Study Of High Temperature Superconductors Research Articles

High frequency response of thick REBCO coated conductors in the framework of the FCC study

A thorough microwave response study of high temperature superconductors, considered as an alternative beam screen coating, has become integral in the design decisions for CERN’s future research infrastructure. Here, we present the surface resistance R_{s} of various text {REBa}_{2}text {Cu}_{3}text {O}_{7-x} (RE = rare earth) coated conductors available in large scale as a function of magnetic field in a broad temperature range measured by a Hakki–Coleman type resonator with resonant frequency nu approx {8},{text {GHz}}. Analysis of the high frequency dissipation supported by DC transport characterization reveals the vortex dynamics in thick text {REBa}_{2}text {Cu}_{3}text {O}_{7-x} films. Determined microscopic vortex parameters span over a wide range of magnitudes and reflect the relevance of the superconducting layer’s microstructure. We demonstrate that the depinning frequencies nu _0 surpass nu, which confirms the operation in high performing, low dissipation pinning regime at measurement conditions. Surface impedance extrapolation to FCC-hh conditions points towards a significant outperformance of copper by coated conductors in terms of surface resistance. The surface resistance margins would open up potential ways for a more efficient frontier circular collider.

Observation of pseudogap in MgB2

We investigate the electronic structure of a specially prepared highly dense conventional high temperature superconductor, MgB2, employing high resolution photoemission spectroscopy. The spectral evolution close to the Fermi energy is commensurate to BCS descriptions as expected. However, the spectra in the wider energy range reveal the emergence of a pseudogap much above the superconducting transition temperature indicating an apparent departure from the BCS scenario. The energy scale of the pseudogap is comparable to the energy of the phonon mode responsible for superconductivity in MgB2 and the pseudogap can be attributed to the effect of electron–phonon coupling on the electronic structure. These results reveal a scenario of the emergence of the superconducting gap within an electron–phonon coupling induced pseudogap and have significant implications in the study of high temperature superconductors.

Phase separation induced by density-spin interaction in one-dimensional extended t-J model

t-J model is one of important theoretical models in the study of high temperature superconductivity. Recent cold molecule experiments indicate that t-J model can be simulated by ultracold polar molecules. In the simulated t-J model, besides long-range dipolar interaction, density-spin interaction has also been introduced. In the present we study the effect of density-spin interaction in the one-dimensional extended t-J model by using the density matrix renormalization group method. We choose three sets of representative parameters, which correspond to three different phases in the ground state phase diagram of t-J model, to calculate the charge and spin density distribution in real space and the structure factor of density-density and spin-spin correlation functions. The results indicate that the nature of the system will not change if the intensity of the density-spin interaction is small, however if the intensity is large enough, the system enters the phase separation, in which the character is quiet different form that of the phase separation in the traditional t-J model.

Iron based superconductors: Pnictides versus chalcogenides

We present a brief review of the present day situation with studies of high-temperature superconductivity in iron pnictides and chalcogenides. Recent discovery of superconductivity with Tc>30K in AxFe2−x/2Se2 (A=K, Cs, Tl, etc) represents the major new step in the development of new concepts in the physics of Fe-based high-temperature superconductors. We compare LDA and ARPES data on the band structure and Fermi surfaces of novel superconductors and those of the previously studied FeAs superconductors, especially isostructural 122-superconductors like BaFe2As2. It appears that electronic structure of new superconductors is rather different from that of FeAs 122-systems. In particular, no nesting properties of electron and hole-like Fermi surfaces is observed, casting doubts on most popular theoretical schemes of Cooper pairing for these systems. Doping of novel materials is extremely important as a number of topological transitions of Fermi surface near the Γ point in the Brillouin zone are observed for different doping levels. The discovery of Fe vacancies ordering and antiferromagnetic (AFM) ordering at pretty high temperatures (TN>500K), much exceeding superconducting Tc makes these systems unique antiferromagnetic superconductors with highest TN observed up to now. This poses very difficult problems for theoretical understanding of superconductivity. We discuss the role of both vacancies and AFM ordering in transformations of band structure and Fermi surfaces, as well as their importance for superconductivity. In particular, we show that system remains metallic with unfolded Fermi surfaces quite similar to that in paramagnetic state. Superconducting transition temperature Tc of new superconductors is discussed within the general picture of superconductivity in multiple band systems. It is demonstrated that both in FeAs-superconductors and in new FeSe-systems the value of Tc correlates with the value of the total density of states (DOSs) at the Fermi level.

