D-wave superconductivity in doped Mott insulators

The effect of proximity to a Mott insulating phase on the charge transport properties of a superconductor is determined. An action describing the low energy physics is formulated and different scenarios for the approach to the Mott phase are distinguished by different variation with doping of the parameters in the action. A crucial issue is found to be the doping dependence of the quasiparticle charge which is defined here and which controls the temperature and field dependence of the electromagnetic response functions. Presently available data on high-T_c superconductors are analyzed. The data, while neither complete nor entirely consistent, suggest that neither the quasiparticle velocity nor the quasiparticle charge vanish as the Mott phase is approached, in contradiction to the predictions of several widely studied theories of lightly doped Mott insulators. Implications of the results for the structure of vortices in high-T_c superconductors are determined.     

Isotopieverschiebung der Bariumnuklide140Ba,138Ba,136Ba und134Ba in der Ba II-Resonanzlinie 6p 2 P 1/2-6s 2 S 1/2

In principle, the surface resistance of superconductors forRF can be calculated in the framework of the BCS-theory. However, even in the simplest case (low rf field strengths and Meissner phase), the theory yields complicated expressions, which need a computer for solving; the results are shown as graphs. To avoid the use of these complicated expressions, we discuss these numerical results in a model which allows an understanding of the unusual dependencies on the material parameters of superconductors. The theoretical results are compared with new experiments on Nb and on alloys showing, that the surface resistance of superconductors give additional information on the superconducting material in the penetration region.     

Temperature dependence of the hole concentration in the model of a hole metal with negative-<Emphasis Type="Italic">U</Emphasis> centers

The temperature dependence of the hole concentration is studied within the model of a hole metal with negative-U centers. Different versions of the model with a simplified representation of allowed bands and a quadratic valence-band dispersion law are considered. It is found that the temperature dependence of the hole concentration can acquire different forms, and the regions of the model parameters corresponding to these forms are determined. The results obtained are used in the discussion of the relation between normal to superconducting properties of high-temperature superconductors.     

层状铜氧化物超导体的有限温Landau理论

采用有限温的Landau理论来描述层状铜氧化物超导体中的竞争序和层间Josephson隧道效应 .通过讨论单层和双层铜氧面结构的超导体中序参量的相图，并与实验相比较来确定理论中 唯象参数的合理范围，同时也解释了在最佳掺杂区附近赝能隙的转变温度远高于与之对应的 能量尺度这个颇具疑惑的实验事实.再将理论应用到多层的超导体系，计算了超导转变温度 T_c随体系层数N的变化，在合理的参数空间里，最佳掺杂区附近的计算结果与现 有的实验相一致，而在极度欠掺杂区和极度过掺杂区的单调性上升可作为理论预言 关键词： 竞争序 层间Josephson隧道效应 电荷不平衡 赝能隙     

Electronic and electron-phonon phenomena in low-dimensional BEDT-TTF-based organic conductors and superconductors

Two related groups of k-phase ion-radical salts (BEDT-TTF) with different electrical properties, namely, superconductors with different transition temperatures and conductors, which transfer to insulating state with decreasing temperature, have been studied by micro-optic spectroscopy. Polarized reflectance spectra of microcrystals have been measured for the three principal crystallographic directions within the 700–40000 cm⁻¹ region, and the corresponding spectra of the optical functions obtained. The anisotropy of the electronic system in the crystals has been established as two-dimensional. The spectra obtained were quantitatively analyzed, the key parameters of the electronic structure and the vibronic coupling constants determined. It is concluded that the conductors have smaller vibronic coupling constants, more narrow allowed electronic bands, and stronger electron-electron interaction compared to those of the superconductors, and that vibronic coupling is the necessary condition for the onset of superconductivity in the superconductors studied. Fiz. Tverd. Tela (St. Petersburg) 41, 897–899 (May 1999)     

Superconductivity as a consequence of an ordering of the electron gas zero-point oscillations

