ABSTRACT
The extremely overdoped cuprates are generally considered to be Fermi liquid metals without exotic orders, whereas the underdoped cuprates harbor intertwined states. Contrary to this conventional wisdom, using Cu L_{3}-edge and O K-edge resonant x-ray scattering, we reveal a charge order (CO) correlation in overdoped La_{2-x}Sr_{x}CuO_{4} (0.35≤x≤0.6) beyond the superconducting dome. This CO has a periodicity of â¼6 lattice units with correlation lengths of â¼20 lattice units. It shows similar in-plane momentum and polarization dependence and dispersive excitations as the CO of underdoped cuprates, but its maximum intensity differs along the c direction and persists up to 300 K. This CO correlation cannot be explained by the Fermi surface instability and its origin remains to be understood. Our results suggest that CO is prevailing in the overdoped metallic regime and requires a reassessment of the picture of overdoped cuprates as weakly correlated Fermi liquids.
ABSTRACT
The recent discovery of superconductivity at the interfaces between KTaO3 and EuO (or LaAlO3) gives birth to the second generation of oxide interface superconductors. This superconductivity exhibits a strong dependence on the surface plane of KTaO3, in contrast to the seminal LaAlO3/SrTiO3 interface, and the superconducting transition temperature Tc is enhanced by one order of magnitude. For understanding its nature, a crucial issue arises: Is the formation of oxide interfaces indispensable for the occurrence of superconductivity? Exploiting ionic liquid (IL) gating, we are successful in achieving superconductivity at KTaO3(111) and KTaO3(110) surfaces with Tc up to 2.0 and 1.0 K, respectively. This oxide-IL interface superconductivity provides a clear evidence that the essential physics of KTaO3 interface superconductivity lies in the KTaO3 surfaces doped with electrons. Moreover, the controllability with IL technique paves the way for studying the intrinsic superconductivity in KTaO3.
ABSTRACT
We report on the observation of a T_{c}â¼0.9 K superconductivity at the interface between LaAlO_{3} film and the 5d transition metal oxide KTaO_{3}(110) single crystal. The interface shows a large anisotropy of the upper critical field, and its superconducting transition is consistent with a Berezinskii-Kosterlitz-Thouless transition. Both facts suggest that the superconductivity is two-dimensional (2D) in nature. The carrier density measured at 5 K is â¼7×10^{13} cm^{-2}. The superconducting layer thickness and coherence length are estimated to be â¼8 and â¼30 nm, respectively. Our result provides a new platform for the study of 2D superconductivity at oxide interfaces.
ABSTRACT
Numerous studies have shown that oxygen vacancies play an important role on the formation of two-dimensional electron gas (2DEG) at SrTiO3-based heterointerfaces. Previously, it is widely believed that the main mechanism is that the oxygen vacancies in SrTiO3 directly contribute electrons to the 2DEG. Here, we performed transport measurements during the creation of 2DEG for depositing amorphous LaAlO3 on SrTiO3 substrates and related heterostructures. Our result suggests that, unlike the previous viewpoint, in this kind of 2DEG the determinant mechanism is the electron transfer from the oxygen vacancies in the film grown on SrTiO3, rather than the oxygen vacancies in SrTiO3 themselves. This effect is so striking that an amorphous film of less than 10% monolayer coverage on SrTiO3, or equivalently 0.04 nm, can already generate a highly conducting 2DEG. The present result may have a general implication and provide a possible way to understand the long-standing debate on the origin of 2DEG at SrTiO3-based heterointerfaces.
ABSTRACT
The single-crystal SrTiO3 (001) has two different surface terminations, TiO2 and SrO. One most remarkable observation in previous studies is that only the heterointerfaces with TiO2-terminated SrTiO3, which usually combines with polar oxides such as LaAlO3, host an electron gas. Here we show that a robust electron gas can be generated between a non-polar oxide, CaHfO3, and SrTiO3 (001) with either termination. Unlike the well-known electron gas of LaAlO3/SrTiO3, the present one of CaHfO3/SrTiO3 essentially has no critical thickness of CaHfO3, can survive a long-time oxygen annealing at high temperature, and its transport properties are stable under exposure to water and other polar solvents. By electrostatic gating through CaHfO3, field-effect devices are demonstrated using CaHfO3/SrTiO3 heterointerfaces with both terminations. These results show that the electron gas reported in the present study is unique and promising for applications in oxide electronics.
