Frontiers of Physics

ISSN 2095-0462

ISSN 2095-0470(Online)

CN 11-5994/O4

2019 Impact Factor: 2.502

封面图片   2011年, 第6卷 第4期
Fermi surface of (Tl, Rb)xFe2-ySe2 superconductor as revealed by high resolution angle-resolved photoemission spectroscopy. Two electron-like Fermi surface sheets are observed around the central point while one electron-like Fermi surface sheet is observed around the zone corner M point which is actually [展开] ...
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2011年, 第6卷 第4期 出版日期:2011-12-05

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EDITORIAL
Energizer keep going: 100 years of superconductivity
Pengcheng Dai, Xing-jiang Zhou, Dao-xin Yao
Frontiers of Physics. 2011, 6 (4): 343-343.  
https://doi.org/10.1007/s11467-011-0227-7

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NEWS AND VIEWS
PERSPECTIVE
Gifts from the superconducting curiosity shop
David Mandrus
Frontiers of Physics. 2011, 6 (4): 347-349.  
https://doi.org/10.1007/s11467-011-0226-8

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A reflection on the contrast between the Cooper pairing in iron-based and conventional superconductors
Fa Wang, Dung-Hai Lee
Frontiers of Physics. 2011, 6 (4): 350-356.  
https://doi.org/10.1007/s11467-011-0212-1

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In this Perspective article we review retrospectively the streamline of our work on iron-based superconductors, and reflect on the mechanism of Cooper pairing in conventional and unconventional, such as iron-based superconductors. The main theme of this review is the concept of effective interaction and renormalization group.

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RESEARCH ARTICLE
Electrodynamics of Abrikosov vortices: the field theoretical formulation
Aron J. Beekman, Jan Zaanen
Frontiers of Physics. 2011, 6 (4): 357-369.  
https://doi.org/10.1007/s11467-011-0205-0

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Electrodynamic phenomena related to vortices in superconductors have been studied since their prediction by Abrikosov, and seem to hold no fundamental mysteries. However, most of the effects are treated separately, with no guiding principles.We demonstrate that the relativistic vortex worldsheet in spacetime is the object that naturally conveys all electric and magnetic information, for which we obtain simple and concise equations. Breaking Lorentz invariance leads to down-to-earth Abrikosov vortices, and special limits of these equations include for instance dynamic Meissner screening and the AC Josephson relation. On a deeper level, we explore the electrodynamics of two-form sources in the absence of electric monopoles, in which the electromagnetic field strength itself acquires the characteristics of a gauge field. This novel framework leaves room for unexpected surprises.

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REVIEW ARTICLE
Mott physics, sign structure, ground state wavefunction, and high-Tc superconductivity
Zheng-Yu Weng
Frontiers of Physics. 2011, 6 (4): 370-378.  
https://doi.org/10.1007/s11467-011-0220-1

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In this article I give a pedagogical illustration of why the essential problem of high-Tc superconductivity in the cuprates is about how an antiferromagnetically ordered state can be turned into a short-range state by doping. I will start with half-filling where the antiferromagnetic ground state is accurately described by the Liang–Doucot–Anderson (LDA) wavefunction. Here the effect of the Fermi statistics becomes completely irrelevant due to the no double occupancy constraint. Upon doping, the statistical signs reemerge, albeit much reduced as compared to the original Fermi statistical signs. By precisely incorporating this altered statistical sign structure at finite doping, the LDA ground state can be recast into a short-range antiferromagnetic state. Superconducting phase coherence arises after the spin correlations become short-ranged, and the superconducting phase transition is controlled by spin excitations. I will stress that the pseudogap phenomenon naturally emerges as a crossover between the antiferromagnetic and superconducting phases. As a characteristic of non Fermi liquid, the mutual statistical interaction between the spin and charge degrees of freedom will reach a maximum in a high-temperature “strange metal phase” of the doped Mott insulator.

