D. H. A. Blank

Polar domains in lead titanate films under tensile strain

Thin films of PbTiO3, a classical ferroelectric, have been grown under tensile strain on single-crystal substrates of DyScO3. The films, of only 5 nm thickness, grow fully coherent with the substrate, as evidenced by synchrotron x-ray diffraction. A mapping of the reciprocal space reveals intensity modulations (satellites) due to regularly spaced polar domains in which the polarization appears rotated away from the substrate normal, characterizing a low-symmetry phase not observed in the bulk material. This could have important practical implications since these phases are known to be responsible for ultrahigh piezoelectric responses in complex systems.

Multiple conducting carriers generated in LaAlO3/SrTiO3 heterostructures

We have found that there is more than one type of conducting carriers generated in LaAlO3/SrTiO3 heterostructures by comparing the sheet carrier density and mobility from optical transmission spectroscopy with those from dc-transport measurements. When multiple types of carriers exist, optical characterization dominantly reflects the contribution from the high-density carriers whereas dc-transport measurements may exaggerate the contribution of the high-mobility carriers even though they are present at low density. Since the low-temperature mobility determined by dc-transport in the LaAlO3/SrTiO3 heterostructures is much higher than that extracted by optical method, we attribute the origin of high-mobility transport to the low-density conducting carriers.

Admixtures to d-Wave Gap Symmetry in Untwinned YBa2Cu3O7 Superconducting Films Measured by Angle-Resolved Electron Tunneling

We report on an ab anisotropy of Jc parallel b/Jc parallel a approximately/= 1.8 IcRn parallelb/IcRn parallel a approximately/= 1.2 and in ramp-edge junctions between untwinned YBa2Cu3O7 and s-wave Nb. For these junctions, the angle theta with the YBa2Cu3O7 crystal b axis is varied as a single parameter. The RnA(theta) dependence presents twofold symmetry. The minima in IcRn at theta approximately/= 50 degrees suggest a real s-wave subdominant component and negligible d(xy)-wave or imaginary s-wave admixtures. The IcRn(theta) dependence is well fitted by 83% dx2-y2-, 15% isotropic s-, and 2% anisotropic s-wave order parameter symmetry, consistent with deltab/deltaa approximately/= 1.5.

Smallest 90° domains in epitaxial ferroelectric films

Periodic ferroelectric-ferroelastic 90° domain patterns with an unprecedented small domain periodicity of 27 nm were observed in thin PbTiO3 films grown on DyScO3 substrates. These patterns contain the narrowest possible a domains 6 nm wide that allow to preserve the lateral coherence in the films, producing highly ordered patterns visible by x-ray diffraction.

Spectroscopic evidence of in-gap states at the SrTiO3/LaAlO3 ultrathin interfaces

Experimental evidence of differences in the electronic properties of an insulating and a conducting SrTiO3/LaAlO3 interface is provided by soft x-ray spectroscopies. While core level photoemission measurements show that only at the conducting interface Ti ions with 3+ ionization state are present, by using resonant photoemission and x-ray absorption spectroscopies, it is shown that in both samples in-gap states with a Ti 3d character are present, but their density is higher at the conducting interface.

Phase-Sensitive order parameter symmetry test experiments utilizing Nd2-xCexCuO4-y/Nb zigzag junctions

Phase-sensitive order parameter symmetry test experiments are presented on the electron-doped high-T(c) cuprate Nd(2-x)Ce(x)CuO(4-y). These experiments have been conducted using zigzag-shaped thin film Josephson structures, in which the Nd(2-x)Ce(x)CuO(4-y) is connected to the low-T(c) superconductor Nb via an Au barrier layer. For the optimally doped as well as for the overdoped Nd(2-x)Ce(x)CuO(4-y), a clear predominant d(x2-y2)-wave behavior is observed at T=4.2 K. Both compounds were also investigated at T=1.6 K, presenting no indications for a change to a predominant s-wave symmetry with decreasing temperature.

Two-dimensional confinement of 3d{1} electrons in LaTiO_{3}/LaAlO{3} multilayers.

We report spectroscopic ellipsometry measurements of the anisotropy of the interband transitions parallel and perpendicular to the planes of (LaTiO3)n(LaAlO3)5 multilayers with n=1-3. These provide direct information about the electronic structure of the two-dimensional (2D) 3d{1} state of the Ti ions. In combination with local density approximation, including a Hubbard U calculation, we suggest that 2D confinement in the TiO2 slabs lifts the degeneracy of the t{2g} states leaving only the planar d{xy} orbitals occupied. We outline that these multilayers can serve as a model system for the study of the t{2g} 2D Hubbard model.

Two-Dimensional Confinement of 3d1 Electrons in LaTiO3/LaAlO3 Multilayers

We report spectroscopic ellipsometry measurements of the anisotropy of the interband transitions parallel and perpendicular to the planes of (LaTiO3)n(LaAlO3)5 multilayers with n=1-3. These provide direct information about the electronic structure of the two-dimensional (2D) 3d{1} state of the Ti ions. In combination with local density approximation, including a Hubbard U calculation, we suggest that 2D confinement in the TiO2 slabs lifts the degeneracy of the t{2g} states leaving only the planar d{xy} orbitals occupied. We outline that these multilayers can serve as a model system for the study of the t{2g} 2D Hubbard model.

Two-Dimensional Confinement of3d1Electrons inLaTiO3/LaAlO3Multilayers

We report spectroscopic ellipsometry measurements of the anisotropy of the interband transitions parallel and perpendicular to the planes of (LaTiO3)n(LaAlO3)5 multilayers with n=1-3. These provide direct information about the electronic structure of the two-dimensional (2D) 3d{1} state of the Ti ions. In combination with local density approximation, including a Hubbard U calculation, we suggest that 2D confinement in the TiO2 slabs lifts the degeneracy of the t{2g} states leaving only the planar d{xy} orbitals occupied. We outline that these multilayers can serve as a model system for the study of the t{2g} 2D Hubbard model.

Periodic nanodomains in PbTiO3 films under tensile strain

In epitaxial ferroelectric thin films under tensile strain, a transition from 180° domains to 90° domains can be achieved by increasing the film thickness. In many cases, the 90° a/c domains appear in the form of wide c-domains separated by narrow a-stripes. We have deposited thin films of PbTiO3 on DyScO3 substrates and show that the condition of horizontal coherence forces the a-domains to adopt discrete sizes, the smallest of which is substrate-independent and depends only on the tetragonality of the ferroelectric film. Growing PbTiO3 on DyScO3, we have achieved the narrowest (6 nm) possible a-domains, as well as the shortest periodicity that allows to maintain the lateral coherence in the film.