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Iron-based layered superconductor La[O1-xFx]FeAs (x= 0.05-0.12) with Tc = 26 K
Abstract
We report that a layered iron-based compound LaOFeAs undergoes superconducting transition under doping with F- ions at the O2- site. The transition temperature (Tc) exhibits a trapezoid shape dependence on the F- content, with the highest Tc of ∼26 K at ∼11 atom %.
... Subsequently, in another copper oxide Y-Ba-Cu-O system, the critical temperature of the phase transition was above 90K [9], higher than the liquid nitrogen temperature for the first time. In 2008, iron-based superconductors were discovered to exist in the oxide LaFeAs[O1-xFx] [10][11][12][13], and the transition temperature has ever reached 43 K in SmFeAs[O0.9F0.1] [14] and the highest one reached 55K in SmFeAsO~0.85 ...
... It reveals that the Debye temperature θD is not only dependent on the acoustic speed v but also dependent on the Fermi momentum PF. Furthermore, the elastic waves in cubic crystals, based on the lattice dynamics, give both the speeds of longitudinal and transverse waves in the [10] direction proportional to −1 2 ⁄ , where M is the atomic mass and ρ is the mass density of the material [2]. The linear dispersion relation = is also mentioned where v comes from the average of the long-wavelength phase velocities of the three acoustic modes that [6] ...
... So, we can express = √ ⁄ in Eqs. (10) and (11) where β is the other constant. Therefore, the Debye temperature can be expressed by ...
We study the critical temperature Tc and Debye temperature θD for 12 single-element superconductors and find a relation TcθD=const. for every two metals of each group in the periodic table of elements. Especially, even the Tc ratio of Hg to Zn is as high as 4.77 but their TcθD are still close to each other. This formula is beneficial to explain why the single-element insulator at room temperature will not show the superconducting characteristic since it has no free-electron gas. The relation reveals that Tc is inversely proportional to ωD or θD, and also inversely proportional to the acoustic velocity v and the momentum of the Fermi electrons PF. Furthermore, two special formulae are also found for all chosen transition metals and post-transition metals, respectively, and show an important thing that the critical temperature Tc is also related to the valence electrons or the largest positive oxidation number in those superconductors.
... Despite the abundance of identified superconductors, platforms remain scarce, where superconductivity manifests across a series of materials. This is particularly true in compounds involving d electrons [2][3][4][5][6][7][8][9][10][11][12]. Recently, seven materials from a family of d-element-rich compounds, Sc 6 M Te 2 , have been identified as possessing bulk superconductivity [12]. ...
... Thus, we use the SCPH theory to obtain a stable phonon dispersion with real frequencies. Also, equations (1) and (3) indicate that imaginary frequency phonons would give an ill-defined EPC matrix element and Eliashberg spectral function, which should be handled carefully for phonon-mediated superconductors [21-24, 38, 39]. While one may artificially set the imaginary phonon frequencies to zero or their absolute values or introduce a sizeable electronic smearing parameter [40,41], in this work, we employ a treatment that uses the SCPH theory. ...
We perform a systematic ab initio study on phonon-mediated superconductivity in the transition-metal-based superconductors Sc$_6M$Te$_2$ ($M$ = Fe, Co, Ni). Firstly, our charge analysis reveals significant electron transfer from Sc to $M$ due to the substantial difference in the electronegativity, filling the 3$d$ orbitals of $M$ and suppressing magnetic instability. Secondly, we show that Sc$_6$FeTe$_2$ exhibits strong lattice anharmonicity. Moreover, for $M =$ Fe and Co, we find low-frequency soft phonon bands of $M$ which can be interpreted as "rattling phonons" in the framework formed by Sc. While not observed in the case of $M=$ Ni, the rattling phonons give rise to a prominent peak or plateau in the Eliashberg spectral function and enhance the pairing instability. By reproducing the experimental trend of superconducting transition temperatures, our study underscores the potential of designing phonon-mediated superconductors by strategically combining non-superconducting and magnetic transition-metal elements.
... More findings on other metal compounds, such as the iron-based superconductor [14] and the recently discovered nickelate superconductor [15], were falling behind the Tc of the cuprate's 135 K mark. A new report illustrated a relatively high Tc of 80 K for a superconducting nickelate La3Ni2O7. ...
... We hope that the new technology utilizing advanced equipment and new procedures of chemical syntheses can ultimately resolve the synthetic problems and obtain much purer Th compounds than those made many years ago. The growth of the single crystals for the Th salts can be attempted, on the purpose to prepare the highly qualified testing 14 samples for the superconducting assessments, such as the growth through utilizing the traveling solvent floating zone (TSFZ) method. Crystallization would not only further purify the materials but also prepare more suitable single crystals for the sample's research. ...
An innovative superconducting mechanism was developed based on a patented discovery of room temperature superconductivity in several thorium (Th) salts or compounds. The rationale behind this discovery is through the experimental results about the unique electrical and magnetic properties of these Th salts. Accordingly, this new superconducting mechanism for these Th salts was obtained through the analyses of the compounds’ molecular configurations and the packings of their relevant crystal structures. Owing to the distinctive properties of the Th compounds in this discovery, we further established a different electron pairing scheme as opposed to the pairing of the Cooper pairs. Compared to the long distance electron pairing of the Cooper pairs, the new electron pairing in our mechanism is constructed by two closely correlated electrons that reside on the same atomic orbital of the Th cation as an electron lone pair. In this new superconducting mechanism, the Th cation is sitting on a relevant crystallographic lattice arrangement or a network of the relevant Th compounds. This network is constructed by many Th cations along certain crystallographic lattice arrangement where each Th cation contains one electron lone pair. This network allows the electron lone pairs on the Th cations to be delocalized over the network, “in pair”, and thus, reveals the electron lone pair in presence in a long distance fashion. The closely correlated electron lone pairs hop on this network and thus, construct the supercurrent over the network. This explanation about the superconductivity laid the basis for our superconducting mechanism. All these conclusions came from the previous studies about these compounds that illustrated their unusual features of a coexistence of high electrical conductivity and diamagnetism. These electrical and magnetic properties fall in line with that of superconductors but, surprisingly, these Th compounds have such properties at the ambient condition, meaning under room temperature and atmosphere pressure. We expect that this mechanism may be appropriate to be utilized to other superconducting cases, either conventional or unconventional, and therefore, can be adapted to describe the superconducting phenomenon for other superconductors.
... More findings on other metal compounds, such as the iron-based superconductor [14] and the recently discovered nickelate superconductor [15], were falling behind the Tc of the cuprate's 135 K mark. A new report illustrated a relatively high Tc of 80 K for a superconducting nickelate La3Ni2O7. ...
... 14 applications but also offer copious knowledge for the comprehensive understanding of the superconducting phenomenon in a manner not only to develop new approaches to design and synthesize new high or room temperature superconductors but also to pave a way for fully understanding the mechanism of superconductivity. The work with the new superconducting mechanism may shed light on realizing the over 100 year dream of obtaining the applicable superconductors. ...