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Properties of the multiorbital Hubbard models for the iron-based superconductors
Elbio Dagotto, Adriana Moreo, Andrew Nicholson, Qinglong Luo, Shuhua Liang, Xiaotian Zhang
Frontiers of Physics. 2011, 6 (4): 379-397.  
https://doi.org/10.1007/s11467-011-0222-z

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A brief review of the main properties of multiorbital Hubbard models for the Fe-based superconductors is presented. The emphasis is on the results obtained by our group at the University of Tennessee and Oak Ridge National Laboratory, Tennessee, USA, but results by several other groups are also discussed. The models studied here have two, three, and five orbitals, and they are analyzed using a variety of computational and mean-field approximations. A “physical region” where the properties of the models are in qualitative agreement with neutron scattering, photoemission, and transport results is revealed. A variety of interesting open questions are briefly discussed such as: what are the dominant pairing tendencies in Hubbard models? Can pairing occur in an interorbital channel? Are nesting effects of fundamental relevance in the pnictides or approaches based on local moments are more important? What kind of magnetic states are found in the presence of iron vacancies? Can charge stripes exist in iron-based superconductors? Why is transport in the pnictides anisotropic? The discussion of results includes the description of these and other open problems in this fascinating area of research.

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MINI-REVIEW ARTICLE
Vortex lattices in type-II superconductors studied by small-angle neutron scattering
Morten Ring Eskildsen
Frontiers of Physics. 2011, 6 (4): 398-409.  
https://doi.org/10.1007/s11467-011-0214-z

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Here we review recent small-angle scattering studies of the vortex lattice in a range of type-II superconductors carried out by our group. Emphasis is placed on providing examples of the kind of information which can be obtained by such measurements, focusing in particular on studies of the vortex lattice structure and form factor in LuNi2B2C, TmNi2B2C, CeCoIn5 and Ba(Fe0.93Co0.07)2As2.

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REVIEW ARTICLE
Structural, magnetic and electronic properties of the iron–chalcogenide AxFe2-ySe2 (A=K, Cs, Rb, and Tl, etc.) superconductors
Dai-xiang Mou, Lin Zhao, Xing-jiang Zhou
Frontiers of Physics. 2011, 6 (4): 410-428.  
https://doi.org/10.1007/s11467-011-0229-5

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The latest discovery of a new iron–chalcogenide superconductor AxFe2-ySe2 (A = K, Cs, Rb, and Tl, etc.) has attracted much attention due to a number of its unique characteristics, such as the possible insulating state of the parent compound, the existence of Fe-vacancy and its ordering, a new form of magnetic structure and its interplay with superconductivity, and the peculiar electronic structures that are distinct from other Fe-based superconductors. In this paper, we present a brief review on the structural, magnetic and electronic properties of this new superconductor, with an emphasis on the electronic structure and superconducting gap. Issues and future perspectives are discussed at the end of the paper.

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Superconductivity and spin fluctuations
Shiliang Li, Pengcheng Dai
Frontiers of Physics. 2011, 6 (4): 429-439.  
https://doi.org/10.1107/s11467-011-0221-0

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In the conventional superconductors, the Cooper pairs are mediated by phonons, which is a process where only the correlations between the phonons and the charge properties of the electrons are needed. However, superconductivity can also be derived from other types of elementary excitations. The spin fluctuations are arguably the most promising candidate that can mediate such unconventional superconductivity. In some of the important systems such as cuprates, Fe-based superconductors and heavy-fermion superconductors, spin fluctuations play a key role in the mechanism of their superconductivity although there are still many debates. In this paper, we will give a brief review on the correlation between the spin fluctuations and superconductivity.

