FN - 7th International Conference
Science and Engineering of Novel Superconductors


Session FN-1 - Materials, Structure, Physical Chemistry and General Properties

FN-1:IL01  Ferroelectricity and Magnetoelectric Coupling in Underdoped Cuprates
C. PANAGOPOULOS, Division of Physics and Applied Physics, Nanyang Technological University, Singapore

In high-Tc cuprates, a wide range of exotic ground states is accessible by charge carrier-doping an antiferromagnetic Mott insulator, including short-range glassy phases and superconductivity. However, the nature of the charge correlations coexistent with antiferromagnetism in underdoped cuprates has remained unclear. I will discuss experimental evidence for carrier-doping resulting in the onset of ferroelectricity accompanied by slow charge fluctuations. The presence of ferroelectricity and magnetoelectricity indicate spin-charge coupling in lightly doped high temperature copper-oxide superconductors. The observed magnetoelectric coupling leads to several novel effects including a feedback enhancement of the magnetization below the ferroelectric transition and a predicted magnetocapacitive effect. Our experimental studies have been addressed in terms of material-dependent parameters sensitive to changes such as chemical doping. The discovery of ferroelectricity and magnetoelectric effect in the cuprate materials due to charge carrier doping have sparked many questions for future studies. In particular, what happens at higher charge carrier concentration and the relation of the above mentioned observations to the unconventional ground state of the cuprate phase diagram.

FN-1:IL03  Highest Zero-resistance Temperature (153K) of Hg1223 at High Pressures
A. YAMAMOTO, RIKEN Center for Emergent Matter Science, Saitama, Japan

Since discovery of Hg-based copper oxide HgBa2Ca2Cu3O8+d(Hg1223) with Tc of 134K in 1993, the highest Tc has not been updated for 20 years. Although Gao and Monteverde reported Tc=160K at 31GPa and Tc=164K at 23GPa for pure and fluorinated Hg1223, respectively, these values were ambiguous due to the broad transitions accompanied with high residual resistivity. The maximum value of Tc is of great importance not only for understanding a mechanism of the superconductivity but also for practical application. In order to confirm this ceiling and break through it by experiment, we prepared Hg1223 at high pressure and measured electrical resistivity under high pressure. The Tc increased and zero resistivity was achieved at 153K at 15GPa. This is the first report of Tc above 150K. Pressure dependence of resistivity was studied in samples with various doping. The absolute value of resistivity reduced with increasing pressure and Tc increased with a rate of 2.5K/GPa. The slope decreases but does not saturate up to 15GPa, indicating the possibility that even higher Tc is available. Further issues discussed include: the maximum achievable Tc, the doping dependence of ΔTc/ΔP, anisotropy of compression, the contribution of outer and inner CuO2 layers and oxygen rearrangement of HgO2+

FN-1:L04  Superconductivity in(Cu1-X MoX)Sr2RECu2O7+δ (RE = Y, Er, Tm) Molibdocuprates
M.A. ALARIO-FRANCO1, S. MARIK1,  2, E. MORÁN1, C. LABRUGERE2, O. TOULEMONDE2, A. DOS SANTOS1, I. HERRERO-ANSORREGUI1, 1Dpto. Química Inorgánica, Facultad de CC. Químicas, U. Complutense de Madrid, Madrid, Spain; 2CNRS, Université de Bordeaux, ICMCB, Pessac, France

A systematic study is reported on the range of stability and structure-composition-properties correlation of molybdenum substituted Sr-based 123 compounds synthesized at ambient pressure. A detailed investigation of the structure-composition-properties is carried out for the as-prepared and oxygen annealed pure Mo0.3Cu0.7Sr2RECu2Oy materials. Their crystal structures were characterized by combining X-Ray/neutron powder diffraction and electron diffraction techniques. For the samples with RE = Y and Er, both the as-prepared and oxygen annealed materials show tetragonal symmetry crystallizing in the P4/mmm space group (S. G.). But a tetragonal (Space Group P4/mmm) to orthorhombic (Pmmm) phase transformation has been observed, associated with an oxidation reaction using the combination of neutron powder diffraction and electron diffraction techniques for the Mo0.3Cu0.7Sr2TmCu2Oy sample.
The influence of oxygen annealing in the electronic states for these systems associated with an oxidation reaction from a non-superconducting state to a superconducting one has also been investigated by means of X-ray photoelectron spectroscopy (XPS). these measurements show the predominance of the MoV oxidation state over the MoVI one on both as-synthesized and annealed phases; annealing under flowing oxygen enhances both the MoVI and CuII amounts. On the other hand, the as-prepared Mo0.3Cu0.7Sr2ErCu2Oy material shows the existence of ferromagnetic clusters while the as-prepared Mo0.3Cu0.7Sr2TmCu2Oy material is found to be a re-entrant spin glass (RSG) system.
All the oxygen annealed samples are not magnetic but superconducting (SC) with Tc up to ~85 K (much higher than the high pressure phase CuSr2YCu2O7 : Tc ~60 K).

FN-1:IL05  New Phenomenon Akin to Superconductivity and yet not Superconductivity: The Zero Magnetoresistance in Polymer Nanofibers
YUNG WOO PARK, Department of Physics and Astronomy, Seoul National University, Seoul, Korea

The zero magneto resistance (ZMR) observed in polyacetylene nanofibers1-4 reported a few years ago, is a seminal discovery which shows the ZMR in high electric field. It is named as “zero M resistivity” in analogy to the zero resistance of superconductivity. The ZMR is that the resistance change is zero under magnetic field at above critical electric field. Namely the resistance remains constant under magnetic field up to 35 tesla when the applied electric field is higher than 2ⅹ104 V/cm. It is not the Meissner effect expelling the magnetic field at temperature below Tc. And yet, the ZMR is effectively similar to the Meissner effect since the magnetic field does not affect the sample resistance, i.e., the sample is insensitive to the applied magnetic field as if there is no magnetic field inside the sample (expelled). The observed ZMR is explained with the de-confined spinless charge carriers (the local excitations called solitons) phenomenologically. The mechanism could be more fundamental such as BCS type condensation with one order higher effect, i.e., not the resistance itself zero but the derivative w.r.t. magnetic field is zero. In addition, it is interesting that the soliton state (the mid gap state) in polyacetylene is formed by half of the state from conduction band and half of the state from valence band, which is akin to the Majorana fermion. And it is this soliton state which is responsible for the zero magneto resistance (ZMR). The mid gap state which is conducting, is in the boundary of insulating π-π* band state (the topological soliton in polyacetylene) akin to an one-dimensional topological insulator.
1. A. Choi, et.al., Synthetic Metals, 160, 1349 (2010),
2. Y.W. Park, Chemical Society Reviews, 39, 2428 (2010)
3. N. Kirova, et.al., Physica B 407, 1939 (2012)
4. A. Choi, et.al., Phys. Rev. B 86, 155423 (2012)

FN-1:IL06  2D Superconductivity by Ionic Gating
Y. IWASA, University of Tokyo and RIKEN, Tokyo, Japan

Ion gated superconductivity has been so far demonstrated on the atomically flat surfaces of several substances including SrTiO3, ZrNCl, La2CuO4, YBa2Cu3Ox, and MoS2. In the presentation, discussion is given on the nature of superconductivity, such as the phase diagrams and difference from the bulk superconductors, with particular emphasis on the two dimensionality of superconductivity. Based on the transport measurements and anisotropy of upper critical fields Hc2, we found that superconductivity is highly two-dimensional with a thickness of 1~2 nm, which is much smaller than the in-plane coherence lenght. These values are close to the Thomas-Fermi screening lengths, indicating that the carriers are accumulated by the electrostatic mechanism. Another important aspect of electric field effect is that the spatial inversion symmetry is inherently broken in the present device, and might produce a serious impact of paring symmetry, particularly in systems with strong spin-orbit interactions. In fact, the in-plane Hc2 was found considerably enhanced by a factor of more than three in comparison to the value of the Pauli limit. This strongly indicates considerable mixture of triplet nature in the paring in electric field induced superconductivity.

