Hans Boschker

Interface superconductor with gap behaviour like a high-temperature superconductor

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 40u2009microelectronvolts 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.

Quantum-Matter Heterostructures

Combining the power and possibilities of heterostructure engineering with the collective and emergent properties of quantum materials, quantum-matter heterostructures open a new arena of solid-state physics. Here we provide a review of interfaces and heterostructures made of quantum matter. Unique electronic states can be engineered in these structures, giving rise to unforeseeable opportunities for scientific discovery and potential applications. We discuss the present status of this nascent field of quantum-matter heterostructures and its limitations, perspectives, and challenges.

Evidence for superconducting phase coherence in the metallic/insulating regime of the LaAlO3–SrTiO3 interface electron system

A superconducting phase with an extremely low carrier density of the order of 10^13 cm^-2 is present at LaAlO3-SrTiO3 interfaces. If depleted from charge carriers by means of a gate field, this superconducting phase undergoes a transition into a metallic/insulating state that is still characterized by a gap in the spectral density of states. Measuring and analyzing the critical field of this gap, we provide evidence that macroscopically phase-coherent Cooper pairs are present in the metallic/insulating state. This is characterized by fluctuating vortex-antivortex pairs, and not by individual, immobile Cooper pairs. The measurements furthermore yield the carrier-density dependence of the superconducting coherence length of the two-dimensional system.

Artificial atoms based on correlated materials

Low-dimensional electron systems fabricated from quantum matter have in recent years become available and are being explored with great intensity. This article gives an overview of the fundamental properties of such systems and summarizes the state of the field. We furthermore present and consider the concept of artificial atoms fabricated from quantum materials, anticipating remarkable scientific advances and possibly important applications of this new field of research. The surprising properties of these artificial atoms and of molecules or even of solids assembled from them are presented and discussed.

Coherently strained epitaxial YBa2Cu3O7−δ films grown on NdGaO3 (110)

YBa

Investigation of LaAlO3-SrTiO3 field-effect transistors under hydrostatic pressure

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Monolithically Integrated Circuits from Functional Oxides

Cu

WITHDRAWN: Nanoscale patterning of complex-oxide materials

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Oxide Microelectronics: Monolithically Integrated Circuits from Functional Oxides (Adv. Mater. Interfaces 1/2014)

O