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3 pages, 162 KiB

Open AccessEditorial

Layered Superconductors

by Yoshikazu Mizuguchi

Condens. Matter 2018, 3(1), 4; https://doi.org/10.3390/condmat3010004 - 01 Feb 2018

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Abstract

Since the discovery of cuprates (Cu-oxide superconductors) in 1986 [1–4], layered superconductors have attracted much attention, due to the emergence of high-transition-temperature (high-T_c) and unconventional superconductivity. Full article

(This article belongs to the Special Issue Layered Superconductors)

Research

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3655 KiB

Open AccessArticle

Single Crystal Growth and Superconducting Properties of Antimony-Substituted NdO_0.7F_0.3BiS₂

by Satoshi Demura, Satoshi Otsuki, Yuita Fujisawa, Yoshihiko Takano and Hideaki Sakata

Condens. Matter 2018, 3(1), 1; https://doi.org/10.3390/condmat3010001 - 27 Dec 2017

Cited by 2 | Viewed by 3286

Abstract

Antimony (Sb) substitution of less than 8% was examined on a single crystal of a layered superconductor NdO_0.7F_0.3BiS₂. The superconducting transition temperature of the substituted samples decreased as Sb concentration increased. A lattice constant along the c [...] Read more.

Antimony (Sb) substitution of less than 8% was examined on a single crystal of a layered superconductor NdO_0.7F_0.3BiS₂. The superconducting transition temperature of the substituted samples decreased as Sb concentration increased. A lattice constant along the c-axis showed a large decrease compared with that along the a-axis. Since in-plane chemical pressure monotonically decreased as Sb concentration increased, the suppression of the superconductivity is attributed to the decrease in the in-plane chemical pressure. Full article

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9778 KiB

Open AccessArticle

Edge Contamination, Bulk Disorder, Flux Front Roughening, and Multiscaling in Type II Superconducting Thin Films

by Michel Geahel, Isabelle Jouanny, Dominique Gorse-Pomonti, Marie Poirier-Quinot, Javier Briatico and Cornelis Jacominus Van der Beek

Condens. Matter 2017, 2(3), 27; https://doi.org/10.3390/condmat2030027 - 21 Aug 2017

Cited by 4 | Viewed by 4280

Abstract

We have investigated the effect of different types of disorder on the propagation, roughness, and scaling properties of magnetic flux fronts in a type II superconductor. A progression from the usual (Kardar–Parisi–Zhang-type) scaling to multiscaling is observed as the disorder strength is increased. [...] Read more.

We have investigated the effect of different types of disorder on the propagation, roughness, and scaling properties of magnetic flux fronts in a type II superconductor. A progression from the usual (Kardar–Parisi–Zhang-type) scaling to multiscaling is observed as the disorder strength is increased. A hierarchy of disorder strengths is established for YBa

_{2}

Cu

_{3}

O

_{7 - δ}

thin films. The results cast light on the physical origin of the roughening of flux fronts, and they are of interest for the design and elimination of flux noise in microscopic superconducting thin-film devices. Full article

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6105 KiB

Open AccessArticle

by John H. Miller, Jr. and Martha Y. S. Villagrán

Condens. Matter 2017, 2(2), 21; https://doi.org/10.3390/condmat2020021 - 17 Jun 2017

Cited by 3 | Viewed by 5224

Abstract

The nucleation and dynamics of Josephson and Abrikosov vortices determine the critical currents of layered high-T_c superconducting (HTS) thin films, grain boundaries, and coated conductors, so understanding their mechanisms is of crucial importance. Here, we treat pair creation of Josephson and [...] Read more.

The nucleation and dynamics of Josephson and Abrikosov vortices determine the critical currents of layered high-T_c superconducting (HTS) thin films, grain boundaries, and coated conductors, so understanding their mechanisms is of crucial importance. Here, we treat pair creation of Josephson and Abrikosov vortices in layered superconductors as a secondary Josephson effect. Each full vortex is viewed as a composite fluid of micro-vortices, such as pancake vortices, which tunnel coherently via a tunneling matrix element. We introduce a two-terminal magnetic (Weber) blockade effect that blocks tunneling when the applied current is below a threshold value. We simulate vortex tunneling as a dynamic, time-correlated process when the current is above threshold. The model shows nearly precise agreement with voltage-current (V-I) characteristics of HTS cuprate grain boundary junctions, which become more concave rounded as temperature decreases, and also explains the piecewise linear V-I behavior observed in iron-pnictide bicrystal junctions and other HTS devices. When applied to either Abrikosov or Josephson pair creation, the model explains a plateau seen in plots of critical current vs. thickness of HTS-coated conductors. The observed correlation between theory and experiment strongly supports the proposed quantum picture of vortex nucleation and dynamics in layered superconductors. Full article

