FO - 8th International Conference
Science and Engineering of Novel Superconductors


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

FO-1:IL01  Why is Tc in Cuprates so high?
I. BOZOVIC, Brookhaven National Laboratory, Upton, NY, USA; and Yale University, New Haven, CT, USA

Superconductivity in cuprates has many mysterious facets, but the central question is why the critical temperature (Tc ) is so high. We use atomic-layer-by-layer molecular beam epitaxy to synthesize atomically perfect thin films and multilayers of cuprates and other complex oxides, and optimize the samples for the particular experiment. I will present the results of a comprehensive study that took twelve years and thousands of cuprate samples, perhaps without precedence in Condensed Matter Physics. The large statistics reveals intrinsic properties. We have measured the key physical parameters of the superconducting and normal states and established their precise dependence on doping, temperature, and external fields. The findings bring in some great surprises, rule out many models, and answer our initial question.
Nature 537, 432 (2017); 536, 309 (2016); 472, 458 (2011); 455, 782 (2008); 422, 873 (2003); Science 326, 699 (2009); 316, 425 (2007); 297, 581 (2002); Nature Mater. 12, 877 (2013); 12, 387 (2013); 12, 1019 (2013); 12, 47 (2013); 11, 850 (2012); Nature Phys. 10, 256 (2014); 7, 298 (2011); Nature Nanotech. 9, 443 (2014); 5, 516 (2010); Nature Comm. 2, 272 (2011); PRL 106, 237003 (2011); 102, 107004 (2009); 101, 247004 (2008); 93, 157002 (2004); 89, 107001 (2002).

FO-1:IL03  High Tc Pairing in Size-selected Metal Nanoclusters
V. KRESIN, P. EDWARDS, University of Southern California, Los Angeles, CA, USA; A. HALDER, Argonne National Laboratory, USA

A unique property of metal nanocluster particles is the "superatom"-like shell structure of their delocalized electrons. The electronic shell levels are highly degenerate and therefore represent sharp peaks in the density of states. This can enable exceptionally strong electron pairing in certain clusters composed of just tens to hundreds of atoms. A study of size-resolved free aluminum nanoclusters, Aln, has revealed a novel phenomenon: a rapid rise in the near-threshold density of states of several specific clusters with decreasing temperature. The characteristics of this behavior are fully consistent with a pairing transition, implying a high-temperature superconducting state with Tc>~100 K. This value exceeds that of bulk aluminum by two orders of magnitude, and can be plausibly raised even higher. This observation of pairing in nanocluster particles represents a new class of high-temperature superconducting systems. The results highlight the potential of novel pairing effects in size-quantized systems and the promise of metal nanoclusters as high-Tc building blocks for materials and networks. Associated work toward detecting enhanced superconductivity in individual size-selected nanoclusters soft-landed on a substrate will be described.

FO-1:IL04  Superconductor / Ferromagnet Films and Superconducting Spintronics
M.G. BLAMIRE, Department of Materials Science, University of Cambridge, Cambridge, UK

The discovery in 2010, using superconductor / ferromagnet / superconductor Josephson junctions, that it is possible to controllably create triplet Cooper pairs in which the electrons have parallel spins created the field of superconducting spin electronics (superspintronics). However, even if triplet pairing implies that supercurrents can carry spin, this is not in itself sufficient to create functioning superspintronic devices. In parallel, a variety of other interactions between singlet superconductivity and magnetism have been actively explored. This talk will cover the key developments in superconducting spintronics that have taken place since the first demonstrations of triplet pairing, including the demonstration of superconducting exchange coupling and superconducting spin valve memory devices.

FO-1:L05  Phenomenological Interpretations of DFT Calculations for Superconductors 
J.A. ALARCO, P.C TALBOT, I.D.R. MACKINNON, Institute for Future Environments, and Science and Engineering Faculty, Queensland University of Technology, Brisbane, Queensland, Australia

Density functional theory calculations have contributed to the current understanding of superconducting materials, particularly those displaying strong electron-phonon coupling. For AlB2-type structures, we have shown that the thermal energies associated with phonon anomalies display one-to-one correspondence to the superconducting transition temperature. The correspondence is accurate for a wide range of conditions including isotopes, metal substitutions, pressure and temperature effects. A key region in reciprocal space is the volume between nearly parallel, warped cylindrically shaped s Fermi surfaces. This region corresponds in real space to localized, largely covalent in-plane Boron-Boron bonds, as identified in the early work of An and Pickett. The phonon anomaly disappears for frozen Boron atomic positions displaced away from their equilibrium position. The difference in Fermi energy between the equilibrium and the displaced positions at which the phonon anomaly disappears matches the superconducting gap energy. It is at this point that the electronic band becomes tangential to the Fermi level and the Fermi surface undergoes a topological change from continuous tubes to separated conical cups. We have recently extended this type of correlation to H3S.

