铁基高温超导体研究进展

最近,由于在铁基Ln(O,F)FeAs化合物及其相关化合物中发现具有高于40K的超导电性,层状的铁基化合物引起了凝聚态物理学界很大的兴趣和关注.在随后的研究中发现,在该类材料中最高超导临界温度可达到55K.这些重要的发现使得人们又重新对高温超导体的探索产生了极大的兴趣,并且为研究高温超导的机理提供了新的一类材料.文章主要介绍了作者所在组在新型铁基超导体方面的最新研究进展,包括：（1）铁基超导材料探索研究;（2）铁基超导体的单晶制备及物性研究;（3）铁基超导体的电子相图及自旋密度波（SDW）和超导共存研究;(4）同位素交换对超导转变和SDW转变的效应.最后,在已完成的工作基础上提出了一些今后的研究方向和发展前景.     

铁基高温超导体研究进展

(4)同位素交换对超导转变和SDW转变的效应.最后,在已完成的工作基础上提出了一些今后的研究方向和发展前景.     

Model of vortex states in hole-doped iron-pnictide superconductors

Based on a phenomenological model with competing spin-density-wave (SDW) and extended s-wave superconductivity, the vortex states in Ba(1-x)K(x)Fe2As2 are investigated by solving Bogoliubov-de Gennes equations. Our result for the optimally doped compound without induced SDW is in qualitative agreement with recent scanning tunneling microscopy experiment. We also propose that the main effect of the SDW on the vortex states is to reduce the intensity of the in-gap peak in the local density of states and transfer the spectral weight to form additional peaks outside the gap.     

Low-field anomaly of vortex dynamics in iron-pnictide superconductors

Magnetic relaxations in Ba(Fe_1?xCo_x)₂As₂ before and after swift particle irradiations are measured. The normalized relaxation rate S shows a clear suppression in a pristine sample at low fields below the self-field. Heavy-ion irradiations suppress these anomalies considerably, although there still remain weak features depending on ion species and/or energies. In a proton irradiated sample, the low-field anomaly of S survives although it is smeared compared with that in a pristine sample. All these results indicate that the low-field anomaly of S is suppressed by defects, and correlated defects have stronger effects. Possible origins of the low-field anomaly of S are discussed.     

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.     

Quasiparticle scattering induced by charge doping of iron-pnictide superconductors probed by collective vortex pinning

Charge doping of iron-pnictide superconductors leads to collective pinning of flux vortices, whereas isovalent doping does not. Moreover, flux pinning in the charge-doped compounds is consistently described by the mean-free path fluctuations introduced by the dopant atoms, allowing for the extraction of the elastic quasiparticle scattering rate. The absence of scattering by dopant atoms in isovalently doped BaFe2(As(1-x)P(x))(2) is consistent with the observation of a linear temperature dependence of the low-temperature penetration depth in this material.     

Universal scaling relations in molecular superconductors

Scaling relations between the superconducting transition temperature T(c), the superfluid stiffness rho(s), and the normal state conductivity sigma(0)(T(c)) are identified within the class of molecular superconductors. These new scaling properties hold as T(c) varies over 2 orders of magnitude for materials with differing dimensionality and contrasting molecular structure and are dramatically different from the equivalent scaling properties observed for cuprate superconductors. These scaling relations place strong constraints on theories for molecular superconductivity.     

Magnetic and quasiparticle excitation spectra of an itinerant J1-J2 model for iron-pnictide superconductors

We calculate the magnetic and quasiparticle excitation spectra of an itinerant J(1)-J(2) model for iron-pnictide superconductors. In addition to an acoustic spin-wave branch, the magnetic spectrum has a second, optical branch, resulting from the coupled four-sublattice magnetic structure. The spin-wave velocity has also a planar directional anisotropy, due to the collinear or striped antiferromagnetism. Within the magnetically ordered phase, the quasiparticle spectrum is composed of two Dirac cones, resulting from the folding of the magnetic Brillouin zone. We discuss the relevance of our findings to the understanding of both neutron scattering and photoemission spectroscopy results for SrFe(2)As(2).     

Variational Monte Carlo study of the nematic state in iron-pnictide superconductors with a five-orbital model

We perform a variational Monte Carlo study of the nematic state in iron-pnictide superconductors within a realistic five-orbital model.Our numerical results show that the nematic state,formed by introducing an anisotropic hopping order into the projected wave function,is not stable unless the off-site Coulomb interaction V exceeds a critical value.This demonstrates that V plays a key role in forming the nematic state in iron-pnictide superconductors.In the nematic state,the orbital order and the anisotropic spin correlations are consistent with the experimental observations.We argue that the experimentally observed anisotropic magnetic couplings and structural transition are associated with the nematic state and can be understood in a unified framework.     

