Quasiparticle excitations

The dip disappears when the hole damping rj exceeds the frequency co > 0. Since the main contribution to the damping is made by states with energies — u) < iv — /r < 0 and 0 < k +k — /j < lj, this means that the above consideration is valid when there exist well defined quasiparticle excitations near the Fermi surface. 

The lifetime of the simplest quasiparticle, i.e. a hole in a surface band, can be obtained experimentally from the width of the corresponding peak in ARPES, since the spectral linewidth of a quasiparticle excitation in the energy space is inversely related to its lifetime. The lower panel of Fig. 4 shows the widths of the photoemission peaks at normal emission corresponding to the L-gap surface states. It can be shown [45] that for a 2D band such as these, the widths reflect the initial state (hole) lifetime. For these surface states the lifetime ranges from 30 to 110 femtoseconds (ImeV corresponds to a lifetime of 0.67 x 10 12s). 

A discussion is given of electron correlations in d- and f-electron systems. In the former case we concentrate on transition metals for which the correlated ground-state wave function can be calculated when a model Hamiltonian is used, i.e. a five-band Hubbard Hamiltonian. Various correlation effects are discussed. In f-electron systems a singlet ground-state forms due to the strong correlations. It is pointed out how quasiparticle excitations can be computed for Ce systems. 

Our result for RbaCeo differs from a recent determination by scanning tunneling microscopy [26] (A—77 K), possibly because NMR relaxation probes the minimum quasiparticle excitation energy, while tunneling probes the maximum in the quasiparticle density of states, or because of differences between surface and bulk properties. Our NMR relaxation data for Rb3C5o clearly deviate from an Arrhenius law below 8 K. At these tem-... 

Calculations for finite nuclei will be discussed which demonstrate that the distribution of sp strength in the experimentally accessible energy region can be qualitatively understood. In addition, it becomes possible to interpret both theoretical and experimental results in terms of quasiparticle excitations, the basic concept of Landau s theory of Fermi liquids [19-21]. In contrast to an infinite liquid, the sp basis must be appropriate for the finite system under study and is not composed of the sp momentum states. Apart from this obvious requirement, most notions carry over rather straightforwardly. The ability to calculate the sp strength distribution and compare to experimental data presents an advantage over the approach initiated by Migdal [22,23]. 

The funnel approach restricts the number of the possible pathways. Kinematically, it means that for n local rotations — no matter which path down the funnel has been taken by the molecule — there appears the constraint of small n. This constraint comes from the spectroscopic data on the poorly dimensionally sensitive dispersion laws of the internal quasiparticle excitations [9,10], which stem the same order of magnitude for the two time intervals, for the molecular conformational transition (t), as well as for the (average) time of the local segmental rotations (t,), while bearing t = otj in mind. Certainly, this might be a serious restriction, in principle, for the large molecules conformational transitions in the still (semi-)classical funnel approach. 

Once the specimen turns to a superconducting state, the obtained superconductor-insulator-normal metal (SIN) spectrum probes the quasiparticle excitation in the superconductor, which directly reflects the symmetry of the order parameter A(k). If A(k) has simple s-wave symmetry, as is realized in conventional low-temperature superconductors, one expects a finite gap of A with overshooting peaks just outside the gap in N(E), as illustrated in fig. 6. Even if A(k) possesses anisotropic s-wave symmetry, a finite gap, corresponding to the minimum gap, appears. In dx2-yi superconductors with A(k) = coslkx - cos 2, in contrast, N(E) is gapless with linear N(E) for E A. It is noted that the extended-s wave A(x) = cos 2kx + cos 2ky is also characterized to possess a gapless feature with two singularities bX E = A and A2. 

Wang, H. X., and Mukamel, S., Quasiparticle excition representation of frequency dispensed optical nonlinearities in conjugated polyenes, J. Chem. Phvs., 97, 8019-8036 (1992). 

An intrinsic excitation upon which the rotational band is built, such as, e.g., a vibration or a two-quasiparticle excitation, can be present and is characterized by the projection of its intrinsic angular momentum I on the nuclear symmetry axis. This projection is called the K quantum number. The parity n of the band is also determined entirely by the intrinsic configuration. The entire system can then... 

A particle-hole excitation in the presence of pairing can now easily be described by a two-quasiparticle excitation, where the excitation energy of the final state is simply given by the sum of the quasiparticle energies... 

K Isomers are the nuclear analogues to the bicycle. They form when the K quantum number has to change during a transition, which requires a change of the orientation of the angular momentum vector. K isomers are found, e.g., when two quasiparticle states with large K form as the lowest quasiparticle excitations. This situation is the most common one in the regions around Hf and No [41]. 

