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Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr1-xCaxCoO3). Here, we show that in thin films of the complex perovskite (Pr1-yYy)1-xCaxCoO3-{\delta}, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. The technological implications of this result are accompanied by fundamental prospects, as complete strain control of the electronic ground state is demonstrated, from ferromagnetic metal under tension to nonmagnetic insulator under compression, thereby exposing a potential novel quantum critical point., Comment: 35 pages, 7 figures

The Josephson effect results from the coupling of two superconductors across a spacer such as an insulator, a normal metal or a ferromagnet to yield a phase coherent quantum state. However, in junctions with ferromagnetic spacers, very long-range Josephson effects have remained elusive. Here we demonstrate extremely long-range (micrometric) high-temperature (tens of kelvins) Josephson coupling across the half-metallic manganite La0.7Sr0.3MnO3 combined with the superconducting cuprate YBa2Cu3O7. These planar junctions, in addition to large critical currents, display the hallmarks of Josephson physics, such as critical current oscillations driven by magnetic flux quantization and quantum phase locking effects under microwave excitation (Shapiro steps). The latter display an anomalous doubling of the Josephson frequency predicted by several theories. In addition to its fundamental interest, the marriage between high-temperature, dissipationless quantum coherent transport and full spin polarization brings opportunities for the practical realization of superconducting spintronics, and opens new perspectives for quantum computing. Josephson coupling over micrometres and at tens of kelvins is demonstrated across the half-metallic manganite La0.7Sr0.3MnO3 combined with the superconducting cuprate YBa2Cu3O7.

Semiconductor materials with independently controlled electrical and thermal properties have a unique promise for energy-related applications from thermoelectrics and thermophotovoltaics. Here, using nonthermal plasma synthesized, direct-contact zinc oxide (ZnO) nanocrystal (NC) networks infilled with amorphous $\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$, and amorphous ZnO-$\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ mixture, it is shown that such independent control of electrical and thermal properties is achievable. In this study, in addition to our early reports on control of the electrical properties in these two-phase nanocomposites by tailoring the contact radius between NCs, we demonstrate that the infill composition has a significant impact on the overall thermal conductivity of the NC network and can be used for thermal control. It is also shown that in these heterogeneous systems, the phonons are the dominant heat carriers, and the NC-NC contact radius has a negligible effect on thermal transport. The work suggests that this paradigm of independently controlling the electrical and thermal properties of NC-based materials through tuning the NC-NC contact radius and infill composition can be exploited even further by varying NC and infill materials with potential applications ranging from solar cells and light emitting diodes to solid-state energy converters.

Development of novel magnetic materials is of interest for fundamental studies and applications such as spintronics, permanent magnetics, and sensors. We report on the first experimental realization of single element ferromagnetism, since Fe, Co, and Ni, in metastable tetragonal Ru, which has been predicted. Body-centered tetragonal Ru phase is realized by use of strain via seed layer engineering. X-ray diffraction and electron microscopy confirm the epitaxial mechanism to obtain tetragonal phase Ru. We observed a saturation magnetization of 148 and 160 emu cm−3 at room temperature and 10 K, respectively. Control samples ensure the ferromagnetism we report on is from tetragonal Ru and not from magnetic contamination. The effect of thickness on the magnetic properties is also studied, and it is observed that increasing thickness results in strain relaxation, and thus diluting the magnetization. Anomalous Hall measurements are used to confirm its ferromagnetic behavior., Until now, there have been three choices for a room temperature (RT) single element ferromagnetic material in fundamental studies and applications. Here the authors achieved body-centered tetragonal phase ruthenium thin films by epitaxial growth, which is the 4th RT ferromagnetic single element material.

Ultrathin YBa2Cu3O7−x (YBCO) films were grown on SrTiO3 (STO) substrates using the technique of high-pressure oxygen sputtering. Films were then incorporated in a field effect transistor configuration, which facilitated the control of superconductivity by electrostatic charging. While devices using STO as both the substrate and gate dielectric have produced only relatively small shifts in film electrical properties, very large changes can be realized using an electric double layer transistor configuration employing the ionic liquid DEME-TFSI as the dielectric. By depleting holes an electrostatically tuned superconductor insulator transition was studied using a finite size scaling analysis. The breakdown of scaling at the lowest temperatures suggests the presence of a mixed insulator/superconductor phase separating the two ground states. Further depletion of holes resulted in a change of the majority carriers from holes to electrons and the emergence of what appeared to be very weak re-entrant superconductivity. Also by accumulating holes an underdoped film was tuned into the overdoped regime. A two-step mechanism for electrostatic doping was revealed. Hall effect measurements suggested the presence of an electronic phase transition or a change in the Fermi surface as a function of doping near optimal doping.