Critical current scaling and anisotropy in oxypnictide superconductors

Having succeeded in the fabrication of epitaxial superconducting LaFeAsO(1-x)F(x) thin films we performed an extensive study of electrical transport properties. In the face of multiband superconductivity we can demonstrate that an anisotropic Ginzburg-Landau scaling of the angular dependent critical current densities can be adopted, although being originally developed for single band superconductors. In contrast with single band superconductors the mass anisotropy of LaFeAsO(1-x)F(x) is temperature dependent. A very steep increase of the upper critical field and the irreversibility field can be observed at temperatures below 6 K, indicating that the band with the smaller gap is in the dirty limit. This temperature dependence can be theoretically described by two dominating bands responsible for superconductivity. A pinning force scaling provides insight into the prevalent pinning mechanism and can be specified in terms of the Kramer model.     

c轴织构1111型铁基超导体临界传输电流密的计算

采用最大网络流量模型和Ford-Fulkerson算法思想编写的Matlab程序在计算c轴织构122型铁基超导体临界传输电流密度时表现出色。变动部分参数后,该模型和计算方法也同样适用于其它铁基超导体。这里计算了c轴织构的1111型铁基超导体在不同温度和磁场下的临界电流密度Jc,并研究了材料的相对密度对Jc的影响。     

Universal scaling properties of extreme type-II superconductors in magnetic fields

Anisotropic Ginzburg-Landau superconductors of extreme type-II are considered in an approximation where magnetic field fluctuations are neglected. A formulation of the scaling properties is presented for the singular part of the free energy density in the presence of a magnetic field. From the existence of a magnetization, a diamagnetic susceptibility and superconductivity we determine the limiting behavior of the scaling function in the vicinity of the zero field transition temperature, where critical fluctuations dominate. Our predictions for the temperature and field dependence of magnetization, magnetic torque and melting line etc., uncover the universal critical properties and provide an extension of hitherto used mean-field treatments. The results are consistent with experimental data. Received: 24 April 1998 / Accepted: 5 May 1998     

Temperature-dependent scaling of pinning force data in Bi-based high-Tc superconductors

The scaling of the normalized volume pinning forces, Fp/Fp,max, versus a reduced field h=Ha/Hscale has proven to be a very informative tool to study the origin of the flux pinning in superconductors. Remarkably, on Bi2Sr2Ca2Cu3O10+ (Bi-2223) and Bi2Sr2CaCu2O8+ (Bi-2212) data were mostly analyzed only in a narrow temperature range around 77 K. Here, we present a study of the pinning forces in various Bi-2223 samples at temperatures between 18 K and 80 K. The measurements clearly reveal that there is an apparent non-scaling of the pinning force data; instead, two clearly different temperature regimes for the scaling can be recognized, which are found to be in direct relation to a second step observed in the m(T) curves obtained upon field-cooling and -warming.     

Emergent nesting of the Fermi surface from local-moment description of iron-pnictide high-T<Subscript>c</Subscript> superconductors

We uncover the low-energy spectrum of a t-J model for electrons on a square lattice of spin-1 iron atoms with 3d _xz and 3d _yz orbital character by applying Schwinger-boson-slave-fermion mean-field theory and by exact diagonalization of one hole roaming over a 4 × 4 × 2 lattice. Hopping matrix elements are set to produce hole bands centered at zero two-dimensional (2D) momentum in the free-electron limit. Holes can propagate coherently in the t-J model below a threshold Hund coupling when long-range antiferromagnetic order across the d + = 3d _{(x + iy)z} and d ? = 3d _{(x ? iy)z} orbitals is established by magnetic frustration that is off-diagonal in the orbital indices. This leads to two hole-pocket Fermi surfaces centered at zero 2D momentum. Proximity to a commensurate spin-density wave (cSDW) that exists above the threshold Hund coupling results in emergent Fermi surface pockets about cSDW momenta at a quantum critical point (QCP). This motivates the introduction of a new Gutzwiller wavefunction for a cSDW metal state. Study of the spin-fluctuation spectrum at cSDW momenta indicates that the dispersion of the nested band of one-particle states that emerges is electron-type. Increasing Hund coupling past the QCP can push the hole-pocket Fermi surfaces centered at zero 2D momentum below the Fermi energy level, in agreement with recent determinations of the electronic structure of mono-layer iron-selenide superconductors.