FO-1:L06  New Process for Growing the HgBa2Ca2Cu3O8+δ Super-conductors with the Highest Critical Temperature at Ambient Pressure
B. LORET, A. FORGET, J.-B. MOUSSY, D. COLSON, SPEC, CEA, CNRS-UMR 3680, Université Paris-Saclay, Gif sur Yvette Cedex, France; S. POISSONNET, P. BONNAILLIE, SRMP, DMN, CEA, Université Paris-Saclay, Gif sur Yvette Cedex, France; G. COLLIN, LPS, C.N.R.S. UMR 8502, Université Paris-Sud, Orsay, France; P. THUÉRY, NIMBE, CEA, CNRS, Université Paris-Saclay, Gif sur Yvette Cedex, France; B. LORET, A. SACUTO, Laboratoire Matériaux et Phénomènes Quantiques, Paris Cedex, France

After 30 years of research on high-Tc superconductors, many questions remain concerning the mechanism involved in electron pairing, the high-Tc and its dependence on doping, the relationship between structure and properties and the effect of pressure on superconductivity. Yet this understanding is crucial to achieving higher Tc values that would allow the development of considerable technological applications. Mercury cuprates could be good candidates for this challenge. Indeed, since their discovery in 1993, they still show the highest Tc at ambient pressure, 133 K (160K at 30 GPa) for HgBa2Ca2Cu3O8+δ (Hg1223). Unfortunately, very few studies have been carried out for lack of single crystals, although mercury cuprates can be considered as model structures for understanding the mechanisms of high-Tc, due to their tetragonal symmetry and the record of Tc. In this perspective, we developed an original procedure for the elaboration of high-quality single crystals of superconducting Hg1223 and a method precise control of their oxygen content (doping), which will make it possible to probe the phase diagram of these compounds in a wider range of composition [1]. Structural study, magnetic and Raman measurements will be presented too.
[1] B. Loret et al., Inorg. Chem. 56, 9396 (2017)

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

FO-2:IL01  Magnet Application of Iron-based Superconductors
AKIYASU YAMAMOTO, Department of Applied Physics, Tokyo University of Agriculture and Technology, Tokyo, Japan; J. WEISS, Department of Physics, University of Colorado, Boulder, CO, USA; M. AINSLIE, Department of Engineering, University of Cambridge, UK; A. POLYANSKII, D. LARBALESTIER, E. HELLSTROM, Applied Superconductivity Center, National High Magnetic Field Laboratory, Florida State University, USA  

Iron-based superconductors (IBSCs) with high Tc of ~58 K in 1111 and ~38 K in 122 are interesting for cryocooled high field applications, owing to their high Hc2 with small anisotropy nearly ~1 (in 122). The unique advantage which distinguishes IBSCs from cuprates is high critical misorientation angle of 9 degrees. Thus high critical current over a sample is expected in randomly oriented, polycrystalline form, which leads to low-cost and scalable production. In this study, we report the field trapping properties of untextured polycrystalline 122 bulks. Trapped fields of over 1 T at 5 K and 0.5 T at 20 K have been measured between a stack of magnetized cylinders of bulk polycrystalline Ba0.6K0.4Fe2As2 10 mm in diameter. Magneto-optical imaging revealed a trapped-field distribution corresponding to uniform macroscopic current loops circulating through the sample. A standard numerical modeling technique using the Jc(B, T) characteristics measured from a small specimen performed well in reproducing the experimentally measured trapped fields, again indicating the homogeneous current loops. Since field dependence of Jc is much better than that of MgB2, larger IBSC bulks would be interesting for high field trapping magnet and other applications to be operated by compact cryocoolers.

FO-2:IL02  Laser ARPES Study on High Temperature Superconductors
XINGJIANG ZHOU, National Lab for Superconductivity, Institute of Physics, Chinese Academy of Sciences, Beijing, China

The mechanism of high temperature superconductivity in the copper-based and iron-based superconductors remains a prominent and challenging issue in condensed matter physics. Angle-resolved photoemission spectroscopy (ARPES), as a powerful technique to directly probe the electronic structure of materials, has played a key role in studying high temperature superconductors. In this talk, I will first briefly introduce our latest progress on the development of vacuum ultra-violet laser-based angle-resolved/spin resolved photoemission techniques with superior performance. Then, I will highlight some of our recent results in utilizing the state-of-the-art laser-based ARPES to study the electronic structure and superconductivity of the copper-based superconductors [1-3] and iron-based superconductors [4-8].
[1] Yingying Peng and X. J. Zhou et al., Nature Communications 4, 2459(2013). [2] Junfeng He and X. J. Zhou et al., Physical Review Letters 111, 107005 (2013). [3] J. M. Bok and X. J. Zhou et al., Science Advances 2, e1501329 (2016). [4] Defa Liu and X. J. Zhou et al., Nature Communications 3, 931 (2012). [5] Shaolong He and X. J. Zhou et al., Nature Materials 12, 605 (2013). [6] Xu Liu and X. J. Zhou et al., Nature Communications 5, 5047 (2014). [7] Junfeng He and X. J. Zhou et al., PNAS 111, 18501 (2014). [8] L. Zhao, X. J. Zhou et al., Nature Communications 7, 10608 (2016).

FO-2:IL03  Novel Effects in Multilayer Superconductor/Magnet Films
C. BERNHARD, University of Fribourg, Department of Physics and Fribourg Center of Nanomaterials (FriMat), Fribourg, Switzerland

We observed an intriguing proximity-induced insulator-to-superconductor transition in YBa2Cu3O7/Pr0.5La0.2Ca0.3MnO3 (YBCO/PLCMO) multilayers [1]. The PLCMO layers are designed (by adjusting the tolerance factor) to undergo a magnetic-field-induced transition from an insulating, charge/orbital ordered state at low field to an itinerant, ferromagnetic state at high fields. To our surprise, a coherent superconducting response of the YBCO layer is only observed at high magnetic fields. At low field, where PLCMO is in the charge/orbital ordered state, the low temperature response of the YBCO layer is highly resistive and resembles the one of a granular superconductor or a frustrated Josephson-junction network. This coincidence suggests that an interface effect due to the charge/orbital order of PLCMO is at the heart of this magnetic-field-induced insulator-to-superconductor transition. Evidence for this has been obtained from resonant inelastic x-ray scattering (RIXS) experiments in zero magnetic field which confirm that a static Cu-CDW order is induced (or strongly enhanced) in the YBCO layer.
[1] B.P.P. Mallett et al., Phys. Rev. B 94, 180503(R) (2016).

