arXiv subjects

Xin-Zhong Yan

Publications and source records attributed to Xin-Zhong Yan.

32 records · Page 2Linked to original sources

arXiv2008

Supercurrent transferring through c-axis cuprate Josephson junctions with thick normal-metal-bridge

With simple but exactly solvable model, we investigate the supercurrent transferring through the c-axis cuprate superconductor-normal metal-superconductor junctions with the clean normal metal much thicker than its coherence length. It is shown that the supercurrent as a function of thickness of the normal metal decreases much slower than the exponential decaying expected by the proximity effect. The present result may account for the giant proximity effect observed in the c-axis cuprate SNS junctions.

cond-mat.supr-con↗

arXiv2008

Weak Localization of Dirac Fermions in Graphene

In the presence of the charged impurities, we study the weak localization (WL) effect by evaluating the quantum interference correction (QIC) to the conductivity of Dirac fermions in graphene. With the inelastic scattering rate due to electron-electron interactions obtained from our previous work, we investigate the dependence of QIC on the carrier concentration, the temperature, the magnetic field and the size of the sample. It is found that WL is present in large size samples at finite carrier doping. Its strength becomes weakened/quenched when the sample size is less than a few microns at low temperatures as studied in the experiments. In the region close to zero doping, the system may become delocalized. The minimum conductivity at low temperature for experimental sample sizes is found to be close to the data.

cond-mat.str-el↗

arXiv2007

Electric Transport Theory of Dirac Fermions in Graphene

Using the self-consistent Born approximation to the Dirac fermions under finite-range impurity scatterings, we show that the current-current correlation function is determined by four-coupled integral equations. This is very different from the case for impurities with short-range potentials. As a test of the present approach, we calculate the electric conductivity in graphene for charged impurities with screened Coulomb potentials. The obtained conductivity at zero temperature varies linearly with the carrier concentration, and the minimum conductivity at zero doping is larger than the existing theoretical predictions, but still smaller than that of the experimental measurement. The overall behavior of the conductivity obtained by the present calculation at room temperature is similar to that at zero temperature except the minimum conductivity is slightly larger.

cond-mat.str-el↗

arXiv2007

Study of interacting electrons in graphene under the renormalized-ring-diagram approximation

Using the tight-binding model with long-range Coulomb interactions between electrons, we study some of the electronic properties of graphene. The Coulomb interactions are treated with the renormalized-ring-diagram approximation. By self-consistently solving the integral equations for the Green function, we calculate the spectral density. The obtained result is in agreement with experimental observation. In addition, we also compute the density of states, the distribution functions, and the ground-state energy. Within the present approximation, we find that the imaginary part of the self-energy fixed at the Fermi momentum varies as quadratic in energy close to the chemical potential, regardless the system is doped or not. This result appears to indicate that the electrons in graphene always behave like a moderately correlated Fermi liquid.

cond-mat.str-el↗

arXiv2006

s-Wave-Like excitation in the superconducting state of electron-doped cuprates with d-wave pairing

An intrinsic physical mechanism, based on the doping evolution of the Fermi surface (FS), is explored to reconcile the contradictory experimental results on the superconducting (SC) pairing symmetry in electron-doped cuprates. It is argued that the FS pocket around $(π/2,π/2)$ has not yet formed until doping reaches about the optimal value. Therefore, in the underdoped regime, even if the SC order parameter is $d$ wave which vanishes along the line $k_x=k_y$, the quasiparticle excitation gap is still finite and looks s-wave-like due to the absence of the FS across that line. This makes it possible, with d-wave SC pairing, to understand those experiments which evidenced the s-wave quasiparticle excitation. An explicit theory with consideration of both antiferromagnetic and SC orders is implemented to exhibit the FS evolution from underdoping to overdoping and associated with it the variations of the quasiparticle property, electronic density of states, and low temperature dependences of the physical quantities heat capacity and superfluid density.

cond-mat.supr-con↗

arXiv2006

Study of two-dimensional electron systems in the renormalized-ring-diagram approximation

With a super-high-efficient numerical algorithm, we are able to self-consistently calculate the Green's function in the renormalized-ring-diagram approximation for a two-dimensional electron system with long-range Coulomb interactions. The obtained ground-state energy is found to be in excellent agreement with that of the Monte Carlo simulation. The numerical results of the self-energy, the effective mass, the distribution function, and the renormalization factor of the Green's function for the coupling constants in the range $0 \le r_s \le 30$ are also presented.

cond-mat.str-el↗

arXiv2006

Theory of antiferromagnetism in the electron-doped cuprate superconductors

On the basis of the Hubbard model, we present the formulation of antiferromagnetism in electron-doped cuprates using the fluctuation-exchange approach. Taking into account the spin fluctuations in combination with the impurity scattering effect due to the randomly distributed dopant-atoms, we investigate the magnetic properties of the system. It is shown that the antiferromagnetic transition temperature, the onset temperature of the pseudogap formation, the single particle spectral density, and the staggered magnetization obtained by the present approach are in very good agreement with the experimental results. The distribution function in momentum space at very low temperature is observed to differ significantly from that of the Fermi liquid. Also, we find zero-energy peak in the density of states (DOS) of the antiferromagnetic phase. This DOS peak is sharp in the low doping regime, and disappears near the optimal doping where the AF order becomes weak.

