A number of recent transport and magnetization studies have shown signs of ferromagnetism in the LaAlO 3 /SrTiO 3 heterostructure 1-6 , an unexpected property with no bulk analog in the constituent materials. However, no experiment thus far has provided direct information on the host of the magnetism 7-11 . Here we report spectroscopic investigations of the magnetism using element-specific techniques, including x-ray magnetic circular dichroism and x-ray absorption spectroscopy, along with corresponding model calculations. We find direct evidence for in-plane ferromagnetic order at the interface, with Ti 3+ character in the d xy orbital of the anisotropic t 2g band. These findings establish a striking example of emergent phenomena at oxide interfaces.Recent advances in the atomic-scale synthesis and characterization of perovskite oxide heterostructures have engendered significant interest in their electronic and magnetic structure. SLAC-PUB-15439Division of Materials Sciences and Engineering, under contract DE-AC02-76SF00515 and BES.Given their vast physical properties in bulk form, and their epitaxial compatibility, perovskites provide an ideal arena to explore the competition, interaction, and creation of many ground states at their interfaces 12 . The LaAlO 3 /SrTiO 3 heterostructure is a canonical example, exhibiting interface conductivity 13 , superconductivity 14 , and ferromagnetism 1-6 at the interface between two wide band-gap insulators. From a fundamental perspective, ferromagnetism is perhaps the most important property; although bulk SrTiO 3 can be doped to be metallic and superconducting, neither constituent in bulk form exhibits ferromagnetism. Hence interface ferromagnetism here could be a leading example of truly emergent phenomena. Most previous studies used bulk probes (macroscopic magnetization or torque) 3, 4 ; while scanning SQUID microscopy could localize the magnetism to the near surface region 5, 15 , the specific location where the moments reside is beyond the resolution of the probe. In principle, magnetism could arise from cation/anion defects in the LaAlO 3 or SrTiO 3 , or could be specific to the interface; theoretical scenarios have been proposed for all of these mechanisms 7-11 . Thus it is of central importance to determine the microscopic nature of the observed ferromagnetism.To address this issue, we applied element-specific techniques at the LaAlO 3 /SrTiO 3 (001) interface, namely synchrotron radiation based x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) (see Materials and Methods section). These measurements can uniquely determine whether the observed magnetization is due to a magnetic moment ( ) from one of the constituent elements, or from extrinsic impurities. All spectra were acquired by recording the total electron yield (TEY). Since the maximum probing depth of TEY is approximately 5~10 nm, these measurements are very sensitive to the interface with proper choice of LaAlO 3 thickness. Using the angle dependence of the XMCD signal, whic...

The interactions that lead to the emergence of superconductivity in iron-based materials remain a subject of debate. It has been suggested that electron-electron correlations enhance electron-phonon coupling in iron selenide (FeSe) and related pnictides, but direct experimental verification has been lacking. Here we show that the electron-phonon coupling strength in FeSe can be quantified by combining two time-domain experiments into a "coherent lock-in" measurement in the terahertz regime. X-ray diffraction tracks the light-induced femtosecond coherent lattice motion at a single phonon frequency, and photoemission monitors the subsequent coherent changes in the electronic band structure. Comparison with theory reveals a strong enhancement of the coupling strength in FeSe owing to correlation effects. Given that the electron-phonon coupling affects superconductivity exponentially, this enhancement highlights the importance of the cooperative interplay between electron-electron and electron-phonon interactions.

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