FO-2:IL04  Irradiation-induced Decoupling between Critical Temperature and Energy Gaps in P-doped Ba-122 Films
D. DAGHERO, M. TORTELLO, L. GOZZELINO, R.S. GONNELLI, Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Torino, Italy; T. HATANO, T. KAWAGUCHI, H. IKUTA, Department of Crystalline Materials Science, Nagoya University, Nagoya, Japan   

We studied the effects of Au-ion irradiation on the morphological, structural and superconducting properties of optimally P-doped Ba-122 epitaxial thin films grown on MgO substrate. We used three fluences, up to 7.3 x 10^11 cm^(-2). We found that irradiation suppresses the critical temperature very little (by about 2%) on going from the pristine to the most irradiated film, with a small increase in the transition width. In contrast, the residual resistivity increases by almost 60%, probably because of the additional defects created by ion implantation and heating in the substrate. Point-contact Andreev-reflection spectra taken along the c axis clearly show distinct features associated to two superconducting gaps, apparently nodeless (even though horizontal node lines could be present). Surprisingly, the amplitudes of the two gaps are strongly suppressed by irradiation (by about 25%) so that the relevant gap ratios strongly decrease as a function of fluence. This indicates an unusual decoupling between Tc and the gap amplitudes that could be explained as being due to quasiparticle confinement by defects, which makes the effective density of states available for pairing be higher near Tc than at low temperature.

FO-2:IL05  Electronic and Magnetic Structures of H-doped 1111-type High Tc Superconductors
SOSHI IIMURA, HIDEO HOSONO, Tokyo Institute of Technology, Yokohama, Japan

Hot issue in unconventional superconductors is why the 2nd highest-Tc of 56 K after cuprates is accomplished in electron-doped iron-oxyarsenides LnFeAsO1−xFx (Ln = lanthanide). Electronic phase diagram depicting the evolution of the ground state with doping level is a powerful tool to understand the mechanism of superconductivity (SC). However, owing to a low solubility of the fluorine in the oxygen-site, i. e. x < 0.15, the phase diagram is not well understood, despite the fascinating properties. Here we show crystallographic, magnetic, electronic, and SC properties of hydride-ion (H−)-substituent, LnFeAsO1−xHx, over a wide range of x from 0 to 0.8. We discovered a second SC phase with Tc of 36 K for the x range of 0.18 ≤ x ≤ 0.45 in La-system in addition to the first one (Tc = 29 K, 0.05 ≤ x < 0.18) adjacent to the AFM order at x = 0. Replacing the La with Sm or applying pressure into the La-system, the two-dome structure of Tc(x) merges into a single-dome with an optimum Tc exceeding 50 K. In the over-doped regime of La- and Sm-system, another AFM order was developed. The bipartite AFM phase provide us with an interesting example to illustrate the intimate interplay among the two kinds of magnetic fluctuations and high-Tc superconductivity.

Session FO-3 - Properties of Superconductors

FO-3:IL01  Towards Atomic-scale Andreev Reflection
JOHN Y.T. WEI, University of Toronto & Canadian Institute for Advanced Research, Toronto, Canada

Andreev reflection (AR) is a fundamental particle-hole conversion process that enables current continuity across superconductor/normal-conductor (S/N) interfaces. By virtue of its dependence on spin, momentum and time-reversal invariance, AR can be used to study a host of basic properties, ranging from the spin polarization of itinerant ferromagnets to the pairing symmetry of unconventional superconductors. Traditionally, the spatial resolution of AR measurements is limited by the need for high-transparency S/N contact, such that the probability of AR outweighs that of quasiparticle tunneling (QPT). However, recent studies using near-quantum point contacts have achieved AR through merely a few Landauer conductance channels. In this talk, I will discuss and demonstrate the prospects for AR measurements to reach and even exceed the quantum point-contact limit, by exploiting the QPT process rather than competing against it.
Work supported by NSERC, CFI-OIT and CIFAR Quantum Materials Program.

FO-3:IL02  What do we Really Understand in all Novel High-Tc Super-conductors: Orbitals in Three Dimensions
D.K. SUNKO, Department of Physics, Faculty of Science, University of Zagreb, Zagreb, Croatia 

The dominant paradigm of the study of high-Tc superconducting cuprates has been to consider the two-dimensional copper oxide planes separately from the rest of the material. All exotic properties of the metallic sector are thus automatically ascribed to strong correlations in the corresponding two-dimensional electron gas. Here, arguments are presented that such a separation has become untenable under the accumulated experimental evidence. The main conclusion is that cuprate perovskites are between the textbook metallic and ionic limits, so that the metal in the copper-oxide plane and the dielectric perpendicular to the plane must be considered simultaneously at all points of the phase diagram relevant to superconductivity. Long-standing issues of the Fermi arcs are reviewed from the three-dimensional point of view. Some still open problems are considered under the same principle, in particular the two-dimensional nature of the superconductivity itself. Parallels between cuprates, pnictides, and hydrides are pointed out. A new physical and chemical paradigm is proposed: metallization of the polar bond. Experimental evidence for it is reviewed, especially in cuprate transport measurements, and a new direction in materials research is proposed.

