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Abstracts of Invited Talks
Doping dependent critical current properties in K, Co, and P-doped BaFe2As2 single crystals Dongjoon Song, Shigeyuki Ishida, Hiraku Ogino, Akira Iyo, Hiroshi Eisaki Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8568, Japan Masamichi Nakajima Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan Jun-ichi Shimoyama Department of Physics and Mathematics, Aoyama Gakuin University, Sagamihara 252-5258, Japan Michael Eisterer Atominstitut, TU Wien, Stadionallee 2, 1020 Vienna, Austria ABSTRACT The in-plane critical current density (Jc) of BaFe2As2-based superconductors, Ba1-xKxFe2As2 (K-Ba122), Ba(Fe1-xCox)2As2 (Co-Ba122), and BaFe2(As1-xPx)2 (P-Ba122) in a wide range of doping concentration (x) was investigated by means of magnetization hysteresis loop (MHL) measurements on single crystal samples. Depending on the dopant elements and their concentration, Jc exhibits a variety of magnetic-field (H)- and temperature (T)- dependences. (1) In the case of 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. On the other hand, 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, although the change in Tc is within a few K. (2) For Co-Ba122, the SMP is always present over the entire superconducting (SC) dome from the under- (x ~ 0.05) to the over-doped (x ~ 0.12) region. However, the magnitude of Jc significantly changes with x, exhibiting a sharp maximum at x ~ 0.057, which is a slightly under-doped composition among Co-Ba122. (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 phase boundary between the SC phase and the antiferromagnetic/orthorhombic (AFO) phase. 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, indicating a change in pinning with x. On the other hand, high-Jc samples always attain similar hmax values of 0.40-0.45 for all the dopants, which may suggest that a common pinning source causes the highest Jc. A quantitative analysis of the T-dependent Jc indicates that the two pinning mechanisms, namely, the spatial variations in Tc (referred to as dTc pinning) and the fluctuations in the mean free path (dl pinning), are enhanced for the under-doped samples, which results in the enhancement of Jc. Possible origins for the different pinning mechanism are discussed in connection with the x-dependence of Tc, the residual resistivity, AFO domain boundaries, a possible quantum critical point, etc.
Superconductivity near the BCS-BEC crossover Setsuko Tajima Dept. of Physics, Osaka University, Osaka 560-0043, Japan The electronic phase diagram of the high Tc cuprates is re-examined from the viewpoint of BCS-BEC crossover. The key issue is the formation of preformed bosons far above Tc, which we have found in the optical spectra of YBa2Cu3Oy. The presence of the pseudogap and the preformed pairs is a common property in the strongly interacted particle system such as exciton system or quark matter in nuclear physics. Therefore, superconductivity in the strongly correlated system cannot be treated within the BCS theory even if we replace the pairing glue from phonon to spin. The unusual coexistence of competing orders in the underdoped cuprates is a natural consequence of strong interaction between electrons. Along the same context, the iron-based superconductors are discussed. From the systematic studies of LaFe(P,As)(O,F/H), we have found that superconductivity in some compositional regions can be well described in the Fermi surface nesting scenario, while the heavily electron doped compounds or those with a high pnictogen (chalcogen) height are in the strong correlation regime. Anomalous Phonons and High Temperature Superconductivity in Copper Oxides. Dmitry Reznik Department of Physics, University of Colorado-Boulder, USA It is well known that electron-phonon coupling is responsible for superconductivity in conventional superconductors, but the prevailing view is that it is not important in high temperature superconductivity. Yet, evidence that electron-phonon coupling is very strong for certain phonons in the copper oxides has been building. In particular, Cu-O bond-stretching phonons at 65-85meV in La2−xSrxCuO4 are known to show anomalously large broadening and softening near the reduced wavevector q=(0.3,0,0). I will present results of the systematic investigation of these phonons by inelastic neutron and x-ray scattering and of dispersions of electronic bands to which these phonons should be coupled by angle resolved photoemission (ARPES). These electronic dispersions have kinks around 70 meV that are typically attributed to coupling of electrons to a bosonic mode (which could be a phonon) that mediates superconductivity. Remarkably, the kinks remain strong in the heavily overdoped region of the doping phase diagram of La2−xSrxCuO4, even when the superconductivity completely disappears. Doping