An innovative superconducting mechanism was developed based on a patented discovery of room temperature superconductivity in several thorium (Th) salts or compounds. The rationale behind this discovery is through the experimental results about the unique electrical and magnetic properties of these Th salts. Accordingly, this new superconducting mechanism for these Th salts was obtained through the analyses of the compounds’ molecular configurations and the packings of their relevant crystal structures. Owing to the distinctive properties of the Th compounds in this discovery, we further established a different electron pairing scheme as opposed to the pairing of the Cooper pairs. Compared to the long distance electron pairing of the Cooper pairs, the new electron pairing in our mechanism is constructed by two closely correlated electrons that reside on the same atomic orbital of the Th cation as an electron lone pair. In this new superconducting mechanism, the Th cation is sitting on a relevant crystallographic lattice arrangement or a network of the relevant Th compounds. This network is constructed by many Th cations along certain crystallographic lattice arrangement where each Th cation contains one electron lone pair. This network allows the electron lone pairs on the Th cations to be delocalized over the network, “in pair”, and thus, reveals the electron lone pair in presence in a long distance fashion. The closely correlated electron lone pairs hop on this network and thus, construct the supercurrent over the network. This explanation about the superconductivity laid the basis for our superconducting mechanism. All these conclusions came from the previous studies about these compounds that illustrated their unusual features of a coexistence of high electrical conductivity and diamagnetism. These electrical and magnetic properties fall in line with that of superconductors but, surprisingly, these Th compounds have such properties at the ambient condition, meaning under room temperature and atmosphere pressure. We expect that this mechanism may be appropriate to be utilized to other superconducting cases and therefore, can be adapted to describe the superconducting phenomenon for other superconductors.
... More findings on other metal compounds, such as the iron-based superconductor [14] and the recently discovered nickelate superconductor [15], were falling behind the Tc of the cuprate's 135 K mark. A new report illustrated a relatively high Tc of 80 K for a superconducting nickelate La3Ni2O7. ...
... 14 own their room temperature superconductivity. We also noticed that the stable superconducting states for these Th salt based superconductors can be maintained without a need to administer any external energy, such as high pressure, low temperature or radiation, etc. ...
An innovative superconducting mechanism was developed based on a patented discovery of room temperature superconductivity in several thorium (Th) salts or compounds. The rationale behind this discovery is through the experimental results about the unique electrical and magnetic properties of these Th salts. Accordingly, this new superconducting mechanism for these Th salts was obtained through the analyses of the compounds’ molecular configurations and the packings of their relevant crystal structures. Owing to the distinctive properties of the Th compounds in this discovery, we further established a different electron pairing scheme as opposed to the pairing of the Cooper pairs. Compared to the long distance electron pairing of the Cooper pairs, the new electron pairing in our mechanism is constructed by two closely correlated electrons that reside on the same atomic orbital of the Th cation as an electron lone pair. In this new superconducting mechanism, the Th cation is sitting on a relevant crystallographic lattice arrangement or a network of the relevant Th compounds. This network is constructed by many Th cations along certain crystallographic lattice arrangement where each Th cation contains one electron lone pair. This network allows the electron lone pairs on the Th cations to be delocalized over the network, “in pair”, and thus, reveals the electron lone pair in presence in a long distance fashion. The closely correlated electron lone pairs hop on this network and thus, construct the supercurrent over the network. This explanation about the superconductivity laid the basis for our superconducting mechanism. All these conclusions came from the previous studies about these compounds that illustrated their unusual features of a coexistence of high electrical conductivity and diamagnetism. These electrical and magnetic properties fall in line with that of superconductors but, surprisingly, these Th compounds have such properties at the ambient condition, meaning under room temperature and atmosphere pressure. We expect that this mechanism may be appropriate to be utilized to other superconducting cases and therefore, can be adapted to describe the superconducting phenomenon for other superconductors.
... [4] These properties can be tailored through the different anionic characteristics of the mixed anions, including charge, ionic radius, electronegativity, and polarizability. [4] Currently, mixed-anion compounds are largely investigated in various fields, such as catalysis, [5] battery electrodes, [6] superconductivity, [7] and optical applications. [8] For instance, oxynitride, CaTaO2N and LaTaON2 perovskites present tuneable colours for optical applications stemming from their narrowing band gap governed by the mixed anion with the lowest electronegativity. ...
Mixed-anion compounds, which incorporate multiple types of anions into materials, displays tailored crystal structures and physical/chemical properties, garnering immense interests in various applications such as batteries, catalysis, photovoltaics, and thermoelectrics. However, detailed studies regarding correlations between crystal structure, chemical bonding, and thermal/vibrational properties are rare for these compounds, which limits the exploration of mixed-anion compounds for associated thermal applications. In this work, we investigate the lattice dynamics and thermal transport properties of the metal chalcohalides, CuBiSCl2. A high-purity polycrystalline CuBiSCl2 sample, successfully synthesized via modified solid-state synthetic method, exhibits a low lattice thermal conductivity of 0.9-0.6 W m-1 K-1 from 300 to 573 K. By combining various experimental techniques including 3D electron diffraction with theoretical calculations, we elucidate the origin of low lattice thermal conductivity in CuBiSCl2. The stereo-chemical activity of the 6s2 lone pair of Bi3+ favors an asymmetric environment with neighboring anions involving both short and long bond lengths. This particularity often implies weak bonding, low structure dimensionality, and strong anharmonicity, leading to low lattice thermal conductivity. In addition, the strong two-fold linear S-Cu-S coordination with weak Cu -- Cl interactions induces large anisotropic vibration of Cu or structural disorder, which enables strong phonon-phonon scattering and decreases lattice thermal conductivity. The investigations into lattice dynamics and thermal transport properties of CuBiSCl2 broadens the scope of the existing mixed-anion compounds suitable for the associated thermal applications, offering a new avenue for the search of low thermal conductivity materials in low-cost mixed-anion compounds.
... The couplings between the gradients of these order parameters give vortex solutions, and thus their lattices, interesting new properties [12,13]. Examples of such materials include Sr 2 RuO 4 [14,15], M gB 2 [16], heavy fermion compounds such as U P t 3 [17] and iron based superconductors [18]. Our new method was motivated by, and hence naturally lends itself to, probing the lattices of these unconventional materials. ...
