Recent progress in superconductivity of two-dimensional layered systems

Resume : Two dimensional (2D) phenomena in complex oxides heterointerfaces attract attention of scientific community during the last ten years due to their multitudinous functionalities and promising applications. The progress in this field has been mostly due to the substantial improvments of the deposition techniques such as pulse laser deposition (PLD) and atomic layer-by-layer (ALL) oxide molecular beam epitaxy (MBE). Compared with PLD, the MBE method has several extraordinary advantages: (i) the lowest energy of impinging atoms - providing less intermixing at the interface of different oxides; (ii) a possibility to control the sequences of deposition species and stoichiometry with high precision - providing engineering of interfaces via changing sequence and a stoichiometry of the interface layers; and (iii) in-situ monitoring reflection high energy diffraction (RHEED) - providing a primary feedback for control of the quality of the films growth. All together, all these distinctive features allow to achieve a precise stoichiometry and deposition control to synthesis high quality oxide films and different heterostructures, as well as to understand the dynamics of the deposition process itself, what is crucial for the chemistry and physics of the oxide interfaces. In my talk I focus only on high temperature superconducting (HTSC) interfaces in different complex oxide heterostructures. First I present ALL oxide MBE method, then different heterostructures with 2D HTSC interfaces and their characterizations by using different experimental methods including mutual inductance, electrical resistance measurements, scanning transmission electron microscopy etc. In conclusions I present implications of these finding on future work. The full information on these HTSC interfaces could be found in following publications [1-7]. References : [1] Baiutti, F., et al., in Metal Oxide-Based Thin Film Structures (Ed. Ghenadii Korotcenkov) pp.53-78 (2018) Elsevier Inc. ISBN: 978-0-12-811166-6. [2] Gozar, A. et al., Nature 455 (2008) 782. [3] Bauitti, F. et al., Nature Communications 6 (2015) 8586. [4] Wang, Yi, et al., ACS Appl. Mater. Interfaces. 8 (2016) 6763. [5] Baiutti, F., et al., ACS Appl. Mater. Interfaces, 8 (2016)27368. [5] Suyolcu, Y. E. et al., Scientific Reports 7 (2017) 453. [6] Suyolcu, Y. E. et al., Advanced Materials Interfaces 4 (2017) 1700737. [7] Baiutti, F., et al., Nanoscale, 10 (2018) 8712.

Resume : Coexistence of phases, characterized by different electronic degrees of freedom, commonly occurs in layered superconductors in which the presence of inhomogeneity has been widely observed. In particular, alkaline intercalated FeSe chalcogenides are model systems showing nanoscale coexistence of paramagnetic (PAR) metal and antiferromagnetic (AFM) texture characterized by a large magnetic moment. While the spatial distribution of different phases has been studied by a variety of techniques, their temporal behavior is not yet known. The granular nanostructure suggests the existence of slow phase dynamics with important consequences for the related physical properties unlike homogeneous electronic systems in which fast dynamics prevails. Here, we report the first direct visualization of the atomic motion in the coexisting phases in KxFe2?ySe2 chalcogenide. We have used X-ray Photon Correlation Spectroscopy (XPCS), in which the temporal evolution of X-ray speckle patterns provides direct visualization of the local nano-domains configurations. We have found that, while the PAR phase shows slowing down upon cooling, the AFM texture reveals an anomalous non-equilibrium dynamics with avalanches as in the martensites. When the sample is cooled down across the superconducting Tc the AFM phase shows an anomalous slowing behavior suggesting a nanoscale reorganization of AFM domains and a critical correlation between the AFM phase and superconductivity. This observation provides compelling evidence of avalanche-like collective excitations to drive superconductivity in the layered iron-chalcogenides.

Resume : In this presentation, we will focus on interfacial superconductivity in novel types of heterostructures. In particular, we will present a low-temperature SP-STS study of ultrathin FeTe1-xSex (x = 0, 0.5) films grown on prototypical Bi-based bulk topological insulators. We observe fully developed U-shaped superconducting gaps in FeTe0.5Se0.5 layers of one unit cell (UC) thickness with a transition temperature (Tc) of ~ 11 K, close to the one of the corresponding bulk system (Tc ~ 14.5 K) [1]. Surprisingly, we also find clear evidence for superconductivity up to Tc ~ 6 K for one UC thick FeTe layers grown on Bi2Te3 substrates [2], in contrast to the non-superconducting FeTe bulk compound which exhibits bicollinear antiferromagnetic (AFM) order in a wide temperature range up to 70 K. Even more surprisingly, detailed investigations of the atomic-scale spin-resolved local density of states by SP-STS reveal that superconductivity in one UC layers of FeTe grown on Bi2Te3 appears to spatially coexist with bi-collinear AFM order. By using 3D-vector-resolved SP-STM techniques [3] we find an unusual reorientation of the diagonal double-stripe spin structure at Fe1+yTe thin film surfaces [4]. Moreover, variable-temperature SP-STM studies [5] reveal an enhanced Néel temperature for AFM spin ordering of the ultrathin FeTe films grown on topological insulators [6]. These findings open novel perspectives for theoretical studies of competing orders in Fe-based superconductors as well as for experimental investigations of exotic phases in heterostructures of topological insulators and superconducting layers. References : [1] A. Kamlapure, S. Manna, L. Cornils, T. Hänke, M. Bremholm, Ph. Hofmann, J. Wiebe, and R. Wiesendanger, Phys. Rev. B 95, 104509 (2017): "Spatial variation of the two-fold anisotropic superconducting gap in a monolayer of FeSe0.5Te0.5 on a topological insulator". [2] S. Manna, A. Kamlapure, L. Cornils, T. Hänke, E. M. J. Hedegaard, M. Bremholm, B. B. Iversen, Ph. Hofmann, J. Wiebe, and R. Wiesendanger, Nature Commun. 8, 14074 (2017): "Interfacial superconductivity in a bi-collinear antiferromagnetically ordered FeTe monolayer on a topological insulator". [3] S. Meckler, M. Gyamfi, O. Pietzsch, and R. Wiesendanger, Rev. Sci. Instrum. 80, 023708 (2009): "A low-temperature spin-polarized scanning tunneling microscope operating in a fully rotatable magnetic field". [4] T. Hänke, U. R. Singh, L. Cornils, S. Manna, A. Kamlapure, M. Bremholm, E. M. J. Hedegaard, B. B. Iversen, Ph. Hofmann, J. Hu, Z. Mao, J. Wiebe, and R. Wiesendanger, Nature Commun. 8, 13939 (2017): "Reorientation of the diagonal double-stripe spin structure at Fe1+yTe bulk and thin film surfaces". [5] J. Warmuth, M. Bremholm, Ph. Hofmann, J. Wiebe, and R. Wiesendanger, NPG Quantum Materials 3, 21 (2018): "Domain imaging across the magneto-structural phase transition in Fe1+yTe". [6] U. R. Singh, J. Warmuth, A. Kamlapure, L. Cornils, M. Bremholm, Ph. Hofmann, J. Wiebe, and R. Wiesendanger, Phys. Rev. B 97, 144513 (2018): "Enhanced spin ordering temperature in ultrathin FeTe films grown on a topological insulator".

