Band-structure effects

The interpretation of these unconventional conduction properties is still a challenge for condensed matter physicists. Several models have been proposed including thermally activated hopping [10] band structure effects due to small density of states and narrow pseudo-gap [11,12] or anomalous quantum diffusion [13,14]. Yet all these models are difficult to compare in a quantitative way with experiments. 

For real n-type cuprates, in which the stripons are based on excession and not holon states, the direction of the inequalities is reversed for the QE coefficients (11), but stays the same for the svivon coefficients (12). Deviations from these inequalities, especially for the a and b coefficients, could occur due to band-structure effects, and at specific k points by Eq. (2) they almost disappear for svivons close to point ko. 

As was discussed above, the absence of a kink in the nodal band below Ev [11] in NCCO, supports the possibility that it is also a real n-type cuprate. It is possible that the change in the sign of the TEP slope in NCCO with doping is an anomalous band-structure effect, probably associated with the peculiar evolution of its FS with doping, detected in ARPES [34], The position of the kink (below or above /q.) is determined by the inequality (11) between dq+ and d L, which is less susceptible to band-structure effects than the inequality (11) between bq+ and bq, determining the sign of the TEP slope. Anomalous behavior is observed also in the Hall constant of NCCO [32], which changes... 

The understanding of the near-edge absorption features of intermetallics is far from satisfactory. Band-structure effects are expected to play a role in determining the observed absorption characteristics, but calculations are not available for intermetallic compounds at sufficient energy above the Fermi level. 

Rosenberg, A., Tonucci, R., and Bolden, E., Photonic band-structure effects in the visible and near ultraviolet observed in solid state dielectric arrays, Appl. Phys. Lett, 69, 2639, 1996. 

It has to be stated that the above mentioned works, that invoke the jellium model, are expected to be valid for simple metals such as Al, for which the conduction band can be approximated by an electron gas. In the following we show recent calculations of the position dependent stopping power in which information about the real band structur e of the target is included. The Cu (111) surface is a good candidate to analyze the band structure effects in the stopping due to the presence of a surface band gap and a surface state. 

The phase stability of crystalline electron phases or Hume-Rothery (HR) phases has been explained by the afore-mentioned band-structure effects. For this purpose, the k- as well as r-space representation have been used successfully [5.45,46]. Crystalline HR-phases are well documented and excellent text books or reviews exist in this field [5.13,14, 35]. In the present section, only a few facts are mentioned in order to show how glassy metals belong to this class of phases. 

Surface sensitivity is an intrinsic property of photoemission measurements. The incident light penetrates far into the solid, but the escape depth of excited electrons is very short (Fig. 5), although there are local variations related to direction-dependent band structure effects. Surface sensitivity can be further enhanced by appropriate choice of experimental parameters such as photon energy, angles of incidence and emission, etc., which take advantage of selection rules favouring surface processes. 

It will be clear that this explanation of the difference between the magnetic moment in crystalline and amorphous materials is not restricted to alloys of 3d metals with rare earth. Its general validity follows for instance from the results shown in fig. 49, where the crystalline curves pertaining to the crystalline states are invariably below those of the amorphous states. Here we wish to stress again that the above explanation of the differences in saturation moment does not mean that band structure effects or charge transfer effects can be completely neglected (Malozemoff et al., 1983). The present analysis only shows that CSRO effects play a rather prominent role in the determination of the magnetic properties. 

All of the heavy lanthanide-transition metal amorphous alloys which are magnetic show antiferromagnetic coupling between the lanthanide and transition metal spins. The Curie temperatures as previously noted, are perturbed significantly from the crystalline values and may be either depressed (J -Fe alloys) or increased (R-Co alloys) due to fluctuations in exchange and anisotropy interactions or band structure effects. The latter has been ascribed by Tao et al. (1974) to explain the anomalous increase in the of R-Co alloys. They suggested a reduced electron transfer from the rare earth conduction bands to the Co d-band in the amorphous state compared to the crystalline. In the case of the RF z alloys the situation is more complex due to the population of both minority and majority spin bands of the Fe. 

To evaluate whether trends in chemisorption energies on Pt nanoparticles are consistent with the d-band model, d-band densities of states were projected out for different adsorption sites to determine the corresponding d-band centers relative to the Fermi level. No correlation was observed between the adsorption energies and site-specific d-band centers. Even though metal nanoparticles possess a continuous electronic band structure and, thus, metal-like electronic properties, their catalytic surface properties are not controlled by band structure effects but by the local electronic structure of the adsorption sites. An important conclusion from this study is that... 

Encouraged by these findings it is the aim of this report to study magnetic and transport phenomena of high-temperature superconductors probably in terms of the most simple effective one-band model describing both correlation and band structure effects, the so-called r-r -/model ... 

An alternative explanation of the anomalous resistivity, specific heat and susceptibility of UAlj has been suggested by De Groot et al. (1985). Their explanation of the resistivity behaviour is based on band-structure effects. From a band-structure calculation these authors conclude that the structure of the density of states near the Fermi energy can explain the resistivity variation without the necessity of the introduction of spin fluctuations or any other many-body effect. 

Near the bottom of the conduction band of a rare-gas liquid the energy momentum relation is probably parabolic and may be characterized by an effective mass. At higher energies, possibly as low as -I eV above the band minimum, a significant non-parabolic behavior is likely. This may arise, as in the solid, due to band structure effects, or alternatively due to the interaction with density fluctuations in a way reminiscent of the formation of the "bubble in liquid helium. 

A. Haug, Auger recombination in direct-gap semiconductors band-structure effects. J. Phys. C Solid State Phys. 16(21), 4159-4172 (1983)... 

We specialize the discussion to models that provide a more detailed account of the magnetic behavior of metals. The first step in making the free-electron picture more realistic is to include exchange effects explicitly, that is, to invoke a Hartree-Fock picture, but without the added complications imposed by band-stmcture effects. We next analyze a model that takes into account band-structure effects in an approximate manner. These models are adequate to introduce the important physics of magnetic behavior in metals. 

One important aspect of the impurity-related states is the effective mass of charge carriers (extra electrons or holes) due to band-structure effects, these can have an effective mass which is smaller or larger than the electron mass, described as light or heavy electrons or holes. We recall from our earlier discussion of the behavior of electrons in a periodic potential (chapter 3) that an electron in a crystal has an inverse effective mass which is a tensor, written as [(/n) ]o . The inverse effective mass depends on matrix elements of the momentum operator at the point of the BZ where it is calculated. At a given point in the BZ we can always identify the principal axes which make the inverse effective mass a diagonal tensor, i.e. [(m) ]a = as discussed in chapter 3. The energy of electronic... 

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