A method is developed to compute minimal energy vortex lattices in a general Ginzburg-Landau model of a superconductor subjected to an applied magnetic field. The model may have any number of components and may be spatially anisotropic. Both the period vectors of the vortex lattice, and the orientation of its vortex lines, are determined dynamically; no assumptions are made about them. Methods to compute the first and second critical magnetic fields, $H_{c_1}$ and $H_{c_2}$, in this class of models are also developed. These methods are applied to a simple anisotropic single-component model, and to an anisotropic two-component model of strong current theoretical interest (a so-called $s+id$ model). It is found, in both cases, that at low applied field the vortex lines can tilt very significantly away from the direction of the applied field (by as much as $40^o$ for the single-component and $30^o$ for the $s+id$ model). The optimal lattice in the $s+id$ model is qualitatively very different from the conventional triangular Abrikosov lattice, exhibiting a phase transition from a system of Skyrmion chains when the external field is orthogonal to the basal plane to a deformed Abrikosov lattice when applied in the basal plane.
... One of central research topics in modern materials science and condensed matter physics is the realization of exotic physical properties in two-dimensional (2D) or quasi-2D electron systems, as represented by the quantum Hall effect in graphene, massless Dirac transport in topological insulator (TI) surfaces [1,2,3,4,5], and superconductivity intertwined with spin, charge, and orbital orders in layered high-temperature (T c ) superconductors and transition-metal dichalcogenides [6,7,8,9]. A key to achieve such exotic physical properties partially lies on the technical advancements in obtaining atomically thin flakes and ultrathin films that utilizes, e.g., mechanical exfoliation of bulk crystals [10], chemical vapor deposition (CVD), and molecular-beam epitaxy (MBE) [11,12]. ...
Topotactic chemical reaction (TCR) is a chemical process that transforms one crystalline phase to another while maintaining one or more of the original structural frameworks, typically induced by the local insertion, removal, or replacement of atoms in a crystal. The utilization of TCR in atomic-layer materials and surfaces of bulk crystals leads to exotic quantum phases, as highlighted by the control of topological phases, the emergence of two-dimensional (2D) superconductivity, and the realization of 2D ferromagnetism. Advanced surface-sensitive spectroscopies such as angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) are leading techniques to visualize the electronic structure of such exotic states and provide us a guide to further functionalize material properties. In this review article, we summarize the recent progress in this field, with particular emphasis on intriguing results obtained by combining spectroscopies and TCR in thin films.
... Iron-based superconductors [1], discovered in 2008, are considered to be a new platform to study high-temperature superconducting mechanisms and potential candidates for practical applications [2][3][4]. From the crystalline structure point of view, iron-based superconductors can be divided into two major categories, iron-pnictides with the [FeAs] −1 layer and iron-chalcogenides with the electroneutral FeSe layer. ...
Iron–chalcogenide superconductors continue to captivate researchers due to their diverse crystalline structures and intriguing superconducting properties, positioning them as both a valuable platform for theoretical investigations and promising candidates for practical applications. This review begins with a comprehensive overview of the fabrication techniques employed for various iron–chalcogenide superconductors, accompanied by a summary of their phase diagrams. Subsequently, it delves into the upper critical field, anisotropy, and critical current density. Furthermore, it discusses the successful fabrication of meters-long coated conductors and explores their applications in superconducting radio-frequency cavities and coils. Finally, several prospective avenues for future research are proposed.
... Since the discovery of cuprates in 1986 [1], high temperature superconductivity has been a focus of both theoretical and experimental investigations in condensed matter physics. After twenty-two years, another family of high temperature superconductors, i.e. iron-based superconductors, was also found in 2008 [2]. Such the superconductors with high transition temperatures usually possess the layered crystal structures consisting of the conducting planes, e.g. the CuO 2 planes in the cuprates and the Fe-As(Te, Se) layers in the iron-based superconductors. ...
Motivated by recent scanning tunneling microscopy experiments on Fe atomic line defect in iron-based high temperature superconductors, we explore the origin of the zero energy bound states near the endpoints of the line defect by employing the two-orbit four-band tight binding model. With increasing the strength of the Rashba spin-orbit coupling along the line defect, the zero energy resonance peaks move simultaneously forward to negative energy for s+− pairing symmetry, but split for s++ pairing symmetry. The superconducting order parameter correction due to As(Te, Se) atoms missing does not shift the zero energy resonance peaks. Such the zero energy bound states are induced by the weak magnetic order rather than the strong Rashba spin-orbit coupling on Fe atomic line defect.
... HgBaCaCuO) were found to have the superconducting transition temperature as high as 135 K at normal pressure [11][12][13][14]. At the same time, iron-based layered materials, such as Sm[O 1−x F x ]FeAs, have T c of about 55 K [15][16][17]. Cu and Fe-based compounds are unconventional superconductors that do not follow the BCS theory, and due to their complex electronic structures and strong anharmonic effect and relatively heavy atomic species, it is pretty challenging to disentangle the inner electrons pairing mechanism. On the other hand, hydrides are composed of light chemical elements and have a larger Debye temperature. ...
Leveraging the progress of first-principles modelings in understanding the mechanisms of superconductivity of materials, in this work we investigate the phonon-mediated superconducting properties of transition metal diborides. We report that TaB2 and NbB2 show superconducting transition temperatures as high as 27.0 and 26.0 K at ambient conditions, respectively, comparable with those obtained for CaB2 or MgB2. By mode-by-mode analysis of the electron-phonon-coupling, we reveal that the high superconducting temperature of transition metal diborides is due mainly to the strong coupling between d electrons of the transition metals and the acoustic phonon modes along out-of-plane vibrations. This fact is distinct from that of CaB2 or MgB2, where the superconductivity stems mainly from the boron px and py orbitals, which couple strongly to the optical phonon modes dominated by in-plane B atomic vibrations. Further, we find that transition metal diborides present only a superconducting gap at low temperatures, whereas CaB2 or MgB2 are double superconducting gap superconductors. In addition, we investigate the strain effect on the superconducting transition temperatures of diborides, predicting that Tc can be further enhanced by optimizing the phonon and electronic interactions. This study sheds some light on the exploring high Tc boron-based superconductor materials.
... In 2001, Japanese scientists discovered a new superconducting material, magnesium diboride (MgB2), with a critical temperature of 39 K [16]. In 2008, Japanese and Chinese scientists discovered a new iron-based superconducting material (LaFeAsO1-xFx) [17]. ...
This paper reviews the history of superconductivity first, from the discovery of mercury superconductivity by Onnes to the proposal of the BCS theory, and then to the continuous emergence of high-temperature superconducting materials. This paper points out the limitations of the BCS theory in explaining the phenomenon of high-temperature superconductivity, and proposes a new research method: to explore the nature of superconductivity by analyzing the relationship between the atomic weight and atomic radius of superconductors. In this paper, the statistical analysis of the data of a variety of known superconductors is carried out, and it is found that there is a linear relationship between the critical temperature and the square root of the atomic weight and the atomic radius of the relevant elements, which is in line with the isotopic effect theory of superconductors. Finally, the future direction of high-temperature superconductivity theory is prospected, and it is hoped that this method can provide theoretical guidance for the preparation of high-temperature superconductors or room-temperature superconductors.