Progress in understanding and exploiting the properties of complex oxide heterostructures requires advances in stateof- the-art growth and characterization techniques for these materials. Currently, atomic control of their synthesis is demonstrably inferior to that of their semiconductor counterparts, [ 1 ] despite recent progress for example in creating a high mobility two-dimensional electron gas in semiconducting ZnO [ 2 , 3 ] and in exploring novel interface effects in perovskites. [ 4 ] This superiority in growth control can be attributed in part to the development of quantitative, in situ refl ection high-energy electron diffraction (RHEED) possible in the UHV environment typical of semiconductor fi lm growth. [ 5 ] Oxides with the perovskite structure (chemical formula ABO 3 ) have attracted enormous interest as they display a wide range of exotic properties driven by strong spin-charge-lattice coupling, [ 6 ] such as quantum critical behavior in superconducting cuprates, colossal magnetoresistance in phase-separated manganites, and magnetoelectric coupling in BiFeO 3 . This coupling underlies the strong response of the electronic structure to small distortions of the BO 6 octahedra away from simple cubic coordination, distortions driven either by a Jahn-Teller (J-T) mechanism (removing energy level degeneracy by symmetry reduction) or by octahedral rotations to accommodate imperfect packing within the cubic unit cell that depends on cationic radii. These distortions are signifi cantly infl uenced in a stochastic way by disorder, unavoidably introduced by random A-site doping [ 7 ] or off-stoichiometry (ratio A:B ? 1, or oxygen vacancies) often seen in epitaxial films. In the search for novel electronic properties at interfaces, additional defect mechanisms such as cation intermixing and segregation must be addressed and their precise distribution near the interface known. Ultimately, identifying new electronic properties will depend critically on eliminating these forms of disorder and defects, or precisely controlling them if desired, through atomic design. Here we report evidence of signifi cant improvements in the atomic design of perovskite interfaces made possible by advances in in situ RHEED control of surfaces that exploit the detection of the characteristic octahedral distortions in the surface layer as it is being deposited. This method allows optimization of the phase diagram versus doping of films, and eliminates intermixing and anomalous lattice dilations at interfaces that have previously been observed. Careful structural analysis shows an unusual evolution of octahedral distortions including both J-T type and rotations near the interface not seen in bulk. These new results should be included in electronic structure calculations modeling the properties of real heterointerfaces.

We report large magnetoresistance in ferromagnet/superconductor/ferromagnet structures made of La 0.7 Ca 0.3 MnO 3 and YBa 2 Cu 3 O 7 at temperatures along the resistive transition. We find that the magnetoresistance phenomenon is independent on the orientation of electric current versus field. Furthermore, the effect is also independent on the sweep rate of the magnetic field. This excludes interpretations in terms of spontaneous vortices or anisotropic magnetoresistance of the ferromagnetic layers and supports the view that the magnetoresistance phenomenon originates at the spin-dependent transport of quasiparticles transmitted from the ferromagnetic electrodes into the superconductor.

Using an electronic double layer transistor we have systematically studied the superconductor-to-insulator transition in La${}_{2}$CuO${}_{4+\ensuremath{\delta}}$ thin films grown by ozone-assisted molecular-beam epitaxy. We have confirmed the high crystalline quality of the cuprate films and have demonstrated the suitability of the electronic double layer technique to continuously vary the charge density in a system that is otherwise characterized by the presence of miscibility gaps. The transport and magnetotransport results highlight the role of electron-electron interactions in the mechanism of the transition due to the proximity of the Mott-insulating state.

We successfully tuned an underdoped ultrathin YBa2Cu3O(7-x) film into the overdoped regime by means of electrostatic doping using an ionic liquid as a dielectric material. This process proved to be reversible. Transport measurements showed a series of anomalous features compared to chemically doped bulk samples and a different two-step doping mechanism for electrostatic doping was revealed. The normal resistance increased with carrier concentration on the overdoped side and the high temperature (180 K) Hall number peaked at a doping level of p∼0.15. These anomalous behaviors suggest that there is an electronic phase transition in the Fermi surface around the optimal doping level.

