Structural and magnetic phase diagram of CeFeAsO(1- x)F(x) and its relation to high-temperature superconductivity

Recently, high-transition-temperature (high-Tc) superconductivity was discovered in the iron pnictide RFeAsO(1-x)F(x) (R, rare-earth metal) family of materials. We use neutron scattering to study the structural and magnetic phase transitions in CeFeAsO(1-x)F(x) as the system is tuned from a semimetal to a high-Tc superconductor through fluorine (F) doping, x. In the undoped state, CeFeAsO develops a structural lattice distortion followed by a collinear antiferromagnetic order with decreasing temperature. With increasing fluorine doping, the structural phase transition decreases gradually and vanishes within the superconductivity dome near x=0.10, whereas the antiferromagnetic order is suppressed before the appearance of superconductivity for x>0.06, resulting in an electronic phase diagram remarkably similar to that of the high-Tc copper oxides. Comparison of the structural evolution of CeFeAsO(1-x)F(x) with other Fe-based superconductors suggests that the structural perfection of the Fe-As tetrahedron is important for the high-Tc superconductivity in these Fe pnictides.     

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.     

Hole-doping routes for understanding the relationship between atomic arrangements and superconductivity properties in multi-layered copper oxides

Since the ‘snow’ thawed in the period of 1984–86, the ‘village’ of layered mixed-valent copper oxides has been packed with ‘children’ playing vigorously ‘games’ related to high-T_c superconductivity. In this contribution, we utilize materials chemistry ‘rules’ for the game, and present a systematic view for controlling the charge inhomogeneity over the crystals of layered copper oxides and tailoring their superconductivity properties. We start with a generalized scheme of ‘homologous series’ for categorizing the high-T_c superconductive and related materials. Applying the bond–valence–sum scheme to the crystals of homologous series converges to the concept of ‘hole-doping routes’. Functioning of the different routes for doping the CuO₂ planes with holes is demonstrated for representative high-T_c superconductive materials. Subsequently, the hole distribution over the crystal is shown to be controlled by the choice of the hole-doping routes. It follows further that the control of the hole distribution is essential for tailoring the high-T_c superconductivity properties. A ‘road map’ for refining the concepts of hole-doping routes and hole distribution is presented so that they may be utilized for not only tailoring the high-T_c superconductors but also understanding the nature of high-T_c superconductivity.     

Three-dimensionality of field-induced magnetism in a high-temperature superconductor

Many physical properties of high-temperature superconductors are two-dimensional phenomena derived from their square-planar CuO2 building blocks. This is especially true of the magnetism from the copper ions. As mobile charge carriers enter the CuO2 layers, the antiferromagnetism of the parent insulators, where each copper spin is antiparallel to its nearest neighbours, evolves into a fluctuating state where the spins show tendencies towards magnetic order of a longer periodicity. For certain charge-carrier densities, quantum fluctuations are sufficiently suppressed to yield static long-period order, and external magnetic fields also induce such order. Here we show that, in contrast to the chemically controlled order in superconducting samples, the field-induced order in these same samples is actually three-dimensional, implying significant magnetic linkage between the CuO2 planes. The results are important because they show that there are three-dimensional magnetic couplings that survive into the superconducting state, and coexist with the crucial inter-layer couplings responsible for three-dimensional superconductivity. Both types of coupling will straighten the vortex lines, implying that we have finally established a direct link between technical superconductivity, which requires zero electrical resistance in an applied magnetic field and depends on vortex dynamics, and the underlying antiferromagnetism of the cuprates.     

