Substitution Effect of Pr for Ca in Bi2Sr2CaCu2O8+δ System

The Bi₂Sr₂CaCu₂O₈₊ system samples doped with Pr on Ca sites were synthesized. Resistivity temperature dependence, X-ray powder diffraction, and photoemission experiments have been performed. Both X-ray diffraction and photoemission measurements show that Pr ion enters the lattice with a valence higher than 3⁺, which supports the hole-filling mechanism of the suppression of superconductivity.     

EuSr2RuCu2-xZnxO8中Zn对CU的替代效应

通过对EuSr2RuCu2-xZnxO8超导体系的结构、电阻和磁化强度的观测，研究了Zn对Cu的部分替代对其超导电性和磁性的影响。结果表明，Zn部分替代Cu后EuSr2RuCu2-xZnxO8体系仍能保持单相的上限浓度为x=0.2；替代导致样品发生四方－正交的结构相变，并对其超导电性具有强烈的破坏使用，x=0.01的样品在4.2K以上已不再呈现超导电性；替代增强了Ru离子之间的铁磁相互作用，导致铁磁相变温度的提高。     

纳米Al粉掺杂对MgB2 超导块材超导电性和显微结构影响

以纳米Al粉为掺杂物质，借助超声分散，较好地解决了纳米Al粉的团聚及其与B粉的均匀混合问题，制备出了1mol％、2mol％、5mol％、8mol％纳米Al粉掺杂的MgB2超导块材，并对掺杂效果和机理进行了研究．物相和显微结构分析表明，Al能够替代Mg进入MgB2晶格内，并导致MgB2的晶胞参数a、c逐渐降低，其中c的降低幅度较大．随着Al掺量的增加，MgB2的临界温度疋逐渐降低，由未掺杂时的38．5K降低至掺杂8mol％Al时的35．5K．超导电性研究结果表明，纳米Al粉掺杂，在低温、高场、低掺杂量的条件下可以改善MgB2的超导性能，掺杂量过大反而抑制MgB2的超导性能，2mol％的纳米Al粉掺杂效果最好．     

脉冲激光沉积法制备双层结构铜氧化物界面高温超导（英文）

在2008年发表的一篇经典工作中, Gozar等人报道了由绝缘性的La2CuO4和金属性的La1.55Sr0.45CuO4双层薄膜构成的体系存在界面超导.一个重要的有待回答的问题是该界面超导是否强健以及具有普遍性.在Gozar等人的工作中,铜氧化物双层结构是利用特制的氧化物分子束外延设备生长的,最大Sr掺杂量仅为0.47.在本工作中,我们首次利用脉冲激光沉积法制备了铜氧化物双层结构,并重复出了上述界面超导工作.在此基础上,我们将Sr的掺杂范围大幅扩展到1. 7 0,结果表明在由La2CuO4和过掺杂的La2-xSrxCuO4构成的双层结构中界面超导非常强健和普遍.值得一提的是,我们发现在x>0.8范围内存在一个新的界面超导区间.     

Nano Titanium Monoxide Crystals and Unusual Superconductivity at 11 K

Nano TiO₂ is investigated intensely due to extraordinary photoelectric performances in photocatalysis, new‐type solar cells, etc., but only very few synthesis and physical properties have been reported on nanostructured TiO or other low valent titanium‐containing oxides. Here, a core–shell nanoparticle made of TiO core covered with a ≈5 nm shell of amorphous TiO_1+x is newly constructed via a controllable reduction method to synthesize nano TiO core and subsequent soft oxidation to form the shell (TiO_1+x). The physical properties measurements of electrical transport and magnetism indicate these TiO@TiO_1+x nanocrystals are a type‐?? superconductor of a recorded T_c^onset = 11 K in the binary Ti–O system. This unusual superconductivity could be attributed to the interfacial effect due to the nearly linear gradient of O/Ti ratio across the outer amorphous layer. This novel synthetic method and enhanced superconductivity could open up possibilities in interface superconductivity of nanostructured composites with well‐controlled interfaces.     

