Superconductivity in Sm-doped CaFe_2As_2 single crystals

In this article,the Sm-doping single crystals Ca_(1-x)Sm_xFe_2As_2(x = 0 ~0.2) were prepared by the Ca As flux method,and followed by a rapid quenching treatment after the high temperature growth.The samples were characterized by structural,resistive,and magnetic measurements.The successful Sm-substitution was revealed by the reduction of the lattice parameter c,due to the smaller ionic radius of Sm~(3+)than Ca~(2+).Superconductivity was observed in all samples with onset T_c varying from 27 K to 44 K upon Sm-doping.The coexistence of a collapsed phase transition and the superconducting transition was found for the lower Sm-doping samples.Zero resistivity and substantial superconducting volume fraction only happen in higher Sm-doping crystals with the nominal x 0.10.The doping dependences of the c-axis length and onset T_c were summarized.The high-T_c observed in these quenched crystals may be attributed to simultaneous tuning of electron carriers doping and strain effect caused by lattice reduction of Sm-substitution.     

Two-coupled structural modulations in charge-density-wave state of SrPt_2As_2 superconductor

Transmission electron microscopy(TEM) study of SrPt2As2 reveals two incommensurate modulations appearing in the charge-density-wave(CDW) state below TCDW≈ 470 K. These two structural modulations can be well explained in terms of condensations of two-coupled phonon modes with wave vectors of q1= 0.62a*on the a*–b*plane and q2=0.23a*on the a*–c*plane. The atomic displacements occur along the b-axis direction for q1and along the c-axis direction for q2, respectively. Moreover, the correlation between q1and q2can be generally written as q1=(q2+ a*)/2 in the CDW state, suggesting the presence of essential coupling between q1and q2. A small fraction of Ir doping on the Pt site in Sr(Pt1-xIrx)2As2(x ≤ 0.06) could moderately change these CDW modulations and also affect their superconductivities.     

Superconductivity in an intermetallic oxide Hf_3Pt_4Ge_2O

Discovery of a new superconductor with distinct crystal structure and chemistry often provides great opportunity for further expanding superconductor material base, and also leads to better understanding of superconductivity mechanisms. Here, we report the discovery of superconductivity in a new intermetallic oxide Hf_3Pt_ Ge_2O synthesized through a solid-state reaction. The Hf_3Pt_ Ge_2O crystallizes in a cubic structure(space group Fm-3 m) with a lattice constant of a = 1.241 nm, whose stoichiometry and atomic structure are determined by electron microscopy and x-ray diffraction techniques. The superconductivity at 4.1 K and type-II superconducting nature are evidenced by the electrical resistivity, magnetic susceptibility, and specific heat measurements. The intermetallic oxide Hf_3Pt_ Ge_2O system demonstrates an intriguing structural feature that foreign oxygen atoms can be accommodated in the interstitial sites of the ternary intermetallic framework. We also successfully synthesized a series of Hf_3Pt_ Ge_2O1+δ(-0.25 ≤δ≤ 0.5), and found theδ-dependent superconducting transition temperature Tc. The atomic structure and the electronic structure are also substantiated by first-principles calculations. Our results present an entirely new family of superconductors with distinct structural and chemical characteristics, and could attract research interest in further finding new superconductors and exploring novel physics pertaining to the 5 d-electron in these intermetallic compound systems.     

Electron spin resonance study of a CuIr2S4 single crystal

A spinel sulphide CuIr₂S₄ single crystal, which exhibits an orbitally induced Peierls phase transition at ～230?K, is investigated by electron spin resonance (ESR) spectroscopy. The phase transition is clearly manifested on the ESR spectra. It is suggested that the ESR signals are produced by a few non-dimerized Ir⁴⁺ ions. Moreover, an extra ESR spectrum appears at low temperature in addition to the paramagnetic ESR signals of Ir⁴⁺ ions, which is suggested to be caused by the Jahn–Teller effect of the non-dimerized Ir⁴⁺ ions. From the ESR results, it is found that the Jahn–Teller splitting energy ΔE _JT is much smaller than the spin-dimerization gap.     

