Manipulation of functional properties of the layered materials and their characterization

Resume : The in-plane critical current density (Jc) of Ba1-xKxFe2As2 (K-Ba122), Ba(Fe1-xCox)2As2 (Co-Ba122), and BaFe2(As1-xPx)2 (P-Ba122) is investigated by magnetization hysteresis loop (MHL). Depending on the dopant and x, Jc exhibits a variety of magnetic-field (H)- and temperature (T)- dependences. (1) For K-Ba122, the MHL of the under-doped samples (x < 0.33) exhibits the second magnetization peak (SMP), which sustains high Jc at high H and high T, exceeding 105 A/cm2 at T = 25 K and µ0H = 6 T for x = 0.30. The SMP is missing in the optimally- (x ~ 0.36-0.40) and over-doped (x ~ 0.50) samples, and consequently Jc rapidly decreases by more than one order of magnitude. (2) For Co-Ba122, the SMP is present over the entire superconducting (SC) dome. However, the magnitude of Jc significantly changes with x, exhibiting a sharp maximum at x ~ 0.057 (3) For P-Ba122, the highest Jc is attained at x = 0.30 corresponding to the highest Tc composition. For the over-doped samples, the MHL is characterized by a SMP located close to the irreversibility field Hirr. Common to the three doping variations, Jc becomes highest at the under-doping side of the SC dome near the SC-AF phase boundary. The scaling analysis of the normalized pinning force density fp as a function of the reduced magnetic field h = H/Hirr (Hirr: irreversibility field) shows that the peak in the pinning force position (hmax) depends on x. Possible origins for the different pinning mechanism are discussed.

Resume : Resonant inelastic X-ray scattering (RIXS) is a powerful bulk-sensitive photon-in / photon-out spectro-scopic probe of the electronic structure of condensed matter with atomic and orbital sensitivity. It is a unique tool for studying excitations from the electronic ground state in correlated materials, being directly sensitive to lattice-, charge-, orbital- and spin-degrees of freedom. In this presentation we demonstrate that RIXS at the Fe L3 edge can be used to measure collective magnetic excitations in iron-based superconductors and their parent compounds. Our experiments on hole doped Ba1-xKxFe2As2 and electron doped Ba(Fe1-xCox)2As2 single crystals show well-defined spin-excitations dispersing up to 200 meV and persisting into the superconducting phase. High-energy spin-excitations in Ba(Fe1-xCox)2As2 are in agreement with NaFe1-xCoxAs independent on electron doping, in contrast to hole doped Ba1-xKxFe2As2 for which the spin excitations are clearly softening relative to parent BaFe2As2 highlighting an electron-hole asymmetry of the spin excitations. The situation is different in the BaFe2(As1-xPx)2 series for which superconductivity appears with isovalent doping without changing the number of carriers. Remarkably, the energy of the persistent broad dispersive magnetic excitations in BaFe2(As1-xPx)2 are strongly hardened by doping, arising from the decrease of electronic correlations. However, Fe-Kβ X-rays emission spectroscopy shows a gradual quenching of the local magnetic moment, which is intriguing if compared to the behavior of the spin correlations. We link this unconventional evolution of the magnetism to the shift from 2- to 3-dimensional electronic structure of the system.

Resume : FeSe has the simplest crystal structure among Fe-based high Tc superconductors and has attracted enormous interest so far. One of the intriguing features is its ability to exhibit higher Tc under extreme conditions, such as pressure, intercalation, and a few layers. Another knob includes photo-excitation, which can instantaneously control electronic and crystallographic properties without contact or processing. In this regard, time- and angel-resolved photoemission spectroscopy (TARPES) is a quite powerful tool because it can track transient band structures temporarily emerging after photo-excitation. In this talk, we would like to show our recent results on FeSe measured by the TARPES system employing high harmonic generation from Ar gas for probe pulses. We observed coherent phonon excitations after strong photo-excitation, and would like to discuss the accompanying photo-excited electronic states by comparison with band structure calculations.

Resume : Superconductivity under high pressure has been paid the considerable attention. In particular superconductivity in H2S at 200 K under 150 GPa was discovered using a diamond anvil cell (DAC) [1]. It is predicted that if we can measure the resistivity under extremely higher pressure above 300 GPa, superconductivity at room temperature in hydrogen could be observed [2]. However, resistivity measurement under high pressure is quite difficult because of the small sample sizes (< 100 μm) and the deformation of electrodes by compression. The development of an innovative technique for the transport measurements under high pressure is required. In this study, we have developed a DAC specialized for the resistivity measurements under high pressure [3]. The key component is a special diamonds. It is a heavily boron-doped metallic diamond as an electrode [4,5]. When the boron concentration exceeds 3×10^20 cm-3, diamond shows metallicity and superconductivity at low temperature. The diamond electrodes were nanofabricated by using combination of a microwave-assisted plasma chemical vapor deposition method and an electron beam lithography technique. We have performed a pressure effects of Tc for several superconductors including Fe-based superconductors and High-Tc superconductors. In my presentation, I will give a talk about detailed results of physical properties under pressure. References 1. A. P. Drozdov, M. I. Eremets, I. A. Troyan, V. Ksenofontov and S. I. Shylin, Nature 525, 73(2015). 2. N. W. Ashcroft, Phys. Rev. Lett. 21, 1748 (1968). 3. R. Matsumoto, Y. Sasama, M. Fujioka, T. Irifune, M. Tanaka, T. Yamaguchi, H. Takeya and Y. Takano, Review of Scientific Instruments 87, 076103 (2016) 4. Y. Takano, M. Nagao, I. Sakaguchi, M. Tachiki, T. Hatano, K. Kobayashi, H. Umezawa and H. Kawarada, Appl. Phys. Lett. 85, 2851 (2004). 5. T. Yokoya, T. Nakamura, T. Matsushita, T. Muro, Y. Takano, M. Nagao, T. Takenouchi, H. Kawarada and T. Oguchi, Nature 438, 647 (2005).