D-Wave Superconductivity and Coulomb Correlations in the Two-Dimensional Hubbard Lattice

We considered anisotropic superconductivity within the two-dimensional Hubbard model extended by pairing correlations originating from the electron–phonon interaction. To discuss the onset of superconductivity close to the insulator–metal transition, we used the Hubbard I approximation to account for the formation of the insulating gap and see the role of Coulomb correlations for superconducting pairing. It has been shown that the Hubbard I approximation reflects effective pairing interactions genuine for correlated electron systems and leads to the stabilization of the superconductivity in the d-wave channel. One may expect the cooperation of phonon-free and phonon-induced mechanism in the formation of thed-wave superconducting state.     

On the Presence of Cu1+ in the Superconducting (Hg,M)Sr2CuO4+δ; M = Cr, Mo, or Re

The Cu K-edge measurements on the optimum T _c 1201 compounds (Hg_1–x M _x )Sr₂CuO₄₊ (M = Cr, Mo, or Re; 0.10 x 0.30) show besides Cu²⁺ (predominant) a very weak but distinct signature of Cu¹⁺, only in the superconducting (SC) samples. The Cu¹⁺ feature is conspicuously absent when these compositions are non-superconducting because of different processing conditions. Our finding of monovalent Cu in SC (Hg,M)/Sr-1201 favors the presence of Cu at the Hg-site and this seems to facilitate superconductivity in these cuprates.     

Phase Transition and Superconductivity Enhancement in Se‐Substituted MoTe2 Thin Films

Consecutively tailoring few‐layer transition metal dichalcogenides MX₂ from 2H to T_d phase may realize the long‐sought topological superconductivity in a single material system by incorporating superconductivity and the quantum spin Hall effect together. Here, this study demonstrates that a consecutive structural phase transition from T_d to 1T′ to 2H polytype can be realized by increasing the Se concentration in Se‐substituted MoTe₂ thin films. More importantly, the Se‐substitution is found to dramatically enhance the superconductivity of the MoTe₂ thin film, which is interpreted as the introduction of two‐band superconductivity. The chemical‐constituent‐induced phase transition offers a new strategy to study the s^+? superconductivity and the possible topological superconductivity, as well as to develop phase‐sensitive devices based on MX₂ materials.     

Superhigh Uniform Magnetic Cr Substitution in a 2D Mo2C Superconductor for a Macroscopic-Scale Kondo Effect

Substitutional doping provides an effective strategy to tailor the properties of 2D materials, but it remains an open challenge to achieve tunable uniform doping, especially at high doping level. Here, uniform lattice substitution of a 2D Mo₂C superconductor by magnetic Cr atoms with controlled concentration up to ≈ 46.9 at% by chemical vapor deposition and a specifically designed Cu/Cr/Mo trilayer growth substrate is reported. The concentration of Cr atoms can be easily tuned by simply changing the thickness of the Cr layer, and the samples retain the original structure of 2D Mo₂C even at a very high Cr concentration. The controlled uniform Cr doping enables the tuning of the competition of the 2D superconductor and the Kondo effect across the whole sample. Transport measurements show that with increasing Cr concentration, the superconductivity of the 2D Cr-doped Mo₂C crystals disappears along with the emergence of the Kondo effect, and the Kondo temperature increases monotonously. Using scanning tunneling microscopy/spectroscopy, the mechanism of the doping level effect on the interplay and evolution between superconductivity and the Kondo effect is revealed. This work paves a new way for the synthesis of 2D materials with widely tunable doping levels, and provides new understandings on the interplay between superconductivity and magnetism in the 2D limit.     