拓扑材料中的超导

近来,人们在凝聚态体系中发现了由拓扑不变量定义的物相,其中最重要的有拓扑绝缘体、拓扑半金属和拓扑超导体等.这些物相的拓扑性质由非平凡的拓扑数描述,相应的材料被称为拓扑材料,具有诸多新奇的物理特性.其中拓扑超导体由于边界上有满足非阿贝尔统计的Majorana零能模,成为实现拓扑量子计算的主要候选材料.除了探索本征的拓扑超导体外,由于拓扑性质上的相似性,在不超导的拓扑材料中调制出超导自然成为了实现拓扑超导的重要手段.目前,人们发展了栅极调制、掺杂、高压、近邻效应调制和硬针尖点接触等多种技术,已经成功地在许多拓扑绝缘体和半金属中诱导出了超导,并对超导的拓扑性和Majorana零能模进行了研究.本文回顾了本征拓扑超导候选材料,以及拓扑绝缘体和半金属中诱导出超导的代表性工作,评述了不同实验手段的优势和缺陷、分析了其超导拓扑性的证据,并提出展望.     

Superconductivity in a new NdBa2Ca3Sr4Cu5Ox system

A new compound, NdBa₂Ca₃Sr₄Cu₅O_x, has been synthesised using the conventional solid state reaction technique. The material was characterised by powder XRD, electrical resistivity, ac susceptibility and dc magnetisation studies. The results of powder X-ray diffraction show that the structure is pseudo orthorhombic, having unit cell dimensions a = 5.47 Å, b = 5.46 Å and c = 14.58 Å. Magnetisation studies on a SQUID magnetometer showed a superconducting transition at 52 K. This was confirmed by the measurements of electrical resistance and ac susceptibility of this sample, which also showed superconductivity at 52 K. Details of preparation and characterisation are discussed.     

Substitution Effects on MgB2 Superconductivity

With the help of supercell method, the first-principle calculations were performed for the study of doping crystal Mg_1-xAl_xB₂ and Mg(B_1-yC_y)₂. Analyzing the variations of the charge distribution and the partial densities of states, we found that the compounds with doping Al to MgB₂ compound and/or replacing boron by carbon exhibit new covalent bond effects and unexpected electronic properties, related to superconductivity. The study of the density of states indicates that superconductivity decreases with the increase of Al fraction and carbon concentration. There exists a transition of superconductor to non-superconductor with the change of Al doping fraction. The substitution of boron by carbon results in the decrease of the transition temperature since the decrease of the electron concentration and the lattice constant. The theoretical predictions agree with experimental observations.     

Superconductivity in ultrasmall metallic grains

We review recent experimental and theoretical work on superconductivity in ultrasmall metallic grains, i.e. grains sufficiently small that the conduction electron energy spectrum becomes discrete. The discrete excitation spectrum of an individual grain can be measured by the technique of single‐electron tunneling spectroscopy, and reveals parity effects indicative of pairing correlations in the grain. After introducing the discrete BCS model that has been used to model such grains, we review a phenomenological, grand‐canonical, variational BCS theory describing the paramagnetic breakdown of these pairing correlations with increasing magnetic field. We also review recent canonical theories that have been developed to describe how pairing correlations change during the crossover, with decreasing grain size, from the bulk limit to the limit of few electrons, and compare their results to those obtained using Richardson's exact solution of the discrete BCS model.     