FN-1:L07  Exploring High-Tc Superconductivity by Strain and Electric Field Effect
D. PAVUNA, EPFL, Lausanne, Switzerland

We have studied La2-xSrxCuO4-y (LSCO) strained ultra-thin films (<30nm) heteroepitaxially grown by pulsed laser ablation [1] : We have in-situ measured their electronic structure and determined respective Fermi surfaces. We measured up to 50% enhancement in Tc for in-plane compressively strained films, while the in-plane tensile strain proportionately reduced Tc [2]. Rather than to enhance the role of antinodal 'pseudogap' states, the in-plane compressive strain apparently reduces the out-of-plane disorder scattering and enhances the phase coherence of BCS-like nodal states - hence higher measured Tc [2]. In collaboration with Ivan Bozovic et al. (BNL) we have studied the electric field effect on heteroepitaxial cuprate structures within electric double layer transistor (EDLT) geometry [3]. Very large field induced changes cause shifts in Tc by up to 30K. We are able to substantially vary Tc with the applied electric field and have observed striking quantum phase transition in LSCO films; we have also studied other related materials [3].
1. D. Pavuna et al., Journal of Physics 108, 012040 (2008)
2. D. Oezer, M.Sc. Thesis, EPFL (2009); N. Wooding EPFL Ph.D. Thesis (2013)
3. A.T. Bollinger et al. Nature 472, 458 (2011); G. Dubuy EPFL Ph.D. Thesis (2013)

FN-1:IL08  Fe-vacancy Order and Superconductivity in Tetragonal β-Fe1-xSe
TA-KUN CHEN1, CHUNG-CHIEH CHANG1, HSIAN-HONG CHANG2, AI-HUA FANG1, CHIH-HAN WANG1, WEI-HSIANG CHAO1, CHUAN-MING TSENG1, YUNG-CHI LEE1, 3, YU-RUEI WU1, MIN-HSUEH WEN1, HSIN-YU TANG4, 5, FU-RONG CHEN1, 4, MING-JYE WANG1, 2, MAW-KUEN WU1, 3, 6, DIRK VAN DYCK5, 1Institute of Physics, Academia Sinica, Taipei, Taiwan; 2Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan; Departments of 3Physics and 4Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan; 5Electron Microscopy for Materials Science, Department of Physics, University of Antwerp, Antwerp, Belgium; 6Department of Physics, National Dong Hwa University, Hualien, Taiwan

Several superconducting transition temperatures in the range of 30–46 K were reported in the recently discovered intercalated FeSe system (A1-xFe2-ySe2, A = K, Rb, Cs, Tl). Although the superconducting phases were not yet conclusively decided, more than one magnetic phase with particular orders of iron vacancy and/or potassium vacancy were identified, and some were argued to be the parent phase. Here we show the discovery of the presence and ordering of iron vacancy in nonintercalated FeSe (PbO-type tetragonal β-Fe1-xSe). Three types of iron-vacancy order were found through analytical electron microscopy, and one was identified to be nonsuperconducting and magnetic at low temperature. This discovery suggests that the rich-phases found in A1-xFe2-ySe2 are not exclusive in Fe-Se and related superconductors. In addition, the magnetic β-Fe1-xSe phases with particular iron-vacancy orders are more likely to be the parent phase of the FeSe superconducting system instead of the previously assigned β-Fe1+δTe.

Session FN-2 - New Superconductors of the Pnictides and Related Families

FN-2:IL01  Progress in Epitaxial Thin Films of 122-type Iron-pnictide Superconductors: Non-equilibrium Impurity Doping and Critical Current Density
H. HIRAMATSU1, 3, H. HOSONO1, 2, 3, 1Materials and Structures Laboratory, Tokyo Institute of Technology, Japan; 2Materials Research Center for Element Strategy, Tokyo Institute of Technology, Japan; 3Frontier Research Center, Tokyo Institute of Technology, Japan  

Iron-based superconductors have received a great deal of interest as a new family of high critical-temperature (Tc) superconductors. In particular, 122-type Ba(Fe,Co)2As2 epitaxial films seem to have a great potential for future applications because this system exhibits the highest critical current densities (Jc) ≥ 1 MA/cm2 among the iron-based superconductors.1) In addition, an advantageous grain boundary property of Ba(Fe,Co)2As2 has been clarified 2); i.e., the intergrain Jc at [001]-tilt grain boundaries showed a transition from a strong link to a weak link at a critical misorientation angle of 9o, which is much larger than those of cuprates (3-5o). This characteristic grain boundary property along with high upper critical magnetic fields (Hc2) ≥ 50 T and small anisotropic factors γ = 1-2 is promising for thin-film tapes required for high-field magnets, because the cuprates have high Jc > 1 MA/cm2 at 77 K but γ are high up to 100, and other superconductors such as Nb3Sn and MgB2 have relatively low Hc2 < 30 T.
So far, inserting a conductive buffer-layer of Fe or SrTiO3 has been proposed to produce Ba(Fe,Co)2As2 films exhibiting high intragrain-Jc. On the other hand, we reported the direct growth of high-Jc Ba(Fe,Co)2As2 films on MgO and LSAT substrates without any buffer layers by optimizing the PLD growth conditions. Employing the finely-optimized PLD technique, we succeeded in indirect electron-doping of BaFe2As2 using epitaxial films of rare-earth (RE)-doped BaFe2As2 on MgO substrates. The non-equilibrium PLD film growth process effectively stabilized the metastable doping of RE elements at the Ba sites. Such indirect carrier doping induced superconductivity at maximum Tc ~22 K for (Ba,La)Fe2As2 and ~14 K for (Ba,Ce)Fe2As2. The indirectly electron-doped (Ba,RE)Fe2As2 exhibited lower maximum Tc and narrower widths of the superconductivity domes in the phase diagram than indirectly hole-doped (Ba,K)Fe2As2, while these were almost the same as those of the directly electron-doped Ba(Fe,Co)2As2. These results revealed that polarity and density of the doped carriers mainly control the superconductivity in BaFe2As2. 3,4).
Recently, we have succeeded in growing BaFe2(As,P)2 epitaxial films.5) It should be noted that the high Jc over 1 MA/cm2 was observed even at 9T. Moreover, angular dependence of Jc shows the isotropic properties in a low magnetic field of 1 T. The high Jc over 1 MA/cm2 along with the low anisotropy indicates that BaFe2(As,P)2 is the most promising material for high-field applications.
1) For a review, H. Hiramatsu et al., J. Phys. Soc. Jpn. 81 (2012) 011011.
2) T. Katase, Y. Ishimaru, A. Tsukamoto, H. Hiramatsu, T. Kamiya, K. Tanabe, and H. Hosono, Nat. Commun. 2 (2011) 409.
3) T. Katase, S. Iimura, H. Hiramatsu, T. Kamiya, and H. Hosono, Phys. Rev. B 85 (2012) 140516(R).
4) T. Katase, H. Hiramatsu, T. Kamiya, and H. Hosono, New. J. Phys. 15 (2013) 073019.
5) H. Sato, H. Hiramatsu, T. Kamiya, and H. Hosono, to be submitted.

FN-2:IL03  Fabrication and Properties of Iron-based Superconducting Films
E. BELLINGERI, S. KAWALE, V. BRACCINI, A. GERBI, R. BUZIO, L. PELLEGRINO, A. SALA, A. PALENZONA, M. PUTTI, C. FERDEGHINI, CNR - SPIN Genova, Genova, Italy, Physics Department, University of Genova, Genova, Italy; A. JOST, U. ZEITLER, High Field Magnet Laboratory, Institute for Molecules and Materials, Radboud University Nijmegen, Nijmegen, Netherlands; C. TARANTINI, J. JAROSZYNSKI, Applied Superconductivity Center, National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL, USA; E. REICH, B. HOLZAPFEL, IFW Dresden, Dresden, Germany

Despite their relatively low critical temperature (Tc) values, typically below 15 K, chalcogenides are an interesting sub-class of the iron-based superconductors: in fact, in FeSe Tc can be increased up to 21 K in Fe(Se,Te) thin films due to the residual strain. They also have the simplest structure among the iron-based superconductors and exhibit lower anisotropies, with comparable Hc2(0) well above 50 T. Furthermore, high irreversibility line Hirr very close to the upper critical field Hc2 makes this class of superconductors particularly appealing for high-field and low temperature applications; Jc values as high as a few 10^5 A/cm2 can be sustained in magnetic field of 30 T.
We will present an overview of Fe(Se,Te) thin films grown through pulsed laser deposition (PLD) on different substrates, varying from oxides to fluorides(SrTiO3(001), LaAlO3(001) and CaF2(001), and the effect of the substrate and of the type of growth on the superconducting properties of the films. We will discuss the relationship between structural properties and superconductivity, and we will focus on the comparison of the critical current density and the pinning mechanisms of the different films through the analysis of transport measurements and STM and TEM.

