Charge dynamics in electron-underdoped high-Tc cuprates

We investigate charge dynamics in the antiferromagnetic (AF) phase of electron-doped cuprates by using numerically exact diagonalization technique for an electron-doped t-t′-J model. When AF correlation develops with decreasing temperature, a gap-like behavior emerges in the optical conductivity accompanied by the enhancement of the coherent motion of carriers due to the same sublattice hoppings. This is a remarkable contrast to the behavior of a hole-doped t-t′-J model.     

Quantum interference phenomena between impurity states in d-wave superconductors

We investigate the mutual influence of impurities in two-dimensional d-wave superconductors involving self-consistent solutions of the Bogoliubov-de Gennes equations. The local order parameter suppression, the local density of states as well as the interference of impurity-induced structures are analyzed. We employ an impurity position averaging scheme for the density of states that does not neglect these interference effects, as the commonly used T-matrix approaches do.     

A new numerical method for quasi-particle spectroscopy around a pair of vortices in triplet superconductors

Triplet superconductors such as Sr₂RuO₄ and Na_xCoO₂·yH₂O are now found to be p-wave (p_x±ip_y) or f-wave ((p_x±ip_y)coscp_z) superconductors. In conventional singlet superconductors, vortices are quantized because phase of order parameter must rotate by 2π around a vortex. But triplet superconductors have a degree of freedom of spin, which is described by d-vector. The d-vector and phase can rotate by π around a vortex, separately. Therefore appearance of HQVs is predicted. Theoretically, it is found that a pair of HQVs is more stable than a singly quantized vortex, for several parameter regions.In this study, in order to investigate quasi-particle bound states around two vortices in s-wave superconductors, we have developed a new numerical method to solve the BdG equation for two vortices state, using Mathieu functions. We confirmed the validity of this method for two vortices state and applied it in case of a pair of vortices. And we solved it.     

Competing phases in the cuprates: Charge vs spin order

We have investigated magnetic and charge instabilities of the cuprates within the Gutzwiller approximation RPA (GA+RPA). Interestingly, in GA the dressed Hubbard U is not a single parameter, but has different forms in the spin and charge responses, with distinct doping dependencies. While there are a number of competing magnetic instabilities for hole-doped cuprates, we fail to find any purely electronic charge density waves. The dominant magnetic instabilities are associated with ‘double nesting’, and the phase diagrams are material dependent, with LSCO differing from other cuprates.     

Theory of static and dynamic antiferromagnetic vortices in LSCO superconductors

A key prediction of the SO(5) theory is the antiferromagnetic vortex state. Recent neutron scattering experiment on LSCO superconductors revealed enhanced antiferromagnetic order in the vortex state. Here we review theoretical progress since the original proposal and present a theory of static and dynamic antiferromanetic vortices in LSCO superconductors. It is shown that the antiferromagnetic region induced by the vortices can be greater than the coherence length, due to the light effective mass of the dynamic antiferromagnetic fluctuations at optimal doping, and close proximity to the antiferromagentic state in the underdoped regime. Systematic experiments are proposed to unambiguously determine that the field induced magnetic scattering originates from the vortices and not from the bulk.     

Anisotropic spin fluctuations in CeCoIn5

CeCoIn₅ is an unconventional heavy fermion superconductor with a relatively large transition temperature . NMR measurements of the spin lattice relaxation rate at the In(1) site reveal a significant anisotropy of the fluctuating hyperfine fields, which reverses below 30 K. These results suggest that two-dimensional fluctuations of the Ce 4f moments are relevant for the superconducting pairing.     

Site selection NMR study on electronic state in and near vortex core of YBa2Cu4O8

We report a site-selective ¹⁷O spin-lattice relaxation rate T₁⁻¹ in the vortex state of YBa₂Cu₄O₈. We found that T₁⁻¹ at the planar sites exhibits an unusual nonmonotonic NMR frequency dependence. Based on T₁⁻¹ in the vortex core region, we establish strong evidence that the local density of states within the vortex core is strongly reduced.     