Preparation of underdoped [formula omitted] single crystals

The Bismuth-family of high temperature superconductors has played a vital role in the understanding of high temperature superconductivity. However, for the tri-layer Bi 2 Sr 2 Ca 2 Cu 3 O 10 + δ system, its underdoped regime has been largely inaccessible so far. With elaborate vacuum annealing, we have succeeded in obtaining a series of underdoped tri-layer cuprates Bi 2 Sr 2 Ca 2 Cu 3 O 10 + δ for the first time, with the transition temperatures down to 18 K. Furthermore, the sharp superconducting transitions indicate the homogeneity of the crystals. This provides a new series of samples for the experimental studies of high temperature superconductivity.

Current-voltage curves of a cylinder with a power-law E(J)

The current-voltage V(I) curve of a cylinder of radius a with E=Ec(J∕Jc)∣J∕Jc∣n−1 is numerically calculated for a set of given values of a, Ec, Jc, and the I-sweeping rate R at n=5, 10, 20, and 40. After deriving a scaling law and defining a full penetration point for each calculated I-sweeping-up curve, the results are extended to those for any values of parameters and so can be conveniently used in the study of high-temperature superconductors.

Interpretation of the irreversibility line of polycrystalline MgB 2

The irreversibility line in the H– T phase diagram was investigated by ac-susceptibility and SQUID magnetization measurements on sintered pellets of MgB 2 at magnetic fields up to 9 T. It is shown that the irreversibility line is well described by a simple model. The results are compared with a model by Matsushita [Irreversibility line in high temperature superconductors, Studies of High Temperature Superconductors, vol. 14, Nova Science Publishers Inc., Commack, NY, 1994] that is based on the assumption of flux-creep and thermally activated flux flow which was successfully used to describe the irreversible magnetic behavior of a variety of high- T c superconductors. The reduced applicability of Matsushitas model is attributed to the high pinning strength in MgB 2, which makes MgB 2 a promising candidate for possible technical applications.

High resolution angle-resolved photoemission study of high temperature superconductors: charge-ordering, bilayer splitting and electron–phonon coupling

The latest development of angle-resolved photoemission spectroscopy (ARPES) technique has seen extremely high energy resolution and momentum resolution, as well as multiple angle detection. These advancements have led to new findings through efficient Fermi surface mapping, fine electronic structure resolving, and direct determination of electron self-energy. In this paper, we will highlight some recent high resolution ARPES work on high temperature superconductors. These include: (1) charge-ordering and evolution of electronic structure with doping; (2) bilayer splitting and Fermi surface topology of Bi2212; and (3) strong electron–phonon coupling and electron–electron interaction in high temperature superconductors.

Intrinsic Josephson tunneling for basic studies of high-temperature superconductors

Abstract High-temperature superconductors (HTS) are highly anisotropic with layers of CuO 2 separated by layers of charge reservoirs. The superconducting CuO 2 planes are weakly coupled by Josephson tunneling of Cooper pairs in the c -axis direction. It may be possible to use the intrinsic tunneling to realize a stack of intrinsic Josephson junctions at the surface of a single crystal (or epitaxial film) of HTS. These will form excellent tools in the study of superconducting properties of HTS.

