In high-energy physics, the Higgs field couples to gauge bosons and fermions and gives mass to their elementary excitations. Experimentally, such couplings can be inferred from the decay product of the Higgs boson, i.e., the scalar (amplitude) excitation of the Higgs field. In superconductors, Cooper pairs bear a close analogy to the Higgs field. Interaction between the Cooper pairs and other degrees of freedom provides dissipation channels for the amplitude mode, which may reveal important information about the microscopic pairing mechanism. To this end, we investigate the Higgs (amplitude) mode of several cuprate thin films using phase-resolved terahertz third harmonic generation (THG). In addition to the heavily damped Higgs mode itself, we observe a universal jump in the phase of the driven Higgs oscillation as well as a non-vanishing THG above T c. These findings indicate coupling of the Higgs mode to other collective modes and potentially a nonzero pairing amplitude above T c .
Barium bismuth oxide (BaBiO_{3}) is the end member of two families of high-T_{c} superconductors, i.e., BaPb_{1-x}Bi_{x}O_{3} and Ba_{1-x}K_{x}BiO_{3}. The undoped parent compound is an insulator, exhibiting a charge density wave that is strongly linked to a static breathing distortion in the oxygen sublattice of the perovskite structure. We report a comprehensive spectroscopic and x-ray diffraction study of BaBiO_{3} thin films, showing that the minimum film thickness required to stabilize the breathing distortion and charge density wave is ≈11 unit cells, and that both phenomena are suppressed in thinner films. Our results constitute the first experimental observation of charge density wave suppression in bismuthate compounds without intentionally introducing dopants.
The exchange bias effect is an essential component of magnetic memory and spintronic devices. Whereas recent research has shown that anisotropies perpendicular to the device plane provide superior stability against thermal noise, it has proven remarkably difficult to realize perpendicular exchange bias in thin-film structures. Here we demonstrate a strong perpendicular exchange bias effect in heterostructures of the quasi-two-dimensional canted antiferromagnet La2CuO4 and ferromagnetic (La,Sr)MnO3 synthesized by ozone-assisted molecular beam epitaxy. The magnitude of this effect can be controlled via the doping level of the cuprate layers. Canted antiferromagnetism of layered oxides is thus a new and potentially powerful source of uniaxial anisotropy in magnetic devices. * B.Keimer@fkf.mpg.de
Strain engineering
of complex oxide heterostructures has provided
routes to explore the influence of the local perturbations to the
physical properties of the material. Due to the challenge of disentangling
intrinsic and extrinsic effects at oxide interfaces, the combined
effects of epitaxial strain and charge transfer mechanisms have been
rarely studied. Here, we reveal the local charge distribution in manganite
slabs by means of high-resolution electron microscopy and spectroscopy
via investigating how the strain locally alters the electronic and
magnetic properties of La0.5Sr0.5MnO3–La2CuO4 heterostructures. The charge
rearrangement results in two different magnetic phases: an interfacial
ferromagnetically reduced layer and an enhanced ferromagnetic metallic
region away from the interfaces. Further, the magnitude of the charge
redistribution can be controlled via epitaxial strain, which further
influences the macroscopic physical properties in a way opposed to
strain effects reported on single-phase films. Our work highlights
the important role played by epitaxial strain in determining the spatial
distribution of microscopic charge and spin interactions in manganites
and provides a different perspective for engineering interface properties.
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