Semimetallic state

For (TSeT)2Cl there is a sharper transition to a semimetallic state below 25 K [18]. The origin of this transition is still unclear, but it can be suppressed by applying pressures greater than 5 kbar [66]. 

One of the initial motivations for pressure studies was to suppress the CDW transitions in TTF-TCNQ and its derivatives and thereby stabilize a metallic, and possibly superconducting, state at low temperatures [2]. Experiments on TTF-TCNQ and TSeF-TCNQ [27] showed an increase in the CDW or Peierls transition temperatures (Tp) with pressure, as shown in Fig. 12 [80], Later work on materials such as HMTTF-TCNQ showed that the transitions could be suppressed by pressure, but a true metallic state was not obtained up to about 30 kbar [81]. Instead, the ground state was very reminiscent of the semimetallic behavior observed for HMTSF-TCNQ, as shown by the resistivity data in Fig. 13. One possible mechanism for the formation of a semimetallic state is that, as proposed by Weger [82], it arises simply from hybridization of donor and acceptor wave functions. However, diffuse x-ray scattering lines [34] and reasonably sharp conductivity anomalies are often observed, so in many cases incommensurate lattice distortions must play a role. In other words, a semimetallic state can also arise when the Q vector of the CDW does not destroy the whole Fermi surface (FS) but leaves small pockets of holes and electrons. Such a situation is particularly likely in two-chain materials, where the direction of Q is determined not just by the FS nesting properties but by the Coulomb interaction between CDWs on the two chains [10]. 

An important precursor to the Bechgaard salts, TMTSF-DMTCNQ [2], also showed evidence for a low-temperature semimetallic state at pressures below ca. 12 kbar [83], but in view of the similarities with the Bechgaard salts, it is probable that a fully metallic state is formed at higher pressures [2]. 

The structure of this equation and its curves with N(T) given either by Equation 4 or 5 do not depend generally on dimensionality, d the values for the parameters for the three dimensionalities are of course different, but the evolution of the curves from the semiconductive state to the semimetallic state is the same for all three dimensions. Hence, Equation 5 can be used in Equation 7 to illustrate the predictions of the model. Such is shown in Figure 5 wherein a(T) / cr(200) is plotted for E = 10 eV, n = 1, and Ef is varied from 0 to 2 x 10 eV. In this8sequence, the curves vary from that for a semiconductor at ji to that for a semimetal (a degenerate semiconductor) at d. If curve a were extended to higher temperatures, then it would display a broad maximum, or minimum in the resistivity. The maximum is attributable to the competition... 

It must be emphasized, of course, that the model presented above is intended to describe only part of the evolution from the semiconductive state to the superconductive state and beyond into the semimetallic state. It is intended to describe only the normal state at the onset of the evolution of the superconductive state dictated by the density of carriers. To complete the description it is necessary to recognize the generation of the paired electron state and the resulting strong diamagnetic susceptibility which approaches - —. A thermochemical model describing the equilibria... 

The spectral features in the data shown in Figures 172 and 173 resemble those expected for a density-wave state, for example, as in (TMTSF)2PF6 where an SDW gap is formed at around 100 cm" in the direction perpendicular to the chains [1212]. With this interpretation, the resonance at 70 cm Vould be identified with the pinned collective mode. The spectral weight within the 70 cm resonance corresponds to a small fraction (approximately 10%) of the oscillator strength, which was redistributed from below 2A to above 2A. Alternatively, since the gap spans only a part of the Fermi surface, the results reported here can be interpreted in the context of the correlation-induced semimetallic state proposed by Vescoli et al. [1213]. 

ESR can detect unpaired electrons. Therefore, the measurement has been often used for the studies of radicals. It is also useful to study metallic or semiconducting materials since unpaired electrons play an important role in electric conduction. The information from ESR measurements is the spin susceptibility, the spin relaxation time and other electronic states of a sample. It has been well known that the spin susceptibility of the conduction electrons in metallic or semimetallic samples does not depend on temperature (so called Pauli susceptibility), while that of the localised electrons is dependent on temperature as described by Curie law. 

Figure 20. Electronic structure and transport in mixed conducting perovskites. (a) Band picture of electronic structure in the high-temperature metallic phase of Lai- r tCo03-(5. (Reprinted with permission from ref 109. Copyright 1995 Elsevier.) (b) Localized picture of electron/ hole transport in semimetallic Lai- 3r Fe03-(5, involving hopping of electrons and/or electron holes (depending on the oxidation state of iron).

Tile element uranium also exhibits a formal oxidation number of (II) in a few solid compounds, semimetallic in nature, such as UO and US. No simple uranium ions of oxidation state (II) are known in solution. 

The broad structure in CO2 desorption indicates the existence of a variety of different reaction sites, compatible with varying local geometries around the defect sites. The participation of prism faces is only possible at step edges as the large prism face area of the geometric sample block is passivated by the very first experiment with the sample. Valence band electronic spectra recorded simultaneously with the desorption experiment [90] reveal the transformation of the semimetallic surface into a fully insulating state, compatible with the creation of many surface defects on the (001) plane. 

If the Fermi level is at an energy such that the electronic states are extended, then finite conductivity at zero temperature is expected. This model assumes that the substantial disorder is homogeneous through the isotropic three-dimensional sample. Other external parameters such as magnetic field or pressure can affect the localization/delocalization transition and the localization lengths. This model has received much experimental attention for doped and ion implanted polymers [2,49], although more recent studies of ion-implanted rigid rod and ladder polymers reveals a three-dimensional semimetallic conductor with weak localization effects [50]. 

Curves a and b, or more correctly their reciprocals, resemble the uppermost curves in Figures 1 and 2 the variation from a to b or from top downwards in Figures 1 and 2 are caused in both cases by an increase in the density of carriers. As Ef is increased further the density of carriers increases and the sequence passes through curves like c before the semimetallic curve d is attained. At c the peaked minimum has disappeared, and consequently it corresponds to the lowest curves in Figures 1 and 2 of the "optimum" superconducting state. 

Actinide chalcogenides can be obtained for instance by reaction of the elements, and thermal stability decreases S > Se > Te. Those of a given actinide differ from those of another in much the same way as do the oxides. Nonstoichiometry is again prevalent and, where the actinide appears to have an uncharacteristically low oxidation state, semimetallic behaviour is usually observed. 

Fig. 1. Schematic representation of structure, crystal symmetry and energetic distribution of electronic d-states derived from the transition metal (M-d) in semiconducting dichalcogenides of Group TV, VI and VIII and semimetallic compounds of Group V...

---