Relaxation processes in mesoscopic superconductors

We investigate the possibility of a novel kind of optical pump probe spectroscopy where the two laser pulses are focused on different areas of the sample. The response to the destruction of the superconducting state in a large part of a mesoscopic ring is studied numerically. We use the time dependent Ginzburg-Landau equations with periodic boundary conditions and external magnetic field. We evaluate the relaxation rates of the superconducting order parameter as well as the voltage induced by the charge imbalance. Computer simulations confirm that the perturbation of superconductivity on one part of the ring induces a voltage which decelerates the superconducting electrons on the other part of the ring. This deceleration results in the decrease of the superconducting current and the superfluid density. The relaxation times are of the order of the picosecond, the induced voltage of few millivolts and the variation of the superconducting gap of 10% which we believe to be suitable for time resolved femtosecond optical spectroscopy.     

Doping effect on the carrier scattering in iron-pnictide superconductors studied by charge transport

In order to reveal the role of “carrier doping” in the iron-based superconductors, we investigated the transport properties of the oxygen-deficient iron-arsenides LnFeAsO_1−y (Ln=La, Ce, Pr and Nd) over a wide doping range. We found that the effect of “doping” in this system is mainly on the carrier scattering rather than carrier density, quite distinct from that in high-T_c cuprates. In the case of La system with lower T_c, the low temperature resistivity is dominated by T² term and fairly large magnetoresistance is observed. On the other hand, in the Nd system with higher T_c, carriers are subject to stronger scattering showing nearly T-linear resistivity and small magnetoresistance. Such strong scattering appears intimately correlated with high-T_c superconductivity in the iron-based system.     

Low field AC susceptibility study of intergranular critical current density in Mg-substituted CuBa2Ca3Cu4O12−y high temperature superconductors

Measurements of the a.c.susceptibility (χ=χ′+iχ″) have been made on the Mg substituted high T_C superconducting system, CuBa₂(Mg_xCa_1−x)₃Cu₄O_12−y (Cu-1234) with x=0, 0.10 & 0.20, at different values of the a.c.field amplitude. Estimates of the intergranular critical current density(J_C) made from the field dependent χ″-T curves show an improvement in the Mg-substituted Cu-1234 system. Results have been analysed in the light of the crystal structure and the superconducting anisotropy factor (γ=ξ_ab/ξ_c) of the Cu-1234 system. Lower superconducting anisotropy emanating from Mg substitution has been found to be significant, resulting in better superconducting properties.     

First-principles study of electronic structure and ferromagnetism in Ti-doped ZnO

We perform first-principles spin polarized calculations of the electronic structure of Ti-doped in ZnO. Ferromagnetism in Ti-doped ZnO is identified, which is in agreement with recent experimental and calculated results. A net magnetic moment of 0.715μ_B is found per Ti. At a Ti concentration of 12.5%, total energy calculations show that the ferromagnetic state is 68 meV lower than the antiferromagnetic state. The electronic states near Fermi energy are dominated by strong hybridization between O 2p and Ti 3d, which is just the origin of impurity band in Ti-doped ZnO and also implies that the Ti-O bond is quite covalent instead of purely ionic. Since there is no magnetic element in this compound, Ti-doped ZnO appears to be an unambiguous dilute magnetic semiconductor.     

Effect of polymers doping on flux pinning behavior of MgB2

The effects of polymers doping on irreversibility field (H_irr) and critical current density (J_c) of MgB₂ have been investigated in this work. It is found that both J_c, and H_irr, are improved by doping at relative lower temperature region. The J_c−B curves of all samples studied in this work are well fitted using J_c(B) formula in percolation model. The values of upper critical field anisotropy (γ) are obtained from the fitting result at various temperatures. It is observed that values of γ for polymers doping samples are reduced at these temperatures. This is considered to be responsible for the enhancement of values of J_c for doped samples. Moreover, the percolation threshold, p_c, is found to be enhanced with increasing temperature. It is believed that the grain boundary pinning is still dominating in MgB₂, while the deviation of experimental data from the theoretical values is due to the percolation of suppercurrent in polycrystalline MgB₂.     

