Oscillations, Waves, and Interactions - GWDG

Reflectivity (%)
Conductivity (10 5 Ω −1 cm −1 )
100
80
60
40
20
8
7
6
5
4
3
2
1
Charge order in one-dimensional solids 319
0 10 10 2 10 3 10 4 10 5
KCP
E⏐⏐z
300 K
40 K
E z
Wave number (cm −1 )
0
0 500 1000 1500 2000 2500
Wave number (cm −1 )
Figure 8. (a) Reflectivity of K2Pt(CN)4Br0.3·H2O (abbreviated KCP) measured parallel
and perpendicular to the chains at different temperatures as indicated. (b) Optical conductivity
of KCP for E � stacks at T = 40 K (after Ref. [9]). The excitations across the
single-particle Peierls gap lead to a broad band in the mid-infrared while the small and
sharp peak centered around 15 cm −1 is due to the pinned mode.
from dielectrics. At low temperatures, the single particle gap around 1000 cm −1 becomes
more pronounced, and an additional structure is observed in the far-infrared
conductivity which is assigned to the pinned-mode resonance induced by the CDW
(Fig. 8(b)).
A detailed investigation of the pinned-mode resonance, its center frequency and
lineshape, and furthermore its dependence on temperature and impurity content
turned out to be extremely difficult because it commonly occurs in the range of 3
to 300 GHz (0.1 to 10 cm −1 ); i. e. it falls right into the gap between high-frequency
experiments using contacts and optical measurements by freely travelling waves [16].
Microwave technique based on resonant cavities and quasioptical THz spectroscopy
was advanced over the years in order to bridge this so-called THz gap [17]. Enclosed
resonators have been utilized for decades at the Drittes Physikalisches Institut [18]
and were readily available when in 1971 I. Shchegolev suggested them as a tool for
investigating small and fragile low-dimensional organic crystals like TTF-TCNQ [19].
(a)
(b)