EuMo_6S_8的超导性、电荷密度波和磁性间的相互影响

我们已观察到在120K时Eu_(1.2)Mo_6S_8的热容量、电阻及霍尔系数的反常规象,并确定为一种马氏体结构的相变特征;这种马氏体结构相变产生于一种模糊的电荷密度波的转变,后者在部分费米面上造成能隙。 费米面出现部分能隙说明了在常压下直10mK不出现超导的原因。高压抑制了电荷密度波的转变。从费米面上状态的超导性和这种转变相互竞争出发,计算得到的超导转变温度对压力的依赖性,与实验结果非常一致。在低温时,临界场随温度的变化非常强烈,这种变化已用Jaccarino-Peter补偿效应进行了解释。在高压时与理论的预言比较,EuMo_6S_8的临界场非常高,至少与现有任何的超导体一样高。压力为14Kbar、温度低于2K时在10T到15T范围内已观察到低场下电阻态到超导态的转变,这就是称之为场诱导超导性的一种新效应。     

Crossover from a pseudogap state to a superconducting state

This paper deduces that the particular electronic structure of cuprate superconductors confines Cooper pairs to be first formed in the antinodal region which is far from the Fermi surface,and these pairs are incoherent and result in the pseudogap state.With the change of doping or temperature,some pairs are formed in the nodal region which locates the Fermi surface,and these pairs are coherent and lead to superconductivity.Thus the coexistence of the pseudogap and the superconducting gap is explained when the two kinds of gaps are not all on the Fermi surface.It also shows that the symmetry of the pseudogap and the superconducting gap are determined by the electronic structure,and non-s wave symmetry gap favours the high-temperature superconductivity.Why the high-temperature superconductivity occurs in the metal region near the Mott metal-insulator transition is also explained.     

Editorial

Many excellent articles and textbooks on superconductivity have been published. However, concise and easily-handled data books written for scientists and engineers who work in the field of superconducting materials are very few. This article aims to collect and outline basic properties of superconducting materials and to provide such people with a database. The properties listed include crystal and electronic structures, equilibrium phase diagram, superconducting critical temperature, critical magnetic fields, critical current density, pinning force, specific heat coefficient, Debye temperature, penetration depth, coherence length, GL parameter, energy gap, electronic density of states at the Fermi level, electron-phonon coupling constant, Coulomb pseudopotential, and others. Emphasis is on the materials used in superconductive applications. Structural phase transitions such as martensitic transformations and transitions to the charge-density-wave state in superconducting materials are also reviewed. The properties of superconducting oxides will be summarized in the next article of this series.     

Pseudogap-state superconductivity in a “hot-point” model: Gor’kov equations

The specific features of the superconducting state (with s and d pairing) are considered in terms of a pseudogap state caused by short-range order fluctuations of the “dielectric” type, namely, antiferromagnetic (spin density wave) or charge density wave fluctuations, in a model of the Fermi surface with “hot points.” A set of recurrent Gor’kov equations is derived with inclusion of all Feynman diagrams of a perturbation expansion in the interaction between an electron and short-range order fluctuations causing strong scattering near hot points. The influence of nonmagnetic impurities on superconductivity in such a pseudogap state is analyzed. The critical temperature for the superconducting transition is determined, and the effect of the effective pseudogap width, correlation length of short-range-order fluctuations, and impurity scattering frequency on the temperature dependence of the energy gap is investigated.     

Topological change of the Fermi surface in low-density Rashba gases: application to superconductivity

In this Letter we show how, for small values of the Fermi energy compared to the spin-orbit splitting of Rashba type, a topological change of the Fermi surface leads to an effective reduction of the dimensionality in the electronic density of states in the low charge density regime. We investigate its consequences on the onset of the superconducting instability. We show that the superconducting critical temperature is significantly tuned in this regime by the spin-orbit coupling. We suggest that materials with strong spin-orbit coupling are good candidates for enhanced superconductivity.     

