A. A. Kordyuk

Superconductivity without Nesting in LiFeAs

We have studied the electronic structure of the nonmagnetic LiFeAs (T(c)∼18  K) superconductor using angle-resolved photoemission spectroscopy. We find a notable absence of the Fermi surface nesting, strong renormalization of the conduction bands by a factor of 3, high density of states at the Fermi level caused by a van Hove singularity, and no evidence for either a static or a fluctuating order except superconductivity with in-plane isotropic energy gaps. Our observations suggest that these electronic properties capture the majority of ingredients necessary for the superconductivity in iron pnictides.

Strength of the spin-fluctuation-mediated pairing interaction in a high-temperature superconductor

Although spin fluctuations are believed to have an important role in the mechanism responsible for high-temperature superconductivity, it has been unclear whether the strength of their coupling with fermionic quasiparticles is sufficiently strong. Systematic analysis of angle-resolved photoemission and neutron spectra suggests it is. Theories based on the coupling between spin fluctuations and fermionic quasiparticles are among the leading contenders to explain the origin of high-temperature superconductivity, but estimates of the strength of this interaction differ widely1. Here, we analyse the charge- and spin-excitation spectra determined by angle-resolved photoemission and inelastic neutron scattering, respectively, on the same crystals of the high-temperature superconductor YBa2Cu3O6.6. We show that a self-consistent description of both spectra can be obtained by adjusting a single parameter, the spin–fermion coupling constant. In particular, we find a quantitative link between two spectral features that have been established as universal for the cuprates, namely high-energy spin excitations2,3,4,5,6,7 and ‘kinks’ in the fermionic band dispersions along the nodal direction8,9,10,11,12. The superconducting transition temperature computed with this coupling constant exceeds 150 K, demonstrating that spin fluctuations have sufficient strength to mediate high-temperature superconductivity.

Superconducting gap in the presence of bilayer splitting in underdoped (Pb,Bi)2Sr2CaCu2O8? ?

The clearly resolved bilayer splitting in angle-resolved photoemission spectra of the underdoped Pb-Bi2212 compound raises the question of how the bonding and antibonding sheets of the Fermi surface are gapped in the superconducting state. Here we compare the superconducting gaps for both split components and show that within the experimental uncertainties they are identical. By tuning the relative intensity of the bonding and antibonding bands using different excitation conditions, we determine the precise k dependence of the leading edge gap. Significant deviations from the simple cos(k x )-cos(k y ) gap function for the studied doping level are detected.

Kinks, nodal bilayer splitting, and interband scattering in YBa2Cu3O6+x

We apply the new-generation angle-resolved photoemission spectroscopy methodology to the most widely studied cuprate superconductor YBa2Cu3O(6+x). Considering the nodal direction, we found noticeable renormalization effects known as kinks both in the quasiparticle dispersion and scattering rate, the bilayer splitting, and evidence for strong interband scattering--all the characteristic features of the nodal quasiparticles detected earlier in Bi2Sr2CaCu2O(8+delta). The typical energy scale and the doping dependence of the kinks clearly point to their intimate relation with the spin-1 resonance seen in the neutron scattering experiments. Our findings strongly suggest a universality of the electron dynamics in the bilayer superconducting cuprates and a dominating role of the spin fluctuations in the formation of the quasiparticles along the nodal direction.

Strong electron pairing at the iron 3d(xz) (yz) orbitals in hole-doped BaFe2As2 superconductors revealed by angle-resolved photoemission spectroscopy

D.V.Evtushinsky,1 V.B. Zabolotnyy,1 T.K.Kim,1, 2 A.A.Kordyuk,1, 3 A.N. Yaresko,4 J.Maletz,1 S.Aswartham,1 S.Wurmehl,1 A.V. Boris,4 D.L. Sun,4 C.T. Lin,4 B. Shen,5 H.H.Wen,6 A.Varykhalov,7 R.Follath,7 B.Büchner,1 and S.V.Borisenko1 Institute for Solid State Research, IFW Dresden, P.O.Box 270116, D-01171 Dresden, Germany Diamond Light Source Ltd., Didcot, Oxfordshire, OX11 0DE, United Kingdom Institute of Metal Physics of National Academy of Sciences of Ukraine, 03142 Kyiv, Ukraine Max-Planck-Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China BESSY GmbH, Albert-Einstein-Strasse 15, 12489 Berlin, Germany

