Heterostructures composed of transition metal oxides with strong electron correlation offer a unique opportunity to
design new artificial materials whose electrical, magnetic and optical properties can be manipulated by tailoring the
occupation of the 3d-orbitals of the transition metal in the compound. This possibility is an implication of symmetry
constraints at interfaces resulting in a delicate interplay of spin-, charge-, orbital and lattice interactions of electrons. In
turn, the material properties are sensitive to external perturbations such as strain, electrical and magnetic fields and
photon flux as well. In this contribution we use photon flux exposure to explore the consequences of superlattice
formation of YBa2Cu3O7–δ/La 2/3Ca1/3MnO3 on the entropy transport, especially on the Seebeck coefficient. In addition to the investigation of the fundamental aspects of entropy transport in oxide superlattices, the driving force for this work is the development of optical sensing devices. The method applied is based on the off-diagonal thermoelectric effect (ODTE) appearing in films deposited on substrates with a vicinal cut. This well-known principle serves as a technique to investigate the anisotropic transport properties and the components of the Seebeck tensor in these superlattices. It could be shown that the normalized ODTE signals scale linearly with the number of interfaces in the structures. We observed an enhancement of the ODTE signals by a factor of four due to superlattice formation. The results are discussed with respect to cross-plane coherent backscattering of phonon waves at the superlattice interfaces and the thermal boundary resistance at the YBa2Cu3O7–δ/La2/3Ca1/3MnO3 interfaces.
Electromigration in sub-micron conductors of Cu and CuAl was studied by 1/f noise measurements for the first time. 1/f noise can serve as a very sensitive indicator for electromigration damage: The 1/f noise level is increased by up to two orders of magnitude whereas the resistance of the damaged interconnects is enhanced by less than a factor of two only. The most striking advantage of the 1/f noise measurement technique compared to the methods frequently used at present for electromigration studies (e.g., the Median Time of Failure, MTF technique) is that it is possible to determine the distribution of the activation energies of the processes involved from a single sample at progressive electromigration damaging. In Cu interconnects a strong increase in the number of mobile defects is observed during electromigration damaging whereas the shape of the distribution of the activation energies (maximum between 0.8 and 0.95 eV) does not change much, except shortly before the failure of the interconnect lines where a shift to higher activation energies (maximum: 1.05 eV) is measured. Significantly higher activation energies observed in undamaged and electromigration damaged CuAl0.5wt% interconnects indicate an advanced resistance of CuAl alloys to electromigration when compared to pure Cu lines.
The physical properties of the perovskite-type oxide RuSr2GdCu2O8 have been recently discussed in the view of a simultaneous occurrence of superconductivity and ferromagnetism. In order to explore some peculiarities of these compounds we have prepared superlattices of oxides that are known to be either ferromagnetic [La.67Ca.33MnO3 ] or superconducting [YBa2Cu3O7]. Superlattices of different periodicity serve as model systems for the understanding of the features of the RuSr2GdCu2O8 system and are used to compare their properties with those of single phase epitaxially grown RuSr2GdCu2O8 thin films.
The YBCO/LCMO superlattices have been grown by pulsed laser deposition with individual layer thickness ranging from 4 to 200 unit cells for the YBa2Cu3O7 and 10 to 500 unit cells for the La.67Ca.33MnO3 . Measuring dc-transport and magnetic properties some novel effects have been found due to a coupling between the layers observed in the superlattices. Superlattices with individual thickness of the constituent materials of 4 nm e.g. show a reduced Curie temperature of 120K and a superconducting transition temperature of 52K. Lowering the temperature a reentrant normal state occurs at T = 25K. Switching off the electronic interlayer coupling by the introduction of insulating SrTiO3 spacer layers leads to the intrinsic critical temperatures. For the explanation of the results several novel concepts have to be developed based on a long range ferromagnetic interlayer coupling and a novel long range superconducting proximity effect.
YBa2Cu3O7-x (YBCO) thin films have been grown on vicinal cut SrTiO3 (001) single crystals. The films show a pronounced anisotropic resistivity and flux pinning properties in the substrate plane. UHV scanning tunneling microscopy and transmission electron microscopy have been used to investigate the substrate surface, film morphology and the growth induced defect structure. The anisotropy is caused by planar defects generated via self-organization of the YBCO which lead to an exceptionally large critical current density up to 8 X 1011 A/m2 at 4.2 K. The films are partially detwinned as a consequence of the modification of the growth mode by the terrace-type surface morphology due to the vicinal cut of the substrates.
Electron Beam Induced Current (EBIC) and Cathodoluminescence (CL) contrast of dislocations
in GaAs show a strong temperature dependence. This is not only due to a variation of the
recombination properties of the defects, which are a function of the defect energy level in the gap,
but also due to the variation of diffusion length with temperature and their absolute value, which
depends on doping type and doping concentration of the material. The experimental results are
interpreted in the framework of model calculations for defect contrast and indicate a decreasing
diffusion length and increasing recombination efficiency of dislocations with decreasing
temperature in the range of 20 to 300 K.
Conference Committee Involvement (7)
Oxide-based Materials and Devices VII
14 February 2016 | San Francisco, California, United States
Oxide-based Materials and Devices VI
8 February 2015 | San Francisco, California, United States
Oxide-based Materials and Devices V
2 February 2014 | San Francisco, California, United States
Oxide-based Materials and Devices IV
4 February 2013 | San Francisco, California, United States
Oxide-based Materials and Devices III
22 January 2012 | San Francisco, California, United States
Oxide-based Materials and Devices II
23 January 2011 | San Francisco, California, United States
Oxide-based Materials and Devices
24 January 2010 | San Francisco, California, United States