二维应变作用下超导薄膜LiFeAs的磁性和电子性质

基于密度泛函理论的第一性原理计算,研究了二维应变作用下LiFeAs超导薄膜的磁性结构、电子能带和态密度变化,分析了应变对其超导电性的作用.结果显示,对体系施加1%—6%的二维平面张、压应变均不改变其基态条形反铁磁性结构,费米面附近的电子态密度主要来自于Fe-3d轨道电子以及少量的As-4p电子.研究发现,与无应变情形相比,当施加压应变时,体系中Fe离子的反平行的电子自旋局域磁矩减小,薄膜反铁磁性受到抑制,费米面上电子态密度增加,超导电性来自于以反铁磁超交换耦合作用为媒介的空穴型费米面和电子型费米面间嵌套的Cooper电子对.而在张应变作用时,局域反铁磁性增强,费米面上电子态密度减小,金属性减弱,特别是张应变时费米面上空穴型能带消失, Cooper电子对出现概率显著降低,将抑制超导相变.     

Direct evidence of the charge ordered phase transition of indium nanowires on Si111

A controversial issue of the driving force for the phase transition of the one-dimensional (1D) metallic In wires on Si(111) is studied by low-temperature scanning tunneling microscopy and spectroscopy. The energy gap opening and the longitudinal charge ordering through charge transfer at the Fermi level are unambiguously observed. The vacancy defects induce a local charge ordering decoupled from a lattice distortion above T(c), and pin the phase of charge order below T(c). All these results below and above T(c) including the detailed features such as local fluctuations strongly support the 1D charge-density-wave mechanism for the phase transition.     

Single-gap s-Wave superconductivity near the charge-density-wave quantum critical point in CuxTiSe2

The in-plane thermal conductivity kappa of the layered superconductor CuxTiSe2 was measured down to temperatures as low as Tc/40, at x=0.06 near where the charge-density-wave order vanishes. The absence of a residual linear term at T-->0 is strong evidence for conventional s-wave superconductivity in this system. This is further supported by the slow magnetic field dependence, also consistent with a single gap, of uniform magnitude across the Fermi surface. Comparison with the closely related material NbSe2, where the superconducting gap is 3 times larger on the Nb 4d band than on the Se 4p band, suggests that in Cu0.06TiSe2 the Se 4p band is below the Fermi level and Cu doping into the Ti 3d band is responsible for the superconductivity.     

Quasiparticle density of states of 2H-NbSe2 single crystals revealed by low-temperature specific heat measurements according to a two-component model

Low-temperature specific heat in a dichalcogenide superconductor 2H-NbSe2 is measured in various magnetic fields. It is found that the specific heat can be described very well by a simple model concerning two components corresponding to vortex normal core and ambient superconducting region, separately. For calculating the specific heat outside the vortex core region, we use the Bardeen-Cooper Schrieffer （BCS） formalism under the assumption of a narrow distribution of the superconducting gaps. The field-dependent vortex core size in the mixed state of 2H-NbSe2, determined by using this model, can explain the nonlinear field dependence of specific heat coefficient γ（H）, which is in good agreement with the previous experimental results and more formal calculations. With the high-temperature specific heat data, we can find that, in the multi-band superconductor 2H-NbSe2, the recovered density of states （or Fermi surface） below Tc under a magnetic field seems not to be gapped again by the charge density wave （CDW） gap, which suggests that the superconducting gap and the CDW gap may open on different Fermi surface sheets.     

Novel mechanism of a charge density wave in a transition metal dichalcogenide

The charge density wave (CDW) is usually associated with Fermi surfaces nesting. We here report a new CDW mechanism discovered in a 2H-structured transition metal dichalcogenide, where the two essential ingredients of the CDW are realized in very anomalous ways due to the strong-coupling nature of the electronic structure. Namely, the CDW gap is only partially open, and charge density wave vector match is fulfilled through participation of states of the large Fermi patch, while the straight Fermi surface sections have secondary or negligible contributions.     