It is shown that the temperature dependence of the value of energy gap in superconductors is characteristic for the order–disorder transition. The obtained relationship between the critical parameters of the Bose–Einstein condensate of electrically charged particles is in accordance with measurement data of superconductor critical parameters. The dependence of the critical temperatures for both – I and II – type superconductors and their Sommerfeld constants (and their Fermi energies) is obtained. It is marked that among the high-temperature superconducting ceramics there are the both – I and II – type superconductors. In total the phenomenon of superconductivity is seen as a consequence of ordering into the zero-point oscillation system of the electron gas in a metal at low temperatures. The obtained estimations of the interaction of zero-point oscillations are in satisfactory agreement with the data measurements of critical parameters of I and II type superconductors.     

Spin fluctuations and high temperature superconductivity

Theory of spin fluctuations for itinerant magnetism and its application to high temperature superconductivity are reviewed. After a brief introduction to the whole subject the developments of the self-consistent renormalization theory of spin fluctuations are summarized with particular emphasis on critical properties at the quantum phase transitions. Most of the anomalous properties in the normal state of high-T_c cuprates are understood as due to the critical behaviours for the two dimensional antiferromagnetic metals. By analysing the nuclear magnetic relaxation rate and the T-linear term of resistivity, the set of parameters to specify the spin fluctuations are determined. It is shown that by using the parameters thus obtained one can describe other quantities as well, e.g. optical conductivity. Then we proceed to the theory of superconductivity by the spin fluctuation mechanism. After some discussion on the weak coupling treatments, the strong coupling theory is reviewed. It is shown that the set of parameters determined by the normal state properties of the high-T _c cuprates just give a transition temperature of the right order of magnitude. Among the parameters, the most sensitive one for T _c is the frequency spread of the spin fluctuations. This fact enables us to present a possible unified picture of the antiferromagnetic spin fluctuation-induced superconductors, including heavy fermion superconductors and organic superconductors. This point of view may be confirmed to a certain extent by microscopic calculations based on the fluctuation exchange approximation for the two-dimensional Hubbard models representing not only the cuprates but also organic and trellis lattice compounds. The review is concluded with some discussions on future problems, e.g. the pseudo spin-gap in the under-doped region.     

高磁场大分离间隙的高温和低温超导磁体失超特性分析

本文使用失超分析软件Opera3D对低温和高温超导磁体进行失超分析.这个超导磁体具有两个相互垂直的室温孔,磁体的中心磁场为8～10T.为了合理保护超导磁体,通过变换加热器不同的加热功率密度和加热时间,改变保护电路参数等设置,分析各不同方案计算结果的区别,总结规律,为满足工程需求提出更优的失超保护方案.     

Mechanisms of modification of the energy spectrum in high-temperature superconductors of the bismuth,thallium, and mercury systems upon doping and increase in the number of copper-oxygen layers

This paper reports on the results of a comparative investigation of the specific features of the electron transport and superconducting properties in chain-free high-temperature superconductors of the bismuth, thallium, and mercury systems with different levels and types of doping, as well as with different numbers of copper-oxygen layers. It has been shown that the asymmetric narrow band model provides a means for adequately describing the available data on temperature dependences of the thermopower coefficient for all the studied systems, which has proved the possibility of applying this model as a universal method for describing and analyzing the specific features of the electron transport in high-temperature superconductors of different systems. The parameters of the energy spectrum and the system of charge carriers in the normal phase have been determined, and the general regularities in the transformation of the energy spectrum due to the doping and an increase in the number of copper-oxygen layers in different systems have been revealed. The conclusions have been drawn about the character and mechanisms of influence of the parameters of the energy spectrum in the normal state on the superconducting properties of chain-free high-temperature superconductors in the optimally doped and underdoped regimes. It has been demonstrated that, in the underdoped regime, regardless of the type of doping, the dependence of the critical temperature on the effective width of the conduction band has a close-to-universal character in all the studied systems for each of the phases with different numbers of copper-oxygen layers.     