More advanced teclmiques take into account quasiparticle corrections to the DFT-LDA eigenvalues. Quasiparticles are a way of conceptualizing the elementary excitations in electronic systems. They can be detennined in band stmcture calculations that properly include the effects of exchange and correlation. In the... 

Louie S G 1987 Theory of quasiparticle energies and excitation spectra of semiconductors and insulators Eleotronio Band Struoture and Its Applioations (Leoture Notes in Physios vol 283) ed M Youssouf (Berlin Springer)... 

Straightforward analytical models, however, receive particular attention in the present book, as they are of unique significance in the comprehension of physical phenomena and, moreover, provide the very language to describe them. To exemplify, recall the effect caused on the phase transition theory by the exactly soluble two-dimensional Ising model. Nor can one overestimate the role of the quasiparticle concept in the theory of electronic and vibrational excitations in crystals. As new experimental evidence becomes available, a simplistic physical picture gets complicated until a novel organizing concept is created which covers the facts known from the unified standpoint (thus underlying the aesthetic appeal of science). 

Phenomenological quasiparticle model. Taking into account only the dominant contributions in (7), namely the quasiparticle contributions of the transverse gluons as well as the quark particle-excitations for Nj / 0, we arrive at the quasiparticle model [8], The dispersion relations can be even further simplified by their form at hard momenta, u2 h2 -rnf, where m.t gT are the asymptotic masses. With this approximation of the self-energies, the pressure reads in analogy to the scalar case... 

It is instructive to start with the excitation spectrum in the case of the ordinary 2SC phase when dfi = 0. With the conventional choice of the gap pointing in the anti-blue direction in color space, the blue quarks are not affected by the pairing dynamics, and the other four quasi-particle excitations are linear superpositions of ur>g and dr(J quarks and holes. The quasi-particle is nearly identical with a quark at large momenta and with a hole at small momenta. We represent the quasi-particle in the form of Q(quark, hole), then the four quasiparticles can be represented explicitly as Q(ur,dg), Q(ug, dr), Q(dr,ug) and Q(dg,ur). When S/i = 0, the four quasi-particles are degenerate, and have a common gap A. 

Development of methods related to DFT that can treat this situation accurately is an active area of research where considerable progress is being made. Two representative examples of this kind of work are P. Rinke, A. Qteish, J. Neugebauer, and M. Scheffler, Exciting Prospects for Solids Exact Exchange Based Functional Meet Quasiparticle Energy Calculations, Phys. Stat. Sol. 245 (2008), 929, and J. Uddin, J. E. Peralta, and G. E. Scuseria, Density Functional Theory Study of Bulk Platinum Monoxide, Phys. Rev. B, 71 (2005), 155112. 

That is precisely which is reported say in [123] on example of Pd complexes (and for other systems in Ref. [124]) the TDDFT excitation energies are systematically lower than the experimental ones. In this context it becomes clear that the TDDFT may be quite useful for obtaining the excitation energies in those cases when the ground state is well separated from the lower excited states and can be reasonably represented by a single determinant wave function may be for somehow renormalized quasiparticles interacting according to some effective law, but shall definitely fail when such a (basically the Fermi-liquid) picture is not valid. 

An important prediction of LL theory is that the low-energy elementary excitations of a one-dimensional metal are not electronic quasiparticles, as... 

Now we look on the minimal excitation energies of the quasiparticles which reflect the presence of gaps in the charge-excitation-channel of the superconductor. 

According to equation (3) all the superconductivity gaps become vibronically renormalized. Defining the (larger) pseudogap as the minimal excitation energy of the band b quasiparticle one has... 

Figure one shows the results of calculations of 3gb and 131Sn, one-quasiparticle nuclides. The calculated levels agree very well with experimental excitation states. Figure 1 also shows results using the bare KK interaction and with the addition of a weak QQ potential determined in the two-quasiparticle nucli-... 

Y has properties which are basically determined through the valence nucleons beyond the core 96Zr (or 94Sr). The existence of the isomer of three-quasiparticle character indicates that there is no particular softness against deformation even at high excitation energy. 

Level schemes of Sm-138, Sm-136, Sm-134 and Nd- 132 are given in fig. 4 and the lifetimes of some excited states are summarized in table 1. The excitation energies in these nuclei have been computed on the basis of the interacting boson model, IBM-2 0TS78J. The computations are in good agreement with the experimental results of Sm-136, Sm-134 and Nd-132 but not of Sm-138. The extended IBM-2 which includes the interaction between the bosons and a two-quasiparticle 10+ state can reproduce the experimental situation ... 

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