Proximity-Effect-Induced Superconductivity in Nb/Sb2Te3-Nanoribbon/Nb Junctions

Nanohybrid superconducting junctions using antimony telluride (Sb₂Te₃) topological insulator nanoribbons and Nb superconducting electrodes are fabricated using electron beam lithography and magnetron sputtering. The effects of bias current, temperature, and magnetic field on the transport properties of the junctions in a four-terminal measurement configuration are investigated. Two features are observed. First, the formation of a Josephson weak-link junction. The junction is formed by proximity-induced areas in the nanoribbon right underneath the inner Nb electrodes which are connected by the few tens of nanometers short Sb₂Te₃ bridge. At 0.5 K a critical current of 0.15 µA is observed. The decrease of the supercurrent with temperature is explained in the framework of a diffusive junction. Furthermore, the Josephson supercurrent is found to decrease monotonously with the magnetic field indicating that the structure is in the small-junction limit. As a second feature, a transition is also observed in the differential resistance at larger bias currents and larger magnetic fields, which is attributed to the suppression of the proximity-induced superconductive state in the nanoribbon area underneath the Nb electrodes.     

Scanning tunneling optical spectroscopy in mercury cadmium telluride and related compounds

We consider II–VI narrow gap semiconducting alloys: mercury cadmium telluride, Hg_(1−x)Cd_(x)Te (MCT), mercury zinc telluride, Hg_(1−x)Zn_(x)Te (MZT), and mercury zinc selenide, Hg_(1−x)Zn_(x)Se (MZS). MCT is emphasized for actual calculations, but a table of values needed in all calculations is presented. These materials are of interest because of their application to infrared detectors and related devices, and because they are candidates for low gravity crystal growth to improve uniformity. We present new calculations of the scanning tunneling optical spectroscopy (STOS) current from which the local energy gap, a function of x, and hence the stoichiometry (x) can be determined as a function of position with presumably high spatial resolution. The low temperature tunneling current (vs. photon frequency) has a sharper onset at the band gap than the low temperature optical absorption. This sharp onset originates from the rapid increase in the integrated transmission probabilities and is greatly enhanced by large diffusion lengths. Thus, STOS should be a competitive technique, compared to optical absorption, for determining the local stoichiometry, a property that is important for characterizing crystals.     

Superconductivity in covalent semiconductors

This review covers recent advances in superconductivity of diamond, Si, SiC, group III–V and II–IV semiconductors, metal-intercalated graphite and fullerites. The results are critically analyzed and prospects are given for future research directions. In particular, it is argued that the highest transition temperatures of ∼9 K in diamond and 11.5 K in CaC₆ can further be enhanced and that no reliable evidence exists yet for superconductivity in III-V semiconductors.      

Superconducting state of Sr2RuO4 under magnetic fields

The layered perovskite Sr₂RuO₄ exhibits unconventional superconductivity below 1.5 K. There are now several key experimental facts which indicate that its superconducting symmetry is spin-triplet, p-wave, but some important features of its gap structure still need to be determined. In order to address some of the outstanding issues, we have performed a study of its thermodynamic behavior in the field-temperature phase diagram. Specific heat measurements give evidence for multiple superconducting gaps. We will discuss implications of this in relation to the idea of “orbital-dependent superconductivity”. We will also present the results of a search for a second superconducting transition under the magnetic field applied precisely parallel to the quasi-two-dimensional plane.     

Superconductivity in octagraphene

This article reviews the basic theoretical aspects of octagraphene, an one-atom-thick allotrope of carbon, with unusual two-dimensional(2 D) Fermi nesting, hoping to contribute to the new family of quantum materials. Octagraphene has an almost strongest sp²hybrid bond similar to graphene, and has the similar electronic band structure as iron-based superconductors, which makes it possible to realize high-temperature superconductivity. We have compared various possible mechanisms of superconductivity, including the unconventional s;superconductivity driven by spin fluctuation and conventional superconductivity based on electron–phonon coupling. Theoretical studies have shown that octagraphene has relatively high structural stability. Although many 2 D carbon materials with C;carbon ring and C;carbon ring structures have been reported, it is still challenging to realize the octagraphene with pure square-octagon structure experimentally.This material holds hope to realize new 2 D high-temperature superconductivity.     