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Superconductivity in the cuprates: Deduction of mechanism for d-wave pairing through analysis of ARPES
Han-Yong Choi, Chandra M. Varma, Xing-jiang Zhou
Frontiers of Physics. 2011, 6 (4): 440-449.  
https://doi.org/10.1007/s11467-011-0231-y

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In the Eliashberg integral equations for d-wave superconductivity, two different functions (α2F)n(ω, θ) and (α2F)p,d(ω) determine, respectively, the “normal” self-energy and the “pairing” self-energy. ω is the frequency of fluctuations scattering the fermions whose momentum is near the Fermi-surface and makes an angle θ to a chosen axis. We present a quantitative analysis of the high-resolution laser based Angle Resolved Photoemission Spectroscopy (ARPES) data on a slightly under doped cuprate compound Bi2212 and use the Eliashberg equations to deduce the ω and θ dependence of (α2F)n(ω, θ) for T just above Tc and below Tc. Besides its detailed ω dependence, we find the remarkable result that this function is nearly independent of θ between the (π; π)-direction and 25 degrees from it, except for the dependence of the cut-off energy on θ. Assuming that the same fluctuations determine both the normal and the pairing self-energy, we ask what theories give the function (α2F)p,d(ω) required for the d-wave pairing instability at high temperatures as well as the deduced (α2F)n(θ, ω). We show that the deduced (α2F)n(θ, ω) can only be obtained from antiferromagnetic (AFM) fluctuations if their correlation length is smaller than a lattice constant. Using (α2F)p,d(ω) consistent with such a correlation length and the symmetry of matrix-elements scattering fermions by AFM fluctuations, we calculate Tc and show that AFM fluctuations are excluded as the pairing mechanism for d-wave superconductivity in cuprates. We also consider the quantumcritical fluctuations derived microscopically as the fluctuations of the observed loop–current order discovered in the under-doped cuprates, and which lead to the marginal Fermi–liquid properties in the normal state. We show that their frequency dependence and the momentum dependence of their matrix-elements to scatter fermions are consistent with the θ and ω dependence of the deduced (α2F)n(ω, θ). The pairing kernel (α2F)p,d(ω) calculated using the experimental values in the Eliashberg equation gives d-wave instability at Tc comparable to the experiments.

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Nuclear magnetic resonance studies of vortices in high temperature superconductors
A. M. Mounce, S. Oh, W. P. Halperin
Frontiers of Physics. 2011, 6 (4): 450-462.  
https://doi.org/10.1007/s11467-011-0237-5

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The distinct distribution of local magnetic fields due to superconducting vortices can be detected with nuclear magnetic resonance (NMR) and used to investigate vortices and related physical properties of extreme type II superconductivity. This review summarizes work on high temperature superconductors (HTS) including cuprates and pnictide materials. Recent experimental results are presented which reveal the nature of vortex matter and novel electronic states. For example, the NMR spectrum has been found to provide a sharp indication of the vortex melting transition. In the vortex solid a frequency dependent spin–lattice relaxation has been reported in cuprates, including YBa2Cu3O7-x, Bi2SrCa2Cu2O8+δ, and Tl2Ba2CuO6+δ. These results have initiated a new spectroscopy via Doppler shifted nodal quasiparticles for the investigation of vortices. At very high magnetic fields this approach is a promising method for the study of vortex core excitations. These measurements have been used to quantify an induced spin density wave near the vortex cores in Bi2SrCa2Cu2O8+δ. Although the cuprates have a different superconducting order parameter than the iron arsenide superconductors there are, nonetheless, some striking similarities between them regarding vortex dynamics and frequency dependent relaxation.

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Universal behavior of the upper critical field in iron-based superconductors
Jing-lei Zhang, Lin Jiao, Ye Chen, Hui-qiu Yuan
Frontiers of Physics. 2011, 6 (4): 463-473.  
https://doi.org/10.1007/s11467-011-0235-7

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The newly discovered iron-based high temperature superconductors have demonstrated rich physical properties. Here we give a brief review on the recent studies of the upper critical field and its anisotropy in a few typical series of the iron-based superconductors (FeSCs). In spite of their characters of a layered crystal structure, all the FeSCs possess an extremely large upper critical field and a weak anisotropy of superconductivity, being unique among the layered superconductors. These particular properties indicate potential applications of the FeSCs in the future. Based on the experimental facts of the FeSCs, we will discuss the possible mechanisms of pair breaking in high magnetic fields and its restrictions on the theoretical analysis of the superconducting pairing mechanisms.

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