Electronic interactions and phase transitions at surfaces and in low dimensions

Low-dimensional and magnetic systems provide particularly clear-cut cases for the study of modified electron states that result from coupling with elementary excitations. Electronic quasiparticles of enhanced mass have been observed on the energy scale of phonons, and very recently on that of spin waves. Such mechanisms have bearing for superconducting pair formation, as spin fluctuations are being considered for high-temperature superconductivity. Phase transitions can occur if these interactions are accompanied by a nesting condition in the Fermi surface, leading to charge- or spin-density waves. The phase transition usually involves a structural distortion and is characterized by the opening of an energy gap. Special cases are the presence of multiple bands at the Fermi surface or the occurrence of thermal fluctuations. Recent progress in photoelectron spectroscopy and synchrotron-radiation instrumentation allow us to study these effects directly and resolved in k-space. Observations on surfaces, in magnetic materials, and in quasi-one-dimensional crystals are presented. PACS 73.20.At; 71.38.-k; 75.30.Ds; 71.30.+h; 79.60.-i     

Magnetism and superconductivity in a quasi-2D anisotropic system doped with charge carriers

The theory of multiband superconducting systems with variable density of charge carriers is analyzed. The possibility of emergence of nonphonon high-temperature superconductivity due to the predominance of electron–electron interband interactions over intraband interactions, as well as due to the fact that the thermodynamic and magnetic properties of multiband systems in the superconducting phase differ qualitatively from those of single-band systems, is indicated. Phase transitions in a quasi-2D anisotropic medium upon a change in the carrier concentration, i.e., a transition from the commensurate to the incommensurate state of the spin density wave, are analyzed. Such a transition is observed when the Umklapp processes in the lattice structure are taken into account. These processes facilitate a deviation of wavevector Q of the spin density wave from 2k_F, as well as a displacement of the bandgap relative to the Fermi surface. This leads to the generation of free charge carriers and the possibility of superconductivity. It is shown that superconductivity accompanies the magnetism. The conditions for the coexistence of these two phenomena are determined.     

Distinct fermi surface topology and nodeless superconducting gap in a (Tl0.58Rb0.42)Fe1.72Se2 superconductor

High resolution angle-resolved photoemission measurements have been carried out to study the electronic structure and superconducting gap of the (Tl0.58Rb0.42)Fe1.72Se2 superconductor with a T(c) = 32 K. The Fermi surface topology consists of two electronlike Fermi surface sheets around the Γ point which is distinct from that in all other iron-based superconductors reported so far. The Fermi surface around the M point shows a nearly isotropic superconducting gap of ～12 meV. The large Fermi surface near the Γ point also shows a nearly isotropic superconducting gap of ～15 meV, while no superconducting gap opening is clearly observed for the inner tiny Fermi surface. Our observed new Fermi surface topology and its associated superconducting gap will provide key insights and constraints into the understanding of the superconductivity mechanism in iron-based superconductors.     

Superconductivity at 5.2 K in ZrTe3 polycrystals and the effect of Cu and Ag intercalation

We report the occurrence of superconductivity in polycrystalline samples of ZrTe(3) at temperature 5.2 K at ambient pressure. The superconducting state coexists with the charge density wave (CDW) phase, which sets in at 63 K. The intercalation of Cu or Ag does not have any bearing on the superconducting transition temperature but suppresses the CDW state. The feature of a CDW anomaly in these compounds is clearly seen in the DC magnetization data. Resistivity data are analyzed in order to estimate the relative loss of carriers and reduction in the nested Fermi surface area upon CDW formation in ZrTe(3) and the intercalated compounds.     

Stripes and superconductivity in a one-dimensional self-consistent model

We show that many observable properties of high-temperature superconductors can be obtained in the framework of a one-dimensional self-consistent model with included superconducting correlations. Analytical solutions for spin, charge, and superconductivity order parameters are found. The ground state of the model at low hole doping is a spin-charge solitonic superstructure. Increased doping leads to a transition to the superconducting phase. There is a region of doping where superconductivity, spin density wave, and charged stripe structure coexist. The charge density modulation appears in the vicinity of vortices (kinks in the 1D model) in the superconducting state.     