Resume : We identied quasiparticle states at well-dened energies inside the superconducting gap of the electron system at the LaAlO3-SrTiO3 interface using tunneling spectroscopy. The features are found only in a number of samples and depend upon the thermal-cycling history of the samples. The features consist of a peak at zero energy and other peaks at finite energies, symmetrically placed around zero energy. These peaks disappear, together with the superconducting gap, with increasing temperature and magnetic field. We discuss the likelihood of various physical mechanisms that are known to cause in-gap features in superconductors and conclude that none of these mechanisms can easily explain the results. The conceivable scenarios are the formation of Majorana bound states, Andreev bound states, or the presence of an odd-frequency spin triplet component in the superconducting order parameter [1]. References : [1] L. Kuerten, et al., In-gap features in superconducting LaAlO3 - SrTiO3 - interfaces observed by tunneling spectroscopy, Phys. Rev. B 96, 014513 (2017).

Resume : The two-dimensional electron liquid present at the LaAlO3/SrTiO3 interface exhibits superconductivity and hosts a large spin-orbit interaction. In field effect devices, both phenomena can be tuned by a gate voltage, revealing a dome-like phase diagram for the superconducting state with a critical temperature Tc reaching 300 mK. In a set of field-effect devices based on SrTiO3 interfaces having different amplitudes of polar discontinuity, we have mapped the evolution of the superconducting properties in magnetic fields applied parallel and perpendicular to the conducting plane, revealing that the estimation of the thickness of the superconducting layer changes across the phase diagram and scales with the strength of the polarization mismatch. Funding from Swiss NSF and ERC is acknowledged. References : [1] S. Gariglio, A. Fête and J.-M. Triscone, J. Phys. Condens Matter. 27, 283201 (2015). [2] S. Gariglio, M. Gabay, J. Mannhart and J.-M. Triscone, Physica C 514, 189 (2015). [3] S. Gariglio, M. Gabay and J.-M. Triscone, APL Materials 6, 060701 (2016). [4] D. Li et al., Advanced Science (2018).

Resume : The superconducting state of carrier-doped SrTiO3 is somehow unique since it appears at the lowest carrier concentration among the superconducting materials and is close to a ferroelectric order. Recent work has investigated the influence of ferroelectric fluctuations on superconductivity [1-3]. An enhance-ment of the superconducting transition temperature (Tc) is theoretically predicted in the vicinity of the ferroelectric quantum critical point (QCP) [1]. Experimental studies on ferroelectric SrTi18O3-d [2] and Sr1-xCaxTiO3-d (0.002 < x < 0.02) [3] have indeed revealed that, for the same value of carrier concentration, Tc is higher for 18O and Ca-doped SrTiO3 samples compared with SrTiO3-d samples. In this study, we investigate superconductivity at LaAlO3 (10 u.c.)/Sr1-xCaxTiO3 interfaces (x = 0.0025, 0.005, 0.01, 0.05) to clarify how the 2D electron liquid at the interface is affected by the ferroelectric QCP. A clear metallic behavior is observed in LaAlO3/Sr1-xCaxTiO3 (x = 0.025, 0.005 and 0.01) heterostructures. On the other hand, LaAlO3/Sr1-xCaxTiO3 (x = 0.05) shows a metal-insulator transition at around 70 K. For LaAlO3/Sr1-xCaxTiO3 (x = 0.005 and 0.01) interfaces, we observe a superconducting transition, but not for LaAlO3/Sr1-xCaxTiO3 (x = 0.0025 and 0.05). We discuss the electrostatic modulation of superconductivity and magnetotransport properties in the LaAlO3/Sr1-xCaxTiO3 (x = 0.005 and 0.01) interfacial system. References : 1. J. M. Edge, Y. Kedem, U. Aschauer, N. A. Spaldin, and A. V. Balatsky, Phys. Rev. Lett., 115, 247002 (2015). 2. A. Stucky, G. W. Scheerer, Z. Ren, D. Jaccard, J.-M. Poumirol, C. Barreteau, E. Giannini and D. van der Marel, Sci. Rep. 6, 37582 (2016). 3. C. Rischau, X. Lin, Ch. P. Grams, D. Finck, S. Harms, J. Engelmayer, T. Lorenz, Y. Gallais, B. Fau-qué, J. Hemberger and K. Behnia, Nat. Phys. 13, 643?648 (2017).

