Quasiparticle formation

Heavy Fermion superconductivity is found more frequently in intermetallic U-compounds than in Ce-compounds. This may be related to the different nature of heavy quasiparticles in U-compounds where the 5f-electrons have a considerable, though oibitally dependent, degree of delocalisation. The genuine Kondo mechanism is not appropriate for heavy quasiparticle formation as is the case in Ce-compounds. This may lead to more pronounced delocalised spin fiuctuations in U-compounds which mediate unconventional Cooper pair formation as discussed in sect. 2. The AF quantum critical point scenario invoked for Ce compounds previously also does not seem to be so important for U-compounds with the possible exception of UGe2. On the other hand AF order, mostly with small moments of the order 10 /Ub is frequently found to envelop and coexist with the SC phase in the B-T plane. 

Quasiparticle formation and Kondo screening in the periodic Anderson model... 

Silicon is a model for the fundamental electronic and mechanical properties of Group IV crystals and the basic material for electronic device technology. Coherent optical phonons in Si revealed the ultrafast formation of renormalized quasiparticles in time-frequency space [47]. The anisotropic transient reflectivity of n-doped Si(001) featured the coherent optical phonon oscillation with a frequency of 15.3 THz, when the [110] crystalline axis was parallel to the pump polarization (Fig. 2.11). Rotation of the sample by 45° led to disappearance of the coherent oscillation, which confirmed the ISRS generation,... 

It is remarkable that both quantities show very similar temperature dependences. It means that our microscopic EPR measurements and the macroscopic resistivity measurements by Ando et al. provide evidence of the same phenomenon the formation of hole-rich metallic stripes in lightly doped LSCO well below xcr = 0.06. This conclusion is also supported by a recent angle-resolved photoemission (ARPES) study of LSCO which clearly demonstrated that the metallic quasiparticles exist near the nodal direction below x=0.06 [16],... 

Impurity scattering leads to the formation of the gapless state at some sectors zero energy, N(0), where N(0) is the 2D density of states per spin at the Fermi level, and leads to a universal quasiparticle interlayer conductivity crg(0,0) ... 

A simple physical picture that is consistent with the above results is that above T one has coherent itinerant quasiparticle behavior over the entire Fermi surface, observed as an anomalous Fermi liquid. Below T one loses that coherent behavior for a portion of the Fermi surface near the antinodes the hot quasiparticles (those whose spin-fluctuation-induced interaction is strongest) found there enter the pseudogap state its formation is characterized by a transfer of quasiparticle spectral weight from low to high frequencies that produces the decrease in the uniform spin susceptibility below T. The remainder of the Fermi surface is largely unaffected. 

Figure 10. Theta trajectories for the Be+ (Is-1) Auger pole from the zeroth (bi-variational SCF), second order ( 3), quasiparticle second order (Ej), diagonal Sph-TDA ( 3pA TIM) and quasiparticle diagonal Sph-TDA (E3ph TDA) decouplings of the dilated electron propagator. The disparity between the theta trajectories for the SCF and propagator poles makes apparent the magnitude of correlation and relaxation effects attending the Auger resonance formation.

Taking anharmonicity of the lattice vibrations into account leads to interaction of the phonons with one another. When this interaction turns out to be sufficiently strong, the formation of bound states of quasiparticles becomes possible in addition to the above-mentioned multiparticle states. Such states are... 

In the preceding section it was shown that the formation of bound states of phonons leads to the appearance of a new type of resonance of the dielectric tensor ij(co). It is clear, of course (23), that the nonlinear polarizabilities should have analogous resonances, and this also concerns, besides biphonons, other types of bound states of quasiparticles, such as biexcitons, electron-exciton complexes, etc. 

The very existence of nonlinear polarizabilities is due to the presence of some anharmonicity in the medium. Anharmonicity is usually regarded as a weak perturbation in calculating these polarizabilities. It is clear, however, that when anharmonicity leads to the formation of states of quasiparticles bound to each other, the polarizabilities, in the region of the resonances corresponding to these states, become nonanalytic functions of the anharmonicity constants. For this reason ordinary perturbation theory is found to be inapplicable in their calculation, and more general methods are required. In accordance with two papers,... 

The distinction between Kondo metals, etc. and HF systems is fuzzy at best. As pointed out, the Kondo interaction is, among others, a basic ingredient of HF behavior. In a Kondo-lattice material one observes the effects of the Kondo interaction, for example on the magnetic properties, but very heavy quasiparticles are not formed and in consequence, the Sommerfeld constant is only slightly enhanced, That at least is the basis for a distinction we shall adopt. The hybridization between 4f and conduction electrons can lead to a hybridization gap in the density of states at the Fermi surface. The exact mechanism of gap formation is still under debate and also may vary from compound to compound. If a gap is present, one leaves the realm of Kondo metals and has, depending on the form of the gap (e.g., whether it is open in all crystallographic directions) and on its width, either a Kondo semimetal, semiconductor or insulator. The latter are certainly the most challenging class of Kondo compounds to understand. 

The relatively large Sommerfeld constant (0.79 J/(mol K )) of Sm3Se4 points towards the formation of heavy quasiparticles and, despite the fact that all Sm3X4 compounds are non-metallic, they have been discussed as low-carrier-density HF systems. Optical measurements on Sm3Sc4 (Batlogg et al. 1976) placed the 4f level inside the gap ( 4 eV width) about 0.7 eV below the bottom of the conduction band. Hence, conduction electrons do not play an essential role in the valence fluctuations. 

It is suggestive that the narrow Kondo resonance states of individual 4f impurities will form heavy quasiparticle bands in a periodic lattice of 4f ions. A satisfactory microscopic theory of heavy-band formation has yet to be developed. The Hamiltonian of eq. (107) can be generalized to the lattice by introducing a Bose field h,- at every lattice site. However, in this model it is no longer practicable to restrict to physical states with = 1 at every site. The most successful approach so far consists in a mean-field approximation for the Bose field (Coleman 1985, 1987, Newns and Read 1987) that is valid for large N and r < It can be applied both for the impurity and the lattice model. It starts from the observation that in the limit with QJN= fixed, the rescaled... 

Typically, is near to one for Ce systems with the bare f-level well below Ep. Note that the Anderson width A is much smaller than that conduction-band width, so that N (Ep) is indeed large when compared with the bare conduction-band DOS. On the other hand, it should be clear that singlet formation via an indirect exchange coupling cannot transfer much spectral weight from the bare local level up to Ep. Thus, a narrow resonance near the Fermi level of maximum height (jrAY and width k Tp < A arises. It should be emphasized that the local DOS considered here is not the quasiparticle DOS that is used in Fermi-liquid theory to express thermodynamic quantities via free-particle for-... 

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