Angle-resolved photoemission spectroscopy of superconducting LiFeAs: Evidence for strong electron-phonon coupling

The time of life of fermionic quasiparticles, the distribution of which in the momentum-energy space can be measured by angle resolved photoemission (ARPES), is the first quantity to look for fingerprints of interaction responsible for the superconducting pairing. Such an approach has been recently used for superconducting cuprates, but its direct application to pnictides was not possible due to essential three-dimensionality of the electronic band structure and magnetic ordering. Here, we report the investigation of the quasiparticle lifetime in LiFeAs, a non-magnetic stoichiometric superconductor with a well separated two-dimensional band. We have found two energy scales: the lower one contains clear fingerprints of optical phonon modes while the higher scale indicates a presence of strong electron-electron interaction. The result suggests that LiFeAs is a phonon mediated superconductor with strongly enhanced electronic density of states at the Fermi level.

Estimation of matrix element effects and determination of the Fermi surface in Bi2Sr2CaCu2O8+d systems using angle-scanned photoemission spectroscopy

The strong dependence of the momentum distribution of the photoelectrons on experimental conditions raises the question as to whether angle-resolved photoemission spectroscopy is able to provide an accurate reflection of the Fermi surface in Bi-based cuprate superconductors. In this paper we experimentally prove that the main contribution to the intensity variation comes from matrix-element effects and develop an approach to overcome this problem. We introduce a concept of ‘‘self-normalization’’ that makes the spectra essentially independent of both the matrix elements and particular experimental parameters. On the basis of this concept we suggest a simple and precise method of Fermi-surface determination in quasi-two-dimensional systems.

Electronic band structure and momentum dependence of the superconducting gap in Ca1−xNaxFe2As2 from angle-resolved photoemission spectroscopy

Iron-based high-temperature superconductors form an in-creasingly growing subject for investigation. Unlike othertypes of high-temperature superconductors, iron-based com-pounds can be synthesized in the form of various crystals, ex-hibiting a large variety of electronic band structures, magneticproperties, superconducting order parameters, and electronicproperties in general.

Photoemission-induced gating of topological insulators

The recently discovered topological insulators exhibit topologically protected metallic surface states which are interesting from the fundamental point of view and could be useful for various applications if an appropriate electronic gating can be realized. Our photoemission study of Cu-intercalated Bi2Se3 shows that the surface-state occupancy in this material can be tuned by changing the photon energy and understood as a photoemission-induced gating effect. Our finding provides an effective tool to investigate the new physics coming from the topological surface states and suggests intercalation as a recipe for synthesis of a material suitable for electronic applications.

Momentum dependence of the superconducting gap in Ba1-xKxFe2As2

D. V. Evtushinsky,1 D. S. Inosov,1,2 V. B. Zabolotnyy,1 A. Koitzsch,1 M. Knupfer,1 B. Buchner,1 M. S. Viazovska,3 G. L. Sun,2 V. Hinkov,2 A. V. Boris,2,4 C. T. Lin,2 B. Keimer,2 A. Varykhalov,5 A. A. Kordyuk,1,6 and S. V. Borisenko1 1Institute for Solid State Research, IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany 2Max-Planck-Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany 3Max-Planck-Institute for Mathematics, Vivatsgasse 7, 53111 Bonn, Germany 4Department of Physics, Loughborough University, Loughborough, LE11 3TU. United Kingdom 5BESSY GmbH, Albert-Einstein-Strasse 15, 12489 Berlin, Germany 6Institute of Metal Physics, National Academy of Sciences of Ukraine, 03142 Kyiv, Ukraine Received 24 September 2008; revised manuscript received 30 December 2008; published 17 February 2009