Evolution of the spin-density wave-superconductivity texture in the organic superconductor (TMTSF)2PF6 under pressure

We report an investigation at the endpoint region of the spin density wave state in (TMTSF)₂PF₆ where metal and superconductivity emerge. Thanks to resistivity measurements along the three main crystallographic directions, we are able to follow the texture in this phase coexistence regime. In this respect, superconductivity is used as a decoration technique of the metallic pattern. We show that metal (superconductivity) emerges first along the c^? direction in a counterintuitive manner. Then metal (superconductivity) domains evolves from filaments along the c^? axis towards slabs perpendicular to the a-axis which melt together in the homogeneous phase at high pressure. This evolution is compatible with the proposition of the formation of a soliton phase in the vicinity of the critical pressure of the (TMTSF)₂PF₆ phase diagram.     

Can we explain the coexistence of ferromagnetism and superconductivity based on two parameters model?

A possible explanation about the coexistence of ferromagnetism (FM) and superconductivity (SC) based on a two parameters mean field model in a two-dimensional system is discussed. The key feature of this model is that there are two independent parameters which are responsible for ferromagnetism and superconductivity, respectively. We point out that the coexisting FM and s-wave pairing SC state is energetically not favorable among all possible state. We generalize the two parameter model to include the coexistence of FM with p-wave SC. We find that the phase diagram is not consistent with what experimentally discovered in UGe₂.     

Superconducting clusters in the pseudogap region

The upper boundary of the pseudogap state has been derived as a function of the doping within the impurity mechanism of high-T _c superconductivity under the assumption that, in the region bordering the superconducting phase on the side of the doping less than the optimum level (the pseudogap region), there exist finite superconducting clusters and the boundary of the superconducting phase corresponds to the threshold of the existence of an infinite superconducting cluster. The position of this boundary is in agreement with experiment. The condition imposed on the doping level at which the giant proximity effect should be observed has been derived. It means essentially that the thickness of the nonsuperconducting layer should be small compared to the average size of the superconducting clusters.     

Unconventional superconductivity

‘Conventional’ superconductivity, as used in this review, refers to electron–phonon-coupled superconducting electron pairs described by BCS theory. Unconventional superconductivity refers to superconductors where the Cooper pairs are not bound together by phonon exchange but instead by exchange of some other kind, e.g. spin fluctuations in a superconductor with magnetic order either coexistent or nearby in the phase diagram. Such unconventional superconductivity has been known experimentally since heavy fermion CeCu₂Si₂, with its strongly correlated 4f electrons, was discovered to superconduct below 0.6?K in 1979. Since the discovery of unconventional superconductivity in the layered cuprates in 1986, the study of these materials saw T_c jump to 164?K by 1994. Further progress in high-temperature superconductivity would be aided by understanding the cause of such unconventional pairing. This review compares the fundamental properties of 9 unconventional superconducting classes of materials – from 4f-electron heavy fermions to organic superconductors to classes where only three known members exist to the cuprates with over 200 examples – with the hope that common features will emerge to help theory explain (and predict!) these phenomena. In addition, three new emerging classes of superconductors (topological, interfacial – e.g. FeSe on SrTiO₃, and H₂S under high pressure) are briefly covered, even though their ‘conventionality’ is not yet fully determined.     

Theory of dual fermion superconductivity in hole-doped cuprates

Since the discovery of the cuprate high-temperature superconductivity in 1986, a universal phase diagram has been constructed experimentally and numerous theoretical models have been proposed. However, there remains no consensus on the underlying physics thus far. Here, we theoretically investigate the phase diagram of hole-doped cuprates based on an itinerant-localized dual fermion model, with the charge carriers doped on the oxygen sites and localized holes on the copper d _{x2 ? y2} orbitals. We analytically demonstrate that the puzzling anomalous normal state or the strange metal could simply stem from a free Fermi gas of carriers bathing in copper antiferromagnetic spin fluctuations. The short-range high-energy spin excitations also act as the “magnetic glue” of carrier Cooper pairs and induce d-wave superconductivity from the underdoped to overdoped regime, distinctly different from the conventional low-frequency magnetic fluctuation mechanism. We further sketch out the characteristic dome-shaped critical temperature T _c versus doping level. The emergence of the pseudogap is ascribed to the localization of partial carriers coupled to the local copper moments or a crossover from the strange metal to a nodal Kondo-like insulator. Our work provides a consistent theoretical framework to understand the typical phase diagram of hole-doped cuprates and paves a distinct way to the studies of both non-Fermi liquid and unconventional superconductivity in strongly correlated systems.     