dependence of the magnitude of the giant phonon anomaly is very different from that of the ARPES kink, i.e., the two phenomena are not connected. In fact while the Cu-O bond stretching phonons show giant electron-phonon effects, there are no features in the electronic dispersions of the same samples that can be attributed to these phonons. We show that these results provide indirect evidence that the phonon anomaly originates from novel collective charge excitations as opposed to interactions with electron-hole pairs. Their amplitude follows the superconducting dome so these charge modes may be important for superconductivity. I will also discuss earlier results on a copper oxide with a very high Tc, YBa2Cu3O7, where a similar phonon anomaly becomes greatly enhanced in the superconducting state. Optical spectroscopy study on Fe-pnictides/chalcogenides: coupling effect between itinerant and localized electrons Nan-Lin Wang International Center for Quantum Materials, School of Physics, Peking University, China I present our optical spectroscopic measurements on different Fe-based superconducting systems. For all Fe-based compounds, the optical conductivity spectra contain, in addition to the free carrier response at low frequency, a temperature-dependent gap-like suppression at rather high energy scale near 0.6 eV. This suppression evolves with the As(Se)-Fe-As(Se) bond angle induced by electron- or hole-doping. We elaborate that the feature is mainly caused by the strong Hund's coupling effect between the itinerant electrons and localized electron moment arising from the multiple Fe 3d orbitals. The optical spectroscopy measurement demonstrates the coexistence of itinerant and localized electrons as well as their coupling effect in iron-pnictides/chalcogenides, which would then lead to a more comprehensive picture about the metallic magnetism and superconductivity in the materials.
Quantum Anomalous Hall Effect in a Two-dimensional Ising Ferromagnet BaFe2(PO4)2 Kwan-Woo Lee Department of Display and Semiconductor Physics, Korea Univ., Sejong Department of Applied Physics, Graduate School, Korea Univ. Sejong The quantum anomalous Hall effect (QAHE), also known as Chern insulator, is a two-dimensional ferromagnetic insulator with a nonzero Chern number, resulting in a quantized boundary anomalous Hall conductivity in absence of an external magnetic field. QAHE is a promising candidate for applications of low-energy consuming electronic devices, but has been observed in a very low temperature and only in a transition metal ion-doped topological insulator. So, new QAHE materials are necessary. In contrast to previous studies mostly done in artificial systems, we propose QAHE in an existing bulk material with a quasi-two dimensional Fe honeycomb lattice. We add new understanding of the interplay between Coulomb respulsion strength U and spin-orbit coupling (SOC) in QAHE. The transition from Chern to trivial (Mott) insulator phases occurs, as increasing U. SOC can produce and remain the Chern states with the high Chern number C=3 against U up to 60 times as large. Our results are expected to provide a new direction for pursuing a viable QAH insulator. [Acknowledgement] These researches have been supported by NRF of Korea Grants No. NRF-2016R1A2B4009579 and collaborated with W. E. Pickett (UC, Davis), and Young-Joon Song and Kyo-Hoon Ahn (KU).
Interplay of charge density wave and multiband superconductivity in 2H-PdxTaSe2 D. Bhoi, Chanhee Kim, S. Khim, W. Nam, B. S. Lee, B.-G. Jeon, B. H. Min, S. Park & Kee Hoon Kim Center for Novel States of Complex Materials Research and Institute of Applied Physics, Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Republic of Korea 2H-TaSe2 has been one of the unique transition metal dichalcogenides exhibiting several phase transitions due to a delicate balance among competing electronic ground states. An unusual metallic state at high-T is sequentially followed by an incommensurate charge density wave (ICDW) state at » 122 K and a commensurate charge density wave (CCDW) state at » 90 K, and superconductivity at Tc » 0.14 K. Upon systematic intercalation of Pd ions into TaSe2, we find that CCDW order is destabilized more rapidly than ICDW to indicate a hidden quantum phase transition point at x » 0.09-0.10. Moreover, Tc shows a dramatic enhancement up to 3.3 K at x = 0.08, »24 times of Tc in 2H-TaSe2, in proportional to the density of states N(EF). Investigations of upper critical fields Hc2 in single crystals reveal evidences of multiband superconductivity as temperature-dependent anisotropy factor gH = Hc2ab/Hc2c, quasi-linear increase of Hc2c(T), and an upward, positive-curvature in Hc2ab(T) near Tc. Furthermore, analysis of temperature-dependent electronic specific heat and magnetic field-dependent in-plane thermal conductivity corroborate the presence of multiple superconducting gaps. In zero-field, the absence of the residual linear term of thermal conductivity, κ0/T, at T → 0, suggests the nodeless behavior of superconductivity. Based on above findings and electronic phase diagram vs x, we propose that the increase of N(EF) and effective electron-phonon coupling in the vicinity of CDW quantum phase transition should be a key to the large enhancement of Tc in PdxTaSe2. Reference 1. D. Bhoi et al., Scientific Report 6, 24068 (2016). 2. C. H. Kim et al. unpublished
