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physica status solidi (b)

Density Excitation Spectrum of the One‐Dimensional Fermi Gas

Marian Apostol, A. Corciovei, S. Stoica

Published Jan 1, 1981• 4 citations
Publisher

Abstract

Abstract The collective excitations of the particle‐density fluctuations are studied in the one‐dimensional Fermi gas by means of the generator coordinate method. The dispersion relation are obtained both, for the forward and backward scattering interaction. It is shown that for repulsive interaction a gap occurs in the frequency of the backscattering excitations at wavevectors equal to twice the Fermi momentum.

DOI
10.1002/pssb.2221030145

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Abstract The collective excitations of the particle‐density fluctuations are studied in the one‐dimensional Fermi gas by means of the generator coordinate method. The dispersion relation are obtained both, for the forward and backward scattering interaction. It is shown that for repulsive interaction a gap occurs in the frequency of the backscattering excitations at wavevectors equal to twice the Fermi momentum.
The energy spectrum of the one-dimensional electron gas, solved by Luther and Emery for a particular attractive interaction strength UUU of the backward scattering across the Fermi surface, has been calculated for small deviations from this particular limit. The energy spectrum shows a gap at the Fermi energy, whose magnitude is very sensitive to the interaction UUU. The density of states shows a gap at zero temperature, which is smeared out by the quasiparticle lifetime for $T\ensuremath{\ne}0$.
By the method of ``modes'' introduced by Sawada, the excitation spectra of a fermion system with singular interactions have been obtained in three cases; (1) with one additional particle above and and one hole below the Fermi surface, (2) with one additional particle above the Fermi surface, and (3) with two particles above the Fermi surface. The argument holds not only for the system of low density (nuclear matter with a hard-core repulsion), but also for the case of the interacting electron gas in the high-density region. The results have been compared with the Brueckner-Goldstone perturbation-expansion formulas, by using diagrams in the second- and third-order of the expansion in interaction strength. We show that in the approximation of our treatment, the occupation probability function at temperature T has the same form as the Fermi distribution function.
A well-known feature of the classical monoatomic gas is that its bulk viscosity is strongly suppressed because the single-particle dispersion is quadratic. On the other hand, in condensed matter systems the effective single-particle dispersion is altered by lattice effects and interactions. In this work, we study the bulk viscosity of one-dimensional Fermi gases with generic energy-momentum dispersion relations. As an application, viscous dissipation arising from lattice effects is analyzed for the tight-binding model. In addition, we investigate how weak interactions affect the bulk viscosity. Finally, we discuss viscous dissipation in the regime in which the Fermi gas is not fully equilibrated, as can occur when the system is driven at frequencies that exceed the rate of fermion backscattering. In this case, the Fermi gas is described by three bulk viscosities, which we obtain for a generic single-particle dispersion.
We study the dilute-gas expansion for a two-dimensional Fermi system with arbitrary short-range repulsive interactions. In contrast with the three-dimensional case, we find an unusual pole in the vertex part in the particle-particle channel, for all center-of-mass momenta q2kF{\mathit{k}}_{\mathit{F}}kF​. We show that this represents an excitation consisting of bound hole pairs, which disperses down to zero energy at q=2kF{\mathit{k}}_{\mathit{F}}kF​. We study the effect of these bound states on the single-particle self-energy and find that the quasiparticles are well defined. Thus in the low-density regime, there is no breakdown of Fermi-liquid theory. In the Appendix we discuss the two-particle phase shift in the dilute Fermi gas, its connection with bound states, and the analogy with the potential scattering phase shift.
The excitation spectrum of pair fluctuations in the ground state of a dilute two-dimensional (2D) attractive Fermi-gas is studied for different coupling regimes. In the strong-coupling limit the spectrum has the Bogolubov form, but in the weak-coupling regime there are two types of the pair excitations (different from two-fermion scattering states): (i) long-wavelength collective mode with the cutoff at momentum q(c) similar to 1/xi(0), where xi(0) is the pair size, and (ii) pair excitations with q greater than or equal to root 8m mu.
Phase transitions of a quasi-one-dimensional Fermi gas are studied by the use of the renormalization group method for the fermion system without the bosonization. Intrachain backward and forward scatterings, pair electron hopping between different chains and exchange scattering of electrons in different chains are taken account of. Pair electron hopping gives rise to the singlet and triplet superconductor phase transitions. Electron exchange scattering gives rise to the spin density wave and charge density wave transitions. Calculation is made by the use of the one-dimensional dispersion. Following Menyhárd's method (Ref. 1) for the model with interchain backward scattering only, the change of the fluctuation from the one-dimensional region to the three-dimensional region is described. The transition temperatures are given by the bare couplings at high temperatures.