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540 KiB

Open AccessArticle

Magnetoresistance, Gating and Proximity Effects in Ultrathin NbN-Bi₂Se₃ Bilayers

by Gad Koren

Condens. Matter 2017, 2(2), 14; https://doi.org/10.3390/condmat2020014 - 25 Apr 2017

Cited by 5 | Viewed by 4345

Abstract

Ultrathin

{Bi}_{2} {Se}_{3}

-NbN bilayers comprise a simple proximity system of a topological insulator and an s-wave superconductor for studying gating effects on topological superconductors. Here we report on 3 nm thick NbN layers of weakly connected superconducting islands, overlayed with [...] Read more.

Ultrathin

{Bi}_{2} {Se}_{3}

-NbN bilayers comprise a simple proximity system of a topological insulator and an s-wave superconductor for studying gating effects on topological superconductors. Here we report on 3 nm thick NbN layers of weakly connected superconducting islands, overlayed with 10 nm thick

{Bi}_{2} {Se}_{3}

film which facilitates enhanced proximity coupling between them. Resistance versus temperature of the most resistive bilayers shows insulating behavior but with signs of superconductivity. We measured the magnetoresistance (MR) of these bilayers versus temperature with and without a magnetic field H normal to the wafer (MR = [R(H) − R(0)]/{[R(H) + R(0)]/2}), and under three electric gate-fields of 0 and

\pm 2

MV/cm. The MR results showed a complex set of gate sensitive peaks which extended up to about 30 K. The results are discussed in terms of vortex physics, and the origin of the different MR peaks is identified and attributed to flux-flow MR in the isolated NbN islands and the different proximity regions in the

{Bi}_{2} {Se}_{3}

cap-layer. The dominant MR peak was found to be consistent with enhanced proximity induced superconductivity in the topological edge currents regions. The high temperature MR data suggest a possible pseudogap phase or a highly extended fluctuation regime. Full article

(This article belongs to the Special Issue Layered Superconductors)

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1967 KiB

Open AccessArticle

Discovery of BiS2-Based Superconductor and Material Design Concept

by Yoshikazu Mizuguchi

Condens. Matter 2017, 2(1), 6; https://doi.org/10.3390/condmat2010006 - 31 Jan 2017

Cited by 8 | Viewed by 5555

Abstract

In 2012, we discovered new layered superconductors whose superconducting states emerge in the BiS2 layers. Since their crystal structure, composed of alternate stacks of BiS2 conduction layers and electrically insulating (blocking) layers, is similar to those of cuprate and Fe-based superconductors, many researchers [...] Read more.

In 2012, we discovered new layered superconductors whose superconducting states emerge in the BiS2 layers. Since their crystal structure, composed of alternate stacks of BiS2 conduction layers and electrically insulating (blocking) layers, is similar to those of cuprate and Fe-based superconductors, many researchers have explored new BiS2-based superconductors and have studied the physical and chemical properties of the BiS2-based superconductors. In this paper, we present the histories of the discovery of the first BiS2-based superconductor, Bi4O4S3, and the second one, LaO1−xFxBiS2. The structural variation of the BiS2-based superconductor family is briefly introduced. Then, we show the material design concept for the emergence of bulk superconductivity in BiS2-based compounds. At the end, a possible strategy for the enhancement of the transition temperature in the BiS2-based superconductors is proposed. Full article

(This article belongs to the Special Issue Layered Superconductors)

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Review

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3558 KiB

Open AccessReview

Crystal Growth Techniques for Layered Superconductors

by Masanori Nagao

Condens. Matter 2017, 2(4), 32; https://doi.org/10.3390/condmat2040032 - 16 Oct 2017

Cited by 6 | Viewed by 8445

Abstract

Layered superconductors are attractive because some of them show high critical temperatures. While their crystal structures are similar, these compounds are composed of many elements. Compounds with many elements tend to be incongruent melting compounds, thus, their single crystals cannot be grown via [...] Read more.