Strong coupling superconductivity and prominent superconducting fluctuations in the new superconductor Bi4O4S3

Electric transport and scanning tunneling spectrum(STS)have been investigated on polycrystalline samples of the new superconductor Bi4O4S3.A weak insulating behavior in the resistive curve has been induced in the normal state when the superconductivity is suppressed by applying a magnetic field.Interestingly,a kink appears on the temperature dependence of resistivity near 4 K at all high magnetic fields above 1 T when the bulk superconductivity is completely suppressed.This kink associated with the upper critical field as well as the wide range of excess conductance at low fields and high temperatures is explained as the possible evidence of strong superconducting fluctuation.From the tunneling spectra,a superconducting gap of about 3 meV is frequently observed yielding a ratio of 2Δ/kB TC~16.6.This value is much larger than the one predicted by the BCS theory in the weak coupling regime(2Δ/kB TC~3.53),which suggests the strong coupling superconductivity in the present system.Furthermore,the gapped feature persists on the spectra until 14 K in the STS measurement,which suggests a prominent fluctuation region of superconductivity.Such a superconducting fluctuation can survive at very high magnetic fields,which are far beyond the critical fields for bulk superconductivity as inferred both from electric transport and tunneling measurements.     

Emergence of novel phenomena on the border of low dimensional spin and charge order

Proximity to magnetic order as well as low dimensionality are both beneficial to superconductivity at elevated temperatures. Materials on the border of magnetism display a wide range of novel and potentially useful phenomena: high T_cs, heavy fermions, coexistence of magnetism and superconductivity and giant magnetoresistance. Low dimensionality is linked to enhanced fluctuations and, in the case of heavy fermions, has been experimentally shown to be beneficial for the fluctuations that are responsible for the rich abundance of novel emergent phases. This experimental strategy motivated us to explore 2D insulating magnets with a view to investigate phase evolution across metal-insulator and magnetic-non-magnetic boundaries. This has been a fruitful venture with totally novel results different to our expectations. We present results from 2 distinct systems. The MPS₃ family are highly anisotropic in both their crystal and magnetic structures. FePS₃ in particular is a model insulating honeycomb antiferromagnet. We find that the application of pressure to FePS₃ induces an insulator to metal transition. The second system, Cs₂CuCl₄, is a highly-frustrated quantum spin liquid at low temperature. The competition of the 3 relevant exchange couplings is delicately balanced. It has been shown to become antiferromagnetic at very low temperatures (~1 K). We have found that the application of pressure for 3 days or more followed by a return to ambient pressure stabilises a totally distinct magnetic ground state.     

Appearance of anisotropic non s-wave superconductivity above the critical pressure of antiferromagnetic CeRhIn5

We have carried out ¹¹⁵In nuclear quadrupole resonance (NQR) measurements in CeRhIn₅. At ambient pressure, CeRhIn₅ undergoes an antiferromagnetic AF phase transition at K. The ¹¹⁵In NQR spectrum has shown the appearance of a small internal field in the direction perpendicular to the tetragonal c-axis. With application of a hydrostatic pressure, the AF state is suppressed and the superconductivity appears just above the critical pressure (P = 17 kbar). The nuclear spin lattice relaxation rate 1/T₁ of ¹¹⁵In measured at P = 27 kbar indicates the occurrence of the superconductivity in the nearly AF region. In the superconducting state, 1/T₁ has no Hebel-Slichter coherence peak just below of 2 K and has a power law T-dependence (T³) down to 300 mK. This is consistent with anisotropic superconductivity, with line nodes in the superconducting energy gap: non-s-wave superconductivity occurs in CeRhIn₅. Received 5 July 2000     

Superconductivity in the Lu-Ir alloy system

Superconductivity in the Lu-Ir alloy system is investigated. Although it has been reported that LuIr₂ is superconducting at 2.47 K, it is found that LuIr₂ is not superconducting above 1.3 K. However, alloys of composition between LuIr₂ and Ir are superconducting. This enhancement of superconductivity is not due to the eutectic structure but to the off-stoichiometry of the C15 Laves phase which has a certain homogeneity range. In the C15 Laves phase, the transition temperature increases and the lattice constant decreases monotonically with increasing Ir concentration.     