Magnetic resonance of exchange-coupled copper complexes in perovskite-structure crystals: The potassium tantalate and cuprate superconductors

This paper reports on parallel EPR studies of high-temperature superconductors based on the cuprate perovskites RBa2Cu3O6+x (R=Y, Gd, Nd) and of KTaO3: Cu, which also has a perovskite structure. EPR measurements performed on copper-doped KTaO3 crystals revealed Cu2+-Cu2+ copper pair centers. The copper ions making up pairs are assumed to occupy adjacent tantalum sites. The pair centers are chains consisting of two equivalent Cu2+ ions and three oxygen vacancies aligned in the 〈100〉 direction. The crucial point in the model proposed is the presence of an oxygen vacancy sandwiched between two Cu2+ ions, whereas the outer vacancies do not necessarily occupy neighboring sites. In this structure, complete charge compensation is achieved. Ferromagnetic exchange coupling takes place between the two copper ions. An investigation of the exchange and superhyperfine interactions of copper centers in crystalline potassium tantalate has permitted the estimation of the respective interactions in crystals of the cuprate superconductors which exhibit magnetic resonance signals due to exchange-coupled copper clusters in the case of oxygen deficiency.

Schwinger-Type Boson Realization for the su(4) Algebra: In Relation to the Description of Many-Fermion Systems in Pairing Correlation

Considering the present situation of Lie-algebraic studies of high-temperature superconductivity, a Schwinger-type boson realization of the su(4) algebra is investigated. The basic framework comes from a possible form of the su(n) and the su(n,1) algebras in the Schwinger representation, which was recently proposed by two of the present authors (M.

An Orthogonal Set Constituted by Eight Kinds of Boson Operators

A possible method for obtaining an orthogonal set constituted by eight kinds of boson operators is proposed. The motivation is to apply the set obtained in this paper to three algebras for the study of high temperature superconductivity, i.e., the so(5), the so(4) and the su(2)⊗ su(2) algebras. The basic idea here comes from two recent papers by the present authors, one considering a possible method for obtaining an orthogonal set constituted by four kinds of boson operators and one considering the coupling scheme of two su(1, 1) spins. The raising and lowering operators for the magnitude of the su(2) spin coupled to two su(2) spins are introduced, and it is found that these operators play an essential role in constructing the set.

Two-dimensional periodic frustrated ising models in a transverse field

We investigate the interplay of classical degeneracy and quantum dynamics in a range of periodic frustrated transverse field Ising systems at zero temperature. We find that such dynamics can lead to unusual ordered phases and phase transitions or to a quantum spin liquid (cooperative paramagnetic) phase as in the triangular and kagome lattice antiferromagnets, respectively. For the latter, we further predict passage to a bond-ordered phase followed by a critical phase as the field is tilted. These systems also provide exact realizations of quantum dimer models introduced in studies of high temperature superconductivity.

Microbalance and Related Studies of High-temperature Superconductors

Many properties of oxide superconductors depend on the oxygen concentration and its distribution in the samples. The microgravimetric method is very useful to study of oxide superconductors, as it allows investigations in vacuum and controlled environments in classical applications: thermogravimetric analysis for the study of solid-state reactions, determinations of oxygen contents in redox reactions and the combined measurement of mass and additional parameters, e.g. evolved gas analysis etc. Single-phase samples of high-temperature superconductors were synthesized from stoichiometric mixtures of high-purity oxides and carbonates. Appropriate amounts of the precursor powders were homogenized manually or by a mechanical ball mill and subsequently calcined at temperatures in the 800 to 950°C range with intermediate grinding to ensure homogenous reaction. The lattice parameters of all preparations were controlled, in both initial and final experiments, by the X-ray powder method (CuKα radiation), using a Stadi P (Stoe) diffractometer with a position-sensitive detector. It is well known that temperature and mass can be measured with an accuracy higher by orders of magnitude than it is still possible for the thermoanalyst to determine the transformation temperatures or the mass changes due to overlapping partial reactions. Applications of Cahn microbalance to study of high-temperature superconductors are presented.

