Mott transition in cuprate high-temperature superconductors

In this study, we investigate the metal-insulator transition of charge transfer type in high-temperature cuprates. We first show that we must introduce a new band parameter in the three-band d-p model to reproduce the Fermi surface of high temperature cuprates such as BSCCO, YBCO and Hg1201. We present a new wave function of a Mott insulator based on the improved Gutzwiller function, and show that there is a transition from a metal to a charge-transfer insulator for such parameters by using the variational Monte Carlo method. This transition occurs when the level difference $\Delta_{dp}\equiv \epsilon_p-\epsilon_d$ between d and p orbitals reaches a critical value $(\Delta_{dp})_c$. The energy gain $\Delta E$, measured from the limit of large $\Delta_{dp}$, is proportional to $1/\Delta_{dp}$ for $\Delta_{dp}>(\Delta_{dp})_c$: $\Delta E\propto -t_{dp}^2/\Delta_{dp}$. We obtain $(\Delta_{dp})_c\simeq 2t_{dp}$ using the realistic band parameters

Material-parameter Dependence of Superconductivity in High-temperature Cuprates

AbstractWe show that there is an interesting correlation between material parameters and critical temperature Tc in cuprate high temperature superconductors. Our analysis is based on the d-p model, that is, the three-band Hubbard model including d and p orbitals explicitly. This model contains many parameters; the transfer integrals tdp and tpp, the energy levels ɛp and ɛd, and the Coulomb interaction parameters Ud and Up. Our main results are the following: (a) Tc increases as ɛp−ɛd is increased for Up = 0, (2) Tc is lowered with increase of Up when ɛp−ɛd > 0, (3) Tc is increased with increase of Up when ɛp−ɛd < 0, (4) Tc has a minimum at near ɛp−ɛd = 0 as a function of ɛp−ɛd when Ud and Up are comparable, (5) Ud induces dx2-y2 pairing while Up induces dxy pairing, (6) Tc has a peak as a function of tpp. The results imply that Tc will increase if we can suppress Up. The role of Up is consistent with the experimental tendency that Tc increases as the relative ratio of the hole density at oxygen site to that at copper site is increased, which means that when Up increases, the number of p holes is decreased and Tc is also decreased

Renormalization of hopping integrals in coexistence phase of stripe and d-wave superconductivity in two-dimensional Hubbard model

AbstractWe have performed a variational Monte Carlo simulation on a two-dimensional Hubbard model with first- and second-neighbor hopping terms in order to study the coexistence state of a static stripe state and a modulated d-wave superconductivity in the underdoped cuprates. In addition to a Gutzwiller, a Jastrow and a doublon-holon correlation effects, the band-renormalization effect was considered in the trial wave function. The condensation energies of an 8-period stripe state was computed as a function of a Coulomb energy under the hole-density x=1/8. Our results reveal that the renormalization of higher hopping parameters due to the strong correlation effect enhances the one-dimensional hole motion on a quarter-filled band in the stripe state, and brings quasi-Fermi surface close to the magnetic zone boundary in the coexistence state

Stripe formation in high-Tc superconductors

The non-uniform ground state of the two-dimensional three-band Hubbard model for the oxide high-Tc superconductors is investigated using a variational Monte Carlo method. We examine the effect produced by holes doped into the antiferromagnetic (AF) background in the underdoped region. It is shown that the AF state with spin modulations and stripes is stabilized du to holes travelling in the CuO plane. The structures of the modulated AF spins are dependent upon the parameters used in the model. The effect of the boundary conditions is reduced for larger systems. We show that there is a region where incommensurability is proportional to the hole density. Our results give a consistent description of stripes observed by the neutron- scattering experiments based on the three-band model for CuO plane.Comment: 8 pages, 9 figure