Evolution and control of oxygen order in a cuprate superconductor

The disposition of defects in metal oxides is a key attribute exploited for applications from fuel cells and catalysts to superconducting devices and memristors. The most typical defects are mobile excess oxygens and oxygen vacancies, which can be manipulated by a variety of thermal protocols as well as optical and d.c. electric fields. Here we report the X-ray writing of high-quality superconducting regions, derived from defect ordering, in the superoxygenated layered cuprate, La?CuO(4+y). Irradiation of a poor superconductor prepared by rapid thermal quenching results first in the growth of ordered regions, with an enhancement of superconductivity becoming visible only after a waiting time, as is characteristic of other systems such as ferroelectrics, where strain must be accommodated for order to become extended. However, in La?CuO(4+y), we are able to resolve all aspects of the growth of (oxygen) intercalant order, including an extraordinary excursion from low to high and back to low anisotropy of the ordered regions. We can also clearly associate the onset of high-quality superconductivity with defect ordering in two dimensions. Additional experiments with small beams demonstrate a photoresist-free, single-step strategy for writing functional materials.     

Studies of structural,optical, and electrical properties associated with defects in sodium-doped copper oxide (CuO/Na) nanostructures

In the present paper, we report a detailed study on the sodium (Na) doping-induced modifications in the copper oxide (CuO) nanostructure and its properties. A facile and sustainable sol–gel synthesis approach was employed for the preparation of high-quality pristine CuO- and Na-doped CuO nanostructures(1.0, 3.0, 5.0 and 7.0 mol% doping levels, CuO/Na) with controlled shape and composition. Due to the remarkable difference in the ionic radii of Cu²⁺ (0.73 Å) and Na⁺ (1.02 Å), Na⁺ substitution in place of Cu²⁺ generates strain/distortions in CuO lattice. The XRD analysis reveal the structural alteration from monoclinic to cubic symmetry with increase in doping level and also reveal the phase purity up to 3% doping level, and beyond this (i.e., for 5 and 7% doping level) small amount of impurity phase corresponding to Na₂O was observed. The FTIR results further confirmed the presence of the Na–Cu–O stretching vibrations at higher Na-doped samples. Morphology of the samples indicates that the Na-doped CuO nanostructures exhibit less agglomeration compared to pristine CuO nanoparticles. The presence of Na in CuO lattice were found to greatly enhances optical and electrical properties owing to the formation of defects like copper vacancies and oxygen vacancies at the grain boundaries of the nanoparticles with increased doping of Na.     

钴掺杂氧化铜/可见光协同活化PMS降解罗丹明B及其机理研究

元素掺杂是提升催化剂性能的重要方法。研究采用快速沉淀法制备了钴掺杂氧化铜(Co-doped CuO)纳米催化材料,在可见光条件下,20 min内其活化的过氧硫酸氢钾复合盐(PMS)对罗丹明B染料的降解率达到96%以上,远优于同等条件制备的CuO。本研究还考察了溶液pH、染料初始浓度、催化剂用量等对降解效率影响。钴掺杂后氧化铜纳米颗粒由三维针梭状结构转变为近二维薄带状结构。同时钴掺杂提高了CuO的平带电位进而提升了电荷转移效率。XPS及EPR结果表明钴掺杂能够提高CuO的氧空位含量进而提升催化活性。捕获剂实验结果表明反应过程中的主要活性物种为空穴(h⁺_VB),且羟基自由基(·OH)、单线态氧(¹O₂)、超氧自由基(·O₂^–)、硫酸根自由基(SO₄^–·)也参与了降解反应。最后,本文初步阐明了Co-doped CuO协同可见光活化PMS降解有机污染物的反应机理。     

Laser synthesis of copper oxides 2D structures with high Seebeck coefficient and high thermoelectric figure of merit