Gapless spin-liquid ground state in the S=1/2 kagome antiferromagnet T. Xiang Institute of Physics, Chinese Academy of Sciences, China Frustrated quantum magnetism has moved to the forefront of physics research, posing fundamental questions concerning quantum disordered states, entanglement, topology and the nature of the quantum wavefunction. The defining problem in the field is one of the simplest, the ground state of the nearest neighbor S = 1/2 antiferromagnetic Heisenberg model on the kagome lattice, but has defied all theoretical and numerical methods employed to date. We apply the formalism of tensor-network states, specifically the method of projected entangled simplex states, whose combination of a correct accounting for multipartite entanglement and infinite system size provides qualitatively new insight. By studying the ground-state energy, the staggered magnetization we find at all finite tensor bond dimensions and the effects of a second-neighbour coupling, we demonstrate that the ground state is a gapless spin liquid. We discuss the comparison with other numerical studies and the physical interpretation of the gapless ground state. 1. Z. Y. Xie, J. Chen, J. F. Yu, X. Kong, B. Normand, T. Xiang, Renormalization of quantum many-body systems by the projected entangled simplex states, Phys Rev X 4, 011025 (2014). 2. H. J. Liao, Z. Y. Xie, J. Chen, X. J. Han, H. D. Xie, B. Normand, T. Xiang, Heisenberg antiferromagnet on the Husimi lattice, Phys Rev B 93, 075154 (2016). 3. H. J. Liao, Z. Y. Xie, J. Chen, Z. Y. Liu, H. D. Xie, R. Z. Huang, B. Normand, T. Xiang, Gapless spin-liquid ground state in the S=1/2 kagome antiferromagnet, arXiv:1610.04727
Sign Reversal Gaps in (Li1-xFex)OHFeSe Revealed by STS/STM
Hai-Hu Wen National Laboratory of Solid State Microstructures and Department of Physics, Collaborative Innovation Center for Advanced Microstructures, Nanjing University, Nanjing 210093, China In iron based superconductors, one of the on-going frontier studies is about the pairing mechanism. The Fermi surfaces have different structures among different systems. Some have both electron and hole pockets, for which the early proposed S± pairing manner holds very well. On the other hand, some systems have only or dominantly the electron pockets, such as the monolayer FeSe thin film, K1-xFe2-ySe2 and recently discovered Li1-xFexOHFeSe. We have conducted extensive STM/STS study on the Na(Fe1-xTx)As (T=Co, Cu, Mn)[1], Ba1-xKxFe2As2[2], KFe2As2[3], and Li1-xFexOHFeSe[4] single crystals. We found the clear evidence of the in-gap quasi-particle states induced by the non-magnetic Cu impurities in Na(Fe0.97-xCo0.03Cux)As, giving strong evidence of the S± pairing. Furthermore[1], we show the presence of the bosonic mode with the energy identical to that of the neutron resonance and a simple linear relation W»4.3kBTc, which is explained as a consequence of the S± pairing[2]. The recently discovered (Li1-xFex)OHFeSe phase with Tc » 40 K provides a good platform to check the fundamental problems with systems with only electron pockets[4]. The STS spectrum clearly indicates the presence of double superconducting gaps with maximum magnitudes of D1 » 14.3 meV and D2 » 8.6 meV, and the STS mimics that of the monolayer FeSe thin film. Further analysis based on the quasiparticle interference (QPI) allow us to rule out the d-wave gap, and assign the larger (smaller) gap to the outer (inner) hybridized Fermi pockets associating with the dxy (dxz/dyz) orbitals, respectively. The huge value 2D1/kBTc = 8.7 discovered here undoubtedly proves the strong coupling mechanism in the present superconducting system. Finally we report the spatial mapping of the density of states and follow the theoretical proposal for a robust determination of the gap sign. Together with the magnetic field influence on the in-gap states in (Li1-xFex)OHFe1-dZndSe, our results reveal the sign reversal gaps[5]. We try to give a unified picture about the pairing mechanism in iron based superconductors. 1. H. Yang et al., Nature Communications 4, 2947 (2013). 2. Z. Y. Wang, et al., Nature Physics 9, 42(2013). 3. D. L. Fang, X. Shi et al., Phys. Rev. B 92, 144513 (2015). 4. Zengyi Du, Xiong Yang, Hai Lin, et al., Nature Communications 7, 10565(2016). 5. Xiong Yang, Zengyi Du, Qiangqiang Gu, et al. To be published.
