Oscillations, Waves, and Interactions - GWDG

Charge order in one-dimensional solids 329
Conductivity (Ω -1 cm -1 )
100
300 K
50
0
250 K
50
0
50
0
175 K
50
0
150 K
50
0
130 K
50
0
110 K
50
0
50
0
50
200 K
90 K
82 K
TTF-CA
0
0 20 40 60 80 100
Wavenumber (cm -1 )
Figure 19. Low-frequency conductivity of TTF-CA for T > TNI for different temperatures
as indicated in the panels. As the NI transition is approached by decreasing temperature,
the modes become stronger and an additional band appears as low as 20 cm −1 . To make the
comparison easier, the room temperature spectrum (black line) is replotted in the lowest
frame (after Ref. [32]).
a variety of high-frequency and optical techniques. Purely electronic correlations
between adjacent sites can cause charge disproportionation. Organic conductors are
suitable realizations to investigate the properties at the metal-insulator transitions.
The neutral-ionic transition observed in mixed-stack one-dimensional organic chargetransfer
salts can be a pure change of ionizity, but commonly goes hand in hand with
a Peierls distortion. This can be seen in a softening of the low-frequency phonon
modes above the phase transition.
Optical methods and in particular microwave techniques as developed at the Dritte
Physikalische Institut in Göttingen are powerful tools for investigation of chargeordering
phenomena in solids.
Acknowledgements. The review is based on many years of collaboration with a large
number of people; only some of them can be mentioned here. In particular I would like to
thank N. Drichko, M. Dumm, A. Girlando, B. Gorshunov, G. Grüner, and H.-W. Helberg.