Effect of Oxygen Concentration on Superconducting Properties of Rubidium Tungsten Bronzes Rb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>WO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

Superconducting transition temperature (T _c) as a function of oxygen concentration for hexagonal rubidium tungsten bronzes Rb _x WO _y with 2.80 ≤ y ≤ 3.07 and x = 0.19, 0.23, and 0.27 has been systematically investigated. Three regions corresponding to T _c < 2 K (denoted as superconductivity suppressed region), T _c∼ 3 K (superconductivity uniform region) and T _c > 3 K (superconductivity enhanced region) were identified in T _c–y phase diagram for Rb_0.19WO _y and Rb_0.23WO _y . No superconductivity enhanced region was observed for Rb_0.27WO _y . The superconductivity suppressed region shifts toward higher oxygen content as rubidium concentration increases. The local ordering of the intercalated rubidium atoms might be responsible for the intriguing T _c–y phase diagram of Rb _x WO _y .     

Superconductivity at 14 K in SmFe0.9Co0.1AsO

We report superconductivity in the SmFe_0.9Co_0.1AsO system being prepared by most easy and versatile single-step solid-state reaction route. The parent compound SmFeAsO is non-superconducting but shows the spin density wave (SDW) like antiferromagnetic ordering at around 140 K. To destroy the antiferromagnetic ordering and to induce the superconductivity in the parent system, the Fe²⁺ is partially substituted by Co³⁺. Superconductivity appears in SmFe_0.9Co_0.1AsO system at around 14 K. The Co doping suppresses the SDW anomaly in the parent compound and induces the superconductivity. Magnetization measurements show clearly the onset of superconductivity with T _c^dia at 14 K. The isothermal magnetization measurements exhibit the lower critical fields (H _c1) to be around 200 Oe at 2 K. The bulk superconductivity of the studied SmFe_0.9Co_0.1AsO sample is further established by open diamagnetic M(H) loops at 2 and 5 K. Normal-state (above T _c) linear isothermal magnetization M(H) plots excluded the presence of any ordered magnetic impurity in the studied compound.      

Temperature Dependence of phase Transitions and Superconductivity in YBa_2Cu_3O_(7-X)

By means of X-ray diffraction investigationsand electric resistivity measurements the tempera-ture dependence of phase structure and supercon-ductivity in YBa_2Cu_3O_(7_x) over the temperaturerange of 20℃-950℃ have been studied.The latticeparameters a,b and c as well as conductivityas a function of annealing temperatures canbe roughly divided into three zones which aresuperconductive orthorhombic structure 1(a相似文献   

Pair breaking as a probe of d-wave high-temperature superconductivity

A unified picture is obtained of the Cooper pair-breaking data by Cu-site Zn and Ni in Nd_2–z Ce _z CuO₄, La_2–SrCuO₄, Bi₂Sr₂CaCu₂O₈, Bi_1.8Pb_0.2Sr₂Ca₂Cu₃O₁₀, YBa₂Cu₃O₇, and YBa₂Cu₄O₈. The data are generally inconsistent with spin-fluctuation d-wave pairing mechanisms of superconductivity and with all two-dimensional cuprate-plane models. The data are consistent with superconductivity in the charge reservoirs.     

Superconductivities of Quasi-One-Dimensional and Quasi-Two-Dimensional Tight-Binding Electrons in Magnetic Field

The superconductivity of the tight binding electrons in a magnetic field is studied. We can treat both cases of quasi-one-dimension and quasi-two-dimension in the same manner. We study a various kind of anisotropic superconductivity with line nodes of the energy gap by taking attractive interaction between electrons in nearest sites along each axis. The magnetic field dependence of the transition temperature is calculated for a various pairing symmetry of superconductivity. When a magnetic field is applied in the conducting plane, the transition temperature is shown to increase as the magnetic field increases. In the strong magnetic field the eigenstates approach to those in the absence of hopping between planes, resulting in no orbital frustration.     