Collective properties of superconductors with nontrivial pairing

Three-and two-dimensional models of p-and d-pairing are constructed for superconductors and superfluid quantum liquids using the functional integration formalism. In these models, the collective excitation spectra are calculated for superconductors with nontrivial pairing (such as high-temperature superconductors (HTSC) and heavy-fermion superconductors (HFSC)) for p-and d-pairing. Both three-and two-dimensional systems are considered. Some of recent ideas concerning the realization of the mixture of different states in HTSC are considered. In particular, the mixture of states $d_{x^2 - y^2 } + id_{xy} $ is analyzed. The obtained results of calculations of collective excitation spectra in superconductors with nontrivial pairing may be used for determining the type of pairing and the order parameter in HTSC and HFSC and also for interpreting the experimental results on ultrasound and microwave absorption in these system.     

铁基高温超导体电子结构的角分辨光电子能谱研究

铜氧化物超导体和铁基超导体是人类相继发现的两类高温超导家族,它们的高温超导机理是凝聚态物理领域中长期争论但悬而未决的重大问题.对铁基超导体广泛而深入的研究,以及与铜氧化物高温超导体的对比,对于发展新的量子固体理论、解决高温超导机理、探索新的超导体以及超导实际应用都具有重要意义.固体材料的宏观物性由其微观电子结构所决定,揭示高温超导材料的微观电子结构是理解高温超导电性的前提和基础.由于角分辨光电子能谱技术具有独特的同时对能量、动量甚至自旋的分辨能力,已成为探测材料微观电子结构的最直接、最有力的实验手段,在高温超导体的研究中发挥了重要作用.本文综述了在不同体系铁基超导体中费米面拓扑结构、超导能隙大小和对称性、轨道三维性和选择性、电子耦合模式等的揭示和发现,为甄别和提出铁基超导新理论、解决高温超导机理问题提供重要依据.     

Local density of states around a magnetic impurity in high- T(c) superconductors based on the t-J model

The local density of states (LDOS) around a magnetic impurity in high- T(c) superconductors is studied using the two-dimensional t-J model with a realistic band structure. The order parameters are determined in a self-consistent way within the Gutzwiller approximation and the Bogoliubov-de Gennes theory. In sharp contrast with the nonmagnetic impurity case, the LDOS near the magnetic impurity shows two resonance peaks reflecting the presence of spin-dependent resonance states. It is also shown that these resonance states are approximately localized around the impurity. The present results have an immediate connection with the scanning tunneling spectroscopy observation of Bi2Sr2Ca(Cu1-xNi[Zn](x))(2)O(8+delta).     

The ratio of normal state to superconducting state of spin lattice relaxation rate of unconventional superconductors

In this research, we derive a simple expression for the ratio of normal state superconducting state of spin lattice relaxation rate of unconventional superconductors from the BCS weak-coupling equation. The unconventional superconductors we consider have three types of order parameters as d-wave, ³He A-phase and p-wave three-dimensional order parameter that had been done before by Parker and Haas [D. Parker, S. Haas, Physical Review B 75 (2007) 052501]. After using some numerical approximations and some boundary conditions, we can find the ratio of normal state to superconducting state of spin lattice relaxation rate in power series of temperature dependent order parameters and temperature. Our numerical calculations show the coherence peak below critical temperature clearly that are consistent with Parker and Haas [D. Parker, S. Haas, Physical Review B 75 (2007) 052501]. These results do not agree with the believed that the coherence peak is the only property of s-wave superconductor. However from our calculation, we can conclude that the unconventional superconductors can show the coherence peaks.     

过冷液氮温度下块材的悬浮特性及其仿真研究

随着温度的降低, 超导块材表现出的超导特性也随之变化. 本文结合现有实际应用的车载超导块材冷却方法, 通过改变气压的方式营造过冷液氮温度条件, 探究块材组合与 Halbach 轨道之间的悬浮特性. 由于实验测量存在一定局限性, 只能完成部分工况条件下的研究. 为了更加系统全面的研究, 本文对过冷状态下超导块材的悬浮特性进行仿真计算, 通过与实验结果对比来确定仿真参数. 结果显示, 仿真计算与实验数据吻合度较高, 为后期的研究工作提供了仿真工具.     

