The so-called proximity effect is the manifestation, across an interface, of the systematic competition between magnetic order and superconductivity. This phenomenon has been well documented and ...understood for conventional superconductors coupled with metallic ferromagnets; however it is still less known for oxide materials, where much higher critical temperatures are offered by copper oxide-based superconductors. Here we show that, even in the absence of direct Cu-O-Mn covalent bonding, the interfacial CuO2 planes of superconducting La(1.85)Sr(0.15)CuO(4) thin films develop weak ferromagnetism associated to the charge transfer of spin-polarised electrons from the La(0.66)Sr(0.33)MnO(3) ferromagnet. Theoretical modelling confirms that this effect is general to all cuprate/manganite heterostructures and the presence of direct bonding only affects the strength of the coupling. The Dzyaloshinskii-Moriya interaction, also at the origin of the weak ferromagnetism of bulk cuprates, propagates the magnetisation from the interface CuO2 planes into the superconductor, eventually depressing its critical temperature.
The electric field control of d-electron magnetism in multiferroic transition metal oxides is attracting widespread interest for the underlying fundamental physics and for next generation spintronic ...devices. Here, we report an extensive study of the 3 d magnetism in magnetoelectric Ga0.6 Fe1.4 O3 (GFO) epitaxial films by polarization-dependent x-ray absorption spectroscopy. We found a nonzero integral of the x-ray magnetic circular dichroism (XMCD), with the sign depending upon the relative orientation between the external magnetic field and the crystallographic axes. This finding translates into a sign reversal between the average Fe magnetic orbital and spin moments. Large Fe displacements, among inequivalent octahedral sites, lower the symmetry of the system producing anisotropic paths for the Fe-O bondings giving rise to a large orbital-lattice interaction akin to a preferential crystallographic direction for the uncompensated, or averaged among different sites, magnetic orbital moment. The latter may lead to a partial reorientation of the magnetic orbital moment under an external magnetic field that, combined with the ferrimagnetic nature of the GFO, can qualitatively explain the observed sign reversal of the XMCD integral. The results suggest that control over the local symmetry of the oxygen octahedra in transition metal oxides can offer a suitable leverage over the manipulation of the effective orbital and spin moments in magnetoelectric systems.
N-type organic field-effect transistors (OFETs), based on two perylene diimide semiconductors (PDI-8 and PDI-8CN2) exhibiting very different air sensitivities, have been fabricated on Si/SiO2 ...substrates. These OFETs have been electrically characterized in vacuum both in the dark and under white-light illumination by dc transfer and output curves, bias stress experiments and variable temperature measurements. In particular, the combination of variable temperature and light illumination experiments is shown to be a powerful tool to clarify the influence of charge trapping on the device operation. Even if, in vacuum, the air-sensitive PDI-8 devices display slightly better performances in terms of field-effect mobility and maximum current values, according to our results, charge transport in PDI-8 films is much more affected by charge trap states compared to PDI8-CN2 devices. These trapping centers are mainly active above 180 K, and their physical nature can be basically ascribed to the interaction between silanol groups and water molecules absorbed on SiO2 surface that is more active above the H2O supercooled transition temperature.
We study the superconducting state of the two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 interface using Josephson junctions as spectroscopic probes. The transport properties of these ...devices reveal the presence of two superconducting gap structures and of an unconventional superconducting π channel. These features provide evidence of an unconventional superconducting ground state, possibly related to the interplay between superconductivity and the large Rashba spin-orbit coupling in the 2DEG.
Heterostructures obtained by combining p-type and n-type organic semiconductors are attracting considerable attention for the interesting physical phenomena arising at organic/organic interfaces, as ...well as for their potential application in ambipolar field-effect transistors. Here we report on the observation of an interfacial charge transfer effect in high-quality sexithiophene (T6)/N,N-bis(n-octyl)-dicyanoperylenediimide (PDI-8CN sub(2)) heterostructures, fabricated in situ by a controlled sequential evaporation of T6 and PDI-8CN sub(2) thin films on Si/SiO sub(2) substrates. The electrical characterization of several heterostructures as a function of the thickness of each film shows that the hole and electron transport cannot be explained by invoking only the properties of the individual layers. Electrical characterization and nonlinear optical spectroscopy give clear indications of charge transfer between T6 and PDI-8CN sub(2) layers, accompanied by band offset and interfacial charge accumulation (heterojunction effect) at the organic/organic interface. Signatures of such effect were the enhancement of drain-source current at zero gate voltage, a shift of gate threshold voltage in transfer curves, and an enhanced second-harmonic generation related to a built-in interfacial electric field. Gate-voltage-tunable negative transconductance has been also observed in heterostructures composed by thin T6 layers and explained in terms of gate-voltage-tunable recombination phenomena occurring in the hole/electron accumulation layer.