Resume : BiS2-based layered superconductors Ln(O,F)BiS2 (Ln: rare earth), which consist of alternating stacking of BiS2 conductive layers and LnO block layers have been attracting much attention [1]. Since 4f electrons of Ln ions in the block layers have magnetic degrees of freedom, it is an intriguing question whether the magnetism and superconductivity compete or coexist in this material. We have succeeded in growing high-quality single crystals of Ln(O,F)BiS2 series and have been investigating 4f-electron behaviors. Here we report recent findings of unconventional 4f-electron magnetism in this series [2]. In CeOBiS2, 4f-electrons has a energetically-well-separated doublet ground state and it shows a pronounced -logT divergence in the specific heat at low temperatures [2]. Considering that CeOBiS2 is a nonmetal, this phenomenon cannot be attributed to the competition between Kondo and RKKY interactions as in usual f-electron-based strongly correlated metals, pointing to an unconventional mechanism working in this material. Similar anomaly appears also in Nd(O,F)BiS2. A possible role played by the characteristic feature in the layered crystal structure is discussed. [1] Y. Mizuguchi et al., J. Phys. Soc. Jpn. 81, 114725 (2012). [2] R. Higashinaka et al., J. Phys. Soc. Jpn., 84, 023702 (2015).

Resume : Since 2012, superconductors with BiCh2 layers (Ch: S, Se) have been drawing much attention as a new class of layered superconductors, because of structural similarity to the cuprates and Fe-based family [Y. Mizuguchi et al., JPSJ 2012; PRB 2012]. We have investigated the effects of carrier doping and crystal structure tuning to the superconducting properties of BiCh2-based compounds. We have revealed the essential correlation between superconductivity and in-plane chemical pressure in the conductive Bi-Ch square lattice [Y. Mizuguchi et al., Sci. Rep. 2015]. Furthermore, we have recently reported that the importance of local structural disorder at the in-plane Ch site was strongly correlating with the emergence of superconductivity in the BiCh2 layer [K. Nagasaka et al. JPSJ 2017]. In this presentation, we will review the essential conditions required for the emergence of superconductivity in BiCh2-based compounds. In addition, we will show the crystal structure and physical properties of several new BiCh2-based compounds, which have been recently discovered.

Resume : A mean-field treatment is presented of a square lattice two-band-model for BiS2 taking into account intraband and interband superconductivity. A rich phase diagram involving both types of superconductivity is presented as a function of the ratio between couplings \eta= V_{XX}/V_{XY} and electron doping x. With the help of a quantity we call band-mixing ratio, denoted as R(\phi), the phase diagram is analyzed using a simple and intuitive picture based on how $R(\phi)$ varies as electron doping increases. The predictive power of $R(\phi)$ suggests that it could be a useful tool in qualitatively (or even semi-quantitatively) analyzing multiband superconductivity in BCS-like superconductors.

Resume : We use soft x-ray photoemission spectroscopy (SXPES) to investigate Ce 4f electronic states of a new BiS2 layered superconductor CeO1−xFxBiS2, for polycrystalline and single-crystal samples. The Ce 3d spectrum of the single crystal of nominal composition x = 0.7 has no f0 component and the spectral shape closely resembles the ones observed for Ce trivalent insulating compounds, strongly implying that the CeO layer is still in an insulating state even after the F doping. The Ce 3d-4f resonant SXPES for both polycrystalline and single-crystal samples shows that the prominent peak is located around 1 eV below the Fermi level (EF) with negligible spectral intensity at EF. The F-concentration dependence of the valence band spectra for single crystals shows the increases of the degeneracy in energy levels and of the interaction between Ce 4f and S 3p states. These results give insight into the nature of the CeO1−xFx layer and the microscopic coexistence of magnetism and superconductivity in CeO1−xFxBiS2.

Resume : We report STM/STS studies in underdoped (UD) and optimally (OP) doped crystals of Bi-based high-Tc cuprate Bi2212. In many cases of STM/STS experiments on UD Bi2212 crystals, we observe STS spectra consistent with a two-gap structure consisting of a d-wave SC gap (SCG) and a spatially inhomogeneous pseudogap (PG), whose size varies in nanometer scale over a wide range from an energy of the SCG amplitude to several times larger one. In such UD crystals, we demonstrate checkerboard modulation (CBM) and Cu-O-Cu bond-centered modulation (BCM), which are observed in conductance images at low energies around the SCG and high energies around the PG, respectively; the two electronic superstructures with different characteristic energies coexist with each other in real space. On the other hand, no CBM and BCM are observed in STM/STS experiments on some cleaved surfaces of OP Bi2212; STS spectra exhibit a homogeneous single d-wave like SCG structure within the areas examined. In such OP crystals, we demonstrate that the overall SCG on the entire Fermi surface spatially changes with an amplitude of ~6% of its average and the same period as in the one-dimensional superlattice, which has been predicted by theoretical studies to cause periodic modulation of the antiferromagnetic coupling between Cu-spins.

Resume : Our recent transport and muon-spin-relaxation (muSR) results of the electron-doped high-T_c_ cuprates with the so-called T' structure are reviewed in relation to the undoped (Ce-free) superconductivity in the parent compounds [1-3]. The Hall resistivity of the reduced single crystals of Pr_2-x-y_La_y_Ce_x_CuO_4+d_ (PLCCO) has revealed a nonlinear (linear) behavior in magnetic field in the superconducting (non-superconducting) crystals, suggesting the existence of multiple carriers in the superconducting crystals. The muSR time spectra of reduced superconducting PLCCO with x = 0.10 and y = 0.7 have revealed the development of the Cu-spin correlation at low temperatures. Moreover, the development of the Cu-spin correlation has been found to weaken with increasing x and almost disappear at the end point of the superconducting region in the phase diagram. These results suggest that the multiple carriers and developed Cu-spin correlation are crucial to appear the superconductivity in the T’-cuprates. For the parent compound of undoped La_1.8_Eu_0.2_CuO_4+d_, impurity substitution for Cu has uncovered the superconducting pairing symmetry as a d-wave one. These results are understandable in terms of our proposed novel electronic-structure model based on the strong electron correlation. [1] T. Adachi et al., J. Phys. Soc. Jpn. 82, 063713 (2013). [2] K. Ohashi et al., J. Phys. Soc. Jpn. 85, 093703 (2016). [3] T. Adachi et al., J. Phys. Soc. Jpn. 85, 114716 (2016).