Resume : Enormous research activities in graphene and hexagonal boron nitride (h-BN) directed the two-dimensional (2D) materials science research trends towards the discovery of new 2D materials. Many promising 2D materials such as transition metal dichalcogenides (TMDs), group IV monochalcogenides (MNs) and black phosphorous (BP) have been explored in this decade. These new 2D materials exhibits exciting electrical, optical, optoelectronics and mechanical properties. However, more research activities are mainly focused in TMDs materials especially in MoS2, MoSe2, WS2, WSe2, MoTe2, etc. Recent anisotropic optical properties in BP and GeS have increased the research attentions in many MNs such as SnS, SnSe, GeSe etc. too. [1,2] Since the discoveries of large family of 2D materials, opportunities to build up various 2D heterostructures, multi heterostructures or super lattices for future applications are available. Many TMDs materials are optically and electrically isotropic; and BP, MNs, ReS2, ReSe2 etc. are anisotropic in nature. Moreover, isotropy materials lead to formations of quasi 2D excitons and trions while the anisotropic 2D materials lead to the formation of quasi-one-dimensional (quasi-1D) excitons and trions in a 2D system. Hence, isotropic/anisotropic 2D heterostructures can provide unique particle interactions in between quasi-1D and 2D states. This type of system can help for the understanding of the fundamental scientific phenomenon in 2D limit. Moreover, in-plane isotropic/anisotropic 2D heterostructures may open the possibility of engineering smart devices in the nano-domain. Furthermore, this study will direct the research activities for finding of innovative applications in nano optoelectronics and various other fields such as energy storage, chemical/bio sensors, bio imaging, cancer therapy etc. This work will summaries the recent research progress in in-plane isotropic/anisotropic 2D semiconducting heterostructures, and twill open the potential applications in optoelectronics, thermoelectricity, piezoelectricity, photostriction etc. Keywords: 2D materials, isotropy/anisotropy, hetero structure, trions/excitons. References : 1. Xu, R. et al. Extra ordinarily bound quasi-one-dimensional trions in two-dimensional phosphorene atomic semiconductors. ACS Nano 10, 2046-2053 (2016). 2. Tan, D. et al. Anisotropic optical and electronic properties of two-dimensional layered germanium sulfide. Nano Res. 10, 546-555 (2017).

Resume : The crystalline layered high-Tc superconductor Bi-2212 can be easily cleaved into smoothly faceted flakes which, when placed into intimate physical contact with a variety of layered materials or bulk semiconductors, form heterogeneous junctions. Two such junctions are discussed in this talk: Bi-2212/1T-TaS2 where at low temperatures the 1T-TaS2 is a Mott insulator harboring charge density waves (CDWs) and Bi-2212/n-GaAs Schottky barrier junctions which manifest quantum mechanical tunneling at low bias voltages. The CDW order in the 1T-TaS2 appears to play an important role by coexisting with an unexpected and surprisingly high Tc of the induced proximity gap which, for junctions with high transparencies, is seen to have a surprisingly large value (∼ 20 meV) equal to half that of intrinsic Bi-2212 (∼ 40 meV). Proximity-induced high-Tc superconductivity in the 1T-TaS2 is driven by coupling to the metastable metallic phase coexisting within the Mott commensurate (CCDW) phase and associated with a concomitant change of the CCDW order parameter in the inter-facial region. For the Bi-2212/n-GaAs Schottky barrier junctions, modifications to the thermionic emission equation provide an excellent description of the I-V characteristics even at low temperatures where tunneling is found by differential conductance spectroscopy measurements to be important and capacitance measurements under reverse bias suggest an unexpectedly long electric field screening length in the superconductor.

Resume : Layered black phosphorus (black-P) is a narrow-gap semiconductor, which crystallizes in the orthorhombic A17 structure at ambient pressure and temperature. The layers of black-P consist of six-rings in a so-called chair conformation, and they are weakly bound by van der Waals forces. Under pressure, early studies showed that black-P exhibits several phase transitions [1,2]. The structure changes from A17 (orthorhombic) to semimetallic A7 (rhombohedral) at 4.5 GPa, followed by a metallic primitive cubic phase at 10 GPa. Part of the difficulty of establishing the full electronic phase diagram of black-P has been the unavailability of large, high-quality single crystals. Recent advances in black-P sample preparations [3] and high-pressure techniques have enabled us to perform a series of very detailed experiments on the superconducting and normal state of black-P [4,5]. We have established the detailed phase diagram revealing an anomalous pressure dependence of the critical temperature and the importance of hole carriers in developing superconductivity. Our studies reveal that the intriguing Tc valley, as found in our experiment should be attributed to the Lifshitz transitions. We also find that the experimentally observed Tc plateau at high pressure cannot be fully understood with solely considering an electron-phonon mechanism. Our findings suggest that besides electron-phonon coupling, plasmonic contributions or electronic correlations may be relevant for systems with d-orbital character such as black-P. References : [1] J. C. Jamieson, Science 139, 1291 (1963). [2] H. Kawamura, I. Shirotani, K. Tachikawa, Solid State Commun. 54, 775 (1985). [3] T. Nilges, M. Kersting, T. Pfeifer, Journal of Solid State Chem. 181, 1707 (2008). [4] J. Guo, H. Wang, F.O. von Rohr, W. Yi, Y. Zhou, Z. Wang, S. Cai, S. Zhang, X. Li, Y. Li, J. Liu, K. Yang, A. Li, S. Jiang, Q. Wu, T. Xiang, R.J. Cava, L. Sun, Phys. Rev. B 96, 224513 (2017). [5] X. Wu, H.O. Jeschke, D. Di Sante, F.O. von Rohr, R.J. Cava, R. Thomale, Phys. Rev. Materials 2, 034802 (2018).

Resume : At the interface between monolayer FeSe films and SrTiO3 substrate the superconducting transition temperature (Tc) is unexpectedly high, triggering a surge of excitement. The mechanism for the Tc enhancement has been the central question, as it may present a new strategy for searching for higher Tc materials. To reveal this enigmatic mechanism, by combining advances in high quality interface growth, oxygen isotope substitution, and extensive data from angle resolved photoemission spectroscopy, we provide striking evidence that the high Tc in FeSe/SrTiO3 is the cooperative effect of the intrinsic pairing mechanism in the FeSe and interactions between the FeSe electron and SrTiO3 phonon. Furthermore, our results point to the fascinating prospect that similar cooperation between different Cooper pairing channels may be a general framework to understand and design high-temperature superconductors.