Resonant spin excitations in YBa 2Cu 3O 6+ x and Bi 2Sr 2CaCu 2O 8+ δ

A summary of some recent results of neutron scattering studies of high temperature superconductors is given, with a focus on resonant spin excitations in the superconducting states of YBa 2Cu 3O 6+ x and Bi 2Sr 2CaCu 2O 8+ δ . The opportunities offered by advances in neutron scattering instrumentation, such as focusing and polarization techniques, for these experiments are discussed.

Epitaxial Growth and Characterisation of Artificial and Superconducting Superlattices Deposited by PLD

Thin films and artificial materials deposition, applied to the study of high temperature superconductors, can be a powerful method to achieve better comprehension of the conduction mechanisms in these materials and, in perspective, to obtain new superconducting compounds. We simulate the layered structure of the superconducting oxides by alternating infinite layer compounds ( CaCuO 2, SrCuO 2, BaCuO 2) both pure and doped either with chemical substitutions or stoichiometry variations. By the first way, we obtained superlattices with a semiconducting electrical behaviour, probably because of the disorder induced by such high chemical substitutions. The second way led to superconducting films with transition temperatures around 80 K and low anisotropy. Furthermore, their artificial nature offers an unique possibility of analysing the influence of the structural properties on the superconducting behaviour, such as the correlation between the number of CuO 2 planes contained in the infinite layer block ( CaCuO 2) and the transport properties.

Magnetocaloric Effect in Ferromagnetic and Superconducting Materials

Computer studies are made illustrating the similarities of the interesting change of sign in the magnetocaloric effect between the vortex lattice in superconductors and a system of interacting magnons and phonons in ferromagnetic materials. This change happens in the superconducting system when the external magnetic field approaches a critical value, while the critical point in the ferromagnetic system is where the external magnetic field turns the direction of magnetization from the easy to the hard direction. The usefulness for the study of high-temperature superconductors is suggested.

DTA/TG coupling

With the discovery of high temperature superconductivity an extraordinary field for investigations on new materials containing up to (more than) 8 or 9 components has been open. But, in addition to problems specifically related to superconductivity, we are faced with a prelimary difficulty: the preparation of well defined compounds which may have reproducible properties. This difficulty is smoothed out when the phase relations in the relevant systems are known. Differential Thermal Analysis (DTA) contributes to their establishment. However, in multicomponent systems, an isopletic line is expected to intercept various boundary surfaces and the analysis is obviously complicated, compared to low order systems. In addition, by the nature of high temperature superconductors the oxygen partial pressure used for their preparation is an important thermodynamic parameter which contributes to fix the oxygen content in the solid state. During heat treatments, the composition of the systems may change, due to oxygen or volatile oxides (Tl2O3, PbO2 or HgO) release. A permanent composition control then requires thermogravimetric (TG) measurements associated to DTA. This paper attempts to demonstrate the efficiency of the assembly DTA/TG for the study of high temperature superconductors. Going beyond the frame of preparation and quality control, the system is also particularly useful for kinetic studies. Examples will be principally in the (Tl,Pb)-(Ba,Sr)-Ca-Cu-O systems.

High Resolution Compton Scattering in Fermi Surface Studies: Application to FeAl.

We present a novel theoretical approach to identify Fermi surface contributions in high resolution Compton scattering data. For FeAl Compton profiles we show that the Fermi-surface-related features (occupation of states) can be separated from those originating from the spatial extent of the wave functions. The first high resolution Compton scattering experiment done at the European Synchrotron Radiation Facility confirms these findings. This technique opens up new possibilities especially in Fermiology studies of high temperature superconductors.

The use of far-infrared ellipsometry in the study of high-temperature superconductors: possibilities and limitations

We examine the possibilities of applying spectroscopic ellipsometry for the determination of the far-infrared dielectric function of high temperature superconductors. On the example of YBa 2Cu 3O 7 at 20 K we show that the potential of the method lies in the accurate determination of ϵ 1 rather than the details of the onset of ϵ 2.