FN-2:IL04  Separation of Antiferromagnetism and High-temperature Superconductivity in Ca1-xLaxFe2As2
J. PAGLIONE, S.R. SAHA, T. DRYE, S.K. GOH, L.E. KLINTBERG, J.M. SILVER, F.M. GROSCHE, M. SUTHERLAND, T.J.S. MUNSIE, G.M. LUKE, D. K. PRATT, J.W. LYNN, Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park, MD, USA

We report the effect of applied pressures on magnetic and superconducting order in single crystals of Ca1-xLaxFe2As2. Using electrical transport, elastic neutron scattering and resonant tunnel diode oscillator measurements on samples under both quasi-hydrostatic and hydrostatic pressure conditions, we report a series of phase diagrams spanning the range of substitution concentrations for both antiferromagnetic and superconducting ground states that include pressure-tuning through the antiferromagnetic (AFM) quantum critical point. Our results indicate that the observed high- Tc superconducting phase is intrinsic to these materials, and exists only exclusively from the antiferromagnetic phase in a manner similar to the oxygen- and fluorine-based iron-pnictide superconductors with the highest transition temperatures reported to date. The unusual dichotomy between lower-Tc systems with coexistent superconductivity and magnetism and the  tendency for the highest- Tc systems to show non-coexistence provides an important insight into the distinct transition temperature limits in different members of the iron-based superconductor family. 

FN-2:IL05  Multiband Superconductivity on the Verge of a Fermi-surface Topological Transition in 122 Fe-based Compounds
D. DAGHERO, G.A. UMMARINO, S. GALASSO, R.S. GONNELLI, Dipartimento di Scienza Applicata e Tecnologia (DISAT), Politecnico di Torino, Torino, Italy; G. PROFETA, Dipartimento di Fisica, Università dell’Aquila, L’Aquila, Italy; K. SUZUKI, Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo, Japan

Point-contact Andreev-reflection spectroscopy (PCARS) is a simple but versatile technique to study the number, amplitude and symmetry of the order parameter(s) (OPs) of superconductors. When the direction of current injection is known with respect to the crystallographic axes, the capabilities of PCARS can be considerably improved if a 3D extension of the Blonder-Tinkham-Klapwijk (BTK) model, combined with ab-initio and Eliashberg calculations, is used to analyze the results.  Here we apply this approach to CaFe2As2 (a Fe-based compounds of the 122 family) with particular focus on the region of doping or pressure where the Fermi surface is on the verge of a topological 2D – 3D transition, which causes the disappearance of the holelike Fermi surface (FS) at the centre of the Brillouin zone. We show that this transition is accompanied by the emergence of horizontal node lines in the order parameter and, rather unexpectedly, corresponds to an enhancement of the critical temperature. This indicates that CaFe2As2 can be driven across a transition that makes it bridge the gap between “usual” multiband Fe-based compounds (for which the model of spin-fluctuation-mediated pairing associated to the FS nesting is widely accepted) and “anomalous” single-band ones (like FeSe monolayers on SrTiO3,  KFe2As2, K0.8Fe1.7Se2).

FN-2:IL06  Pairing Interaction in the Fe-based Superconductors Studied with Atomically Resolved Scanning Tunneling Microscopy/Spectroscopy
SHUHENG H. PAN, Department of Physics, University of Houston, Houston, TX, USA; Texas Center for Superconductivity; Institute of Physics, Chinese Academy of Sciences

Novel superconductivity is one of the most challenging subjects in condensed mater physics and material science. Though there have been enough experimental evidences showing that the iron-based superconductivity is not conventional, its microscopic paring mechanism still remains illusive. Scanning tunneling microscopy/spectroscopy (STM/S) has been demonstrated as a unique approach in study of novel superconductivity. In this talk I will discuss some of our experimental results obtained by using a combination of high resolution low temperature scanning tunneling microscopy/spectroscopy and atomic manipulation in the investigation of the microscopic electronic structure and the low energy excitations in various iron-based superconductors. With these experimental results, we demonstrate, at the atomic scale, that the pairing interaction in iron-based superconductors exhibits many local behaviors. These local phenomena not only challenge the current theories based on the itinerant picture, but also provide indispensable information for understanding the microscopic paring mechanism of the iron-based superconductivity.

FN-2:IL07  Angle-resolved Photoemission Spectroscopy Study of Iron-pnictide Superconductors
HONG DING, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, CAS, Beijing, China

Angle-resolved photoemission spectroscopy (ARPES) has been used extensively in studying electronic structure and superconducting gap of the iron-based superconductors (IBSCs). In this talk, I will present our ARPES results on the IBSCs, mainly focus on high-resolution measurements of the superconducting gap of many different IBSCs. Our results strongly suggest that the pairing mechanism of IBSCs is likely to be driven by local antiferromagnetic exchange interactions and collaborative Fermi surface topology, in a fashion similar to the case of cuprate superconductors.

FN-2:L08  Superconductivity at Tc = 36.5 K in Na-substituted SrFe2As2 Single Crystals
L. SHLYK, M. BISCHOFF, R. NIEWA, Institut für Anorganische Chemie, Universität Stuttgart, Stuttgart, Germany; E. ROSE, Physikalisches Institut, Universität Stuttgart, Stuttgart, Germany

Hole doping with Na at the Sr site has been shown to induce superconductivity in polycrystalline Sr1-xNaxFe2As2 with the highest Tc=26 K at x=0.4 [1]. Goko et al. reported mSR measurements on single crystal with a nominal composition of Sr0.5Na0.5Fe2As2 (Tc~35 K)[2], however neither structural information no detailed study of bulk properties was provided. We report here superconductivity, which is achieved in this material at Tc=36.5 K for x=0.47 Na. Single crystals of Na-substituted SrFe2As2 up to 2×2 mm2 with Tc=36.5 K were grown from NaF flux. The upper critical fields Hc2//c(0)=87 T and Hc2//ab(0)=217.5 T are extremely high. The anisotropy ratio is ~ 2.5 The in-plane (H//c) and inter-plane (H//ab) critical currents differ by only a factor of 2.3. Magnetic relaxation measurements reveal that the flux-creep rate in Sr0.53Na0.47Fe2As2 is smaller than in 1111 type phases.
1.R.Cortes-Gil, S.J.Clarke Chem. Mater. 23 1009 (2010)
2.T.Goko et al. Phys. Rev. B 80 024508 (2009)

Session FN-3 - Properties of Superconductors (of Any Type)

FN-3:IL01  Static to Dynamical Antiferromagnetic Correlation in Electron-doped Cuprate Superconductors Studied by ARPES
A. FUJIMORI, Department of Physics, University of Tokyo, Tokyo, Japan  

In electron-doped cuprates, antiferromagnetism is more pronounced than hole-doped cuprates and remains quasi-long-ranged even in the superconducting state [1,2]. The antiferromagnetism manifests itself as band folding and resulting "hot spots" on the Fermi surface. The hot spots are weakened in superconduting samples but never disappear [3].
In the present work, we performed an ARPES study of the Pr1.3-xLa0.7CexCuO4 (x=0.10). Using an improved annealing method, which leads to the nearly complete removal of apical oxygens [4], the hot spots disappeared and a sharp quasi-particle peak is recovered on the entire Fermi surface. Nevertheless, the width of the quasi-particle peak was found momentum-dependent, implying that similar dynamical antiferromagnetic fluctuations persist as in the hole-doped cuprates.
This work was done in collaboration with M. Horio, T. Yoshida, K. Okazaki, Y. Ohta, S. Shin, T. Mizokawa, K. Ono, H. Kumigashira, H. Anzai, M. Arita, H. Namatame, M. Taniguchi, Y. Mori, A. Takahashi, T. Adachi, and Y. Koike.
[1] A. F. Santander-Syro et al., Phys. Rev. Lett. 106, 197002 (2011).

[2] S. R. Park et al., Phys. Rev. B 75, 060501 (2007).
[3] H. Matsui et al., Phys. Rev. B 75, 224514 (2007).
[4] M. Brinkmann et al., Phys. Rev. Lett. 4927 (1995).

