C. Quitmann

Temperature Dependence of the Superconducting Gap Anisotropy in Bi2Sr2CaCu2O8+x

Detailed data on the momentum-resolved temperature dependence of the superconducting gap of Bi2Sr2CaCu2O8+x are presented, complemented by similar data on the intensity of the photoemission superconducting condensate spectral area. The gap anisotropy between the Γ-Mand Γ-X directions increases markedly with increasing temperature, contrary to what happens for conventional anisotropic-gap superconductors, such as lead. Specifically, the size of the superconducting gap along the Γ-X direction decreases to values indistinguishable from zero at temperatures for which the gap retains virtually full value along the Γ-M direction. These data rule out the simplest type of d-wave order parameter.

Observation of a van Hove singularity in Bi2Sr2CaCu2O8+x with angle-resolved photoemission.

Keywords: HIGH-TC SUPERCONDUCTORS ; DENSITY-OF-STATES ; FERMI-SURFACE ; ELECTRONIC-STRUCTURE ; TEMPERATURE Note: Univ wisconsin,ctr appl superconduct,madison,wi 53706. ecole polytech fed lausanne,inst phys appl,ch-1015 lausanne,switzerland. Ma, j, univ wisconsin,dept phys,1150 univ ave,madison,wi 53706.ISI Document Delivery No.: QH079 Reference LSE-ARTICLE-1995-019View record in Web of Science Record created on 2006-10-03, modified on 2017-05-12

Structural and Magnetic Dynamics of a Laser Induced Phase Transition in FeRh

We use time-resolved x-ray diffraction and magneto-optical Kerr effect to study the laser-induced antiferromagnetic to ferromagnetic phase transition in FeRh. The structural response is given by the nucleation of independent ferromagnetic domains (τ(1)~30 ps). This is significantly faster than the magnetic response (τ(2)~60 ps) given by the subsequent domain realignment. X-ray diffraction shows that the two phases coexist on short time scales and that the phase transition is limited by the speed of sound. A nucleation model describing both the structural and magnetic dynamics is presented.

Doping-induced change of superconducting gap anisotropy in Bi2Sr2Ca1Cu2O8+δ

High-resolution angle-resolved photoemission measurements were performed on single crystals of Bi2Sr2Ca1Cu2O8+δ with different oxygen stoichiometries. The data establish that the gap anisotropy (ratio of the gap along Γ-M to the gap along Γ-X) can be reversibly changed from ∼20:1 (optimal or underdoped) to ∼2:1 (overdoped). Differences in sample doping explain the conflicting reports on gap anisotropy in the literature. Possible effects of this change in gap anisotropy on the symmetry of the order parameter are discussed. There remains some ambiguity as to the relation between the order parameter and doping.

INTERLAYER COUPLING AND THE METAL-INSULATOR TRANSITION IN PR-SUBSTITUTED BI2SR2CACU2O8+Y

Substitution of rare-earth ions for Ca in Bi2Sr2CaCu2O8+y is known to cause a metal-insulator transition. Using resonant photoemission we study how this chemical substitution affects the electronic structure of the material. For the partial Cu-density of states at E_F and in the region of the valence band we observe no significant difference between a pure superconducting sample and an insulating sample with 60% Pr for Ca. This suggests that the states responsible for superconductivity are predomi- nately O-states. The partial Pr-4f density of states was extracted utilizing the Super- Koster-Kronig Pr 4d-4f resonance. It consists of a single peak at 1.36eV binding energy. The peak shows a strongly assymetric Doniach-Sunjic line- shape indicating the presence of a continuum of electronic states with sharp cut off at E_F even in this insulating sample. This finding excludes a bandgap in the insulating sample and supports the existance of a mobility gap caused by spatial localization of the carriers. The presence of such carriers at the Pr-site, between the CuO_2 planes shows that the electronic structure is not purely 2-dimensional but that there is a finite interlayer coupling. The resonance enhancement of the photoemission cross section, at the Pr-4d threshold, was studied for the Pr-4f and for Cu-states. Both the Pr-4f and the Cu-states show a Fano-like resonance. This resonance of Cu-states with Pr-states is another indication of coupling between the the Pr-states and those in the CuO_2 plane. Because of the statistical distribution of the Pr-ions this coupling leads to a non-periodic potential for the states in the CuO_2 plane which can lead to localization and thus to the observed metal-insulator transition.

