Orientation-dependent electron-phonon coupling in interfacial superconductors LaAlO3/KTaO3

Abstract

The emergent superconductivity at the LaAlO 3 /KTaO 3 interfaces exhibits a mysterious dependence on the KTaO 3 crystallographic orientations. Here we show, by soft X-ray angle-resolved photoemission spectroscopy, that the interfacial superconductivity is contributed by mobile electrons with unexpected quasi-threedimensional character, beyond the "two-dimensional electron gas" scenario in describing oxide interfaces. At differently-oriented interfaces, the quasi-three-dimensional electron gas ubiquitously exists and spatially overlaps with the small q Fuchs-Kliewer surface phonons. Intriguingly, electrons and the Fuchs-Kliewer phonons couple with different strengths depending on the interfacial orientations, and the stronger coupling correlates with the higher superconducting transition temperature. Our results provide a natural explanation for the orientation-dependent superconductivity, and the first evidence that interfacial orientations can affect electronphonon coupling strength over several nanometers, which may have profound implications for the applications of oxide interfaces in general.

First-principles calculation of electron-phonon coupling in doped KTaO3

“…Our calculations indicate that the measured trends in superconducting T c are not reflected in the calculated electron-phonon coupling strengths λ along the corresponding reciprocal directions, confirming earlier suggestions that the superconductivity is not bulk BCS-like in nature 7 , 17 , 20 , 58 . In this context, recent angle-resolved photoemission spectroscopy measurements implicating coupling of bulk-like electrons to Fuchs-Kliewer surface phonons are highly relevant 59 . The concentration of λ in the lowest frequency optical modes close to Γ hints towards a mechanism in which the polar soft mode plays a role.…”

钽酸钾界面超导的发现与研究进展

“…尽 管有KTO界面存在磁性的报道 [72,73] , 但是综合来看这 一界面并不存在明显的磁有序, 基本可以排除磁涨落 相关机制. 最近, 陈仙辉团队 构是二维的, 这与软X射线角分辨光电子能谱实验结果 不一致 [32] ; 二是T c 与n 2D 的线性关系在实验上并非是普 适的, 比如, 在电场调控实验中, 对于高n 2D 的样品(也 是最典型的KTO界面超导样品), T c 与n 2D 并不呈现明显 依赖关系 [27,31,39] .…”

“…(b) 通过TO声子的超导配对模型 [39] . (c) LAO/KTO(001), LAO/KTO(110)和LAO/KTO(111)沿k [110] 方向的光电子能谱 [32] . (d) 依赖KTO界面取向的电声耦合 [32] Figure 7 Indication of possible superconducting pairing mechanisms.…”

“…(c) LAO/KTO(001), LAO/KTO(110)和LAO/KTO(111)沿k [110] 方向的光电子能谱 [32] . (d) 依赖KTO界面取向的电声耦合 [32] Figure 7 Indication of possible superconducting pairing mechanisms. (a) n 2D dependence of T c for KTO(111) samples [39] .…”

“…(b) pairing via transverse optical phonons [39] . (c) Photoemission spectra along k [110] for LAO/KTO(001), LAO/KTO(110), and LAO/KTO(111) [32] . (d) Orientation-dependent electron-phonon coupling in LAO/KTO interfaces [32] 电声耦合明显依赖于晶面方向.…”