Coherent quasiparticles

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,... 

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. 

An alternative quasiparticle description of the optical response is possible using the nonlinear exciton equations (NEE) (39). The response function is then represented in terms of one-exciton Green functions and exciton-exciton scattering matrix. Four coherent ultrafast 2D techniques have been proposed (16,17), and computer simulations of the 2D response were performed for model aggregates made out of a few two-level chromophores. 

But when does the scaling process stop It must stop when (1) the quantum coherence length = %0el, where 0 Vp/E0, becomes equal to the thermal coherence length th = Vpl nT, at which point the thermal fluctuations take over, (2) E0(/) = E0e l becomes equal to the energy vFq of the quasiparticles involved in the q dependent response functions [the q of Eq. (9a)] or (3) equal to the external Matsubara frequency [Pg.39]

Role of quantum statistics. When considering complex systems by methods of statistical physics, one operates with their time-dependent distributions. In fermionic systems (see Yu. Ozhigov), statistical requirements imply that we must replace the independent-particle description by a quasiparticle formalism for quantum information processing. Effects of statistical fluctuations on coherent scattering processes (see M. Blaauboer et al.) suggest the need for furher exploration of the role of statistics on the dynamics of entangled systems. 

Here a(T) expresses an enhancement factor due to an electron correlation effect. We neglect the temperature dependence of a(T) for the present. A is the hyperfme coupling constant when the S function interaction is assumed. In the case when the transferred hyperfine coupling depends on the neighbouring spins, A is no longer constant but wavenumber dependent as will be mentioned later. E and/(E) are the energy of the quasiparticles and the Fermi function, respectively. The density of states of the quasiparticles NS(E) and the anomalous density of states MS(E) associated with the coherent effect are expressed as follows ... 

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