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Contemporary quantum plasmonics capture subtle corrections to the properties of plasmonic nano-objects in equilibrium. Here, we demonstrate nonequilibrium spill-out redistribution of the electronic density at the ultrafast time scale. As revealed by time-resolved 2D spectroscopy of nanoplasmonic Fe/Au bilayers, an injection of the laser-excited non-thermal electrons induces transient electron spill-out thus changing the plasma frequency. The response of the local electronic density switches the electronic density behavior from spill-in to strong (an order of magnitude larger) spill-out at the femtosecond time scale. The superdiffusive transport of hot electrons and the lack of a direct laser heating indicate significantly non-thermal origin of the underlying physics. Our results demonstrate an ultrafast and non-thermal way to control surface plasmon dispersion through transient variations of the spatial electron distribution at the nanoscale. These findings expand quantum plasmonics into previously unexplored directions by introducing ultrashort time scales in the non-equilibrium electronic systems., Comment: 3 figures

Heterostructures increasingly attracted attention over the past several years to enable various optoelectronic and photonic applications. In this work, atomically thin interfaces of Ir/Al2O3 heterostructures compatible with micro-optoelectronic technologies are reported. Their structural and optical properties were determined by spectroscopic and microscopic techniques (XRR, XPS, HRTEM, spectroscopic ellipsometry, and UV/VIS/NIR spectrophotometry). The XRR and HRTEM analyses reveal a layer-by-layer growth mechanism of Ir in atomic scale heterostructures, which is different from the typical island-type growth of metals on dielectrics. Alongside, XPS investigations imply the formation of Ir-O-Al bonding at the interfaces for lower Ir concentrations, in contrast to the nanoparticle core-shell structure formation. Precisely tuning the ratio of the constituents ensures the control of the dispersion profile along with a transition from effective dielectric to metallic heterostructures. The Ir coating thickness was varied ranging from a few {\AA} to films of about 7 nm in the heterostructures. The transition has been observed in the structures containing individual Ir coating thicknesses of about 2-4 nm. Following this, show epsilon-near-zero metamaterials with tunable dielectric constants by precisely varying the composition of such heterostructures. Overall, a comprehensive study on structural and optical properties of the metal-dielectric interface of Ir/Al2O3 heterostructures was addressed indicating an extension of the material portfolio available for optical system design.

A metal oxide support and a catalytically active metal are the two main ingredients for complex catalysts used in heterogeneous catalysis. The gas environment can change the catalyst during the reaction, modifying its structural and electronic properties. Here, we use monochromated electron energy loss spectroscopy (EELS) to reveal hydrogen-pressure-dependent changes of the electronic structure at the Pd/rutile-TiO$_2$ interface in an environmental transmission electron microscope (ETEM). Hydrogen-induced changes are observed in rutile-TiO$_2$ within $2$~nm from the interface at $10$~Pa of hydrogen pressure, in the Ti $L_{3,2}$ EEL spectra. Lower pressures such as $1$~Pa show no changes in the EEL spectra. We attribute the observed changes in the EEL spectra to hydrogen-induced defects accumulating in the vicinity of the interface. Based on DFT calculations, we developed a thermodynamic multistate defect (TMD) model of the interface and the bulk of the rutile-TiO$_2$. This TMD model predicts high concentrations of positively charged defects accumulating at the interface. The presence of the Schottky barrier stabilizes these defects by significantly lowering their formation energy. Our findings provide important new insights into catalytic processes taking place at metal/metal oxide interfaces in hydrogen gas environments., Comment: 18 pages, 12 figures

Perovskite SrRuO$_3$ is a prototypical itinerant ferromagnet which allows interface engineering of its electronic and magnetic properties. We report synthesis and investigation of atomically flat artificial multilayers of SrRuO$_3$ with the spin-orbit semimetal SrIrO$_3$ in combination with band-structure calculations with a Hubbard $U$ term and topological analysis. They reveal an electronic reconstruction and emergence of flat Ru-4d$_{xz}$ bands near the interface, ferromagnetic interlayer coupling and negative Berry-curvature contribution to the anomalous Hall effect. We analyze the Hall effect and magnetoresistance measurements as a function of the field angle from out of plane towards in-plane orientation (either parallel or perpendicular to the current direction) by a two-channel model. The magnetic easy direction is tilted by about $20^\circ$ from the sample normal for low magnetic fields, rotating towards the out-of-plane direction by increasing fields. Fully strained epitaxial growth enables a strong anisotropy of magnetoresistance. An additional Hall effect contribution, not accounted for by the two-channel model is compatible with stable skyrmions only up to a critical angle of roughly $45^\circ$ from the sample normal. Within about $20^\circ$ from the thin film plane an additional peak-like contribution to the Hall effect suggests the formation of a non-trivial spin structure., Comment: to be published in Phys. Rev. B