Layered superconductors are attractive because some of them show high critical temperatures. While their crystal structures are similar, these compounds are composed of many elements. Compounds with many elements tend to be incongruent melting compounds, thus, their single crystals cannot be grown via the melt-solidification process. Hence, these single crystals have to be grown below the decomposition temperature, and then the flux method, a very powerful tool for the growth of these single crystals with incongruent melting compounds, is used. This review shows the flux method for single-crystal growth technique by self-flux, chloride-based flux, and HPHT (high-pressure and high-temperature) flux method for many-layered superconductors: high-T_c cuprate, Fe-based and BiS₂-based compounds. Full article

(This article belongs to the Special Issue Layered Superconductors)

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1688 KiB

Open AccessReview

In Search of Unambiguous Evidence of the Fulde–Ferrell–Larkin–Ovchinnikov State in Quasi-Low Dimensional Superconductors

by Mihail D. Croitoru and Alexandre I. Buzdin

Condens. Matter 2017, 2(3), 30; https://doi.org/10.3390/condmat2030030 - 11 Sep 2017

Cited by 14 | Viewed by 4239

Abstract

In layered conductors with a sufficiently weak interlayer coupling in-plane magnetic field cause only small diamagnetic currents and the orbital depairing is strongly suppressed. Therefore, the Zeeman effect predominantly governs the spin-singlet superconductivity making the formation of the spatially modulated Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) phase [...] Read more.

In layered conductors with a sufficiently weak interlayer coupling in-plane magnetic field cause only small diamagnetic currents and the orbital depairing is strongly suppressed. Therefore, the Zeeman effect predominantly governs the spin-singlet superconductivity making the formation of the spatially modulated Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) phase possible in such materials. Despite decades of strenuous effort, this state still remains a profound mystery. In the last several years, however, there have been observed several hints indicating the experimental realization of the FFLO state in organic layered superconductors. The emergence of the FFLO phase has been demonstrated mainly based on thermodynamic quantities or microscopically with spin polarization distribution that exhibit anomalies within the superconducting state in the presence of the in-plane magnetic field. However, the direct observation of superconducting order parameter modulation is so far missing. Recently, there have been proposed theoretically several hallmark signatures for FFLO phase, which are a direct consequence of its main feature, the spatial modulation of the order parameter, and hence can provide incontrovertible evidence of FFLO. In this article, a review of these signatures and the underlying theoretical framework is given with the purpose to summarize the results obtained so far, omitting duplications, and to emphasize the ideas and physics behind them. Full article

(This article belongs to the Special Issue Layered Superconductors)

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11896 KiB

Open AccessReview

Superconductivity in 122-Type Pnictides without Iron

by Pan Zhang and Hui-fei Zhai

Condens. Matter 2017, 2(3), 28; https://doi.org/10.3390/condmat2030028 - 23 Aug 2017

Cited by 11 | Viewed by 5126

Abstract

The exploration of superconducting iron-free pnictides with a ThCr₂Si₂-type or related structure and the study of their superconducting properties are important in order to get a deeper understanding of the pairing mechanism of 122 iron pnictides. Here we review [...] Read more.

The exploration of superconducting iron-free pnictides with a ThCr₂Si₂-type or related structure and the study of their superconducting properties are important in order to get a deeper understanding of the pairing mechanism of 122 iron pnictides. Here we review the properties of 122-type iron-free pnictides superconductors with structures similar to that of BaFe₂As₂. Evidence of fully gapped nature of superconducting state has come from the specific heat and thermal conductivity measurements for BaNi₂As₂ and SrNi₂P₂, and nuclear magnetic and quadrupole resonance measurements for CaPd₂As₂ etc. Combined with the fact that no magnetism is observed in 122-type iron-free pnictides superconductors, the majority of evidence suggests that most of these compounds are conventional electron–phonon-mediated superconductors. Full article

(This article belongs to the Special Issue Layered Superconductors)

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7394 KiB

Open AccessReview

Comparative Review on Thin Film Growth of Iron-Based Superconductors

by Yoshinori Imai, Fuyuki Nabeshima and Atsutaka Maeda

Condens. Matter 2017, 2(3), 25; https://doi.org/10.3390/condmat2030025 - 07 Jul 2017

Cited by 21 | Viewed by 6884

Abstract

Since the discovery of the novel iron-based superconductors, both theoretical and experimental studies have been performed intensively. Because iron-based superconductors have a smaller anisotropy than high-Tc cuprates and a high superconducting transition temperature, there have been a lot of researchers working on the [...] Read more.

