Insulating /metalhc

In principle, an RF electrode consists of an insulated metalhc shaft and a non-insulated, active tip of variable length and/or design. To close the electric circuit of the above-described system of generator, electrodes and patient, the active tip is in contact with the target tissue. 

Both phases exhibit metalhc character at RT with <7rt — 90 and 20-70 2 cm for the H2O- and C4H802-derived materials, respectively. The water pseudoiso-morph remains metallic down to 4.2 K while the C4H8O2 pseudoisomorph shows a rather sharp metal-insulator transition below c. 100 K. In this case the metallic state is restored down to 4.2 K by application of hydrostatic pressures of 15 kbar. 

Materials that are formally insulating such as WO3 may become semiconducting by appropriate doping for example, Na WO3 where x < 0.3. Upon further doping, what had been W(V) localized impurity states coalesce into a band and the bronze phase becomes metalhc for x > 0.3. 

The term Ed, stands for charging contributions. This term is absent in spectra of samples exhibiting a finite density of states at the Eermi level. For all practical purposes this is correct for aU true metals and for many semiconductors with intrinsic states near zero binding energy. Many systems relevant in catalysis do, however, not fulfill this condition (all non-black samples, glasses, porous materials, supports) or even worse, are composites of metallic and non-metaUic systems, giving rise to mixed metalhc-insulating behavior of their surface under PES. Such... 

Figure 2. The effect of decreasing particle size (particle nuclearity) on the electronic structure of a metal. The figure attempts to link the bulk, macroscopic regime (with the associated Fermi energy) with the situation of an emerging energy (Kubo) gap arising from the finite size of the particle. The various metalhc and insulating regimes of the mesoscopic regime are identified. Modified from Johnston. ...

Depending on the crystal structure of the one-dimensional stacks and on whether a Peierls transition occurs or not (more on this subject wiU be given in Sect. 9.3), the states in the one-dimensional bands are wholly or partiaUy fUled. The CT crystals can therefore be semiconductors or metalHc conductors. If at high temperature metallic conductivity is present and at a lower temperature Tp a Peierls phase transition occurs, the metal becomes a semiconductor at T< Tp, or an insulator. 

The paramagnetism of the conduction electrons, not observable by ESR, can however be detected by a measurement of the magnetic susceptibility. In the region of metalhc conductivity, one observes in the contribution of the conduction electrons a nearly temperature-independent Pauli susceptibility, well known from metals. In those crystals which undergo a Peierls transition and become semiconductors or insulators at low temperatures, the Pauli susceptibility is transformed into a Curie-law behaviour. The susceptibility then increases with further decreasing tern-... 

Several studies have indicated thatlinePAni)-emeraldine salt(S) / 2-acrylami-do-2 mthyl -1-propane sulfonic acid (AMPSA) [7,8,9,10] cast id (DCAA) is in the metallic regime toward the insulator-metal transition. The metallic beaviour was attributed to the criterion of finite conductivity when TDO K, the positive slope of the temperature dependence reduced activation energy (i.e W (T) = d [log o(T)]/d [log(T)]),liseddelocalized electronic states at the Fermi level that allow condnction without thermal activation. However, this polymer does not show metalhc behavior over the whole range of temperatures. They exhibit semiconducting (da/dT) > 0) and... 

Structural disorder in ICP systems introduces localization phenomena and limits conductivity. The extent of disorder determines whether conductivity is metalHc, insulating, or critical. In the classical definition, a metal should have a positive temperature coefficient of the volume resistivity (p), finite conductivity as T —> 0 K, and the logarithmic derivative of the temperature dependence of conductivity, i.e., the reduced activation energy (W), must show a positive coefficient. The reduced activation energy as a function of temperature is determined by... 

As a striking recent possible application for rare earth nitrides, they have been proposed as possible spintronics material. Aerts etal. have made a theoretical study where they report that the RN compounds can take a wide spectrum of properties over the different rare earths, from insulating to half-metallic to metalhc behavior. SmN and EuN are... 

---