Hot spots and pseudogaps for hole- and electron-doped high-temperature superconductors

Using cluster perturbation theory, it is shown that the spectral weight and pseudogap observed at the Fermi energy in recent angle resolved photoemission spectroscopy of both electron- and hole-doped high-temperature superconductors find their natural explanation within the t-t(')-t(")-U Hubbard model in two dimensions. The value of the interaction U needed to explain the experiments for electron-doped systems at optimal doping is in the weak to intermediate coupling regime where the t-J model is inappropriate. At strong coupling, short-range correlations suffice to create a pseudogap, but at weak-coupling long correlation lengths associated with the antiferromagnetic wave vector are necessary.     

Angle-resolved photoemission spectroscopy of electron-doped cuprate superconductors: isotropic electron-phonon coupling

We have performed high resolution angle-resolved photoemission (ARPES) studies on electron-doped cuprate superconductors Sm2-xCexCuO4 (x=0.10, 0.15, 0.18), Nd2-xCexCuO4 (x=0.15), and Eu2-xCexCuO4 (x=0.15). Imaginary parts of the electron removal self energy show steplike features due to an electron-bosonic mode coupling. The steplike feature is seen along both nodal and antinodal directions but at energies of 50 and 70 meV, respectively, independent of the doping and rare earth element. Such energy scales can be understood as being due to preferential coupling to half- and full-breathing mode phonons, revealing the phononic origin of the kink structures. Estimated electron-phonon coupling constant lambda from the self energy is roughly independent of the doping and momentum. The isotropic nature of lambda is discussed in comparison with the hole-doped case where a strong anisotropy exists.     

Superfluid response in electron-doped cuprate superconductors

We propose a weakly coupled two-band model with dx(2)(-y(2)) pairing symmetry to account for the anomalous temperature dependence of superfluid density rho(s) in electron-doped cuprate superconductors. This model gives a unified explanation to the presence of an upward curvature in rho(s) near T(c) and a weak temperature dependence of rho(s) in low temperatures. Our work resolves a discrepancy in the interpretation of different experimental measurements and suggests that the pairing in electron-doped cuprates has predominately dx(2)(-y(2)) symmetry in the whole doping range.     

Research trends in electron-doped cuprate superconductors

In this review, we look back on some intriguing and puzzling issues in electron-doped cuprate superconductors, such as electron-hole asymmetry, two types of carriers, quantum critical points, order-parameter symmetry, etc. The necessity of study on this family is invoked in comparison with the hole-doped counterparts from several aspects. The related progress, especially in last few years, has been outlined point to point, as well as other hot topics like the discovery of ambipolar superconductors, the applications in superconducting electronics, and the emergency of superconductivity in parent compounds. In perspective, the utilization of blooming advanced techniques, electric double layer transistor and combinatorial film deposition, will bring some new insights into the mechanism such as electron-doped cuprate superconductors.     

Tunneling spectroscopy and order parameter symmetry of hole- and electron-doped cuprate superconductors

In tunneling spectroscopy of superconductors the density of states close to the surface or the interface to an insulating tunneling barrier is probed. For d-wave superconductors the particle–hole coherence results in interesting new phenomena at surfaces such as the formation of bound surface states at the Fermi level by Andreev reflection due to a sign change of the order parameter field in different k -directions. The probing of these states represents a phase-sensitive experiment allowing the determination of the order parameter symmetry in superconductors. We summarize the present experimental status with respect to the study of high-temperature superconductors (HTS). We discuss theoretically predicted consequences of a dominating d-wave order parameter in the hole-doped HTS on their tunneling spectra as well as on the physics of high-temperature superconductor Josephson junctions. A comparison of the tunneling spectra obtained for hole- and electron-doped HTS leads to the conclusion that the former have a d-wave, whereas the latter most likely have an anisotropic s-wave order parameter. We also address some unsettled questions related to the presence of a state with broken time-reversal symmetry at surfaces and interfaces of d-wave HTS and discuss specific features of d-wave tunnel junctions that have been predicted theoretically but still not been confirmed in experiments.     

Microwave electrodynamics of electron-doped cuprate superconductors

We report microwave cavity perturbation measurements of the temperature dependence of the penetration depth, lambda(T), and conductivity, sigma(T) of Pr(2-x)Ce(x)CuO(4-delta) (PCCO) crystals, as well as parallel-plate resonator measurements of lambda(T) in PCCO thin films. Penetration depth measurements are also presented for a Nd(2-x)Ce(x)CuO(4-delta) (NCCO) crystal. We find that Deltalambda(T) has a power-law behavior for T相似文献   

Electronic Raman response in electron-doped cuprate superconductors

The electronic Raman response in the electron-doped cuprate superconductors is studied based on the t-t^′-J model. It is shown that although the domelike shape of the doping dependent peak energy in the B2g symmetry is a common feature for both electron-doped and hole-doped cuprate superconductors, there are pronounced deviations from a cubic response in the B1g channel and a linear response in the B2g channel for the electron-doped case in the low energies. It is also shown that these pronounced deviations are mainly caused by a nonmonotonic d-wave gap in the electron-doped cuprate superconductors.     