Resume : Sr2IrO4 crystalizes in K2NiF4-type structure and is an antiferromagnetic insulator with TN = 240 K, whose character has been recognized as a consequence of large spin-orbit interaction as well as Coulomb interaction in Ir 5d electrons. Recent observations of anisotropic energy gap in carrier-doped samples [1,2] provided experimental support to theoretical claims that doped Sr2IrO4 can be a superconductor, while little is known about how this state evolves from parent insulating compound [3]. In order to clarify this point, we have performed angle-resolved photoelectron spectroscopy on lightly-doped (Sr,La)2IrO4 single crystals. We have found that a dispersive state develops inside the Mott gap by La doping, whose momentum dependence corresponds to the d-wave gapped state at higher doping [2] but its energy position is altered by the presence of anistropic gap, resulting in a remnant Fermi surface state. These observations suggest that the nodal liquid state observed in higher-doped Sr2IrO4 samples evolves in a highly corresponding manner as in cuprate superconductors. [1] Y. K. Kim et al., Nature Physics 12, 37 (2015) [2] A. de la Torre et al., Phys. Rev. Lett. 115, 176402 (2015) [3] V. Brouet et al., Phys. Rev. B 92, 081117(R) (2015)

Resume : The Sr2IrO4 material has been in the lime light during last years due to the discovery of a novel Jeff = ½ Mott state in the large spin-orbit regime. The sensitivity of this quantum state to local coordination and structural distortions suggests strain and confinement as ideal routes for studying and manipulating its properties. Recently, it was demonstrated that oxygen K-edge resonant inelastic X-ray scattering (RIXS) is capable to capture magnetic excitations in 5d-oxides [1]. Due to the higher surface sensitivity compared to the hard X-ray counterpart, the soft X-ray RIXS is the most suitable for the investigation of thin films. From O K-edge RIXS on roughly 20 nm-thick Sr2IrO4 films, grown on several substrates by Pulsed Laser Deposition (PLD), we detected the low-energy elementary excitations encompassing single magnons, bimagnons and spin-orbital excitations and their dispersion relations as a function of strain. In this talk, I will present a direct observation of the evolution of these low-energy quasiparticle excitations and the spin dynamics upon epitaxial strain in the Jeff = ½ Mott insulator Sr2IrO4. [1] X. Lu et al, “Oxygen K-edge Resonant Inelastic X-Ray Scattering as a Probe of Single Magnons and Excitonic Quasiparticles in Sr2IrO4 and Sr3Ir2O7”, manuscript submitted

Resume : A unique iron-based superconductor FeSe and related materials have attracted much attention lately. One of the interesting features of FeSe is the effect of the excess irons. There are actually several possibilities for the effect : carrier doping, appearance of impurity bands, or affecting the original band structure itself. Here we have performed a band calculation of FeSe and related materials using a supercell that contains the excess iron, and then unfolding the band structure to compare with the original one. We find that all of the above three effects take place, depending on the quantity of the excess irons. We will discuss these effects in detail.

Resume : The strength and the role of electron-electron correlations in iron-based superconductors remain debated. We argue that solution of the debate lies in the orbital-selective character of the correlations, with some orbitals being significantly more correlated than others. This reconciles conflicting evidences and accounts for the dependence of the effective mass on doping and on the specific material considering the cases of electron- and hole-doped BaFe2As2 [1] and the comparison between FeSe and FeS [2]. The orbital-selective Mott scenario also establishes a novel link between the iron-based superconductors and the cuprates, which suggests that the role of electron-electron correlations may be also the source of superconductivity. In this perspective in the second part of the talk we also discuss how such orbital-selective correlations influence the tendency towards a nematic ordering in FeSe and in other compounds [3] and the role of selective correlations in determining the properties and the symmetry of the superconducting state [4]. [1] L. de' Medici, G. Giovannetti and M. Capone, Phys. Rev. Lett. 112, 177001 (2014) [2] L. Fanfarillo. G. Giovannetti, M. Capone and E. Bascones, Phys. Rev. B 95, 144511 (2017) [3] C. Tresca, G. Giovannetti, M. Capone and G. Profeta, Phys. Rev B 95, 95, 205117 (2017) [4] L. Fanfarillo, A. Valli and M. Capone, in preparation

Resume : The superconductivity in the thin limit or at the interface has been a new trend to explore novel quantum phenomena, which are accelerated by the discovery of high transition temperature (Tc) superconductivity above 40 K in monolayer FeSe on SrTiO3. It has been proposed so far that the high Tc superconductivity originates from the interfacial effects between the film and substrate including electronic band modulation, charge transfer, and electron-phonon coupling. However, what effect plays a central role is still under devate. In this talk, we introduce that the multi-functionalities of electric double layer transistor, containing electrochemical etching and electrostatic doping, provide a new route to study the thin film or interfacial superconductivity [1], and then demonstrate our systematic studies on the high Tc superconductivity of FeSe as a function of thickness, substrate material, and electric field [2]. The maximum value of the onset Tc reaches 40 K for all the FeSe on various substrates, such as SrTiO3, MgO and KTaO3, with thinning. In addition, we found a universal relationship between the Hall coefficient in normal state and Tc regardless of different film thickness and substrate materials. The finding evidences that the high-Tc superconductivity does not primarily originate from a specific interface combination but from a charge carrier filling at specific electronic band structure. We will also discuss how the superconductivity develops using the estimated length scales of charge accumulation layers at the Ionic liquid/FeSe and FeSe/substrate interfaces. [1] J. Shiogai et al., Nat. Phys. 12, 42 (2016). [2] J. Shiogai et al., Phys. Rev. B 95, 115101 (2017).

