Shiping Feng

A gauge invariant dressed holon and spinon description of the normal state of underdoped cuprates

A partial charge?spin separation fermion-spin theory is developed to study the normal-state properties of the underdoped cuprates. In this approach, the physical electron is decoupled as a gauge invariant dressed holon and spinon, with the dressed holon behaving like a spinful fermion, representing the charge degree of freedom together with the phase part of the spin degree of freedom, while the dressed spinon is a hard-core boson, representing the amplitude part of the spin degree of freedom. The local electron constraint for single occupancy is satisfied. Within this approach, the charge and spin dynamics of the underdoped cuprates are studied based on the model. It is shown that the charge dynamics is mainly governed by the scattering from the dressed holons due to the dressed spinon fluctuation, while the scattering from the dressed spinons due to the dressed holon fluctuation dominates the spin dynamics.

Kinetic energy driven superconductivity in doped cuprates

Within the t-J model, the mechanism of superconductivity in doped cuprates is studied based on the partial charge-spin separation fermion-spin theory. It is shown that dressed holons interact, occurring directly through the kinetic energy by exchanging dressed spinon excitations, leading to a net attractive force between dressed holons; then the electron Cooper pairs originating from the dressed holon pairing state are due to the charge-spin recombination. and their condensation reveals the superconducting ground state. The electron superconducting transition temperature is determined by the dressed holon pair transition temperature and is proportional to the concentration of doped holes in the underdoped regime. With the common form of the electron Cooper pair, we also show that there is a coexistence of the electron Cooper pair and antiferromagnetic short-range correlation, and hence the antiferromagnetic short-range fluctuation can persist into the superconducting state. Our results are qualitatively consistent with experiments.

Magnetic nature of superconductivity in doped cuprates

Within the kinetic energy driven superconducting mechanism, the magnetic nature of cuprate superconductors is discussed. It is shown that the superconducting state is controlled by both charge carrier gap function and quasiparticle coherent weight. This quasiparticle coherent weight grows linearly with the hole doping concentration in the underdoped and optimally doped regimes, and then decreases with doping in the overdoped regime, which leads to that the maximal superconducting transition temperature occurs around the optimal doping, and then decreases in both underdoped and overdoped regimes. Within this framework, we calculate the dynamical spin structure factor of cuprate superconductors, and reproduce all main features of inelastic neutron scattering experiments, including the energy dependence of the incommensurate magnetic scattering at both low and high energies and commensurate resonance at intermediate energy.

Electronic structure of kinetic energy driven superconductors

Within the framework of the kinetic energy driven superconductivity, we study the electronic structure of cuprate superconductors. It is shown that the spectral weight of the electron spectrum in the antinodal point of the Brillouin zone decreases as the temperature is increased. With increasing the doping concentration, this spectral weigh increases, while the position of the sharp superconducting quasiparticle peak moves to the Fermi energy. In analogy to the normal-state case, the superconducting quasiparticles around the antinodal point disperse very weakly with momentum. Our results also show that the striking behavior of the superconducting coherence of the quasiparticle peaks is intriguingly related to the strong coupling between the superconducting quasiparticles and collective magnetic excitations.

ELECTRONIC STRUCTURE OF KINETIC ENERGY DRIVEN CUPRATE SUPERCONDUCTORS

In this paper, we review the low energy electronic structure of the kinetic energy driven d-wave cuprate superconductors. We give a general description of the charge-spin separation fermion-spin theory, where the constrained electron is decoupled as the gauge invariant dressed holon and spin. In particular, we show that under the decoupling scheme, the charge-spin separation fermion-spin representation is a natural representation of the constrained electron defined in a restricted Hilbert space without double electron occupancy. Based on the charge-spin separation fermion-spin theory, we have developed the kinetic energy driven superconducting mechanism, where the superconducting state is controlled by both superconducting gap parameter and quasiparticle coherence. Within this kinetic energy driven superconductivity, we have discussed the low energy electronic structure of the single layer and bilayer cuprate superconductors in both superconducting and normal states, and qualitatively reproduced all main features of the angle-resolved photoemission spectroscopy measurements on the single layer and bilayer cuprate superconductors. We show that the superconducting state in cuprate superconductors is the conventional Bardeen-Cooper-Schrieffer like with the d-wave symmetry, so that the basic Bardeen-Cooper-Schrieffer formalism with the d-wave gap function is still valid in discussions of the low energy electronic structure of cuprate superconductors, although the pairing mechanism is driven by the kinetic energy by exchanging spin excitations. We also show that the well-pronounced peak-dip-hump structure of the bilayer cuprate superconductors in the superconducting state and double-peak structure in the normal state are mainly caused by the bilayer splitting.

