materials, investigation of thermal expansion ͑TE͒ could give valuable information about the correlation of length with the CDW deformation, as thermal cycling creates a uniform in average deformation of the CDW of the maximum possible value. 4 TE study near the Peierls transition temperature can be also helpful in understanding the lattice distortion and the formation of the collective electron state. 5 The length changes of needle-like samples have been studied by various methods.…”