Structural tuning of unconventional superconductivity in PuMGa5 (M=Co,Rh)

The superconducting properties of the recently discovered PuMGa5 (M=Co,Rh) superconductors, including the power law behavior of the specific heat, the evolution of the superconducting transition T(c) temperature with pressure, and the linear relation between T(c) and ratio of tetragonal lattice parameters c/a, are compared to those of the heavy fermion CeMIn5 (M=Co,Rh,Ir) unconventional superconductors. The striking similarity of the properties between the two families of superconductors suggests a common physics and a common (magnetically mediated) mechanism of superconductivity.     

Transport properties between unconventional superconductors and ferromagnets

The properties of spin-polarized tunneling between ferromagnets and d-wave superconductors are studied based on a scattering theory. Conductance spectra are given for arbitrary barrier height cases, and the effects of anisotropy in the pair potential are analyzed. It is shown that the polarization can be well estimated from the heights of zero-bias peak in the case of d-wave superconductors.     

Investigate the effect of anisotropic order parameter on the specific heat of anisotropic two-band superconductors

The effect of anisotropic order parameter on the specific heat of anisotropic two-band superconductors in BCS weak-coupling limit is investigated. An analytical specific heat jump and the numerical specific heat are shown by using anisotropic order parameters, and the electron–phonon interaction and non-electron–phonon interaction. The two models of anisotropic order parameters are used for numerical calculation that we find little effect on the numerical results. The specific heat jump of MgB₂, Lu₂Fe₃Si₅ and Nb₃Sn superconductors can fit well with both of them. By comparing the experimental data with overall range of temperature, the best fit is Nb₃Sn, MgB₂, and Lu₂Fe₃Si₅ superconductors.     

Evidence for nodal quasiparticles in the nonmagnetic superconductor YNi2B2C via field-angle-dependent heat capacity

Field-angle dependent heat capacity of the nonmagnetic borocarbide superconductor YNi2B2C reveals a clear fourfold oscillation, the first observation of its kind. The observed angular variations were analyzed as a function of magnetic field angle, field-intensity, and temperature to provide its origin. The quantitative agreement between experiment and theory strongly suggests that we are directly observing nodal quasiparticles generated along <100> by the Doppler effect. The results demonstrate that field-angle heat capacity can be a powerful tool in probing the momentum-space gap structure in unconventional superconductors such as high T(c) cuprates, heavy-fermion superconductors, etc.     

On the effect of boundary conditions on I–V characteristics of HTSC tunnel junctions

The pairing potential distribution over the thickness of superconducting CuO₂ layers in cuprate HTSCs is determined within the Ginzburg–Landau (GL) theory using the microscopic justification of this theory by Gor’kov. It is found that the pairing potential in them is significantly suppressed due to the effect of non-superconducting interlayers, which results in a decrease in the critical temperature of these superconductors. The temperature dependences of the effective energy gap and current–voltage (I–V) characteristic of tunnel junctions of the “break junction” type made of these superconductors are calculated.     

Band structure of SrFeAsF and CaFeAsF—the base phases of a new group of oxygen-free FeAs superconductors

The results of ab initio FLAPW-GGA calculations of the band structure of the recently synthesized four-component fluorine arsenides SrFeAsF and CaFeAsF, which are the base phases of a new group of oxygen-free FeAs superconductors, are presented. The energy bands, electron state density distributions, effective atomic charges, Fermi surface topology, low-temperature electronic specific heat, and molar Pauli paramagnetic susceptibility have been determined for SrFeAsF and CaFeAsF and are compared to similar data for oxyarsenide LaFeAsO, which is the base phase of the family of the recently discovered high-temperature (T _c ～ 26–56 K) FeAs oxygen-containing superconductors.