Phase diagram of Pr1-xLaCexCuO4-d as function of electron number and possible absence of disparity between electron- and hole-doped cuprate phase diagrams Dongjoon Song,1 Garam Han,2,3 Wonshik Kyung,2,3 Jeongjin Seo,2,4 Soohyun Cho,2,4 Beom Seo Kim,2,3 Masashi Arita,5 Kenya Shimada,5 Hirofumi Namatame,5 Masaki Taniguchi,5 Y. Yoshida,1 H. Eisaki,1 Seung Ryong Park,6 and C. Kim2,3 1Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8568, Japan 2Center for Correlated Electron Systems, Institute for Basic Science, Seoul 151-742, Republic of Korea 3Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Republic of Korea 4Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Republic of Korea 5Hiroshima Synchrotron Radiation Center, Hiroshima University, Hiroshima 739-0046, Japan 6Department of Physics, Incheon National University, Incheon 22012, Republic of Korea Although oxygen non-stoichiometry is common in oxide materials and may affect the properties of high-Tc copper oxide superconductor (cuprate), the phase diagram of electron-doped cuprates has been drawn as a function of the dopant Ce content alone. However, it has been recently claimed based on angle resolved photoemission (ARPES) results that annealing and oxygen vacancy can induce a sufficient change in the carrier density in the Pr1.3La0.6Ce0.1CuO4 single crystal [1] and T’-La2CuO4 thin film [2]. These results clearly indicate that the doping should be considered in conjunction with the annealing and oxygen non-stoichiometry and that it is essential to use the actual carrier (electron) density in building the phase diagram. To find the true phase diagram and resolve the remaining issues, we have performed systematic ARPES experiments on Pr1-xLaCexCuO4-d (PLCCO) single crystals (x = 0.10, 0.15, and 0.18) prepared under various annealing conditions. From the Fermi surface volume, we extracted electron number (n) and successfully established the phase diagram of PLCCO as a function of n. We found that, different from the Ce based case [3], the maximum Tc (Tc,max) occurs at around n = 0.15 and that the deduced the AF phase boundary dose not extends into superconducting dome. As a result, so determined phase diagram shows lack of disparity in comparison to the hole doped cuprate phase diagram. Besides, we observed a normal state gap feature at nodal region in the under-n samples, which suggests that a nodeless d-wave gap symmetry is the characteristic of under-doped cuprates. In this talk, we will present the detailed doping and annealing condition dependence of those features and discuss their origins, as well as their relation with Tc. Contact : dj-son@aist.go.jp [1] M. Horio et al. Nature Comm. 7, 10567 (2016). [2] Haofei I. Wei et al. Phys. Rev. Lett. 117, 147002 (2016). [3] M. Fujita et al., Phys. Rev. Lett. 101, 107003 (2008).
Superconducting Fluctuations in the Normal State of the Two-Dimensional Hubbard Model Emanuel Gull Department of Physics, University of Michigan, USA We compute the two-particle quantities relevant for superconducting correlations in the two-dimensional Hubbard model within the dynamical cluster approximation. In the normal state we identify the parameter regime in density, interaction, and second-nearest-neighbor hopping strength that maximizes the dx2−y2 superconducting transition temperature. We find in all cases that the optimal transition temperature occurs at intermediate coupling strength, and is suppressed at strong and weak interaction strengths. Similarly, superconducting fluctuations are strongest at intermediate doping and suppressed towards large doping and half-filling. We find a change in sign of the vertex contributions to dxy superconductivity from repulsive near half filling to attractive at large doping. p-wave superconductivity is not found at the parameters we study, and s-wave contributions are always repulsive. For negative second-nearest-neighbor hopping the optimal transition temperature shifts towards the electron-doped side in opposition to the van Hove singularity which moves towards hole doping. We surmise that an increase of the local interaction of the electron-doped compounds would increase Tc. This is for Phys. Rev. Lett. 115, 116402 (2015). The talk will also contain a substantial part of earlier and later work. HunPyo Lee Kangwon National University, Korea We would like to propose that a combination of the semiclassical approximation (SCA) with Monte Carlo tool can be an efficient and reliable impurity solver for large cluster extension calculations of a dynamical mean field theory, where continuous-time quantum Monte Carlo approaches could not access due to computational burden. In order to show the reliability of the method, we consider two test cases: (i) the one-dimensional frustrated single-band Hubbard model within a dynamical cluster approximation (DCA) with 24-site clusters and (ii) the doped two-dimensional extended Hubbard model within the DCA with 4-site clusters. In both test cases, we compare our results to ones obtained from other numerical tools and observe reasonable physical properties, while Luttinger sum rule in paramagnetic metallic regions cannot be captured by the SCA due to the freezing of dynamical fluctuations.