Semiconductors are considered promising materials of thermo-converters and thermo-sensors. The photons generated by a KrF laser source (λ?=?248 nm, τ_FWHM?≤?25 ns) were used for the synthesis of stable crystalline phases of copper oxides 2D single-layered semiconductor structures via the reaction of ablated copper atoms with oxygen molecules by reactive pulsed laser deposition (RPLD). Obtained semiconductor 2D single-layered structures of (23–75) nm thickness were deposited on 293, 600 or 800 K?<?100?>?Si substrates in oxygen atmosphere at 1.0, 3.0 and 5.0 Pa. X-ray diffraction analysis evidenced polycrystalline structures of the deposits reviled of two crystalline semiconductor phases CuO (002) and CuO(111). Semiconductor temperature trend was detected with variable energy band gap (E_g) in the range of (0.10–1.5) eV depending on substrate temperature, oxygen pressure and structure thickness. The optimum conditions were found out when the S coefficient was being homogeneously increased from 2.0 mV/K up to 10.5 mV/K and thermoelectric figure of merit (ZT) from 0.0035 up to 9.0 in the range of (290–340) K. The interpretation of behaviour for these 2D single-layered semiconductor structures of thermoelectric properties was provided. Obtained 2D single-layered structures based on copper oxides with such high S coefficient and high ZT are exceptionally strong candidates for a new effective thermo-sensors and thermo-converters operating at moderate temperature. Moreover, RPLD serves as an up to date method for the synthesis of 2D structures with such superior thermo-sensor and thermo-converter properties. These 2D single-layered structures based on copper oxides are among the most promising candidates based on non-toxic precursors for “green” technology fabrication of efficient thermo-sensors and thermo-converters.

     

Preparation of nanocrystalline Cu<Subscript>2</Subscript>O thin film by pulsed laser deposition

In this paper, the synthesis of nanocrystalline copper oxides Cu₂O and CuO thin films on glass substrates using a pulsed 532 nm Nd:YAG laser is presented. Deposition of films is achieved at two different substrate temperatures. The influence of substrate temperature on the structural and optical properties of copper oxide films are discussed and analyzed. The X-ray diffraction (XRD) results show that the deposited films are crystalline in nature. Films prepared at 300 °C substrate temperature were Cu₂O and has (111) and (200) diffracted peaks, while films grown at 500 °C were CuO and has (111) and (020) planes. The morphology of deposited films were characterized by scanning electron microscope (SEM) and atomic force microscope (AFM). The optical energy gap of Cu₂O and CuO films have been determined and found to be 2.04 and 1.35 eV respectively.     

Oxygen Isotope Effect in Cuprates Results from Polaron-induced Superconductivity

The planar oxygen isotope effect coefficient measured as a function of hole doping in the Pr- and La-doped YBa₂Cu₃O₇ (YBCO) and the Ni-doped La_1.85Sr_0.15CuO₄ (LSCO) superconductors quantitatively and qualitatively follows the form originally proposed by Kresin and Wolf [Phys. Rev. B 49, 3652 (1994)], which was derived for polarons perpendicular to the superconducting planes. Interestingly, the inverse oxygen isotope effect coefficient at the pseudogap temperature also obeys the same formula. These findings allow the conclusion that the superconductivity in YBCO and LSCO results from polarons or rather bipolarons in the CuO₂ plane. The original formula, proposed for the perpendicular direction only, is obviously more generally valid and accounts for the superconductivity in the CuO₂ planes.     

Photo-Deformation Effect in HTSC Copper Oxides

HTSC copper oxides demonstrate phase separation with typical size of coherent areas from few to hundreds unit cells. Radiation leads to photo-doping of these materials, the domain structure and doping state of the main fracture change at the same time. As charge carrier concentration in CuO₂ planes controls their degree of distortion (“buckling”), so radiation causes photo-deformation effect in considered materials. Photo-deformation effect has been investigated for Bi₂Sr₂Ca₂Cu₃O _x with different oxygen content. It has been shown that radiation of both under-doped and optimally-doped samples leads to the increase of material fraction with optimally doped CuO₂-planes due to charge exchange with reservoir block BiO _x .     