Interest for layered Ruddlesden-Popper strongly correlated manganites of Pr$_{0.5}$Ca$_{1.5}$MnO$_4$ as well as to their thin film polymorphs is motivated by the high temperature of charge orbital ordering above room temperature. We report on the tailoring of the c-axis orientation in epitaxial RP-PCMO films grown on SrTiO$_3$ (STO) substrates with different orientations as well as the use of CaMnO$_3$ (CMO) buffer layers. Films on STO(110) reveal in-plane alignment of the c-axis lying along to the [100] direction. On STO(100), two possible directions of the in-plane c-axis lead to a mosaic like, quasi two-dimensional nanostructure, consisting of RP, rock-salt and perovskite building blocks. With the use of a CMO buffer layer, RP-PCMO epitaxial films with c-axis out-of-plane were realized. Different physical vapor deposition techniques, i.e. ion beam sputtering (IBS), pulsed laser deposition (PLD) as well as metalorganic aerosol deposition (MAD) are applied in order to distinguish between the effect of growth conditions and intrinsic epitaxial properties. For all deposition techniques, despite their very different growth conditions, the surface morphology, crystal structure and orientation of the thin films reveal a high level of similarity as verified by X-ray diffraction, scanning and high resolution transmission electron microscopy. We found that for different epitaxial relations the stress in the films can be relaxed by means of a modified interface chemistry. The charge ordering in the films estimated by resistivity measurements occurs at a temperature close to that expected in bulk material.

The rich phase diagram of bulk Pr$_{1-x}$Ca$_{x}$MnO$_3$ resulting in a high tunability of physical properties gave rise to various studies related to fundamental research as well as prospective applications of the material. Importantly, as a consequence of strong correlation effects, electronic and lattice degrees of freedom are vigorously coupled. Hence, it is debatable whether such bulk phase diagrams can be transferred to inherently strained epitaxial thin films. In this paper, the structural orthorhombic to pseudo-cubic transition for $x=0.1$ is studied in ion-beam sputtered thin films and point out differences to the respective bulk system by employing in-situ heating nano-beam electron diffraction to follow the temperature dependence of lattice constants. In addition, it is demonstrated that controlling the environment during heating, i.e. preventing oxygen loss, is crucial in order to avoid irreversible structural changes, which is expected to be a general problem of compounds containing volatile elements under non-equilibrium conditions.

Spinel lithium manganese oxide (LixMn2O4) is used as an active material in battery cathodes. It is a relatively inexpensive and environmentally friendly material, but suffers from capacity fade during use. The capacity losses are generally attributed to the formation of the tetragonal phase (x > 1) due to overpotentials at the surfaces of the micron-sized particles that are used in commercial electrodes. In this study, we investigate the mechanisms of tetragonal phase formation by performing electrochemical lithiation (discharging) in-situ in the transmission electron microscope (TEM) utilizing diffraction and high resolution as well as spectroscopy. We observe a sharp interface between the cubic spinel (x = 1) and the tetragonal phase (x = 2), that moves under lithium diffusion-control. The tetragonal phase forms as a complex nanotwinned microstructure, presumably to relieve the stresses due to expansion during lithiation. We propose that the twinned microstructure stabilizes the tetragonal phase, adding to capacity loss upon deep discharge., Comment: 31 pages, 7 figures plus additional SI, submitted to ACS Nano

Multilayered heterostructures of Ce0.85Sm0.15O2-delta and Y0.16Zr0.92O2-delta of a high crystallographic quality were fabricated on (001) - oriented MgO single crystal substrates. Keeping the total thickness of the heterostructures constant, the number of ceria-zirconia bilayers was increased while reducing the thickness of each layer. At each interface Ce was found primarily in the reduced, 3+ oxidation state in a layer extending about 2 nm from the interface. Concurrently, the conductivity decreased as the thickness of the layers was reduced suggesting a progressive confinement of the charge transport along the YSZ layers. The comparative analysis of the in-plane electrical characterization suggests that the contribution to the total electrical conductivity of these interfacial regions is negligible. For the smallest layer thickness of 2 nm the doped ceria layers are electrically insulating and the ionic transport only occurs through the zirconia layers. This is explained in terms of a reduced mobility of the oxygen vacancies in the highly reduced ceria.