Resume : An interface between wide band-gap insulators, LaAlO3 and SrTiO3 (LAO/STO) possesses exciting properties such as a quasi-two-dimensional electron gas, two-dimensional superconductivity, ferromagnetic behavior and giant spin-orbit coupling. We have systematically investigated superconductivity in the LAO/STO nanostructures with dimensions of 100 ? 300 nm [1]. The nanostructures were fabricated using our patterning method based on low-energy Ar ion beam irradiation [2]. Analysis of current-voltage characteristics suggests that our nanostructures behave like clean superconducting filaments without formation of weak links. Moreover, we observed a SQUID-like periodic modulation of the critical current in nano-rings corresponding to the Little-Parks fluxoid quantization. A most remarkable observation is an enhancement of the critical current by a small perpendicular magnetic field. This effect may be explained by the suppression of spin flip scattering on magnetic domains by external magnetic field or by induced unconventional pairing. The fact that the dip in critical current at zero magnetic field is similar in structures with different geometries suggests that it is of local origin. Our results point towards the coexistence of a homogeneous interfacial superconductivity and spatially separated magnetism at the LAO/STO interface. References : [1] A. Kalaboukhov et al., Phys. Rev. B 96, 184525 (2017). [2] P.P. Aurino et al., Appl. Phys. Lett. 102, 201610 (2013).

Resume : The intercalation chemistry of iron chalcogenide superconductors has developed well in the last years. Motivated by the very high critical temperatures observed with FeSe monolayers [1-3] deposited on SrTiO3 substrates, it appeared conceivable that intercalated FeSe layers may mimic this scenario. Indeed, several groups have reported superconductivity up to 45 K through intercalation of molecular species (mostly amines) in FeSe and additional electron doping [4-8]. Unfortunately, the sample qualities of these materials are often rather poor and hamper the precise structural characterization. One reason is the still insufficient knowledge about and control of phase formation under hydro- or solvothermal conditions. Even the seemingly simple hydrothermal synthesis of FeSe leads to perfectly stoichiometric, but oddly enough, non-superconducting samples [9]. On the other hand, the solvothermal synthesis in ethylenediamine/glycerol yields superconducting FeSe, where the typical tetragonal-to-orthorhombic (nematic) phase transition no longer occurs. More reliable is the hydrothermal synthesis of LiOH-intercalated FeSe and FeS. We found that substitution of selenium by sulfur in [(Li,Fe)OH]Fe(Se1-xSx) gradually suppresses superconductivity, while the ferromagnetic properties of the [(Li,Fe)OH] layer persist [10]. Substitution of iron by cobalt likewise suppresses superconductivity of [(Li,Fe)OH]FeSe, whereby cobalt replaces iron in the FeSe and (Li,Fe)OH layers. stoichiometric [LiOH]FeSe has been shown to be non-superconducting [11], while this is still unclear for FeSe intercalated with neutral spacers like ethylenediamine (en). We have synthesized non-superconducting Fe1-xSe(en)0.3 [12], while a recent report by Gao et al. shows superconductivity at 10 K in this undoped compound [13]. This is unexpected given the fact that superconductivity was mostly observed by electron doping in FeSe intercalates as well as in FeSe@STO. References : [1] Q.-Y. Wang et al., Chin. Phys. Lett., 2012, 29, 037402. [2] S. He et al., Nat Mater, 2013, 12, 605-610. [3] J.-F. Ge et al., Nat Mater, 2015, 14, 285-289. [4] M. Burrard-Lucas et al., Nat. Mater., 2013, 12, 15-19. [5] T. Hatakeda et al., J. Phys. Soc. Jpn., 2013, 82, 123705. [6] T. Noji et al, Physica C, 2014, 504, 8-11. [7] T. Hatakeda et al., Journal of Physics: Conference Series, 2014, 568, 022032. [8] S. J. Sedlmaier et al., J. Am. Chem. Soc., 2013, 136, 630-633. [9] U. Pachmayr et al., Chem. Commun., 2016, 52, 194-197 [10] U. Pachmayr et al., Chem. Commun., 2015, 51, 4689-4692. [11] H. Sun et al., Inorg. Chem. 2015, 54, 1958-1964. [12] J. Stahl et al., Dalton Trans. 2018, 47, 3264-3271. [13] Z. Gao et al., Sci. Chin. Mater. 2018, https://doi.org/10.1007/s40843-017-9230-2.

Resume : Recent advances have developed methods to produce ideal two-dimensional (2D) electron systems, which are highly-crystalline with the minimal disorder [1]. Here, we introduce the recent developments of highly-crystalline 2D superconductors and a series of unprecedented physical properties discovered originating from inversion symmetry in these systems. First of all, we highlight the quantum phases, i.e., quantum metallic state [2] and the quantum Griffiths phase [3] in out-of-plane magnetic fields. In addition, we focus two novel phenomena owing to broken inversion symmetry originating from crystal structure in ion-gated MoS2: one is the experimental observation of enhanced in-plane upper critical field up to 52 Tesla by spin-valley locking (Ising superconductivity) [4] and the other is the nonreciprocal superconducting transport, the latter of which is later expected to be universal phenomena in noncentrosymmetric superconductors [5]. This nonreciprocal transport can be regarded as the intrinsic ratchet effect originating from the noncentrosymmetric structure. These series of unprecedented phenomena suggest that highly-crystalline 2D superconductors evidently offer tremendous opportunities to unveil the intrinsic exotic nature of superconductors, leading to a new era of 2D superconductivity. References : [1] Y. Saito, T. Nojima and Y. Iwasa Nature Reviews Materials 2, 16094 (2016). [2] Y. Saito et al. Science 350, 409 (2015). [3] Y. Saito et al. Nature Communications 9, 778 (2018). [4] Y Saito et al. Nature Physics 12, 144 (2016). [5] R. Wakatsuki and Y. Saito et al. Science Advances 3, e1602390 (2017).