... Finally, we discuss an application to s ±wave paring states in FeSCs. After the discovery of the superconductivity of F-doped LaFeAsO in 2008 [62], several related families of FeSCs have been discovered [63][64][65][66][67][68][69][70]. The superconducting states of FeSCs are expected from their high T c to be an unconventional pairing such as d-wave and s ± -wave pairings, and the antiferromagnetic spin fluctuation theory supports an s ± pairing [71,72]. ...
We show that spin-singlet $s$-wave multi-band superconductors have a topological phase protected by rotation symmetry and time-reversal symmetry without spin-orbit coupling in two and three dimensions. This topological phase, an example of a representation-protected topological phase, has a $\mathbb{Z}_2$ topological index and is stable as long as the bands at the Fermi energy are formed by a doublet of orbital states with finite angular momenta. In the limit of weak superconducting pair potential, the $\mathbb{Z}_2$ index gives a Fermi-surface formula and is related to the winding number of three-dimensional strong topological superconductors of class CI. We present a model of a topological $s_{\pm}$-wave superconductor that has gapless surface states with a quadratic dispersion and suggest a connection with iron-based superconductors.
... These compounds can be chemically doped or subjected to external pressure to induce superconductivity [4]. The discovery of superconductivity in iron-pnictide compounds [5][6][7][8], which emerges in close proximity to magnetic instability [9][10][11][12], opens up new avenues for studying quantum critical phenomena in systems with multiple order parameters. The presence of spin-density wave (SDW) and superconducting (SC) orders in ironpnictide superconductors (FeSC) [13][14][15][16][17][18][19][20][21][22] suggests that the SDW transition line extends into the superconducting state. ...
We hereby present a comprehensive study of polycrystalline YbFe2As2 which includes detailed investigation on the specific heat capacity and Raman spectroscopy measurements at high magnetic fields (up to 16 T) and low temperatures. Low temperature x-ray diffraction investigations at 30 K on YbFe2As2 reveal an orthorhombic unit cell structure, which indicates structural transitions from monoclinic to orthorhombic as temperature decreases. In specific heat capacity measurements, a slope change is observed around 70 K. It may be a signature of spin density wave (SDW) phase transition. The specific heat data has been fitted in the temperature ranging from 20 to 90 K. It has been found that linear coefficient of specific heat (γ) is quite enhanced suggesting the existence of quasi particles with heavy effective mass. Its value changes slowly with magnetic field below 70 K. Raman spectra have been studied in the temperature range of 4 to 300 K. It is observed that there is a trend of shifting the B1G peak position in Raman spectra on the higher side as the temperature decreases. A change of slope in shift of B1G versus temperature plot is noticed around 70 K suggesting phase transition which is consistent with specific heat and previous resistivity measurements. Overall, our results provide new insights into the physical properties and phase transitions of YbFe2As2.
... Since the discovery of cuprates in 1986 [1], high temperature superconductivity has been a focus of both theoretical and experimental investigations in condensed matter physics. After twenty-two years, another family of high temperature superconductors, i.e. iron-based superconductors, was also found in 2008 [2]. Such the superconductors with high transition temperatures usually possess the layered crystal structures consisting of the conducting planes, e.g. the CuO 2 planes in the cuprates and the Fe-As(Te, Se) layers in the iron-based superconductors. ...
Motivated by recent scanning tunneling microscopy experiments on Fe atomic line defect in iron-based high temperature superconductors, we explore the origin of the zero energy bound states near the endpoints of the line defect by employing the two-orbit four-band tight binding model. With increasing the strength of the Rashba spin-orbit coupling along the line defect, the zero energy resonance peaks move simultaneously forward to negative energy for $s_{+-}$ pairing symmetry, but split for $s_{++}$ pairing symmetry. The superconducting order parameter correction due to As(Te, Se) atoms missing does not shift the zero energy resonance peaks. Such the zero energy bound states are induced by the weak magnetic order rather than the strong Rashba spin-orbit coupling on Fe atomic line defect.
... The experimental discovery of high transition temperature T c in superconducting oxides [1] and the subsequent discoveries of strontium ruthenate [2], magnesium diboride [3], and iron pnictides [4,5] have motivated an intense theoretical investigation in the superconducting properties of these materials. Several experiments have found that one of these compounds, magnesium diboride (MgB 2 ), with a transition temperature of ≈ 40 K [6], has two superconducting gaps [7][8][9][10][11][12][13] and, consequently, was classified as a two-band superconductor (SC) [14,15]. ...
We investigate the effects of disorder in a hybridized two-dimensional two-band s-wave superconductor model. The situation in which electronic orbitals form these bands with angular momentum such that the hybridization $V_{i,j}$ among them is antisymmetric, under inversion symmetry, was taken into account. The on-site disorder is given by a random impurity potential $W$. We find that while the random disorder acts to the detriment of superconductivity, hybridization proceeds favoring it. Accordingly, hybridization plays an important role in two-band models of superconductivity, in order to hold the long-range order against the increase of disorder. This makes the present model eligible to describe real materials, since the hybridization may be induced by pressure or doping. In addition, the regime from moderate to strong disorder, reveals that the system is broken into superconductor islands with correlated local order parameters. These correlations persist to distances of several order lattice spacing which corresponds to the size of the SC-Islands.
... Iron-based superconductors (IBSs) discovered in 2008 [1] have sparked great interest thanks to their relatively high critical temperatures T c , their very high critical fields H c2 and their critical current densities J c , which match those required for applications in different sectors [2][3][4][5]. Among the many IBS families, compounds belonging to the socalled 11-system are very interesting: despite their relatively low critical temperature (around 15 K for bulk Fe(Se,Te) [2], which reach up to 21 K in thin films due to compressive strain [6]), they exhibit lower toxicity if compared to other iron-based superconductors containing arsenic [7], easy phase synthesis and high and nearly isotropic upper critical fields. ...
Iron-based superconductors are under study for their potential for high-field applications due to their excellent superconducting properties such as low structural anisotropy, large upper critical fields and low field dependence of the critical current density. Between them, Fe(Se,Te) is simple to be synthesized and can be fabricated as a coated conductor through laser ablation on simple metallic templates. In order to make all the steps simple and fast, we have applied the spark plasma sintering technique to synthesize bulk Fe(Se,Te) to obtain quite dense polycrystals in a very short time. The resulting polycrystals are very well connected and show excellent superconducting properties, with a critical temperature onset of about 16 K. In addition, when used as targets for pulsed laser ablation, good thin films are obtained with a critical current density above 105 A cm−2 up to 16 T.
... The observation of high-temperature superconductivity in Fe-based compounds challenged the theories of superconductivity [1,2]. Several ideas have been proposed, suggesting how one may revise, extend or totally rethink the existing theories [3,4]. ...