cond-mat.str-el↗

arXiv2006

Fluctuation-Exchange Study of Antiferromagnetism in Electron-Doped Cuprate Superconductors with Disorder

On the basis of the Hubbard model, we extend the fluctuation-exchange (FLEX) approach to investigating the properties of antiferromagnetic (AF) phase in electron-doped cuprate superconductors. Furthermore, by incorporating the effect of scatterings due to the disordered dopant-atoms into the FLEX formalism, our numerical results show that the antiferromagnetic transition temperature, the onset temperature of pseudogap due to spin fluctuations, the spectral density of the single particle near the Fermi surface, and the staggered magnetization in the AF phase as a function of electron doping can consistently account for the experimental measurements.

cond-mat.supr-con↗

arXiv2005

Long-Range Coulomb Effect on the Antiferromagnetism in Electron-doped Cuprates

Using mean-field theory, we illustrate the long-range Coulomb effect on the antiferromagnetism in the electron-doped cuprates. Because of the Coulomb exchange effect, the magnitude of the effective next nearest neighbor hopping parameter increases appreciably with increasing the electron doping concentration, raising the frustration to the antiferromagnetic ordering. The Fermi surface evolution in the electron-doped cuprate Nd$_{2-x}$Ce$_x$CuO$_4$ and the doping dependence of the onset temperature of the antiferromagnetic pseudogap can be reasonably explained by the present consideration.

cond-mat.supr-con↗

arXiv2005

Density of states modulation in the pseudogap state of high-$T_c$ superconductors

The density of states modulation recently observed by the scanning tunneling microscopic experiment in the pseudogap state of high-$T_c$ superconductors is explained by the pairing assisted particle transitions under perturbation of a periodic pairing interaction. In such a transition process, a particle with momentum ${\bf k}$ firstly picks up another particle of inverse spin to produce a pair leaving a hole. Under the perturbation of periodic pairing interaction of modulation wave vector ${\bf Q}$, the pair absorbs a momentum ${\bf Q}$ and then breaks into two single-particles: one propagates with momentum ${\bf k+Q}$, and another one fills in the hole. The transition is significant at low energies within the pseudogap since where the pairing excitations most favorably survive. We calculate the Fourier component of the modulated density of states using two different models. Both of the theoretical results are consistent with the experiment.

cond-mat.supr-con↗

arXiv2004

Superconductivity in the quasi-two-dimensional Hubbard model

On the basis of spin and pairing fluctuation-exchange approximation, we study the superconductivity in quasi-two-dimensional Hubbard model. The integral equations for the Green's function are self-consistently solved by numerical calculation. Solutions for the order parameter, London penetration depth, density of states, and transition temperature are obtained. Some of the results are compared with the experiments for the cuprate high-temperature superconductors. Numerical techniques are presented in details. With these techniques, the amount of numerical computation can be greatly reduced.

cond-mat.supr-con↗

arXiv2002

Pairing-fluctuation effect in quasi-two-dimensional superconductivity

We present a self-consistent approach to deal with the pairing-fluctuation effects in quasi-two-dimensional superconducting systems. Besides the Cooper pairs in the Bose-Einstein condensate, there are pairs occupying the excited states, which results in the predominant fluctuations. The low-lying excited states are the collective modes. On the basis of ladder-diagram approximation, we treat the single particles and the pairs on an equal-footing manner. The Green's function of single particles is obtained as an analytic solution to a cubic equation. The bosonic degrees of freedom are relevant to the pseudogap physics in the high-T_c cuprates. The superconducting order parameter and the transition temperature are substantially reduced from the values of the mean-field theory. The calculated phase boundary of superconducting state can reasonably describe the experiment data for cuprates.

cond-mat.supr-con↗

arXiv2001

Fluctuation Effect in Superconductivity in an Electron Model with d-Wave Attraction

On the basis of $t$-matrix approximation, we study the superconductivity in the tight-binding model with d-wave attraction. The low-lying collective modes are considered as the predominant long-range fluctuations in the system. The Green's function is obtained as an analytic solution to a cubic equation. The superconducting order parameter and the transition temperature are substantially reduced from the values of the mean-field theory. The results for the pseudogap parameter at $T_c$ are compared with experiments. The calculated phase boundary of the superconductivity can reasonably describe the experiment results for the cuprates. The results for density of states are presented.

cond-mat.supr-con↗

arXiv2000

Theory of electron transport in normal metal/superconductor junctions

On the basis of the Keldysh method of non-equilibrium systems, we develop a theory of electron tunneling in normal-metal/superconductor junctions. By using the tunneling Hamiltonian model (being appropriate for the tight-binding systems), the tunneling current can be exactly obtained in terms of the equilibrium Green functions of the normal metal and the superconductor. We calculate the conductance of various junctions. The discrepancy between the present treatment and the well-known scheme by Blonder, Tinkham, and Klapwijk is found for some junctions of low interfacial potential barrier.

cond-mat.supr-con↗