Superconductivity in FeSe thin films driven by the interplay between nematic fluctuations and spin-orbit coupling Jian Kang School of Physics and Asyronomy, University of Minnesota, USA The origin of the high-temperature superconducting state observed in FeSe thin films, with phase diagram displaying no sign of magnetic order, remains a hotly debated topic. Here we investigate whether fluctuations arising due to the proximity to a nematic phase, which is observed in the phase diagram of this material, can promote superconductivity. We find that nematic fluctuations alone promote a highly degenerate pairing state, in which both s-wave and d-wave symmetries are equally favored, and Tc is consequently suppressed. However, the presence of a sizable spin-orbit coupling or inversion symmetry-breaking at the film interface lifts this harmful degeneracy and selects the s-wave state, in agreement with recent experimental proposals. The resulting gap function displays a weak anisotropy, which agrees with experiments in monolayer FeSe.
Magnetic Excitations in doped Cuprates and Iridates from Raman Scattering and RIXS Institut fürFestkörperphysik, Karlsruher Institut für Technologie, Germany Superconductivity in high temperature superconducting cuprates arises upon doping of half-filed insulating and antiferromagnetically ordered CuO2 planes. We report on the details of the evolution of the magnetic excitation spectra from the undoped to the overdoped regime using resonant inelastic x-ray scattering (RIXS) at the Cu L3-edge [1,2,3]. We observe a cross-over in the nature of these excitations, which are collective, magnon-like excitations at low doping and evolve into incoherent particle-hole excitations in the Fermi-liquid state. In the underdoped regime, using Raman scattering, we observe a feedback of the superconducting transition on the high energy magnetic excitations suggesting their implication in the pairing [4]. We have also studied the doping dependence of the magnetic excitations in HTSC candidate electron-doped Sr2-xLaxIrO4 using Raman scattering and Ir L3-edge RIXS [3,4]. The long range magnetic order is rapidly lost with increasing doping, but two-dimensional short-range order (SRO) and dispersive magnon excitations with nearly undiminished spectral weight persist up to x = 0:10. The magnons in the SRO phase are heavily damped and exhibit anisotropic softening. Similarities to the case of cuprates are discussed. [1] M. Le Tacon et al. Nature Physics 7 725 (2011) [2] M. Minola et al., Phys. Rev. Lett. 114, 217003 (2015). [3] M. Minola et al., in preparation. [4] Y. Li et al. Phys. Rev, Lett. 108, 227003 (2012) [3] H. Gretarsson et al., Phys. Rev. Lett.116, 136401 (2016). [4] H. Gretarsson et al., Phys. Rev. Lett. 117, 107001 (2016)
Quantitative Determination of the Pairing Interactions for High Temperature Superconductivity in Cuprates Xingjiang Zhou National Lab for Superconductivity, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Email: XJZhou@iphy.ac.cn
A profound problem in modern condensed matter physics is to discover and understand the nature of the fluctuations and their coupling to fermions in cuprates which lead to high temperature superconductivity and the associated strange-metal normal state. In this talk, we will report the quantitative determination of the normal and pairing self-energies, made possible by laser-based angle-resolved photoemission measurements with unprecedented accuracy and stability. Through a precise inversion procedure, both the effective interactions in the attractive d-wave symmetry and the repulsive part in the full symmetry are determined. Besides finding the pairing self-energy and the attractive interactions for the first time, these results expose a central paradox of the high Tc problem: how the same frequency independent fluctuations can dominantly scatter at angles +-pi/2 in the attractive channel as well as lead to angle-independent repulsive scattering. The experimental results will be compared with the available theoretical calculations based on antiferromagnetic fluctuations, Hubbard model and the quantum-critical fluctuations of loop-current order. *This work is done in collaboration with Jin Mo Bok, Jong Ju Bae, Han-Yong Choi, Chandra M. Varma, Wentao Zhang, Junfeng He, Yuxiao Zhang and Li Yu [1]. J. M. Bok et al., Science Advances 2, e1501329 (2016). Itinerant vs. local magnetisms in correlated materials Zheng-Yu Weng Institute for Advanced Study, Tsinghua University, China Magnetism is an essential characteristic of electron correlations. The itinerant and local magnetisms usually represent two limits of correlations from weak to strong according to a conventional wisdom. In this talk, I will first overview the magnetisms related phenomena in various correlated systems, including single-band cuprates and multiband iron-based superconductors. Then I will show how both the itinerant and local magnetisms must be combined to play a critical role in metallic and superconducting phases of these systems. Several simplified theoretical cases will be discussed as illustrative examples.
