Charge-density-wave transport

Thus far, in Sect. 9.6 we have considered the electronic structure and the singleparticle excitations of (Fa)2PF6 crystals as examples of radical-cation salts. For single-particle charge transport at T T, a thermal activation of the charge carriers across the energy gap is necessary. It corresponds to a frequency of = 4.4 10 Hz, thus an excitation in the infrared spectral range. 

Complementary to the real part a co) of the conductivity is the real part s (co) of the dielectric function. s (co) exhibits a frequency dependence at temperatures between 105 K and 40 K which corresponds to Debye relaxation. The resulting polarisation is described in simplified form by 

oo and Sst are the dielectric constants for cu - oo and for cu 0 and r is the relaxation time. A quantitative evaluation of the temperature dependence of the relaxation time shows a thermally-activated relaxation probability. The value for its activation energy A is the same as the value of A obtained from the dc conductivity. It follows that the mechanism for the relaxation must be scattering of the charge-density wave from free charge carriers [41]. 

Problem 9.1. The optically-induced reversed Peierls transition  

In the Cu(DCNQI)2 radical-anion salt in the neighbourhood of the Peierls phase-transition temperature, it was found that the insulating state can be switched optically on a time scale of less than 20 ps into the conducting state (F. O. Karutz, H. C. Wolf et al, Phys. Rev. Lett. 81,140 (1998)). This optical switching process was termed a reversed Peierls transition . From the current transients, it was found that the switched volume must be at least 100 times larger than the directly photo- 

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Liq Cryst 62 181 (1980) (b) W. Briitting and W. Riess Peierls Instability and Charge-Density-Wave Transport in Fluoranthene and Perylene Radical-Cation Salts Acta Phys. Pol. A 87 785 (1995). 

Furthermore, in treating the electrical conductivity we have thus far considered only single-particle excitations and, in particular at T< T, only thermal excitations of the charge carriers across the Peierls band gap 2A. As we shall see later, the charge-density wave itself can also be transported. This charge-density-wave transport is strongly frequency and electric-field dependent (see Sect. 9.6.6). 

A0.3M0O3 (A = K, Rb) shows a M-NM transition associated with charge density waves, nonohmic transport and quasi one-dimensional character (Schlenker et ai, 1985). Similar nonohmic transport is found in NbSc3 and Ta 3. 

A very important feature of the Frohlich model is that the lattice distortion and the charge density wave need not be fixed to the frame of reference of the lattice (i.e., the phase of the distortion need not be fixed). The electrons which make up the charge density wave may then move as a unit (collective charge transport) with a large effective charge and large effective mass leading to enhanced conductivity. 

Charge injection, 19-2, 19-10, 19-24, 19-40 Charge injection barriers, 21-19-21-20 Charge neutrality levels, 19-7-19-8 Charge transport, 8-3, 8-25-8-34, 19-3 Charge vibrational coupling, 21-2 Charge-density wave (CDW), 16-10, 16-12,... 

As expected for a charge density wave (CDW) system, below the M-I transition, and for electrical fields high enough to depin the CDW, non-linear transport phenomena are observed in many a-phase compounds, namely those with M = Au and Pt [92—94]. In spite of the non-linear transport being a recent aspect of the research in this family of compounds, the experimental data already existing provides by far the strongest and more complete evidence for non-linear transport as due to a CDW motion in molecular conductors. The Au and Pt compounds with low transition temperatures at 8.2 and 12.2 K, have been more studied because their... 

The lattice constant of the x = 3 face-centered cubic unit cell is 14.28 A (4). Accordingly, the carrier density is 4.1 x 10 cm , with four C o molecules and twelve donated electrons p>er unit cell. This charge density corresponds to a Fermi wave vector kf = 0.50 A which, when substituted into a Boltzmann equation description of the minimum resistivity gives = 2.3 A for the electronic mean free path. This unphysically small implies that, even at X = 3, the Boltzmann equation is inadequate for describing a system where intergranular transport may still be limiting the conductivity. 

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