FN-3:IL02  Charge Order Driven by Fermi-arc Instability in Underdoped Cuprates
A. DAMASCELLI, University of British Columbia, Vancouver, Canada  

A common theoretical description of the remarkably high Tc in cuprates has been hindered by the apparent diversity of intertwining electronic orders out of which superconductivity emerges. The unresolved link between these competing orders and the pseudogap phase with its "Fermi arc" phenomenology remains key to understanding these materials. Here we combine resonant X-ray scattering (REXS), scanning-tunneling microscopy (STM), and angle-resolved photoemission spectroscopy (ARPES) to report the discovery of a single unifying phenomenon: charge order whose wavevector spans the Fermi arc tips and appears consistently in surface and bulk, and momentum and real space, within one cuprate family. The observed wavevector rules out simple antinodal nesting in the single particle limit, but matches well with a phenomenological model of a many-body instability of the Fermi arcs. These converging findings reveal the existence of a universal charge-ordered state in underdoped cuprates, and suggest its connection to the pseudogap regime.

FN-3:IL06  STM Spectroscopy Probe of Magnetic Field Depairing and Vortex Lattice Transition in a Multiband Superconductor
I. FRIDMAN1, V. LUKIC2, C. KLOC3, C. PETROVIC4, J.Y.T. WEI1, 5, 1University of Toronto, Canada; 2Stevens Institute of Technology, USA; 3Nanyang Technological University, Singapore; 4Brookhaven National Laboratory, USA; 5Canadian Institute for Advanced Research, Canada  

The Cooper pairing in a number of superconductors involves carriers from multiple bands, which can optimize the pairing phase space and provide novel pairing interactions. This talk will present a fundamentally new approach for probing multiband pairing: using a directional diamagnetic supercurrent to perturb the quasiparticle density-of-states spectrum, and measuring the spectral evolution -- due to pair breaking by finite superfluid momentum -- to distinguish the different pairing gaps. This approach is demonstrated on the layered superconductor 2H-NbSe2, using a scanning tunneling microscope (STM) at 300 mK with an in-plane magnetic field up to 9 T [1]. The STM spectroscopy measurements revealed unambiguous evidence for multiband pairing [2], as well as a novel reorientation transition of the in-plane vortex lattice [3]. We will discuss the first-order and quantum-critical characteristics of this transition, in terms of the geometric frustration of a distorted hexagonal vortex lattice, and show that this transition is intimately related to the multiband pairing.
[1] I. Fridman et al., Applied Physics Letters 99, 192505 (2011).
[2] I. Fridman et al., arXiv:1110.6490, [cond-mat.supr-con].
[3] I. Fridman et al., arXiv:1303.3559, [cond-mat.supr-con].

FN-3:IL07  High Tc Superconductivity in Cuprate/Titanate Heterostructures
D. DI CASTRO, G. BALESTRINO, A. TEBANO, D. INNOCENTI, F. RIDOLFI, University of Rome Tor Vergata and CNR-SPIN, Rome, Italy; C. ARUTA, CNR-SPIN, Rome, Italy

In the superconducting superlattice CaCuO2/SrTiO3, significant experimental evidences suggest the confinement of superconductivity within few unit cells at the CaCuO2/SrTiO3 interface and the important role of additional oxygen atoms entering in the interfaces during growth in oxygen rich environment. In particular, the overall experimental data points toward an interfacial reconstruction by oxygen redistribution that, in case of excess oxygen, generates holes in the CuO2 planes. Therefore, the interfaces in these SLs behave as a charge reservoir for the CuO2 planes, allowing superconductivity in the CaCuO2 layer. Starting from these results, we deeply investigated also the behavior of a single interface in CaCuO2/SrTiO3 bilayers. We found that the transport properties are strongly dependent on the oxidizing power of the growth atmosphere, as the superlattices, but also on the layer sequence. In particular, on NdGaO3 substrate, the sequence NdGaO3/CaCuO2/SrTiO3 is superconducting, whereas the sequence NdGaO3/SrTiO3/CaCuO2 is non-superconducting. The transport properties of the bilayers seem to be affected much also by the lattice mismatch with the substrate used (NdGaO3, SrTiO3, LaAlO3). We will give an overview of the investigations performed on both superlattices and bilayers.

FN-3:L08  Role of Mg-B-O Nanostructural Inhomogenities on the Performance of Superconducting MgB2
T.A. PRIKHNA1, M. EISTERER2, V.V. KOVYLAEV3, H.W. WEBER2, V. MOSHCHIL1, W. GAWALEK4, X. CHAUD5, 1Institute for Superhard Materials of the National Academy of Sciences of Ukraine, Kiev, Ukraine; 2Atominstitut, Vienna University of Technology, Vienna, Austria; 3Institute for Problems in Material Science of the National Academy of Sciences of Ukraine, Kiev, Ukraine; 4Institut für Photonische Technologien, Jena, Germany; 5CNRS/LNMP, Grenoble Cedex, France

The distribution of nanostructural inhomogenities, such as areas with high concentration of B and impurity O, pays a key role on the variation of the jc of MgB2. A wide range of pressure and temperature (0.1 MPa-2 GPa and 600-1100 °C) was applied on Mg:2B powder mixtures or MgB2 powders with different amounts of impurity O and C and initial grain sizes for producing various MgB2 samples. The different preparation conditions lead to differences in connectivity (17-98%) and bulk density (55-99%) and allowed to conclude that a high amount of such rather homogeneously distributed O- and B-enriched nanolayers and nanoinclusions are formed in all materials and can act as pinning centers influencing jc. The increase in manufacturing temperature leads to an enhanced jc in low and medium magnetic fields (up to 1-1.2 MA/cm2 at 20 K at 0-1T) independently from the applied pressure as long as the MgB2 stoichiometry of the material matrix is preserved. The Auger and SEM study showed that a transformation of the discontinuous oxygen enriched 15-20 nm thick nano-layers (MgB0.9-3.5O1.6-2) into distinct dispersed Mg-B-O inclusions (MgB0.6-0.8O0.8-0.9) takes place with increasing manufacturing temperature. These structural variations correlate well with the type of pinning and increase of the jc.

FN-3:L09  Microwave Measurements of Surface Resistance and Conductivity of Optimised NdBaCuO Films
J. MAZIERSKA, K. LEONG, James Cook University, Townsville, Australia; J. KRUPKA, Warsaw University of Technology, Poland

Microwave measurements of rare earth copper oxide superconductors started almost immediately after HTS materials were discovered. Resonant techniques of HTS' surface resistance, RS, started with copper cavities followed by niobium cavities, stripline and microstripline resonators, a parallel-plate resonator, and finally dielectric rod resonators. Recently a different approach was introduced [1] in which, instead of RS, the complex conductivity is determined from measured resonance frequency and Q-factor of sapphire resonator with the films. In [1] YBCO films were used for illustration of the concept and the theory behind it.
In this paper we will present microwave properties of optimized NdBaCuO films on MgO substrates and access their suitability for microwave applications. NdBCO is known to have higher TC and JC than YBCO but not much is known about their loss at microwave frequencies. Measured RS of NdBCO will be also compared to those of other REBCO films, and a comparison of films' RS of thickness 50nm and 700nm obtained using the RS and the conductivity approaches will be given and discussed. The films under test have been manufactured by THEVA, with TC of 94K and JC of 4MA/cm2 at 77K.
J. Krupka et al, IEEE Transactions on Applied Superconductivity, v. 23, 64501232, 2013

Session FN-4 - Theory and Mechanisms (for Normal and Superconducting States)

FN-4:IL01  Spin-Polarised Supercurrents: A Paradigm Shift for Spintronics
J.W.A. ROBINSON1, A. DI BERNARDO1, T. HIGGS1, A. WELLS1, N. BANERJEE1, M. EGILMEZ, M.G. BLAMIRE1, Y. KALCHEIM2, O. MILLO2, 1Department of Material Science and Metallurgy, University of Cambridge, Cambridge, UK; 2Racah Institute of Physics and the Hebrew University Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem, Israel