We used x-ray absorption spectroscopy to study the orbital symmetry and the energy band splitting of (111) LaAlO3/SrTiO3 and LaAlO3/EuTiO3/SrTiO3 heterostructures, hosting a quasi-two-dimensional ...electron system (q2DES), and of a Ti-terminated (111) SrTiO3 single crystal, also known to form a q2DES at its surface. We demonstrate that the bulk tetragonal Ti-3d D4h crystal field is turned into a trigonal D3d crystal field in all cases. The symmetry adapted a1g and egπ orbitals are nondegenerate in energy and their splitting, Δ, is positive at the bare STO surface but negative in the heterostructures, where the a1g orbital is lowest in energy. These results demonstrate that the interfacial symmetry breaking induced by epitaxial engineering of oxide interfaces has a dramatic effect on their electronic properties, and it can be used to manipulate the ground state of the q2DES.
The new frontier for spintronics is the realization of devices in which the spin can be controlled by electric fields. Multiferroics, materials exhibiting strong interplay between spin and orbital ...degrees of freedom, are candidates for the realization of such a paradigm. In this work, we study the magnetoelectric coupling in epitaxial
BiMnO
3
thin films which exhibit a large saturation magnetization. By combining X-ray absorption spectroscopy data and theoretical modeling, we demonstrate that
BiMnO
3
thin films have an improper magnetoelectric behavior, characterized by competing antiferromagnetic and ferromagnetic correlations. As a consequence, we show that in these materials the Mn-3d orbital and magnetic orders can be tuned via the ferroelectric polarization, opening perspectives for the realization of novel spintronic devices.
Pervasive fluctuationsAmong the many intertwined phases in the cuprate superconductor phase diagram is the charge density wave (CDW) order, which has been detected in all major cuprate families. It ...is thought that CDW competes with superconductivity, but whether it has bearing on the mechanism of superconductivity remains unclear. Arpaia et al. undertook a comprehensive study of charge density fluctuations in a cuprate family, varying doping and temperature. They found that short-range dynamic charge fluctuations were present in a large portion of the phase diagram, at temperatures considerably higher than those at which the CDW order disappears.Science, this issue p. 906Charge density modulations have been observed in all families of high–critical temperature (Tc) superconducting cuprates. Although they are consistently found in the underdoped region of the phase diagram and at relatively low temperatures, it is still unclear to what extent they influence the unusual properties of these systems. Using resonant x-ray scattering, we carefully determined the temperature dependence of charge density modulations in YBa2Cu3O7–δ and Nd1+xBa2–xCu3O7–δ for several doping levels. We isolated short-range dynamical charge density fluctuations in addition to the previously known quasi-critical charge density waves. They persist up to well above the pseudogap temperature T*, are characterized by energies of a few milli–electron volts, and pervade a large area of the phase diagram.
Studies on oxide quasi-two dimensional electron gas (q2DEG) have been a playground for the discovery of novel and sometimes unexpected phenomena, like the reported magnetism at the surface and at the ...interface between LaAlO$_{3}$ and SrTiO$_{3}$ non-magnetic materials. However, magnetism in this system is weak and there are evidences of a not intrinsic origin. Here, by using in-situ high-resolution angle resolved photoemission we demonstrate that ferromagnetic EuTiO$_{3}$, the magnetic counterpart of SrTiO$_{3}$ in the bulk, hosts a q2DEG at its (001) surface. This is confirmed by density functional theory calculations with Hubbard U terms in the presence of oxygen divacancies in various configurations, all of them leading to a spin-polarized q2DEG related to the ferromagnetic order of Eu-4f magnetic moments. The results suggest EuTiO$_{3}$(001) as a new material platform for oxide q2DEGs, characterized by broken inversion and time reversal symmetries.
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