We report on the magnetic coupling of La0.7Sr0.3MnO3 layers through SrTiO3 spacers in La0.7Sr0.3MnO3/SrTiO3 epitaxial heterostructures. Combined aberration-corrected microscopy and electron-energy-loss spectroscopy evidence charge transfer to the empty conduction band of the titanate. Ti d electrons interact via superexchange with Mn, giving rise to a Ti magnetic moment as demonstrated by x-ray magnetic circular dichroism. This induced magnetic moment in the SrTiO3 controls the bulk magnetic and transport properties of the superlattices when the titanate layer thickness is below 1 nm.

Colossal ionic conductivity was recently discovered in YSZ/SrTiO_(3) multilayers and was explained in terms of strain- and interface-enhanced disorder of the O sublattice. In the present paper we use a combination of scanning transmission electron microscopy and electron energy loss spectroscopy (EELS) and theoretical EELS simulations to confirm the presence of a disordered YSZ O sublattice in coherent YSZ/SrTiO_(3) multilayers. O K-edge fine structure simulated for the strained disordered O sublattice phase of YSZ possesses blurred-out features compared to that of ordered cubic bulk YSZ, and experimental EELS fine structure taken from the strained YSZ of coherent YSZ/SrTiO_(3) thin films is similarly blurred out. Elemental mapping is shown to be capable of resolving ordered YSZ O sublattices. Elemental mapping of O in the coherent YSZ/STO multilayers is presented in which the O sublattice is seen to be clearly resolved in the STO but blurred out in the YSZ, indicating it to be disordered. In addition, we present imaging and EELS results which show that strained regions exist at the incoherent interfaces of YSZ islands in STO with blurred out fine structure, suggesting these incoherent regions may also support high ionic conductivities. Recently, Cavallaro et al. reported electronic conductivities in samples of incoherent disconnected islands embedded in STO that are similar to the islands described herein. The presence of a region of O depleted STO at the interface with incoherent YSZ islands is revealed by EELS elemental mapping, implying the n-type doping of STO/YSZ nanocomposites with disconnected incoherent YSZ islands.

In systems with strong electron-lattice coupling, such as manganites, orbital degeneracy is lifted, causing a null expectation value of the orbital magnetic moment. Magnetic structure is thus determined by spin-spin superexchange. In titanates, however, with much smaller Jahn-Teller distortions, orbital degeneracy might allow non-zero values of the orbital magnetic moment, and novel forms of ferromagnetic superexchange interaction unique to t(2g) electron systems have been theoretically predicted, although their experimental observation has remained elusive. In this paper, we report a new kind of Ti(3+) ferromagnetism at LaMnO(3)/SrTiO(3) epitaxial interfaces. It results from charge transfer to the empty conduction band of the titanate and has spin and orbital contributions evidencing the role of orbital degeneracy. The possibility of tuning magnetic alignment (ferromagnetic or antiferromagnetic) of Ti and Mn moments by structural parameters is demonstrated. This result will provide important clues for understanding the effects of orbital degeneracy in superexchange coupling.

et al., Direct evidence for charge leakage at the interface of epitaxial SrTiO 3/LaMnO3 superlattices with atomically sharp interfaces is provided. The direction of charge leakage can be reversed by changing the LMO/STO thickness ratio. This result will be important for the understanding of some of the reported limitations of oxide devices involving manganite/titanate interfaces. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim., Research at UCM was supported by MICINN grant MAT200806517.

The magnetic properties of La0.7Ca0.3MnO3 (LCMO) manganite thin films were studied with magnetometry and ferromagnetic resonance as a function of film thickness. They maintain the colossal magnetoresistance behavior with a pronounced metal-insulator transition around 150-200 K, except for the very thinnest films studied (3 nm). Nevertheless, LCMO films as thin as 3 nm remain ferromagnetic, without a decrease in saturation magnetization, indicating an absence of dead-layers, although below approx. 6 nm the films remain insulating at low temperature. Magnetization hysteresis loops reveal that the magnetic easy axes lie in the plane of the film for thicknesses in the range of 4-15 nm. Ferromagnetic resonance studies confirm that the easy axes are in-plane, and find a biaxial symmetry in-plane with two, perpendicular easy axes. The directions of the easy axes with respect to the crystallographic directions of the cubic SrTiO3 substrate differ by 45 degrees in 4 nm and 15 nm thick LCMO films., Presented at Intermag conference (Madrid, 2008). Accepted for publication in IEEE Transactions on Magnetics

We studied ferromagnet/superconductor/ferromagnet trilayers based on La_(0.7)Ca_(0.3)MnO_(3) manganite and YBa_(2)Cu_(3)O_(7−δ) (YBCO) high-T_(c) cuprate with magnetoresistance and magnetization measurements. We find an inverse superconducting spin-switch behavior, where superconductivity is favored for parallel alignment of the magnetization in the ferromagnetic layers. We argue that this inverse superconducting spin switch originates from the transmission of spin-polarized carriers into the superconductor. In this picture, the thickness dependence of the magnetoresistance yields the spin-diffusion length in YBCO as 13 nm. A comparison of bilayers and trilayers allows ruling out the effect of the stray fields of the domain structure of the ferromagnet as the source of the inverse superconducting spin switch.