Resume : Nowadays, environmental pollution is a very hot topic. Contamination of the environment and therefore food with heavy metals is one of the most urgent subject among scientists. That is why, presented studies refers to the tests on the extraction of heavy metal ions (Pb2+, Cu2+, Cd2+) from aqueous samples by means of surface functionalized ferrite nanoparticles doped with calcium, cobalt, nickel and manganese. In order to increase the extraction of the heavy metals, nanoparticles surface was modified with five different ligands (phthalic anhydride, succinic anhydride, acetic anhydride, 3-phosphonopropionic acid, and 16-phosphonohexadecanoic acid). Aqueous samples after extraction were examined by FAAS method. Nanoparticles with and without attached ions have been tested with EDX. Inorganic ferrite cores used for tests, firstly were characterized by physicochemical methods (TEM, X-ray diffraction, IR spectroscopy, and Mössbauer spectroscopy). It is shown that surface modification of nanoparticles can significantly influence on the extraction of tested heavy metal ions and the effectiveness is related not only to surface modification but also to core composition. The work was partially financed by EU founds via project with contract number POPW.01.03.00-20-034/09-00, POPW.01.03.00-20-004/11-00 and by NCN founds, project number 2014/13/N/ST5/00568.

Resume : All the iron-based superconductors identified so far share a square lattice composed of iron atoms as a common feature. In copper-based high-Tc materials, the superconducting phase emerges not only in square-lattice structures but also in ladder structures, which give nice hints for elucidating the microscopic mechanism of the superconductivity. Here, we report the discovery of pressure-induced superconductivity in the iron-based ladder material BaFe2S3, a Mott insulator with striped-type magnetic ordering below 120 K. On the application of pressure, this compound exhibits a metal–insulator transition at about 11 GPa, followed by the appearance of superconductivity below Tc = 24 K. Our findings indicate that the square lattice is not the necessary ingredients of the superconductivity and that not only the spin and orbital fluctuations but also the charge fluctuations could play a key role in the emergence of the superconductivity in iron-based superconductors. [1] Y. Nambu, et al., Phys. Rev. B 85, 064413 (2012). [2] F. Du, et al., Phys. Rev. B 85, 214436 (2012). [3] F. Du, et al., Phys. Rev. B 90, 085143 (2014). [4] D. Ootsuki, et al., Phys. Rev. B 91, 014505 (2015). [5] T. Hawai, et al., Phys. Rev. B 91, 184416 (2015). [6] H. Takahashi, et al., Nat. Mater. 14, 1008 (2015). [7] Y. Hirata, et al., Phys. Rev. B 92, 205109 (2015). [8] T. Yamauchi, et al., Phys. Rev. Lett. 115, 246402 (2015). [9] Songxue Chi, et al., Phys. Rev. Lett. 117, 047003 (2016). [10] Kou Takubo, et al., arXiv:1704.04864.

Resume : Layer structured iron selenide, FeSe has the simplest crystal structures among iron-based superconductors. It shows superconductivity with transition temperature (Tc) around ~10 K under ambient pressure. When potassium is intercalated between FeSe layers, the Tc increases more than 30 K, and it has been reported as potential superconductors with Tc’s of 30-48 K. However, the intercalated crystals, KxFe2-ySe2 intrinsically separate into superconducting 122-phase and insulating 245-phase. Then it has been had difficulties for the identification of the relationship between the surface morphology, compositional ratio and its crystal structure. We have been investigated growth mechanism and superconducting properties of the single crystals using In-situ high-temperature single crystal X-ray diffraction measurements [1] and TEM measurements supported by a micro-sampling technique [2]. These diffraction studies directly revealed the effect of heating treatment and correspondence between compositional ratio and the crystal structure. Finally, it was concluded that a perfect FeSe layers in the crystals provides the higher Tc superconducting phase of ~44 K. References [1] M. Tanaka et al. J. Phys. Soc. Jpn., 85, 044710 (2016). [2] M. Tanaka et al. Appl. Phys. Express, 10, 023101 (2017).

Resume : Recently enhanced superconductivity (SC) in single-layer FeSe/STO (Tc > 65 K) [1] and electron doped FeSe (Tc >40K) was observed. Here, I will present STM study on the pairing symmetry of FeSe/STO [2] and Li0.8Fe0.2OHFeSe [3]. We studied SC state and the pairing symmetries by field dependent quasi-particle interference (QPI). The result suggests plain s-wave pairing, without phase change. Substrate effects and doping dependence will also be presented [4]. We compare the SC in FeSe films grown on SrTiO3 and graphitized SiC. The results indicate that FeSe/SrTiO3 interface can further enhance SC beyond mere doping. The enhancement decays exponentially with thickness, pointing to interfacial electron−phonon coupling. Besides, we studied the electronic structure and SC of (Li, Fe)OHFeSe via surface K-dosing. We found that the Γ-centered electron band can be tuned to cross EF by K-dosing, resulting in a Lifshitz transition. With further K -dosing, a new SC dome emerges on the Γ-pocket. Eventually, the system evolves in to an anomalous insulating phase with gradually depleted DOS near EF. Our results provide clues to understand FS topology vs. SC, and reveal the anomalous correlation effect derived SC. [1]. Q. Wang, et al., Chin. Phys. Lett. 29, 037402 (2012). [2]. Q. Fan, et al., Nat. Phys. 11, 946 (2015). [3]. Y. J. Yan, et al, Phys. Rev. B 94, 134502 (2016). [4]. W.H. Zhang, et al, Nano Lett., 16, 1969 (2016).

