Temperature dependence of the Peierls gap in (TaSe₄)₂I

Abstract

We have determined the longitudinal optical conductivity of (TaSe4),I in the energy range from 50 meV to 2 eV at different temperatures between 15 K and 420 K. We find a clear evidence that the free carriers are condensed into a charge density wave ground state not only below the transition temperature of 263 K but also at higher temperature up to the limits of the chemical stability of this compound.

The spectral, structural and transport properties of the pseudogap system (TaSe 4 ) 2 I

“…The optical conductivity of (TaSe 4 ) 2 I has been measured over a range of temperatures [8] and also shows a surprising lack of evolution with temperature.…”

“…Strangely however, the gap (or, above 263 K the pseudogap) measured by different experiments is not of the same size, as is illustrated in Table 1, where gaps and their corresponding mean-field transition transition temperatures are listed. [3]) 180 meV/2100 K 263 K -Optical Conductivity [8] 200 meV/2300 K --ARPES [4,3] 520 meV/6000 K -892 K Magnetic Susceptibility [5] --860 K…”

Electronic crystals: an experimental overview

“…Apparently the observation of the 1.1 -THz mode above the Peierls transition does not seem compatible with the interpretation of the ω 1 mode as a bound CDW collective mode. However as seen below, in (TaSe 4 ) 2 I, all the gap structure is already visible in optical conductivity above T P [707].…”

“…The temperature dependence of the optical conductivity of (TaSe 4 ) 2 I, exhibiting the strong absorption due to the CDW gap, is shown in figure 7.20(a). It is remarkable to note [707] that a gap is already present in the electronic excitation spectrum above the Peierls transition, manifesting that, even at high temperature, free carriers are condensed into a CDW ground state. However, above T c = 263 K, as shown by neutron or X-ray scattering, the CDW does not exhibit long-range order, but has the physical properties of a liquid.…”

Unusual plasmon dispersion in the quasi-one-dimensional conductor(TaSe4)2I: Experiment and theory

“…18 Hence the reason for the sudden increase in the width of the plasmon peak lies mainly in the fact that with increasing momentum transfer the plasmon enters the continuum of interband transitions. From optical measurements 19 or tight-binding ͑TB͒ band-structure calculations 20 this continuum is known to begin slightly below 2 eV. Thus one can speak of a well-defined plasmon excitation up to momentum transfers of about 0.4 Å Ϫ1 .…”