Color Superconductivity in Cold Dense Quark Matter

We investigate the phase structure of color superconducting quark matter at intermediate densities for two- and three-flavor systems. We thereby focus our attention on the influence of charge neutrality conditions as well as -equilibrium of the different phases. These constraints are relevant in the context of quark matter at the interior of compact stars. We analyze the implications of color superconductivity on compact star configurations using different hadronic and quark equations of state.Presented at the 15th Indian-Summer School of Physics: Understanding Dense Matter, Prague, Czech Republic, 27 August–1 September, 2003.     

Superconductivity and x—ray photoemission study of MgB2 thin films

Highly c-axis oriented MgB2 thin films with Tc^onset of 39.6K were fabricated by magnesium diffusing into pulsedlaser-deposited boron precursors.The estimation of critical current density Jc,using hysteresis loops and the Bean model,has given the value of 10^7A/cm^2(15K,0T),which is one of the highest values ever reported.The x-ray photoemission study of the MgB2 thin films has revealed that the binding energies of Mg 2p and B 1s are at 49.4eV and 186.9eV,which are close to those of metallic Mg and transition-metal diborides,respectively.     

Superconductivity under uniaxial compression in β-(BDA-TTP) salts

In order to clarify the mechanism of organic superconductor β-(BDA-TTP) salts. We study the superconductivity under uniaxial compression with non-dimerized two-band Hubbard model. We have calculated the uniaxial compression dependence of T_c by solving the Eliashberg’s equation using the fluctuation exchange (FLEX) approximation. The transfer integral under the uniaxial compression was estimated by the extended Huckel method.We have found that non-monotonic behaviors of T_c in experimental results under uniaxial compression are understood taking the spin frustration and spin fluctuation into account.     

Hydrostatic pressure effect on Tc of new BiS2‐based Bi4O4S3 and NdO0.5F0.5BiS2 layered superconductors

We investigate the external hydrostatic pressure effect on the superconducting transition temperature (T_c) of new layered superconductors Bi₄O₄S₃ and NdO_0.5F_0.5BiS₂. Though the T_c is found to have a moderate decrease from 4.8 K to 4.3 K (dT_c^onset/dP = –0.28 K/GPa) for Bi₄O₄S₃ superconductor, the same increases from 4.6 K to 5 K (dT_c^onset/dP = 0.44 K/GPa) up to 1.31 GPa followed by a sudden decrease from 5 K to 4.7 K up to 1.75 GPa for NdO_0.5F_0.5BiS₂ superconductor. The variation of T_c in these systems may be correlated to an increase or decrease of the charge carriers in the density of states under externally applied pressure. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cu2ZnSnS4 类四元硫族半导体的理论研究——以二元、三元、四元半导体的演化为思路

在过去60多年中,人们对半导体的研究集中在一元、二元和三元半导体方面,最近,出于寻找新型廉价、环保、高效光伏转换材料的需要,Cu2ZnSnS4 类 I2-II-IV-VI4型四元硫族半导体吸引了人们越来越多的关注,它在光催化和热电等多方面的应用也不断被发掘. 然而,对于这类四元半导体的基本性质,如晶体结构和电子结构,人们知之甚少,很多研究还停留在经验阶段. 文章首先简要回顾了这类半导体的由来和在应用方面的最新进展,然后详细介绍了文章作者对这类四元半导体的第一性原理计算研究工作的进展,其中包括：系统研究了这类硫族半导体在从二元向三元再向四元的演化过程中晶体结构和电子能带结构变化的规律,总结了元素成分对其影响的一般趋势,并结合实验结果分析了这类四元半导体晶格结构表征和带隙测量中易于出现的混淆;文章作者还以Cu2ZnSnS4 为例,考察了这类四元化合物相对二元、三元化合物的相稳定性和本征缺陷性质. 文章介绍的研究结果将为一系列I2-II-IV-VI4型四元半导体的深入研究提供基础.