Resume : Cubic chalcogenide, SnTe has been attracting interest after presenting the possibility of a topological crystalline insulator. Doping In achieved the highest transition temperature (Tc) superconductivity. Surface sensitive spectroscopy revealed that Sn1-xInxTe shows topologically protected unique surface states. This interest brought the material family of SnTe into the intensive studies, such as alloy compound with Pb, Pb1-xSnxTe. In doped superconductors drew attention due to the limited valence state Sn, Pb and In show. In pursuit of new cubic superconductor with the unique surface state, we focused on one of the candidate materials, Ag1-xSn1+xSe2. 　 We investigated the properties, mostly lattice constant and Tc by substituting Se site for other chalcogens, Te and S. The obtained result indicates that the fundamental behavior stays unchanged by the chalcogen substitution, while the correlation between the lattice constant and Tc does not fall onto a single curve in chalcogen substitution. This indicates that the relation between these two parameters indicates the superconductivity is governed by Ag/Sn site rather than chalcogen sites. The substitution of Se also gives a slight modulation on Tc, which is expected from the similarity to SnTe where the bands at the Fermi energy consist of both Sn and Te. In contrast to the Se site substitution, substitution of Ag/Sn site by numerous elements gives a single curve when Tc is plotted against the ratio of Sn2+ and chalcogen. While there are several findings of the valence of elements, the behavior indicates that the Sn2+ is the key parameter for defining the superconductivity, which contrasts from other SnCh superconductors.

Resume : A number of structural models are under discussion for the In2Se3 semiconductor, based on its crystallographic studies and in the context of previous first principles calculations - see, e.g., [1,2]. A way to match the stoichiometry with the covalent (typically 4-coordinated) bonding can be found within either (i) a bulk crystal containing (ordered) cation vacancies, or (ii) a layered structure, that offers, in its turn, variations in layer stackings and nearest-neighbour coordinations. For practical / predictive applications, the issues of correct representation of the dispersive interactions (hence getting accurate interatomic distances) and of the correct description of the optical gap (in order to adequately simulate electron excitations) are of primary importance. Keeping an affordable computational effort as a relative priority, we probe schemes (variations of those by Grimme and Tkatchenko - Scheffler), to take into account the van der Waals interaction, in view of making reliable predictions of total energies and the precision lattice relaxations. Moreover we compare the performance of the mBJ (meta-GGA) vs the HSE (hybrid) exchange-correlation potentials within the density-functional theory in what concerns the description of the electron bands. For some phases which are competitive in energy, a connecting structural path will be suggested, and the vibration-related corrections to the static total energies estimated. [1] L.Debbichi et al., J.Phys.Chem.Lett.6, 3098 (2015); [2] J.Srour et al., Phys.Stat.Sol.B (2017); DOI 10.1002/pssb.201700120

Resume : I will discuss our coherent effort in Singapore to modulate the properties of multilayer architectures for functional skyrmion devices at room temperature. First, I will introduce a materials recipe to tune the skyrmion size and density, thermodynamic stability parameter, as well as the crossover between isolated and disordered-lattice configurations. Second, I will present results on the electrical signature of skyrmions using transport and imaging experiments, and their spin dynamics and collective spin excitation modes from high frequency measurements. Finally, I will address their nucleation, stability, current-induced formation and dynamics in design-nanostructures.

Resume : In this work we show that by manipulating the lattice of incipient ferroelectrics we are able to directly control the quantum criticality in this system. Pressure is the ?applied external field? that couples directly to the lattice and its excitations - phonons. We use pressure to tune the strength of quantum critical fluctuations in incipient ferroelectrics, as well as the quantum paraelectric state and observe the evolution of a novel phase emergent from the quantum critical point. We have carried out unprecedented measurements of the dielectric response of a quantum critical ferroelectric to extremely high precision. The study of quantum criticality in ferroelectric systems is of significance as they provide a model system for the fundamental mechanisms of quantum criticality, without the complications of gapless electrons. Also recent understanding of superconductivity in interfacial systems and doped ferroelectrics has been attributed to the ferroelectric quantum criticality.

Resume : The superconducting state of doped SrTiO3 is somehow unique: it appears at the lowest carrier concentration in a material close to a ferroelectric order. Recent work has investigated the influence of ferroelectric fluctuations on superconductivity [1]. This theoretical study predicts an enhancement of the superconducting transition temperature (Tc) for samples in the vicinity of the ferroelectric quantum critical point (QCP). Experimental studies in ferroelectric SrTi18O3 [2] and Sr1-xCaxTiO3 (0.002 < x < 0.02) [3] have indeed revealed that Tc is higher for 18O and Ca-doped SrTiO3 samples. In this study, we investigate superconductivity at LaAlO3/Sr1-xCaxTiO3 interfaces to clarify how the 2D electron liquid at the interface is affected by the ferroelectric QCP. A clear metallic behavior is observed in SrTiO3 (6 u.c.)/LaAlO3 (5 u.c.)/Sr1-xCaxTiO3 (x = 0.005 and 0.01) heterostructures, with sheet resistance values similar to the ones of the LaAlO3/SrTiO3 system. We will discuss the superconducting properties of the LaAlO3/Sr1-xCaxTiO3 interfacial system under field effect. References: [1] J. M. Edge et al, Phys. Rev. Lett., 115 (2015) 247002., [2] A. Stucky et al., Sci. Rep. 6 (2016) 37582., [3] C. Rischau et al., Nat. Phys. 4085 (2017). DOI: 10.1038/NPHYS4085.

Resume : Extensive research has been carried out in the field of optoelectronic materials and devices. In this study, intercalation of lithium ions is used to manipulate the optical response of lithium manganese oxide (LMO) films, a material normally used as cathode in secondary batteries. To study the optical behaviour, firstly platinum, a current collector, followed by the active LMO layer is deposited on oxidised silicon wafers using ion-beam sputtering. The multi-layered sample is characterized using optical reflectometry at different lithiation states of the LixMn2O4 layer between x=0 to x=1. The reversibility of the optical response is verified by in-situ optical reflectometry during charging and discharging cycles of LMO in the wavelength range of 450-1000 nm. Furthermore, the changes in the optical constants between different charge states is revealed by fitting the reflectance data with Clausius-Mossotti dispersion relation. The equation to fit the curve considers (1) three oscillators to simulate the photon-electron interaction, (2) Fresnel’s equations for normal incidence and (3) multiple reflections experienced by light in the thin film samples. The study reveals a reversible change of the optical properties during an electrochemical reaction, making it a promising candidate for the application of optical switching via phase modulation. The kinetics of switching is evaluated.