We report on large magnetoresistance in ferromagnet/superconductor/ferromagnet trilayer structures made of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ and ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$. We find that the shape and height of the magnetoresistance peaks are not modified when the relative orientation of current and magnetic field is changed from parallel to perpendicular. Furthermore, we find that the temperature shift of the resistance curves is independent of current and of the sweep rate of the magnetic field. These observations favor the view that the magnetoresistance phenomenon originates in the spin dependent transport of quasiparticles transmitted from the ferromagnetic electrodes into the superconductor, and rule out interpretations in terms of spontaneous vortices or anisotropic magnetoresistance of the ferromagnetic layers.

We report magnetoresistance in ferromagnet / superconductor / ferromagnet structures made of La0.7Ca0.3MnO3 and YBa2Cu3O7 in the current in plane (CIP) geometry when the temperature is fixed along the superconducting transition and the magnetic field is swept in an hysteresis loop sequence. We describe experiments changing the geometry of current versus field. We find that the shape and height of the magnetoresistance peaks is not modified no matter the field is directed parallel or perpendicular to the current. This excludes interpretations in terms of spontaneous vortices or anisotropic magnetoresistance of the ferromagnetic layers and supports the view that the magnetoresistance phenomenon originates at the spin dependent transport of quasiparticles transmitted from the ferromagnetic electrodes into the superconductor.

We report on the interplay between magnetism and superconductivity in La_(0.7)Ca_(0.3)MnO_(3)/YBa_(2)Cu_(3)O_(7) structures. We have grown heterostructures (bilayers and trilayers) with a constant thickness of the ferromagnetic layer of 40 unit cells (15 nm) and changing the thickness of the superconductor between 1 (1.2 nm) and 40 unit cells (48 nm). The critical temperature of the bilayers decreases when the thickness of the superconductor is reduced below 10 unit cells, thus providing an estimate of the length scale of superconductivity suppression by spin-polarized quasiparticles in YBa_(2)Cu_(3)O_(7-δ) (YBCO) of 10 nm, much larger than the coherence length. For thickness of the YBCO layer smaller than 4 unit cells; a second mechanism of superconductivity depression comes into play, probably related to the ferromagnetic/superconducting proximity effect. The relative importance in depressing the critical temperature of intrinsic mechanisms (quasiparticle diffusion and proximity effect) and extrinsic ones (intralayer disorder, interface roughness, or reduced dimensionality of ultrathin layers) is discussed.

Ultrathin YBa2Cu3O7−x (YBCO) films were grown on SrTiO3 (STO) substrates using a high-pressure oxygen sputtering system. The films were incorporated in a field effect transistor configuration to study the control of superconductivity by electrostatic charging. While devices using STO as both the substrate and gate dielectric have produced only small Tc shifts, a clear transition between superconducting and insulating behaviour was realized using an electronic double layer transistor employing the ionic liquid DEME-TFSI as a gate dielectric. Employing a finite size scaling analysis, curves of resistance vs. temperature were found to collapse into two branches over the temperature range from 6 K to 22K suggesting the existence of a quantum critical point. However the scaling failed at temperatures below this range, indicating the possible presence of an additional phase between the superconducting and insulating regimes. Further depletion of holes appears to result in the accumulation of electrons resulting in a change of the majority carriers from holes to electrons and the emergence of what appears to be very weak re-entrant superconductivity. By changing the polarity of the gate voltage, an underdoped film was tuned into the overdoped regime by accumulating additional holes. An unexpected two-step mechanism for electrostatic doping was revealed. Hall effect measurements exhibited anomalous features, which suggest the occurrence of an electronic phase transition near optimal doping.

Interfaces between correlated oxides are attracting great interest. Electron correlations give rise to novel forms of couplings between electronic ground states at both sides of the interface. The bonding discontinuity at the interface between magnetic and nonmagnetic oxides is at the origin of a form of low dimensional magnetism in the otherwise nonmagnetic material. Its origin is the splitting of its bands due to the hybridization with the exchange split bands of the magnetic material. This induced magnetism could find interesting functionalities in devices with operation controlled by the interface such as tunnel or field effect devices of interest in spintronics.