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We study the structural, magnetic, and transport properties of LaAlO3/EuTiO3/SrTiO3 heterostructures grown by pulsed laser deposition. The samples are characterized in situ by electron diffraction and scanning probe microscopy and ex situ by transport measurements and x-ray absorption spectroscopy. LaAlO3/EuTiO3/SrTiO3 films show a ferromagnetic transition at T ⩽ 7.5 K, related to the ordering of Eu2+ spins, even in samples characterized by just two EuTiO3 unit cells. A finite metallic conductivity is observed only in the case of samples composed by one or two EuTiO3 unit cells and, simultaneously, by a LaAlO3 thickness greater than or equal to four unit cells. The effect of ferromagnetic EuTiO3 on the transport properties of δ-doped LaAlO3/EuTiO3/SrTiO3 is critically discussed. [ABSTRACT FROM AUTHOR]

Optical second harmonic generation (SHG) spectra of LaA1O3/SrTiO3, LaGaO3/SrTiO3,andNdGaO3/SrTiO3 interfaces with SrTiO3 substrates have been measured up to 4.2 eV as a function of film thickness and temperature. This spectral range is characterized by two-photon transitions from the valence to conduction bands of SrTiO3 which are classified with a model based on symmetry, selection rules, and atomic orbital overlaps. This model is further confirmed in spectral measurements as a function of film thickness, material overlayer, and temperature. SHG enhancement at low temperature indicates an increase of the interfacial polarity with decreasing temperature. This confirms the relation between SHG and the spatial translation of Ti ions which are more prone to move at lower temperatures because of the quantum-paraelectric nature of SrTiO3. We finally show that SHG spectroscopy captures structural details of the interfaces. In particular, we find evidence for proximity effects such as LaA1O3-induced distortions of the TiO6 octahedra or bond buckling. We also observe a correlation between SHG and the in-plane lattice mismatch between the SrTiO3 substrate and the LaA1O3, LaGaO3, and NdGaO3 overlayers. [ABSTRACT FROM AUTHOR]

We present femtosecond pump-probe spectroscopy studies of time-resolved optical reflectivity of all- oxide YBa2Cu3O7/La07Sr0.3MnO3 (YBCO/LSMO) superconductor/ferromagnet (S/F) bilayers consisting of a 100-nm-thick YBCO base layer and either 10 or 35 nm LSMO cap thickness. At temperatures far below the YBCO superconducting transition Tc, samples with a 10 nm F overlayer show a photoresponse that is similar to, but faster than, pure-YBCO, 100-nm-thick reference samples, while close to Tc and above (up to ∼ 160 K) we observe a signature of both the electronic and spin response that cannot be interpreted as an incoherent sum of contributions from the two layers. The photoresponse of the S/F structures with the 35-nm LSMO layer always qualitatively follows that of the pure LSMO, but with a shorter relaxation time. In all cases, the YBCO/LSMO nonequilibrium dynamics can be modeled using a generalized multitemperature model, which is a superposition of the dynamics of the three-temperature models that are used to describe the superconductor and ferromagnet subsystems, respectively. The long term of the photoresponse signal of the S/F bilayer can be well fitted with the two characteristic relaxation times. [ABSTRACT FROM AUTHOR]

Phase-sensitive, spatially resolved optical second-harmonic-generation experiments were performed on LaAlO3/SrTiO3 heterostructures. Lateral inhomogeneities on a length scale of ≈ 30 μm are found when a one-unit-cell-thick epitaxial monolayer of LaAIO3 is grown on TiO2-terminated SrTiO3 single crystals. The inhomogeneity is absent in samples with LaAlO33 layers of more than one unit cell. The results are discussed in the framework of electronic, oxidic, and chemical inhomogeneities. [ABSTRACT FROM AUTHOR]

The polarization induced at the LaAlO3/SrTiO3 interface has been explored by optical second harmonic generation (SHG) spectroscopy. Our data reveal that an orbital reconstruction of the SrTiO3 surface occurs when three epitaxial monolayers of LaAlO3 are deposited on top of it. The reconstruction manifests as a spectral modification of a Ti4+(3d) state at the LaAlO3/SrTiO3 interface which is accompanied by a displacement of the Ti4+ ions in adjacent SrTiO3 monolayers in order to compensate for the polarization discontinuity induced at the interface. According to the SHG experiment, the carriers transferred to the interface are trapped because in spite of the orbital reconstruction, the interface remains insulating. The onset of conductivity with the formation of a two-dimensional electron liquid appears to be a subordinate effect related to a minority of carriers and occurs upon further increase of the LaAlO3 coverage. [ABSTRACT FROM AUTHOR]