We report on the use of time-resolved optical ellipsometry to monitor the deposition of single atomic layers with subatomic sensitivity. Ruddlesden-Popper thin films of SrO(SrTiO3)n=4 were grown by means of metalorganic aerosol deposition in the atomic layer epitaxy mode on SrTiO3(100), LSAT(100) and DyScO3(110) substrates. The measured time dependences of ellipsometric angles, ${\Delta}(t)$ and ${\Psi}(t)$, were described by using a simple optical model, considering the sequence of atomic layers SrO and TiO2 with corresponding bulk refractive indices. As a result, valuable online information on the growth process, the film structure and defects were obtained. Ex situ characterization techniques, i.e. transmission electron microscopy (TEM), X-ray diffraction (XRD) and X- ray reflectometry (XRR) verify the crystal structure and confirm the predictions of optical ellipsometry., Comment: 19 pages, 5 figures

The electronic structure of Pr$_{1-x}$Ca$_x$MnO$_3$ has been investigated using a combination of first-principles calculations, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), electron-energy loss spectroscopy (EELS), and optical absorption. The full range of compositions, $x=0,1/2,1$, and a variety of magnetic orders have been covered. Jahn-Teller as well as Zener polaron orders are considered. The free parameters of the local hybrid density functionals used in this study has been determined by comparison with measured XPS spectra. A model Hamiltonian, valid for the entire doping range, has been extracted. A simple local-orbital picture of the electronic structure for the interpretation of experimental spectra is provided. The comparison of theoretical calculations and different experimental sprectra provide a detailed and consistent picture of the electronic structure. The large variations of measured optical absorption spectra are traced back to the coexistence of magnetic orders respectively to the occupation of local orbitals. A consistent treatment of the Coulomb interaction indicate a partial cancellation of Coulomb parameters and support the dominance of the electron-phonon coupling., Comment: 25 pages, 15 figures

Using the sensitivity of magneto-induced second harmonic generation to spin currents (SC), we demonstrate in Fe/Au/Fe/MgO(001) pseudo spin valves the generation of 250 fs-long SC pulses. Their temporal profile indicates that superdiffusive hot electron transport across a sub-100~nm Au layer is close to the ballistic limit and the pulse duration is primarily determined by the thermalization time of laser-excited hot carriers in Fe. Considering the calculated spin-dependent Fe/Au interface transmittance we conclude that a non-thermal spin-dependent Seebeck effect is responsible for the generation of ultrashort SC pulses. We also show that hot electron spins rotate upon interaction with non-collinear magnetization at the Au/Fe interface, which holds high potential for future spintronic devices., Comment: 5 pages, 4 figures

Spintronics had a widespread impact over the past decades due to transferring information by spin rather than electric currents. Its further development requires miniaturization and reduction of characteristic timescales of spin dynamics combining the sub-nanometer spatial and femtosecond temporal ranges. These demands shift the focus of interest towards the fundamental open question of the interaction of femtosecond spin current (SC) pulses with a ferromagnet (FM). The spatio-temporal properties of the impulsive spin transfer torque exerted by ultrashort SC pulses on the FM open the time domain for probing non-uniform magnetization dynamics. Here we employ laser-generated ultrashort SC pulses for driving ultrafast spin dynamics in FM and analyzing its transient local source. Transverse spins injected into FM excite inhomogeneous high-frequency spin dynamics up to 0.6 THz, indicating that the perturbation of the FM magnetization is confined to 2 nm., Comment: 6 pages, 4 figures. Supplementary Information: 5 pages, 4 figures

A time domain approach to probe hot carrier-induced spin dynamics is demonstrated. The experiments are performed in epitaxial Au/Fe/MgO(001), where spin-polarized hot carriers are excited in the Fe layer by 35 fs laser pulses. They propagate to the Au surface where the transient spin polarization is detected by magneto-induced second harmonic generation. Different energies of majority and minority hot carriers excited in the exchange-split Fe band structure lead to their spindependent lifetimes in Au. Accordingly, two spin-polarized current contributions which propagate superdiffusively at different velocities result in a spin current pulse of about 100 fs duration., Comment: 5 pages, 4 figures