Structural and magnetic dynamics in the magnetic shape-memory alloy Ni2MnGa

Magnetic shape-memory Heusler alloys are multiferroics stabilized by the correlations between electronic, magnetic, and structural order. To study these correlations we use time-resolved x-ray diffraction and magneto-optical Kerr effect experiments to measure the laser induced dynamics in a Heusler alloy Ni2MnGa film and reveal a set of time scales intrinsic to the system. We observe a coherent phonon which we identify as the amplitudon of the modulated structure and an ultrafast phase transition leading to a quenching of the incommensurate modulation within 300 fs with a recovery time of a few ps. The thermally driven martensitic transition to the high temperature cubic phase proceeds via nucleation within a few ps and domain growth limited by the speed of sound. The demagnetization time is 320 fs, which is comparable to the quenching of the structural modulation.

Direct observation of antiferromagnetically oriented spin vortex states in magnetic multilayer elements

We report on the coupling of spin vortices in magnetic multilayer elements. The magnetization distribution in thin film disks consisting of two ferromagnetic layers separated by a nonmagnetic spacer is imaged layer-resolved by using x-ray microscopy. We directly observe two fundamentally different vortex coupling states, namely antiferromagnetic and ferromagnetic orientation of the flux directions. It is found that these states are predetermined for systems that involve a sufficiently strong interlayer exchange coupling, whereas for the case of a purely dipolar interaction both states are transformable into each other.

Coherent phonon dynamics at the martensitic phase transition of Ni2MnGa

We use time-resolved optical reflectivity to study the laser stimulated dynamics in the magnetic shape memory alloy Ni2MnGa. We observe two coherent optical phonons, at 1.2 THz in the martensite phase and at 0.7 THz in the pre-martensite phase, which we interpret as a zone-folded acoustic phonon and a heavily damped amplitudon, respectively. In the martensite phase the martensitic phase transition can be induced by a fs laser pulse on a timescale of a few ps.

Magnetization dynamics of Landau structures: tuning the response of mesoscopic magnetic objects using defects

Magnetic vortex cores are interacting with and can even be annihilated by artificial defects, such as holes. These defects have been fabricated by focused ion beam milling (FIB) into the magnetic domains, domain walls and the center of square-shaped vortices, known as Landau structures. We report the imaging of the magnetization dynamics of Landau structures containing holes by means of x-ray magnetic circular dichroism photo-emission electron microscopy (XMCD-PEEM). Due to the high lateral and temporal resolution of this method, the magnetic excitation spectrum, which is characteristic for the vortex-hole interaction, is investigated in detail. We find that the vortex core as well as domain walls can be trapped by small holes. With the help of micromagnetic simulations we show that the vortex gyrotropic motion frequency is enhanced, whereas the amplitude is significantly reduced in the case of non-centric holes in domain walls.

Metal-insulator transition and electronic structure in Pr-doped Bi2Sr2(Caz, Pr1 − z)Cu2O8 + y

Abstract Substitution of Ca for Pr in Bi 2 Sr 2 (Ca z , Pr 1 − x )Cu 2 O 8 + y leads to an insulator-metal transition at a critical concentration z c = 0.52. The partial Pr-4f density of states of the insulating sample ( z = 0.4) was extracted utilizing the Pr 4d−4f Super-Coster-Kronig resonance in the resonant photoemission. The 4f-emission consists of a single peak at 1.36 eV binding energy and shows a strongly asymmetric Doniach-Sunjic line shape. This indicates an electronic band continuum with a sharp cutoff at E F even in the insulating sample. This finding excludes a band gap at E F in the insulator, but supports the existence of localized states at E F due to disorder.