The origin of superconductivity in FeSe monolayer on SrTiO$_3$ belongs to one of the unresolved mysteries in condensed-matter physics. Here by investigation of the temperature evolution of the dynamic charge response of FeSe/SrTiO$_3$ we demonstrate that the response of the monolayer itself is nearly temperature independent. This indicates a constant Fermi surface over a wide range of temperature, in stark contrast to that of the bulk FeSe and other Fe-based superconductors. Our results, which manifest the peculiarity of the electronic structure of the FeSe monolayer, may help for a microscopic understanding of the superconductivity in Fe-chalcogenide monolayers on oxide surfaces in general.
... La2O3(Fe1-xMnx)2Se2 (x=0, 0.5) tetragonal systems (space group I4/mmm that are not superconducting) are closely related to very intensively studied superconducting iron pnictides and chalcogenides. In the known iron pnictide and chalcogenide systems the Fe ion is tetrahedrally coordinated [1,2] and the environment is non-planar. The superconductivity in the latter systems appear either under pressure or via substitutions which alters the local Fe geometry. ...
... The recent discovery of superconductivity in the NiAs-type structure high-entropy compound M 1−x Pt x Sb (M = equimolar Ru, Rh, Pd and Ir) [8] triggered our interest in the exploration of isostructural systems. Concurrently, the discovery of Fe-based pnictide and chalcogenide superconductors with high superconducting transition temperature (T c ) [10] has led to enormous excitement and opening of new perspective to develop a promising new platform to realize high-T c superconductors. Among these, FeZnSb 2 stands out -an unexplored Fe-based system that has a NiAs-type structure with disordered atomic positions of Fe and Zn (2a Wyckoff position) while Sb atoms occupy the 2c position according to [2] who first reported this system, derived from FeSb compound. ...
We present a first-principles investigation of electronic structure , lattice dynamics, and electron-phonon coupling of NiAs-type structure FeZnSb2 and the isostructural parent compound FeSb within the framework of density functional theory. The calculation on partial disordered system FeZnSb2 was performed in a fixed configuration. This hypothetical ordered structure is predicted to be superconducting. General Terms NiAs-type structure, partial disordered systems, superconductivity in Iron-based compounds
... Finally, we also compute the monopole-antimonopole deconfinement transition temperature T d as a function of the superconducting critical temperature T c . The results are plotted in Figure 5. From Figure 5, we can see that due to the magnetic energy term in Equation (23), the possible monopole and antimonopole production at T ∈ (T d , T c ) will only exist above T c ≈ 70 K, and T d increases to about 115 K as T c reaches 130 K. Since the discovery of the F-doped superconductor LaFeAsO 1−x F x with T c ∼ 26 K in 2008 [20], other Fe-based superconducting systems have also generated great interest in the scientific community. The parent compounds of these superconductors are usually semi-metallic and the contribution of all five 3d electrons to the Fermi surface manifests the multi-band electronic structure in the materials. ...
The condensed matter Bose system may contain effective monopole quasiparticles in its excitation spectrum. In this paper, we first accomplish the mapping of the two-band Ginzburg–Landau theory to the extended CP1 model, and then perform the Monte Carlo simulations on the 50×50×50 cubic lattice with periodic boundary conditions. With the numerical data of monopole density and magnetic susceptibility, we indicate that there exists a monopole–antimonopole deconfinement transition for the two-band superconducting system with the critical temperature above 70 K. We also suggest the possible detection of this new monopole plasma phase in high-Tc iron-based superconductors.
The critical current I c of single crystals of the iron pnictide superconductor BaFe 2 (As 1 − x P x ) 2 has been studied through measurements of magnetic hysteresis cycles. We show that the introduction of micrometer-scale irregularities on the surface significantly increases I c , primarily near the irreversibility magnetic field H irr . The observed increase can be attributed to a non-dissipative surface current that arises from the collective bending of the vortex lattice at the sample surface, enabled by the surface irregularities. This mechanism, which is not pinning in the proper sense, has previously been studied in clean, low- T c , metallic superconductors, but had not been investigated in Fe-based superconductors. The observed increase in I c is consistent with a theoretical estimate based on the Mathieu-Simon continuum theory of the vortex state.
Breakthrough in Superconducting Magnet Strength and Stability
Iron-based superconductors are promising for uses like quantum computing and superstrong magnets. However, improving their superconducting properties is challenging. This study aimed to improve these properties in a specific superconductor, K-doped Ba122, using Bayesian optimization. The researchers made samples under different conditions and measured their superconducting properties to refine the process. Two large disk-shaped samples were made using the best processing conditions found from data-driven and researcher-driven methods. The superconducting properties of these samples, and their ability to act as magnets, were tested at low temperatures. The results showed significant improvements, proving the optimization process’s effectiveness and resulting in an iron-based superconducting magnet with unprecedented strength. The study concludes that machine learning, especially Bayesian optimization, can significantly advance high-performance superconducting materials development. This could lead to more efficient and powerful superconducting magnets for various uses. This summary was initially drafted using artificial intelligence, then revised and fact-checked by the authors.
We investigate the electronic structure and topological properties of iron-based superconductors LaFe2As2 using density functional theory plus dynamical mean-field theory. We find that the uncollapsed tetragonal LaFe2As2 is in a nontrivial Z2 topological phase and has topological Dirac surface states near the Fermi energy which suggests there could be Majorana zero modes in the superconducting LaFe2As2. In light of the nontrivial topological properties and superconductivity of LaFe2As2 and CaKFe4As4, we predict a new iron-based compound LaBaFe4As4 and find it possesses two sets of topological Dirac surface states near the Fermi energy despite of a trivial Z2 topological index. These topological surface states are induced by a nontrivial high-order topological index Z8, a new mechanism that is distinct from all-known iron-based superconductors. Our study not only demonstrates that both LaBaFe4As4 and uncollapsed tetragonal LaFe2As2 can be good platforms for exploring topological superconductivity but also paves a new way to realize it with a nontrivial high-order topological index.
Recent experimental observations have showed some signatures of superconductivity close to 80 K in La3Ni2O7 under pressure and have raised the hope of achieving high-temperature superconductivity in bulk nickelates. However, a zero-resistance state—a key characteristic of a superconductor—was not observed. Here we show that the zero-resistance state does exist in single crystals of La3Ni2O7−δ using a liquid pressure medium at up to 30 GPa. We also find that the system remains metallic under applied pressures, suggesting the absence of a metal–insulator transition proximate to the superconductivity. Moreover, analysis of the normal state T-linear resistance reveals a link between this strange-metal behaviour and superconductivity. The association between strange-metal behaviour and high-temperature superconductivity is very much in line with other classes of unconventional superconductors, including the cuprates and Fe-based superconductors. Further investigations exploring the interplay of strange-metal behaviour and superconductivity, as well as possible competing electronic or structural phases, are essential to understand the mechanism of superconductivity in this system.