Resume : When juxtaposed to superconductors, normal materials are expected to develop superconducting behavior in a region close to the interface. This effect usually survives in mesoscopic scales, hardly surpassing tenths of micrometers in reach. Here, we probe the electrical transport properties of macroscopic (millimeter-size) graphite, bismuth and antimony samples with current electrodes made out of superconducting alloys. Our results reveal the induction of a partial superconducting-like behavior in the bulk of our devices, occurring concomitantly to superconductivity in the current injection leads. Our results suggest the triggering of a network of superconducting grains in the materials studied, which persist in scales of the order of hundredths of micrometers.

Resume : A high-mobility electron gas was observed in 2004 [1] at the interface of heterostructure LaAlO3 (LAO) and SrTiO3 (STO). Such heterointerfaces involving two insulating nonmagnetic oxides were comprehensively studied. In particular, it was found that the metallic phase (quasi-two-dimensional electron gas, 2DEG) is formed in the STO layers at the LAO/STO interface when the number of LAO layers is larger than three [2]. Such a system undergoes a transition to a superconducting state at temperatures below 300 mK [3]. We report the tailoring quasi-two-dimensional electron gas (q2DEG) state as well as a high-Tc quasi-two-dimensional superconductivity (HTq2DSC) in heterostructure Ba0.8Sr0.2TiO3/La2CuO4 consisting of an insulating ferroelectric film deposited by magnetron sputtering on a non-atomically-flat surface of an insulator single crystal. First, the ab-initio calculation of the structural and electronic properties of the BaTiO3/La2CuO4 heterostructure have been performed. It is shown that already with one BTO layer the band gap of the heterostructure is closed. Second, the temperature dependence of the electrical resistance for heterostructure formed by antiferromagnetic La2CuO4 (LCO) single crystals with epitaxial films of ferroelectric Ba0.8Sr0.2TiO3 (BSTO) deposited onto them has been studied. The measured electrical resistance is compared to that exhibited by LCO single crystals without the films. The interface of that heterostructure shows superconducting behavior with transition temperature Tc is about 30 K, which 100 time larger than Tc in LaAlO3/SrTiO3.The beginning of a transition to superconducting state occurs around 40 K, similar to what is observed in bulk La2-xSrxCuO4 single crystals at optimal doping. Important that, when we applied electrodes for resistance measurements on the back side of the heterostructure (from single crystal side, without contact with interface), superconducting state is not observed. Therefore, we conclude that this is not a case of surface superconductivity, and that the oxygen does not penetrate the surface layer during the sputtering of the film. Our results open a new page in creating interfaces with HTq2DSC, since it has been shown experimentally that using of a ferroelectric material in the heterostructures with parent compound of high temperature superconductor can leads to q2DEG and as well as HTq2DSC state. The heterostructure in that case is created by a relatively simple method at the non-atomically-flat boundary of the two insulating oxides with different elementary cell structures. It opens up new physics, when a polarization catastrophe is associated not with the polar oxides, but with the ferroelectric oxides. If a weak magnetic field is applied perpendicularly to the interface of the heterostructure, a resistance appears. This confirms a quasi-two-dimensional nature of the superconductive state. This highly robust phenomenon is confined within the interface area. The proposed concept promises ferroelectrically controlled interface superconductivity which offers the possibility of novel design of electronic devices. The reported study was funded by Russian Scientific Foundation, research project No. 18-12-00260. References : [1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004); [2] S. Thiel, et.al., Science 313, 1942 (2006). [3] N. Reyren, S. et.al., Science 317, 1196 (2007).

Resume : The material design to get high temperature superconductors in anisotropic quantum matter tuned at a topological Lifshitz transition in multi-valley metals was proposed on Dec 7, 1993 [1] . It was based on the coexistence of strong coupling spots in the k-space and free carriers. The chemical potential is tuned to a Lifshitz transition by control of anisotropic strain and electronic density. The predictions have been confirmed in cuprates [3-6], diborides [7], iron based superconductors [8-10], pressurized H3S [11-14]. A major step in the field has been the development of the theory of superconductivity of with a steep and a flat band by numerical solution of Bogolibov de Gennes equations in the BCS and in the BCS-BEC crossover near Lifshitz transitions [6,15]. All these materials are described by multi-band-Hubbard models with a complex phase separation occurring at the Lifshitz transitions [6,15] observed by using scanning micro XANES [2] and scanning micro x-ray diffraction [3]. Here we discuss the emerging role of Fano resonances in sulfur hydrides, in organics and related matter where high energy intrinsic localized modes and the interplay between the flat band at the topological Lifshitz transition and a steep band gives high temperature superconductivity [16-17]. We show that the physics of systems showing high temperature superconductivity coexists with fluctuations of short range Periodic Spin Charge Lattice Orbital modulations driven by the proximity to Lifshitz transitions controlled by strain, charge density or pressure. References : [1] A. Bianconi, M. Missori Sol. State Commun. 91, 287 (1994) doi:10.1016/0038-1098(94)90304-2. [2] A. Bianconi, US Patent 6,265,019 (2001). [3] G. Campi et al., Nature 525, 359 (2015). doi:10.1038/nature14987. [4] A. Shengelaya and K. A. Müller, EPL (Europhysics Letters) 109, 27001 (2015). [5] A. Bianconi, Solid State Communications 91, 1 (1994). [6] A. Perali, et al. Supercond. Sci. Technol. 25, 124002 (2012). [7] A. Bianconi, Journal of Superconductivity 18, 625 (2005) doi:10.1007/s10948-005-0047-5. [8] M. Fratini, et al., Supercond. Sci. Technol 21, 092002 (2008). [9] R. Caivano, et al., Sup. Sci. and Tech. 22, 014004 (2009).  doi:10.1088/0953-2048/22/1/014004. [10] A. Bianconi, Nature Physics 9, 536-537 (2013). doi:10.1038/nphys2738. [11] A. Bianconi, T. Jarlborg, EPL (Europhysics Letters) 112, 37001 (2015). doi:10.1209/0295-5075/112/37001. [12] A. Bianconi, T. Jarlborg, Nov. Supercond. Mater. 1, 37 (2015) doi:10.1515/nsm-2015-0006. [13] T. Jarlborg, A. Bianconi, Scientific Reports 6, 24816 (2016). doi:10.1038/srep24816. [14] M.V. Mazziotti, et al. EPL (Europhysics Letters) 118, 37003 (2017). doi:10.1209/0295-5075/118/37003. [15] G. Campi, A. Bianconi, Journal of Superconductivity and Novel Magnetism 29, 627 (2016). doi:10.1007/s10948-015-3326-9. [16] Bussmann-Holder, A., Köhler, J., Simon, A., Whangbo, M.-H., Bianconi, A., and Perali, A. Bussmann-Holder, A. et al.. Condensed Matter 2, 24 (2017). doi:10.3390/condmat2030024. [17] Bussmann-Holder, A., Köhler, J., Simon, A., Whangbo, M. & Bianconi, Journal of Superconductivity and Novel Magnetism 30, 151 (2017). doi:10.1007/s10948-016-3947-7.

