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Register a new userManipulating electronic states at oxide interfaces using focused micro X-rays from standard lab-sources
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Recently, x-ray illumination, using synchrotron radiation, has been used to manipulate defects, stimulate self-organization and to probe their structure. Here we explore a method of defect-engineering low-dimensional systems using focused laboratory-scale X-ray sources. We demonstrate an irreversible change in the conducting properties of the 2-dimensional electron gas at the interface between the complex oxide materials LaAlO3 and SrTiO3 by X-ray irradiation. The electrical resistance is monitored during exposure as the irradiated regions are driven into a high resistance state. Our results suggest attention shall be paid on electronic structure modification in X-ray spectroscopic studies and highlight large-area defect manipulation and direct device patterning as possible new fields of application for focused laboratory X-ray sources.
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Here we study the electronic properties of cuprate/manganite interfaces. By means of atomic resolution electron microscopy and spectroscopy, we produce a subnanometer scale map of the transition metal oxidation state profile across the interface between the high $T_c$ superconductor YBa$_2$Cu$_3$O$_{7-delta}$ and the colossal magnetoresistance compound (La,Ca)MnO$_3$. A net transfer of electrons from manganite to cuprate with a peculiar non-monotonic charge profile is observed. Model calculations rationalize the profile in terms of the competition between standard charge transfer tendencies (due to band mismatch), strong chemical bonding effects across the interface, and Cu substitution into the Mn lattice, with different characteristic length scales.
We describe the ground- and excited-state electronic structure of bulk MnO and NiO, two prototypical correlated electron materials, using coupled cluster theory with single and double excitations (CCSD). As a corollary, this work also reports the first implementation of unrestricted periodic ab initio equation-of motion CCSD. Starting from a Hartree-Fock reference, we find fundamental gaps of 3.46 eV and 4.83 eV for MnO and NiO respectively for the 16 unit supercell, slightly overestimated compared to experiment, although finite-size scaling suggests that the gap is more severely overestimated in the thermodynamic limit. From the character of the correlated electronic bands we find both MnO and NiO to lie in the intermediate Mott/charge-transfer insulator regime, although NiO appears as a charge transfer insulator when only the fundamental gap is considered. While the lowest quasiparticle excitations are of metal 3d and O 2p character in most of the Brillouin zone, near the {Gamma} point, the lowest conduction band quasiparticles are of s character. Our study supports the potential of coupled cluster theory to provide high level many-body insights into correlated solids.
By using a realist microscopic model, we study the electric and magnetic properties of the interface between a half metallic manganite and an insulator. We find that the lack of carriers at the interface debilitates the double exchange mechanism, weakening the ferromagnetic coupling between the Mn ions. In this situation the ferromagnetic order of the Mn spins near the interface is unstable against antiferromagnetic CE correlations, and a separation between ferromagnetic/metallic and antiferromagnetic/insulator phases at the interfaces can occur. We obtain that the insertion of extra layers of undoped manganite at the interface introduces extra carriers which reinforce the double exchange mechanism and suppress antiferromagnetic instabilities.
The metallic interface between two oxide insulators, such as LaAlO3/SrTiO3 (LAO/STO), provides new opportunities for electronics and spintronics. However, due to the presence of multiple orbital populations, tailoring the interfacial properties such as the ground state and metal-insulator transitions remains challenging. Here, we report an unforeseen tunability of the phase diagram of LAO/STO by alloying LAO with a ferromagnetic LaMnO3 insulator without forming lattice disorder and at the same time without changing the polarity of the system. By increasing the Mn-doping level, x, of LaAl1-xMnxO3/STO, the interface undergoes a Lifshitz transition at x = 0.225 across a critical carrier density of nc= 2.8E13 cm-2, where a peak TSC =255 mK of superconducting transition temperature is observed. Moreover, the LaAl1-xMnxO3 turns ferromagnetic at x >=0.25. Remarkably, at x = 0.3, where the metallic interface is populated by only dxy electrons and just before it becomes insulating, we achieve reproducibly a same device with both signatures of superconductivity and clear anomalous Hall effect. This provides a unique and effective way to tailor oxide interfaces for designing on-demand electronic and spintronic devices.
Two-dimensional electron gas (2DEG) confined in quantum wells at insulating oxide interfaces have attracted much attention as their electronic properties display a rich physics with various electronics orders such as superconductivity and magnetism. A particularly exciting features of these hetero-structures lies in the possibility to control their electronic properties by electrostatic gating, opening up new opportunities for the development of oxide based electronics. However, unexplained gating hysteresis and time relaxation of the 2DEG resistivity have been reported in some bias range, raising the question of the precise role of the gate voltage. Here we show that in LaTiO3/SrTiO3 and LaAlO3/SrTiO3 heterostructures, above a filling threshold, electrons irreversibly escape out of the well. This mechanism, which is directly responsible for the hysteresis and time relaxation, can be entirely described by a simple analytical model derived in this letter. Our results highlight the crucial role of the gate voltage both on the shape and the filling of the quantum well. They also demonstrate that it is possible to achieve a low-carrier density regime in a semiconductor limit, whereas the high-carrier density regime is intrinsically limited.
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