Optical data are reported on a spectral weight transfer over a broad frequency range of Bi2Sr2CaCu2O8+delta, when this material became superconducting. Using spectroscopic ellipsometry, we observed the removal of a small amount of spectral weight in a broad frequency band from 10(4) cm(-1) to at least 2 x 10(4) cm(-1), due to the onset of superconductivity. We observed a blue shift of the ab-plane plasma frequency when the material became superconducting, indicating that the spectral weight was transferred to the infrared range. Our observations are in agreement with models in which superconductivity is accompanied by an increased charge carrier spectral weight. The measured spectral weight transfer is large enough to account for the condensation energy in these compounds.

The drive to develop materials with new multifunctional capabilities has rekindled interest in multiferroics-systems which are characterized by the simultaneous presence of, and coupling between, magnetic and electric order parameters. In naturally occurring multiferroics the magnetoelectric coupling is often weak, and new classes of artificially structured composite materials that combine dissimilar magnetic and ferroelectric systems are being developed to optimize order parameter coupling.[1-6] Here, we describe direct, charge-mediated magnetoelectric coupling in a heterogeneous multiferroic that takes advantage of the sensitivity of a strongly correlated magnetic system to competing electronic ground states. Using magneto-optic Kerr effect magnetometry, we observe large magnetoelectric coupling in ferroelectric/lanthanum manganite heterostructures, including electric field-controlled on/off switching of magnetism. These results open a new vista for the development of novel magnetoelectric devices with large charge coupling between electric and magnetic degrees of freedom.Doped lanthanum manganites are complex oxides characterized by a strong interplay between electron transport, magnetism, and crystal lattice distortions, leading to a rich variety of electronic behavior, including magnetic and charge-ordered states, colossal magnetoresistance (CMR), and a diversity of electron transport behavior. Underlying the competition between these ground states is the prominent role of charge in double exchange, hopping, and orbital overlap. [7,8] To date, controlling charge as a parameter has most often been achieved using chemical doping, which is robust, and permanent. An alternative approach to modulate carrier density is to use an electrostatic field, [9][10][11][12][13][14][15] which has been used successfully to modulate charge-dependent phenomena, including superconductivity [16] and dilute magnetic semiconducting behavior. [12,13,17] In these systems, the nature of the electron correlations results in a strong sensitivity of the material properties to the charge-carrier concentration.Magnetism has also been controlled at interfaces using field effects. Magnetotransport measurements (planar and anomalous Hall effect, magnetoresistance, resistance) indicate large changes in critical temperature, [12,13,[18][19][20] while changes in coercivity have also been observed. [14] Moreover, magnetoelectric effects at interfaces have been predicted to arise from spin density accumulation in metallic ferromagnet/ferroelectric structures, induced by charge screening of the electric field.[5] These experimental and theoretical results point to the potential of these types of structures for nanostructured multiferroics.Here, we demonstrate a large charge-driven magnetoelectric coupling effect in a Sr-doped lanthanum manganite/ferroelectric composite structure resulting from direct control of magnetism via charge carrier density. This approach has the advantage that its physical mechanism is transparent and the size of the effec...

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The synthesis of materials with well-controlled composition and structure improves our understanding of their intrinsic electrical transport properties. Recent developments in atomically controlled growth have been shown to be crucial in enabling the study of new physical phenomena in epitaxial oxide heterostructures. Nevertheless, these phenomena can be infl uenced by the presence of defects that act as extrinsic sources of both doping and impurity scattering. Control over the nature and density of such defects is therefore necessary to fully understand the intrinsic materials properties and exploit them in future device technologies. Here, it is shown that incorporation of a strontium copper oxide nano-layer strongly reduces the impurity scattering at conducting interfaces in oxide LaAlO 3 -SrTiO 3 (001) heterostructures, opening the door to high carrier mobility materials. It is proposed that this remote cuprate layer facilitates enhanced suppression of oxygen defects by reducing the kinetic barrier for oxygen exchange in the hetero-interfacial fi lm system. This design concept of controlled defect engineering can be of signifi cant importance in applications in which enhanced oxygen surface exchange plays a crucial role.