Effect of Mn substitution on the delicate balance between structure and properties of Gd1.4Ce0.6Sr2RuCu2O10

Some new members of a ruthenocuprate(2212) series have been synthesized by Mn substitution for Ru in Gd_1.4Ce_0.6Sr₂RuCu₂O₁₀. Characterization by x-ray diffraction (XRD) phase analysis has been carried out. Changes in structural features on substitution, including a significant change in lattice parameter for a very low substitution level, have been observed. Four-probe resistivity studies indicate the coexistence of superconductivity and magnetism for the pristine compound and a semiconductor-like upturn in resistivity and the absence of superconductivity even for very low levels of Mn substitution. AC susceptibility measurements show a progressive suppression of the magnetic transition temperature as well as a smearing of the magnetic transition as a function of Mn substitution. Possible reasons for the absence of superconductivity have been discussed.     

The superconductivity and structure of equiatomic ternary transition metal pnictides

Recent superconductivity and crystallographic data for the MM′X ternary compounds with M = Ti, Zr, or Hf, M′ = Ru or Os, and X = P or As are presented. Moderate to high superconducting transition temperatures (T _c 's)are exhibited by the ZrRuSi-type hexagonal phase which is found to be metastable at low temperatures. The low-temperature phase of ZrRuP has the TiNiSi-type orthorhombic structure and exhibits superconductivity at 3.9K. The low-temperature phase of HfRuAs has the TiFeSi-type orthorhombic superstructure and remains normal at 1.0K. TiRuAs exists only with the superstructure and remains normal at 0.35K. The details of these various structural modifications point to the importance of undistorted zig-zag chains of metal atoms for the occurrence of superconductivity in these compounds. The lowT _c for TiRuP and the absence of superconductivity for TiOsP above 0.35K are not accounted for by these structural modifications.     

Quenched superconductivity by rapid cooling down to low temperatures below Tc2 in single-crystal HoMo6S8

Electrical resistivity of single-crystal HoMo₆S₈ has been measured at low temperatures down to 0.01 K. By slow cooling, the resistivity in the re-entrant normal state below T_c2 is recovered up to 98 % of its value above T_c1. This suggests that there still remains a little superconductive region not due to Bloch walls, but rather to ferromagnetic magnetic microdomains of comparatively small sizes. By rapid cooling, on the other hand, the superconductivity above T_c2 has been found to be quenched at low temperatures below T_c2. This quenched superconductivity, coexisting with the quenched modulated magnetic structure, is destroyed as a function of time and magnetic field, which seems attributed to the growth of the microdomains.     

STRUCTURE AND SUPERCONDUCTIVITY OF Mg1-xLixB2

The effects of the substitution of Li for Mg in Mg_1-xLi_xB₂(x=0,0.1,0.2) on their structure and superconductivity have been investigated. It has been found by X-ray diffraction that the substitution of Li for Mg with x=0.1 and 0.2 does not cause phase transformation in these samples. However,the measurements of temperature-dependent normalized magnetization indicate the loss of superconductivity with the increase of Li content in these samples.     

The origin of the pseudogap phase: precursor superconductivity versus a competing energy gap scenario

In the last few years evidence has been accumulating that there are a multiplicity of energy scales which characterize superconductivity in the underdoped cuprates. In contrast to the situation in BCS superconductors, the phase coherence temperature T_c is different from the energy gap onset temperature T^∗. In addition, thermodynamic and tunneling spectroscopies have led to the inference that the order parameter Δ_sc is to be distinguished from the excitation gap Δ; in this way, pseudogap effects persist below T_c. It has been argued by many in the community that the presence of these distinct energy scales demonstrates that the pseudogap is unrelated to superconductivity. In this paper, we show that this inference is incorrect. We demonstrate that the difference between the order parameter and excitation gap and the contrasting dependences of T^∗ and T_c on hole concentration x and magnetic field H follow from a natural generalization of BCS theory. This simple generalized form is based on a BCS-like ground state, but with self-consistently determined chemical potential in the presence of arbitrary attractive coupling g. We have applied this mean field theory with some success to tunneling, transport, thermodynamics, and magnetic field effects. We contrast the present approach with the phase fluctuation scenario and discuss key features which might distinguish our precursor superconductivity picture from that involving a competing order parameter.     

