Conductivity, optical

The dynamic conductivity calculated from reflectivity data in the far IR in an as-grown single crystal with Xd.c. 2Q cm amounts to x p l 5Q cm at 300 K, weakly dependent on wave number in the measured range 5 to 300 cm Faymonville et al. [17]. 

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It is possible to use the quantum states to predict the electronic properties of the melt. A typical procedure is to implement molecular dynamics simulations for the liquid, which pemiit the wavefiinctions to be detemiined at each time step of the simulation. As an example, one can use the eigenpairs for a given atomic configuration to calculate the optical conductivity. The real part of tire conductivity can be expressed as... 

Fig. 3. Reflectivity (a) and optical conductivity spectra (b) of oriented CNTs films along the an and aj directions. Bruggeman (BM) and Maxwell-Garnett (MG) fits (see text and Table 2) are also presented.

Fig. 4. The reflectivity (a) and the optical conductivity (b) in the p direction are similar to the ones along the a directions (Fig. 3). However, the absence of data above 4 eV changes the high energy spectrum of the optical conductivity. These changes are not relevant for the low frequency spectral range. The Maxwell-Garnett (MG) fit is also displayed as well as the intrinsic reflectivity and conductivity calculated from the fit (see Table 2 for the parameters).

Fig. 7. Model calculations for the reflectivity (a) and the optical conductivity (b) for a simple (bulk) Drude metal and an effective medium of small metallic spherical particles in a dielectric host within the MG approach. The (bulk) Drude and the metallic particles are defined by the same parameters set the plasma frequency = 2 eV, the scattering rate hr = 0.2 eV. A filling factor/ = 0.5 and a dielectric host-medium represented by a Lorentz harmonic oscillator with mode strength fttOy, 1 = 10 eV, damping ftF] = I eV and resonance frequency h(H = 15 eV were considered for the calculations.

This suggests an intrinsic metallic behaviour of the single CNTs. In this respect. Fig. 12 presents the intrinsic reflectivity (a) and optical conductivity spectra (b) of a hypothetical "bulk" (i.e., / = 1) CNTs specimen, using the parameters of Table 2. The low frequency metallic behaviour is easily recognised. (The reflectivity tends to 100 % when the frequency goes to zero and... 

Fig. 12. (a) The intrinsic reflectivity and (b) optical conductivity calculated for a "bulk" CNTs specimen (i.e. /= 1). They were calculated within the MG framework with the parameters of Table 2 for both an and a. ... 

Alternative evaluations of the absorption spectrum (or the optical conductivity) of Peierls systems within the FGM have been reported in Refs. (451 and [46], In those... 

Fig. 4.14 (a) Optical transmittance of graphene on a polyethylene terephthalate (PET) flexible substrate [19]. Optical and electrical data for PEDOT PSS-based composites with SWCNT (b) transmittance at 550 nm, (c) sheet resistance, (d) DC conductivity and (e) ratio of DC to optical conductivity. 

Among many fascinating properties, quasicrystals with high structural quality, such as the icosahedral AlCuFe and AlPdMn alloys, have unconventional conduction properties when compared with standard intermetallic alloys. Their conductivities can be as low as 450-200 (Qcm) [7]. Furthermore the conductivity increases with disorder and with temperature, a behaviour just at the opposite of that of standard metal. In a sense the most striking property is the so-called inverse Mathiessen rule [8] according to which the increases of conductivity due to different sources of disorder seems to be additive. This is just the opposite that happens with normal metals where the increases of resistivity due to several sources of scattering are additive. Finally the Drude peak which is a signature of a normal metal is also absent in the optical conductivity of these quasicrystals. 

An important result is also that many approximants of these quasicrystalline phases have similar conduction properties. For example the crystalline a-AlMnSi phase with a unit cell size of about 12 A and 138 atoms in the unit cell has a conductivity of about 300(Qcm) at low temperature [7,9]. The conductivity has the same defect and temperature dependence as that of the AlCuFe and AlPdMn icosahedral phase. There is, to our knowledge, no experimental result on the optical conductivity of this a-AlMnSi phase, but it is very likely that it is similar to that of AlCuFe and AlPdMn icosahedral phase. 

Fig. 3.8 Generic layout of a system suitable for studying very fast and ultrafast processes. Appropriate radiation sources may be a flash lamp, a laser or an electron accelerator, while optical, conductivity, or ESR detection systems may be employed.

Fig. 1. Optical conductivity spectra of AXC60 (x = 0, 3, 4, and 6) [7]. K3C60 is a metal, which shows a Drude-like behavior at low energy region, while K4C6o is an insulator, which does not show such a behavior.

Dia-Log Co. manufactures p-jump units with optical conductivity detection capabilities. A photograph of the p-jump unit with conductivity detection is shown in Fig. 4.6. Relaxation times of 50 >Lts—100 s can be measured. The conductivity range is 200 S m-1 to 0.05 S m-1, the temperature range is 273-343 K, and a sample volume of 0.5 ml or more can be used with a readout digitizer that has a memory of up to 256 values. It provides automatic data processing and data reduction with a microprocessor. The data can also be evaluated with PET, HP 67, or Wang 600 and 720 hand-held calculators. 

The aim of this article is to show that the new quasi-two-dimensional organic conductor p -(BEDO-TTF)5[CsHg(SCN)4]2 [hereafter called (BEDO)CsHg] (BEDO-TTF - bis-(ethylenedioxy)tetrathiafulvalene) which contains closed and open orbits displays rather complicated oscillatory spectra associated with magnetic breakdown (MB) and quantum interference (QI) effects. Tight binding band structure calculations for this compound are proposed to characterise its Fermi surface. The aim of the article includes also an investigation of the optical conductivity anisotropy with polarized infrared reflectance spectra. 

The IR reflectivity measurements were performed on single crystals of 2 0.5 0.3 mm3 in size. A FT-IR Perkin-Elmer 1725X spectrometer equipped with microscope and a helium cryostat was used. Polarized reflectivity spectra (R(ro)) were measured from the conducting plane in two principal directions. Optical conductivity a(co) was obtained by Kramers-Kronig transformation. 

Figure 3. Optical conductivity spectra of p -BEDO-TTF)5[CsHg(SCN)4]2 for E L L and E L at 300, 200, 100 and 10 K (L is BEDO-TTF stack direction). The fit with Drude-Lorenz model for T=10 K is shown by thin solid line.

The behaviour of the polarized reflectivity and optical conductivity spectra of new quasi-two-dimensional organic conductor p -(BEDO-TTF)5[CsHg(SCN)4]2 versus temperature for E L and E1. L are quite different. For E . L, the temperature changes of R(ro) and ct(co) are due to the decrease of the optical relaxation constant of the free carriers as expected for a metal. For E L at temperatures below 200 K, the energy gaps in the ct(co) spectra at about 4000 cm 1 and at frequencies below 700 cm 1 appear simultaneously with the two new bands of ag vibrations of the BEDO-TTF molecule activated by EMV coupling. This suggests a dimerization of the BEDO-TTF molecules in the stacks, which leads to a metal-semiconductor transition.. In the direction perpendicular to L, the studied salt shows metallic properties due to a very favourable overlap of the BEDO-TTF molecular orbitals. 

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