Evolution from Unconventional Spin Density Wave to Superconductivity and a Pseudogaplike Phase in NaFe_{1-x}Co_{x}As

We report the doping, temperature, and spatial evolutions of the electronic structure of NaFe_{1-x}Co_{x}As studied by scanning tunneling microscopy. In the parent state we directly observe the spin density wave gap, which exhibits unconventional features that are incompatible with simple Fermi surface nesting. The optimally doped sample has a single superconducting gap, but in the overdoped regime a novel pseudogaplike feature emerges. The pseudogaplike phase coexists with superconductivity in the ground state, persists well into the normal state, and shows strong spatial variations. The characteristics of the three distinct electronic states revealed here shed important new lights on the microscopic models for the iron-based superconductors.     

The high temperature superconductivity in cuprates: physics of the pseudogap region

We discuss the physics of the high temperature superconductivity in hole doped copperoxide ceramics in the pseudogap region. Starting from an effective reduced Hamiltonianrelevant to the dynamics of holes injected into the copper oxide layers proposed in aprevious paper, we determine the superconductive condensate wavefunction. We show that thelow-lying elementary condensate excitations are analogous to the rotons in superfluid⁴He. We arguethat the rotons-like excitations account for the specific heat anomaly at the criticaltemperature. We discuss and compare with experimental observations the London penetrationlength, the Abrikosov vortices, the upper and lower critical magnetic fields, and thecritical current density. We give arguments to explain the origin of the Fermi arcs andFermi pockets. We investigate the nodal gap in the cuprate superconductors and discussboth the doping and temperature dependence of the nodal gap. We suggest that the nodal gapis responsible for the doping dependence of the so-called nodal Fermi velocity detected inangle resolved photoemission spectroscopy studies. We discuss the thermodynamics of thenodal quasielectron liquid and their role in the low temperature specific heat. We proposethat the ubiquitous presence of charge density wave in hole doped cuprate superconductorsin the pseudogap region originates from instabilities of the nodal quasielectrons drivenby the interaction with the planar CuO₂ lattice. We investigate the doping dependence of thecharge density wave gap and the competition between charge order and superconductivity. Wediscuss the effects of external magnetic fields on the charge density wave gap andelucidate the interplay between charge density wave and Abrikosov vortices. Finally, weexamine the physics underlying quantum oscillations in the pseudogap region.     

Hole core in superconductors and the origin of the Spin Meissner effect

It is proposed that superconductors possess a hidden ‘hole core’ buried deep in the Fermi sea. The proposed hole core is a small region of the Brillouin zone (usually at the center of the zone), where the lowest energy states in the normal state reside. We propose that in the superconducting state these energy states become singly occupied with electrons of a definite spin helicity. In other words, that holes of a definite spin helicity condense from the top to the bottom of the band in the transition to superconductivity, and electrons of that spin helicity ‘float’ on top of the hole core, thus becoming highly mobile. The hole core has radius q₀ = 1/2λ_L, with λ_L the London penetration depth, and the electrons expelled from the hole core give an excess negative charge density within a London penetration depth of the real space surface of the superconductor. The hole core explains the development of a spin current in the transition to superconductivity (Spin Meissner effect) and the associated negative charge expulsion from the interior of metals in the transition to superconductivity, effects we have proposed in earlier work to exist in all superconductors and to be at the root of the Meissner effect.     

Duality between unidirectional harge-density-wave order and superconductivity

The existence of a duality between a unidirectional charge-density-wave order and a superconducting order is shown. This duality predicts the existence of a charge-density wave near a superconducting vortex, and the existence of superconductivity near a charge-density-wave dislocation.     

Influence of the charge concentration on the density of states in a two-dimensional superconducting system

The gap and the density of states of high-T_c superconductors have been a subject of paramount interest. In order to explain the observed experimental behavior several pairing mechanisms in high-temperature superconductivity have been considered, by theoretical calculations. In this work, within the BCS scheme, a two-band model with energy band overlapping is introduced. The gap parameter and the density of states in a two-dimensional superconducting system are studied as functions of the charge concentration. This model is applied to Bi2212 in order to obtain numerical results.