scholarly journals SPECTRAL FUNCTION OF A d-p HUBBARD MODEL

2011 ◽  
Vol 25 (01) ◽  
pp. 41-53 ◽  
Author(s):  
E. J. CALEGARI ◽  
S. G. MAGALHAES
Keyword(s):  
Correlation Function ◽  
Fermi Surface ◽  
Hubbard Model ◽  
Spectral Function ◽  
Coulomb Interaction ◽  
Green's Functions ◽  
Green’S Functions ◽  
Pole Approximation ◽  
Antiferromagnetic Correlations ◽  
Important Correlation

This work investigates a d-p Hubbard model by the n-pole approximation in the hole-doped regime. In particular, the spectral function A(ω, k) is analyzed varying the filling, the local Coulomb interaction and the d-p hybridization. It should be remarked that the original n-pole approximation (Phys. Rev.184, 451 1969) has been improved in order to include adequately the k-dependence of the important correlation function 〈Sj·Si〉 present in the poles of the Green's functions. It has been verified that the topology of the Fermi surface (defined by A(ω = 0, k)) is deeply affected by the doping, the strength of the Coulomb interaction and also by the hybridization. Particularly, in the underdoped regime, the spectral function A(ω = 0, k) presents very low intensity close to the antinodal points (0, ±π) and (±π, 0). Such a behavior produces an anomalous Fermi surface (pockets) with pseudogaps in the region of the antinodal points. On the other hand, if the d-p hybridization is enhanced sufficiently, such pseudogaps vanish. It is precisely the correlation function 〈Sj·Si〉, present in the poles of the Green's functions, plays an important role in the underdoped situation. In fact, antiferromagnetic correlations coming from 〈Sj·Si〉 strongly modify the quasiparticle band structure. This is the ultimate source of anomalies in the Fermi surface in the present approach.

ANISOTROPIC PSEUDOGAP IN THE HALF-FILLING TWO-DIMENSIONAL HUBBARD MODEL AT FINITE TEMPERATURE

2000 ◽  
Vol 14 (21) ◽  
pp. 2271-2286
Author(s):  
TAIICHIRO SAIKAWA ◽  
ALVARO FERRAZ
Keyword(s):  
Fermi Surface ◽  
Fermi Level ◽  
Hubbard Model ◽  
Spectral Function ◽  
Density Of States ◽  
Spin Fluctuation ◽  
The Self ◽  
Two Dimensional ◽  
Self Energy ◽  
Half Filling

We have studied the pseudogap formation in the single-particle spectra of the half-filling two-dimensional Hubbard model. Using a Green's function with the one-loop self-energy correction of the spin and charge fluctuations, we have numerically calculated the self-energy, the spectral function, and the density of states in the weak-coupling regime at finite temperatures. Pseudogap formations have been observed in both the density of states and the spectral function at the Fermi level. The pseudogap in the spectral function is explained by the non-Fermi-liquid-like nature of the self-energy. The anomalous behavior in the self-energy is caused by both the strong antiferromagnetic spin fluctuation and the nesting condition on the non-interacting Fermi surface. In the present approximation, we find a logarithmic singularity in the integrand of the self-energy imaginary part. The pseudogap in the spectral function is highly momentum dependent on the Fermi surface. This anisotropy of the pseudogap is produced by the flatness of the band dispersion around the saddle point rather than the nesting condition on the Fermi level.

scholarly journals Composite Operator Method Analysis of the Underdoped Cuprates Puzzle

2014 ◽  
Vol 2014 ◽  
pp. 1-29 ◽  
Author(s):  
Adolfo Avella
Keyword(s):  
Fermi Surface ◽  
Hubbard Model ◽  
Cuprate Superconductors ◽  
Operator Method ◽  
Coulomb Repulsion ◽  
Pauli Principle ◽  
Composite Operator ◽  
Antiferromagnetic Correlations ◽  
Underdoped Cuprates ◽  
Composite Operator Method

The microscopical analysis of the unconventional and puzzling physics of the underdoped cuprates, as carried out lately by means of the composite operator method (COM) applied to the 2D Hubbard model, is reviewed and systematized. The 2D Hubbard model has been adopted as it has been considered the minimal model capable of describing the most peculiar features of cuprates held responsible for their anomalous behavior. COM is designed to endorse, since its foundation, the systematic emergence in any SCS of new elementary excitations described by composite operators obeying noncanonical algebras. In this case (underdoped cuprates—2D Hubbard model), the residual interactions—beyond a 2-pole approximation—between the new elementary electronic excitations, dictated by the strong local Coulomb repulsion and well described by the two Hubbard composite operators, have been treated within the noncrossing approximation. Given this recipe and exploiting the few unknowns to enforce the Pauli principle content in the solution, it is possible to qualitatively describe some of the anomalous features of high-Tc cuprate superconductors such as large versus small Fermi surface dichotomy, Fermi surface deconstruction (appearance of Fermi arcs), nodal versus antinodal physics, pseudogap(s), and kinks in the electronic dispersion. The resulting scenario envisages a smooth crossover between an ordinary weakly interacting metal sustaining weak, short-range antiferromagnetic correlations in the overdoped regime to an unconventional poor metal characterized by very strong, long-but-finite-range antiferromagnetic correlations leading to momentum-selective non-Fermi liquid features as well as to the opening of a pseudogap and to the striking differences between the nodal and the antinodal dynamics in the underdoped regime.

scholarly journals Энергетический спектр и спектр оптического поглощения фуллерена С-=SUB=-28-=/SUB=- в модели Хаббарда

2020 ◽  
Vol 62 (11) ◽  
pp. 1960
Author(s):  
А.В. Силантьев
Keyword(s):  
Group Theory ◽  
Hubbard Model ◽  
Green's Functions ◽  
Green’S Functions ◽  
Analytical Form ◽  
Energy Spectra ◽  
Symmetry Groups ◽  
Energy States ◽  
Static Fluctuation

Anticommutator Green’s functions and energy spectra of fullerene C28 and endofullerene Zr@C28 with the Td symmetry groups have been obtained in an analytical form within the Hubbard model and static fluctuation approxima-tion. The energy states have been classified using the methods of group theory, and the allowed transitions in the energy spectra of fullerene C28 and endofull-erene Zr@C28 with the Td symmetry groups have been determined.

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