V. B. Zabolotnyy

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.

(pi-pi) electronic order in iron arsenide superconductors

The distribution of valence electrons in metals usually follows the symmetry of the underlying ionic lattice. Modulations of this distribution often occur when those electrons are not stable with respect to a new electronic order, such as spin or charge density waves. Electron density waves have been observed in many families of superconductors, and are often considered to be essential for superconductivity to exist. Recent measurements seem to show that the properties of the iron pnictides are in good agreement with band structure calculations that do not include additional ordering, implying no relation between density waves and superconductivity in these materials. Here we report that the electronic structure of Ba1-xKxFe2As2 is in sharp disagreement with those band structure calculations, and instead reveals a reconstruction characterized by a (π, π) wavevector. This electronic order coexists with superconductivity and persists up to room temperature (300 K).

Unusual band renormalization in the simplest iron-based superconductor FeSe 1 − x

The electronic structure of the iron chalcogenide superconductor FeSe_{1-x} was investigated by high- resolution angle-resolved photoemission spectroscopy (ARPES). The results were compared to DFT calculations showing some significant differences between the experimental electronic structure of FeSe_{1-x}, DFT calculations and existing data on FeSe_{x}Te_{1-x}. The bands undergo a pronounced orbital dependent renormalization, different from what was observed for FeSe_{x}Te_{1-x} and any other pnictides.

Pseudogap and Charge Density Waves in Two Dimensions

Using angle-resolved photoemission spectroscopy we demonstrate that a normal-state pseudogap exists above T(N-IC) in one of the most studied two-dimensional charge-density wave (CDW) dichalcogenides 2H-TaSe(2). The initial formation of the incommensurate CDW is confirmed as being driven by a conventional nesting instability, which is marked by a pseudogap. The magnitude, character, and anisotropy of the 2D-CDW pseudogap bear considerable resemblance to those seen in superconducting cuprates.

One-Sign Order Parameter in Iron Based Superconductor

The onset of superconductivity at the transition temperature is marked by the onset of order, which is characterized by an energy gap. Most models of the iron-based superconductors find a sign-changing (s±) order parameter (1-6), with the physical implication that pairing is driven by spin fluctuations. Recent work, however, has indicated that LiFeAs has a simple isotropic order parameter (7-9) and spin fluctuations are not necessary (7,10), contrary to the models (1-6). The strength of the spin fluctuations has been controversial (11,12), meaning that the mechanism of superconductivity cannot as yet be determined. We report the momentum dependence of the superconducting energy gap, where we find an anisotropy that rules out coupling through spin fluctuations and the sign change. The results instead suggest that orbital fluctuations assisted by phonons (13,14) are the best explanation for superconductivity.

Momentum dependence of the superconducting gap in Ba 1 − x K x Fe 2 As 2

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

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.

Specific heat and angle-resolved photoemission spectroscopy study of the superconducting gaps in LiFeAs

U. Stockert, 2, ∗ M. Abdel-Hafiez, D. V. Evtushinsky, V. B. Zabolotnyy, A. U. B. Wolter, S. Wurmehl, I. Morozov, 1 R. Klingeler, S. V. Borisenko, and B. Büchner Institute for Solid State Research, IFW Dresden, D-01171 Dresden, Germany MPI for Chemical Physics of Solids, D-01187 Dresden, Germany Moscow State University, Moscow 119991, Russia Kirchhoff Institute for Physics, Heidelberg University, D-69120 Heidelberg (Dated: November 19, 2010)

Fermi surface nesting in several transition metal dichalcogenides

By means of high-resolution angle-resolved photoelectron spectroscopy (ARPES), we have studied the fermiology of 2H transition metal dichalcogenide polytypes TaSe2, NbSe2 and Cu0.2NbS2. The tight-binding model of the electronic structure, extracted from ARPES spectra for all three compounds, was used to calculate the Lindhard function (bare spin susceptibility), which reflects the propensity to charge density wave (CDW) instabilities observed in TaSe2 and NbSe2. We show that though the Fermi surfaces of all three compounds possess an incommensurate nesting vector in the close vicinity of the CDW wave vector, the nesting and ordering wave vectors do not exactly coincide, and there is no direct relationship between the magnitude of the susceptibility at the nesting vector and the CDW transition temperature. The nesting vector persists across the incommensurate CDW transition in TaSe2 as a function of temperature despite the observable variations of the Fermi surface geometry in this temperature range. In Cu0.2NbS2, the nesting vector is present despite different doping levels, which leads us to expect a possible enhancement of the CDW instability with Cu intercalation in the CuxNbS2 family of materials.