Upon injection into a ferromagnet from a superconductor, spin-singlet supercurrents rapidly decay within a few nanometers unless the superconductor-ferromagnet interface (S/F) allows spin-aligned triplet Cooper pairs to form [1]. It is now established that such triplet pairs form when the magnetisation at the S/F interface is non-collinear with respect to the magnetisation in the F layer [2]. Because triplet pairs carry spin in addition to charge it is possible that triplet supercurrents could be used in spintronics in order to control the electronic state of a device ("superconducting spintronics"); however, unlike non-superconducting spin-polarised currents triplet currents are dissipationless and so could offer an energy efficient solution for low-temperature applications such in large scale data handling facilities.
My group has discovered a variety of ways to generate spin triplet pairs and in this talk I will provide an overview of our recent experimental work in this area; in particular, I will discuss S/F junctions containing spin-density wave antiferromagnetism, rare earth magnetic spirals such as Ho and Gd, and ferromagnetic multilayers with controlled anisotropy.
[1] F Bergeret et al., Rev. Mod. Phys. 77, 1321 (2005).
[2] M Eschrig, Phys. Today 64, 43-49 (January)

FN-4:IL02  Peeping at Pairing Interactions with Coherent Charge Fluctuation Spectroscopy
J. LORENZANA, Institute for Complex Systems - CNR and Physics Department, Sapienza, University of Rome, Italy; B. MANSART, A. MANN, A. ODEH, M. CHERGUI, F. CARBONE, EPFL, Lausanne, Switzerland  

It has been recently demonstrated that a pump pulse can generate coherent charge fluctuations in a superconductor through a stimulated Raman process [1] paving the way to a new technique, Coherent Charge Fluctuation Spectroscopy (CCFS), to investigate pairing interactions and manipulate the superconducting wave function. We will present the basic phenomena involved and show how the technique bears a strong analogy with nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR). The reflectivity of the system gets modulated by the coherentcharge fluctuation of the condensate. A resonance at the Mott scale (2.6eV) allows to identify a high-energy excitation which is coupled to the superconducting quasiparticles which suggest that non-retarded interactions are involved in the pairing as suggested by unconventional theories of superconductivity [2].
[1] B. Mansart, J. Lorenzana, A. Mann, A. Odeh, M. Scarongella, M. Chergui, F. Carbone, Proc. Natl. Acad. Sci. USA, 110 4539 (2013); J. Lorenzana, B. Mansart, A. Mann, A. Odeh, M. Chergui, and F. Carbone, Europ. Phys. J ST 222,1223 (2013).
[2]P. W. Anderson, Science 316, 1705 (2007).

FN-4:IL03  Recent Progress in Modeling Electronic Structure and Spectroscopy of Novel Superconductors and Topological Insulators
A. BANSIL, Physics Department, Northeastern University, Boston, Massachusetts, USA

I will discuss some of our recent results aimed at understanding the electronic structure and spectroscopy of novel superconductors, topological materials, and atomically thin 2D films. [1-8]  Illustrative examples include: (i) Asymmetry of the scanning tunneling (STM) spectrum of the cuprate high-Tc superconductors between positive and negative bias voltages and the extent to which it comes about within the conventional picture, and how strong correlation effects modify the STM spectra; (ii) The character of the doped holes in the curpate superconductor La-Sr-Cu-O as revealed by the analysis of the doping dependent high-resolution Compton scattering studies; (iii) How by exploiting electronic structure techniques we have been able to predict and understand the characteristics of many new classes of binary, ternary and quaternary topologically interesting systems; (iv) How atomically thin 2D materials such as silicene and MoSe2 offer exciting new possibilities for manipulating electronic structures and the associated topological phases, providing novel platforms for applications. Work supported by Basic Energy Sciences, United States Department of Energy.
[1] H. Lin et al., Nano Letters 13, 1915 (2013).
[2] Y. Okada et al., Science 341, 1496 (2013).
[3] M. R. Scholz et al., Physical Review Letters 110, 216801 (2013).
[4] W-F Tsai et al., Nature Communications 4, 1500 (2013).
[5] F.-C. Chuang et al., Applied Physics Letters 102, 022424 (2013).
[6] Y. Sakurai et al., Science 332, 698 (2011).
[7] Y. Okada et al., Nature Materials 12, 707 (2013).
[8] Yi Zhang et al., Nature Nanotechnology (2013).

FN-4:IL04  Sorting Chicken from Eggs: Polaronic or Spin Fluctuation Mediated Superconductivity in High Temperature Copper Oxides
A. BUSSMANN-HOLDER, Max-Planck-Institute for Solid State Research, Stuttgart, Germany

Starting from the antiferromagnetic parent compounds of high temperature superconducting copper oxides, we use a Heisenberg Hamiltonian where the Cu spins are coupled quadratically to the local lattice displacements. This additional term guarantees the survival of antiferromagnetic fluctuations also in the non-magnetic and superconducting phase. It has the important consequence that the magnetic susceptibility deviates from paramagnetic behavior at high temperatures and follows the local structural anomalies observed by EXAFS in a subtle manner. Specifically, it is shown that antiferromagnetic fluctuations are present in the superconducting phase activated by the coupling to a polaronic mode. In this scenario the spin fluctuations are not the origin of the electron pairing but caused by the coupling to polarizability induced anharmonic local lattice displacements.

FN-4:IL05  Density Functional Theory for Plasmon-assisted Superconductivity
R. AKASHI, R. ARITA, RIKEN Center for Emergent Matter Science (CEMS), Saitama, Japan

Non-empirical calculation of superconducting transition temperature (Tc) has been one of the most fascinating challenges in condensed matter theory. Recently, density functional theory for superconductors (SCDFT) was formulated [M. Lüders et al., Phys. Rev. B 72, 024545 (2005), M. A. L. Marques et al., Phys. Rev. B 72, 024546 (2005)], and has been successfully applied to various phonon-mediated superconductors. There, without introducing any adjustable parameters, the experimental Tcs were reproduced with Kelvin-scale accuracy.
In the conventional SCDFT, the screened Coulomb interaction is treated within the static approximation, following the Migdal-Eliashberg theory. As the first step toward DFT for unconventional superconductors, we recently formulated a new scheme taking account of the dynamical structure of the screened Coulomb interaction [Phys. Rev. Lett. 111 057006 (2013)]. In the new formalism, pairing mechanisms involving plasmons or excitons can be described. We applied the new scheme to elemental lithium under high pressures and layered nitride superconductors. We found that the contribution of plasmons is appreciable, and our new scheme significantly improves the agreement between theory and experiment.

FN-4:IL06  Magnetic and Ionic Effects in High-temperature Superconductors
G. NIKSIC, D.K. SUNKO, S. BARISIC, Department of Physics, Faculty of Science, University of Zagreb, Zagreb, Croatia

Experimental magnetic responses of high-temperature superconductors are reviewed with particular regard to a minimal theoretical interpretation, compatible with the known data, which can account for the incommensurate response between T* and the superconducting Tc. Strong correlations dynamically redistribute the spectral weight of the effective low-energy fermions in the Brillouin zone so as to suppress incommensurabilities in the high-temperature particle-hole response of the doped metal in the relevant parameter space. Assuming additional order parameters below T* are of secondary importance, the magnetic pseudogap transition at T* is suggested to be a discommensuration of a high-energy dispersive mode of the copper spins in interaction with the predominantly oxygen nodal metal, with the appearance of a central peak. The physical origin of the effective interactions involved is described. Ionic (3D) effects on the metallic (2D) picture are qualitatively discussed.

Session FN-5 - Vortex Lattice Physics

FN-5:IL01  Vortex Molecules in Layered Thin Films Superconductors
A. BUZDIN, University of Bordeaux, LOMA, Bordeaux, France; A. MELNIKOV, A. SAMOKHVALOV, Institute for Physics of Microstructures, Nizhny Novgorod, Russia

In bulk layered superconductors the vortices tilted with respect to the anisotropy axes attract each other at long distance, which leads to the vortex chains structures. In thin film the intervortex interaction is modified by an extremely slow decay of the supercurrent induced by a single vortex line (Pearl's effect). The interplay between these interactions in thin films is responsible for a formation of a minimum of the interaction potential vs the intervortex distance. This minimum exists only for relatively strong tilting. Depending on the strength and the tilt of the magnetic field we may expect the formation of the vortex molecules rearranging in multiquanta flux lattices. The increase in the field tilting should be accompanied by the series of the phase transitions between the vortex lattices with different number of vortices per unit cell.