FO-3:IL03  Collapse of High-Tc Superconductivity via Ultrafast Quenching of the Phase Coherence
F. BOSCHINI1, E. RAZZOLI1, E.H. DA SILVA NETO2, M. ZONNO1, G. LEVY1, G.G. GU3, D.J. JONES1, G. GIANNETTI4, A. DAMASCELLI1, 1Quantum Matter Institute, University of British Columbia, Vancouver, Canada; 2UC Davis, Davis, USA; 3Brookhaven National Laboratory, Upton, USA; 4Università Cattolica del Sacro Cuore, Brescia, Italy

One of the most fascinating properties of low-density condensates is the emergence of phase transitions driven solely by the fragility of the phase coherence. This intriguing physics has triggered an intense search for tools to control the rigidity of superconducting phases and investigate the collapse of superconductivity induced by phase fluctuations. Electrically-gated oxide interfaces, ultracold Fermi atoms and cuprate superconductors, which are characterized by an intrinsically small phase-stiffness, are paradigmatic examples. Here, we use ultrashort light pulses to probe and drive the phase fragility of the Bi2Sr2CaCu2O8+δ cuprate superconductor, up to the point of completely quenching the phase coherence without affecting the electron pairing. Time-resolved photoemission is used to track and disentangle the dynamics of phase fluctuations and charge excitations. This work demonstrates the dominant role of phase coherence in the emergence of high-temperature superconductivity and offers a new benchmark for non-equilibrium investigations of the phase diagram.

FO-3:IL04  Analyzing Supercurrents with x-ray Eyes 
J. ALBRECHT, Research Institute for Innovative Surfaces FINO, Aalen University, Germany; J. SIMMENDINGER, S. RUOSS, G SCHÜTZ, MPI for Intelligent Systems, Stuttgart, Germany 

Scanning transmission x-ray microscopy has been used to image electric currents in superconducting films at temperatures down to 20 K. The magnetic stray field of supercurrents in a thin YBaCuO film is mapped into a soft-magnetic coating of permalloy. The so created local magnetization of the ferromagnetic film can be detected by dichroic absorption of polarized x-rays. To enable high-quality measurements in transmission geometry the whole heterostructure of ferromagnet, superconductor and single-crystalline substrate has been thinned to an overall thickness of less than 1 µm. With this novel technique local supercurrents can be analyzed in a wide range of temperatures and magnetic fields. A magnetic resolution of less than 100nm together with simultaneuosly obtained nanostructural data allow the correlation of local supercurrents with the micro- and nanostructure of the superconducting film.

FO-3:L06  Comprehensive Phase Diagram of Two-dimensional Space Charge Doped Bi2Sr2CaCu2O8+x
E. STERPETTI1, J. BISCARAS1, A. ERB2, A. SHUKLA1, 1Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Sorbonne Université Paris 06, UMR CNRS 7590, MNHN, IRD UMR 206, Paris, France; 2Walther Meissner Institut fur Tieftemperaturforschung, Bayerische Akademie der Wissenschaften, Garching Germany

The phase diagram of hole-doped high critical temperature superconductors as a function of doping and temperature has been intensively studied with chemical variation of doping. Chemical doping can provoke structural changes and disorder, masking intrinsic effects. Alternatively a field-effect transistor geometry with an electrostatically doped, ultra-thin sample can be used. However to probe the phase diagram, carrier density modulation beyond 1014cm-2 and transport measurements performed over a large temperature range are needed. Here we use the space charge doping method1,2 to measure transport characteristics from 330~K to low temperature. We extract parameters and characteristic temperatures over a large doping range and establish a comprehensive phase diagram for one unit cell thick Bi2Sr2CaCu2O8+x as a function of doping, temperature and disorder3.
1. Johan Biscaras et al., Nature Communications 6, 8826 (2015). 2. A. Paradisi et al., Applied Physics Letters 107, 143103 (2015). 3. E.Sterpetti et al., Nature communications (Accepted 7 November 2017)

FO-3:L07  Effects of Dy2O3 doping on the Anisotropy and Transport of MgB2 Wires 
M.D. SUMPTION, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH, USA   

In this work Dy2O3 doping in MgB2 strands is shown to enhance critical current density and irreversibility fields in different temperature regimes. For higher doping amounts (4-6wt%) increase in Bc2 and Birr are seen in C doped MgB2 strands which already have enhanced Bc2. On the other hand, for lower co-doping levels, transport and Birr are enhanced at higher temperature (10 K to 30 K), without changing the transition temperature Tc or upper critical magnetic field Bc2 of MgB2. For a series of carbon and Dy2O3 co-doped MgB2 wires with 2.0 wt. % C and Dy2O3 concentrations ranging from 0 to 6.0 wt. %. the transport critical current density of the co-doped MgB2 wire was increased by x 2 at 4 K (6 T), and doubled at 20 K (1 T), which (coupled with the Bc2 and Birr results) suggests a reduction in the anisotropy induced by Dy2O3 doping. The specific heats of the MgB2 wires were measured to investigate the anisotropy, and the results are consistent with a reduction an anisotropy, with the practical result that Dy2O3 is shown to be the first dopant since C to be truly effective for MgB2, and it is particularly effective at higher temperatures, in contrast to C.
Acknowledgements: This work was supported by the NIH under R01EB018363