Competition between superconductivity and insulating magnetic state in κ-(BEDT-TTF)2Cu[N(CN)2]Cl: Effects of pressure, magnetic field, and partial anion substitution

Systematic experimental study of the electrical transport and static magnetization reveals a direct and delicate interplay between magnetism and superconductivity in the organic material-(BEDT-TTF)₂Cu[N(CN)₂]Cl, displayed through the specific temperature-pressure phase diagram, and such new effects for organic solids as the reentrant superconductivity in zero applied magnetic field and high-field-induced interchange between superconductivity and the nonmetallic resistive phase in the high-pressure region.     

The Mechanism of High-Tc Superconductivity: “Nonlinear” Superconductivity

The main purpose of the paper is to present an overview of the current situation in the development of the understanding of the mechanism of high-T _c superconductivity which arises due to moderately strong, nonlinear electron–phonon interactions and due to spin fluctuations. The former are responsible for electron pairing, and the latter mediate the phase coherence. In addition, a key experiment for superconductivity in cuprates is proposed.     

On the Possibility of s+d Wave Superconductivity Within a Two-Band Scenario for High Tc Cuprates

A variety of different experimental results show substantial evidence that the order parameter in high-temperature superconducting copper oxides is not of pure d-wave symmetry, but that an s-wave component exists, which especially shows up in experiments that test the c-axis properties. These findings are modeled theoretically within a two-band model with interband interactions, where the superconducting order parameters in the two bands are allowed to differ in symmetry. It is found that the coupling of order parameters with different symmetries (s+d) leads to substantial enhancements of the superconducting transition temperature T _c as compared to order parameters with only s-wave symmetry. An additional enhancement factor of T _c is obtained from the coupling of the bands to the lattice where moderate couplings favor superconductivity while too strong couplings lead to electron (hole) localization and consequently suppress superconductivity.     

Decompression‐Driven Superconductivity Enhancement in In2Se3

An unexpected superconductivity enhancement is reported in decompressed In₂Se₃. The onset of superconductivity in In₂Se₃ occurs at 41.3 GPa with a critical temperature (T_c) of 3.7 K, peaking at 47.1 GPa. The striking observation shows that this layered chalcogenide remains superconducting in decompression down to 10.7 GPa. More surprisingly, the highest T_c that occurs at lower decompression pressures is 8.2 K, a twofold increase in the same crystal structure as in compression. It is found that the evolution of T_c is driven by the pressure‐induced R‐3m to I‐43d structural transition and significant softening of phonons and gentle variation of carrier concentration combined in the pressure quench. The novel decompression‐induced superconductivity enhancement implies that it is possible to maintain pressure‐induced superconductivity at lower or even ambient pressures with better superconducting performance.     

Theory of superconductivity at high temperatures

Superconductivity is generally explained by an electron-lattice interaction which results in the pairing of electrons and the condensation of these pairs into a state of lower entropy. In high-temperature superconductivity the pairs consist of hybrids in which the top of the oxygen band of the crowded perovskite layer is mixed with the bottom of unoccupiedd- orf-bands from monoxide layers in the crystal. Only electrons and phonons with low quasi-momentum (k) values participate. This makes it possible to localize the lattice perturbation into broad regions in which the van der Waals forces are reduced and the perovskite planes are contracted. The low entropy state associated with superconductivity manifests itself as the formation of a superlattice of lattice distortions which is in actual motion in the current-carrying states. The observability of this superlattice is discussed.     

Superconductivity and magnetism in 11-structure iron chalcogenides in relation to the iron pnictides

Abstract

This is a review of the magnetism and superconductivity in ‘11’-type Fe chalcogenides, as compared to the Fe-pnictide materials. The chalcogenides show many differences from the pnictides, as might be anticipated from their very varied chemistries. These differences include stronger renormalizations that might imply stronger correlation effects as well as different magnetic ordering patterns. Nevertheless the superconducting state and mechanism for superconductivity are apparently similar for the two classes of materials. Unanswered questions and challenges to theory are emphasized.