Resume : The present work addresses the investigation of the optimal conditions for amplification of the superconducting critical temperature Tc in a system of multiple electronic components. We use mean field theory for multiband superconductivity focusing on the case where the first electronic component is assumed to have a vanishing Fermi velocity corresponding to a case of the intermediate polaronic regime, and the second electronic component is in the weak coupling regime with standard high Fermi velocity in agreement with the experimental results in cuprates, diborides and iron based superconductors. By keeping a constant and small exchange interaction between the two electron fluids, we search for the optimum coupling strength in the electronic polaronic component which gives the largest amplification of the superconducting critical temperature in comparison with the case of a single electronic component. References : [1] Annette Bussmann-Holder, Jürgen Köhler, Arndt Simon, Myung-Hwan Whangbo, Antonio Bianconi and Andrea Perali, Condens. Matter 2017, 2(3), 24. a.bussmann-holder@fkf.mpg.de

Resume : In two-dimensional crystals, crystalline inversion asymmetry or Fermi surface instabilities can lead to the electronic spin degeneracy being lifted by Rashba spin-orbit coupling. This has unique implications for superconductivity, where spin splitting ensures that Cooper pairs form in mixed singlet-triplet states. The result is a gap function with unconventional symmetry, and an atypical doping dependence arising from the split single-particle spectrum. In this talk we explore these consequences in a two-dimensional tight-binding model on a square lattice with Rashba spin-orbit coupling as well as a correlated hopping term arising from an off-diagonal part of the electron-electron interaction. It is known that the Rashba coupling typically decreases the critical temperature in the presence of attractive interactions. Here we show that with correlated hopping, Rashba coupling actually enhances the critical temperature as well as the tunnelling asymmetry. This may explain some recent observations in the LaAlO3/SrTiO3 interface where Rashba physics is important.

Resume : We address a long standing problem concerning the origin of bosonic excitations that strongly interact with charge carriers. We show that the time - resolved pump - probe experiments are capable to distinguish between regular bosonic degrees of freedom, e.g. phonons, and the hard-core bosons, e.g., magnons. The ability of phonon degrees of freedom to absorb essentially unlimited amount of energy renders relaxation dynamics nearly independent on the absorbed energy or the fluence. In contrast, the hard core effects pose limits on the density of energy stored in the bosonic subsystems resulting in a substantial dependence of the relaxation time on the fluence and/or excitation energy. Very similar effects can be observed also in a different setup when the system is driven by multiple pulses. J. Kogoj, M. Mierzejewski and J. Bon?a, Phys. Rev. Lett., 117, 227002 (2016).

Resume : The effects specific to multi-gap superconductors include novel vertical and skyrmionic states, giant-paramagnetic response, hidden criticality, and time-reversal symmetry breaking. In this talk, I will discuss the evolution of multigap superconductivity in MgB2 at its ultrathin limit. Atomically thin MgB2 is distinctly different from bulk MgB2, in that both free Mg- and B-surfaces contribute additional surface electronic state, comparable in electronic density to the bulk-like σ- and π-bands. Using the ab initio electron-phonon coupling and anisotropic Eliashberg equations, we show that these surface states strongly recast superconductivity in MgB2 at few-monolayer thickness, providing the gap that hybridizes with σ-gap for 4+ monolayer films, but shifts towards the π-gap in thinner structures. As a consequence, the observable superconducting properties and vortex matter radically change with every added monolayer in the thinnest limit. Furthermore, a single monolayer of MgB2 develops three distinct superconducting gaps, on completely separate parts of the Fermi surface. We show that this three-gap superconductivity is robust over the entire temperature range that stretches up to considerably high critical temperature of 20 K. We also reveal that Tc can be boosted to > 55 K under tensile strain of ~ 4%. Finally, I will discuss how hydrogenation hybridizes with all prominent coupling channels in monolayer MgB2, and enhances superconductivity to even higher temperatures!

Resume : Origin of characteristic spin excitations in doped Mott insulator is a fascinating issue, since it is relevant to the mechanism of high-Tc superconductivity. To gain a deep insight into the nature of spin excitations, we studied the doping and the thermal evolutions of spin excitations in La2-xSrxCuO4 by neutron and resonant inelastic X-ray scattering measurements. From the systematic study, we found the evidence of two energy scales in the hour-glass shaped spin excitations, which is separated by the waist energy. The feature will be discussed in connection with the two spin degrees of freedom, that is, the spins of itinerant and localized electrons.

Resume : Ising-type superconductivity in monolayer TMD?s becomes a platform of intensive studies experimentally and theoretically. The strong Zeeman type spin-orbit coupling (SOC) aligns the spins of electrons in Cooper pairs along the out-of-plane direction, making a superconducting state robust against external magnetic field applied parallel to the crystal plane. As the SOC originates from broken inversion symmetry, restoring the symmetry is expected to suppress the Ising protection. Here, we use bilayer MoS2 - crystal which is globally centrosymmetric but possesses broken local inversion symmetry. By implementing it into electric double layer transistor (EDLT), we are able to induce superconductivity in both layers and gradually tune a coupling strength ? a parameter that can reveal hidden degrees of freedom.