Resume : Graphene has enormous potential due to its outstanding physical properties compared even to single-wall carbon nanotubes. One of the fundamental issues is identifying layers of graphene structures. High-resolution TEM (HRTEM) images revealing edge lines of planar view graphene provide uncontroversial evidence of the presence pure graphene. Due to the difficulty and the limitation of HRTEM observation, Raman approaches (e.g. shape of 2D-peak or an intensity ratio of 2D-peak to G-peak, I2D/IG) have been suggested as effective method for confirming graphene layers. However, the Raman data do not provide direct evidence of graphene because the shape of 2D-peak for monolayer graphene has been reported to be similar to those for multilayer graphene and the intensity ratio is an indication of electronic decoupling of graphene layers. In this presentation, a simple methodology to analyse graphene structures using Raman signals in the range ~100-~500 cm-1 will be reported. The low energy signals on Raman spectra of plasma seeded grown graphene sheets are originated from nano-curvature of mono- and bilayer graphene [1]. I will explain the appearance of the low energy Raman signals from curved graphene with radial mode (RM) of Raman Eigen vectors. The RM based new analysis provides a standard way of identifying and evaluating mono- and bilayer graphene with Raman spectroscopy. [1]. J.-K. Lee et al., The Journal of Physical Chemistry Letters. 8, 2597-2601 (2017).

Resume : The studies of iron-pnictides have shown that the multi-orbital low energy electronic states formed in the FeAs layer are moderately correlated due to finite Hund's coupling and on-site Coulomb interaction. However, so far, only little is known about the influence of the spacer layer on the low energy electronic states of the FeAs layer. Using angle-resolved photoemission spectroscopy, supported with DFT calculations, we determined the electronic structure of 112 Fe-pnictide Ca1-xLaxFeAs2 which has a metallic spacer layer between FeAs planes from which the superconductivity originates. Our studies reveal that, comparing to other superconducting iron-pnictides with insulating spacer layers the electron-electron correlations are not weakened by the metallic spacer layers. Furthermore, due to spin-orbit coupling the electronic structure of each CaAs layer is topologically non-trivial. These results indicate that below superconducting transition temperature Ca1-xLaxFeAs2 could be an alternating combination of a quantum spin Hall insulator and a superconductor.

Resume : In this talk, I will present our recent work on high temperature cuprate superconductors, the iron-based superconductors, and topological materials by using vacuum ultra-violet laser-based angle-resolved photoemission systems. These include: (1). Quantitative determination of pairing interactions in high temperature cuprate superconductors[1]; (2). Common electronic structures and superconducting gap structure in (Li,Fe)OHFeSe, AxFe2-ySe2 and single-layer FeSe/SrTiO3 superconductors[2]; (3). Direct evidence of interaction-induced Dirac cones in single-layer silicene/Ag(111)[3]; (4). Temperature-induced Liftshitz transition in ZrTe5 and HfTe5 and their topological nature[4]; and (5). Spectroscopic evidence of type-II Weyl semimetal states in WTe2[5] and MoTe2[6]. References [1]. J. M. Bok, X. J. Zhou et al., Science Advances 2, e1501329 (2016). [2]. L. Zhao, X. J. Zhou et al., Nature Communications 7, 10608 (2016). [3]. Y. Feng, X. J. Zhou et al., PNAS 133, 14656 (2016). [4]. Y. Zhang, G. D. Liu, X. J. Zhou et al., Nature Communications 8, 15512 (2017). [5]. C. L. Wang, G. D. Liu, X. J. Zhou et al., Phys. Rev. B 94, 241119(R) (2016). [6]. A. J. Liang, X. J. Zhou et al., arXiv:1604.01706.

Resume : We use static Hartree-Fock, time-dependent mean-field and nonequilibrium GW calculations to investigate the response of excitonic insulators to photo-excitation. The Hartree-Fock calculations suggest that the Hartree shifts associated with the charge transfer between bands can lead to more resonant states and a transient enhacement of excitonic order after photo-excitation, in agreement with recent experimental observations [1]. The simulation of the dynamics within time-dependent mean field theory furthermore shows that the coupling to phonons may play an additional important role [2], since it produces a mass for the phase mode. The resulting dynamics is thus qualitatively different from the system without phonon coupling, and substantial photo-induced enhancements of the order can be realized. While the transient state is protected by the relatively slow carrier relaxation across the gap, the thermalization will lead to a reduction or melting of the excitionic order. We study this thermalization process using nonequilibrium GW calculations and show that it involves two stages, a slow prethermalization controlled by a nonthermal fixed point, followed by a faster thermalization [3]. [1] S. Mor et al., arxiv (2016) [2] Y. Murakami, D. Golez, M. Eckstein and P. Werner, submitted (2017) [3] D. Golez, P. Werner and M. Eckstein, PRB (2016).

Resume : Two-dimensional (2D) semiconductors have emerged as a class of materials that may impact our future electronics technologies [1]. A key issue is controlling their electronic band structure widely to overcome the limit of natural properties. In this talk, I will introduce our recent angle-resolved photoemission spectroscopy studies on the widely tunable band gap in a 2D semiconductor, black phosphorus. We found that the in situ deposition of alkali-metal atoms modulates the band gap by the giant Stark effect [2,3], resulting in the transition from a trivial narrow-gap semiconductor to a 2D topological Dirac semimetal. After the band inversion, the band topology of semi-metallic black phosphorus can be described by a pair of Dirac cones, similar to that of grapheme [4]. References 1. X. Ling et al., PNAS 112, 4523 (2014). 2. J. Kim et al., Science 349, 723 (2015). 3. M. Kang et al., Nano Lett. 17, 1610 (2017). 4. J. Kim et al., under review (2017).