By measuring the dynamical and conventional magnetization relaxation of Ba0.64K0.36Fe2As2 single crystals, we found a strong second peak effect on the magnetization hysteresis loops. It is found that there is a kink of magnetization at a field between the valley and maximum magnetization. Interestingly, the magnetization relaxation rate has a deep minimum at the field with the kink, indicating a diminished vortex creep. The relaxation rate at this field is clearly smaller than the so-called universal lower limit of the relaxation rate characterized by S0≈Gi1/2(T/Tc). This diminished vortex creep is associated with the origin of the second magnetization peak effect and attributed to the strongly hindered flux motion when experiencing the transition from the quasi-ordered to disordered vortex phases.
This chapter introduces Mössbauer spectrometry in the structural analysis of various Fe-based compounds that exhibit superconductivity and magnetoresistance. The structure and magnetic properties studied using 57Fe Mössbauer spectrometry in conventional superconductors, high TC superconductors, Fe-based superconductors and manganites are presented.
FeSe 0.4 Te 0.6 polycrystal bulks were synthesized using the solid‐state sintering method. We developed a novel multistep sintering process and studied the impact of the sintering cycle number. Different cycles of multistep sintering, ranging from one to four cycles were systematically arranged. It was found that the superconducting properties of the FeSe 0.4 Te 0.6 polycrystal bulk using three cycles of sintering surpassed those of other samples, reaching a maximum of = 13.4 K, = 14.7 K, and a minimum of ∆ = 1.3 K. And the superconducting current density at 4 K reached 1.77 × 10 ⁴ A/cm ² , exhibiting a weak fishtail effect. At 9 K, a composite pinning effect emerged, consisting of normal point pinning and ∆ κ surface pinning. The enhancement in superconductivity is attributed to improve uniformity in grain arrangement and more effective element diffusion. After multiple sintering cycles, the stacked grains exhibited increased thickness and the grain arrangement became dense.
Superconductors, materials that can conduct electricity without resistance, have captivated scientists and engineers since their discovery in 1911. This paper provides a comprehensive analysis of the future of superconductors, examining recent advancements, potential applications, and the challenges that remain. With a focus on high-temperature superconductors, novel materials, and the theoretical underpinnings of superconductivity, this research aims to present a thorough understanding of where the field is heading. We also explore the societal and technological impacts of widespread superconductor utilization, particularly in energy transmission, medical technology, and quantum computing.
As one of the typical iron-based superconductors, Ba(Fe 1 − x Co x ) 2 As 2 exhibits a dome-like evolution of superconducting transition temperature T c with the Co doping ( x ), which provides an ideal platform to explore the mechanism of unconventional superconductivity. Through ab initio molecular dynamics (AIMD) studies, we find that the spin, charge, and lattice degrees of freedom in Ba(Fe 1 − x Co x ) 2 As 2 are intimately coupled with a characteristic frequency around 5.5 THz, which originates from the A 1 g phonon mode of the As atoms. Compared with the undoped BaFe 2 As 2 , the spin dynamics in Ba(Fe 1 − x Co x ) 2 As 2 ( x = 0.0625, 0.1250, 0.1875) shows enhanced spatial inhomogeneities, though the features of the charge dynamics are rarely modified. Along with the increasing Co doping, the spatial inhomogeneities of Fe spin dynamics display nonmonotonic changes, meanwhile the lattice vibrational spectra first show peak broadening and then restore the main peak around 5.5 THz, being both consistent with the nonmonotonic behavior of superconducting T c . Our study provides a molecular dynamics approach to investigate the couplings of spin, charge, and lattice in doped BaFe 2 As 2 , which could also be applied to explore the dome-like evolution of superconductivity in other Fe-based superconductors.
Ba1-xKxFe2As2 superconductors have strong potential for magnet applications through their very high upper critical field, relatively high superconducting transition temperature and manufacturability through the powder-in-tube (PIT) route. However, the critical current density in PIT tapes is still low compared to the incumbent technologies, so a greater understanding of the limiting factors is required. We have measured in-field critical currents (Ic) of stainless steel and silver double-sheathed monofilament Ba0.6K0.4Fe2As2 superconductor tapes at elevated temperatures from 15 K to 35 K. At 20 K the critical current density is up to 140 kA/cm2 in low (optimal) field and 22 kA/cm2 in 8 T. In the low-field region we observe an anomalous and sharp suppression of Ic centred at zero field. This feature is non-hysteretic for lower temperatures and perpendicular field, but becomes hysteretic for higher temperatures in perpendicular field and all temperatures in parallel field. The low-field suppression is reflected also in the n-values which can otherwise be very high, in excess of 100, in optimal field. Magnetic-field hysteresis of Ic is generally attributed to flux exclusion / flux trapping in granular superconductors and this is likely to be the case also in the present conductors. The low-field Ic anomaly also likely has its origin in the planar granularity, while magnetic phases in grains or grain boundaries may also play a role.
Decomposition of superconductors sometimes becomes crucial when studying essential physical properties of the superconductors. For example, the cuprate superconductor YBa2Cu3O7-d decomposes by long-time air exposure. In this study, we investigate the aging effects on superconducting properties of BiS2-based superconductors Bi4O4S3 and LaO0.5F0.5BiS2, both were first synthesized in 2012, using their polycrystalline samples synthesized several years ago. We find that 12-year-old Bi4O4S3 samples exhibit bulk superconductivity with a slight degradation of the superconducting transition temperature (Tc) of 0.2 K. For a high-pressure-synthesized LaO0.5F0.5BiS2 sample, clear decrease in Tc is observed, which suggests that high-pressure strain is reduced by aging.
K- and P-doped SrFe2As2 belong to hole doping and isovalent doping, respectively, and their effects on the band, Fermi surface, and partial density of states of Fe 3d and As 4p orbitals are also significantly different. However, these two types of doping consistently follow the rule that reducing the Fe ions height can quickly suppress antiferromagnetic order. Compared with the As-Fe-As angle, Fe-As bond length, and Fe-Fe distance, the decrease in Fe ions height more directly and quickly suppresses antiferromagnetic order, and the total density of states in the range of – 1.5 eV to ~ 1.5 eV exhibits similar properties in K and P doping in the process of 25% K- and 12.5% P-doped SrFe2As2.
We propose a theoretical framework for generating gapless topological superconductivity (GTSC) hosting Majorana flat edge modes (MFEMs) in the presence of a two-dimensional (2D) array of magnetic adatoms with noncollinear spin texture deposited on top of an unconventional superconductor. Our observations reveal two distinct topological phase transitions within the emergent Shiba band depending on the exchange coupling strength (J) between magnetic adatom spins and superconducting electrons: the first one designates transition from gapless non-topological to gapless topological phase at lower J, while the second one denotes transition from gapless topological to a trivial gapped superconducting phase at higher J. The gapless topological superconducting phase survives at intermediate values of J, hosting MFEMs. Furthermore, we investigate the nature of the bulk effective pairings, which indicate that GTSC appears due to the interplay between pseudo “s-wave” and pseudo “px+py” types of pairing. Consequently, our study opens a promising avenue for the experimental realization of GTSC in 2D Shiba lattice based on d-wave superconductors as a high-temperature platform.