Resume : High Tc cuprates are especially attractive since they exhibit so far the record high transition temperatures, i.e. up to 130 K at ambient conditions. An interesting question is whether there is space for cuprates to further increase Tc. Here we will present our work on pressure tuned cuprates with variants modulation structures related to oxygen ordering in system with simple composition consisting of copper alkaline earth oxides wherein Tc enhancements are promising upon improvement of dopant ordering.

Resume : With advances in thin-film growth, atomic-level control of interfaces has reached an unprecedented level of sophistication. At the ultrathin range, interface phenomena can induce unusual changes of some electronic properties in a way that is impossible in the bulk materials. One of the possibilities is the modification of Coulomb interaction between charge carriers by changing effective dielectric constant of the material. Such an effect which was called the Coulomb interaction engineering was demonstrated in a thin semiconducting layers sandwiched by insulators with a different dielectric constant. It would be interesting to check if similar effect can exist in thin films of functional oxide materials. Following this idea we recently proposed to create a multilayer structure in which an underdoped copper-oxide high-temperature superconductor is sandwiched between high-dielectric-constant insulator layers such as ferro- or ferri-electrics, thereby reducing the Coulomb repulsion between the intrinsically present clusters or stripes in the CuO2 layers of the pseudogap phase. This might lead to an increase in the size of such clusters, resulting in smaller distances between them and coherence at higher temperature, i.e., a higher Tc. In order to check the possibility of Coulomb interaction engineering, epitaxial heterostructures of cuprate superconductors and high dielectric constant insulator layers were grown by means of pulsed laser deposition technique. In-situ Reflection High-Energy Electron Diffraction (RHEED) system was used to verify the layer-by-layer growing condition. High quality fully epitaxial heterosturctures were produced and characterized by various experimental techniques. Obtained results will be presented and remaining challenges will be discussed.

Resume : Molecularly intercalated layered iron monochalcogenides, {M}FeSe1-xChx ({M} = organic or metal-organic donor, Ch = S, Te) are currently the subject of intensive research due to their fascinating chemistry and tunable electric and magnetic properties related both to the possible doping of host material and the specific nature of guests residing in the structure. The rich chemistry and physics of these novel iron-based systems affect the intricate relations between crystal structure, superconductivity and magnetic properties. Controlling of the mutual interactions could be achieved by subtle adjustment of the crystal structure through substitutions either on iron or selenium sites and most favourably by intercalation of electron donating guests between the FeCh sheets, which in the parent inorganic host, FeSe1-xChx , are held together by weak van der Waals forces. In recent years, significant progress has been made in the field of molecular intercalates of layered iron selenides, revealing superconducting inorganic-organic hybrid materials with critical temperatures, Tc, up to 46 K. Many of these novel materials have been synthesized by low-temperature intercalation of molecular species, such as adducts/solvates of alkali metals and nitrogen-containing donor molecules into layered structure of inorganic hosts. It has been shown that both the chemical nature as well as orientation of organic molecules in the van der Waals gaps of the inorganic host are crucial for structural modifications and may be used for alteration of superconducting properties of these hybrid organic-inorganic superconductors. In this talk, the most important aspects of intercalation chemistry leading to the formation of novel molecularly intercalated superconducting phases with respect to the effect of the chemical nature of intercalating guest molecules on the crystal structure and physical properties will be discussed. This work was partly supported by the Department of the Navy, Office of Naval Research Global under ONR GRANT CONTRACT NO. N62909-17-1-2126

Resume : The pairing mechanism in novel Fe-based superconductors is still an opened question. Conventional and pressure Mössbauer studies of simplest FeSe superconductor and its derivatives are very important for the understanding of the mechanisms of superconductivity in these systems. Application of nuclear inelastic scattering of synchrotron radiation to study of the local phonon DOS in FeSe-based SC as function of temperature and pressure could prove that electron-phonon interactions could not be the main mechanism for superconductivity in these systems. Mössbauer experiments appeared very informative in study of influence of static non-compensated magnetic moments on superconductivity in iron-based superconductors. Doping of small amounts of Cu into the FeSe suppresses superconductivity and introduces local static moments at the Fe sites. High-pressure studies using the Synchrotron Mössbauer Source (SMS) revealed that restoration of superconductivity under pressure in Cu-doped FeSe occurs because of the suppression of the static spin-glass state. We conclude that only nano-scale phase separation of insulating antiferromagnetic and metallic non-magnetic FeSe-similar domains provides conditions for coexistence of static magnetism and superconductivity. Many experimental facts provide evidence that antiferromagnetic spin fluctuations can mediate superconductivity acting as ?glue? for Cooper pairs in Fe-based superconductors. Our Mössbauer studies of FeSe intercalated with Li/NH3 spacer layers with a superconducting transition temperature of TC = 43 K support this idea. They demonstrate that simultaneously with superconducting transition in 57Fe Mössbauer spectra appears a magnetic subspectrum of dynamic nature and its intensity scales with a transition curve when passing to the superconducting state. Pressure measurements using the 57Fe-SMS confirmed important role of the antiferromagnetic spin fluctuations in pairing mechanism in FeSe-based superconductors.

Resume : Transition metal dichalcogenides (TMDs), a family of 2D layered materials like graphene, have been subject to tremendous experimental and theoretical studies not only due to their exciting physical properties but also as systems that may solve critical technological problems. I will describe a few TMD systems with novel electronic properties, where the application of hydrostatic pressure lead to large and unexpected effects. These include the Weyl semimetal Td-MoTe2 and metallic NbSe2 (where a strong pressure effect on superfluid density, its linear scaling with Tc in both, as well as a possible sign changing s+- gap in MoTe2 were observed), and semiconducting 2H-MoTe2 (where we provide the first evidence for involvement of magnetic ordering in the physics of TMDs). I will discuss these results from an experimental perspective using a combination of muon-spin rotation, X-ray/Neutron powder diffraction, and atomic-resolution scanning tunneling microscopy techniques. References : [1] Z. Guguchia et. al., Nature Communications 8, 1082 (2017). [2] Z. Guguchia et. al., arXiv:1711.05392v2 (2017).