Study of CuO Nano-particles/CuTl-1223 Superconductor Composite

Synthesis and characterization of (CuO) _x /Cu_0.5Tl_0.5Ba₂Ca₂Cu₃O_10?δ ; {(CuO) _x /CuTl-1223} composites with x=0 %, 10 %, 15 % and 20 % have been reported. The fluctuations induced conductivity (FIC) analysis of (CuO) _x /CuTl-1223 composite has been carried out using Aslamazov-Larkin (AL) and Lawrence-Doniach (LD) models in the temperature regime well above the critical temperature (T>T _c ). The electrical resistivity versus temperature curves of as-prepared and oxygen post-annealed (CuO) _x /CuTl-1223 composite were fitted by using above mentioned models to extract the microscopic parameters such as zero temperature coherence length along c-axis{ξ _c (0)}, inter-layer coupling (J), dimensional critical exponent (λ) and inter-grain coupling constant (α) etc. It has been observed that the cross-over temperature (T _o ) fits very well the two-dimensional (2D) and three-dimensional (3D) AL equations and shifts towards the lower temperature regime with the enhanced weight percentage of CuO nano-particles. The shifting of AL 3D region to higher temperature after oxygen post-annealing indicates the restoration of oxygen and optimization of charge carriers in conducting CuO₂ planes. The gradual decrease in the value of inter-grain coupling constant (α) with the increase of CuO nano-particles content reflects an improvement in the inter-grain coupling resulting into an increase in the coherence length (ξ _c ) along the c-axis. Almost all superconductivity parameters have been improved after oxygen post-annealing. The suppression of superconductivity parameters in the composite with x=20 % limits the optimum doping level of CuO nano-particles in (CuO) _x /CuTl-1223 composite.     

Analysis of partial substitutions in one Cu-O layer superconductors

In the paper a model is proposed that is able to explain the superconductor volume/total volume fraction shape vs. the doping concentration for copper oxide-based superconductors with a single Cu-O layer. The results of this model are in good agreement with the experimental determinations. The model is based on a postulate—the necessity for loss of overlap of copper and oxygen orbitals for the appearance of superconductivity, i.e., a distortion of superconducting layers. The distortion is achieved by substituting a kind of ion from the parent compound with another kind of ion that have different charge in comparison with the former. The electrostatic interactions of the doped ions with the lattice lead to superconductivity.     

Roles of oxygen defects in copper oxide superconductors

Oxygen vacancy and interstitial defects can have a profound effect on the superconducting properties of copper oxide compounds. Recent work on compounds such as La₂CuO_4+x and HgBa₂CuO_4+x has provided new insight into the role of interstitial oxygen defects as a doping mechanism. The number of carriers created by each interstitial defect depends on the local defect structure. Studies of (La, Sr, Ca)₃Cu₂O_6+x with various metal compositions and metalsite ordering show that interstitial oxygen defects that form between the CuO₂ layers in this structure systematically lowerT _c and eventually destroy superconductivity. Conversely, oxygen vacancies in the CuO₂ planes have surprisingly little effect at concentrations below 3%. The infinite-layer compounds, ACuO₂, where A=La, Sr, Ca, Nd, etc., in solid-solution combinations, could offer a similar environment for the formation of interstitial oxygen defects between the CuO₂ planes, allowing interstitial oxygen defects to contribute to the doping of these compounds. However, neutron diffraction experiments on Sr_0.9La_0.1CuO₂ (T _c = 42 K) have not found any interstitial oxygen.     

Further Investigation on the Correlation between Ionization Potential and Superconductivity of Oxides

The relations of oxygen content, critical temperature TC and mean ionization potential < U0> of superconducting oxides are reported. We found that in oxides oxygen is the major element which governs their < U0> te values and thus oxygen content plays an jmportant role in their superconducting properties. As for Y(123)- and T1-systems oxygenation may improve their TC values, but for Biand Hg-systems oxygenation process is not so important. In Y(123), as the oxygen content increase results in the increase of TC upon < U0>to, while the substitution of Ba2+ by Sr2+ causes the decrease of TC upon < U0 >. These results may provide some clues about the superconductivity of oxides and support further that < Uo > is really a good criterion for oxide superconductivity. Two kinds of possible superconducting oxides are predicted in this paper according to the mean ionization potential criterion.     

Discovery of superconductivity in KTaO₃ by electrostatic carrier doping

Superconductivity at interfaces has been investigated since the first demonstration of electric-field-tunable superconductivity in ultrathin films in 1960(1). So far, research on interface superconductivity has focused on materials that are known to be superconductors in bulk. Here, we show that electrostatic carrier doping can induce superconductivity in KTaO(3), a material in which superconductivity has not been observed before. Taking advantage of the large capacitance of the self-organized electric double layer that forms at the interface between an ionic liquid and KTaO(3) (ref. 12), we achieve a charge carrier density that is an order of magnitude larger than the density that can be achieved with conventional chemical doping. Superconductivity emerges in KTaO(3) at 50 mK for two-dimensional carrier densities in the range 2.3 × 10(14) to 3.7 × 10(14) cm(-2). The present result clearly shows that electrostatic carrier doping can lead to new states of matter at nanoscale interfaces.     