Iron-based superconductors display a large degree of variability in electronic structure at the Fermi surface, resulting in superconducting gap structures, Tc's, and other properties that vary considerably from family to family. Recently, it was noted that across many different families of Fe-based systems, the ratio Δmax/Tc is found to be quasiuniversal with a large value of ∼3.5 compared to the one-band BCS weak-coupling result of 1.76. Here, Δmax is the measured maximum gap across the Fermi surface. This remarkable fact was attributed to strong-coupling effects arising from Hund's metal physics. Here, we perform a “high-throughput” scan across band masses, Fermi energies, and interaction parameters in a weak-coupling Suhl-Matthias-Walker model. We find that unexpectedly large values of Δmax/Tc can be achieved within weak coupling, and that quasiuniversal behavior similar to experiment emerges for those systems where interband interactions dominate intraband ones. However, within the current framework, a large mass contrast between bands is required.
High-Tc superconductivity (SC) has been found experimentally in the bilayer material La3Ni2O7 under high pressure recently, in which the Ni-3d3z2−r2 and 3dx2−y2 orbitals are expected to play a key role in the electronic structure and the SC. Here we study the two-orbital electron correlations and the nature of the SC in the framework of the dynamical mean-field theory using the bilayer two-orbital Hubbard model downfolded from the band structure of La3Ni2O7. We find that each of the two orbitals forms s±-wave SC pairing. Because of the interorbital hoppings, the two-orbital SCs are concomitant, and furthermore they transition to Mott insulating states simultaneously when tuning the system to half filling. The Hund's coupling induced local interorbital spin coupling enhances the electron correlations pronouncedly and is crucial to the SC.
Structural, elastic, and electronic properties of SmO1− xFxBiS2 were studied using first principles method, and effects of F− doping were clearly observed. By increasing x, the increase of lattice parameter ‘a’ and decrease of ‘c’ are mainly due to the displacement of S1 and Sm atoms respectively. For x = 0.3, new (Bi–S1)2 bonds formation was observed. The gap between Bi- and S1-planes decreases and disappeared at x = 0.9. Cauchy pressures, B/G, and ν indicate ductile nature having metallic bonds with small ionic contribution. B, G, and E are
highly anisotropic and show anomaly at x = 0.3, 0.5, and 0.8. Sm 4f states are mainly responsible for metallicity.
For x = 0.3, total DOS decreases due to new bonds formation. Anomalous displacement of Sm-sites by doping at
O- or Sm-sites induces anomalous displacement of S1 atoms which may be the main reason for change of
different properties of SmO1-xFxBiS2.
4Hb−TaS2 is a naturally formed quasi-two-dimensional heterojunction material composed of alternating monolayers of insulating (1T-) and superconducting (1H-) TaS2. We report on a comprehensive high-pressure study on the interplay between charge-density wave (CDW) and superconductivity (SC) in 4Hb−TaS2. The results uncover a dome-shaped strong-coupling superconductor on the border of a 3 × 3 commensurate CDW at Pc1≈2.0 GPa: (1) nearly one order enhancement of upper critical field Bc2(0); (2) the derived 2Δ0/kBTc beyond the BCS theory, decaying to a conventional one above 4.5 GPa; (3) the exponent of normal-state resistivity n≈1.50 and triply enhanced electronic effective mass. Under pressure, a third CDW emerging from 2.0 GPa is related to the original two CDWs, then disappears above 11.5 GPa. The temperature dependence of Bc2(T) collapses into a universal curve and the comparison of Bc2(T) to a polar-state function in 4Hb−TaS2. Theoretical calculations proposed the stronger interlayer coupling and band hybridization responsible for strong-coupling SC; the suppression of new CDWs is related to band inversions between the 1T-Ta-dxy (GM1+) and S-pz bands (GM2-) at the ΓA point; above 5.0 GPa, the coexisting weak-coupling SC and linear magnetoresistance can be attributed to the formation of electronic bands along the M−K lines and the ΓA point. Our discovery provides an excellent example to demonstrate the interplay of the strong-coupling superconducting state and topological electronic state in van der Waals heterojunctions.
В данной работе впервые проведены систематические исследования плотности критического тока Jc в двух ориентациях магнитного поля B ab и B c в монокристалле сверхпроводящего пниктида EuCsFe4As4, имеющего магнитный порядок ниже Tc и относящегося к семейству 1144. Определена анизотропия Jc и ее температурная зависимость. Показатели степенной зависимости α кривых Jc(B) в полях от 0.1 до 1 Тл хорошо согласуются с теоретическими предсказаниями для сильного пиннинга вихрей Абрикосова, что также подтверждается значениями силы пиннинга, оцененными с помощью модели Дью-Хьюза.
The sensitive response of antiferromagnetic order to As ions height in pressure environment makes the reduction of As ions height beneficial for rapid suppression of antiferromagnetism. The analysis from lattice parameters and electron density indicates that As ions height has a significant impact on antiferromagnetic suppression. The interrelationship between As ions height and antiferromagnetic suppression makes it easier for uniaxial pressure along the c‐axis environments to suppress antiferromagnetism, cause Lifshitz transition compared and induce Fermi surface nesting than uniaxial pressure along the a‐axis (or b‐axis) environments, which can explain the fluctuation of superconducting critical pressure measurements in experiments with uneven pressure application from a microscopic perspective.
Superconductivity at Tc = 80 K has recently been reported above 14 GPa in La3Ni2O7, which thus introduces a new family of high-temperature superconductors. Using a first-principles calculation with Coulomb repulsion, we unveil a surprising new route to obtain superconductivity in La3Ni2O7 at ambient pressure by introducing compressive strain along the [001] direction. The shape of the NiO6 octahedra affect the Ni-3d z 2 density of states (DOS) at Fermi level, and it can be modulated by applying compressive strain instead of hydrostatic pressure. Notably, when the octahedral regularity parameter defined herein is R ≈ 4%, La3Ni2O7 acquires a high Ni-3d z 2 DOS and hole Fermi pocket. Our study thus indicates a path for obtaining superconductivity in La3Ni2O7 at ambient pressure and elucidates the relationship between structural properties and superconductivity.
Since the discovery of high-transition-temperature (high-T(c)) superconductivity in layered copper oxides, many researchers have searched for similar behaviour in other layered metal oxides involving 3d-transition metals, such as cobalt and nickel. Such attempts have so far failed, with the result that the copper oxide layer is thought to be essential for superconductivity. Here we report that Na(x)CoO2*yH2O (x approximately 0.35, y approximately 1.3) is a superconductor with a T(c) of about 5 K. This compound consists of two-dimensional CoO2 layers separated by a thick insulating layer of Na+ ions and H2O molecules. There is a marked resemblance in superconducting properties between the present material and high-T(c) copper oxides, suggesting that the two systems have similar underlying physics.