FO-3:L08  Properties and Structure of MgB2-based Superconductors
T. PRIKHNA1, V. ROMAKA2, M. EISTERER3, A. KOZYREV1, A. SHAPOVALOV1, A. SHATERNIK1, 1V. Bakul Institute for Superhard Materials of the National Academy of Sciences of Ukraine (NASU), Kiev, Ukraine; 2Lviv Polytechnic National University, Lviv, Ukraine; 3Atominstitut, TU Wien, Vienna, Austria 

The structures of MgB2-based materials with high SC characteristics manufactured under different pressure-temperature conditions in the form of blocks, wires and thin-films usually contain admixed oxygen as х-ray, SEM and Auger spectroscopy study showed. The higher synthesis temperatures lead to higher critical current densities, Jc, in low and medium magnetic fields, but also to lower Jc at higher fields and higher critical magnetic fields Bc2 and Birr. Usually three main phases were present in the materials: (1) near MgB2 matrix which contained a small amount of admixed oxygen (MgB2.2-1.7O0.4-0.6); (2) inclusions of magnesium borides MgBx, x>>2; (3) nanolayers (if the synthesis temperature was low) or separate oxygen enriched inclusions (if it was higher) with near MgBO stoichiometry. Rietveld refinement of EDX data showed that the main phases had MgB1.68-1.8O0.2-0.32 compositions. Ab-initio modeling of boron substitution by oxygen in MgB2 (ΔHf =-150.6 meV/atom) demonstrated that this is energetically favorable up to the composition MgB1.75O0.25 (ΔHf =-191.4 meV/atom) and that oxygen can be present only in one B plane of MgB2 unit cell due to the tendency to agglomerate, but each second B plane can stay clean (the last was supported by the experimental data found in literature).

FO-3:IL10  Density Waves of HTSC in Atomic Scale 
JINHO LEE, Dept. of Physics and Astronomy, Seoul National University, Republic of Korea CCES, Institute of Basic Science, South Korea   

Pseudo-Gap (PG) phase of the cuprates is believed to hold a key to the understanding of the mechanism of high Tc superconductivity. Recently, discoveries of CDW phase in PG phase have been reported on cuprate superconductors, but its detailed origin is still not fully understood. We studied PG phase of BSCCO 2212 using variable temperature STM and observed density modulations similar to the reported CDW’s wave vectors. Unlike the previously reported CDW’s, however, we find these density modulations disperse in particle - hole asymmetric manner and their correlation length is not a conventional one. On the other hand, we succeeded in realizing the Scanning Josephson Tunneling Microscopy (SJTM) and discovered wave-like features in the Cooper pair density by directly probing the Josephson current. In this presentation, we will discuss a new length scale in PG phase – phase coherence length of these density modulations in real-space and their possible origin as well as their relation to the superconductivity by comparing these PG density waves to the recently discovered PDW (Pair Density Wave).

FO-3:IL11  A Fresh View of the Unusual Properties of the Cuprates
N. BARISIC, Institute of Solid State Physics, TU Wien, Wien, Austria; Department of Physics, Faculty of Science, University of Zagreb, Zagreb, Croatia

One of the major challenges in condensed matter physics is to understand the enigmatic behavior exhibited by the cuprates. We have performed thorough experimental study of HgBa2CuO4+δ, which in many respects is a model cuprate compound. From the comparison with data for other cuprates we are able to separate universal underlying behavior from compound-specific features. Perhaps the most remarkable finding is the existence of an underlying Fermi-liquid scattering rate that remains essentially unchanged across the phase diagram [1,2]. Guided by established universalities, and by the knowledge that the cuprates are inherently inhomogeneous, we propose a simple model in which exactly one localized hole per planar copper-oxygen unit is delocalized and becomes itinerant with increasing doping and temperature [3]. The model is percolative in nature, with parameters that are nearly compound- and doping-independent and experimentally constrained. It comprehensively captures pivotal unconventional experimental results, including the temperature and doping dependence of the pseudogap phenomenon, the strange-metal linear temperature dependence of the planar resistivity, and the doping dependence of the superfluid density.
[1] N. Barišić et al., arXiv:1507.07885. [2] Y. Li et al., Phys. Rev. Lett. 117, 197001 (2016). [3] D. Pelc et al., arXiv:1710.10221.

Session FO-4 - Theory and Mechanisms

FO-4:IL02  Superconductivity in Time Reversal Symmetry Breaking Compounds
HUIQIU YUAN, Center for Correlated Matter and Department of Physics, Zhejiang University, China

Unconventional superconductivity with additional symmetry breaking has been one of the most attractive topics in condensed matter physics during recent decades. Here we will present our recent studies on a few superconductors where the time reversal symmetry (TRS) is broken upon entering the superconducting state. LaNiC2 and LaNiGa2 display simple metallic behavior in the normal state and our measurements demonstrate the presence of two fully open superconducting gaps in these two compounds [1,2]. On the other hand, the muSR measurements provide evidence for TRS breaking below Tc [3,4]. These results allow us to propose a nonunitary triplet pairing state, which both breaks TRS and leads to nodeless two-gap superconductivity, and therefore accounts well for the seemingly contradictory experimental results of LaNiC2 and LaNiGa2 [1]. In addition, we have recently synthesized a series of new superconductors, which also lack inversion symmetry and have broken TRS [5]. We performed various measurements to characterize the superconducting order parameter, and the possible pairing states will be discussed.
References: [1] Z.F. Weng et al. Phys. Rev. Lett. 117, 027001 (2016). [2] J. Chen et al. New J. Phys. 15, 053005 (2013). [3] A.D. Hillier, et al. Phys. Rev. Lett. 102, 117007 (2009). [4] A.D. Hillier, et al. Phys. Rev. Lett. 109, 097001 (2012). [5] W.B. Jiang, T. Shang et al., unpublished.

