Rolf Heid

Phonon dispersion and electron-phonon coupling in MgB2 and AlB2.

We present a first principles investigation of the lattice dynamics and electron-phonon coupling of the superconductor MgB2 and the isostructural AlB2 within the framework of density functional perturbation theory using a mixed-basis pseudopotential method. Complete phonon dispersion curves and Eliashberg functions alpha2F are calculated for both systems. The main differences are related to high frequency in-plane boron vibrations, which are strongly softened in MgB2 and exhibit an exceptionally strong electron-phonon coupling. We also report on Raman measurements, which support the theoretical findings. Implications for the superconducting transition temperature are briefly discussed.

Inelastic X-ray scattering in YBa2Cu3O6.6 reveals giant phonon anomalies and elastic central peak due to charge-density-wave formation

1 Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany 2 European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France 3 CNR-SPIN, CNISM and Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy 4 Institut für Festkörperphysik, Karlsruher Institut für Technologie (KIT), P.O.B. 3640, D-76021 Karlsruhe, Germany

Evidence of Strong Correlations and Coherence-Incoherence Crossover in the Iron Pnictide Superconductor KFe2As2

Using resistivity, heat-capacity, thermal-expansion, and susceptibility measurements we study the normal-state behavior of KFe2As2. Both the Sommerfeld coefficient (γ≈103 mJ mol(-1) K(-2)) and the Pauli susceptibility (χ≈4×10(-4)) are strongly enhanced, which confirm the existence of heavy quasiparticles inferred from previous de Haas-van Alphen and angle-resolved photoemission spectroscopy experiments. We discuss this large enhancement using a Gutzwiller slave-boson mean-field calculation, which shows the proximity of KFe2As2 to an orbital-selective Mott transition. The temperature dependence of the magnetic susceptibility and the thermal expansion provide strong experimental evidence for the existence of a coherence-incoherence crossover, similar to what is found in heavy fermion and ruthenate compounds, due to Hunds coupling between orbitals.

Kohn anomaly in MgB2 by inelastic X-ray scattering.

We study phonons in MgB2 using inelastic x-ray scattering (1.6 and 6 meV resolution). We clearly observe the softening and broadening of the crucial E(2g) mode through the Kohn anomaly along GammaM, in excellent agreement with ab initio calculations. Low temperature measurements (just above and below T(c)) show negligible changes for the momentum transfers investigated and no change in the E(2g) mode at A between room temperature and 16 K. We report the presence of a longitudinal mode along GammaA near in energy to the E(2g) mode that is not predicted by theory.

Lattice Dynamics and Electron-Phonon Interaction in (3,3) Carbon Nanotubes

We present a detailed study of the lattice dynamics and electron-phonon coupling for a (3,3) carbon nanotube which belongs to the class of small diameter based nanotubes which have recently been claimed to be superconducting. We treat the electronic and phononic degrees of freedom completely by modern ab initio methods without involving approximations beyond the local density approximation. Using density functional perturbation theory we find a mean-field Peierls transition temperature of approximately 240 K which is an order of magnitude larger than the calculated superconducting transition temperature. Thus in (3,3) tubes the Peierls transition might compete with superconductivity. The Peierls instability is related to the special 2k(F) nesting feature of the Fermi surface. Because of the special topology of the (n,n) tubes we also find a phonon softening at the Gamma point.

Ab initio calculations of multilayer relaxations of stepped Cu surfaces

We present trends in the multilayer relaxations of several vicinals of Cu(100) and Cu( 111) of varying terrace widths and geometries. The electronic structure calculations are based on density-functional theory in the local-density approximation with norm-conserving, nonlocal pseudopotentials in the mixed basis representation. While relaxations continue for several layers, the major effect is concentrated near the step and comer atoms. On all surfaces the step atoms contract inward, in agreement with experimental findings. Additionally, the corner atoms move outward and the atoms in the adjacent chain undergo a large inward relaxation. Correspondingly, the largest contraction (4%) is in the bond length between the step atom and its bulk nearest neighbor (BNN), while that between the corner atom and the BNN is somewhat enlarged. The surface atoms also display changes in registry of up to 1.5%. Our results are in general in good agreement with low-energy electron-diffraction data including the controversial case of Cu(511). Subtle differences are found with results obtained from semiempirical potentials.

Momentum dependence of the electron-phonon coupling and self-energy effects in superconducting YBa2Cu3O7 within the local density approximation.

Using the local density approximation and a realistic phonon spectrum we determine the momentum and frequency dependence of alpha(2)F(k,omega) in YBa(2)Cu(3)O(7) for the bonding, antibonding, and chain band. The resulting self-energy Sigma is rather small near the Fermi surface. For instance, for the antibonding band the maximum of ReSigma as a function of frequency is about 7 meV at the nodal point in the normal state and the ratio of bare and renormalized Fermi velocities is 1.18. These values are a factor of 3-5 too small compared to the experiment showing that only a small part of Sigma can be attributed to phonons. Furthermore, the frequency dependence of the renormalization factor Z(k,omega) is smooth and has no anomalies at the observed kink frequencies which means that phonons cannot produce well-pronounced kinks in stoichiometric YBa(2)Cu()3)O(7), at least, within the local density approximation.

Unveiling mode-selected electron-phonon interactions in metal films by helium atom scattering

The quasi two-dimensional electron gas on a metal film can transmit to the surface even minute mechanical disturbances occurring in the depth, thus allowing the gentlest of all surface probes, helium atoms, to perceive the vibrations of the deepest atoms via the induced surface-charge density oscillations. A density functional perturbation theory (DFPT) and a helium atom scattering study of the phonon dispersion curves in lead films of up to 7 mono-layers on a copper substrate show that: (a) the electron-phonon interaction is responsible for the coupling of He atoms to in-depth phonon modes; and (b) the inelastic HAS intensity from a given phonon mode is proportional to its electron-phonon coupling. The direct determination of mode-selected electron-phonon coupling strengths has great relevance for understanding superconductivity in thin films and two-dimensional systems.

Strain-Driven Approach to Quantum Criticality in AFe2As2 with A=K, Rb, and Cs

The iron-based superconductors AFe_{2}As_{2} with A=K, Rb, Cs exhibit large Sommerfeld coefficients approaching those of heavy-fermion systems. We have investigated the magnetostriction and thermal expansion of this series to shed light on this unusual behavior. Quantum oscillations of the magnetostriction allow identifying the band-specific quasiparticle masses which by far exceed the band-structure derived masses. The divergence of the Grüneisen ratio derived from thermal expansion indicates that with increasing volume along the series a quantum critical point is approached. The critical fluctuations responsible for the enhancement of the quasiparticle masses appear to weaken the superconducting state.Felix Eilers, Kai Grube, Diego A. Zocco, Thomas Wolf, Michael Merz, Peter Schweiss, Rolf Heid, Robert Eder, Rong Yu, Jian-Xin Zhu, Qimiao Si, Takasada Shibauchi, 6 and Hilbert v. Löhneysen 7 Institut für Festkörperphysik, Karlsruher Institut für Technologie, 76021 Karlsruhe, Germany Department of Physics, Renmin University of China, Beijing 100872, China Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA Department of Physics and Astronomy, Rice University, Houston, TX 77005, USA Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561, Japan Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan Physikalisches Institut, Karlsruhe Institut für Technologie, 76049 Karlsruhe, Germany (Dated: October 8, 2015)

Ab initio calculations of adsorbate-induced stress on Ni(100)

We have calculated the surface stress induced on adsorption of