The ternary phosphide compounds LaT4P12 are superconductors for T = Fe or Ru with transition temperatures of 4.08 K and 7.20 K, respectively. Magnetic ordering occurs when La is substituted by a magnetic rare earth element.
Calculated volumes of formation for substitutional impurities in alkali halides are shown to be approximately consistent with the empirical rule that for miscible crystals the average volume per ion pair scales linearly between the two single-component extremes (Vegard's rule). The small deviations found from the rule are found to be in good agreement with available X-ray measurements.
Convincing evidence has been discovered for bulk superconductivity in UPtâ at 0.54 K based on specific-heat, resistance, and ac susceptibility measurements. In addition, new evidence is presented that indicates that UPtâ is a spin-fluctuation system. If true, this is the first coexistent superconductor--spin-fluctuation system.
The electronic structure of the superconducting material LaOFeP is investigated by means of ab initio calculations using density functional theory. The concept of two-dimensional building blocks as well as Bader analysis are used to obtain more insight about the charge transfer in this layered material. The band structure and the Fermi surface are presented in order to be compared with future experiments. It is found that the intralayer chemical bonding present a significant part of covalency, whereas the interlayer bonding is almost completely ionic. Also, four sheets of the Fermi surface have a significant two-dimensional character.
Superconductivity — one of the best understood many-body problems in physics — has again become a challenge following the discovery of unconventional superconducting materials: these include heavy-fermion1, organic2 and the high-transition-temperature copper oxide3 superconductors. In conventional superconductors, the electrons form superconducting Cooper pairs in a spin-singlet state, which has zero total spin (S = 0). In principle, Cooper pairs can also form in a spin-triplet state (S = 1), analogous to the spin-triplet 'p-wave' state of paired neutral fermions in superfluid 3He (ref. 4). At present, the heavy-fermion compound UPt3 is the only known spin-triplet superconductor5,6, although the layered oxide superconductor Sr2RuO4 (ref. 7) is believed, on theoretical grounds8, to be a promising candidate. The most direct means of identifying the spin state of Cooper pairs is from measurements of their spin susceptibility, which can be determined by the Knight shift (as probed by nuclear magnetic resonance (NMR)). Here we report Knight-shift measurements of Sr2RuO2 using 17O NMR. Our results show no change in spin susceptibility on passing through the superconducting transition temperature, which provides the definitive identification of Sr2RuO4 as a spin-triplet superconductor.
FOLLOWING the discovery of superconductivity at ~30 K in
La2−xBaxCuO4 (ref. 1), a
large number of related compounds have been found that are superconducting at
relatively high temperatures. The feature common to all of these materials is a
layered crystal structure based on a perovskite template and containing planar
networks of copper and oxygen. This raises the question of whether
superconductivity can occur in layered perovskites that do not contain copper.
To the best of our knowledge, no such material has been found to date, despite
nearly a decade of searching. We describe here the discovery of
superconductivity in Sr2RuO4, a layered perovskite
isostructural with
La2−xBaxCuO4 (Fig. 1). Our
results demonstrate that the presence of copper is not a prerequisite for the
existence of superconductivity in a layered perovskite. But the low value of
the superconducting transition temperature (T
c = 0.93 K)
points towards a special role for copper in the high-temperature
superconductors.
Metallic, oxygen-deficient compounds in the Ba–La–Cu–O system, with the composition Ba
x
La5–x
Cu5O5(3–y) have been prepared in polycrystalline form. Samples withx=1 and 0.75,y>0, annealed below 900C under reducing conditions, consist of three phases, one of them a perovskite-like mixed-valent copper compound. Upon cooling, the samples show a linear decrease in resistivity, then an approximately logarithmic increase, interpreted as a beginning of localization. Finally an abrupt decrease by up to three orders of magnitude occurs, reminiscent of the onset of percolative superconductivity. The highest onset temperature is observed in the 30 K range. It is markedly reduced by high current densities. Thus, it results partially from the percolative nature, bute possibly also from 2D superconducting fluctuations of double perovskite layers of one of the phases present.
The equiatomic quaternary compounds RFeAsO (R=La–Nd, Sm, Gd), RRuAsO (R=La–Nd, Sm, Gd–Dy) and RCoAsO (R=La–Nd) crystallize with the tetragonal ZrCuSiAs type structure, which was refined from single-crystal X-ray diffractometer data of PrFeAsO: P4/nmm, Z=2, a=398.5(1) pm, c=859.5(3) pm, R=0.058 for 167 structure factors and 12 variable parameters. All atomic positions are fully occupied. Chemical bonding in PrFeAsO can be rationalized with oxidation numbers corresponding to the formula Pr+3Fe+2As−3O−2. An electron count of 18 for the iron atoms can only be achieved if Fe–Fe interactions of 281.8 pm are considered as bonding.
We have investigated the anisotropic resistivities of YBa2Cu3O7−y using detwinned crystals with various oxygen contents (6.68 ≤7-y≤ 6.93). The out-of-plane resistivity ρc shows a crossover from high-T metallic to low-T semiconducting behavior while the in-plane resistivity ρa deviates in the low-T region from T-linear dependence. We find that the crossover in ρc is linked with the onset of nonlinearity in ρa, which seems to be associated with the ‘‘spin gap’’ suggested by neutron and NMR studies.
Transport and magnetic properties of (La1-xSrx)2CuO4 are systematically investigated over a wide composition range up to x=0.175 including a nonsuperconducting metal phase in the heavily doped region. Remarkable changes associated with the superconductor-to-nonsuperconductor transition are observed both in the Hall coefficient and the magnetic susceptibility, suggesting the modification of both charge and spin states.
We report superconductivity in an iron-based layered oxy-pnictide LaOFeP. LaOFeP is composed of an alternate stack of lanthanum oxide (La3+O2-) and iron pnictide (Fe2+P3-) layers. Magnetic and electrical resistivity measurements verify the occurrence of the superconducting transition at approximately 4 K.
A layered oxyphosphide, LaNiOP, was synthesized by solid-state reactions. This crystal was confirmed to have a layered structure composed of an alternating stack of (La(3+)O(2-))(+) and (Ni(2+)P(3-))(-). We found that the resulting LaNiOP shows a superconducting transition at approximately 3 K. This material exhibited metallic conduction and Pauli paramagnetism in the temperature range of 4-300 K. The resistivity sharply dropped to zero and the magnetic susceptibility became negative at <4 K, indicating that a superconducting transition occurs. The volume fraction of the superconducting phase estimated from the diamagnetic susceptibility reached approximately 40 vol % at 1.8 K, substantiating that LaNiOP is a bulk superconductor.
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