FO-4:IL03  Superconductivity in Topological Materials: Insights from Superconducting Density Functional Theory
RYOTARO ARITA, RIKEN Center for Emergent Matter Science, Saitama, Japan

Superconductivity in doped topological insulators or topological crystalline insulators is of great interest, since there have been several experimental studies suggesting that they are promising candidates of topological superconductors. To perform first-principles calculation of superconductivity in such topological materials, there are several problems: First, since the energy scale of the Fermi energy is very small, we need to consider some unconventional pairing mechanism such as the cooperation between plasmon and phonon. Second, we have to take account of the wave number dependence (such as sign changes) of the pairing gap function. Third, the spin-orbit coupling can affect the phonon frequencies, electron-phonon coupling and electronic structure. Recently, we have performed a fully-nonempirical calculation for doped SnTe (a topological crystalline insulator) and Bi2Se3 (a topological insulator) based on superconducting density functional theory. We will discuss how the relativistic effect or plasmon play a crucial role, and in which situation odd-parity (topological) superconductivity can emerge in these topological materials.
This work was done in collaboration with T. Nomoto, T. Koretsune and M. Kawamura.

FO-4:IL06  Robust Dynamical Charge Density Waves in High-Tc Superconducting Cuprates 
M. GRILLI, Dipartimento di Fisica, Università di Roma "Sapienza", Rome, Italy

There is increasing consensus that charge modulations are a common occurrence in all families of high temperature superconducting cuprates. Charge ordering (CO) arises from a zero-temperature phase transition (a quantum critical point) around optimal doping. Due to their quantum nature and competition with superconductivity, the charge fluctuations are dynamical and could provide a possible mechanism for the non-Fermi-liquid transport and (contribute to) the high superconducting Tc. Commensuration, pinning and/or surface effects make the CO more static and its presence was confirmed in the underdoped region by inelastic neutron scattering, STM, or NMR. However, at higher doping and temperature, where the short-range fluctuating character of the CO is more pronounced, RIXS is most suitable to investigate the presence and the character of CO fluctuations and it provides a clear evidence that strong quite long-ranged CO fluctuations extend even in the overdoped region of cuprates [1]. Moreover, the important role of a van Hove singularity in driving CO in the overdoped regime indicates that this occurs as an instability of a metallic Fermi liquid state with no influence of long-ranged spin excitations and `Mottness’.
[1] Y. Y. Peng, et al., arxiv:1705.06165

FO-4:IL07  Fermi Surface Reconstruction in the Pseudogap State 
J. STOREY, Robinson Research Institute, Victoria University of Wellington, Wellington, New Zealand

Hole-doped cuprate high-temperature superconductors are distinguished by a partial gap in the electronic spectrum, known as the pseudogap, which persists far above the superconducting transition temperature. Key questions concern its origin and whether it is essential in any way to high-temperature superconductivity. Here I will demonstrate that the physical properties observed experimentally in the pseudogap state are accounted for by a class of Fermi surface reconstruction models, in which the Fermi surface collapses in size via an intermediate regime of hole- and electron-like pockets. Finally I will discuss some of the important remaining questions to be answered, such as identifying the mechanism behind the reconstruction, as well as the prospects for raising the superconducting transition temperature.
Session FO-5 - Vortex Lattice Physics

FO-5:IL01  Anomalous Enhancement of Critical Current Density in Iron-based Superconductors with Splayed Columnar Defects Anomalous Enhancement of Critical Current Density in Iron-based Superconductors with Splayed Columnar Defects
TSUYOSHI TAMEGAI, A. PARK, N. ITOH, N. YAMAOKA, S. PYON, Department of Applied Physics, The University of Tokyo, Tokyo, Japan; T. KAMBARA, A. YOSHIDA, Nishina Center, RIKEN, Wako, Saitama, Japan; S. Okayasu, Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki, Japan; A. ICHINOSE, Central Research Institute of Electric Power Industry, Electric Power Engineering Research Laboratory, Yokosuka, Kanagawa, Japan

Iron-based superconductors can sustain a large critical current density (Jc) over 1 MA/cm2 at low temperatures. Further enhancement of Jc can be achieved by introducing artificial defects in terms of high-energy particle irradiation. We have demonstrated five-fold enhancement of Jc by introducing columnar defects (CDs) by irradiating 200 MeV Au ions into Ba(Fe,Co)2As2. Further enhancement of Jc up to ~15 MA/cm2 has been achieved in (Ba,K)Fe2As2 by irradiating 2.6 GeV U ions. Even stronger enhancement of Jc is predicted by splaying the direction of CDs, and has been implemented in YBa2Cu3O7. We applied this idea to (Ba,K)Fe2As2, and achieved Jc = 19.5 MA/cm2 by irradiating 2.6 GeV U ions at angles of ±5°. Interestingly, when the splay angle is increased to ±20°, an anomalous peak effect (PE) shows up at ~BΦ/3 when the field is applied along the average direction of splayed CDs. By changing the ion species and/or their energy, the splay angle range for the anomalous PE changes. We compare this anomalous PE in (Ba,K)Fe2As2 with similar PE in cuprates with parallel CDs, and discuss its origin.