电子型掺杂铜氧化物超导体准粒子激发谱研究

文中工作采用t-t′-t″-J模型研究了电子型掺杂铜氧化物超导体在超导序和反铁磁序共存情况下的准粒子激发谱,分别计算了在欠掺杂区、最佳掺杂区和过掺杂区的谱权重,计算结果与角分辨光电子谱实验结果符合较好.     

Quasiparticle scattering interference in electron-doped cuprate superconductors

By considering the nonmonotonic d-wave gap effect, the energy and momentum dependence of quasiparticle scattering interference is studied in the presence of a single impurity. It is shown that the pattern of the quasiparticle scattering peaks in the full Brillouin zone of electron-doped cuprate superconductors is very different from that in the hole-doped case described by the Octet model. This difference is the result of the nonmonotonic d-wave superconducting gap in the electron-doped case. As the energy increases, the position of the local peaks in the Brillouin zone moves rapidly. In particular, the characteristic peaks of the electron-doped cuprate superconductors appear between the antinodal and nodal directions, unlike in the hole-doped case.     

Evolution of spin dynamics in electron-doped cuprate superconductors

Within the kinetic energy driven superconducting mechanism, the evolution of the magnetic excitations of the electron-doped cuprates in the superconducting state is studied. It is shown that there is a broad commensurate low energy magnetic scattering peak, while the magnetic resonance energy is located among this broad commensurate low energy scattering range. This broad commensurate low energy magnetic scattering disperses outward into a continuous ring-like incommensurate magnetic scattering at high energy.     

Fluctuation-exchange study of antiferromagnetism in disordered electron-doped cuprate superconductors

On the basis of the Hubbard model, we extend the fluctuation-exchange (FLEX) approach to investigating the properties of the 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.     

Doping dependence of charge order in electron-doped cuprate superconductors

Abstract

In the recent studies of the unconventional physics in cuprate superconductors, one of the central issues is the interplay between charge order and superconductivity. Here the mechanism of the charge-order formation in the electron-doped cuprate superconductors is investigated based on the t-J model. The experimentally observed momentum dependence of the electron quasiparticle scattering rate is qualitatively reproduced, where the scattering rate is highly anisotropic in momentum space, and is intriguingly related to the charge-order gap. Although the scattering strength appears to be weakest at the hot spots, the scattering in the antinodal region is stronger than that in the nodal region, which leads to the original electron Fermi surface is broken up into the Fermi pockets and their coexistence with the Fermi arcs located around the nodal region. In particular, this electron Fermi surface instability drives the charge-order correlation, with the charge-order wave vector that matches well with the wave vector connecting the hot spots, as the charge-order correlation in the hole-doped counterparts. However, in a striking contrast to the hole-doped case, the charge-order wave vector in the electron-doped side increases in magnitude with the electron doping. The theory also shows the existence of a quantitative link between the single-electron fermiology and the collective response of the electron density.     

From semiconductors to superconductors: a simple model for pseudogaps

We consider a two-dimensional semiconductor with a local attraction among the carriers. We study the ground state of this system as a function of the semiconductor gap. We find a direct transition from a superconducting to an insulating phase for no doping at a critical value, the single particle excitations being always gapped. For finite doping we find a smooth crossover. We calculate the critical temperature due to both the particle excitations and the Berezinkii-Kosterlitz-Thouless transition. Received 8 December 1998     

Phase-sensitive evidence for d-wave pairing symmetry in electron-doped cuprate superconductors

We present phase-sensitive evidence that the electron-doped cuprates Nd(1.85)Ce(0.15)CuO(4-y) (NCCO) and Pr(1.85)Ce(0.15)CuO(4-y) (PCCO) have d-wave pairing symmetry. This evidence was obtained by observing the half-flux quantum effect, using a scanning SQUID microscope, in c-axis-oriented films of NCCO or PCCO epitaxially grown on tricrystal [100] SrTiO3 substrates designed to be frustrated for a d(x(2)-y(2)) order parameter. Samples with two other configurations, designed to be unfrustrated for a d-wave superconductor, do not show the half-flux quantum effect.     