Resume : Transition metal dichalcogenides (TMDs), and in particular MoS2, are 2D layered materials whose electronic properties are sensitive to structural changes and interlayer interactions. The number of layers, strain and stacking between layers, the last one induced by weak interlayer van der Waals forces, affect electronic properties and electron transport, and therefore can be tuned in order to optimize their use in electronic devices. Only few studies in the literature focus on the effects of these factors on transport properties. The main aim of our work is to show the effects of the rotation/sliding between layers on the transport properties of multilayer MoS2. We have considered five high-symmetry stacking configurations and performed electronic calculations. Our approach is based on ab initio simulations of structural properties and quantum simulation of electron transport. We have observed different behaviours for the configurations where sulphur atoms sit ‘on top’ with respect to the configurations where they are placed in hollow sites. An improvement in the transmission is shown for some configurations, giving clues about the possible configurations that can be considered in the devices design.

Resume : Graphene-based nanostructures have been investigated as very promising candidates for water filtration and/or separation membranes. Experimental evidences have shown that graphene oxide (GO) can be impermeable to liquids, vapors and gases, while it allows a fast permeation of water molecules [1-2]. The understanding of filtration mechanisms came mostly from studies dedicated to water desalination, while very few works have been dedicated for water distillation. In this work, we have investigated the molecular mechanism underlying the alcohol/water separation inside GO membranes. A series of Molecular Dynamics and Grand-Canonical Monte Carlo simulations were performed to probe the ethanol/water and methanol/water separation through GO membranes composed of multiple layered graphene-based sheets with different interlayer distance and number of functional groups. To rationalize the preferred selectivity of water in nano-confining structure and the effective blocking (rejection) of alcohol molecules, we have carried out detailed atomistic studies. Our results show that both the size exclusion and water molecular arrangement within GO channels are responsible for the separation. The 2D geometry channel is crucial for an effective separation mechanism, since it forces the formation of water monolayers with robust H-bond networks. We conclude that the interlayer separation values and the number of functional groups are key factors for designing more efficient separation membranes. Our results are consistent with the available experimental data and contribute to clarify important aspects of the separation behavior of confined alcohol/water in GO membranes. [1] R. R. Nair et. al. Science 2012, 335 (6067), 442-444 ; [2] R K. Joshi et. al. Science 2014, 343 (6172), 752-754

Resume : Molybdenum disulfide, the most studied member of the transition metal dichalcogenides family, has been used as solid lubricant for several decades, showing extremely low friction coefficients[1] and stability to high temperature. Its lubricating properties are ascribed to the weak van der Waals interactions between sulfur atoms in the crystalline layered structure. Moreover MoS2, even when prepared in the amorphous state or made of randomly oriented domains, can undergo shear induced structural transitions to the more ordered layered state affecting its tribological properties[2]. Exploiting a recently developed classical force field[3] able to treat explicitly formation and breaking of bonds, we investigate by molecular dynamics simulations, the shear induced structural changes and possible layer formation in the amorphous molybdenum disulfide. The ordering process is studied in details, with particular regard to the estimation of the thermodynamic properties that govern the process itself. A connection with crystallization theories is finally found, conferring a predictive power to the achieved results. Overall, this study aims at gaining an atomic level understanding of the dynamics of layer formation process in MoS2, thus controlling and possibly improving its tribological properties. [1] J.M. Martin et al., Phys. Rev. B, 48, 10583(R) (1993). [2] J. Moser, F. Lévy, Thin Solid Films, 228, 257 (1993). [3] T. Liang et al., Phys. Rev. B, 79, 245110 (2009).

Resume : I will present novel scanning potoemission microscope capable of acquiring angle resolved photoemission data with sub-micrometer sized beam provided by synchrotron radiation demagnified using multilayer mirrors of a Schwarzschild objectives. This solution provides enough photons for angle resolved photoemission measurements and makes possible investigation of electronic structure of inhomogeneous samples such as superconductors embedded in an insulator as well as artificially prepared heterostructures and devices. Technical solutions and several application examples will be presented.

Resume : Combination of functional complex oxides and 2D layered materials let us explore novel physical phenomena and possible device applications in the unprecedented heterostructures. As an example, observation of quantum transport behavior in graphene near the functional transition metal oxides provides new possibilities of understanding the constituent materials. In this presentation, we present examples of synergetic behavior in monolayer graphene on SrTiO3 (STO) epitaxial thin film, using electrical transport measurements. In the first part of the presentation, we show that a large gate-voltage scaling in graphene transport is plausible using STO thin film with the high dielectric constant. The graphene on epitaxial STO thin film shows quantum Hall state which survives up to 200 K at a magnetic field of 14 T. In addition, the substantial shift of charge neutrality point in graphene seems to correlate with the temperature-dependent dielectric constant of the STO thin film, and its effective dielectric properties could be deduced from the universality of quantum phenomena in graphene. [1] In the second part, we show the probing of the creation and annihilation of oxygen vacancies at graphene/STO surface using the quantum conductance of graphene. Since monolayer graphene is highly sensitive to the surrounding electronic environment without modifying it, we can exclusively understand the electronic reconstruction occurring in the STO layer due to oxygen vacancy formation. By analyzing the hysteretic current-voltage loops, we can quantitatively estimate the relation between the thickness, dielectric constant, and oxygen vacancy concentration in oxygen deficient STO layer. [2] [1] J. Park et al., Nano Lett. 16, 1754-1759 (2016). [2] K. T. Kang et al., Adv. Mater. 29, 1700071 (2017).