Controlling unconventional superconductivity via tunable magnetic quantum critical points S. Seo1, E. Park1, E. D. Bauer2, F. Ronning2, J. D. Thompson2, J. N. Kim3, J.-H. Shim3, Tuson Park1, a Sungkyunkwan University, Department of Physics & Center for Quantum Materials and Superconductivity, Suwon 16419, Korea b Los Alamos National Laboratory, Los Alamos, NM 87545, USA c Pohang University of Science and Technology, Department of Physics, Pohang 37673, Korea The discovery of a magnetic critical point inside the pressure-induced unconventional superconducting phase of CeRhIn5 has raised a question on if the superconductivity could arise in spite of the QCP or because of the QCP [1, 2]. A lack of further explicit examples has been laid against the role of quantum critical fluctuations in producing superconductivity. Only recently has theory reported the possibility that the Cooper pair density may be enhanced near a local-type of QCP where both magnetic and quasiparticles- degrees-of-freedom are critical [3, 4]. In this presentation, we discuss study on tuning the AFM QCP of CeRhIn5 by a small Sn-doping. The pressure-induced superconducting phase was found to move with the tunable QCPs. The precise control of the superconducting phase by the magnetic QCPs underlines that unconventional superconductivity is produced because of the T=0 K instability in CeRhIn5 [5, 6]. + Author for correspondence: tp8701@skku.edu 1. T. Park et al., Nature 440, 65 (2006) 2. T. Park et al., Nature 456, 366 (2008) 3. J. H. Pixley et al., Phys. Rev. B 91, 201109(R) (2015) 4. M. A. Metlitski et al., Phys. Rev. B 92, 115111 (2015) 5. S. Seo et al., Nat. Communications 6, 6433 (2015) 6. S. Seo et al., Nat. Phys. 10, 120 (2014).
Phase transitions of optimally doped BaFe1.87Co0.13As2 manifested in the nonequilibrium dynamics
Kyungwan Kim Department of Physics, Chungbuk National University, Cheongju, Korea The magnetic order and structural transition are known to be suppressed near and beyond the optimal doping in Fe-based superconductors. Even in the paramagnetic state of a tetragonal structure, however, signatures of strong nematic fluctuations have been reported. The origin of the nematic fluctuation and its role for the superconductivity are being discussed intensively. The photo-induced reflectivity presents one of the clearest evidences for the nematic state. The ultrafast dynamics of the optimally doped BaFe1.87Co0.13As2 in the superconducting state presents two distinct anisotropic components. As temperature increases across Tc, the superconducting response disappears but the response remains anisotropic. When the temperature continues to increase further, the ultrafast dynamics suggests that at least two more phase transitions exist until the nematic response disappears. We will discuss the possible origins and their implications.
Electron doping studies of iron-based superconductors
Changyoung Kim
Department of Physics & Astronomy, Seoul National University
The Cooper pairs in iron-based superconductors are believed to be mediated by spin density wave fluctuations which are enhanced by the Fermi surface nesting condition. Therefore, existence of the Fermi surface nesting condition is thought to be the necessary condition for the high Tc. However, recent developments in the field show that the nesting condition may not be as essential as once believed to be. Mono-layer FeSe film grown on SrTiO3 shows a much higher Tc but has only an electron pocket. This appears to invalidate the notion that the nesting is essential in the iron-based superconductivity. However, this was only limited to FeSe system, and thus one can say that such phenomena is inherent only to chacogenide systems. We have performed doping dependent ARPES studies of iron-based superconductors. We find as the doping increases past the “optimal doping” (in terms of the nesting), the Tc determined from the gap size keeps increasing, as high as 44K. These results suggest that the nesting condition is not a necessary condition for both iron-pnictide and iron-chalcogenide systems. These results provide us an important clue on the paring mechanism.