Variational approach to strong correlation in the photoemission of electron-doped superconductors

We use a variational approach with strictly strong-correlated constraint to gain insight into low-energy states of t-t^′-t^″-J model in the electron-doped regime. Compared with the recent results on the electron-doped cuprates obtained by angle-resolved photoemission spectroscopy (ARPES), we show that based on the long-range ordered antiferromagnetic metallic state prohibiting vacant sites, our results lead to qualitatively similar trends in ARPES spectra and Fermi surface topology. Additionally, the results about the evolution of the energy gap and spectral weight as a function of doping will be discussed.     

Magnetic resonance in the spin excitation spectrum of electron-doped cuprate superconductors

We study the emergence of a magnetic resonance in the superconducting state of the electron-doped cuprate superconductors. We show that the recently observed resonance peak in the electron-doped superconductor Pr0.88LaCe0.12CuO4-delta is consistent with an overdamped spin exciton located near the particle-hole continuum. We present predictions for the magnetic-field dependence of the resonance mode as well as its temperature evolution in those parts of the phase diagram where dx2-y2-wave superconductivity may coexist with an antiferromagnetic spin-density wave.     

Incommensurate spin-density wave order in electron-doped BaFe2 As2 superconductors

Neutron diffraction studies of Ba(Fe(1-x)Co(x))(2)As)(2) reveal that commensurate antiferromagnetic order gives way to incommensurate magnetic order for Co compositions between 0.056 < x < 0.06. The incommensurability has the form of a small transverse splitting (0, ± ε, 0) from the commensurate antiferromagnetic propagation vector Q(AFM) = (1,0,1) (in orthorhombic notation) where ε ≈ 0.02-0.03 and is composition dependent. The results are consistent with the formation of a spin-density wave driven by Fermi surface nesting of electron and hole pockets and confirm the itinerant nature of magnetism in the iron arsenide superconductors.     

浅析电子型掺杂铜氧化物超导体的退火过程

铜氧化物高温超导体的发现, 打破了基于电声子相互作用BCS理论所预言的超导转变温度极限, 掀开了高温超导材料探索和高温超导机理研究的序幕. 根据掺杂类型的不同, 铜氧化物超导材料可以分为空穴型掺杂和电子型掺杂两类. 受限于样品, 对电子型掺杂铜氧化物的研究工作远少于空穴型掺杂体系. 本文简要回顾有关电子型掺杂铜氧化物超导体近期研究成果, 通过对比电子型掺杂和空穴型掺杂铜氧化物的相图来阐明电子型掺杂铜氧化物的研究对探索高温超导机理的必要性, 并特别针对电子型掺杂样品制备中的关键因素“退火过程”展开讨论. 结合课题组最新实验结果和相关实验报道我们发现电子型掺杂铜氧化物超导体在制备过程中除受到温度和氧分压的影响外, 退火效果还受到界面应力的强烈调制. 在综合考虑样品生长过程中温度、气氛及应力等多种因素的基础上, 探讨了“保护退火”方法导致电子型体系化学掺杂相图变化的起因.     

Spin fluctuation spectrum of electron-doped high-Tc superconductors

The magnetic excitation spectrum of electron-doped copper oxide superconductors is calculated using the Hubbard model on a square lattice. First, the on-site repulsion is treated with the random phase approximation. The spectrum of electron-doped systems in the superconducting state is compared with that of hole-doped systems, and the relationship between the frequency at which a peak grows in the spectrum and the superconducting energy gap at a hot spot (an intersection of the Fermi surface and the magnetic Brillouin zone boundary) is investigated. As compared with the hole-doped systems, the resonance condition is difficult to be satisfied in the electron-doped systems because of the small density of states around the hot spot. Moreover, the correlation effect in the Hubbard model is treated by the fluctuation-exchange approximation (FLEX), and the spin fluctuation spectra in the superconducting state in a wide region of the wave vector and frequency are calculated. We have found that the intensity of the magnetic spectrum at incommensurate wave vectors obtained with the FLEX is considerably weaker than that obtained with the RPA. The validity of the Fermi-liquid approach is also discussed.     

Origin of the anomalous low temperature upturn in the resistivity of the electron-doped cuprate superconductors

The temperature, doping, and field dependences of the magnetoresistance (MR) in Pr2-xCexCuO4-delta films are reported. We distinguish between orbital MR, found when the magnetic field is applied perpendicular to the ab planes, and the nearly isotropic spin MR. The latter, the major MR effect in the superconducting samples, appears in the region of the doping-temperature phase diagram where drho/dT<0, or an upturn in the resistivity appears. We conclude that the upturn originates from spin scattering processes.