Resume : The temperature dependence of the in-plane magnetic penetration depth (\lambda_ab) in an extensively characterized sample of superconducting CaKFe_4As_4 (T_c = 35K) was investigated using muon-spin rotation (\muSR). A comparison of \lambda_ab^-2(T) measured by \muSR with the one inferred from ARPES data confirms the presence of multiple gaps at the Fermi level. An agreement between \muSR and ARPES requires the presence of additional bands, which are not resolved by ARPES experiments. These bands are characterised by small supercondcting gaps with an average zero-temperature value of \Delta_0 =2.4(2)meV. Our data suggest that in CaKFe_4As_4 the s+- order parameter symmetry acquires a more sophisticated form by allowing a sign change not only between electron and hole pockets, but also within pockets of similar type.

Resume : Unconventional superconductivity in iron pnictides and chalcogenides is widely believed to be controlled by the interplay of low-energy antiferromagnetic spin fluctuations and the particular topology of the Fermi surface in these materials. Based on these ingredients one would also expect the large class of isostructural and isoelectronic iron germanide compounds to be good superconductors. In reality however, they superconduct at very low temperatures or not at all. We establish that superconductivity in iron germanides is suppressed by strong ferromagnetic tendencies [1,2], which surprisingly do not occur due to changes in bond-angles or -distances compared to iron pnictides and chalcogenides, but due to changes in the high-energy electronic structure upon exchange of elements. Our results indicate that superconductivity in iron-based materials can not be fully understood based on low-energy model Hamiltonians. This work was done in collaboration with D. Guterding, I. I. Mazin, J. K. Glasbrenner, E. Bascones, R. Valenti. References : [1] H. O. Jeschke, I. I. Mazin, R. Valenti, Phys. Rev. B 87, 241105(R) (2013). [2] D. Guterding, H. O. Jeschke, I. I. Mazin, J. K. Glasbrenner, E. Bascones, R. Valenti, Phys. Rev. Lett. 118, 017204 (2017).

Resume : Most of the results of electric-field-induced superconductivity in MoS2 have so far been interpreted in terms of population of the conduction band minima at the K point of the First Brillouin Zone (FBZ). By performing low-temperature electric transport measurements in ion-gated few-layer MoS2 devices we observed two maxima in the residual square resistance at the increase of charge doping. According to recent theoretical calculations they correspond to the Fermi level crossing of the high-energy, spin-orbit-coupling split minima (Q1 and Q2) at the Q point in the FBZ. Superconductivity appears only in proximity of the crossing of the high-energy sub-band at Q2 and maximum Tc is obtained only when this sub-band is highly populated. These results highlight for the first time the essential role of the Lifshitz transition associated to the crossing of the sub-band at Q2 in the induction and optimization of superconductivity in MoS2. In a second experiment we overcame the limits in the maximum surface carrier density achievable in the electrostatic regime by exploiting a strong electric field to drive ions in between the van-der-Waals-bonded layers. After optimization of this intercalation process we observed pronounced ?hump? structures around 200 K in the T dependence of the resistivity of ultrahighly-doped LixMoS2 thin flakes. We interpret these anomalies as a sign of the appearance of a charge-density-wave phase that was recently theoretically predicted in highly-doped MoS2.

Resume : The coexistence of superconductivity and ferromagnetism in doped EuFe2As2 is not only a fascinating fundamental state of matter but also might be potentially interesting for spintronics and high magnetic field applications. In a brief overview I shall summarize the results of experimental investigations of the effects of applied magnetic field, hydrostatic pressure and chemical substitution on the crystal structure, magnetic ordering and superconductivity of EuFe2As2. I will present recent results of our investigations of peculiar transport and magnetic properties of EuFe2-xCoxAs2 single crystals as well as neutron diffraction studies of the magnetic structure evolution with Co doping or under influence of high pressure. EuFe2-xNixAs2 is exceptionally intriguing because in this system Ni doping although suppresses the SDW order, it does not lead to the appearance of superconductivity. Finding the reason of the absence of superconductivity in EuFe2-xNixAs2 could make a significant contribution to a better understanding of the superconductivity mechanism in the iron pnictides. I present the magnetic phase diagram as a function of Ni doping derived from magnetization and resistivity data obtained on single-crystalline material. The search for superconductivity in this system was carried out down to 0.55 K and also under high pressure up to 2.5 GPa. The ab initio calculations of the electronic structure properties were performed using density functional theory.

Resume : Magnetic measurements at ambient and at hydrostatic pressure were performed for single crystals of Fe0.99Te0.66Se0.34 and Fe0.994Ni0.007Te0.66Se0.34 in order to study influence of pressure on the basic parameters describing the superconducting state. An enhancement of all the investigated parameters for both Fe0.99Te0.66Se0.34 and Fe0.994Ni0.007Te0.66Se0.34 under hydrostatic pressure was noticed. The critical current density (jc) at ambient pressure is much smaller for Fe0.994Ni0.007Te0.66Se0.34 than that for both Fe0.99Te0.66Se0.34 and FeTe0.5Se0.5, but under hydrostatic pressure the Ni-substituted sample exhibits bigger improvement of jc than that for both undoped Fe Te-Se samples. Superconducting state properties of Fe0.99Te0.66Se0.34 single crystal, such as lower and upper critical fields, were found to be poorer at both ambient and hydrostatic pressure than those observed for multiphase FeTe0.5Se0.5 sample. However, the recovery of superconducting state properties was not observed in Fe0.994Ni0.007Te0.66Se0.34 crystal as a result of disorder introduced by Ni substitution, as compared with those in Fe0.99Te0.66Se0.34. It means that an inhomogeneous spatial distribution of ions with nanoscale phase separation is essential for enhancement of superconductivity but disorder introduced by simple substitution of another kind of ions alone is not sufficient.