Interplay between the magnetic fluctuations and superconductivity in lanthanum cuprates

We report an analysis of the magnetic fluctuations in superconducting La_2?xSr_xCuO₄ and related lanthanum cuprates that have different symmetry of the low-temperature structure. NMR and ESR investigations revealed a dynamical coexistence of superconductivity and the antiferromagnetic correlations in most of the superconductivity region of the phase diagram. We show that, for all compounds, regardless of their low-temperature symmetry and their superconducting properties, the enhancement of the spin stiffness near 1/8 doping takes place.     

Quasiparticles in a strongly correlated liquid with the fermion condensate: applications to high-temperature superconductors

A model of a strongly correlated electron liquid based on fermion condensation (FC) is extended to high-temperature superconductors. Within our model, the appearance of FC presents a boundary separating the region of a strongly interacting electron liquid from the region of a strongly correlated electron liquid. We study the superconductivity of a strongly correlated liquid and show that, under certain conditions, the superconductivity vanishes at temperatures T > T _c ≈ T _node, with the superconducting gap being smoothly transformed into a pseudogap. As a result, the pseudogap occupies only a part of the Fermi surface. The gapped area shrinks with increasing the temperature and vanishes at T = _T*. The single-particle excitation width is also studied. The quasiparticle dispersion in systems with FC can be represented by two straight lines, characterized by the effective masses and, intersecting near the binding energy that is on the order of the superconducting gap. It is argued that this strong change of the quasiparticle dispersion upon binding can be enhanced in underdoped samples because of strengthening the FC influence. The FC phase transition in the presence of the superconductivity is examined, and it is shown that this phase transition can be considered as driven by the kinetic energy.     

Superconductivity in the new Nb5Si3−xBx phase

In the Nb–Si system the αNb₅Si₃ phase is not a superconductor. Considering the high solubility of this phase for boron, in this work it has been evaluated the effect of boron doping in αNb₅Si₃ on the electrical and magnetic properties of the produced materials. It has been shown that boron doping promoted superconductivity for all studied compositions. The Nb₅Si_2.4B_0.6 sample presented the maximum superconducting critical temperature (7.8 K) with upper critical magnetic field close to μ_oH～4.2 T. Thus, this work shows for the first time superconductivity in a phase with Cr₅B₃-prototype structure.     

YBa2-xSrxCu3O7-δ系统的超导电性与结构的关联

本文系统地研究了YBa_2-xSr_xCu₃O_7-δ中超导电性与物相以及结构变化之间的关系。发现当0c并不随Sr的含量而单调变化。在x=1.0处,T_c出现一峰值。同时,超导临界温度与样品的晶体结构之间呈现出强烈的相关性:随着正交性的增强(即a,b间差值的增大),T_c单调地上升。本文还就S 关键词：     

Superconductivity with transition temperature up to 80 K for TbSr2Cu2.7Mo0.3O7+δ

A Tb-123 phase with the composition, TbSr₂Cu_2.7Mo_0.3O_7+δ, has been synthesized in single-phase form by the solid-state reaction route. Its phase purity has been confirmed from neutron powder diffraction studies. The as-synthesized Tb-123 sample does not show superconductivity down to 5 K. On the other hand, an unusually high antiferromagnetic ordering temperature (T_N) of around 7 K is seen for the Tb moments. After 120-atm-O₂ post-annealing, bulk superconductivity is achieved in this compound with a superconducting transition temperature (T_c) of about 30 K, without any significant effect on T_N. To achieve higher oxygen content and higher T_c, the as-synthesized sample was subjected to high-pressure oxygenation, carried out in a closed cell, at 5 GPa and 400 °C in the presence of AgO as an excess-oxygen source. This sample exhibited superconductivity onset at around 80 K with a Meissner fraction larger than 10% at 5 K. Our observation of superconductivity at 80 K is the highest T_c to-date for the Tb-123 type compounds.     

Superconductivity in Hg-substituted BaPb0.75Bi0.25O3

A series of Hg-doped BaPb_0.75Bi_0.25O₃ (BPBO) with a nominal composition of BaPb_0.75 − xHg_xBi_0.25O₃ (x=0-0.40 with 0.05 intervals) has been synthesized by solid state reaction. The system shows a lattice parameter expansion and lattice symmetry distortion with Hg doping. Superconducting transition temperature Tc and superconducting volume fraction of the system decrease with Hg doping level in the low doping level region (0?x?0.25) and are nearly fully suppressed at x=0.25. However, the superconductivity is recovered with further increasing Hg content at x>0.3. The possible mechanisms of the superconductivity in the low doping level region and the recovery of superconductivity in the high doping level region for Hg-doped BPBO system have been discussed.