Layered High Temperature Superconductors Research Articles

Effective magnetic monopole mechanism for localized electron pairing in HTS

The mechanism responsible for spatially localized strong coupling electron pairing characteristic of high-temperature superconductors (HTS) remains elusive and is a subject of hot debate. Here we propose a new HTS pairing mechanism which is the binding of two electrons residing in adjacent conducting planes of layered HTS materials by effective magnetic monopoles forming between these planes. The pairs localized near the monopoles form real-space seeds for superconducting droplets and strong coupling is due to the topological Dirac quantization condition. The pairing occurs well above the superconducting transition temperature Tc. Localized electron pairing around effective monopoles promotes, upon cooling, the formation of superconducting droplets connected by Josephson links. Global superconductivity arises when strongly coupled granules form an infinite cluster, and global superconducting phase coherence sets in. The resulting Tc is estimated to fall in the range from hundred to thousand Kelvins. Our findings pave the way for tailoring materials with elevated superconducting transition temperatures.

Effect of Anisotropy on the Transport Properties of Layered High-temperature Superconductors with Extended Magnetic and Nonmagnetic Defects

Effect of Anisotropy on the Transport Properties of Layered High-temperature Superconductors with Extended Magnetic and Nonmagnetic Defects

Electronic Properties and Fermi Surface for New Layered High-Temperature Superconductors CaAFe4As4 (A = K, Rb, and Cs): FLAPW-GGA Calculations

Recently, new FeAs-based high-temperature superconductors CaAFe4As4 (A = K, Rb, and Cs) with a layered tetragonal crystal structure were synthesized (T C ∼ 30 K). In this letter, we report for the first time the band structures, Fermi surface topology, total and partial densities of electronic states, and interatomic interactions for CaAFe4As4 as estimated by means of the first-principle FLAPW-GGA calculations. The obtained data for CaAFe4As4 phases are analyzed in comparison with each other and with other related FeAs systems. The interatomic bonding picture can be represented as a highly anisotropic mixture of metallic, covalent, and ionic contributions, which are realized inside Fe4As4-layered blocks and between these blocks and Ca, A atomic sheets. The Fermi surfaces of these systems have a multisheet character and are compiled of a large number of cylinders at the edges and in the central part of the Brillouin zone. It is established that the high-temperature superconductivity in CaAFe4As4 compounds as in other related systems correlates well with such structure parameters as bond angles and anion height.

Magnetization of layered superconductors with ferromagnetic nanorods

The magnetization of layered high-temperature superconductors (HTSC) with ferromagnetic nanorods as bulk pinning centers is studied in the 2D model of layered HTSC by using Monte Carlo method. Magnetic part of the interaction energy between a ferromagnetic cylinder of arbitrary radius and constant magnetization and an Abrikosov vortex was calculated in London approximation. The magnetization at different radius of magnetic defects was investigated. The results of calculations were compared with the results for extended nonmagnetic defects.

Chaos induced by coupling between Josephson junctions

It is found that, in a stack of intrinsic Josephson junctions in layered high temperature superconductors under external electromagnetic radiation, the chaotic features are triggered by interjunction coupling, i.e., the coupling between different junctions in the stack. While the radiation is well known to produce chaotic effects in the single junction, the effect of interjunction coupling is fundamentally different and it can lead to the onset of chaos via a different route to that of the single junction. A precise numerical study of the phase dynamics of intrinsic Josephson junctions, as described by the CCJJ+DC model, is performed. We demonstrate the charging of superconducting layers, in a bias current interval corresponding to a Shapiro step subharmonic, due to the creation of a longitudinal plasma wave along the stack of junctions. With increase in radiation amplitude chaotic behavior sets in. The chaotic features of the coupled Josephson junctions are analyzed by calculations of the Lyapunov exponents. We compare results for a stack of junctions to the case of a single junction and prove that the observed chaos is induced by the coupling between the junctions. The use of Shapiro step subharmonics may allow longitudinal plasma waves to be excited at low radiation power.

Magnetization of Layered High-temperature Superconductors with Extended Ferromagnetic Defects

The magnetization of layered high-temperature superconductors (HTSC) with ferromagnetic nanorods as bulk pinning centers is studied in the 2D model of layered HTSC by using a Monte Carlo method. Magnetic part of the interaction energy between a ferromagnetic cylinder of arbitrary radius and constant magnetization and an Abrikosov vortex was calculated in London approximation. The periodic and non-periodic lattices of magnetic defects were considered. The magnetization was investigated at different radii of magnetic defects. The results of calculations were compared with the results for extended nonmagnetic defects.

Strong thermal fluctuations in cuprate superconductors in magnetic fields aboveTc

Recent measurements of fluctuation diamagnetism in high temperature superconductors show distinct features above and below Tc, which can not be explained by simple gaussian fluctuation theory. Self consistent calculation of magnetization in layered high temperature superconductors, based on the Ginzburg-Landau-Lawrence-Doniach model and including all Landau levels is presented. The results agree well with the experimental data in wide region around Tc, including both the vortex liquid below Tc and the normal state above Tc. The gaussian fluctuation theory significantly over estimates the diamagnetism for strong fluctuations. It is demonstrated that the intersection point of magnetization curves appears in the region where the lowest Landau level contribution dominates.

Method of interlayer tunneling for studies of layered high temperature superconductors and charge density wave materials

AbstractA brief review of recent results on interlayer tunneling spectroscopy of layered high temperature superconductors (HTS) and charge density wave (CDW) materials is presented demonstrating high capability of this method for studies of both electron condensed states. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Monte Carlo simulation of layered high-temperature superconductors

We present results from Monte Carlo simulations of the vortex state in layered high temperature superconductors. We use a set of potentials derived from the Lawrence-Doniach free-energy functional which incorporates (i) intra-layer coupling (ii) inter-layer Josephson and electromagnetic interactions. We have employed a novel technique for performing an in-plane infinite lattice summation for the intra-layer interactions. This provides a minimum 50,000 times faster speed-up in the simulations over previous naïve methods which add periodic images in shells of increasing radius. We present results of the numerical B-T phase diagram in the pure system and obtain good agreement with available experimental/theoretical results.

Gap anisotropy in the layered high temperature superconductors.

We present a new strategy for constructing semiphenomenological models which deal with the electronic structure of the normal and superconducting states on equal footing. We demonstrate that it can reproduce all the striking features of the high-resolution photoemission experiments on the layered high ${T}_{c}$ superconductors.

Secondary Retarded Effect in Layered High-Temperature Superconductors

Assuming a weak electron-phonon coupling mechanism in the high-temperature superconductivity in copper oxides, the possibility of effective electron-electron interaction mediated by the secondary retarded effect associated with inter- CuO2 layer coupling has been examined. It is pointed out that under the appropriate phonon phase there is an attraction even for |εn,k±q-εn,k|>\\hbarωq, circumventing the conventional BCS restriction.

Anomalies in low field magnetic response near T c in layered high temperature superconductors

Magnetic response at low fields is observed to be hysteretic and anomalous in a JCLS for T ∗ <T<T c(0). These results inter alia confirm that ϱ c =0 and ϱ ab ≠0 in this temperature interval.

Transition temperature variation in layered high temperature superconductors

We describe a mathematical technique for studying T c variation with the number of layers in HTSCs. The T c in a uniform magnetic field is shown to be bounded. The upper critical field temperature dependence and the effect of introducing a single inequivalent layer on T c variation are studied.