FN-5:IL02  Vortex Structures in Iron-based Superconductors by Small-angle Neutron Scattering
H. KAWANO-FURUKAWA, Ochanomizu University, Bunkyo-ku, Tokyo, Japan  

Small angle neutron scattering (SANS) technique can be used to explore the vortex state of the type-II superconductors, since it measures the Fourier components of the magnetic field distribution in a crystal. SANS is also a powerful technique to observe directly the vortex lattice (VL) structure.
Recently, we succeeded in observing clear VL Bragg spots on KFe2As2 (KFA) single crystals for the first time in iron-based superconductors [1]. With fields parallel to the c axis, a nearly isotropic hexagonal VL is observed up to 2/3 Hc2(0). We concluded that KFA is the s+- superconductor with "horizontal" gap. With fields away from the c-axis, the VL shows distortion. By fitting it with the anisotropic London model, we estimate the anisotropy factor for penetration depths. Furthermore, we tried to interpret VL phase transitions by the ``Hairy Ball" theorem [2].
Very recently, we further performed SANS measurements on another iron-based superconductor, BaFe2(As0.67,P0.33)2 (BFAP) and succeeded in observing clear Bragg spots [3]. We will discuss the difference between KFA and BFAP from the viewpoint of different gap structures that they have.
[1] H. Kawano-Furukawa et al., Phys. Rev. B 84, 024507 (2011)
[2] H. Kawano-Furukawa et al., PRB in press
[3] R. Ishii et al., unpublished.

FN-5:IL03  How the Macroscopic Current in a Superconductor Correlates with the Microscopic Flux-line Distribution: An Experimental Study
M. ZEHETMAYER, J. HECHER, Atominstitut, Vienna University of Technology, Vienna, Austria  

We have investigated the flux-line distribution of type-II superconductors via tunneling microscopy. Introducing new defects, capable of flux-line pinning, via neutron irradiation allowed us to study the influence of the vortex pinning energy. The macroscopic critical current of the single crystals was measured in a SQUID and its field dependence found to form a pronounced second peak as a consequence of neutron irradiation. We studied the corresponding vortex lattice at different magnetic fields and different irradiation levels. In the second peak regime, we observed that the vortex lattice disorder is fairly proportional to the critical current density below the field of maximum critical current but that the disorder further grows although the current becomes smaller at higher fields. Time correlation measurements showed that the vortex displacement does not significantly change with disorder or with magnetic field. Recording up to two thousand vortices per image made it possible to discover vortex-defect clusters. We analyzed the size, density, and shape of these clusters at different magnetic fields, critical currents, and neutron irradiation levels.

Session FN-6 - Synthesis and Processing

FN-6:IL02  High-resolution Electron Microscopy and Electron Energy-loss Spectroscopy as Tools to Study the Structure and Bonding Nature of Complex Oxides
G.A. BOTTON, M. BUGNET, N. GAUQUELIN, G.Z. ZHU, Department of Materials Science and Engineering, McMaster University, Hamilton, Canada  

Over the last decade, electron microscopy has dramatically evolved because of major breakthroughs in aberration correction, stability of the instruments and electron monochromators allowing sub-angstrom resolution with synchrotron-quality spectroscopic capabilities.
Using a combination of scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) with better than 0.1eV energy resolution, we provide here a review of recent studies where EELS and STEM have allowed us to probe the structure, the local chemistry and the nature of the local electronic structure of a range of complex oxides. These studies show that it is possible to determine the location of particular atomic species in a crystal used as dopants [1], the local coordination and valence of atoms in crystals and surfaces [2,3], and also the nature of the hybridization and valence in perovskites [4] and superconductors [5]. With these techniques, we explore defects in materials and the nature of the electronic structure at interfaces.
[1] G.Z. Zhu, et al. Phys. Chem. Chem. Phys. 15, 11420 (2013)
[2] G.Z. Zhu, et al. Nature, 490, 384, (2012)
[3] S. Turner, et al., Chem. Mater., 24, 1904−1909 (2012)
[4] M. Bugnet, G. Radtke and G.A. Botton, submitted
[5] N. Gauquelin, et al., submitted

FN-6:L03  Synthesis and Characterization of Phenacene Superconducting Hydrocarbons
G.A. ARTIOLI, L. MALAVASI, Dip. di Chimica, Università di Pavia-INSTM UdR di Pavia, Pavia, Italy  

The discovery of 18 K Tc in K-doped picene (K3picene) reported by Mitsuhashi and colleagues in March 2010 has triggered a huge interest towards superconducting hydrocabons. Starting from this discovery a new era in the superconducting research has been opened and the potential to discovered analogous new HTSC has been extremely exciting. Since the paper of Mitsuhashi different works were published on this topic but the crystal structures of the superconducting phases are still unclear as well as the composition. The reasons can be found in the dramatic difficulties in synthesizing samples by a conventional solid state reaction. The aim of the present work was devoted to the synthesis of alkali-metal doped phenacene compounds. New synthetic routes were explored in order to understand the intercalation process in order to obtain high quality crystalline samples. Starting from a low temperature intercalation process, related to solution intercalation method with anhydrous ammonia, different synthetic processes have been developed. The efforts have been directed to the systematic investigation of the alkali metal doped phenacene as a function of dopant and stoichiometry (and intercalation procedure) followed by a thorough study of magnetic and structural properties of the samples.

FN-6:IL05  Topological Electronic Structure and Properties of Single-layer Films for Spintronics Applications: Silicene and Related Materials
HSIN LIN, Graphene Research Centre and Department of Physics, National University of Singapore, Singapore; WEI-FENG TSAI, CHENG-YI HUANG, FENG-CHUAN CHUANG, ZHI-QUAN HUANG, CHIA-HSIU HSU, YU-TZU LIU, HUA-RONG CHANG, Department of Physics, National Sun Yat-Sen University, Kaohsiung, Taiwan; TAY-RONG CHANG, HORNG-TAY JENG, Department of Physics, National Tsing Hua University, Hsinchu, Taiwan; GAURAV GUPTA, MANSOOR BIN ABDUL JALIL, GENGCHIAU LIANG, Department of Electrical and Computer Engineering, National University of Singapore, Singapore; ARUN BANSIL, Department of Physics, Northeastern University, Boston, MA, USA  

Two-dimensional thin films with significant spin-orbit coupling (SOC) strength enable potential applications for spintronics devices because SOC provides a new route for generating spin-split states in nonmagnetic materials without the need of exchange coupling. Moreover, spin-orbit coupling can induce nontrivial topological phases, i.e. quantum spin Hall phases, which could harbor back-scattering-free spin-polarized current at the edge. We have shown via first-principles calculations that field-gated silicene possesses two gapped Dirac cones exhibiting nearly 100% spin-polarization. Band gaps as well as the band topology can be tuned with an external E-field perpendicular to the plane. We propose a design for a silicene-based spin-filter that should enable the spin-polarization of an output current to be switched electrically, without the need to switch external magnetic fields. Our quantum transport calculations indicate that the proposed designs will be highly efficient and robust against weak disorder and edge imperfections. We also propose a Y-shaped spin/valley separator that produces spin-polarized current at two output terminals with opposite spins. Other spin-orbit thin films will be discussed.

FN-6:IL06  Single Crystal Growth of BaFe2As2-based Superconductors by Self-flux Method
H. EISAKI, K. KIHOU, S. ISHIDA, M. NAKAJIMA, A. IYO, C.H. LEE, AIST, Tsukuba, Japan; T. SAITO, H. FUKAZAWA, Y. KOHORI, Chiba University,, Japan; S. UCHIDA, Tokyo University, Japan

Single crystal samples of (Ba,K)Fe2As2 and BaFe2(As,P)2 have been successfully synthesized by a self-flux method. Concentrations of K and P in the grown crystals are systematically changed by changing the mixing ratio of the starting elements and by employing adequate precursors. The crystals have maximum size of 5mm x 5mm x 0.2mm and possess flat surfaces corresponding to the crystallographic (001) planes. The superconducting transition temperatures systematically change with changing the doping ratio of K and P, reaching 38K for K-doped and 30K for P-doped samples, respectively.