Unusual electronic phase transitions in a natural heterostructure of iron pnictides and vanadium oxides, Sr2VO3FeAs Jun Sung Kim Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea Heterostructure of correlated electron materials offers a versatile platform to introduce a rich variety of phase transition. Subtle balance of competing interactions between the layers can generate new exotic phases that would otherwise not be stabilized in each layer. Here, we report that such an example is a natural heterostructure of the Mott-insulating vanadium oxides and the high-T_c superconducting iron pnictides. Clear evidence of the phase transition at T_0 = 155 K was observed in the iron pnictide layer, not in the vanadium oxide layer, using bulk and NMR measurements on single crystalline Sr_2VO_3FeAs. However, neither magnetic ordering with sufficient spin moment nor symmetry change in the crystal structure has been detected at T_0. At T_mag ≈ 45 K, far below T_0, unusual magnetic transition occurs in the iron pnictide layer, while the vanadium oxide layer remains nonmagnetic at low temperatures. The complex evolution of various phases in Sr_2VO_3FeAs is drastically distinct from the phase transitions found in other iron pnictides or vanadium oxides, highlighting the importance of the interlayer coupling between the layers.
Spin-polarized STM of unconventional superconductors and 2D spin lattice systems Jhinhwan Lee Department of Physics, KAIST, Korea Spin-polarized STM is a powerful atomic scale imaging and manipulating tool to investigate 2D spin lattice systems and various unconventional superconductors where strong correlation and antiferromagnetism play important roles in superconductivity. To demonstrate its potential we investigated a tetragonal Fe-base superconductor Sr2VO3FeAs to see if we can observe a Fe-layer C4 symmetric magnetism unperturbed by lattice distortion. The parent-state superconductor Sr2VO3FeAs with Tc~37 K has also an interesting structure with two perovskite Sr2VO3 layers cladding the FeAs layer in a unit cell, making it an ideal choice to investigate the role of interfacial phonons on the enhancement of Fe-based superconductivity in association with the recent discovery of Tc~100 K in a monolayer FeSe on SrTiO substrate. Our observation shows a SPSTM signature consistent with plaquette antiferromagnetic order in the Fe layer expected to be preferred over the diagonal double stripe order by quantum fluctuations. Also observed in our QPI experiment is the existence of shake-off bands which is the spectroscopic signature of electron-phonon interaction. Further position-dependent gap-map-based QPI analysis shows the signatures of pairing enhancement due to interfacial phonons which is consistent with the self-consistent Migdal-Eliashberg T-matrix QPI simulation.
Investigation of superconducting paring symmetry by magnetic force microscopy
Jeehoon Kim
Department of Physics, Postech, Pohang, Korea
We have constructed a He magnetic force microscope (MFM), operating within a vector magnet with the base temperature of 300 mK and magnetic field range of 2-2-9 T in the x-y-z direction. We demonstrated magnetic imaging capabilities at very low temperature by imaging simultaneously superconducting vortices and magnetic stripes at T= 500 mK in the ferromagnetic superconductor ErNi2B2C which has a ferromagnetic transition below Twfm=2.3 K. The direct visualization of coexistence between superconductivity and magnetism was carried out in ErNi2B2C. The vector field performance of the apparatus was also demonstrated by the creation and imaging of Abrikosov vortices within a superconducting Nb film using a vector field. For example, an in-plane field allows creating a vortex-antivortex pair which is confined through a single flux tube, and thus showing a linear potential in distance. The 3D-magnet MFM also allows us to measure the paring symmetry of superconductors by MFM via Meissner force detection. We will show anisotropic angular dependence on the magnetic penetration depth in CeCoIn5, which signals the d-wave paring symmetry.
Pairing interaction of cuprate high Tc superconductors
Han-Yong Choi Department of Physics, Sung Kyun Kwan University, Suwon, Korea
After some thirty years of intense research into the cuprates, the mechanism of high Tc superconductivity still remains elusive. The root of the high Tc conundrum lies in the requirement that the superconductivity must be understood together with the strange metallic and pseudogap states in their totality. Leading ideas for high Tc superconductivity mechanism are the resonating valence bond, the loop current order, and the antiferromagnetic spin fluctuations scenarios. The best way to differentiate among the proposals is, perhaps, to determine the frequency and momentum dependence of the normal and pairing self-energies from experiments. Thus determined self-energies (or, corresponding Eliashberg functions if necessary) can be checked against those from the proposed ideas. This procedure has been implemented by analyzing the ultra high resolution laser ARPES data on Bi2212 samples. This reveals a central paradox of the high Tc problem: how the angle-independent fluctuations apparent in the normal channel can predominantly scatter at angles
* J. M. Bok et al., Science Advances 2, e1501329 (2016).
