Weak Pseudogap Behavior Research Articles

Fermi arcs, pseudogap, and collective excitations in dopedSr2IrO4: A generalized fluctuation exchange study

Motivated by recent experimental measurements, we study the quasiparticle spectra and the collective excitations in doped ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$, in which the interesting interplay between the electronic correlations and strong spin-orbital coupling (SOC) exists. To include the SOC, we use the Hugenholtz diagrams to extend the fluctuation exchange (FLEX) approach to the case where the SU(2) symmetry can be broken. By using this generalized FLEX method, we find a weak pseudogap behavior near $(\ensuremath{\pi},0)$ in the slightly electron-doped system, with the corresponding Fermi arc formed by the partial destruction of Fermi surface. Similar features also appear in the hole-doped system; however, the position of the Fermi arc is rotated ${45}^{\ensuremath{\circ}}$ with respect to the former. These results are consistent with the recent angle-resolved photoemission spectra in ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$. We suggest that these anomalous phenomena are caused by the scatterings of quasiparticles off the isospin fluctuation derived from the effective ${J}_{\text{eff}}=1/2$ doublet.

Spin response and weak pseudogap behavior in electron-doped cobaltates

The spin dynamics of electron doped Mott insulators on a triangular lattice is studied based on the t–J model. It is found that the particularly universal behaviors of integrated dynamical spin structure factor seen in the doped Mott insulators on a square lattice, are absent in the doped Mott insulators on a triangular lattice, suggesting the presence of a weak pseudogap in the normal state.

Band structure renormalization and weak pseudogap behavior inNa0.33CoO2: Fluctuation exchange study based on a single-band model

Based on a single-band Hubbard model and the fluctuation exchange approximation, the effective mass and the energy band renormalization in ${\mathrm{Na}}_{0.33}\mathrm{Co}{\mathrm{O}}_{2}$ is elaborated. The renormalization is observed to exhibit certain kind of anisotropy, which agrees qualitatively with the angle-resolved photoemission spectroscopy measurements. Moreover, the spectral function and density of states in the normal state are calculated, with a weak pseudogap behavior being seen, which is explained as a result of the strong Coulomb correlations. Our results suggest that the large Fermi surface associated with the ${a}_{1g}$ band plays likely a central role in the charge dynamics.

Origin of Weak Pseudogap Behaviors in Na0.35CoO2: Absence of Small Hole Pockets

We analyze the ``normal electronic states'' of Na_{0.35}CoO_2 based on the effective d-p model with full d-orbital freedom using the fluctuation-exchange (FLEX) approximation. They sensitively depend on the topology of the Fermi surfaces, which changes as the crystalline electric splitting (CES) due to the trigonal deformation. We succeed in reproducing the weak pseudo-gap behaviors in the density of states (DOS) and in the uniform magnetic susceptibility below 300K, assuming that six small hole-pockets predicted by LDA band calculations are absent. When they exist, on the contrary, then ``anti-pseudo-gap behaviors'' should inevitably appear. Thus, the present study strongly supports the absence of the small hole-pockets in Na_{0.35}CoO_2, as reported by recent ARPES measurements. A large Fermi surface around the \Gamma-point would account for the superconductivity in water-intercalated samples.

Microscopic theory of weak pseudogap behavior in the underdoped cuprate superconductors: General theory and quasiparticle properties

We use a solution of the spin fermion model which is valid in the quasistatic limit $\ensuremath{\pi}T\ensuremath{\gg}{\ensuremath{\omega}}_{\mathrm{sf}},$ found in the intermediate (pseudoscaling) regime of the magnetic phase diagram of cuprate superconductors, to obtain results for the temperature and doping dependence of the single particle spectral density, the electron-spin fluctuation vertex function, and the low frequency dynamical spin susceptibility. The resulting strong anisotropy of the spectral density and the vertex function lead to the qualitatively different behavior of hot [around $\mathbf{k}=(\ensuremath{\pi},0)]$ and cold [around $\mathbf{k}=(\ensuremath{\pi}/2,\ensuremath{\pi}/2)]$ quasiparticles seen in ARPES experiments. We find that the broad high energy features found in ARPES measurements of the spectral density of the underdoped cuprate superconductors are determined by strong antiferromagnetic (AF) correlations and incoherent precursor effects of an SDW state, with reduced renormalized effective coupling constant. Due to this transfer of spectral weight to higher energies, the low frequency spectral weight of hot states is strongly reduced but couples very strongly to the spin excitations of the system. For realistic values of the antiferromagnetic correlation length, their Fermi surface changes its general shape only slightly but the strong scattering of hot states makes the Fermi surface crossing invisible above a pseudogap temperature ${T}_{*}.$ The electron spin-fluctuation vertex function, i.e., the effective interaction of low energy quasiparticles and spin degrees of freedom, is found to be strongly anisotropic and enhanced for hot quasiparticles; the corresponding charge-fluctuation vertex is considerably diminished. We thus demonstrate that, once established, strong AF correlations act to reduce substantially the effective electron-phonon coupling constant in cuprate superconductors.

Weak Pseudogap Behavior in the Underdoped Cuprate Superconductors

We report on an exact solution of the nearly antiferromagnetic Fermi liquid spin fermion model in the limit \pi T << \omega_{sf}, which demonstrates that the broad high energy features found in ARPES measurements of the spectral density of the underdoped cuprate superconductors are determined by strong antiferromagnetic (AF) correlations and precursor effects of an SDW state. We show that the onset temperature, T^{cr}, of weak pseudo-gap (pseudoscaling) behavior is determined by the strength, \xi, of the AF correlations, and obtain the generic changes in low frequency magnetic behavior seen in NMR experiments with \xi(T^{cr}) \approx 2, confirming the Barzykin and Pines crossover criterion.