FN-6:L07  Optimally Processed MgB2 Bulk Magnets with High Critical Currents
M. MURALIDHAR1, K. INOUE1, M.R. KOBLISCHKA2, M. MURAKAMI1, 1Superconducting Materials Laboratory, Department of Materials Science and Engineering, Shibaura Institute of Technology, Tokyo, Japan; 2Institute of Experimental Physics, Saarland University, Saarbrücken, Germany

We report on the fabrication and characterization of disk-shaped bulk MgB2 superconductors with high performance as superconducting super-magnets. Several samples were fabricated using a solid state reaction in pure Ar atmosphere from 750 to 950 °C to obtain the highest critical current density as well as large trapped field values. Microstructural observations with scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that the grain size is the crucial parameter to improve the critical currents as well as the field trapping abilities. The MgB2 sample processed at 775 °C for 3 h showed the highest critical current density of 176 kA cm-2 and 55 kA cm-2 at 20 K in self field and a trapped field of 1 T, respectively. Furthermore, the critical current density values increased further to 250 kA cm-2 and 150 kA cm-2 with decreasing temperature down to 10K. As a result, samples of 20 mm diameter and 7 mm thickness, prepared with an optimized processing temperature, exhibited a trapped field of 1.5 T at 20 K. The present results imply that the continuing development of large size bulk MgB2 with optimized processing conditions, and flux pinning properties will benefit numerous new industrial applications.

Session FN-7 - Applications

FN-7:IL02  Modelling and Measurement of AC Loss in Coated Conductor Windings
E. PARDO, J. SOUC, J. KOVAC, Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava, Slovakia  

Changing magnetic fields in coated conductors generate AC loss, which reduces efficiency and difficultates cooling. Therefore, it is necessary to develop measurement and modelling techniques in order to estimate the AC loss. This contribution models and measures both stacks of pancake coils with up to thousands of turns in AC current and racetrack coils in AC magnetic field. We also study the effect of a bias DC current. Our measurements are done by electrical means. The numerical model takes the interaction between magnetization currents into account, as well as the anisotropic magnetic-field-dependent Jc and the smooth E(J) relation. We found that the model agrees with the measurements. In addition, a DC current increases the AC loss. The model also reveals that, for stacks of pancakes with a bias DC current, the power loss is higher in the raising part of the current cycle than in the decreasing one. In order to model the AC loss in coils, the interaction between magnetization currents cannot be neglected, also when the coil contains a large number of turns. In conclusion, the presented numerical model is suitable to predict the AC loss in magnet-size windings (including SMES and transformers). In addition, ripple AC loss can be accurately measured by electrical means.

FN-7:IL04  The Superconducting Order Parameter in Iron-based Superconductors
T. SHIBAUCHI, Department of Advanced Materials Science, University of Tokyo, Chiba, Japan

Determination of the structure of the order parameter, or superconducting gap, that describes the superconducting state is of primary importance for understanding the mechanism of high-temperature superconductivity in iron pnictide materials. We provide bulk evidence that even in the closely related materials the superconducting gap shows different structures; some have nodes (zero points in certain momentum directions) and some do not.
In BaFe2As2-based materials, near the optimum compositions of Co and K doping systems, nodeless gap has been established, while in the isovalent P-substitution and the overdoped regime of K-doping systems we found that the superconducting gap has nodes which give substantial low-energy excitations of quasiparticles. By using electron irradiation which creates point defects, we show that the nodes in P-substituted system can be lifted by impurity scattering. This indicates that the nodes in this system is not protected by the order parameter symmetry, which provides compelling evidence for A_1g (s-wave) symmetry. We conclude that the gap structure is non-universal but the order-parameter symmetry is universally A_1g in 122 iron pnictide superconductors.

FN-7:IL05  Low Loss Cable from High-temperature Superconducting Tapes
F. GOMORY, J. SOUC, M. VOJENCIAK, M. SOLOVYOV, Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava, Slovakia

The 2nd generation of tapes from high-temperature superconductors exhibits excellent electric transport capacity, however magnetization currents could cause significant dissipation when exposed to variable magnetic field. Promising solution of the latter problem is the helical layout of tapes on a flexible round former (recently called the Coated Conductors on Round Core or CORC cables). Then it is possible to consider these cables as an option for the low-voltage winding of a power transformer or for the design of large superconducting magnets.
We present the study of AC loss in single-layer CORC cables made from coated conductors with non-magnetic substrate. The regimes of AC transport (sometimes called the self-field conditions) and of AC magnetization due to external field have been investigated. Numerical simulations have been performed assuming there are no coupling currents between the tapes. The results are in good agreement with experimental data obtained on the conductors with the length equal to the entire number of tape transpositions. Depending on the gap between tapes the loss component caused by the interaction of magnetic fields in neighboring tapes could vary.

FN-7:L06  Light MgB2 Superconductor with Ti-barrier and Al-stabilization Suitable for Mass-limited Applications
P. KOVAC, I. HUŠEK, T. MELIŠEK, L. KOPERA, Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava, Slovakia

MgB2 superconducting compound is a light and cheap material promising material for application in superconducting cables and windings. Applicable MgB2 wire are made in the form of filamentary composite structures containing superconducting filaments surrounded by metallic barriers (mostly Nb) protecting the MgB2 from well conductive metallic stabilization (usually Cu). Recently, MgB2 superconducting wires have been made by using of Mg, Ti and Al components, which offer the lightest superconducting wire. Low temperature measurements have shown that high critical current densities can be obtained if Al-stabilization is protected against the interdiffusion with Ti. This material can be attractive for aviation and space applications and also for offshore wind turbines, where the total mass should be limited as much as possible.

FN-7:IL08  Development of Roebel Cables for High Current Applications
W. GOLDACKER, A. KARIO, A. KLING, Karlsruhe Institute of Technology, Institute for Technical Physics, Eggenstein, Germany

Roebel cables from Coated Conductors are low AC loss cables with the potential to carry high currents. The flat structure is ideal for applications in windings and the layout is flexible for transposition length of the strands and applied strand number. The applications of interest are rotating machinery, transformer and large magnets. The behavior of single cables in background fields and different temperatures is quite well understood and described by means of FEM models. Transport AC loss measurements were performed and demonstrate the effect of reduced losses. Investigations on windings are quite new and rare and will be presented for a sample made from 5 m Roebel cable. Pancake coils and layered coils show a significantly different behavior with respect to the current capacity, the self field induced effects and the transport AC losses. Data were measured on different coil designs and calculated with FEM codes. Consequences for the different applications will finally be discussed.

FN-7:IL09  Physics and Applications of High-Tc Superconductors
J.L. TALLON, MacDiarmid Institute & Callaghan Innovation, Lower Hutt, New Zealand  

High-Tc cuprate superconductors (HTS) represent a revolutionary development in the field of superconductivity. Not only does their essential physics mark out completely new territory in the physics of condensed electron systems, the materials themselves exhibit astonishing properties that have already opened up a rather broad range of applications. This talk will attempt to (i) summarise some of the key new understandings and remaining challenges in HTS physics, and (ii) will also traverse the range of current applications of these materials and the remaining challenges in fulfilling the high expectations that emerged with their discovery a quarter of a century ago. The talk will represent a personal perspective from involvement across the full spectrum of fundamental and applied research through to commercialisation.

FN-7:IL11  Recent Progress in Terahertz Emission and Detection of High Temperature Superconducting Bi2Sr2CaCu2O8+d
L. OZYUZER, Y. DEMIRHAN, F. TURKOGLU, H. KOSEOGLU, H. ALABOZ, M. KURT, Department of Physics, Izmir Institute of Technology, Izmir, Turkey; K. KADOWAKI, University of Tsukuba, Tsukuba, Japan

The generation of terahertz (THz) radiation emitted from high temperature superconductor Bi2Sr2CaCu2O8+d (Bi2212) and responses to THz waves increase the importance of applications of layered superconductors [1]. In this work, rectangular Au/Bi2212 mesa structures were fabricated on Bi2212 single crystals. c-axis resistance versus temperature, current-voltage characteristics and bolometer measurements were analyzed. The radiation at 0.54 THz was detected using the Michelson interferometric setup [2]. The emission frequency calculated by a Fourier transformation is consistent with Josephson frequency-voltage relation. We also demonstrated that the necessity of small critical current density for decreasing the heating effect on emission can be satisfied by driving spin polarized current. An emitted power as high as 0.06 mW was detected from Bi2212 mesas. We also fabricated hot electron bolometer using Bi2212 single crystals for THz wave detection. The recent results will be discusssed.
[1] L. Ozyuzer et al., Science 318, 1291 (2007).
[2] F. Turkoglu et al., Supercond. Sci. Technol. 25, 125004 (2012).
This research is supported by Science, Industry and Technology Ministry of Turkey under project number SANTEZ 1386.STZ.2012-1

Poster Presentations

FN:P02  Thermopower Studies of Multiband Superconductors
K. OGANISIAN, K. ROGACKI, Institute of Low Temperature and Structure Research, Wroclaw, Poland; N.D. ZHIGADLO, J. KARPINSKI, Laboratory for Solid State Physics, ETH, Zurich, Switzerland

It is well known that the intra- and interband scatterings play the main role in determination of the superconducting properties of multiband superconductors. A simple method to characterize the transport properties of a multiband superconductor is to measure the thermoelectric power (S) and its anisotropy. To study the influence of the intra- and interband scattering on the normal state properties of the two-band superconductor we have chosen MgB2 as an example of clearly two-band anisotropic material with the orthogonal σ and π bands. In this report we present results of the detailed studies of S(T) for high quality single crystals of MgB2. We analyze the electronic properties of MgB2 separating the thermopower for each band, for the case of a weak interband scattering. We have found that Sab is mainly determined by Sπ instead Sσ as expected. Thus, the π-band has been revealed as determining the electronic transport properties of MgB2 with the Sπ negative contribution and the weak π-band scattering.