Response to comment on “Controlled injection of spin triplet supercurrents into a strong ferromagnet”

The comment by Lin He proposes that at the interface between holmium and cobalt an interesting magnetic proximity effect occurs in which the spiral magnetic wavelength (λ) of holmium is enhanced to 5.4 nm. Consequently, the optimum holmium layer thickness for the superconductor spin triplet proximity effect to occur in Nb/Ho/Co/Ho/Nb Josephson junctions of ≈4.5 and ≈10 nm would then correspond to the presence of ≈λ/2 and ≈3λ/2 spiral wavelengths. Although intriguing, this idea seems to be at odds with the conventional micromagnetics of either thin film (polycrystalline) holmium or single-crystal Ho at low temperatures (10 K). Although an increase in λ in Ho/metal multilayer thin films with epitaxially matched interfaces has been reported, the possibility of such an effect in a polycrystalline system seems unlikely.     

Fermi arc formation by chiral spin textures in high-temperature superconductors

In angle-resolved photoemission spectroscopy pseudogap phenomenon in high-temperature superconductors is observed as Fermi arcs, or truncated Fermi surface. Here I argue that the hole induced chiral spin texture scenario naturally leads to Fermi arcs by including hole hopping processes. Disappearance of part of the Fermi surface is associated with the effect of the coherence factor. Suppressed spectral weight of the holes turns out to be an electron-like component which has weight near (π,0) only and has some charge instability.     

Resonant inelastic X-ray scattering and its magnetic circular dichroism

Magnetic circular dichroism in resonant inelastic X-ray scattering (MCD-RIXS) of ferromagnetic systems is discussed for the longitudinal geometry (LG), where the directions of incident X-ray and magnetization are parallel, and the transverse geometry (TG), where they are perpendicular. MCD-RIXS in LG is represented by two successive real processes, X-ray absorption and X-ray emission, while MCD-RIXS in TG is given by the interference process. We describe an application of MCD-RIXS in LG to the detection of Ce 4f² contribution in Ce, L₃ absorption edge of CeFe₂, and theoretical and experimental studies of MCD-RIXS in TG for Gd and Sm systems.     

铁基超导体中的反铁磁序和自旋动力学

类似于其他非常规超导材料,铁基高温超导电性通常出现在静态长程反铁磁序被抑制之后,并且强烈的自旋涨落始终与超导电性相伴相生,因此理解磁性相互作用是建立铁基超导微观机理的重要前提.中子散射作为研究凝聚态物质中磁性相互作用的有力工具,在揭示铁基超导电性的磁性起源方面起到了关键作用.本文系统总结了近十年来铁基超导材料的中子散射研究结果,包括铁基超导材料中的静态磁结构、磁性相变、动态磁激发、电子向列相等,并探讨它们与超导电性之间的关系.     

Excitons in optical spectra of boron-nitride (BN) nanotubes

Exciton effects are studied in single-wall boron-nitride nanotubes. The Coulomb interaction dependence of the band gap, the optical gap, and the binding energy of excitons are discussed. The optical gap of the (5,0) nanotube is about 6 eV at the on-site interaction U=2t with the hopping integral t=1.1 eV. The binding energy of the exciton is 0.50 eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the binding energy are almost independent of the geometries of nanotubes. This novel property is in contrast with that of the carbon nanotubes, which show metallic and semiconducting properties depending on the chiralities.     

Linear response function around a localized impurity in a superconductor

Imaging the effects of an impurity like Zn in high-T_c superconductors [Nature 61 (2000) 746] has rekindled interest in defect problems in the superconducting phase. This has prompted us here to re-examine the early work of March and Murray [Phys. Rev. 120 (1960) 830] on the linear response function in an initially translationally invariant Fermi gas. In particular, we present corresponding results for a superconductor at zero temperature, both in the s- and in the d-wave case, and mention their direct physical relevance in the case when the impurity potential is highly localized.     

Three-dimensional simulation of quasi-particle structures in nano-scaled superconductors

We have developed numerical simulation method for quasi-particle structures in the three-dimensional nano-sized superconductors, using the three-dimensional finite element method and the Bogoliubov-de Gennes equation. Using this method, we analyzed the superconducting state in the nano-sized cubic superconductors. We found the spatial oscillations of order parameter because of the confinement of superconducting electrons, and also we found the quasi-particle bound states at the corners of the cubic superconductors because of suppression of superconductivity at the corners.