Giant spin-induced polarizations and directional dichroism at spin-wave excitations in BiFeO3 Jun Hee Lee School of Energy & Chemical Engineering, UNIST , Korea
Superconductivity in high temperature superconducting cuprates arises upon doping of half-filed insulating and antiferromagnetically ordered CuO2 planes. We report on the details of the evolution of the magnetic excitation spectra from the undoped to the overdoped regime using resonant inelastic x-ray scattering (RIXS) at the Cu L3-edge [1,2,3]. We observe a cross-over in the nature of these excitations, which are collective, magnon-like excitations at low doping and evolve into incoherent particle-hole excitations in the Fermi-liquid state. In the underdoped regime, using Raman scattering, we observe a feedback of the superconducting transition on the high energy magnetic excitations suggesting their implication in the pairing [4]. We have also studied the doping dependence of the magnetic excitations in HTSC candidate electron-doped Sr2-xLaxIrO4 using Raman scattering and Ir L3-edge RIXS [3,4]. The long range magnetic order is rapidly lost with increasing doping, but two-dimensional short-range order (SRO) and dispersive magnon excitations with nearly undiminished spectral weight persist up to x = 0:10. The magnons in the SRO phase are heavily damped and exhibit anisotropic softening. Similarities to the case of cuprates are discussed. [1] J. H. Lee* et al., New Journal of Physics 18, 043025 (2016). [2] S. Calder, J. H. Lee* et al., Nature Communications 6, 8916 (2015). [3] H. Dixit, J. H. Lee* et al., Scientific Reports 5, 12969 (2015). [4] J. H. Lee* and R. Fishman, Physical Review Letters 115, 207203 (2015). [5] I. Kezsmarki, U. Nagel, S. Bordacs, R. Fishman, J. H. Lee et al., Physical Review Letters 115, 127203 (2015). [6] R. Fishman, J. H. Lee et al., Physical Review B 92, 094422 (2015). Nematicity and superconductivity in Fe-based and cuprate superconductors: Impact of Aslamazov-Larkin vertex corrections H. Kontani Department of Physics, Nagoya University, Japan Nematicity and high-Tc superconductivity are key features in Fe-based superconductors [1-3]. Especially, the high-Tc mechanism absent of hole-pockets in heavily electron-doped FeSe attracts great attention. To attack this issue, we study the higher-order many-body effect called the vertex correction (VC) that has been neglected in conventional Migdal-Eliashberg (ME) gap equation. Due to the VC, the dressed effective Coulomb interaction possesses nontrivial spin and orbital-dependences. In FeSe, we find that (i) the VC not only induces the orbital fluctuations, but also strongly magnifies the orbital-fluctuation-mediated pairing interaction. In addition, (ii) sizable pairing glue is given by the exchange of the orbital+spin composite fluctuations. Because of both important beyond-ME effects, which are caused by the interplay between orbital and spin fluctuations, the anisotropic s-wave state in heavily electron-doped FeSe is satisfactorily explained. The proposed hole-pocket-less pairing mechanism would be important for various Fe-based superconductors. We also present the theoretical study of the charge-density-wave state in cuprate superconductors [4,5]. Toward solving this unsolved problem, we employ the recently developed functional renormalization-group method, by which we can calculate the higher-order VCs in a systematic and unbiased way with high numerical accuracy. We discover the critical development of the p-orbital-density-wave (p-ODW) instability in the strong-spin-fluctuation region. The obtained p-ODW state possesses the key characteristics of the charge ordering pattern in Bi- and Y-based superconductors, such as the wave vector parallel to the nearest Cu-Cu direction, and the d-symmetry form factor with the antiphase correlation between px and py orbitals in the same unit cell. [1] Y. Yamakawa and H. Kontani, arXiv:1609.09618. [2] S. Onari, Y. Yamakawa, and H. Kontani, Phys. Rev. Lett. 116, 227001 (2016). [3] Y. Yamakawa, S. Onari, and H. Kontani,Phys. Rev. X 6, 021032 (2016). [4] M. Tsuchiizu, Y. Yamakawa, and H. Kontani, Phys. Rev. B 93, 155148 (2016). [5] Y. Yamakawa and H. Kontani, Phys. Rev. Lett. 114, 257001 (2015) |
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