Since the discovery of the novel iron-based superconductors, both theoretical and experimental studies have been performed intensively. Because iron-based superconductors have a smaller anisotropy than high-Tc cuprates and a high superconducting transition temperature, there have been a lot of researchers working on the film fabrication of iron-based superconductors and their application. Accordingly, many novel features have been reported in the films of iron-based superconductors, for example, the fabrication of the epitaxial film with a higher Tc than bulk samples, the extraction of the metastable phase which cannot be obtained by the conventional solid state reaction, and so on. In this paper, we review the progress of research on thin film fabrications of iron-based superconductors, especially the four categories: LnFeAs(O,F) (Ln = Lanthanide), AEFe2As2 (AE = Alkaline-earth metal), FeCh (Ch = Chalcogen), and FeSe monolayer. Furthermore, we focus on two important topics in thin films of iron-based superconductors; one is the substrate material for thin film growth on the iron-based superconductors, and the other is the whole phase diagram in FeSe1-xTex which can be obtained only by using film-fabrication technique. Full article

(This article belongs to the Special Issue Layered Superconductors)

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1885 KiB

Open AccessReview

Novel Electronic State and Superconductivity in the Electron-Doped High-T_c T’-Superconductors

by Tadashi Adachi, Takayuki Kawamata and Yoji Koike

Condens. Matter 2017, 2(3), 23; https://doi.org/10.3390/condmat2030023 - 04 Jul 2017

Cited by 23 | Viewed by 5357

Abstract

In this review article, we show our recent results relating to the undoped (Ce-free) superconductivity in the electron-doped high-

T_{c}

cuprates with the so-called T’ structure. For an introduction, we briefly mention the characteristics of the electron-doped T’-cuprates, including the reduction annealing, [...] Read more.

In this review article, we show our recent results relating to the undoped (Ce-free) superconductivity in the electron-doped high-

T_{c}

cuprates with the so-called T’ structure. For an introduction, we briefly mention the characteristics of the electron-doped T’-cuprates, including the reduction annealing, conventional phase diagram and undoped superconductivity. Then, our transport and magnetic results and results relating to the superconducting pairing symmetry of the undoped and underdoped T’-cuprates are shown. Collaborating spectroscopic and nuclear magnetic resonance results are also shown briefly. It has been found that, through the reduction annealing, a strongly localized state of carriers accompanied by an antiferromagnetic pseudogap in the as-grown samples changes to a metallic and superconducting state with a short-range magnetic order in the reduced superconducting samples. The formation of the short-range magnetic order due to a very small amount of excess oxygen in the reduced superconducting samples suggests that the T’-cuprates exhibiting the undoped superconductivity in the parent compounds are regarded as strongly correlated electron systems, as well as the hole-doped high-

T_{c}

cuprates. We show our proposed electronic structure model to understand the undoped superconductivity. Finally, unsolved future issues of the T’-cuprates are discussed. Full article

(This article belongs to the Special Issue Layered Superconductors)

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10545 KiB

Open AccessReview

Titanium Pnictide Oxide Superconductors

by Takeshi Yajima

Condens. Matter 2017, 2(1), 4; https://doi.org/10.3390/condmat2010004 - 12 Jan 2017

Cited by 14 | Viewed by 6909

Abstract

In 2012, a novel superconductor BaTi₂Sb₂O was found in the layered titanium pnictide oxides ATi₂Pn₂O. A related superconductor BaTi₂Bi₂O was subsequently discovered in 2013. The structure of these materials consists [...] Read more.

In 2012, a novel superconductor BaTi₂Sb₂O was found in the layered titanium pnictide oxides ATi₂Pn₂O. A related superconductor BaTi₂Bi₂O was subsequently discovered in 2013. The structure of these materials consists of alternate stacking of superconducting Ti₂Pn₂O layers and Ba blocking layers, which is somewhat similar to high-T_c cuprates since the Ti₂Pn₂O layer contains an anti-CuO₂-type Ti₂O square lattice. In addition to the structural similarity to the well-known high-T_c superconductors, BaTi₂Pn₂O shows unique physical properties: two superconducting domes appear in the electronic phase diagram for solid solutions of BaTi₂(Sb_1‒xBi_x)₂O and a unique density-wave instability which coexists with superconductivity. In this short review, the early studies of titanium pnictide oxides, the discovery of novel superconductors BaTi₂Pn₂O, and recent progress are summarized. Full article

(This article belongs to the Special Issue Layered Superconductors)

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