Electronic instability

Raghu, C., Anusooya Pati, Y., Ramasesha, S. Structural and electronic instabilities in polyacenes density-matrix renormalization group study of a long-range interacting model. Phys. Rev. B 2002, 65(15), 155204. 

Classical BCS theory dictated that superconductors with the highest Tc s would not be thermodynamically stable phases. Softening the phonons would raise Tc but would ultimately lead to structural instabilities. Increasing the density of states at the Fermi level would raise Tc but would eventually lead to an electronic instability. 

Not all the anomalies occur around the weaker cations. For instance, on the basis of size alone, one would expect to find Cr in six coordination but it is only ever found tetrahedrally coordinated. The absence of any observed examples of octahedral coordination around Cr is the result of an electronic instability discussed in Section 8.3.2. 

Canadell, E. and Whangbo, M.-H. (1991). Conceptual aspects of structure-property correlations and electronic instabilities with application to low dimensional transition-metal oxides. Chem. Rev. 91, 965-1034. 

These C-T salts are metallic at room temperature and are also two-dimensional. Upon lowering the temperature some of these salts become superconducting, provided that they can avoid the electronic instability toward a metal-insulator transition. The actual mechanism of superconductivity is not known at present. 

Here, an is the Bohr orbit radius of the isolated center and nc is the critical carrier density at the M-NM transition. Another way of viewing the transition is that of an electronic instability which ensues when the trapping of an electron into a localized level also removes one electron from the Fermi gas of electrons. This must clearly lead to a further reduction in the screening properties (which are themselves directly related to the conduction electron density) and a catastrophic situation then ensures the localization of electrons from the previously metallic electron gas. 

In order to compensate for variations during sample analysis (e.g. thermal instabilities, variability in flow rate and also electronic instability in the mass analyzer), samples are usually analyzed together with an internal standard, which is always added to the sample in the same amount. All measured peak areas or peak heights can be normalized on the signal of the internal standard, which helps to eliminate fluctuations during the individual measurement. 

Figure 7.7. The Peierls distortion of a one-dimensional metallic chain, (a) An undistorted chain with a half-filled band at the Fermi level (filled levels shown in bold) has an unmodulated electron density, (b) The Peierls distortion lowers the symmetry of the chain and modulates the electron density, creating a CDW and opening a band gap at the Fermi level, (c) The Fermi surface nesting responsible for the electronic instability.

Almost two decades ago theoreticians predicted that in a quasi-one-dimensional metal, characterized as having a one-dimensional gas of weakly interacting electrons, instabilities could arise leading to transitions to various ground states, such as charge density wave (CDW), spin density wave (SDW), or superconducting (61). Previously, it had been predicted that electron-phonon coupling in a one-dimensional... 

In the present contribution, we will examine the fundamentals of such an approach. We first describe some basic notions of the tight-binding method to build the COs of an infinite periodic solid. Then we consider how to analyze the nature of these COs from the viewpoint of orbital interaction by using some one-dimensional (ID) examples. We then introduce the notion of density of states (DOS) and its chemical analysis, which is especially valuable in understanding the structure of complex 3D sohds or in studying surface related phenomena. Later, we introduce the concept of Fermi surface needed to examine the transport properties of metallic systems and consider the different electronic instabilities of metals. Finally, a brief consideration of the more frequently used computational approaches to the electronic structure of solids is presented. 

The concept of Fermi surface does not only play a key role in understanding the dimensionality of the transport properties of metals but also in explaining the electronic instabilities of partially filled band systems. When a piece of a Fermi surface can be translated by a vector q and superimposed on another piece of the Fermi surface, this Fermi surface is said to be nested by the vector q. Since the Fermi surface of (92a) consists of two parallel fines, it is nested by an infinite number of wave vectors, two examples of which are shown in (96). In discussing Fermi surface nesting, it is important to consider Fermi surfaces in the entire reciprocal space, which is achieved by repeating the Fermi surface pattern of the first... 

A number of the distortions found in cation coordination environments are the result of electronic instabilities in the atoms themselves. Such distortions are typically found among the transition metal cations and the main group elements in low oxidation states. The origins of these instabilities are different and they... 

The largest distortions among transition metal cations are not shown by cations with partially filled d shells but by those whose d shells are empty or nearly empty (Kunz and Brown, 1995). There is still an electronic instability caused by degeneracy, but in these cations the degeneracy is accidental, and the... 

A fourth possibility was suggested by Tosatti and Anderson [140], the basis of which was that reconstructions derive from an electronic instability of the ideal surface, leading either to a charge density wave plus lattice distortions (a kind of Jahn—Teller effect) or possibly to a surface density wave. 

There are two main advantages of double-beam operation over single-beam operation. Very rapid monitoring of sample and reference helps to eliminate errors due to drift in source intensity, electronic instability, and any changes in the optical system. Also, double-beam operation lends itself to automation—the spectra can be recorded by a strip-chart recorder. 

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