Resume : BiS2-based layered compounds, such as LaOBiS2, are an attractive compound that shows unique functionality for superconductivity, thermoelectricity, and so forth. As an example, LaOBiSSe exhibits thermoelectric figure of merit ZT of 0.36 at 650 K (A. Nishida et al. Appl. Phys. Express, 8, 111801 (2015)). This is because of increased carrier mobility and reduced lattice thermal conductivity of LaOBiS2 as a result of Se substitution. In the present study, we report the effect of elemental substitution on thermoelectric properties of LaOBiSSe. So far, Sb, Pb, and Sn have been studied as a dopant. Almost all of doped LaOBiSSe exhibit higher electrical resistivity and Seebeck coefficient than undoped sample, indicating these dopants acted to decrease carrier density. Electrical power factor at moderate temperature (300600 K) can be increased by optimization of amount of substitution. This increased performance is attractive because thermoelectric generator for waste heat harvesting should be operated at a moderate temperature range. In the conference, we will report the further detail of elemental substitution for LaOBiSSe to optimize the thermoelectric performance. Theoretical calculation to discuss the electronic structure will also be presented.

Resume : Planar 2D materials are possibly the ideal channel candidates for future field effect transistors (FETs), due to their unique electronic properties. However, the performance of FETs based on 2D materials is yet to exceed those of conventional silicon based devices. Here, a 2D channel thin film made from liquid phase exfoliated molybdenum oxide nanoflake inks with highly controllable substoichiometric levels is presented. The ability to induce oxygen vacancies by solar light irradiation in an aqueous environment allows the tuning of electronic properties in 2D substoichiometric molybdenum oxides (MoO3−x). The highest mobility is found to be ≈ 600 cm2 V−1 s−1 with an estimated free electron concentration of ≈1.6 × 1021 cm−3 and an optimal IOn/IOff ratio of >105 for the FETs made of 2D flakes irradiated for 30 min (x = 0.042). These values are significant and represent a real opportunity to realize the next generation of tunable electronic devices.

Resume : It is now generally accepted that further pro-gress of the semiconductor industry will be governed by achievements in 2D material science in terms of both processing technolo-gies and the incorporation of new materials. Currently there are about 200 companies, involved in the production of graphene – powerfull 2D carbon material of the future. Most of them utilize theliquid chemical exfo-liation processes, which involves use of toxic and dangerous chemicals i. e. hydrazine for reduction purposes. These chemicals can be substituted by non-toxic, environment and human-friendly “green” chemicals, such as ascorbic acid (vitamin C), glucose, caffeic acid or even fruits juice extracts. Their exact influence on the graphene oxide (GO) is not well known yet and deserves detailed study for future application in graphene in-dustrial production. For this purpose, we have studied synthesis of GO by modified Hummers method and its further reduction by various green reagents, engineering its surface (Fig.1). Raman spectra of GO has demonstrated both D and G bands, as well as 2D line, peculiar for 2D carbon materials. The presence of carbonyl (40%), carboxyl (17%), epoxy (3%) and ox-ygen substitution functional groups was iden-tified and quantified on GO surface using XPS spectroscopy (Fig. 2). Upon GO reaction with diverse “green” reductants (Glucose, Vitamin C, Caffeic Acid and Blueberry juice) it was revealed that Vitamin C has the best reducing performance. It turned out, howev-er, that it creates most defects in the reduced rGO material, accordingly to the D/G lines ratio. While Glucose was observed to be the best green reductant in terms of least defects in rGO and best oxygen content decrease performance. Detailed data analysis allows us to claim that glucose as environmentally friendly and non-toxic “green” reductant can be used in the liquid chemical exfoliation processes for graphene production.

Resume : Two dimensional (2D) materials attracts attention of scientific world since the discovery of graphene in 2004. These materials characterise the large variety, one can find such materials as MoS2, WS2, black phosphorus or boron nitride – the best known materials so far. All of 2D materials are unique due to its properties and potential further application as for example photodetectors, solar cells, anode materials for ion-lithium batteries or thermoelectric materials. Despite the fact, the intense works are in progress still a lot of properties of 2D materials are unknown. Here we present the Raman spectroscopy measurements in dependence of temperature which give us better insight to find out thermal properties of layered materials. The knowledge about thermal properties is crucial when operating the electronic devices, especially in terms of the heat conduction and the heat removal due to the power dissipation, which limits electronic devices performance. Herein 2D materials like ReSe2, SnSe2, GeSe, PbSnS2 and black phosphorous were investigated in range of temperature from 70 – 450 K in vacuum conditions. All of these materials shows the non-linear in low temperatures, what is caused by anharmonic phonon decay – three or four phonon decay process. In higher temperature, one can observed the linear behaviour what stays in good agreement with the other known 2D materials. In this range of temperature the first order coefficient were calculated. Finding presented here are important in further analysis of phonon and thermal properties of 2D materials.

Resume : The stomatopod (mantis shrimp) telson serves as an example of natural biological armour whose high impact resistance property is believed to arise from the hierarchical organization of highly mineralised alpha-chitin nanofibres and twisted plywood (Bouligand) structures at the nano and micron scale [1,2]. To determine the structural and mechanical changes of the stomatopod cuticle under external loading can help us to identify the key mechanisms enabling their functional optimization. However, the complex hierarchical structure in biological and synthetic fibrous nanocomposites entails considerable difficulties in the interpretation of the crystallographic texture from diffraction data. Here, we present a novel reconstruction method to image the 3D distribution of fibres from 2D wide-angle X-ray diffraction (WAXD) pattern [3]. Furthermore, we develop a new 3D X-ray nanostrain imaging method combining analytical modelling of the diffraction signal, fibre-composite theory and in situ deformation, to determine the strain and structure change in 3D for different length scale [4]. Our method enables separation of deformation dynamics at multiple hierarchical levels, a critical consideration in the cooperative mechanics characteristic of biological and bioinspired materials. The nanostrain reconstruction technique is general, and can be applied to determine the in situ dynamics of advanced nanostructured materials with 3D hierarchical design.