The physics of the superconducting state in two-dimensional (2D) electron systems is relevant to understanding the high-transition-temperature copper oxide superconductors and for the development of future superconductors based on interface electron systems. But it is not yet understood how fundamental superconducting parameters, such as the spectral density of states, change when these superconducting electron systems are depleted of charge carriers. Here we use tunnel spectroscopy with planar junctions to measure the behaviour of the electronic spectral density of states as a function of carrier density, clarifying this issue experimentally. We chose the conducting LaAlO3-SrTiO3 interface as the 2D superconductor, because this electron system can be tuned continuously with an electric gate field. We observed an energy gap of the order of 40 microelectronvolts in the density of states, whose shape is well described by the Bardeen-Cooper-Schrieffer superconducting gap function. In contrast to the dome-shaped dependence of the critical temperature, the gap increases with charge carrier depletion in both the underdoped region and the overdoped region. These results are analogous to the pseudogap behaviour of the high-transition-temperature copper oxide superconductors and imply that the smooth continuation of the superconducting gap into pseudogap-like behaviour could be a general property of 2D superconductivity.

These authors contributed equally to this work.The ability to control materials properties through interface engineering is demonstrated by the appearance of conductivity at the interface of certain insulators, most famously the {001} interface of the band insulators LaAlO 3 (LAO) and TiO 2 -terminated SrTiO 3 (STO) 1,2 . Transport and other measurements in this system display a plethora of diverse physical phenomena 3-14 . To better understand the interface conductivity, we used scanning superconducting quantum interference device (SQUID) microscopy to image the magnetic field locally generated by current in an interface. At low temperature, we found that the current flowed in highly conductive narrow paths oriented along the crystallographic axes, embedded in a less conductive background. The configuration of these paths changed upon thermal cycling above the STO cubic to tetragonal structural transition temperature, implying that local conductivity is strongly modified by STO tetragonal domain

The rich array of conventional and exotic electronic properties that can be generated by oxide heterostructures is of great potential value for device applications. However, only single transistors bare of any circuit functionality have been realized from complex oxides. Here, monolithically‐integrated n‐type metal‐oxide‐semiconductor logic circuits are reported that utilize the two‐dimensional electron liquid generated at the LaAlO3/SrTiO3 interface. Providing the capability to process the signals of functional oxide devices such as sensors directly on oxide chips, these results illustrate the practicability and the potential of oxide electronics.

Using the metal-insulator transition that takes place as a function of carrier density at the LaAlO3-SrTiO3 interface, oxide diodes have been fabricated with room-temperature breakdown voltages of up to 200 V. With applied voltage, the capacitance of the diodes changes by a factor of 150. The diodes are robust and operate at temperatures up to 270 C.

Phase-sensitive, spatially resolved optical second-harmonic-generation experiments were performed on LaAlO 3 /SrTiO 3 heterostructures. Lateral inhomogeneities on a length scale of ≈ 30 μm are found when a one-unit-cell-thick epitaxial monolayer of LaAlO 3 is grown on TiO 2 -terminated SrTiO 3 single crystals. The inhomogeneity is absent in samples with LaAlO 3 layers of more than one unit cell. The results are discussed in the framework of electronic, oxidic, and chemical inhomogeneities.