FO-5:IL02  Point-like Defects for Enhanced Flux Pinning in Technical Nb3Sn Superconductors
J. BERNARDI, S. PFEIFFER, USTEM, Technische Universität Wien, Wien, Austria; T. BAUMGARTNER, M. EISTERER, Atominstitut, Technische Universität Wien, Wien, Austria; L. BOTTURA, C. SCHEUERLEIN, A. BALLARINO, CERN, Geneva, Switzerland 

Within the the FCC study the research on Nb3Sn wires was intensified during the last years, because for the FCC project superconducting magnets with high critical current Jc, surpassing the properties of current magnets, are required. Recently it was shown (1), that neutron irradiation with fast neutrons (E > 0.1 MeV) influences the volume pinning force within these wires most likely by the formation of point-like pinning centers, and leads to a significant increase of Jc. In the present investigation we explore the microstructure of state-of-the-art Ti-doped Nb3Sn multifilamentary RRP-wires which were exposed to different fluences. We apply electron microscopy and correlate the findings with magnetometry measurements.
1) T. Baumgartner et al., Supercond. Sci. Technol. 27 (2014), 015005.

FO-5:IL03  Flux Pinning in Oxypnictide Thin Films
KAZUMASA IIDA1,2, C. TARANTINI3, J. HÄNISCH4, F. KURTH5, J. JAROSZYNSKI3, T. OHMURA2, T. MATSUMOTO1, T. URATA1,2, T. HATANO1,2, S. MEYER4, S. KAUFFMANN-WEISS4, B. HOLZAPFEL4, D.C. LARBALESTIER3, H. IKUTA1,2, 1Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, Japan; 2Department of Materials Physics, Graduate School of Engineering, Nagoya University, Japan; 3Applied Superconductivity Center, National High Magnetic Field Laboratory, Florida State University, USA; 4Institute for Technical Physics, Karlsruhe Institute of Technology, Germany; 5Institute for Metallic Materials, IFW Dresden, Germany

Since the discovery of Fe-based superconductors (FBS), a lot of progresses in the growth of single crystals and thin films have been made. Now high quality FBS thin films are available, which spurs progress of fundamental as well as application research. We have focused on NdFeAs(O,F) thin films because of the highest Tc amongst FBS in the bulk form. By exploring transport properties we found a vortex trapping and lock-in by the Nd(O,F) blocking layer in NdFeAs(O,F) when the applied magnetic field is close to the ab-plane. The vortex trapping is hallmarked by a drop of exponent n in E~Jn (E and J are electric field and current density), whereas a local peak in the dip of n is a sign of the vortex lock-in. Additional evidence for the vortex lock-in is an almost field-independent critical current density Jc for H//ab at low temperature. We have also fabricated artificial grain boundaries (GBs) on NdFeAs(O,F) by using [001]-tilt MgO bicrystal substrates. It was found that Jc is decreased exponentially with increasing the grain boundary angle (θGB) and the decay rate of Jc against θGB is similar to YBa2Cu3O7 (YBCO).
This work was partially supported by JSPS KAKENHI Grant Number 16H04646.

FO-5:L04  Nature of the Second Magnetization Peak in Superconducting Single Crystals
L. MIU, National Institute of Materials Physics, Bucharest-Magurele, Romania

The nature of the second magnetization peak (SMP) appearing on the dc magnetic hysteresis curves of superconducting single crystals with random pinning is still under debate. Many interesting SMP models and mechanisms have been proposed and considered so far, and it is believed at present that this effect is system dependent. However, by analyzing the dc magnetization relaxation in the SMP domain for various single crystal specimens (superconducting cuprates and iron-based superconductors), of different pinning strength, with the external magnetic field H oriented along the crystallographic c axis or perpendicular to it, we found several sample independent, characteristic aspects, such as the absence of single-vortex collective pinning around the SMP onset field, and the sign changing of the vortex creep exponent between the onset field and the peak field. This general behavior and the peculiar disappearance of the SMP in some underdoped cuprates and pnictides attributed by us to the presence of static magnetic and charge order support strongly the pinning-induced disordering of the low-H quasi-ordered vortex solid as the actual scenario for the SMP.

FO-5:IL05  STM Studies of Vortices in FeSe Single Crystals
M. IAVARONE, Department of Physics, Temple University, Philadelphia, PA, USA

In spite of its simple crystal structure, the electronic properties of the iron-based superconductor FeSe (Tc ~ 9 K) are rich and attractive. Superconductivity in FeSe takes place in a so-called nematic phase that is associated with orbital ordering. Another interesting aspect is that Fermi wave length is as long as the coherence length therefore, placing FeSe most likely in the BCS-BEC crossover regime. These features should result in non-trivial electronic states around the local defects such as vortices and impurities. We have performed low temperature STM/STS experiments on FeSe to investigate its electronic properties. Vortex matter in this system will be discussed and connected to the multiband superconductivity nature of this material.