Theory of photoemission from copper oxide materials

A mean-field theory which satisfies the electron on-site local constraint in the relevant regime of density for the high-temperature superconductors is developed. Within this approach, the electron spectral function, the electron dispersion, and the electron density of states of copper oxide materials are discussed, and the results are qualitatively consistent with the numerical simulations.

GAUGE INVARIANT DRESSED HOLON AND SPINON IN DOPED CUPRATES

We develop a partial charge-spin separation fermion-spin theory implemented by the gauge invariant dressed holon and spinon. In this novel approach, the physical electron is decoupled as the gauge invariant dressed holon and spinon, with the dressed holon behaviors like a spinful fermion, and represents the charge degree of freedom together with the phase part of the spin degree of freedom, while the dressed spinon is a hard-core boson, and represents the amplitude part of the spin degree of freedom, then the electron single occupancy local constraint is satisfied. Within this approach, the charge transport and spin response of the underdoped cuprates is studied. It is shown that the charge transport is mainly governed by the scattering from the dressed holons due to the dressed spinon fluctuation, while the scattering from the dressed spinons due to the dressed holon fluctuation dominates the spin response.

Slave-particle studies of the electron-momentum distribution in the low-dimensional t-J model.

The electron-momentum distribution function in the t-J model is studied in the framework of the slave-particle approach. Within the decoupling scheme used in gauge-field and related theories, we treat formally phase and amplitude fluctuations as well as constraints without further approximations. Our result indicates that the electron Fermi surface observed in high-resolution angle-resolved photoemission and inverse-photoemission experiments cannot be explained within this framework, and the sum rule for the physical electron is not obeyed. A correct scaling behavior of the electron-momentum distribution function near k approximately k(F) and k approximately 3k(F) in one dimension can be reproduced by considering the nonlocal string fields [Z. Y. Weng et al., Phys. Rev. B 45, 7850 (1992)], but the overall momentum distribution is still not correct, at least at the mean-field level.

Two gaps with one energy scale in cuprate superconductors

The interplay between the superconducting gap and normal-state pseudogap in cuprate superconductors is studied based on the kinetic energy driven superconducting mechanism. It is shown that the interaction between charge carriers and spins directly from the kinetic energy by exchanging spin excitations in the higher power of the doping concentration induces the normal-state pseudogap state in the particle-hole channel and superconducting state in the particle-particle channel, therefore there is a coexistence of the superconducting gap and normal-state pseudogap in the whole superconducting dome. This normal-state pseudogap is closely related to the quasiparticle coherent weight, and is a necessary ingredient for superconductivity in cuprate superconductors. In particular, both the normal-state pseudogap and superconducting gap are dominated by one energy scale, and they are the result of the strong electron correlation.

Enhancement of superconducting transition temperature by the additional second neighbor hopping t′ in the t–J model

Abstract Within the kinetic energy driven superconducting mechanism, the effect of the additional second neighbor hopping t ′ on the superconducting state of the t – J model is discussed. It is shown that t ′ plays an important role in enhancing the superconducting transition temperature of the t – J model. It is also shown that the superconducting-state of cuprate superconductors is the conventional Bardeen–Cooper–Schrieffer like, so that the basic Bardeen–Cooper–Schrieffer formalism is still valid in quantitatively reproducing the doping dependence of the superconducting gap parameter and superconducting transition temperature, and electron spectral function at [ π , 0 ] point, although the pairing mechanism is driven by the kinetic energy by exchanging dressed spin excitations.