Chengjian Li

Two-dimensional superconductivity at (110) LaAlO3/SrTiO3 interfaces

Novel low dimensional quantum phenomena at (110) LaAlO3/SrTiO3 (LAO/STO) interfaces are expected after the quasi two dimensional electron gas similar to that of (001) LAO/STO interfaces was found at this (110) system. Here, we report the two dimensional superconductivity with a superconducting transition temperature of ≅ 184 mK at (110) LAO/STO interfaces. The two dimensional characteristics of the superconductivity are consistent with our analysis based on a Berezinskii-Kosterlitz-Thouless transition. The estimated superconducting layer thickness is about 18 nm. This discovery may inspire new studies of LAO/STO interfaces and open additional opportunities for design of novel oxide electronic devices.

Gate dependence of upper critical field in superconducting (110) LaAlO3/SrTiO3 interface

The fundamental parameters of the superconducting state such as coherence length and pairing strength are essential for understanding the nature of superconductivity. These parameters can be estimated by measuring critical parameters such as upper critical field, Hc2. In this work, Hc2 of a superconducting (110) LaAlO3/SrTiO3 interface is determined through magnetoresistive measurements as a function of the gate voltage, VG. When VG increases, the critical temperature has a dome-like shape, while Hc2 monotonically decreases. This relationship of independence between the variation of Tc and of Hc2 suggests that the Cooper pairing potential is stronger in the underdoped region and the coherence length increases with the increase of VG. The result is as for high temperature superconducting cuprates and it is different than for conventional low temperature superconductors.

Formation of Two-dimensional Electron Gas at Amorphous/Crystalline Oxide Interfaces

Experimentally, we found the percentage of low valence cations, the ionization energy of cations in film, and the band gap of substrates to be decisive for the formation of two-dimensional electron gas at the interface of amorphous/crystalline oxide (a-2DEG). Considering these findings, we inferred that the charge transfer from the film to the interface should be the main mechanism of a-2DEG formation. This charge transfer is induced by oxygen defects in film and can be eliminated by the electron-absorbing process of cations in the film. Based on this, we propose a simple dipole model that successfully explains the origin of a-2DEG, our experimental findings, and some important properties of a-2DEG.

Interface-enhanced sensitivity of photoconductivity to the electric current and magnetic field in La0.67Ca0.33MnO3/Nb-SrTiO3 p-n junctions

The effects of electric current and magnetic field on the photoconductivity (PC) of La0.67Ca0.33MnO3 (LCMO) films have been investigated within a comparative study on LCMO films on an n-type semiconducting Nb-doped SrTiO3 (NSTO) and insulating SrTiO3 (STO) substrates separately. At room temperature, LCMO on NSTO (LCMO/NSTO) was found to show remarkable PC effect than that on STO (LCMO/STO). More attractive is that, compared with that of LCMO/STO, the room-temperature PC of LCMO/NSTO also shows a high sensitivity to the change of current or magnetic field: for LCMO/NSTO under a light illumination with an intensity of 60 mW/cm2, the increment of current yields a decreases of PC at a rate of 7.6%/μA, while an application of magnetic field of 1.2 T can suppress PC by a percentage as high as ∼20%. In contrast, under the same stimulus of current or field, PC of LCMO/STO remains almost unchanged. The phenomena can be explained based on the interface effects correlated with the photocarrier injection, current shu...

Influence of In-Gap States on the Formation of Two-Dimensional Election Gas at ABO 3 /SrTiO 3 Interfaces

We explored in-gap states (IGSs) in perovskite oxide heterojunction films. We report that IGSs in these films play a crucial role in determining the formation and properties of interfacial two-dimensional electron gas (2DEG). We report that electron trapping by IGSs opposes charge transfer from the film to the interface. The IGS in films yielded insulating interfaces with polar discontinuity and explained low interface carrier density of conducting interfaces. An ion trapping model was proposed to explain the physics of the IGSs and some experimental findings, such as the unexpected formation of 2DEG at the initially insulating LaCrO3/SrTiO3 interface and the influence of substitution layers on 2DEG.

Conductivity and band alignment of LaCrO3/SrTiO3 (111) heterostructure

In this work, we investigate the electrical transport property and electronic structure of oxide heterostructure LaCrO3/SrTiO3 (111). The interface grown under relatively low oxygen partial pressure is found to be metallic with a conducting critical thickness of 11 unit cells of LaCrO3. This criticality is also observed by x-ray photoelectron spectroscopy, in which the Ti3+ signal intensity at the spectrum edge of the Ti-2p3/2 core level increases rapidly when the critical thickness is reached. The variations of the valence band offset and full width at half maximum of the core-level spectrum with LaCrO3 thickness suggest that the built-in fields exist both in LaCrO3 and in SrTiO3. Two possible origins are proposed: the charge transfer from LaCrO3 and the formation of a quantum well in SrTiO3. Our results shed light on the understanding of the doping mechanism at the polar/non-polar oxide interface. Moreover, due to the interesting lattice and spin structure of LCO in the (111) direction, our work provides a basis for further exploring the novel topological quantum phenomena in this system.