Multiple strain-induced phase transitions in LaNiO3 thin films

Weber, M.C. et al.Strain effects on epitaxial thin films of LaNiO3 grown on different single crystalline substrates are studied by Raman scattering and first-principles simulation. New Raman modes, not present in bulk or fully-relaxed films, appear under both compressive and tensile strains, indicating symmetry reductions. Interestingly, the Raman spectra and the underlying crystal symmetry for tensile and compressively strained films are different. Extensive mapping of LaNiO3 phase stability is addressed by simulations, showing that a variety of crystalline phases are indeed stabilized under strain which may impact the electronic orbital hierarchy. The calculated Raman frequencies reproduce the principal features of the experimental spectra, supporting the validity of the multiple strain-driven structural transitions predicted by the simulations.J.K., M.W., M.G., and J.I. acknowledge support from the National Research Fund, Luxembourg through a Pearl grant (Grant No. FNR/P12/4853155). ICMAB-CSIC authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015- 0496) and the MAT2014-56063-C2-1-R and MAT2013- 40581-P projects, and from Generalitat de Catalunya (2014 SGR 734). UB authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, project MAT2013-41506-P and from Generalitat de Catalunya (2014 SGR 672).Peer reviewe

Untangling Electrostatic and Strain Effects on the Polarization of Ferroelectric Superlattices

Khestanova, Ekaterina et al.The polarization of ferroelectric superlattices is determined by both electrical boundary conditions at the ferroelectric/paraelectric interfaces and lattice strain. The combined infl uence of both factors offers new opportunities to tune ferroelectricity. However, the experimental investigation of their individual impact has been elusive because of their complex interplay. Here, a simple growth strategy has permitted to disentangle both contributions by an independent control of strain in symmetric superlattices. It is found that fully strained short-period superlattices display a large polarization whereas a pronounced reduction is observed for longer multilayer periods. This observation indicates that the electrostatic boundary mainly governs the ferroelectric properties of the multilayers whereas the effects of strain are relatively minor.Financial support by the Spanish Government [Projects MAT2014- 56063-C2-1-R and MAT2013-41506 ] and Generalitat de Catalunya ( 2014-SGR-734 and 2014-SGR-672 ) is acknowledged. ICMAB-CSIC authors acknowledge fi nancial support from the Spanish Ministry of Economy and Competitiveness , through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV- 2015-0496 ). I.F. acknowledges Juan de la Cierva – Incorporación postdoctoral fellowship (IJCI-2014- 19102) from the Spanish Ministry of Economy and Competitiveness. The transmission electron microscopy works were conducted in the Laboratorio de Microscopias Avanzadas at Instituto de Nanociencia de Aragón (Universidad de Zaragoza). The authors acknowledge the LMA-INA for offering access to their instruments and expertise. The authors thank Massimiliano Stengel for useful discussions.Peer reviewe

Spin Hall magnetoresistance as a probe for surface magnetization in Pt/CoFe2_2O4_4 bilayers

We study the spin Hall magnetoresistance (SMR) in Pt grown $\textit{in situ}$ on CoFe$_2$O$_4$ (CFO) ferrimagnetic insulating (FMI) films. A careful analysis of the angle-dependent and field-dependent longitudinal magnetoresistance indicates that the SMR contains a contribution that does not follow the bulk magnetization of CFO but it is a fingerprint of the complex magnetism at the surface of the CFO layer, thus signaling SMR as a tool for mapping surface magnetization. A systematic study of the SMR for different temperatures and CFO thicknesses gives us information impossible to obtain with any standard magnetometry technique. On one hand, surface magnetization behaves independently of the CFO thickness and does not saturate up to high fields, evidencing that the surface has its own anisotropy. On the other hand, characteristic zero-field magnetization steps are not present at the surface while they are relevant in the bulk, strongly suggesting that antiphase boundaries are the responsible of such intriguing features. In addition, a contribution from ordinary magnetoresistance of Pt is identified, which is only distinguishable due to the low resistivity of the $\textit{in-situ}$ grown Pt.Comment: 19 pages, 8 figures, Supplemental Materia

Untangling electrostatic and strain effects on the polarization of ferroelectric superlattices

The polarization of ferroelectric superlattices is determined by both electrical boundary conditions at the ferroelectric/paraelectric interfaces and lattice strain. The combined influence of both factors offers new opportunities to tune ferroelectricity. However, the experimental investigation of their individual impact has been elusive because of their complex interplay. Here, a simple growth strategy has permitted to disentangle both contributions by an independent control of strain in symmetric superlattices. It is found that fully strained short‐period superlattices display a large polarization whereas a pronounced reduction is observed for longer multilayer periods. This observation indicates that the electrostatic boundary mainly governs the ferroelectric properties of the multilayers whereas the effects of strain are relatively minor

Kerr measurements on single-domain SrRuO3 thin films

We report on the magneto-optical measurements of an epitaxial SrRuO3 film grown on SrTiO3 (0 0 1), which previously was determined to be single domain orientated by x-ray diffraction and Raman spectroscopy techniques. Our experiments reveal a large Kerr rotation, which reaches a maximum value of about 0.5° at low temperature. By measuring magnetic hysteresis loops at different temperatures, we determined the temperature dependence of the Kerr rotation in the polar configuration. Values of the anisotropic magnetoresistance ~ 20% have been measured. These values are remarkably higher than those of other metallic oxides such as manganites. This striking difference can be attributed to the strong spin-orbit interaction of the Ru 4d ion in the SrRuO3 compound

Engineering two-dimensional superconductivity and Rashba spin-orbit coupling in LaAlO/SrTiO quantum wells by selective orbital occupancy

The discovery of two-dimensional electron gases (2DEGs) at oxide interfaces-involving electrons in narrow d -bands-has broken new ground, enabling the access to correlated states that are unreachable in conventional semiconductors based on s - and p - electrons. There is a growing consensus that emerging properties at these novel quantum wells-such as 2D superconductivity and magnetism-are intimately connected to specific orbital symmetries in the 2DEG sub-band structure. Here we show that crystal orientation allows selective orbital occupancy, disclosing unprecedented ways to tailor the 2DEG properties. By carrying out electrostatic gating experiments in LaAlO₃/SrTiO₃ wells of different crystal orientations, we show that the spatial extension and anisotropy of the 2D superconductivity and the Rashba spin-orbit field can be largely modulated by controlling the 2DEG sub-band filling. Such an orientational tuning expands the possibilities for electronic engineering of 2DEGs at LaAlO₃/SrTiO₃ interfaces. Two-dimensional electron gases at oxide interfaces induce exotic behaviours. By studying samples with different crystal orientation, Herranz et al. show that the extension and anisotropy of the oxide quantum well properties can be controlled through selective sub-band filling via orientational tuning

Global data set of long-term summertime vertical temperature profiles in 153 lakes

Climate change and other anthropogenic stressors have led to long-term changes in the thermal structure, including surface temperatures, deepwater temperatures, and vertical thermal gradients, in many lakes around the world. Though many studies highlight warming of surface water temperatures in lakes worldwide, less is known about long-term trends in full vertical thermal structure and deepwater temperatures, which have been changing less consistently in both direction and magnitude. Here, we present a globally-expansive data set of summertime in-situ vertical temperature profiles from 153 lakes, with one time series beginning as early as 1894. We also compiled lake geographic, morphometric, and water quality variables that can influence vertical thermal structure through a variety of potential mechanisms in these lakes. These long-term time series of vertical temperature profiles and corresponding lake characteristics serve as valuable data to help understand changes and drivers of lake thermal structure in a time of rapid global and ecological change