The crystal field in rare earth based high-temperature superconductors

Neutron spectroscopy is a powerful tool to determine unambiguously the crystal-field (CF) potential in rare-earth (R) based high-T _c superconducting materials. This technique provides detailed information on the electronic ground state of the R ions which is important to understand the thermodynamic magnetic properties as well as the observed coexistence between superconductivity and long-range magnetic ordering of the R ion sublattice at low temperatures. Moreover, the decay of the antiferromagnetic state of the parent compound as well as the evolution of the superconducting state upon doping can be directly and quantitatively monitored. It is found that the observed CF spectra separate into different local components whose spectral weights distinctly depend on the doping level, i.e., there is clear experimental evidence for cluster formation. The onset of superconductivity can be shown to result from percolation which means that the superconductivity is an inhomogeneous materials property. Since the linewidths of CF transitions directly probe the static electronic susceptibility, we discuss temperature-dependent experiments of the relaxation rate of CF excitations in both optimally doped and underdoped regimes. It is shown that there is clear evidence for the opening of an electronic gap in the normal state of underdoped superconductors. Furthermore, the relaxation behavior appears to be extremely dependent upon the energy at which the static susceptibility is being probed. The main observed features can be reproduced by considering a strongly anisotropic gap function.     

Interpretation of doping phenomena in high-T c superconductors

On the basis of an indirect-exchange pairing mechanism of superconductivity we present a consistent interpretation of doping phenomena in both hole-doped as well as electron-doped high-T _c superconductors. We argue that in all these materials the unifying feature is the existence of a correlated narrow band of electron states formed due to doping. Numerous experimental evidences for the occurrence of such a band (reflectivity, thermoelectric power, electrical resistivity, X-ray absorption, point-contact tunneling etc.) now exist. Assuming the existence of such a band it was earlier shown that the indirect-exchange (superexchange) coupling between electrons in this band via closed-shell oxygen anions is attractive in the s-wave channel and leads to high-T _c superconductivity. Within the framework of this pairing mechanism, recent doping experiments (for both types of doping) can be given a unified interpretation. In addition, definitive predictions of the doping conditions under which critical temperatures are expected to enhance, are made.     

Nanocrystalline copper doped zinc oxide produced from copper doped zinc hydroxide nitrate as a layered precursor

Undoped and copper doped nanostructured zinc oxides were synthesized by using a series of synthetic layered material, undoped and copper doped zinc hydroxide nitrates at various molar percentages of copper (2–10) within the layers as precursors. The layered materials were heat-treated at 500 °C to produce zinc oxide nanostructures with crystallite sizes in the range of 23–35 nm. Optical studies of the nanostructured copper doped zinc oxides showed the decrease in band gap with increasing content of the doping agent, copper.     

Large enhancement of the thermopower in Na(x)CoO2 at high Na doping

Research on the oxide perovskites has uncovered electronic properties that are strikingly enhanced compared with those in conventional metals. Examples are the high critical temperatures of the cuprate superconductors and the colossal magnetoresistance in the manganites. The conducting layered cobaltate Na(x)CoO2 exhibits several interesting electronic phases as the Na content x is varied, including water-induced superconductivity and an insulating state that is destroyed by field. Initial measurements showed that, in the as-grown composition, Na(x)CoO2 has moderately large thermopower S and conductivity sigma. However, the prospects for thermoelectric cooling applications faded when the figure of merit Z was found to be small at this composition (0.60.75, S undergoes an even steeper enhancement. At the critical doping x(p) approximately 0.85, Z (at 80 K) reaches values approximately 40 times larger than in the as-grown crystals. We discuss prospects for low-temperature thermoelectric applications.