Jihong Qin

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.

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.

Electronic structure of kinetic energy driven superconductors in the presence of bilayer splitting

Within the framework of the kinetic energy driven superconductivity, the electronic structure of bilayer cuprate superconductors in the superconducting state is studied. It is shown that the electron spectrum of bilayer cuprate superconductors is split into the bonding and antibonding components by the bilayer splitting, then the observed peak-dip-hump structure around the [�,0] point is mainly caused by this bilayer splitting, with the superconducting peak being related to the antibonding component, and the hump being formed by the bonding component. The spectral weight increases with increasing the doping concentration. In analogy to the normal state case, both electron antibonding peak and bonding hump have the weak dispersions around the [�,0] point.

Diamagnetism versus paramagnetism in charged spin-1 Bose gases

It has been suggested that either the diamagnetism or paramagnetism of Bose gases, due to the charge or spin degrees of freedom respectively, appears solely to be extraordinarily strong. We investigate the magnetic properties of charged spin-1 Bose gases in an external magnetic field, focusing on the competition between the diamagnetism and paramagnetism, using the Lande-factor g of particles to evaluate the strength of the paramagnetic effect. We propose that a gas with g < 1/√8 exhibits diamagnetism at all temperatures, while a gas with g > 1/2 always exhibits paramagnetism. Moreover, a gas with the Lande-factor in between shows a shift from paramagnetism to diamagnetism as the temperature decreases. The paramagnetic and diamagnetic contributions to the total magnetization density are also calculated in order to demonstrate some details of the competition.

Thermodynamic properties of the itinerant-boson ferromagnet

Thermodynamics of a spin-1 Bose gas with ferromagnetic interactions is investigated via the mean-field theory. It is apparently shown in the specific-heat curve that the system undergoes two phase transitions, the ferromagnetic transition and Bose-Einstein condensation, with the Curie point above the condensation temperature. Above the Curie point, the susceptibility fits the Curie-Weiss law perfectly. At a fixed temperature, the reciprocal susceptibility is also in a good linear relationship with the ferromagnetic interaction.

Pseudogap and charge dynamics in doped cuprates

Abstract Within the microscopic theory of the normal-state pseudogap state, the doping and temperature dependence of the charge dynamics in doped cuprates is studied in the whole doping range from the underdoped to heavily overdoped. The conductivity spectrum in the underdoped and optimally doped regimes contains the low-energy non-Drude peak and unusual midinfrared band. However, the position of the midinfrared band shifts towards to the low-energy non-Drude peak with increasing doping. In particular, the low-energy non-Drude peak incorporates with the midinfrared band in the heavily overdoped regime, and then the low-energy Drude behavior recovers. It is shown that the striking behavior of the low-energy non-Drude peak and unusual midinfrared band in the underdoped and optimally doped regimes is closely related to the emergence of the doping and temperature dependence of the normal-state pseudogap.

Optical and transport properties in a doped two-leg ladder antiferromagnet

Within the t-J model, the optical and transport properties of the doped two-leg ladder antiferromagnet are studied based on the fermion-spin theory. It is shown that the optical and transport properties of the doped two-leg ladder antiferromagnet are mainly governed by holon scattering. The low-energy peak in the optical conductivity is located at a finite energy, while the resistivity exhibits a crossover from the high-temperature metalliclike behavior to the low-temperature insulatinglike behavior, which is consistent with the experiments.

Competition between paramagnetism and diamagnetism in charged Fermi gases

Abstract The charged Fermi gas with a small Lande-factor g is expected to be diamagnetic, while that with a larger g could be paramagnetic. We calculate the critical value of the g -factor which separates the dia- and paramagnetic regions. In the weak-field limit, g c has the same value both at high and low temperatures, g c = 1 / 12 . Nevertheless, g c increases with the temperature reducing in finite magnetic fields. We also compare the g c value of Fermi gases with those of Boltzmann and Bose gases, supposing the particle has three Zeeman levels σ = ± 1 , 0 , and find that g c of Bose and Fermi gases is larger and smaller than that of Boltzmann gases, respectively.

Magnetic properties of two-dimensional charged spin-1 Bose gases

Abstract Within the mean-field theory, we investigate the magnetic properties of a charged spin-1 Bose gas in two dimensions. In this system the diamagnetism competes with paramagnetism, where the Lande factor g is introduced to describe the strength of the paramagnetic effect. The system presents a crossover from diamagnetism to paramagnetism with the increasing of the Lande factor. g c denotes the critical value of the Lande factor. We get the same value of g c both in the low temperature and strong magnetic field limit. Our results also show that in very weak magnetic field no condensation happens in the two-dimensional charged spin-1 Bose gas.

Effect of the pseudogap on the infrared response in cuprate superconductors

One of the most essential aspects of cuprate superconductors is a large pseudogap coexisting with a superconducting gap, then some anomalous properties can be understood in terms of the formation of the pseudogap. Within the kinetic energy-driven superconducting mechanism, the effect of the pseudogap on the infrared response of cuprate superconductors in the superconducting state is studied. By considering the interplay between the superconducting gap and pseudogap, the electron current–current correlation function is evaluated based on the linear response approach and it then is employed to calculate finite-frequency conductivity. It is shown that in the underdoped and optimally doped regimes, the transfer of the part of the low-energy spectral weight of the conductivity spectrum to the higher energy region to form a midinfrared band is intrinsically associated with the presence of the pseudogap.