FN:P03  Critical Current Density and Pinning Energy of Partial Melted Sm-based Superconductor
H. IMAO, Department of Control Engineering, Matsue National College of Technology, Matsue, Japan

The relationship between critical current density (Jc) and pinning energy in partially melted Sm-based superconductors was investigated. The pinning force and the pinning energy of the superconductor were measured by a system combining an electronic balance and permanent magnet. The partial melted Sm-Ba-Cu-O superconducting bulks were composed of SmBa2Cu3Oy (Sm-123) superconductor and Sm2BaCuOx (Sm-211) insulator. The pinning energy of the samples was influenced by the concentration of Sm-211. The Jc of the samples depended on the pinning energy. However, in the partial melting process, changes in melting times altered the Jc values but had little effect on the value of the pinning energy. The change of the thickness of the bulk shows the same dependence. The results indicated that Jc was influenced by the dispersion of the insulator acting as a pinning center within the superconducting bulks.

FN:P04  Critical Current of Bi-2212 Single Crystal by Doping Oxides as a Pinning Center
T. AGOU, H. IMAO, Department of Control Engineering, Matsue National College of Technology, Matsue, Japan

It is necessary to form pinning centers in superconductors to allow the flow of large currents through the specimens. To clarity the properties of pinning centers, it is preferable to investigate single crystals. In this study, heat treatment was used to dope various oxides into Bi_2Sr_2CaCu_2O_x (Bi-2212) single crystals prepared by self-flux methods and the critical current (Ic) was measured. The oxides used in this study were Al_2O_3 and the rare earth oxides Er_2O_3 and Nd_2O_3. At 77K, Nd_2O_3 and Er_2O_3 are magnetic, whereas Al_2O_3 is nonmagnetic. The Ic of the samples was measured as a current per width of 1 cm (Ics). The resulting Ics of Bi-2212 single crystal was 2.8A/cm and that of the Al_2O_3 -doped Bi-2212 sample was 4.5A/cm. Compared with these samples, the other rare earth oxides doping gave Ics values in excess of 10A/cm. The results indicated that the doping oxides were effective to operate as pinning centers in the samples. We assumed the current path in a single crystal, and calculated the Ics by the superconducting current simulation. The results indicated that the oxides permeated from a crystal surface in the porous shape. The oxides increase the current which flow in the Cu-O_2 planes that are parallel to the a-b plane.

FN:P05  High Critical Currents in Single Grain YBa2Cu3Oy Bulk Superconductors Produced by Infiltration-growth Process
K. NAKAZATO1, M. MURALIDHAR1, K. INOUE1, M.R. KOBLISCHKA2, M. MURAKAMI1, 1Superconducting Materials Laboratory, Department of Materials Science and Engineering, Shibaura Institute of Technology, Tokyo, Japan; 2Institute of Experimental Physics, Saarland University, Saarbrücken, Germany

Large single grain YBa2Cu3Oy "Y-123" superconductors have been grown by infiltrating a Ba3Cu5Oy "Y-035" liquid source into a Y2BaCuO5 "Y-211" precursor block. Samples of 27 mm in diameter and 5 mm thickness were produced by a slow cooling process adopting the optimum growth profile which was constricted on the basis of isothermal growth conditions. The microstructure was studied by means of atomic force microscopy (AFM) and magnetization measurements using a SQUID magnetometer. Magnetization measurements indicated a sharp superconducting transition with Tc (onset) around 93.2 K. The critical current density (Jc) at 77 K and 0T was 175 kAcm-2, which is the highest value of all samples reported in the literature without controlling the initially added Y-211 secondary phase particle size or other nano-sized additions. The Jc values at higher temperatures, like that of 87 K or 90 K were 95 kAcm-2 and 24 kAcm-2, respectively. AFM measurements revealed that uniform nano-sized secondary Y-211 particles are embedded in the Y-123 matrix. The results suggest that the Y-123 bulk samples processed by optimized growth condition has uniform, nano-sized secondary phase particles embedded in the Y-123 matrix, which leads to a dramatic improvement of the performance at 77 K as well as at 90 K.

FN:P06  Terahertz Emission from Triple Mesa Structures Fabricated from Intrinsic Josephson Junctions in Cuprate Superconductors
Y. DEMIRHAN, L. OZYUZER, H. SAGLAM, F. TURKOGLU, H. KOSEOGLU, M. KURT, H. ALABOZ, Department of Physics, Izmir Institute of Technology, Urla, Izmir, Turkey; K. KADOWAKI, University of Tsukuba, Tsukuba, Japan

Although electromagnetic waves in the terahertz frequency region host potential applications as imaging, spectroscopy, information technology, environmental monitoring and medical diagnosis, they have been limited by the lack of powerful, continuous wave and compact solid-state sources. Among the cuprates, Bi2Sr2CaCu2O8+d (Bi2212) is a potential candidate of compact devices operating at the terahertz frequency range. In this study, under optimized doping conditions we aimed to investigate powerful terahertz emission. THz emission in the µW range can be obtained fabricating rectangular-shaped mesa structures on the Bi2212 crystal[1-2]. Mesa structures were fabricated on Bi2212 crystals using e-beam lithography and ion beam etching systems. c-axis resistance-temperature (R-T), and current-voltage behavior (I -V) were measured in a He flow cryostat. During I-V characterization, we used Golay Cell to detect the THz emission.
[1] L. Ozyuzer et al., Science 318, 1291 (2007).
[2] F. Turkoglu et al., Supercond. Sci. Technol. 25 125004 (2012).
This research is supported by Science, Industry and Technology Ministry of Turkey under project number SANTEZ 1386.STZ.2012-1

FN:P07  Transition to the Normal State Induced by High Current Densities in High-Tc Superconductor Microbridges under Thermal Smallness Conditions
J.M. DOVAL, J. MAZA, J.A. VEIRA, M. TELLO*, F. VIDAL, LBTS. Universidad de Santiago de Compostela, Spain; *Facultad de Ciencias, Universidad del País Vasco, Bilbao, Spain

Some of our results on the behavior of different high-Tc cuprate superconductors submitted to current densities high enough to induce their full transition to the normal state (quenching phenomena) are summarized here1,2. We will focus on the influence on the quenching of the so-called "thermal smallness" condition, when the superconductor dimensions becomes smaller than the thermal diffusion length of both the own superconductor and its refrigerant (the substrate, in the case of thin films), which breaks the thermal dimensional scaling. The measurements were performed in YBa2Cu3O7 thin film microbridges with different widths (between 5 µm and 100 µm), under pulsed high current densities with characteristics times in the millisecond range1. These results shows the thermal instability origin of the observed quenching. This conclusion was confirmed, even for commertial superconducting wires, by using an analytical approach for the thermal instabilities2.
1. M. Ruibal, G. Ferro, M. Osorio, J. Maza, J.A. Veira, F. Vidal, Phys. Rev. B 75, 01254 (2007).
2. J. Maza, G. Ferro, J.A. Veira and F. Vidal, Phys. Rev. B 78, 094512 (2008); ibid, B 84,21453 (20011); Supercond. Sci. Technol. 26, 105004 (2013).


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