Session FO-6 - Synthesis and Processing

FO-6:IL01  Growth and Properties of Novel Superconducting Materials
G. LOGVENOV, GIDEOK KIM, Max Planck Institute for Solid State Research, Stuttgart, Germany

Oxide Molecular Beam Epitaxy (oxide MBE) is a unique technique for the synthesis of high quality oxide thin films and heterostructures devoted to fundamental studies and to the fabrication of nano-sized devices. In recent times, such a growth method has been customized for the realization of different multicomponent oxide materials, including superconducting heterostructures. In the present article, the most important aspects related to the oxide MBE are discussed, including design, control of the growth process, different deposition schemes, advantages and challenges of the method. A focus will be put on the synthesis of novel superconducting oxides heterointerfaces by using atomic layer-by-layer (ALL) oxide MBE, an advanced deposition method which allows for designing materials down to the atomic layer level. Several successful examples of high temperature superconducting oxide heterostructures and heterointerfaces, which have been fabricated by oxide MBE, will be presented, demonstrating the power of the ALL oxide MBE method [1,2,3].
References: [1] Baiutti F., et al., Nat. Commun. 6 (2015), 8586. [2] Suyolcu Y.E., et al., Scientific Reports, 7:453 (2017) 1. [3] Kim G., et al., accepted in Phys. Rev. Mater. (2018).

Session FO-7 - Superconductor Applications

FO-7:IL01  High Temperature Superconductors for Rotating Machinery and Power Applications
J.L. TALLON, Robinson Research Institute, Victoria University of Wellington, Lower Hutt, New Zealand

Cuprate high-temperature superconductors have matured to the extent that many applications are now possible and indeed are currently being rolled out. Conductor performance is truly impressive and consistency of performance is now sufficiently reliable. The main obstacle to widespread applications is probably only cost and the best means to drive this down is ongoing improvement in performance and, perhaps more importantly, increased volumes – preferably by several orders of magnitude. This talk summarizes our experience in New Zealand in developing HTS conductors and devices, especially in the present context, for energy applications but also for NMR and MRI. We traverse the fundamentals of conductor physics, cable manufacture and ac losses, progressing to charging using flux pumping and then to engineered products, including rotating machinery, transformers, next-generation aerospace propulsion and MRI.

FO-7:IL02  Superconducting Thin-film Quantum Circuits: Coherence Limits
A. USTINOV, Karlsruhe Institute of Technology, Karlsruhe, Germany 

Over the past 15 years, superconducting thin-film devices as microwave resonators and qubits have shown amazing improvements of their quality factors and coherence times. Superconducting quantum circuits are presently the most favored candidates towards building future quantum computers and simulators. Nevertheless, the quantum coherence and microwave losses in these nearly-perfect superconducting components are limited by several materials related problems. I will review known physical mechanisms limiting the coherence of superconducting qubits and resonators and will discuss, in particular, our recent experiments [1,2] detecting and manipulating individual two-level defects on the surfaces and interfaces of these devices.
[1] J. Lisenfeld, G.J. Grabovskij, C. Mueller, J.H. Cole, G. Weiss, A.V. Ustinov, Nature Commun. 6, 6182 (2015). [2] J. Lisenfeld, A. Bilmes, S. Matityahu, S. Zanker, M. Marthaler, M. Schechter, G. Schön, A. Shnirman, G. Weiss, A.V. Ustinov, Sci. Rep. 6, 23786 (2016).

FO-7:IL03  On Progress in Superconducting Electronics  
S. PAGANO1,2, N. MARTUCCIELLO2,1, 1Physics Department, University of Salerno, Fisciano (SA), Italy; 2C.N.R. SPIN Salerno, Fisciano (SA), Italy 

Energy efficiency has become a key issue for modern high performance computing. During the last fifty years the density of integration has followed an almost exponential increase. However, a large integration level comes with a large energy dissipation, that has reached the level of 100 W/cm2. This limits the maximum clock frequency to about 4 GHz and poses severe limitation to the number of active elements in a chip that can be powered at any time. Josephson junction based superconducting electronics has been proposed for high performance computing mainly for the higher clock frequency (up to 200 GHz) and energy efficiency. The major difficulty for using superconducting technologies is the lower integration scale available today (about five orders of magnitude). Improving the integration level requires investments both in the fabrication technology and in the logic cell design. In particular memory elements represent a critical point, as the minimum memory cell size spans about 100 μm2. Current state of the art in energy efficient superconducting digital circuits and memories will be reviewed. In addition, a novel proposal of non Josephson superconducting memory elements, based on magnetic etherostructures with superconducting nanowires, will be discussed in details.

FO-7:IL04  Novel Josephson Junctions with Non-zero Ground State Phase  
E. GOLDOBIN, R. MENDITTO, D. KOELLE, R. KLEINER, University of Tübingen, Tübingen, Germany

Nowadays researchers are able to fabricate Josephson junctions (JJs) that have a non-zero phase in the ground state. In comparison, conventional JJs have a ground state phase equal to zero. Several groups already demonstrated the JJs with the ground state phase equal to pi, some arbitrary phase varphi_0 and doubly degenerate ±varphi. Different physical mechanisms, such as d-wave superconductivity, interaction of superconductivity with ferromagnetism or spin-orbit interaction can be used to achieve this. The constant phase can be used as a phase battery[4, 5] to phase-bias superconducting electronic circuits and pave the road towards truely low-dissipation superconducting electronics. In addition, such devices like varphi JJs with degenerate ground state provide more functionality. First, their ground state is degenerate and can be used as 1 bit of information storage as demonstrated by our group. Second, the (asymmetry of the) Josephson energy profile can be controlled in-situ so that one can build ratchet-like devices. Such experiments showed rather good figures of merit in deterministic regime. In the future, we expect the appearance of other novel functional Josephson devices based on new physical effects that will allow to enchance the performance of Josephson circuits.

Cimtec 2018

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